View GPS downlink on spectrum analyzer - Page 1
Forum section: electronics / rf-microwave

author: cncjerry
author_rank: Unknown
timestamp: February 07, 2022, 07:02:06 pm
content: should I be able to see the GPS downlink spectrum on an analyzer? I had a GPS antenna that seemed flakey and instead of waiting for the GPSDO to tell me if I fixed it, I thought I would just hook up my R&S spectrum analyzer. I made a T and fed the antenna 5v with a DC block on the analyzer. Nothing, even when I ultimately found it was working on the Spectracomm GPSDO.I then found an R&S paper about recording the GPS signal using an SA but it looks like the signal was below -174dB, something about it being like 16db below the thermal noise level. I haven't thought about this much and I'm sure it is something that will be "duh". The analyzer I am using was showing a noise floor of -145 at 10hz RBW. It goes lower but I didn't want to stick around and wait for it to sweep.ideas?Jerry

author: hagster
author_rank: Frequent Contributor
timestamp: February 07, 2022, 07:17:14 pm
content: With a omnidirectional antenna GPS is below the noise floor. You will not be able to see it.The antennas normally have an amplifier (you have powered with 5v and a biastee) and a filter. In this case you might be able to see the raised noise pedestal from the filtered and amplified input, but what you are seeing is the noise.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 07, 2022, 07:22:03 pm
content: It's a spread spectrum signal. There is about 40 dB of processing gain to pull it above the noise. Google and learn about it.

author: hagster
author_rank: Frequent Contributor
timestamp: February 07, 2022, 07:34:29 pm
content: The thing that amazes me with GPS is that most receivers use a 1bit ADC. It's literally just sensing if the voltage is positive or negative every clock cycle.

author: cdev
author_rank: Super Contributor
timestamp: February 07, 2022, 07:56:48 pm
content: All I've ever seen looks like a hump of more noise around 1572.42 MHz (quite wide) . That's all. And Ive tried hard to see more.

author: hagster
author_rank: Frequent Contributor
timestamp: February 07, 2022, 09:12:00 pm
content: You need a directional antenna. Probably at least 20dBi to make it clear.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 07, 2022, 11:04:39 pm
content: Quote from: hagster on February 07, 2022, 09:12:00 pmEven with that, it's like you said previously, "you might be able to see the raised noise pedestal from the filtered and amplified input, but what you are seeing is the noise". Tx power is 44.8 watts with 12 dBi antenna gain. The received power is going to be about -129 dBm plus whatever antenna gain you have. The noise power in the 2 MHz bandwidth is -111 dBm. And.. the signal looks like noise.

author: TheSteve
author_rank: Unknown
timestamp: February 08, 2022, 04:05:34 am
content: This is the noise I can see with a 25 dB gain antenna, second picture is with the antenna blocked. So if nothing else it is picking up some noise from the sky.

author: hagster
author_rank: Frequent Contributor
timestamp: February 08, 2022, 06:46:33 am
content: Did you aim at a particular satellite?It might be hard to isolate a single signal clearly. There are Beidou, GLONASS, Galileo signals all at about the same frequency these days.

author: cncjerry
author_rank: Unknown
timestamp: February 09, 2022, 06:13:11 pm
content: I think the other issue was that I didn't have the span wide enough. I was just using a puck as well, but it had clear sky. I ultimately found the issue though, cheap BNC to SMA connectors.