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Comment by planteen | Hacker News Reader

Does anyone know if any of these SDR solutions for hobbyists can get survey grade position solutions?

http://www.rtklib.com/ is an open source implementation of real time kinematic centimeter-level positioning. The link on that page to another SDR GPS project is dead, but https://gnss-sdr.org/docs/tutorials/understanding-data-types... claims support for RTCM and RINEX, which you might be able to feed into RTKLIB.

We've been trying to use it with a hackrf and bladerf and no dice at receiving actual GPS, even with an antenna exactly tuned to 1.57542GHz. Only when we transmit GPS signals ourselves so the signal is much stronger, are we able to obtain any satellites at all.

The software works, but you need a low noise amplifier or something else that you'd probably have to order from America or China (that would have taken too long for our school project).

Don't most external GPS antennas patch include an LNA right on the antenna?

Of course, it's still a very faint signal.

Not really, actually a while ago I ordered a bunch of unamplified ceramic patch antennas because they were cheap. Managed to get fix with a 4x4 array and a uBlox NEO-6M chip; at 5x5 gain was good but the beam too narrow. In either case the signal was still under the noise floor.

Are you using a bias-tee to insert power on the center pin of the coax? Your antenna probably has a preamp built in, which needs to be powered otherwise it acts as an attenuator.

I was under the impression that gps antennas were powered...and special.

The receiver has to pull the signal from below the noise floor so needs a little help but, admittedly, my understanding on how it all works is limited.

A sibling comment mentioned using a Bias-T, and that's pretty much always needed unless you're using a COTS Rx (which would generally have one built-in)

It doesn't even have to be a fancy one. In a pinch, once, I use a random-valued inductor and the smallest-valued capacitor I had on-hand, and soldered them to a little PCB I made by slicing up copper-clad board with an x-acto knife. (0603 parts, if I recall.) Worked like a charm!

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