Receiving Differential GPS Beacons with a HackRF
Differential GPS (DGPS) are signals that exist between 285 – 325 kHz and are used to enhance the accuracy of GPS receivers. The system can improve GPS accuracy from 15m down to 10cm in some cases. It works using a network of ground stations at a very accurate known location that continuously measure the GPS error they receive. They then broadcast this error to DGPS capable receivers. The receiver can then use this error knowledge to correct their own readings.
With an VLF capable radio these DGPS beacons can be received and decoded on your PC. Over on swling.com guest poster Mario has submitted a post showing that these DGPS beacons can be received with a HackRF SDR and the MultiPSK software. The HackRF is a $299 SDR that can tune down to VLF (at reduced sensitivity). We note that the same or better results could also be achieved with a HackRF or RTL-SDR with upconverter.
I’ve received DGPS signals on 300 and 320 khz as well as an NDB from my local airport on 250 khz, with an RTL-SDRblog radio and a homebrew Low Pass filter. (Just a coil of wire in series with the longwire antenna). NO up-converter. Just tuned there in Direct/Q mode. A proper ferrite stick antenna and maybe a pre-amp might work better. Then it’d be directional too. One station was in Richmond, BC, Canada, and the other said it was in Appleton, WA. I’m in Nanaimo, BC which is roughly 40-60 km from Richmond/Vancouver, BC. Appleton is on the far side of Washington state, and only came through at night. I love my RTL-SDR.