Some interesting things with GNSS toolkit

[Makmerfy]

Hello Everyone!

Spoiler

Have found some interesting thing in GNSS toolkit. 

Why when numbers of satellite increase power decrease? 

For example: I have an USRP 2942R, and GNSS Toolkit.

When I generate 3 GPS satellite measurement power from RX/TX out is -15 dBm and for 10 satellites its -21 dBm.

And for GLONASS too.

Sorry for my pronunciation)

[Jupiter02]

Hi Makmerfy,

In general, when considering power, we are going to be looking at the combination of all satellites. As such, interference between satellites can change the waveform and potentially affect power. 

How are you measuring power and defining power (average or peak)? In addition, what are the generation settings and power settings on the hardware?

[Makmerfy]

Hi Jupiter02,

I have Anritsu spectrum analyzer and measuring power with it. Power in Band gives me average power. Also

i have connect I16 stream direct to FFT and Power Spectrum.vi, with conversion to Complex DBL and got the same result.

USRP generation setting is GAIN = 0dB, Frequency 1,57542GHz for GPS and 1,602G for GLONASS IQ RAte = 10MHz, coerced values are the same.

GNSS toolkit IQ RATE 10 MS/s for each GPS and GLONASS.

Reference Power -50dBm

Max Relative Power 10dBm

and so on by default.

thx.

[Jupiter02]

Hi Makmerfy,

We tested the generation settings you mentioned with 3 and 10 satellites using a PXIe-5673 Vector Signal Generator and saw results consistent with the formula for peak power linked here. We were able to see the total channel power increase as the number of satellites increased, consistent with Total channel power = Pref + 10log(N).

As such, we are wondering if the decrease in power is due to the USRP hardware or if there is something else going wrong. Are you using the GPS Simulation with NI USRP and LabVIEW example to perform this test? Please feel free to post screenshots of your configuration/settings for further investigation.