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INSTALL
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==================================
OGN DVB-T receiver version 0.2.2
==================================
Install guide
===============
Important change: to comply with open-source licenses
=======================================================
we split the executable into two parts:
1. ogn-rf - RF signal acquisition, GSM cablibration, FFT
2. ogn-decode - packet demodulator and decoder
ogn-rf user open source RTLSDR and FFT code and is now open source,
while ogn-decode is closed source.
The two executable run in parallel and ogn-rf passes
Fourier spectra to ogn-decode through the named Unix FIFO: ogn-rf.fifo
You need to create this FIFO with this command:
mkfifo ogn-rf.fifo
in the same directory as the executables.
To run on Raspberry PI with executable using GPU for faster FFT
=================================================================
For the executables to be able to use GPU the following needs to be done:
sudo chown root ogn-rf
sudo chmod a+s ogn-rf
sudo chown root gsm_scan
sudo chmod a+s gsm_scan
sudo mknod gpu_dev c 100 0
Frequency calibration
=======================
DVB-T USB dongle use cheap crystals and these are not very accurate.
They can be off by as much to 100ppm, which at 868MHz is 86kHz away
- far too much for correct reception of FSK modulation with +/-50kHz deviation
Fortunately, this can be corrected, we only need to know by how much the crystal is off.
GSM signals are a good reference and not far away in frequency thus they can be used
for precise frequency calibration.
To measure the frequency offset of your DVB-T stick use the gsm_scan tool, like this:
./gsm_scan --ppm 50 --gain 20
-ppm is your initial guess for the offset and --gain is the RF front end gain.
As you can see using gsm_scan is not fully automatic. You may need to make few guesses
and the frequency offset and the optimal gain. Too little gain and you don't get any signals.
Too much gain and the signals get so distorted you get plenty of false readings.
The most popular big black DVB-T sticks with R820T tuner have crystals with offest between 40and 80ppm.
The tiny black sticks with R820T are around zero.
With a good antenna in a GSM-dense area you may need to go down with the gain to 15-20dB
while far from civilation you may need to go to maximum 50-60dB.
After few tries you should be able to catch several broadcast channels and get a consistent reading.
Note that as the DVB-T dongle acquires data, it warms up and frequency offset starts drifting by few ppm
this is normal. However the small DVB-T sticks can get too hot and then they suddenly jump in frequency.
If you identify such stick, use it with good cooling - thus don't close it in a small box without ventilation.
Here is sample output of: ./gsm_scan --ppm 40 --gain 40
935.400MHz: +14.7dB: -25.84 -25.81 -25.71 [ppm]
936.000MHz: +14.4dB: -25.68 -25.47 -25.44 -25.63 -25.62 [ppm]
936.400MHz: +9.7dB: -25.51 -25.47 -25.38 -25.34 -25.28 -25.22 [ppm]
938.000MHz: +6.4dB: -25.18 -25.10 -25.22 -25.17 -25.09 [ppm]
939.000MHz: +4.5dB: -24.90 -25.12 -24.38 -25.14 -24.84 [ppm]
948.000MHz: +23.1dB: -25.69 -25.77 -25.78 -25.71 -25.72 [ppm]
949.600MHz: +14.9dB: -25.32 -25.21 [ppm]
950.000MHz: +24.8dB: -25.46 -25.65 [ppm]
952.000MHz: +18.1dB: -25.28 -25.36 -25.29 -25.17 -25.34 [ppm]
952.400MHz: +24.0dB: -25.90 -25.90 -25.77 -25.87 -25.92 [ppm]
953.200MHz: +21.3dB: -24.83 -24.87 [ppm]
956.400MHz: +11.6dB: -25.73 -25.28 -25.46 -24.49 -24.89 [ppm]
956.600MHz: +30.9dB: -24.70 -25.10 -24.83 -25.19 [ppm]
957.000MHz: +9.4dB: -25.20 -25.20 -25.19 -25.35 [ppm]
957.200MHz: +23.5dB: -24.56 -14.85 [ppm]
957.600MHz: +28.7dB: -25.12 -25.14 -25.12 -25.06 -25.14 [ppm]
958.200MHz: +6.5dB: -24.40 -24.61 -24.21 -23.86 -24.30 -24.52 [ppm]
959.000MHz: +10.1dB: -25.00 -25.02 -25.22 -24.76 -25.31 -25.08 [ppm]
959.200MHz: +28.9dB: -24.72 -24.71 -24.66 -24.69 -24.71 -24.70 [ppm]
959.800MHz: +12.3dB: -23.80 -23.65 -24.06 -24.08 -23.88 -23.49 -23.95 [ppm]
Receiver Xtal correction = 40-25.135 = +14.865 (0.188) ppm [46]
which means we guessed about 25 ppm to high,
and so we retry: ./gsm_scan --ppm 15 --gain 40
935.400MHz: +16.0dB: +0.47 +0.31 +0.24 +0.37 +0.41 [ppm]
936.000MHz: +15.3dB: +0.40 +0.07 +0.63 +0.44 +0.51 [ppm]
936.400MHz: +7.9dB: -0.13 +0.19 -0.08 -0.18 -0.25 [ppm]
938.000MHz: +6.2dB: -0.08 -0.06 -0.06 -0.03 +0.01 [ppm]
948.000MHz: +23.3dB: +0.33 +0.31 +0.33 +0.33 +0.32 [ppm]
949.600MHz: +15.0dB: -0.21 -0.16 -0.15 -0.21 -0.17 [ppm]
950.000MHz: +25.1dB: -0.23 -0.30 -0.08 -0.15 -0.28 [ppm]
950.800MHz: +6.8dB: +0.16 +0.16 +0.10 [ppm]
952.000MHz: +16.3dB: +0.28 +0.28 +0.27 +0.27 +0.26 [ppm]
952.400MHz: +24.7dB: -0.34 -0.31 -0.33 -0.32 -0.29 -0.34 [ppm]
952.800MHz: +12.1dB: -0.21 -0.26 -0.26 -0.28 -0.33 [ppm]
953.200MHz: +21.7dB: -0.25 -0.25 -0.25 -0.24 -0.26 [ppm]
954.000MHz: +15.3dB: -0.26 -0.19 -0.23 -0.21 -0.24 [ppm]
954.400MHz: +9.7dB: +0.10 +0.13 +0.03 +0.07 [ppm]
955.600MHz: +4.2dB: +0.07 +0.59 [ppm]
956.600MHz: +31.2dB: -0.30 -0.38 -0.34 -0.30 -0.32 [ppm]
957.000MHz: +8.5dB: -0.35 -0.53 -0.27 -0.59 -0.19 -0.31 [ppm]
957.200MHz: +23.2dB: -0.31 -0.31 -0.29 -0.28 -0.30 [ppm]
957.600MHz: +29.6dB: -0.29 -0.25 -0.24 -0.28 -0.27 [ppm]
958.800MHz: +13.3dB: +0.11 +0.04 +0.15 +0.08 +0.14 +0.12 [ppm]
959.200MHz: +28.1dB: +0.14 +0.16 +0.18 +0.14 +0.15 [ppm]
959.800MHz: +15.6dB: +0.13 +0.19 +0.17 +0.18 +0.14 [ppm]
Receiver Xtal correction = 15-0.064 = +14.936 (0.199) ppm [81]
Which means that our guess 15ppm is just correct.
Here we should choose a frequency with several signals within 1MHz,
for example 952.800MHz is a good choice because there are good calibration signals
just below and just above.
These three numbers we need to put int he config file:
1. frequency offset = +15ppm
2. GSM frequency for fine calibration: 952.800MHz
3. Optimum RF gain to find the signals but not oversdrive the receiver: 40dB
Whith correct numbers the ogn-rf should output such messages:
GSM freq. calib. = -1.298 +/- 0.067 ppm, 8 points
GSM freq. calib. = -1.231 +/- 0.066 ppm, 6 points
This means the GSM calibration is succesfully receiving the signals and calibrating
and we are just on the frequency (jus 1.3ppm away, which is unimportant)
Config file
=============
about ready to make the configuration file, here is a template:
# ------------------------------------------------------------------------------------------------------------------
RF:
{
FreqCorr = +15; # [ppm] "big" R820T sticks have 40-80ppm correction factors, measure it with gsm_scan
GSM: # for frequency calibration based on GSM signals
{ CenterFreq = 952.8; # [MHz] find the best GSM frequency with gsm_scan
Gain = 40.0; # [dB] RF input gain (beware that GSM signals are very strong !)
} ;
} ;
Position:
{ Latitude = +48.0000; # [deg] Antenna coordinates
Longitude = +9.0000; # [deg]
Altitude = 100; # [m] Altitude above sea leavel
GeoidSepar = 48; # [m] Geoid separation: FLARM transmits GPS altitude, APRS uses means Sea level altitude
} ;
APRS:
{ Call = "SampleAPRSnameToChange"; # APRS callsign (max. 9 characters)
} ;
# ------------------------------------------------------------------------------------------------------------------
The elements are:
1. Station name: up to 9 characters -> replace "SampleAPRSnameToChange" with your station name
2. Station coordinates
3. Frequency offset and GSM parameters aspreviously discussed
Station coordinates does not have to be precise, in case you prefere not to disclose your geographical position.
Running the receiver as a service
===================================
We use shellbox: a tool to run any number of processes in the background with the capability to restart should they crash.
You need to install two file: rtlsdr-ogn and rtlsdr-ogn.conf like this:
sudo apt-get -y install procserv telnet nano
sudo cp -v rtlsdr-ogn /etc/init.d/rtlsdr-ogn
sudo cp -v rtlsdr-ogn.conf /etc/rtlsdr-ogn
sudo update-rc.d rtlsdr-ogn defaults
then you should edit the rtlsdr-ogn.conf like this:
sudo nano -w /etc/rtlsdr-ogn.conf
which should look like this:
50000 pi /home/pi/rtlsdr-ogn ./ogn-rf SampleAPRSnameToChange.conf
50001 pi /home/pi/rtlsdr-ogn ./ogn-decode SampleAPRSnameToChange.conf
this is example from Raspberry PI wheere the receiver runs under user "pi" and files are in the directory "rtlsdr-ogn".
Replace SampleAPRSnameToChange.conf with your configuration file.
To start the service you type:
sudo service rtlsdr-ogn start
and this will start the executables listed in /etc/rtlsdr-ogn.conf
To stop:
sudo service rtlsdr-ogn stop
Monitoring
============
To check if things work alright you can watch the output of the executables by telnet'ing to ports 50000 and 50001:
telnet localhost 50000
or
telnet localhost 50001
On the ogn-rf output you should see the GSM caibration messages and on the ogn-decode output there should be
APRS keep-alive messages and decoded position packets from aircrafts.
Quesions and comments
=======================
please send to contact@glidernet.org