LabSat

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.

A more in depth technical explanation of this process is this: 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 PC receives these packets of words and stores them in a simple binary file.

Spectrum analyser showing RF output

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.