Garmin GPS155 TSO for FlightGear

Copyright (c) 2022 Tobias Dammers (tdammers@gmail.com)

This is free software; see enclosed COPYING file for licensing details.

Introduction

The Garmin GPS155 TSO was one of the first GPS units to be certified for RNAV IFR operations in GA aircraft. Unlike most newer devices, it does not feature a map display; it has a text-only 20x3 character cell display, and it can drive a HSI or similar instrument to provide visual guidance to the pilot.

Installation

This simulation is intended for aircraft model authors to add to existing or new FlightGear aircraft; it cannot be installed as an add-on, and it is not intended to be user-installable.

With that said, the installation method goes something like this:

1. Copy all the relevant files into your aircraft model's directory tree. You need the following directories and everything below them:
   • Fonts
   • Models
   • Nasal The easiest way to achieve this is by using the included to-aircraft.sh script (requires Bash, which is available or emulated on all Linux and OS X systems, and on Windows via various Unix layers such as git-bash, MSYS, Cygwin, etc.).
2. Register the Nasal scripts with your aircraft. In your aircraft-set.xml, add the following to the <nasal> section:

   <gps155>
    <file>Nasal/gps155.nas</file>
   </gps155>

   This will load the GPS155's main entry point; all the other scripts are loaded automatically from there.
3. If you want the GPS155TSO to persist settings between FG sessions, add the following to your aircraft-set.xml (merge with existing <sim> and <aircraft-data> elements, if any):

   <sim>
      <aircraft-data>
        <path>instrumentation/gps155/settings</path>
      </aircraft-data>
   </sim>

4. Add the following to your aircraft-set.xml (again, merge with existing <autopilot> and <route-manager> elements, if any):

   <autopilot>
     <route-manager>
         <disable-fms type="bool">1</disable-fms>
     </route-manager>
   </autopilot>

   This is necessary to prevent the Route Manager from performing the built-in automatic sequencing, which the GPS155TSO will override.
5. Add the 3D model to your cockpit. This can be achieved by adding something like the following to your cockpit model XML:

    <model>
      <name>gps155</name>
      <path>Aircraft/Lockheed1049h/Models/Instruments/gps155.xml</path>
      <offsets>
        <x-m>-15.649</x-m>
        <y-m>-0.480</y-m>
        <z-m>0.760</z-m>
        <pitch-deg>0</pitch-deg>
      </offsets>
    </model>

   FIXME: the GPS155 backlight is currently hard-coded to feed off of the /controls/lighting/panel-norm property; this should of course become a property alias, and aircraft authors can select whichever property suits them to drive the backlight.
6. Adapt your aircraft so that the GPS can drive the autopilot and HSI. The easiest way to do that is to use the "slaving" feature of the built-in GPS, which effectively sends quasi-VOR signals to the HSI and autopilot, synthesized from the GPS. However, you can also use a 3-way switch (NAV1, NAV2, GPS) and feed the autopilot and instruments directly from the GPS.

Some Technical Details

The GPS155TSO is designed to use the built-in GPS device as much as possible; it mostly acts as a frontend for that, so almost all the functionality is also accessible through the Equipment → GPS dialog. However, a couple additional things are necessary, and most of these are reflected in the property tree under /instrumentation/gps155. What it inherits from the built-in GPS remains in /instrumentation/gps, so in order to interact with the unit via properties, you will need to use both of these subtrees.

Satellite data is currently not a proper simulation, but uses random data according to a relatively simple algorithm; when the unit says "acquiring", it just shows a selection of random satellite IDs, with signal strengths fluctuating randomly, until a timer runs out and the unit becomes functional.

Likewise, just like the built-in GPS itself, the GPS155TSO does not simulate GPS inaccuracy; it always produces perfect lateral positioning data, and the altitude is read off of the first altimeter.

This also means that the unit interacts with the route manager in the same way the built-in GPS does - there is no route editing functionality yet, but when flying a route entered via the route manager, and the GPS set to LEG mode, the GPS155TSO will display the legs on the NAV CDI page.

Development Status

Working / Done:

• 3D Model
• Font and canvas display
• NAV CDI and NAV Position screens
• DTO key from NAV Position screen
• RTE
• SET (partial)
• Waypoint Confirm screen
• Waypoint Select screen

Features To Add

• NAV menus
• NRST
• WPT
• MSG
• Approaches
• Animate buttons & knobs
• Various bits & pieces