Using satellite doppler effect to determine position

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AD5NL
Posts: 34
Joined: Fri Sep 16, 2016 7:00 pm

Using satellite doppler effect to determine position

Post by AD5NL » Sat Apr 01, 2017 5:01 am

Hi everyone,

I was watching an old episode of James Burke's "Connections" documentary, and in one he uses an old satellite navigation system that relies on measuring the doppler effect of a passing satellite, to calculate the position of the user on earth.

The idea being, that the doppler shift frequency at any given point in time will be different for different positions on Earth.

Has anyone considered trying to come up with a way of reproducing this technique with an RTL-SDR? Although it certainly wouldn't be as accurate as GPS, it might be an interesting grad student project.

Paixillation
Posts: 4
Joined: Fri Mar 10, 2017 7:15 pm
Location: Ankara, Turkey

Re: Using satellite doppler effect to determine position

Post by Paixillation » Sat Apr 01, 2017 7:40 am

I am wondering the same thing, since it also my grad project. We have recieved signals from a satellite so far, but haven't been able to calculate its doppler shift yet. My guess is, doppler shift will only provide us the satellite's radial velocity.

But that is probably the case for only 1 station observation.

hotpaw2
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Re: Using satellite doppler effect to determine position

Post by hotpaw2 » Sat Apr 08, 2017 6:17 pm

This requires a frequency reference much more stable than the amount of satellite doppler shift. Not sure the oscillator ppm spec on an RTL-SDR meets this criteria. Maybe by wiring in an external higher quality frequency reference?

CF20852
Posts: 1
Joined: Wed Sep 13, 2017 12:26 am

Re: Using satellite doppler effect to determine position

Post by CF20852 » Wed Sep 13, 2017 12:42 am

I'm old enough to remember doing this stuff for a living ;)

Take a look at https://github.com/CF20852/Lightning-Location-Algorithm for another application of some very similar math, with some references to the integrated Doppler navigation algorithm used by the Navy Navigation Satellite System (TRANSIT) satellite navigation receivers.

You need a pretty stable reference oscillator to get accurate position fixes with integrated Doppler navigation. Back in the '70s, we used $300 (in 1975 US$) ovenized quartz crystal reference oscillators. The oscillator doesn't have to be dead-nuts accurate on frequency, but it should be stable over the duration of a satellite pass. The algorithm will solve for the oscillator frequency offset. With a TCXO-based dongle, you may be able to get some fixes, but they may not be all that accurate. If it's just a fun project, that may not be all that important.

The adaptation of the software in the github posting mentioned above to integrated Doppler navigation is left as an exercise for the student. Hey, I adapted the integrated Doppler nav program to LORAN hyperbolic (TDOA) and TOA navigation before I got my undergraduate degree. It ain't rocket surgery.

myself
Posts: 1
Joined: Thu Nov 23, 2017 1:22 pm

Re: Using satellite doppler effect to determine position

Post by myself » Thu Nov 23, 2017 1:33 pm

I think you're describing Transit or something like it: https://en.wikipedia.org/wiki/Transit_(satellite)

And inversely, the old EPIRBs were located by Cospas/Sarsat satellites by measuring the doppler shift of the ground station as the satellites went over: https://en.wikipedia.org/wiki/Emergency ... out_GPS.29

Yes, you'll need a better oscillator. There are TCXO's available for 28.8MHz, but OCXO's are pretty rare. I was playing with http://www.analog.com/designtools/en/si ... eFilters=0 and it's possible to take a standard 10MHz reference (like you'd get from a bog-standard rubidium or super-common OCXO) and multiply it up to 28.8MHz, but it's nearing the edge of the AD9851's capability -- it really wants a higher-frequency input.

Of course, you could just get a better SDR with better reference-oscillator support, but where's the fun in that?

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