[Hackrf-dev] Attempting to Transmit GPS Using the HackRF One

[Karl Koscher]

Sooooo several comments:

1) It's very possible your signal is too strong. IIRC, GPS signals are
usually around -150 to -160 dBm at the receiver. The HackRF can transmit at
about +10 dBm. That is SIGNIFICANTLY stronger than most GPS receivers
expect.

2) BE VERY CAREFUL ABOUT TRANSMITTING GPS SIGNALS. Even Faraday cages don't
provide perfect isolation -- I've used one with 90 dB of isolation before.
Of course, +10 dBm - 90 dB is... -80 dBm, which is 80 dB louder than a
typical GPS signal! Even if you hard-wire your HackRF to your receivers
(through attenuators, of course), you'll likely be leaking a relatively
strong signal.

3) You will likely have to transmit a signal for at least 15 minutes to get
a lock, as your transmitter isn't synced to actual GPS time. See
https://en.wikipedia.org/wiki/Time_to_first_fix. Phones may also have
trouble locking because they might rely on A-GPS for the ephemeris and
almanac data, as well as coarse time sync.

On Thu, Jul 9, 2020 at 1:59 PM John Akin <jakin at asset-intertech.com> wrote:

> I am working on a project that involves a GPS receiver and I need a GPS
> generating device to produce test data.  I thought the HackRF One sounded
> perfect for the job.
>
> I have gone through the tutorial videos on Great Scott Gadget’s site and
> implemented the FM radio project, so I believe that I have verified my
> HackRF is definitely working as an FM receiver.  Now I am seeking guidance
> in the transmitting of GPS.
>
> I found several tutorial videos on line that make it look easy enough and,
> following their examples, I have been using the HackRF transfer command to
> transmit the GPS signal (the command syntax I have used is,
> “hackrf_transfer -t .\gpssim.bin -f 1575420000 -s 2600000 -a 1 -x 0 -C 2
> -R.”)
>
> I see the Transmit LED is on, as I would expect, but I am not having luck
> in verifying the transfers are occurring using apps for either Android
> phones, or iPhones.  The GPS location reported by these aps is my physical
> location no matter what coordinates I supply.
>
> I have also connected, via SMA cable, the HackRF to my project and the GPS
> receiver does not appear to be receiving GPS coordinates either.  The GPS
> sentences it reports contain no latitude or longitude values.
>
> Anyone have any recommendations?
>
> Thanks!
>
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