Recording GPS RF data

In order to record the GPS signals contained within the RF data received from a GPS antenna, the first process is to downconvert the RF signal into a signal which can then be digitised and stored in a binary file. This is an identical process to that which is employed in the front end of almost all GPS receivers.

For the LabSat 1, an image reject mixer downconverts the RF signal to a 4.092 MHz intermediate frequency (IF). The integrated IF filter feeds an IQ combiner, limiter and output latch. The output signal is a 1-bit quantized 4.092 MHz digital IF sampled at 16.368MHz.

Once the GPS data has been digitised into a bit stream, the bits are then packed into 16bit words and transmitted via a high speed USB link to the PC. An internal 4Mbyte buffer gives a 2 second window to smooth out any fluctuations in the speed of the USB link.

The LabSat 2 uses a different method whereby the data is sampled into both I and Q data as 1 bit each, or 2 bits of I and Q data. This results in greater fidelity, which is useful for sensitivity testing and re-producing multi-path and scintillation effects.

Spectrum analyser showing RF output

Replaying GPS RF data

The PC streams the stored binary file to the LabSat via a high speed USB link. The 4Mbyte buffer ensures the data stream is consistent during replay. Each 16bit word is then converted into a 1 bit data stream which is then fed into an up-converter at 16.368MHz to generate a 1575Mhz GPS RF signal which can be directly connected to the input of almost any GPS engine.

In line with the output is a digital attenuator, which allows adjustment of the output level by up to 31dB in 0.5dB steps.