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osqzss / gps-sdr-sim

Software-Defined GPS Signal Simulator
MIT License
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gps signal strength #138

Closed robellebor closed 6 years ago

robellebor commented 6 years ago

I can receive the signals using usrp x310 ... but it's not fixing a 3D position because the satellite signals are weak and they change rapidly ... I tried increasing the gain but still, i cant get a lock...

i saw issue #35 and changed the signal gain gain[i] = 100 and it did change the signals strength but still the number of satellites changes rapidly so the reciever cant get a fix ...

Mictronics commented 6 years ago

I had the same issue with the ADALM-Pluto that is using a 12bit DAC. Actually I got an improvement by using 9-bit sin/cos tables generating the baseband signal. During experiments with gain, offset and bitshift values I noticed that you cannot simply increase gain. First this will decrease SNR, second, at some point there will be overflows in math that completely disturb your baseband signal. So use small steps when you make changes. Also keep in mind that baseband IQ of each channel will be summed up, hence the more channels you simulated lower the gain should be.

I found a good indication for signal quality when using the app SatStat on Android. When I see PRNs showing up in the SBAS, GLONASS and BEIDOU range than I know the signal is bad.

osqzss commented 6 years ago

The "gain" parameter is used to create a "relative" signal power difference among GPS satellites up to about 10dB. The maximum signal amplitude is set 128, and you can only reduce the signal power from it to avoid DAC overflows as Mictronics mentioned. If you want to improve the "absolute" signal power, please increase the gain (or reduce the attenuation) of your signal transmitter.