Over on his blog “RTL-SDR DX” dewdude has been exploring the reception and decoding of Differential GPS (DGPS) signals. DGPS signals are transmitted by government authorities in the long wave band at around 300 kHz. These beacons are used to dramatically improve the accuracy of GPS (Global Positioning System) devices from their default accuracy of about 15 m down to about 10 cm. Unlike GPS signals which originate from satellites, the DGPS signal is terrestrial based and is broadcast from multiple known fixed positions. The signal itself contains information about the difference between the DGPS stations received GPS position and it’s known exact position. These differences can be used to correct other GPS receivers that receive DGPS signal.
By using his RTL-SDR (with upconverter or HF modification) dewdude was able to receive the DGPS beacon in SDR#. Then by piping the output audio into SpectrumLab’s DGPS decoder he was able to decode the data contained within the DGPS signal. His post contains a tutorial showing how to set up SpectrumLab to decode DGPS. If you’re interested in hearing what a DGPS signal sounds like, dewdude has uploaded a sound sample at the bottom of another post of his.