GPS positioning with RTL2832U

[Carles Fernandez]

Dear all,

we have been doing some experiments with RTL2832U-based DVB-T receiver USB dongles using a GNSS software receiver. The result is that we got a position fix after succesfully acquiring, tracking, and demodulating the navigation message of four GPS satellites.

More details are available here: http://www.gnss-sdr.org/documentation/gnss-sdr-operation-realtek-rtl2832u-usb-dongle-dvb-t-receiver

Best regards,

Carles

[Ravindra]

Hi,

      I am very much attracted to work on GNSS SDR after reading the article "GNSS-SDR operation with a Realtek RTL2832U USB dongle DVB-T receiver" in GNSS-SDR website.

I purchased a USB DVB-T RTL2832U+R820T dongle and I also ordered a GPS L1 active antenna 3 weeks back (expecting this weekend) antenna. I installed SDR sharp software for Windows OS and using it.

Actually I am interested in GNSS receiver, I did M.Tech thesis on SW based GPS L1 receiver (till tracking only). I used MATLAB program for offline processing of IF data. I referred [2] reference given in the article and other books and papers.

The IF data I used was downloaded from Danish GPS center's website. The data was 40 seconds long and has following parameters.

Sampling frequency = 16367600
IF frequency = 4130400

I am aiming to dump the IF data from dongle and use my code to process it for acquisition and tracking. By reading the article I think the IF frequency will be VCO drift (I measured drift for my dongle; it was 80 KHz less the L1 carrier frequency). I wanted to understand, whether the down converted signal from dongle will not have carrier component in it? Kindly correct me if I am wrong. Also can we use this on Windows OS?
I request your help in understanding the concept.

Thanks and regards,
Ravindra H J

[jdow]

Welllll, let's try that again, Kemo Sabe. What do you mean by "carrier
component"? In general the modulators are the equivalent of a pair of
double balanced mixers in quadrature with each other for L1 and L2 (on
Phase 2 birds a least). The P code and C/A code, both, have a very
tiny DC term in the sequences. So theoretically there is some DC or
carrier component as transmitted. But, just off hand, I do not think
that is what you're looking to hear.

For receiving you want to use a Costas loop sort of receiver. Designing
a full receiver should keep you quite occupied for a few days as your
marriage disintegrates - or perhaps a few weeks if you want to give
your family some time. {^_-}

{^_^} Joanne, W6MKU, Designed GPS satellite FSDU, Pre launch test
receiver (and its BITE transmitter), and a few other things
on those birds. FSDU is Frequency Synthesizer and Distribution
Unit - 2 uHz steps at 10.23MHz for fine tuning out oscillator
errors (better than mag field adjustment) and for screwing up
the signal in a very controlled manner to fsck with enemy
cruise missiles trying to drop down Jimmy Carter's toity, a
feature made mostly useless by differential GPS, something
the Air Force "overseers" REALLY did not want to hear. I had
some perverse fun when annoyed. {^_-}

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[Adam Nielsen]

> {^_^} Joanne, W6MKU, Designed GPS satellite FSDU

Wow, that's fascinating, I would love to hear any stories you might have!!

What was the idea behind disrupting enemy cruise missiles? Wouldn't that
disrupt your own stuff too?

Cheers,
Adam.

[jdow]

Of course not. We KNEW what we were doing to our signals. But commercial
users didn't so they got limited to 100 meters accuracy. Military still
got its rated 10 meters easily since what was done on the satellites
could be undone on the ground.

{^_^} Joanne, W6MKU

[Adam Nielsen]

>> What was the idea behind disrupting enemy cruise missiles? Wouldn't that
>> disrupt your own stuff too?
>
> Of course not. We KNEW what we were doing to our signals.

But doesn't that mean you have to constantly update your ground equipment?
Isn't that impractical? Well maybe not today, but back then?

Cheers,
Adam.

[jdow]

That was thought of and dealt with. The inaccuracy was changing
continuously and prescheduled using crypto key devices. So the military
knowing the schedule and key could unwind the perturbations. They were
subtle enough that detecting them required some fancy equipment. You
might say it is frequency hopping on a very small scale.

{^_-} Joanne, W6MKU