On 12/16/11 7:46 AM, Azelio Boriani wrote:
A used Spirent is only 26K to 37K. Interesting: playing back bits from RAM... can it be that simple? Obvious: a DAC is required.
No.. you don't even need a DAC. The underlying waveform is a binary code that is BPSK modulated.
there are a variety of commercial GPS record/playback units that basically are single bit digitizers with no mixer (other than the sampler with a well chosen sample rate to put the GPS signal at a good place in the passband)
The GPS receiver we're flying for the CoNNeCT project samples the three bands with a single bit at about 38.6 MHz, which aliases the L1 of 1575 down to about 9 MHz: close to fs/4, which is a convenient place. The other bands alias to convenient places as well.
There is some art in picking a good sample rate... you want something that aliases to somewhere convenient, and you want to not have the Doppler push you past a folding point. Doppler from GPS satellite motion to a stationary platform is on order of 5kHz max. For a receiver LEO satellite (probably a worst case) you'd need to add a platform motion of 7 km/s, which is about 10 kHz. Real worst case would be something that's reached escape velocity, which I think is 40-50 kHz Doppler.
So don't go picking sample rates that are exact fractions of L1. You'll wind up in alias unwrapping hell.
For generating, though, sample rates that are a multiple of the chiprate (1.023 MHz) might be the best strategy. Say you clock the bits out at 10.23 MHz, and you could use the 154th harmonic of the same oscillator as your L1 carrier and all that.
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