> I think, a specialized GPS SDR can be build for less than 500 USD > in low (a dozen at max) volumes.
The USRP works for GPS L1 (though P/Y is a little undersampled at 8 Ms/s complex), but I didn't find a way to acquire both L1 and L2 simultaneously at useful sample rates (maybe current USRP hardware is better). Also 16 or 8 bits is too much precision---2 bits is more appropriate and for some reason wasn't a standard option. It was fun to acquire and track L1 and L2C separately, but what I really want is a no-holds-barred geodetic reference receiver. A dedicated tri-band GPS front end could be built for less than $500, I agree. Software can handle acquisition, tracking, and conversion to RINEX. The hardware just needs to translate RF to bits on the wire (gigabit Ethernet say) and be phase-stable over temperature. One possible inexpensive design: - RF input passively split three ways, with LC filters for the three channels: L5/E5, L2, and L1/E1/Glonass - For each channel, a downconverter (Maxim MAX2121) feeding a ~65 Ms/s ADC (e.g. MAX19505) - A low-cost FPGA (e.g. Spartan-6) that quantizes the channels to 2 bits, does AGC, assembles Ethernet packets - Ethernet PHY, power (PoE?), etc. For a timing receiver, one could inexpensively add one more ADC that samples a 10 MHz input signal and a 1PPS input signal. 1PPS packets would be emitted only when transitions are detected, and the 10 MHz signal could be downconverted to a low-bandwidth signal to be sent over Ethernet with the others. This way one has reference signals coherently sampled with the GPS signals. I think LC filters would provide enough protection against strong out-of-band interferers; semicustom ceramic-resonator filters or, worse, full-custom SAW filters are not hobbyist-friendly and may not be as stable over temp. Also I think the phase noise of the MAX2121 is acceptable. Possibly the FPGA should be doing the pulse blanking for L5 since the FPGA still has the 8-bit signal available. Is there a publically-available antenna design that's easy to fabricate, has a stable phase center, covers 1100--1600 MHz, and has a good pattern over this band with low cross-polarization? Even a large choke-ring design would be okay if it's fully specified. Cheers, Peter _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
