diff --git a/sw/ground_segment/cockpit/gcs.ml b/sw/ground_segment/cockpit/gcs.ml
index 08bc053387a..14b1fb1925e 100644
--- a/sw/ground_segment/cockpit/gcs.ml
+++ b/sw/ground_segment/cockpit/gcs.ml
@@ -26,6 +26,7 @@ module G = MapCanvas
 open Printf
 open Latlong
 
+
 let locale = GtkMain.Main.init ~setlocale:false ()
 
 let soi = string_of_int
@@ -377,7 +378,7 @@ let options =
     "-auto_hide_fp", Arg.Unit (fun () -> Live.auto_hide_fp true; hide_fp := true), "Automatically hide flight plans of unselected aircraft";
     "-timestamp", Arg.Set timestamp, "Bind on timestampped telemetry messages";
     "-ac_ids", Arg.String (fun s -> Live.filter_ac_ids s), "comma separated list of AC IDs to show in GCS";
-    "-no_confirm_kill", Arg.Unit (fun () -> confirm_kill := false), "Disable kill confirmation from strip button"; 
+    "-no_confirm_kill", Arg.Unit (fun () -> confirm_kill := false), "Disable kill confirmation from strip button";
   ]
 
 
@@ -498,7 +499,7 @@ let resize = fun (widget:GObj.widget) orientation size ->
 
 let rec pack_widgets = fun orientation xml widgets packing ->
   let size = try Some (ExtXml.int_attrib xml "size") with _ -> None in
-  match String.lowercase (Xml.tag xml) with
+  match Compat.bytes_lowercase (Xml.tag xml) with
       "widget" ->
         let name = ExtXml.attrib xml "name" in
         let widget =
@@ -525,7 +526,7 @@ and pack_list = fun resize orientation xmls widgets packing ->
 
 let rec find_widget_children = fun name xml ->
   let xmls = Xml.children xml in
-  match String.lowercase (Xml.tag xml) with
+  match Compat.bytes_lowercase (Xml.tag xml) with
       "widget" when ExtXml.attrib xml "name" = name -> xmls
     | "rows" | "columns" ->
       let rec loop = function
@@ -539,7 +540,7 @@ let rec find_widget_children = fun name xml ->
 
 let rec replace_widget_children = fun name children xml ->
   let xmls = Xml.children xml
-  and tag = String.lowercase (Xml.tag xml) in
+  and tag = Compat.bytes_lowercase (Xml.tag xml) in
   match tag with
       "widget" ->
         Xml.Element("widget",
@@ -561,7 +562,7 @@ let rec update_widget_size = fun orientation widgets xml ->
     if orientation = `HORIZONTAL then rect.Gtk.width else rect.Gtk.height
   in
   let xmls = Xml.children xml
-  and tag = String.lowercase (Xml.tag xml) in
+  and tag = Compat.bytes_lowercase (Xml.tag xml) in
   match tag with
       "widget" ->
         let name = ExtXml.attrib xml "name" in
diff --git a/sw/ground_segment/cockpit/live.ml b/sw/ground_segment/cockpit/live.ml
index d9130033dd6..ec4ddbd9d72 100644
--- a/sw/ground_segment/cockpit/live.ml
+++ b/sw/ground_segment/cockpit/live.ml
@@ -27,6 +27,7 @@ open Latlong
 module LL = Latlong
 open Printf
 
+
 module Tele_Pprz = PprzLink.Messages(struct let name = "telemetry" end)
 module Ground_Pprz = PprzLink.Messages(struct let name = "ground" end)
 module Alert_Pprz = PprzLink.Messages(struct let name = "alert" end)
@@ -167,7 +168,7 @@ let select_ac = fun acs_notebook ac_id ->
     if !active_ac <> "" then begin
       let ac' = find_ac !active_ac in
       ac'.strip#hide_buttons ();
-      ac'.notebook_label#set_width_chars (String.length ac'.notebook_label#text);
+      ac'.notebook_label#set_width_chars (Compat.bytes_length ac'.notebook_label#text);
       if !_auto_hide_fp then hide_fp ac'
     end;
 
@@ -339,12 +340,12 @@ let mark = fun (geomap:G.widget) ac_id track plugin_frame ->
 let attributes_pretty_printer = fun attribs ->
   (* Remove the optional attributes *)
   let valid = fun a ->
-    let a = String.lowercase a in
+    let a = Compat.bytes_lowercase a in
     a <> "no" && a <> "strip_icon" && a <> "strip_button" && a <> "pre_call"
     && a <> "post_call" && a <> "key" && a <> "group" in
 
   let sprint_opt = fun b s ->
-    if String.length b > 0 then
+    if Compat.bytes_length b > 0 then
       sprintf " %s%s%s" s b s
     else
       ""
@@ -631,7 +632,7 @@ let create_ac = fun ?(confirm_kill=true) alert (geomap:G.widget) (acs_notebook:G
   let settings_file = Http.file_of_url settings_url in
   let settings_xml =
     try
-      if String.compare "replay" settings_file <> 0 then
+      if Compat.bytes_compare "replay" settings_file <> 0 then
         ExtXml.parse_file ~noprovedtd:true settings_file
       else
         Xml.Element("empty", [], [])
@@ -731,7 +732,7 @@ let create_ac = fun ?(confirm_kill=true) alert (geomap:G.widget) (acs_notebook:G
         and airspeed = sprintf "%.1f" ac.airspeed in
 
         let msg_items = ["WIND_INFO"; ac_id; "42"; wind_east; wind_north;airspeed] in
-        let value = String.concat ";" msg_items in
+        let value = Compat.bytes_concat ";" msg_items in
         let vs = ["ac_id", PprzLink.String ac_id; "message", PprzLink.String value] in
         Ground_Pprz.message_send "dl" "RAW_DATALINK" vs;
       with
diff --git a/sw/ground_segment/cockpit/page_settings.ml b/sw/ground_segment/cockpit/page_settings.ml
index 4d0c29f498c..8b6dbf3fbcb 100644
--- a/sw/ground_segment/cockpit/page_settings.ml
+++ b/sw/ground_segment/cockpit/page_settings.ml
@@ -24,6 +24,7 @@
 
 open Printf
 
+
 let (//) = Filename.concat
 
 
@@ -90,7 +91,7 @@ let one_setting = fun (i:int) (do_change:int -> float -> unit) ac_id packing dl_
       1.
   in
   (* get number of digits after decimal dot *)
-  let digits = try String.length (ExtXml.attrib dl_setting "step") - String.index (ExtXml.attrib dl_setting "step") '.' - 1 with _ -> 0 in
+  let digits = try Compat.bytes_length (ExtXml.attrib dl_setting "step") - Compat.bytes_index (ExtXml.attrib dl_setting "step") '.' - 1 with _ -> 0 in
   let page_incr = step_incr
   and page_size = step_incr
   and show_auto = try ExtXml.attrib dl_setting "auto" = "true" with _ -> false in
@@ -248,7 +249,7 @@ let one_setting = fun (i:int) (do_change:int -> float -> unit) ac_id packing dl_
 
   (** Insert the related buttons in the strip and prepare the papgets DnD *)
   List.iter (fun x ->
-    match String.lowercase (Xml.tag x) with
+    match Compat.bytes_lowercase (Xml.tag x) with
         "strip_button" ->
           let label = ExtXml.attrib x "name"
           and sp_value = ExtXml.float_attrib x "value"
@@ -292,7 +293,7 @@ let same_tag_for_all = function
   | x::xs ->
     let tag_first = Xml.tag x in
     List.iter (fun y -> assert(ExtXml.tag_is y tag_first)) xs;
-    String.lowercase tag_first
+    Compat.bytes_lowercase tag_first
 
 
 (** Build the tree of settings *)
@@ -365,4 +366,3 @@ object (self)
     let settings = Array.fold_right (fun setting r -> try (setting#index, setting#xml, setting#last_known_value)::r with _ -> r) variables [] in
     SaveSettings.popup airframe_filename (Array.of_list settings) do_change
 end
-
diff --git a/sw/ground_segment/cockpit/saveSettings.ml b/sw/ground_segment/cockpit/saveSettings.ml
index 6a7fbdb3c41..f0a8c6870ac 100644
--- a/sw/ground_segment/cockpit/saveSettings.ml
+++ b/sw/ground_segment/cockpit/saveSettings.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 module U = Unix
 
 (** How to have them local ? *)
@@ -198,7 +199,7 @@ let fill_data = fun (model:GTree.tree_store) settings airframe_xml ->
 
   (* Warning if needed *)
   if !not_in_airframe_file <> [] then begin
-    GToolbox.message_box ~title:"Warning" (Printf.sprintf "Some parameters not writable in the airframe file:\n\n%s" (String.concat "\n" !not_in_airframe_file));
+    GToolbox.message_box ~title:"Warning" (Printf.sprintf "Some parameters not writable in the airframe file:\n\n%s" (Compat.bytes_concat "\n" !not_in_airframe_file));
   end
 
 
diff --git a/sw/ground_segment/cockpit/sectors.ml b/sw/ground_segment/cockpit/sectors.ml
index 5c777126487..d1b032880e9 100644
--- a/sw/ground_segment/cockpit/sectors.ml
+++ b/sw/ground_segment/cockpit/sectors.ml
@@ -2,11 +2,12 @@
 
 open Printf
 
+
 let (//) = Filename.concat
 
 let rec display = fun (geomap:MapCanvas.widget) r ->
 
-  match String.lowercase (Xml.tag r) with
+  match Compat.bytes_lowercase (Xml.tag r) with
       "disc" ->
         let rad = float_of_string (ExtXml.attrib r "radius")
         and geo = Latlong.of_string (ExtXml.attrib (ExtXml.child r "point") "pos") in
diff --git a/sw/ground_segment/cockpit/strip.ml b/sw/ground_segment/cockpit/strip.ml
index f69ceaf1200..9fd3cbaedd0 100644
--- a/sw/ground_segment/cockpit/strip.ml
+++ b/sw/ground_segment/cockpit/strip.ml
@@ -23,6 +23,7 @@
  *)
 
 open Printf
+
 module LL=Latlong
 
 let (//) = Filename.concat
@@ -327,7 +328,7 @@ object
 
     (* add a button widget in a vertical box if it belongs to a group (create new group if needed) *)
   method add_widget ?(group="") w =
-    let (vbox, pack) = match String.length group with
+    let (vbox, pack) = match Compat.bytes_length group with
         0 -> (GPack.vbox ~show:true (), true)
       | _ -> try (Hashtbl.find button_tbl group, false) with
           Not_found ->
diff --git a/sw/ground_segment/tmtc/broadcaster.ml b/sw/ground_segment/tmtc/broadcaster.ml
index 5f81760dd61..91471055f0a 100644
--- a/sw/ground_segment/tmtc/broadcaster.ml
+++ b/sw/ground_segment/tmtc/broadcaster.ml
@@ -1,3 +1,4 @@
+
 open Printf
 
 let () =
@@ -24,12 +25,12 @@ let () =
 
     (* Forward telemetry on Ivy *)
     let buffer_size = 256 in
-    let buffer = String.create buffer_size in
+    let buffer = Compat.bytes_create buffer_size in
     let get_tcp = fun _ ->
       begin
         try
           let n = input i buffer 0 buffer_size in
-          let data = String.sub buffer 0 n in
+          let data = Compat.bytes_sub buffer 0 n in
 
           Ivy.send (sprintf "%s %s" !ivy_to (Base64.encode_string data))
         with
diff --git a/sw/ground_segment/tmtc/fw_server.ml b/sw/ground_segment/tmtc/fw_server.ml
index 7d9c051dfcc..0ed3b0465f0 100644
--- a/sw/ground_segment/tmtc/fw_server.ml
+++ b/sw/ground_segment/tmtc/fw_server.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 open Printf
 open Server_globals
 open Aircraft
@@ -57,10 +58,10 @@ let ivalue = fun x ->
     | _ -> failwith "Receive.log_and_parse: int expected"
 
 let format_string_field = fun s ->
-  let s = String.copy s in
-  for i = 0 to String.length s - 1 do
+  let s = Compat.bytes_copy s in
+  for i = 0 to Compat.bytes_length s - 1 do
     match s.[i] with
-        ' ' -> s.[i] <- '_'
+        ' ' ->  Compat.bytes_set s i '_'
       | _ -> ()
   done;
   s
@@ -356,4 +357,3 @@ let log_and_parse = fun ac_name (a:Aircraft.aircraft) msg values ->
       a.datalink_status.downlink_rate <- ivalue "downlink_rate";
       a.datalink_status.downlink_msgs <- ivalue "downlink_nb_msgs"
     | _ -> ()
-
diff --git a/sw/ground_segment/tmtc/ivy2serial.ml b/sw/ground_segment/tmtc/ivy2serial.ml
index 8f8386ec347..a01c4be83b2 100644
--- a/sw/ground_segment/tmtc/ivy2serial.ml
+++ b/sw/ground_segment/tmtc/ivy2serial.ml
@@ -38,6 +38,7 @@ module Tm_Pprz = PprzLink.Messages(struct let name = "telemetry" end)
 module Dl_Pprz = PprzLink.Messages(struct let name = "datalink" end)
 module PprzTransport = Protocol.Transport(Pprz_transport.Transport)
 
+
 open Printf
 
 let () =
@@ -80,12 +81,12 @@ let () =
 
   (* The function to be called when data is available *)
   let buffer_size = 256 in
-  let buffer = String.create buffer_size in
+  let buffer = Compat.bytes_create buffer_size in
   let get_datalink_message = fun _ ->
     begin
       try
         let n = input (Unix.in_channel_of_descr fd) buffer 0 buffer_size in
-        let b = String.sub buffer 0 n in
+        let b = Compat.bytes_sub buffer 0 n in
         Debug.trace 'x' (Debug.xprint b);
 
         let use_dl_message = fun payload ->
diff --git a/sw/ground_segment/tmtc/ivy2udp.ml b/sw/ground_segment/tmtc/ivy2udp.ml
index a2b7cba5a52..59118339339 100644
--- a/sw/ground_segment/tmtc/ivy2udp.ml
+++ b/sw/ground_segment/tmtc/ivy2udp.ml
@@ -28,6 +28,7 @@ module Tm_Pprz = PprzLink.Messages(struct let name = "telemetry" end)
 module Dl_Pprz = PprzLink.Messages(struct let name = "datalink" end)
 module PprzTransport = Protocol.Transport(Pprz_transport.Transport)
 
+
 open Printf
 
 let () =
@@ -61,7 +62,7 @@ let () =
       let (msg_id, vs) = Tm_Pprz.values_of_string args.(0) in
       let payload = Tm_Pprz.payload_of_values msg_id (int_of_string !id) vs in
       let buf = Pprz_transport.Transport.packet payload in
-      let n = String.length buf in
+      let n = Compat.bytes_length buf in
       let n' = Unix.sendto socket buf 0 n [] sockaddr in
       assert (n = n')
     with _ -> () in
@@ -74,12 +75,12 @@ let () =
   Unix.bind socket sockaddr;
 
   let buffer_size = 256 in
-  let buffer = String.create buffer_size in
+  let buffer = Compat.bytes_create buffer_size in
   let get_datalink_message = fun _ ->
     begin
       try
         let n = input (Unix.in_channel_of_descr socket) buffer 0 buffer_size in
-        let b = String.sub buffer 0 n in
+        let b = Compat.bytes_sub buffer 0 n in
         Debug.trace 'x' (Debug.xprint b);
 
         let use_dl_message = fun payload ->
diff --git a/sw/ground_segment/tmtc/ivy_tcp_aircraft.ml b/sw/ground_segment/tmtc/ivy_tcp_aircraft.ml
index e02a961b020..3d5532df85b 100644
--- a/sw/ground_segment/tmtc/ivy_tcp_aircraft.ml
+++ b/sw/ground_segment/tmtc/ivy_tcp_aircraft.ml
@@ -3,6 +3,7 @@ module Tm_Pprz = PprzLink.Messages(struct let name = "telemetry" end)
 module Dl_Pprz = PprzLink.Messages(struct let name = "datalink" end)
 module PprzTransport = Protocol.Transport(Pprz_transport.Transport)
 
+
 open Printf
 let () =
   let ivy_bus = ref Defivybus.default_ivy_bus  in
@@ -40,12 +41,12 @@ let () =
 
   (* Forward a datalink command on the bus *)
   let buffer_size = 256 in
-  let buffer = String.create buffer_size in
+  let buffer = Compat.bytes_create buffer_size in
   let get_datalink_message = fun _ ->
     begin
       try
         let n = input i buffer 0 buffer_size in
-        let b = String.sub buffer 0 n in
+        let b = Compat.bytes_sub buffer 0 n in
         Debug.trace 'x' (Debug.xprint b);
 
         let use_dl_message = fun payload ->
diff --git a/sw/ground_segment/tmtc/ivy_tcp_controller.ml b/sw/ground_segment/tmtc/ivy_tcp_controller.ml
index 92d5498cb8a..27173d97c20 100644
--- a/sw/ground_segment/tmtc/ivy_tcp_controller.ml
+++ b/sw/ground_segment/tmtc/ivy_tcp_controller.ml
@@ -1,3 +1,4 @@
+
 open Printf
 
 module Tm_Pprz = PprzLink.Messages(struct let name = "telemetry" end)
@@ -28,12 +29,12 @@ let () =
   let (i, o) = Unix.open_connection sockaddr in
 
   let buffer_size = 256 in
-  let buffer = String.create buffer_size in
+  let buffer = Compat.bytes_create buffer_size in
   let get_message = fun _ ->
     begin
       try
         let n = input i buffer 0 buffer_size in
-        let b = String.sub buffer 0 n in
+        let b = Compat.bytes_sub buffer 0 n in
         Debug.trace 'x' (Debug.xprint b);
 
         let use_tele_message = fun payload ->
diff --git a/sw/ground_segment/tmtc/kml.ml b/sw/ground_segment/tmtc/kml.ml
index 841f1a3d1d1..e1ab28b6f03 100644
--- a/sw/ground_segment/tmtc/kml.ml
+++ b/sw/ground_segment/tmtc/kml.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 open Aircraft
 open Latlong
 open Printf
@@ -91,7 +92,7 @@ let circle = fun geo radius alt ->
     let wgs84 = of_utm WGS84 utm in
     coordinates wgs84 alt in
   let points = Array.init 360 degree_point in
-  String.concat " " (points.(359) :: Array.to_list points)
+  Compat.bytes_concat " " (points.(359) :: Array.to_list points)
 
 
 let ring_around_home = fun utm0 fp ->
@@ -236,7 +237,7 @@ let update_horiz_mode =
       let alt = ac.desired_altitude in
       match ac.horiz_mode with
           Segment (p1, p2) ->
-            let coordinates = String.concat " " (List.map (fun p -> coordinates p alt) [p1; p2]) in
+            let coordinates = Compat.bytes_concat " " (List.map (fun p -> coordinates p alt) [p1; p2]) in
             let kml_changes = update_linear_ring url_flight_plan "horiz_mode" coordinates in
             print_xml ac.name "route_changes.kml" kml_changes
         | Circle (p, r) ->
diff --git a/sw/ground_segment/tmtc/link.ml b/sw/ground_segment/tmtc/link.ml
index e486890492f..6f1d9ec90f4 100644
--- a/sw/ground_segment/tmtc/link.ml
+++ b/sw/ground_segment/tmtc/link.ml
@@ -24,6 +24,7 @@
     the Ivy software bus.
 *)
 
+
 open Latlong
 open Printf
 
@@ -194,7 +195,7 @@ let update_ms_since_last_msg =
       statuss
 
 let use_tele_message = fun ?udp_peername ?raw_data_size payload ->
-  let raw_data_size = match raw_data_size with None -> String.length (Protocol.string_of_payload payload) | Some d -> d in
+  let raw_data_size = match raw_data_size with None -> Compat.bytes_length (Protocol.string_of_payload payload) | Some d -> d in
   let buf = Protocol.string_of_payload payload in
   Debug.call 'l' (fun f ->  fprintf f "pprz receiving: %s\n" (Debug.xprint buf));
   try
@@ -272,13 +273,13 @@ module XB = struct (** XBee module *)
 
       | Xbee_transport.RX_Packet_64 (addr64, rssi, options, data) ->
         Debug.trace 'x' (sprintf "getting XBee RX64: %Lx %d %d %s" addr64 rssi options (Debug.xprint data));
-        use_tele_message ~raw_data_size:(String.length frame_data + oversize_packet) (Protocol.payload_of_string data)
+        use_tele_message ~raw_data_size:(Compat.bytes_length frame_data + oversize_packet) (Protocol.payload_of_string data)
       | Xbee_transport.RX868_Packet (addr64, options, data) ->
         Debug.trace 'x' (sprintf "getting XBee868 RX: %Lx %d %s" addr64 options (Debug.xprint data));
-        use_tele_message ~raw_data_size:(String.length frame_data + oversize_packet) (Protocol.payload_of_string data)
+        use_tele_message ~raw_data_size:(Compat.bytes_length frame_data + oversize_packet) (Protocol.payload_of_string data)
       | Xbee_transport.RX_Packet_16 (addr16, rssi, options, data) ->
         Debug.trace 'x' (sprintf "getting XBee RX16: from=%x %d %d %s" addr16 rssi options (Debug.xprint data));
-        use_tele_message ~raw_data_size:(String.length frame_data + oversize_packet) (Protocol.payload_of_string data)
+        use_tele_message ~raw_data_size:(Compat.bytes_length frame_data + oversize_packet) (Protocol.payload_of_string data)
 
 
   let send = fun ?ac_id device rf_data ->
@@ -309,7 +310,7 @@ let local_broadcast_address =
 
 let udp_send = fun fd payload peername ->
   let buf = Pprz_transport.Transport.packet payload in
-  let len = String.length buf in
+  let len = Compat.bytes_length buf in
   let addr = if !udp_broadcast then (Unix.inet_addr_of_string !udp_broadcast_addr) else peername in
   Debug.call 'u' (fun f -> fprintf f "udp_send to %s port %i\n" (Unix.string_of_inet_addr addr) !udp_uplink_port);
   let sockaddr = Unix.ADDR_INET (addr, !udp_uplink_port) in
@@ -364,7 +365,7 @@ let parser_of_device = fun device ->
   match device.transport with
     | Pprz ->
         let use = fun s ->
-          let raw_data_size = String.length (Protocol.string_of_payload s) + 4 (*stx,len,ck_a, ck_b*) in
+          let raw_data_size = Compat.bytes_length (Protocol.string_of_payload s) + 4 (*stx,len,ck_a, ck_b*) in
           let udp_peername =
             if !udp then
               Some !last_udp_peername
@@ -471,7 +472,7 @@ let () =
 
     (** Listen on a udp port or a serial device or on pipe *)
     let on_serial_device =
-      String.length !port >= 4 && String.sub !port 0 4 = "/dev" in (* FIXME *)
+      Compat.bytes_length !port >= 4 && Compat.bytes_sub !port 0 4 = "/dev" in (* FIXME *)
     let fd =
       if !udp then
         begin
diff --git a/sw/ground_segment/tmtc/modem.ml b/sw/ground_segment/tmtc/modem.ml
index a1270baa10e..8686e802569 100644
--- a/sw/ground_segment/tmtc/modem.ml
+++ b/sw/ground_segment/tmtc/modem.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 open Printf
 
 module Protocol = struct
@@ -32,20 +33,20 @@ module Protocol = struct
   let stx = Char.chr 0x02
   let etx = 0x03
   let index_start = fun buf ->
-    String.index buf stx
+    Compat.bytes_index buf stx
 
   let payload_length = fun buf start ->
     Char.code buf.[start+1] - 1
 
   let length = fun buf start ->
-    let len = String.length buf - start in
+    let len = Compat.bytes_length buf - start in
     if len >= 2 then
       Char.code buf.[start+1] + 3
     else
       raise Serial.Not_enough
 
   let checksum = fun msg ->
-    let l = String.length msg in
+    let l = Compat.bytes_length msg in
     let ck_a = ref 0 in
     for i = 1 to l - 3 do
       ck_a := Char.code msg.[i] lxor !ck_a
@@ -53,9 +54,9 @@ module Protocol = struct
     !ck_a = Char.code msg.[l-2] && Char.code msg.[l-1] = etx
 
   let payload = fun msg ->
-    let l = String.length msg in
+    let l = Compat.bytes_length msg in
     assert(l >= 4);
-    Serial.payload_of_string (String.sub msg 2 (l-4))
+    Serial.payload_of_string (Compat.bytes_sub msg 2 (l-4))
 
   let packet = fun _payload ->
     failwith "Modem.Protocol.packet not implemented"
@@ -95,12 +96,12 @@ let valim = fun x -> float x *. 0.0162863 -. 1.17483
 let parse_payload = fun payload ->
   let payload = Serial.string_of_payload payload in
   status.detected <- 1;
-  let len = String.length payload in
+  let len = Compat.bytes_length payload in
   status.nb_byte <- status.nb_byte + len;
   status.nb_msg <- status.nb_msg + 1;
   let id = Char.code payload.[0] in
   if id = msg_data then
-    Some (String.sub payload 1 (len-1))
+    Some (Compat.bytes_sub payload 1 (len-1))
   else begin
     begin
       match id with
diff --git a/sw/ground_segment/tmtc/rotorcraft_server.ml b/sw/ground_segment/tmtc/rotorcraft_server.ml
index 72a6b14df81..6266393dc5f 100644
--- a/sw/ground_segment/tmtc/rotorcraft_server.ml
+++ b/sw/ground_segment/tmtc/rotorcraft_server.ml
@@ -69,10 +69,10 @@ let foi32value = fun x ->
     | _ -> failwith "Receive.log_and_parse: int32 expected"
 
 let format_string_field = fun s ->
-  let s = String.copy s in
-  for i = 0 to String.length s - 1 do
+  let s = Compat.bytes_copy s in
+  for i = 0 to Compat.bytes_length s - 1 do
     match s.[i] with
-        ' ' -> s.[i] <- '_'
+        ' ' -> Compat.bytes_set s i '_'
       | _ -> ()
   done;
   s
diff --git a/sw/ground_segment/tmtc/server.ml b/sw/ground_segment/tmtc/server.ml
index a9f738d3497..7c4ff523c71 100644
--- a/sw/ground_segment/tmtc/server.ml
+++ b/sw/ground_segment/tmtc/server.ml
@@ -27,6 +27,7 @@ let gps_mode_3D = 3
 let no_md5_check = ref false
 let replay_old_log = ref false
 
+
 open Printf
 open Latlong
 open Server_globals
@@ -66,7 +67,7 @@ let wind_msg_period = 5000 (* ms *)
 let aircraft_alerts_period = 1000 (* ms *)
 let send_aircrafts_msg = fun _asker _values ->
   assert(_values = []);
-  let names = String.concat "," (Hashtbl.fold (fun k _v r -> k::r) aircrafts []) ^ "," in
+  let names = Compat.bytes_concat "," (Hashtbl.fold (fun k _v r -> k::r) aircrafts []) ^ "," in
   ["ac_list", PprzLink.String names]
 
 
@@ -449,9 +450,9 @@ let send_aircraft_msg = fun ac ->
 
 (** Check if it is a replayed A/C (c.f. sw/logalizer/play.ml) *)
 let replayed = fun ac_id ->
-  let n = String.length ac_id in
-  if n > 6 && String.sub ac_id 0 6 = "replay" then
-    (true, String.sub ac_id 6 (n - 6), "/var/replay/", ExtXml.parse_file (Env.paparazzi_home // "var/replay/conf/conf.xml"))
+  let n = Compat.bytes_length ac_id in
+  if n > 6 && Compat.bytes_sub ac_id 0 6 = "replay" then
+    (true, Compat.bytes_sub ac_id 6 (n - 6), "/var/replay/", ExtXml.parse_file (Env.paparazzi_home // "var/replay/conf/conf.xml"))
   else
     (false, ac_id, "", conf_xml)
 
@@ -479,7 +480,7 @@ let check_md5sum = fun ac_name alive_md5sum aircraft_conf_dir ->
     match alive_md5sum with
         PprzLink.Array array ->
           let n = Array.length array in
-          assert(n = String.length md5sum / 2);
+          assert(n = Compat.bytes_length md5sum / 2);
           for i = 0 to n - 1 do
             let x = int_of_string (sprintf "0x%c%c" md5sum.[2*i] md5sum.[2*i+1]) in
             assert (x = PprzLink.int_of_value array.(i))
@@ -490,7 +491,7 @@ let check_md5sum = fun ac_name alive_md5sum aircraft_conf_dir ->
       match alive_md5sum with
           PprzLink.Array array ->
             let n = Array.length array in
-            assert(n = String.length md5sum / 2);
+            assert(n = Compat.bytes_length md5sum / 2);
             for i = 0 to n - 1 do
               let x = 0 in
               assert (x = PprzLink.int_of_value array.(i))
@@ -788,8 +789,8 @@ let jump_block = fun logging _sender vs ->
 let raw_datalink = fun logging _sender vs ->
   let ac_id = PprzLink.string_assoc "ac_id" vs
   and m = PprzLink.string_assoc "message" vs in
-  for i = 0 to String.length m - 1 do
-    if m.[i] = ';' then m.[i] <- ' '
+  for i = 0 to Compat.bytes_length m - 1 do
+    if m.[i] = ';' then Compat.bytes_set m i ' '
   done;
   let msg_id, vs = Dl_Pprz.values_of_string m in
   let msg = Dl_Pprz.message_of_id msg_id in
diff --git a/sw/ground_segment/tmtc/server_globals.ml b/sw/ground_segment/tmtc/server_globals.ml
index 636a35da33e..97171cfea3c 100644
--- a/sw/ground_segment/tmtc/server_globals.ml
+++ b/sw/ground_segment/tmtc/server_globals.ml
@@ -1,5 +1,6 @@
 exception Telemetry_error of string * string
 
+
 (* options for serving xml config files and kml via http *)
 let hostname = ref "localhost"
 let port = ref 8889
@@ -18,4 +19,4 @@ let horiz_modes = [|"WAYPOINT";"ROUTE";"CIRCLE";"ATTITUDE";"MANUAL"|]
 let if_modes = [|"OFF";"DOWN";"UP"|]
 
 let string_of_values = fun values ->
-  String.concat " " (List.map (fun (_, v) -> PprzLink.string_of_value v) values)
+  Compat.bytes_concat " " (List.map (fun (_, v) -> PprzLink.string_of_value v) values)
diff --git a/sw/lib/ocaml/Makefile b/sw/lib/ocaml/Makefile
index b022fa8d568..719430a07d9 100644
--- a/sw/lib/ocaml/Makefile
+++ b/sw/lib/ocaml/Makefile
@@ -49,25 +49,39 @@ ifeq ($(LABLGTK2GNOMECANVAS),)
 LABLGTK2GNOMECANVAS = $(shell ocamlfind query -p-format lablgtk2.gnomecanvas 2>/dev/null)
 endif
 
+CAMLP4_DEFS ?=
 NETCLIENT_VER := $(shell ocamlfind query -format '%v' netclient)
 NETCLIENT_MAJOR := $(shell echo $(NETCLIENT_VER) | cut -f1 -d.)
+NETCLIENT_MINOR1 := $(shell echo $(NETCLIENT_VER) | cut -f2 -d.)
+NETCLIENT_MINOR2 := $(shell echo $(NETCLIENT_VER) | cut -f3 -d.)
 ifeq ($(shell test $(NETCLIENT_MAJOR) -ge 4; echo $$?),0)
-CAMLP4_DEFS = -DNETCLIENT_V_4
+CAMLP4_DEFS += -DNETCLIENT_V_4
+ifeq ($(shell test $(NETCLIENT_MINOR2) -ge 4; echo $$?),0)
+CAMLP4_DEFS += -DNETCLIENT_V_404
+else ifeq ($(shell test $(NETCLIENT_MINOR1) -g 0; echo $$?),0)
+CAMLP4_DEFS += -DNETCLIENT_V_404
+endif
+endif
+OCAMLC_VER := $(shell ocamlc -version)
+OCAMLC_MAJOR := $(shell echo $(OCAMLC_VER) | cut -f1 -d.)
+OCAMLC_MINOR := $(shell echo $(OCAMLC_VER) | cut -f2 -d.)
+ifeq ($(shell test $(OCAMLC_MAJOR) -ge 4; echo $$?),0)
+ifeq ($(shell test $(OCAMLC_MINOR) -ge 2; echo $$?),0)
+# the Bytes module is available since OCaml 4.02.0
+CAMLP4_DEFS += -DHAS_BYTES_MODULE
+endif
 endif
-CAMLP4_DEFS ?=
 PP_OPTS = -pp "camlp4o pa_macro.cmo $(CAMLP4_DEFS)"
 
 # which source files to run through caml4p
-PP_SRC = http.ml
-PP_CMO = $(PP_SRC:.ml=.cmo)
-PP_CMX = $(PP_SRC:.ml=.cmx)
+PP_SRC =  http.ml compat.ml
 
 INCLUDES=
 PKGCOMMON=pprzlink,xml-light,netclient,glibivy,lablgtk2
 XINCLUDES=
 XPKGCOMMON=pprzlink,xml-light,glibivy,$(LABLGTK2GNOMECANVAS),lablgtk2.glade
 
-SRC = fig.ml debug.ml base64.ml serial.ml ocaml_tools.ml expr_syntax.ml expr_parser.ml expr_lexer.ml extXml.ml env.ml xml2h.ml latlong.ml egm96.ml srtm.ml http.ml maps_support.ml gm.ml iGN.ml geometry_2d.ml cserial.o ubx.ml xmlCom.ml os_calls.ml editAirframe.ml defivybus.ml fp_proc.ml gen_common.ml
+SRC = compat.ml fig.ml debug.ml base64.ml serial.ml ocaml_tools.ml expr_syntax.ml expr_parser.ml expr_lexer.ml extXml.ml env.ml xml2h.ml latlong.ml egm96.ml srtm.ml http.ml maps_support.ml gm.ml iGN.ml geometry_2d.ml cserial.o ubx.ml xmlCom.ml os_calls.ml editAirframe.ml defivybus.ml fp_proc.ml gen_common.ml
 CMO = $(SRC:.ml=.cmo)
 CMX = $(SRC:.ml=.cmx)
 
@@ -131,22 +145,21 @@ ml_gtk_drag.o : ml_gtk_drag.c
 	@echo OC $<
 	$(Q)$(OCAMLC) $(INCLUDES) -package $(PKGCOMMON) -c -ccopt "$(GTKCFLAGS)" $<
 
-# bit of a hack: only run some specifc files through caml4p
-$(PP_CMO) : $(PP_SRC)
-	@echo OC $<
-	$(Q)$(OCAMLC) $(INCLUDES) $(PP_OPTS) -package $(PKGCOMMON) -c $<
-
-$(PP_CMX) : $(PP_SRC)
-	@echo OOC $<
-	$(Q)$(OCAMLOPT) $(INCLUDES) $(PP_OPTS) -package $(PKGCOMMON) -c $<
-
 %.cmo : %.ml
 	@echo OC $<
-	$(Q)$(OCAMLC) $(INCLUDES) -package $(PKGCOMMON) -c $<
+	@if test $(findstring $<,$(PP_SRC)); then \
+		$(OCAMLC) $(INCLUDES) $(PP_OPTS) -package $(PKGCOMMON) -c $<; \
+	else \
+		$(OCAMLC) $(INCLUDES) -package $(PKGCOMMON) -c $<; \
+	fi;
 
 %.cmx : %.ml
 	@echo OOC $<
-	$(Q)$(OCAMLOPT) $(INCLUDES) -package $(PKGCOMMON) -c $<
+	@if test $(findstring $<,$(PP_SRC)); then \
+		$(OCAMLOPT) $(INCLUDES) $(PP_OPTS) -package $(PKGCOMMON) -c $<; \
+	else \
+		$(OCAMLOPT) $(INCLUDES) -package $(PKGCOMMON) -c $<; \
+	fi;
 
 %.cmi : %.mli
 	@echo OC $<
diff --git a/sw/lib/ocaml/base64.ml b/sw/lib/ocaml/base64.ml
index 922ca789237..545ad7be820 100644
--- a/sw/lib/ocaml/base64.ml
+++ b/sw/lib/ocaml/base64.ml
@@ -28,6 +28,8 @@
 (** Exception raised when there's an attempt to encode a chunk incorrectly *)
 exception Invalid_encode_chunk of int
 
+
+
 (** The character map of all base64 characters *)
 
 let char_map = [|
@@ -48,27 +50,27 @@ let encode_chunk chars =
   let llength = List.length chars in
   if(llength = 0 || llength > 3) then
     raise (Invalid_encode_chunk(llength));
-  let chunk = String.make 4 '=' in
+  let chunk = Compat.bytes_make 4 '=' in
   let a = List.hd chars in
   let tmpa = (((Char.code a) land 3) lsl 4) in
-  chunk.[0] <- char_map.( (Char.code a) lsr 2);
+  Compat.bytes_set chunk 0 char_map.( (Char.code a) lsr 2);
   (* Check for another character *)
   if (llength < 2) then (
-    chunk.[1] <- char_map.(tmpa);
+    Compat.bytes_set chunk 1 char_map.(tmpa);
     chunk;
   ) else (
     let b = List.nth chars 1 in
     let tmpb = ((Char.code b) lsr 4) in
     let tmpa2 = ((Char.code b) land 0x0f) lsl 2 in
-    chunk.[1] <- char_map.(tmpa lor tmpb);
+    Compat.bytes_set chunk 1 char_map.(tmpa lor tmpb);
     if (llength < 3) then (
-      chunk.[2] <- char_map.(tmpa2);
+      Compat.bytes_set chunk 2 char_map.(tmpa2);
       chunk
     ) else (
       let c = List.nth chars 2 in
       let tmpb2 = ((Char.code c) land 0xc0) lsr 6 in
-      chunk.[2] <- char_map.(tmpa2 lor tmpb2);
-      chunk.[3] <- char_map.((Char.code c) land 0x3f);
+      Compat.bytes_set chunk 2 char_map.(tmpa2 lor tmpb2);
+      Compat.bytes_set chunk 3 char_map.((Char.code c) land 0x3f);
       chunk
     )
   )
@@ -200,7 +202,7 @@ let decode_string s = decode_to_string (Stream.of_string s)
 (** Simple test function. *)
 
 let test() =
-  let wordlist = ["A";"AB";"ABC";"Dustin";String.create 128] in
+  let wordlist = ["A";"AB";"ABC";"Dustin";Compat.bytes_create 128] in
   print_endline("String:");
   List.iter (fun x -> print_endline(encode_string x))
     wordlist;
diff --git a/sw/lib/ocaml/compat.ml b/sw/lib/ocaml/compat.ml
new file mode 100644
index 00000000000..0d888af199f
--- /dev/null
+++ b/sw/lib/ocaml/compat.ml
@@ -0,0 +1,77 @@
+(*
+ * Abstract some String/Byte functions for compatibility between OCaml 3.x and 4.x
+ *
+ * Copyright (C) 2016 Felix Ruess <felix.ruess@gmail.com>
+ *
+ * This file is part of paparazzi.
+ *
+ * paparazzi is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * paparazzi is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with paparazzi; see the file COPYING.  If not, write to
+ * the Free Software Foundation, 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ *)
+
+ IFDEF HAS_BYTES_MODULE THEN
+ module BYTES = Bytes
+ ELSE
+ module BYTES = String
+ END
+
+let bytes_create = fun len ->
+  BYTES.create len
+
+let bytes_contains = fun c s ->
+  BYTES.contains c s
+
+let bytes_length = fun len ->
+  BYTES.length len
+
+let bytes_make = fun n c->
+  BYTES.make n c
+
+let bytes_copy = fun s->
+  BYTES.copy s
+
+let bytes_lowercase = fun s->
+  BYTES.lowercase s
+
+let bytes_uppercase = fun s->
+  BYTES.uppercase s
+
+let bytes_blit = fun src srcoff dst dstoff len->
+  BYTES.blit src srcoff dst dstoff len
+
+let bytes_sub = fun s start len->
+  BYTES.sub s start len
+
+let bytes_index = fun c s ->
+  BYTES.index c s
+
+let bytes_concat = fun sep sl->
+  BYTES.concat sep sl
+
+let bytes_index_from = fun s i c ->
+  BYTES.index_from s i c
+
+let bytes_get = fun s n->
+  BYTES.get s n
+
+let bytes_compare = fun s1 s2->
+  BYTES.compare s1 s2
+
+let bytes_set = fun s n c->
+  BYTES.set s n c
+
+let bytes_iter = fun f s->
+  BYTES.iter f s
diff --git a/sw/lib/ocaml/compat.mli b/sw/lib/ocaml/compat.mli
new file mode 100644
index 00000000000..df0f3ebdc25
--- /dev/null
+++ b/sw/lib/ocaml/compat.mli
@@ -0,0 +1,40 @@
+(*
+ * Abstract some String/Byte functions for compatibility between OCaml 3.x and 4.x
+ *
+ * Copyright (C) 2016 Felix Ruess <felix.ruess@gmail.com>
+ *
+ * This file is part of paparazzi.
+ *
+ * paparazzi is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * paparazzi is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with paparazzi; see the file COPYING.  If not, write to
+ * the Free Software Foundation, 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ *)
+
+val bytes_create : int -> string
+val bytes_contains : string -> char -> bool
+val bytes_length : string -> int
+val bytes_make : int -> char -> string
+val bytes_copy : string -> string
+val bytes_lowercase : string -> string
+val bytes_uppercase : string -> string
+val bytes_blit : string -> int -> string -> int -> int -> unit
+val bytes_sub : string -> int -> int -> string
+val bytes_index : string -> char -> int
+val bytes_concat : string -> string list -> string
+val bytes_index_from : string -> int -> char -> int
+val bytes_get : string -> int -> char
+val bytes_compare : string -> string -> int
+val bytes_set : string -> int -> char -> unit
+val bytes_iter : (char -> unit) -> string -> unit
diff --git a/sw/lib/ocaml/debug.ml b/sw/lib/ocaml/debug.ml
index 23674976632..0c688424ae8 100644
--- a/sw/lib/ocaml/debug.ml
+++ b/sw/lib/ocaml/debug.ml
@@ -22,11 +22,12 @@
  *
  *)
 
+
 let level = ref (try Sys.getenv "PPRZ_DEBUG" with Not_found -> "")
 let log = ref stderr
 let call lev f =
   assert( (* assert permet au compilo de tout virer avec l'option -noassert *)
-    if (String.contains !level '*' || String.contains !level lev)
+    if (Compat.bytes_contains !level '*' || Compat.bytes_contains !level lev)
     then begin
       f !log;
       flush !log
@@ -36,11 +37,11 @@ let call lev f =
 let trace lev s = call lev (fun f -> Printf.fprintf f "%s\n" s)
 
 let xprint = fun s ->
-  let n = String.length s in
-  let a = String.make (3*n) ' ' in
+  let n = Compat.bytes_length s in
+  let a = Compat.bytes_make (3*n) ' ' in
   for i = 0 to n - 1 do
     let x = Printf.sprintf "%02x" (Char.code s.[i]) in
-    a.[3*i] <- x.[0];
-    a.[3*i+1] <- x.[1]
+    Compat.bytes_set a (3*i) x.[0];
+    Compat.bytes_set a (3*i+1) x.[1]
   done;
   a
diff --git a/sw/lib/ocaml/defivybus.ml b/sw/lib/ocaml/defivybus.ml
index b87b0a1e4d9..eabc6480198 100644
--- a/sw/lib/ocaml/defivybus.ml
+++ b/sw/lib/ocaml/defivybus.ml
@@ -21,7 +21,9 @@
  * Boston, MA 02111-1307, USA.
  *
  *)
-let default_ivy_bus = String.copy (
+
+
+let default_ivy_bus = Compat.bytes_copy (
   try (Sys.getenv "IVY_BUS" )
   with  Not_found ->
     (if Os_calls.contains (Os_calls.os_name) "Darwin" then
diff --git a/sw/lib/ocaml/egm96.ml b/sw/lib/ocaml/egm96.ml
index 306ab6f345e..fafed11ffd0 100644
--- a/sw/lib/ocaml/egm96.ml
+++ b/sw/lib/ocaml/egm96.ml
@@ -24,6 +24,7 @@
 
 open Latlong
 
+
 let (//) = Filename.concat
 
 let ncols = 1440
@@ -33,7 +34,7 @@ let data =
     let path = [Env.paparazzi_home // "data" // "srtm"] in
     let f = Ocaml_tools.open_compress (Ocaml_tools.find_file path "WW15MGH.DAC") in
     let n = ncols * nrows * 2 in
-    let buf = String.create n in
+    let buf = Compat.bytes_create n in
     really_input f buf 0 n;
     buf)
 
diff --git a/sw/lib/ocaml/env.ml b/sw/lib/ocaml/env.ml
index abbd76caccf..bcd2f5bec4d 100644
--- a/sw/lib/ocaml/env.ml
+++ b/sw/lib/ocaml/env.ml
@@ -24,6 +24,7 @@
 
 open Printf
 
+
 let (//) = Filename.concat
 let space_regexp = Str.regexp "[ \t]+"
 let make_element = fun t a c -> Xml.Element (t,a,c)
@@ -80,8 +81,8 @@ let filter_absolute_path = fun path ->
 (* filter settings and keep the ones without brackets *)
 let filter_settings = fun settings ->
   let sl = Str.split (Str.regexp "[ ]+") settings in
-  let sl = List.filter (fun s -> not (s.[0] = '[' && s.[String.length s - 1] = ']')) sl in
-  String.concat " " sl
+  let sl = List.filter (fun s -> not (s.[0] = '[' && s.[Compat.bytes_length s - 1] = ']')) sl in
+  Compat.bytes_concat " " sl
 
 (* filter on modules based on target *)
 let filter_modules_target = fun module_file ->
@@ -170,7 +171,7 @@ let expand_ac_xml = fun ?(raise_exception = true) ac_conf ->
 let read_process command =
   let buffer_size = 2048 in
   let buffer = Buffer.create buffer_size in
-  let string = String.create buffer_size in
+  let string = Compat.bytes_create buffer_size in
   let in_channel = Unix.open_process_in command in
   let chars_read = ref 1 in
   while !chars_read <> 0 do
@@ -196,5 +197,4 @@ let key_modifiers_of_string = fun key ->
     | "Meta" -> "<Meta>"
     | x -> x
   ) key_split in
-  String.concat "" keys
-
+  Compat.bytes_concat "" keys
diff --git a/sw/lib/ocaml/expr_lexer.mll b/sw/lib/ocaml/expr_lexer.mll
index eb50d083274..a0583142478 100644
--- a/sw/lib/ocaml/expr_lexer.mll
+++ b/sw/lib/ocaml/expr_lexer.mll
@@ -1,6 +1,6 @@
-(* 
- * Lexical tokens à la C
- *  
+(*
+ * Lexical tokens � la C
+ *
  * Copyright (C) 2003-2010 Antoine Drouin, Pascal Brisset, ENAC
  *
  * This file is part of paparazzi.
@@ -22,6 +22,7 @@
  *)
 {
 open Expr_parser
+
 }
 rule token = parse
     [' ' '\t' '\n'] { token lexbuf}
@@ -29,7 +30,7 @@ rule token = parse
   | ['0'-'9']+ { INT (int_of_string (Lexing.lexeme lexbuf)) }
   | ['0'-'9']+'.'['0'-'9']* { FLOAT (float_of_string (Lexing.lexeme lexbuf)) }
   | '$'?['a'-'z' '_' 'A'-'Z'] (['a'-'z' 'A'-'Z' '_' '0'-'9']*) { IDENT (Lexing.lexeme lexbuf) }
-  | '\''[^'\'']+'\'' { let s = Lexing.lexeme lexbuf in IDENT (String.sub s 1 (String.length s - 2)) }
+  | '\''[^'\'']+'\'' { let s = Lexing.lexeme lexbuf in IDENT (Compat.bytes_sub s 1 (Compat.bytes_length s - 2)) }
   | ',' { COMMA }
   | '.' { DOT }
   | ';' { SEMICOLON }
@@ -38,7 +39,7 @@ rule token = parse
   | ')' { RP }
   | '{' { LC }
   | '}' { RC }
-  | '[' { LB } 
+  | '[' { LB }
   | ']' { RB }
   | "->" { DEREF }
   | "==" { EQ }
diff --git a/sw/lib/ocaml/expr_syntax.ml b/sw/lib/ocaml/expr_syntax.ml
index f600a9cb2d1..e8bbffad557 100644
--- a/sw/lib/ocaml/expr_syntax.ml
+++ b/sw/lib/ocaml/expr_syntax.ml
@@ -1,5 +1,5 @@
 (*
- * Syntax of expressions à la C
+ * Syntax of expressions � la C
  *
  * Copyright (C) 2003-2010 Antoine Drouin, Pascal Brisset, ENAC
  *
@@ -24,6 +24,7 @@
 
 open Printf
 
+
 type ident = string
 
 type operator = string
@@ -45,7 +46,7 @@ let sprint = fun ?call_assoc expr ->
   | Some (n, l) -> Some n, l
   in
   let rec eval = function
-    | Ident i when i.[0] = '$' -> sprintf "%s" (c_var_of_ident (String.sub i 1 (String.length i - 1)))
+    | Ident i when i.[0] = '$' -> sprintf "%s" (c_var_of_ident (Compat.bytes_sub i 1 (Compat.bytes_length i - 1)))
     | Ident i -> sprintf "%s" i
     | Int i -> sprintf "%d" i
     | Float i -> sprintf "%f" i
@@ -59,7 +60,7 @@ let sprint = fun ?call_assoc expr ->
         sprintf "%d" index
     | Call (i, es) ->
         let ses = List.map eval es in
-        sprintf "%s(%s)" i (String.concat "," ses)
+        sprintf "%s(%s)" i (Compat.bytes_concat "," ses)
     | Index (i,e) -> sprintf "%s[%s]" i (eval e)
     | Field (i,f) -> sprintf "%s.%s" i f
     | Deref (e,f) -> sprintf "(%s)->%s" (eval e) f
diff --git a/sw/lib/ocaml/extXml.ml b/sw/lib/ocaml/extXml.ml
index 10d674359c3..25f7bb05e30 100644
--- a/sw/lib/ocaml/extXml.ml
+++ b/sw/lib/ocaml/extXml.ml
@@ -24,6 +24,7 @@
 
 open Printf
 
+
 exception Error of string
 
 let sep = Str.regexp "\\."
@@ -74,7 +75,7 @@ let attrib_option = fun xml attr ->
   try Some (Xml.attrib xml attr)
   with Xml.No_attribute _ -> None
 
-let tag_is = fun x v -> String.lowercase (Xml.tag x) = String.lowercase v
+let tag_is = fun x v -> Compat.bytes_lowercase (Xml.tag x) = Compat.bytes_lowercase v
 
 let attrib_or_default = fun x a default ->
   try Xml.attrib x a
@@ -88,7 +89,7 @@ let buffer_attr = fun indent tab (n,v) ->
   Buffer.add_char tmp ' ';
   Buffer.add_string tmp n;
   Buffer.add_string tmp "=\"";
-  let l = String.length v in
+  let l = Compat.bytes_length v in
   for p = 0 to l-1 do
     match v.[p] with
       | '\\' -> Buffer.add_string tmp "\\\\"
@@ -147,7 +148,7 @@ let my_to_string_fmt = fun tab_attribs x ->
 
 
 let to_string_fmt = fun ?(tab_attribs = false) xml ->
-  let l = String.lowercase in
+  let l = Compat.bytes_lowercase in
   let rec lower = function
     | Xml.PCData _ as x -> x
     | Xml.Element (t, ats, cs) ->
@@ -158,7 +159,7 @@ let to_string_fmt = fun ?(tab_attribs = false) xml ->
 
 
 let subst_attrib = fun attrib value xml ->
-  let u = String.uppercase in
+  let u = Compat.bytes_uppercase in
   let uattrib = u attrib in
   match xml with
   | Xml.Element (tag, attrs, children) ->
diff --git a/sw/lib/ocaml/fig.ml b/sw/lib/ocaml/fig.ml
index 36d60f562c4..cfb1cd3d688 100644
--- a/sw/lib/ocaml/fig.ml
+++ b/sw/lib/ocaml/fig.ml
@@ -24,6 +24,7 @@
 
 open Printf
 
+
 let version = "3.2"
 
 type units = int
@@ -466,7 +467,7 @@ let font_flags = fun rigid special font hidden ->
     bit hidden 3
 
 let code_string = fun f s ->
-  for i = 0 to String.length s - 1 do
+  for i = 0 to Compat.bytes_length s - 1 do
     if Char.code s.[i] > 0o177 then
       fprintf f "\\%3o" (Char.code s.[i])
     else
@@ -499,8 +500,8 @@ let rec read_comments = fun s ->
   bscanf s " %0c" (fun c ->
     if c = '#' then
       bscanf s " %s@\n" (fun l ->
-        let n = String.length l in
-        String.sub l 2 (n-2) :: read_comments s)
+        let n = Compat.bytes_length l in
+        Compat.bytes_sub l 2 (n-2) :: read_comments s)
     else
       [])
 
@@ -621,9 +622,9 @@ let read_text = fun s ->
   bscanf s " %d %d %d %d %d %d %f %d %f %f" (fun st c depth _ps ft fs angle ff h l ->
     let p = read_point s in
     bscanf s "%c%s@\n" (fun _space text ->
-      let n = String.length text in
-      assert (String.sub text (n-4) 4 = "\\001");
-      let text = String.sub text 0 (n - 4) in
+      let n = Compat.bytes_length text in
+      assert (Compat.bytes_sub text (n-4) 4 = "\\001");
+      let text = Compat.bytes_sub text 0 (n - 4) in
       Text (justification_of_int st, c, depth, font_of_int (bit2 ff) ft, fs, angle, ff, h, l, p,text)))
 
 
@@ -648,7 +649,7 @@ let read = fun file ->
   let s = Scanning.from_file file in
   bscanf s "#FIG %s@\n%s %s@\n %s %s %f %s %d" (fun v o j u p m multi t ->
 
-    if String.sub v 0 3 <> "3.2" then
+    if Compat.bytes_sub v 0 3 <> "3.2" then
       failwith ("Unknown FIG format version: "^v);
 
     let comments = read_comments s in
diff --git a/sw/lib/ocaml/gen_common.ml b/sw/lib/ocaml/gen_common.ml
index 44cfba23787..87e87cc8d7b 100644
--- a/sw/lib/ocaml/gen_common.ml
+++ b/sw/lib/ocaml/gen_common.ml
@@ -24,6 +24,7 @@
 
 open Printf
 
+
 (** simple boolean expressions *)
 type bool_expr =
   | Var of string
@@ -91,8 +92,8 @@ let targets_of_field =
   let pipe = Str.regexp "|" in
   fun field default ->
     let f = ExtXml.attrib_or_default field "target" default in
-    if String.length f > 0 && String.get f 0 = '!' then
-      Not (expr_of_targets (fun x y -> Or(x,y)) (Str.split pipe (String.sub f 1 ((String.length f) - 1))))
+    if Compat.bytes_length f > 0 && Compat.bytes_get f 0 = '!' then
+      Not (expr_of_targets (fun x y -> Or(x,y)) (Str.split pipe (Compat.bytes_sub f 1 ((Compat.bytes_length f) - 1))))
     else
       expr_of_targets (fun x y -> Or(x,y)) (Str.split pipe f)
 
@@ -114,7 +115,7 @@ let get_autopilot_of_airframe = fun xml ->
  * Returns the boolean expression of targets of a module *)
 let get_targets_of_module = fun xml ->
   Xml.fold (fun a x ->
-    match String.lowercase (Xml.tag x) with
+    match Compat.bytes_lowercase (Xml.tag x) with
     | "makefile" when a = Var "" -> targets_of_field x Env.default_module_targets
     | "makefile" -> Or (a, targets_of_field x Env.default_module_targets)
     | _ -> a
@@ -344,4 +345,3 @@ let is_element_unselected = fun ?(verbose=false) target modules name ->
         unselected
     | _ -> false
   with _ -> false
-
diff --git a/sw/lib/ocaml/gm.ml b/sw/lib/ocaml/gm.ml
index ae4f7993bce..1425a8fda69 100644
--- a/sw/lib/ocaml/gm.ml
+++ b/sw/lib/ocaml/gm.ml
@@ -27,6 +27,7 @@ let (//) = Filename.concat
 open Latlong
 open Printf
 
+
 let tile_size = 256, 256
 let zoom_max = 22
 let zoom_min = 18
@@ -131,7 +132,7 @@ let tile_of_key = fun keyholeStr ->
   and lat       = ref (-1.)
   and latLonSize = ref 2. in
 
-  for i = 1 to String.length keyholeStr - 1  do
+  for i = 1 to Compat.bytes_length keyholeStr - 1  do
     latLonSize /.= 2.;
 
     match keyholeStr.[i] with
@@ -148,14 +149,14 @@ let tile_of_key = fun keyholeStr ->
 
 
 let is_prefix = fun a b ->
-  String.length b >= String.length a &&
-    a = String.sub b 0 (String.length a)
+  Compat.bytes_length b >= Compat.bytes_length a &&
+    a = Compat.bytes_sub b 0 (Compat.bytes_length a)
 
 (** Get the tile or one which contains it from the cache *)
 let get_from_cache = fun dir f ->
   let files = Sys.readdir dir in
   (* sort files to have the longest names first *)
-  Array.sort (fun a b -> String.length b - String.length a) files;
+  Array.sort (fun a b -> Compat.bytes_length b - Compat.bytes_length a) files;
   let rec loop = fun i ->
     if i < Array.length files then
       let fi = files.(i) in
@@ -172,11 +173,11 @@ let get_from_cache = fun dir f ->
 
 (** Get the tile or one which contains it from the a hash table *)
 let get_from_hashtbl = fun tbl key ->
-  let l = String.length key in
+  let l = Compat.bytes_length key in
   let rec loop = fun i ->
     if i = 0 then raise Not_found;
     try
-      let subkey = String.sub key 0 i in
+      let subkey = Compat.bytes_sub key 0 i in
       let file = Hashtbl.find tbl subkey in
       (tile_of_key subkey, file)
     with _ -> loop (i-1)
@@ -188,7 +189,7 @@ let get_from_hashtbl = fun tbl key ->
 let xyz_of_qsrt = fun s ->
   let x = ref 0
   and y = ref 0
-  and n = String.length s in
+  and n = Compat.bytes_length s in
   for i = 1 to n - 1 do (* Skip the first t *)
     x := !x * 2;
     y := !y * 2;
@@ -202,17 +203,17 @@ let xyz_of_qsrt = fun s ->
   (!x, !y, n-1)
 
 let ms_key = fun key ->
-  let n = String.length key in
+  let n = Compat.bytes_length key in
   if n = 1 then invalid_arg "Gm.ms_key";
-  let ms_key = String.create (n-1) in
+  let ms_key = Compat.bytes_create (n-1) in
   for i = 1 to n - 1 do
-    ms_key.[i-1] <-
-      match key.[i] with
+    Compat.bytes_set ms_key (i-1)
+      (match key.[i] with
           'q' -> '0'
         | 'r' -> '1'
         | 's' -> '3'
         | 't' -> '2'
-        | _ -> invalid_arg "Gm.ms_key"
+        | _ -> invalid_arg "Gm.ms_key")
   done;
   (ms_key, ms_key.[n-2])
 
@@ -257,7 +258,7 @@ let set_policy = fun p ->
 let get_policy = fun () ->
   !policy
 
-let remove_last_char = fun s -> String.sub s 0 (String.length s - 1)
+let remove_last_char = fun s -> Compat.bytes_sub s 0 (Compat.bytes_length s - 1)
 
 
 type hashtbl_cache = (string, string) Hashtbl.t
@@ -277,7 +278,7 @@ let get_image = fun ?tbl key ->
   let cache_dir = get_cache_dir !maps_source in
   mkdir cache_dir;
   let rec get_from_http = fun k ->
-    if String.length k >= 1 then
+    if Compat.bytes_length k >= 1 then
       let url = url_of_tile_key !maps_source k in
       let jpg_file = cache_dir // (k ^ ".jpg") in
       try
@@ -301,7 +302,7 @@ let get_image = fun ?tbl key ->
       | Some ht -> get_from_hashtbl ht key
     in
     (* if not exact match from cache, try http if CacheOrHttp policy *)
-    if !policy = CacheOrHttp && (String.length t.key < String.length key) then
+    if !policy = CacheOrHttp && (Compat.bytes_length t.key < Compat.bytes_length key) then
       try get_from_http key with _ -> (t, f)
     else (t, f)
   with
diff --git a/sw/lib/ocaml/gtk_tools.ml b/sw/lib/ocaml/gtk_tools.ml
index 146c90f74e5..f39654c53f3 100644
--- a/sw/lib/ocaml/gtk_tools.ml
+++ b/sw/lib/ocaml/gtk_tools.ml
@@ -25,6 +25,7 @@
 (** GTK utilities
 *)
 
+
 class pixmap_in_drawin_area = fun ?drawing_area ?width ?height ?packing () ->
   let da =
     match drawing_area with
@@ -148,7 +149,7 @@ let tree_values = fun ?(only_checked=true) (tree : tree) ->
   store#foreach (fun _ row ->
     let v = store#get ~row ~column:name
     and c = store#get ~row ~column:check in
-    let space = if String.length !values > 0 then " " else "" in
+    let space = if Compat.bytes_length !values > 0 then " " else "" in
     let v =
       if c then v else
         if only_checked then ""
@@ -171,9 +172,9 @@ let get_selected_in_tree = fun  (tree : tree) ->
 let add_to_tree = fun ?(force_unselect=false) (tree : tree) string ->
   let (store, name, check, _) = tree_model tree in
   let row = store#append () in
-  let l = String.length string in
+  let l = Compat.bytes_length string in
   let checked = not (string.[0] = '[' && string.[l - 1] = ']') in
-  let string = if not checked then String.sub string 1 (l - 2) else string in
+  let string = if not checked then Compat.bytes_sub string 1 (l - 2) else string in
   store#set ~row ~column:check (checked && not force_unselect);
   store#set ~row ~column:name string
 
diff --git a/sw/lib/ocaml/http.ml b/sw/lib/ocaml/http.ml
index 22249b04218..fd01bc65297 100644
--- a/sw/lib/ocaml/http.ml
+++ b/sw/lib/ocaml/http.ml
@@ -9,8 +9,8 @@ module H = Http_client
 END
 
 let file_of_url = fun ?dest url ->
-  if String.sub url 0 7 = "file://" then
-    String.sub url 7 (String.length url - 7)
+  if Compat.bytes_sub url 0 7 = "file://" then
+    Compat.bytes_sub url 7 (Compat.bytes_length url - 7)
   else
     let tmp_file =
       match dest with
@@ -19,9 +19,13 @@ let file_of_url = fun ?dest url ->
     let call = new H.get url in
     call#set_response_body_storage (`File (fun () -> tmp_file));
     let pipeline = new H.pipeline in
-    pipeline#set_proxy_from_environment ();
-    pipeline#add call;
-    pipeline#run ();
+    IFDEF NETCLIENT_V_404 THEN
+    pipeline # set_proxy_from_environment (~insecure:false) ()
+    ELSE
+    pipeline # set_proxy_from_environment ()
+    END;
+    pipeline # add call;
+    pipeline # run ();
     match call#status with
       | `Successful ->
         (* prerr_endline (Printf.sprintf "file sucessfull: %s, '%s'" tmp_file url); *)
diff --git a/sw/lib/ocaml/mapFP.ml b/sw/lib/ocaml/mapFP.ml
index f494690c567..9d68f392a69 100644
--- a/sw/lib/ocaml/mapFP.ml
+++ b/sw/lib/ocaml/mapFP.ml
@@ -24,6 +24,7 @@
 
 open Printf
 open Latlong
+
 let (//) = Filename.concat
 
 let sof = string_of_float
@@ -33,7 +34,7 @@ let float_attr = fun xml a -> float_of_string (ExtXml.attrib xml a)
 let rec assoc_nocase at = function
 [] -> raise Not_found
   | (a, v)::avs ->
-    if String.uppercase at = String.uppercase a then v else assoc_nocase at avs
+    if Compat.bytes_uppercase at = Compat.bytes_uppercase a then v else assoc_nocase at avs
 
 (** Returns the WGS84 coordinates of a waypoint, either from its relative x and
     y coordinates or from its lat and long *)
@@ -63,7 +64,7 @@ XmlEdit.Deleted -> wp#delete ()
       let wgs84 = geo_of_xml utm_ref float_attrib in
 
       wp#geomap#edit_georef_name wp#name (assoc_nocase "name" attribs);
-      wp#set wgs84;      
+      wp#set wgs84;
       wp#set_name (assoc_nocase "name" attribs)
     with
         _ -> ()
@@ -202,7 +203,7 @@ let display_kml = fun ?group color geomap xml ->
   try
     let document = ExtXml.child xml "Document" in
     let rec loop = fun child ->
-      let tag = String.lowercase (Xml.tag child) in
+      let tag = Compat.bytes_lowercase (Xml.tag child) in
       match tag with
         | "linestring" | "linearring" ->
             let coordinates = ExtXml.child child "coordinates" in
@@ -265,7 +266,7 @@ class flight_plan = fun ?format_attribs ?editable ~show_moved geomap color fp_dt
     let waypoints = ExtXml.child xml "waypoints" in
     try
       List.fold_left (fun l x ->
-        match String.lowercase (Xml.tag x) with
+        match Compat.bytes_lowercase (Xml.tag x) with
             "kml" ->
               let file = ExtXml.attrib x "file" in
               display_kml ~group:wpts_group#group color geomap (ExtXml.parse_file (Env.flight_plans_path // file));
diff --git a/sw/lib/ocaml/mapGoogle.ml b/sw/lib/ocaml/mapGoogle.ml
index 779241af585..2c627443248 100644
--- a/sw/lib/ocaml/mapGoogle.ml
+++ b/sw/lib/ocaml/mapGoogle.ml
@@ -27,6 +27,7 @@ let array_forall = fun f a ->
 
 open Printf
 
+
 module LL = Latlong
 
 (** Quadtreee of displayed tiles *)
@@ -48,7 +49,7 @@ let char_of = function
 let mem_tile = fun tile_key ->
   let rec loop = fun i tree ->
     tree = Tile ||
-    i < String.length tile_key &&
+    i < Compat.bytes_length tile_key &&
     match tree with
         Empty -> false
       | Tile -> true
@@ -58,7 +59,7 @@ let mem_tile = fun tile_key ->
 (** Adding a tile to the store *)
 let add_tile = fun tile_key ->
   let rec loop = fun i tree j ->
-    if i < String.length tile_key then
+    if i < Compat.bytes_length tile_key then
       match tree.(j) with
           Empty ->
             let sons = Array.make 4 Empty in
@@ -103,7 +104,7 @@ let display_tile = fun (geomap:MapCanvas.widget) wgs84 level ->
   let key = desired_tile.Gm.key in
   if not (mem_tile key) then
     let (tile, jpg_file) = Gm.get_image key in
-    display_the_tile geomap tile jpg_file (String.length tile.Gm.key)
+    display_the_tile geomap tile jpg_file (Compat.bytes_length tile.Gm.key)
 
 
 exception New_displayed of int
@@ -130,7 +131,7 @@ let fill_window = fun (geomap:MapCanvas.widget) zoomlevel ->
   (** Go through the quadtree and look for the holes *)
   let rec loop = fun twest tsouth tsize trees i zoom key ->
     (* Check for intersection *)
-    if not (twest > east || (twest+.tsize < west && (east < 1. (* Standard case *) || twest+.2.>east (* Over 180° *))) || tsouth > north || tsouth+.tsize < south) then
+    if not (twest > east || (twest+.tsize < west && (east < 1. (* Standard case *) || twest+.2.>east (* Over 180� *))) || tsouth > north || tsouth+.tsize < south) then
       let tsize2 = tsize /. 2. in
       try
         match trees.(i) with
@@ -138,16 +139,16 @@ let fill_window = fun (geomap:MapCanvas.widget) zoomlevel ->
           | Empty ->
             if zoom = 1 then
               let tile, image = Gm.get_image ~tbl key in
-              let level = String.length tile.Gm.key in
+              let level = Compat.bytes_length tile.Gm.key in
               display_the_tile geomap tile image level;
-              raise (New_displayed (zoomlevel+1-String.length tile.Gm.key))
+              raise (New_displayed (zoomlevel+1-Compat.bytes_length tile.Gm.key))
             else begin
               trees.(i) <- Node (Array.make 4 Empty);
               loop twest tsouth tsize trees i zoom key
             end
           | Node sons ->
             let continue = fun j tw ts ->
-              loop tw ts tsize2 sons j (zoom-1) (key^String.make 1 (char_of j)) in
+              loop tw ts tsize2 sons j (zoom-1) (key^Compat.bytes_make 1 (char_of j)) in
 
             continue 0 twest (tsouth+.tsize2);
             continue 1 (twest+.tsize2) (tsouth+.tsize2);
@@ -195,10 +196,10 @@ let pixbuf = fun sw ne zoomlevel->
         if zoom = 1
         then
           let tile, image = Gm.get_image key in
-          raise (To_copy (zoomlevel+1-String.length tile.Gm.key, image))
+          raise (To_copy (zoomlevel+1-Compat.bytes_length tile.Gm.key, image))
         else begin
           let continue = fun j tw ts ->
-            loop tw ts tsize2 (zoom-1) (key^String.make 1 (char_of j)) in
+            loop tw ts tsize2 (zoom-1) (key^Compat.bytes_make 1 (char_of j)) in
           continue 0 twest (tsouth+.tsize2);
           continue 1 (twest+.tsize2) (tsouth+.tsize2);
           continue 2 (twest+.tsize2) tsouth;
diff --git a/sw/lib/ocaml/os_calls.ml b/sw/lib/ocaml/os_calls.ml
index 22ccb1745fe..b7ec496be47 100644
--- a/sw/lib/ocaml/os_calls.ml
+++ b/sw/lib/ocaml/os_calls.ml
@@ -21,12 +21,14 @@
  * Boston, MA 02111-1307, USA.
  *
  *)
+
+
 let current_os = ref "not_set"
 
 let read_process_output command =
   let buffer_size = 2048 in
   let buffer = Buffer.create buffer_size in
-  let string = String.create buffer_size in
+  let string = Compat.bytes_create buffer_size in
   let in_channel = Unix.open_process_in command in
   let chars_read = ref 1 in
   while !chars_read <> 0 do
@@ -41,7 +43,7 @@ let contains s substring =
   try ignore (Str.search_forward (Str.regexp_string substring) s 0); true
   with Not_found -> false
 
-let os_name = String.copy (
+let os_name = Compat.bytes_copy (
   if contains !current_os "not_set" then (
     current_os := read_process_output "uname" );
   !current_os
diff --git a/sw/lib/ocaml/papget.ml b/sw/lib/ocaml/papget.ml
index 440e9287f8a..b69da8032de 100644
--- a/sw/lib/ocaml/papget.ml
+++ b/sw/lib/ocaml/papget.ml
@@ -23,6 +23,7 @@
  *)
 
 open Printf
+
 module PC = Papget_common
 module PR = Papget_renderer
 module E = Expr_syntax
@@ -367,7 +368,7 @@ object (self)
     let (x, y) = item#xy in
     let attrs =
       [ "type", msg_obj#type_;
-        "display", String.lowercase item#renderer#tag;
+        "display", Compat.bytes_lowercase item#renderer#tag;
         "x", sprintf "%.0f" x; "y", sprintf "%.0f" y ] in
     Xml.Element ("papget", attrs, scale_prop::val_props@renderer_props)
 end
@@ -386,7 +387,7 @@ object
     let (x, y) = item#xy in
     let attrs =
       [ "type", type_;
-        "display", String.lowercase item#renderer#tag;
+        "display", Compat.bytes_lowercase item#renderer#tag;
         "x", sprintf "%.0f" x; "y", sprintf "%.0f" y ] in
     Xml.Element ("papget", attrs, properties@props)
 end
@@ -415,9 +416,8 @@ object (self)
     let (x, y) = item#xy in
     let attrs =
       [ "type", "video_plugin";
-        "display", String.lowercase item#renderer#tag;
+        "display", Compat.bytes_lowercase item#renderer#tag;
         "x", sprintf "%.0f" x; "y", sprintf "%.0f" y ] in
     Xml.Element ("papget", attrs, properties@props)
   initializer ignore(adj#connect#value_changed (fun () -> self#update_zoom (string_of_float adj#value)))
 end
-
diff --git a/sw/lib/ocaml/serial.ml b/sw/lib/ocaml/serial.ml
index 42edcf2cfa1..795519cf10d 100644
--- a/sw/lib/ocaml/serial.ml
+++ b/sw/lib/ocaml/serial.ml
@@ -24,6 +24,7 @@
 
 open Printf
 
+
 type speed =
     B0
   | B50
@@ -87,11 +88,11 @@ type 'a closure = Closure of 'a
 (*let buffer_len = 256 *)
 let buffer_len = 2048
 let input = fun ?(read = Unix.read) f ->
-  let buffer = String.create buffer_len
+  let buffer = Compat.bytes_create buffer_len
   and index = ref 0 in
 
   let wait = fun start n ->
-    String.blit buffer start buffer 0 n;
+    Compat.bytes_blit buffer start buffer 0 n;
     index := n in
 
   Closure (fun fd ->
@@ -102,7 +103,7 @@ let input = fun ?(read = Unix.read) f ->
     Debug.call 'T' (fun f -> fprintf f "input: %d %d\n" !index n);
     let rec parse = fun start n ->
       Debug.call 'T' (fun f -> fprintf f "input parse: %d %d\n" start n);
-      let nb_used = f (String.sub buffer start n) in
+      let nb_used = f (Compat.bytes_sub buffer start n) in
       (*  Printf.fprintf stderr "n'=%d\n" nb_used; flush stderr; *)
       if nb_used > 0 then
         parse (start + nb_used) (n - nb_used)
@@ -113,5 +114,3 @@ let input = fun ?(read = Unix.read) f ->
       else
         wait start n in
     parse 0 n)
-
-
diff --git a/sw/lib/ocaml/srtm.ml b/sw/lib/ocaml/srtm.ml
index 22d3b7a1aed..7a19b3d5454 100644
--- a/sw/lib/ocaml/srtm.ml
+++ b/sw/lib/ocaml/srtm.ml
@@ -24,6 +24,7 @@
 
 open Latlong
 
+
 let (//) = Filename.concat
 
 type error = string
@@ -56,7 +57,7 @@ let find = fun tile ->
         try
           let f = open_compressed (tile_name ^".hgt") in
           let n = tile_size*tile_size*2 in
-          let buf = String.create n in
+          let buf = Compat.bytes_create n in
           really_input f buf 0 n;
           Hashtbl.add htiles tile buf;
           buf
@@ -147,6 +148,3 @@ let horizon_slope = fun geo r psi alpha d ->
     (*  Printf.printf "debut calcul \n"; *)
     calc_horizon 0.0 0.0 0.0;
   end
-
-
-
diff --git a/sw/lib/ocaml/ubx.ml b/sw/lib/ocaml/ubx.ml
index 5b534b75aac..8875ef28afe 100644
--- a/sw/lib/ocaml/ubx.ml
+++ b/sw/lib/ocaml/ubx.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 module UbxProtocol = struct
   (** SYNC1 SYNC2 CLASS ID LENGTH(2) UBX_PAYLOAD CK_A CK_B
       LENGTH is the lentgh of UBX_PAYLOAD
@@ -32,8 +33,8 @@ module UbxProtocol = struct
   let offset_length=4
   let index_start = fun buf ->
     let rec loop = fun i ->
-      let i' = String.index_from buf i sync1 in
-      if String.length buf > i'+1 && buf.[i'+1] = sync2 then
+      let i' = Compat.bytes_index_from buf i sync1 in
+      if Compat.bytes_length buf > i'+1 && buf.[i'+1] = sync2 then
         i'
       else
         loop (i'+1) in
@@ -43,20 +44,20 @@ module UbxProtocol = struct
     Char.code buf.[start+5] lsl 8 + Char.code buf.[start+4] + 4
 
   let length = fun buf start ->
-    let len = String.length buf - start in
+    let len = Compat.bytes_length buf - start in
     if len >= offset_length+2 then
       payload_length buf start + 4
     else
       raise Protocol.Not_enough
 
   let payload = fun buf ->
-    Protocol.payload_of_string (String.sub buf offset_payload (payload_length buf 0))
+    Protocol.payload_of_string (Compat.bytes_sub buf offset_payload (payload_length buf 0))
 
   let uint8_t = fun x -> x land 0xff
   let (+=) = fun r x -> r := uint8_t (!r + x)
   let compute_checksum = fun buf ->
     let ck_a = ref 0 and ck_b = ref 0 in
-    let l = String.length buf in
+    let l = Compat.bytes_length buf in
     for i = offset_payload to l - 1 - 4 do
       ck_a += Char.code buf.[i];
       ck_b += !ck_a
@@ -70,15 +71,15 @@ module UbxProtocol = struct
 
   let packet = fun payload ->
     let payload = Protocol.string_of_payload payload in
-    let n = String.length payload in
+    let n = Compat.bytes_length payload in
     let msg_length = n + 4 in
-    let m = String.create msg_length in
-    m.[0] <- sync1;
-    m.[1] <- sync2;
-    String.blit payload 0 m 2 n;
+    let m = Compat.bytes_create msg_length in
+    Compat.bytes_set m 0 sync1;
+    Compat.bytes_set m 1 sync2;
+    Compat.bytes_blit payload 0 m 2 n;
     let (ck_a, ck_b) = compute_checksum m in
-    m.[msg_length-2] <- Char.chr ck_a;
-    m.[msg_length-1] <- Char.chr ck_b;
+    Compat.bytes_set m (msg_length-2) (Char.chr ck_a);
+    Compat.bytes_set m (msg_length-1) (Char.chr ck_b);
     m
 end
 
@@ -134,7 +135,7 @@ type message_spec = Xml.xml
 
 let ubx_payload = fun msg_xml values ->
   let n = int_of_string (ExtXml.attrib msg_xml "length") in
-  let p = String.make n '#' in
+  let p = Compat.bytes_make n '#' in
   let fields = Xml.children msg_xml in
   List.iter
     (fun (label, value) ->
@@ -147,19 +148,19 @@ let ubx_payload = fun msg_xml values ->
       match fmt with
         | "U1" ->
           assert(value >= 0 && value < 0x100);
-          p.[pos] <- byte value
+          Compat.bytes_set p (pos) (byte value)
         | "I1" ->
           assert(value >= -0x80 && value <= 0x80);
-          p.[pos] <- byte value
+          Compat.bytes_set p pos (byte value)
         | "I4" | "U4" ->
           assert(fmt <> "U4" || value >= 0);
-          p.[pos+3] <- byte (value asr 24);
-          p.[pos+2] <- byte (value lsr 16);
-          p.[pos+1] <- byte (value lsr 8);
-          p.[pos+0] <- byte value
+          Compat.bytes_set p (pos+3) (byte (value asr 24));
+          Compat.bytes_set p (pos+2) (byte (value lsr 16));
+          Compat.bytes_set p (pos+1) (byte (value lsr 8));
+          Compat.bytes_set p (pos+0) (byte value)
         | "U2" | "I2" ->
-          p.[pos+1] <- byte (value lsr 8);
-          p.[pos+0] <- byte value
+          Compat.bytes_set p (pos+1) (byte (value lsr 8));
+          Compat.bytes_set p (pos+0) (byte value)
         | _ -> failwith (Printf.sprintf "Ubx.make_payload: unknown format '%s'" fmt)
     )
     values;
@@ -176,13 +177,13 @@ let message = fun class_name msg_name ->
 let payload = fun class_name msg_name values ->
   let class_id, msg_id, msg = message class_name msg_name in
   let u_payload = ubx_payload msg values in
-  let n = String.length u_payload in
+  let n = Compat.bytes_length u_payload in
 
   (** Just add CLASS_ID, MSG_ID and LENGTH(2) to the ubx payload *)
-  let m = String.create (n+4) in
-  m.[0] <- Char.chr class_id;
-  m.[1] <- Char.chr msg_id;
-  m.[2] <- Char.chr (n land 0xff);
-  m.[3] <- Char.chr ((n land 0xff00) lsr 8);
-  String.blit u_payload 0 m 4 n;
+  let m = Compat.bytes_create (n+4) in
+  Compat.bytes_set m 0 (Char.chr class_id);
+  Compat.bytes_set m 1 (Char.chr msg_id);
+  Compat.bytes_set m 2 (Char.chr (n land 0xff));
+  Compat.bytes_set m 3 (Char.chr ((n land 0xff00) lsr 8));
+  Compat.bytes_blit u_payload 0 m 4 n;
   Protocol.payload_of_string m
diff --git a/sw/lib/ocaml/xmlEdit.ml b/sw/lib/ocaml/xmlEdit.ml
index 3427e8f1b0f..9a2f468a024 100644
--- a/sw/lib/ocaml/xmlEdit.ml
+++ b/sw/lib/ocaml/xmlEdit.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 let default_background = "white"
 
 type gtkTreeViewDropPosition =
@@ -104,7 +105,7 @@ let string_of_attribs = fun attribs ->
   match attribs with
       ["PCData", data] -> data
     | _ ->
-      String.concat " " (List.map (fun (a,v) -> sprintf "%s=\"%s\"" a v) attribs)
+      Compat.bytes_concat " " (List.map (fun (a,v) -> sprintf "%s=\"%s\"" a v) attribs)
 
 type id = int
 let gen_id =
@@ -131,18 +132,18 @@ let encode_crs =
 *)
 
 let recode_crs = fun s ->
-  let n = String.length s in
-  let s' = String.create n in
+  let n = Compat.bytes_length s in
+  let s' = Compat.bytes_create n in
   let i = ref 0 and j = ref 0 in
   while !i < n do
     if !i < n-1 && s.[!i] == '\\' && s.[!i+1] == 'n' then begin
-      s'.[!j] <- '\n';
+      Compat.bytes_set s' (!j)  '\n';
       incr i
     end else
-      s'.[!j] <- s.[!i];
+      Compat.bytes_set s' (!j) s.[!i];
     incr i; incr j
   done;
-  String.sub s' 0 !j
+  Compat.bytes_sub s' 0 !j
 
 
 
@@ -351,21 +352,21 @@ let set_attribs = fun ((model, path):node) attribs ->
 let rec replace_assoc a v = function
 [] -> [(a, v)]
   | (a', v')::l ->
-    if a = String.uppercase a'
+    if a = Compat.bytes_uppercase a'
     then (a, v)::l
     else (a', v')::replace_assoc a v l
 
 let set_attrib = fun node (a, v) ->
   let atbs = attribs node in
-  set_attribs node (replace_assoc (String.uppercase a) v atbs)
+  set_attribs node (replace_assoc (Compat.bytes_uppercase a) v atbs)
 
 let attrib = fun node at ->
-  let at = String.uppercase at in
+  let at = Compat.bytes_uppercase at in
   let ats = attribs node in
   let rec loop = function
   [] -> raise Not_found
     | (a,v)::avs ->
-      if String.uppercase a = at then v else loop avs in
+      if Compat.bytes_uppercase a = at then v else loop avs in
   loop ats
 
 let tag = fun ((model, path):node) ->
diff --git a/sw/logalizer/sd2log.ml b/sw/logalizer/sd2log.ml
index c4fab9d892c..b9d714aeaee 100644
--- a/sw/logalizer/sd2log.ml
+++ b/sw/logalizer/sd2log.ml
@@ -24,6 +24,7 @@
  *)
 
 open Printf
+
 module U = Unix
 let (//) = Filename.concat
 let var_path = Env.paparazzi_home // "var"
@@ -86,11 +87,11 @@ let hex_of_array = function
   | PprzLink.Array array ->
       let n = Array.length array in
       (* One integer -> 2 chars *)
-      let s = String.create (2*n) in
+      let s = Compat.bytes_create (2*n) in
       Array.iteri
         (fun i dec ->
           let hex = sprintf "%02x" (PprzLink.int_of_value array.(i)) in
-          String.blit hex 0 s (2*i) 2)
+          Compat.bytes_blit hex 0 s (2*i) 2)
         array;
       s
   | value ->
@@ -110,8 +111,8 @@ let search_conf = fun md5 ->
   let files = Sys.readdir dir in
   let rec loop = fun i ->
     if i < Array.length files then begin
-      if String.length files.(i) > (md5_ofs + md5_len)
-      && String.sub files.(i) md5_ofs md5_len = md5 then
+      if Compat.bytes_length files.(i) > (md5_ofs + md5_len)
+      && Compat.bytes_sub files.(i) md5_ofs md5_len = md5 then
         dir // files.(i)
       else
         loop (i+1)
diff --git a/sw/simulator/diffusion.ml b/sw/simulator/diffusion.ml
index 5f50e243671..4b6d31c24c6 100644
--- a/sw/simulator/diffusion.ml
+++ b/sw/simulator/diffusion.ml
@@ -1,3 +1,4 @@
+
 open Printf
 
 module Ground_Pprz = PprzLink.Messages(struct let name = "ground" end)
@@ -67,10 +68,10 @@ let send_on_ivy = fun () ->
     ys := sprintf "%.6f" ((Rad>>Deg)wgs84.posn_long) :: !ys;
     vs := sprintf "%d" plume.value :: !vs)
     plumes ;
-  let ids = String.concat "," !ids
-  and xs = String.concat "," !xs
-  and ys = String.concat "," !ys
-  and vs = String.concat "," !vs in
+  let ids = Compat.bytes_concat "," !ids
+  and xs = Compat.bytes_concat "," !xs
+  and ys = Compat.bytes_concat "," !ys
+  and vs = Compat.bytes_concat "," !vs in
   Ground_Pprz.message_send my_id "PLUMES"
     [ "ids", PprzLink.String ids;
       "lats", PprzLink.String xs;
diff --git a/sw/simulator/sim.ml b/sw/simulator/sim.ml
index e44af59b2ab..8a3ba909511 100644
--- a/sw/simulator/sim.ml
+++ b/sw/simulator/sim.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 open Printf
 open Stdlib
 open Latlong
@@ -239,9 +240,9 @@ module Make(AircraftItl : AIRCRAFT_ITL) = struct
       (*    and theta_ = s.Gps.course *)
           and (phi, theta, psi) = FlightModel.get_attitude !state in
           fg_msg buffer lat lon alt phi theta psi;
-      (**       for i = 0 to String.length buffer - 1 do fprintf stderr "%x " (Char.code buffer.[i]) done; fprintf stderr "\n"; **)
+      (**       for i = 0 to Compat.bytes_length buffer - 1 do fprintf stderr "%x " (Char.code buffer.[i]) done; fprintf stderr "\n"; **)
           try
-            ignore (Unix.sendto socket buffer 0 (String.length buffer) [] sockaddr)
+            ignore (Unix.sendto socket buffer 0 (Compat.bytes_length buffer) [] sockaddr)
           with
               Unix.Unix_error (e,f,a) -> Printf.fprintf stderr "Error sending to FlightGear: %s (%s(%s))\n" (Unix.error_message e) f a; flush stderr
     in
@@ -274,7 +275,7 @@ module Make(AircraftItl : AIRCRAFT_ITL) = struct
           let inet_addr = Unix.inet_addr_of_string !fg_client in
           let socket = Unix.socket Unix.PF_INET Unix.SOCK_DGRAM 0 in
           (* Unix.connect socket (Unix.ADDR_INET (inet_addr, 5501)); *)
-          let buffer = String.create (fg_sizeof ()) in
+          let buffer = Compat.bytes_create (fg_sizeof ()) in
           let sockaddr = (Unix.ADDR_INET (inet_addr, 5501)) in
           Stdlib.timer ~scale:time_scale fg_period (fg_task socket buffer sockaddr);
           fprintf stdout "Sending to FlightGear at %s\n" !fg_client; flush stdout
diff --git a/sw/supervision/pc_common.ml b/sw/supervision/pc_common.ml
index f274770cde3..c46474095f9 100644
--- a/sw/supervision/pc_common.ml
+++ b/sw/supervision/pc_common.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 open Printf
 
 let (//) = Filename.concat
@@ -43,12 +44,12 @@ let run_and_log = fun log exit_cb com ->
   let channel_out_fd = Unix.descr_of_in_channel com_stdout in
   let channel_out = GMain.Io.channel_of_descr channel_out_fd in
   let cb = fun _ ->
-      let buf = String.create buf_size in
+      let buf = Compat.bytes_create buf_size in
       (* loop until input returns zero *)
       let rec log_input = fun out ->
         let n = input out buf 0 buf_size in
         (* split on beginning of new line *)
-        let s = Str.split (Str.regexp "^") (String.sub buf 0 n) in
+        let s = Str.split (Str.regexp "^") (Compat.bytes_sub buf 0 n) in
         List.iter (fun l -> log l) s;
         if n = buf_size then (log_input out) + n else n
       in
@@ -60,18 +61,18 @@ let run_and_log = fun log exit_cb com ->
   pid, channel_out, com_stdout, io_watch_out
 
 let strip_prefix = fun dir file subdir ->
-  let n = String.length dir in
-  if not (String.length file > n && String.sub file 0 n = dir) then begin
+  let n = Compat.bytes_length dir in
+  if not (Compat.bytes_length file > n && Compat.bytes_sub file 0 n = dir) then begin
     let home = Env.paparazzi_home in
-    let nn = String.length home in
-    if (String.length file > nn && String.sub file 0 nn = home) then begin
-      ".." // String.sub file (nn+1) (String.length file - nn -1)
+    let nn = Compat.bytes_length home in
+    if (Compat.bytes_length file > nn && Compat.bytes_sub file 0 nn = home) then begin
+      ".." // Compat.bytes_sub file (nn+1) (Compat.bytes_length file - nn -1)
     end else
      let msg = sprintf "Selected file '%s' should be in '%s'" file dir in
      GToolbox.message_box ~title:"Error" msg;
      raise Exit
   end else
-    subdir // String.sub file (n+1) (String.length file - n - 1)
+    subdir // Compat.bytes_sub file (n+1) (Compat.bytes_length file - n - 1)
 
 
 let choose_xml_file = fun ?(multiple = false) title subdir cb ->
diff --git a/sw/supervision/pc_control_panel.ml b/sw/supervision/pc_control_panel.ml
index 60c542f1ab8..3ef9ba84bc5 100644
--- a/sw/supervision/pc_control_panel.ml
+++ b/sw/supervision/pc_control_panel.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 open Printf
 module Utils = Pc_common
 
@@ -44,7 +45,7 @@ let program_command = fun x ->
     if cmd.[0] = '/' then
       cmd
     else if cmd.[0] = '$' then
-      String.sub cmd 1 ((String.length cmd) - 1)
+      Compat.bytes_sub cmd 1 ((Compat.bytes_length cmd) - 1)
     else
       Env.paparazzi_src // cmd
   with Not_found ->
@@ -76,7 +77,7 @@ let flash_modes =
     let options = List.map (fun o ->
       sprintf "%s=%s" (Xml.attrib o "name") (Xml.attrib o "value")
       ) (List.filter (fun t -> Xml.tag t = "variable") (Xml.children m)) in
-    let options = String.concat " " options in
+    let options = Compat.bytes_concat " " options in
     (* add to hash tables *)
     Hashtbl.add modes mode options;
     List.iter (fun b ->
@@ -117,9 +118,9 @@ let parse_process_args = fun (name, args) ->
   (* Mark spaces into args *)
   let marked_space = Char.chr 0 in
   let in_quotes = ref false in
-  for i = 0 to String.length args - 1 do
+  for i = 0 to Compat.bytes_length args - 1 do
     match args.[i] with
-      ' ' when !in_quotes -> args.[i] <- marked_space
+      ' ' when !in_quotes -> Compat.bytes_set args i marked_space
     | '"' -> in_quotes := not !in_quotes
     | _ -> ()
   done;
@@ -127,19 +128,19 @@ let parse_process_args = fun (name, args) ->
   let args = Str.split (Str.regexp "[ ]+") args in
   (* Restore spaces and remove quotes *)
   let restore_spaces = fun s ->
-    let n = String.length s in
+    let n = Compat.bytes_length s in
     for i = 0 to n - 1 do
-      if s.[i] = marked_space then s.[i] <- ' '
+      if s.[i] = marked_space then Compat.bytes_set s i ' '
     done;
     if n >= 2 && s.[0] = '"' then
-      String.sub s 1 (n-2)
+      Compat.bytes_sub s 1 (n-2)
     else
       s in
   let args = List.map restore_spaces args in
   (* Remove the first "arg" which is the command *)
   let args = List.tl args in
   (* Build the XML arg list *)
-  let is_option = fun s -> String.length s > 0 && s.[0] = '-' in
+  let is_option = fun s -> Compat.bytes_length s > 0 && s.[0] = '-' in
   let rec xml_args = function
       [] -> []
     | option::value::l when not (is_option value) ->
@@ -173,7 +174,7 @@ let save_session = fun gui session_combo ->
       name
 
 let double_quote = fun s ->
-  if String.contains s ' ' then
+  if Compat.bytes_contains s ' ' then
     sprintf "\"%s\"" s
   else
     s
@@ -245,7 +246,7 @@ let supervision = fun ?file gui log (ac_combo : Gtk_tools.combo) (target_combo :
     Gtk_tools.add_to_combo session_combo Gtk_tools.combo_separator;
     let strings = ref [] in
     Hashtbl.iter (fun name _session -> strings := name :: !strings) sessions;
-    let ordered = List.sort String.compare !strings in
+    let ordered = List.sort Compat.bytes_compare !strings in
     List.iter (fun name -> Gtk_tools.add_to_combo session_combo name) ordered
   in
 
@@ -336,4 +337,3 @@ let supervision = fun ?file gui log (ac_combo : Gtk_tools.combo) (target_combo :
   in
   ignore (gui#menu_item_delete_session#connect#activate ~callback);
   session_combo, execute_custom
-
diff --git a/sw/tools/generators/gen_aircraft.ml b/sw/tools/generators/gen_aircraft.ml
index e9f7c749b34..a8e851c7d16 100644
--- a/sw/tools/generators/gen_aircraft.ml
+++ b/sw/tools/generators/gen_aircraft.ml
@@ -22,6 +22,7 @@
  *
  *)
 
+
 open Printf
 module U = Unix
 
@@ -57,14 +58,14 @@ let check_unique_id_and_name = fun conf conf_xml ->
 
 let configure_xml2mk = fun f xml ->
   (* all makefiles variables are forced to uppercase *)
-  let name = String.uppercase (ExtXml.attrib xml "name")
+  let name = Compat.bytes_uppercase (ExtXml.attrib xml "name")
   and value = ExtXml.attrib_or_default xml "value" ""
   and default = ExtXml.attrib_or_default xml "default" ""
   and case = ExtXml.attrib_or_default xml "case" "" in
   (* Only print variable if value is not empty *)
-  if String.length value > 0 then
+  if Compat.bytes_length value > 0 then
     fprintf f "%s = %s\n" name value
-  else if String.length default > 0 then
+  else if Compat.bytes_length default > 0 then
     fprintf f "%s ?= %s\n" name default;
   (* also providing lower and upper case version on request *)
   if Str.string_match (Str.regexp ".*lower.*") case 0 then
@@ -115,11 +116,11 @@ let file_xml2mk = fun f ?(arch = false) dir_name target xml ->
 let module_xml2mk = fun f target firmware m ->
   let name = ExtXml.attrib m.xml "name" in
   let dir = try Xml.attrib m.xml "dir" with Xml.No_attribute _ -> name in
-  let dir_name = String.uppercase dir ^ "_DIR" in
+  let dir_name = Compat.bytes_uppercase dir ^ "_DIR" in
   (* print global flags as compilation defines and flags *)
   fprintf f "\n# makefile for module %s in modules/%s\n" name dir;
   List.iter (fun flag ->
-    match String.lowercase (Xml.tag flag) with
+    match Compat.bytes_lowercase (Xml.tag flag) with
     | "configure" -> configure_xml2mk f flag
     | "define" -> define_xml2mk f ~target flag
     | _ -> ()) m.param;
@@ -140,7 +141,7 @@ let module_xml2mk = fun f target firmware m ->
       in
       Xml.iter
       (fun field ->
-          match String.lowercase (Xml.tag field) with
+          match Compat.bytes_lowercase (Xml.tag field) with
           | "configure" -> configure_xml2mk f field
           | "define" -> define_xml2mk f ~target field
           | "include" -> include_xml2mk f ~target ~vpath:m.vpath field
@@ -164,7 +165,7 @@ let modules_xml2mk = fun f target xml fp ->
   (** include modules directory for ALL targets, not just the defined ones **)
   fprintf f "$(TARGET).CFLAGS += -Imodules -Iarch/$(ARCH)/modules\n";
   List.iter
-    (fun dir -> fprintf f "%s_DIR = modules/%s\n" (String.uppercase dir) dir
+    (fun dir -> fprintf f "%s_DIR = modules/%s\n" (Compat.bytes_uppercase dir) dir
     ) dir_list;
   (* add vpath for external modules *)
   List.iter
@@ -198,7 +199,7 @@ let subsystem_xml2mk = fun f firmware s ->
   List.iter (fun def -> define_xml2mk f def) s_defines;
   (* include subsystem *) (* TODO test if file exists with the generator ? *)
   let s_name = name ^ s_type ^ ".makefile" in
-  let s_dir = "CFG_" ^ String.uppercase (Xml.attrib firmware "name") in
+  let s_dir = "CFG_" ^ Compat.bytes_uppercase (Xml.attrib firmware "name") in
   fprintf f "ifneq ($(strip $(wildcard $(%s)/%s)),)\n" s_dir s_name;
   fprintf f "\tinclude $(%s)/%s\n" s_dir s_name;
   fprintf f "else\n";
@@ -352,13 +353,13 @@ let () =
     (* remove settings if not supported for the current target *)
     let settings = List.fold_left (fun l s -> if Gen_common.is_element_unselected ~verbose:true target modules s then l else l @ [s]) [] (Str.split (Str.regexp " ") settings) in
     (* update aircraft_xml *)
-    let aircraft_xml = ExtXml.subst_attrib "settings" (String.concat " " settings) aircraft_xml in
+    let aircraft_xml = ExtXml.subst_attrib "settings" (Compat.bytes_concat " " settings) aircraft_xml in
     (* add modules settings *)
     let settings_modules = try Env.filter_settings (value "settings_modules") with _ -> "" in
     (* remove settings if not supported for the current target *)
     let settings_modules = List.fold_left (fun l s -> if Gen_common.is_element_unselected ~verbose:true target modules s then l else l @ [s]) [] (Str.split (Str.regexp " ") settings_modules) in
     (* update aircraft_xml *)
-    let aircraft_xml = ExtXml.subst_attrib "settings_modules" (String.concat " " settings_modules) aircraft_xml in
+    let aircraft_xml = ExtXml.subst_attrib "settings_modules" (Compat.bytes_concat " " settings_modules) aircraft_xml in
     (* finally, concat all settings *)
     let settings = settings @ settings_modules in
     let settings = if List.length settings = 0 then
@@ -366,7 +367,7 @@ let () =
         fprintf stderr "\nInfo: No 'settings' attribute specified for A/C '%s', using 'settings/dummy.xml'\n\n%!" aircraft;
         "settings/dummy.xml"
       end
-      else String.concat " " settings
+      else Compat.bytes_concat " " settings
     in
 
     (** Expands the configuration of the A/C into one single file *)
diff --git a/sw/tools/generators/gen_airframe.ml b/sw/tools/generators/gen_airframe.ml
index f44a7508ff5..53b86bc0d94 100644
--- a/sw/tools/generators/gen_airframe.ml
+++ b/sw/tools/generators/gen_airframe.ml
@@ -24,6 +24,7 @@
 
 let max_pprz = 9600. (* !!!! MAX_PPRZ From paparazzi.h !!!! *)
 
+
 open Printf
 open Xml2h
 
@@ -104,7 +105,7 @@ let rec string_from_type = fun name v t ->
   let sprint_array = fun v t ->
     let vs = Str.split array_sep v in
     let sl = List.map (fun vl -> string_from_type name vl t) vs in
-    "{ "^(String.concat " , " sl)^" }"
+    "{ "^(Compat.bytes_concat " , " sl)^" }"
   in
   let rm_leading_trailing_spaces = fun s ->
     let s = Str.global_replace (Str.regexp "^ *") "" s in
@@ -291,8 +292,8 @@ let rec parse_section = fun ac_id s ->
       let servos = Xml.children s in
       let nb_servos = List.fold_right (fun s m -> Pervasives.max (int_of_string (ExtXml.attrib s "no")) m) servos min_int + 1 in
 
-      define (sprintf "SERVOS_%s_NB" (String.uppercase driver)) (string_of_int nb_servos);
-      printf "#include \"subsystems/actuators/actuators_%s.h\"\n" (String.lowercase driver);
+      define (sprintf "SERVOS_%s_NB" (Compat.bytes_uppercase driver)) (string_of_int nb_servos);
+      printf "#include \"subsystems/actuators/actuators_%s.h\"\n" (Compat.bytes_lowercase driver);
       nl ();
       List.iter (parse_servo driver) servos;
       nl ()
@@ -357,14 +358,14 @@ let rec parse_section = fun ac_id s ->
 let h_name = "AIRFRAME_H"
 
 let hex_to_bin = fun s ->
-  let n = String.length s in
+  let n = Compat.bytes_length s in
   assert(n mod 2 = 0);
-  let b = String.make (2*n) 'x' in
+  let b = Compat.bytes_make (2*n) 'x' in
   for i = 0 to n/2 - 1 do
-    b.[4*i] <- '\\';
-    Scanf.sscanf (String.sub s (2*i) 2) "%2x"
+    Compat.bytes_set b (4*i) '\\';
+    Scanf.sscanf (Compat.bytes_sub s (2*i) 2) "%2x"
       (fun x ->
-        String.blit (sprintf "%03o" x) 0 b (4*i+1) 3)
+        Compat.bytes_blit (sprintf "%03o" x) 0 b (4*i+1) 3)
   done;
   b