What's your budget? Put a white-rabbit switch (3.5keur) in the middle, and install a mile of single-mode fiber to each rx-station. Then use TDC or FDEL SPEC-cards (1.5keur each) at the RX-stations to time-stamp the incoming pulse. <1 ns systematic and <50 ps RMS random error should be doable. The systematic constant error in time-stamp for each rx-station can maybe be calibrated out in the TDOA-algorithm? The FDEL-card can time-stamp up to 100 kEdges/s (that results in a ca 4 Mb/s datastream).
Anders On Thu, Mar 26, 2015 at 4:27 AM, Robert Watzlavick <roc...@watzlavick.com> wrote: > I'm working on a project that I could use some advice on and also might be > of interest to the list. If it's not appropriate for the list, my > apologies. > > I want to develop a tracking system for an amateur rocket that can allow > me to track the rocket even if onboard GPS is lost (as is typical during > ascent and sometimes during descent) or if telemetry is lost. The idea is > to use a transmitter in the rocket and have 4 or more ground stations about > a mile apart each receive the signal. Multilateration based on TDOA (time > difference of arrival) measurements would then be used to determine x, y, > z, and t. With at least 4 ground stations, you don't need to know the time > the pulse was transmitted. The main problem I'm running into is that most > of the algorithms I've come across are very sensitive to the expected > uncertainty in the time measurements. I had thought 100 ns of timing > accuracy in the received signals would be good enough but I think I need to > get down less than 40 ns to keep the algorithms from blowing up. My > desired position accuracy is around 100 ft up to a range of 100k ft. > > There were two different methods I thought of. The first method would > transmit a pulse from the rocket and then use a counter or TDC on the > ground to measure the time difference between a GPS PPS and the pulse > arrival. This is the most straightforward method but I'm worried about the > timing accuracy of the pulse measurement. I should be able to find a > timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 > sigma) so that portion is in the ballpark. There also seem to be TDCs that > have accuracy and resolution in the tens of picosecond range but they also > have a maximum interval in the millisecond range. I'm not sure I can > ensure the pulse sent from the rocket will be within a few miilliseconds of > the 1 PPS value on the ground. I will have onboard GPS before launch so in > theory I could initialize a counter to align the transmit pulse within a > millisecond or so to the onboard PPS. But, once GPS is lost on ascent, > unless I put an OCXO onboard that pulse may drift too far away (due to > temperature, acceleration, etc.) for the TDC on the ground to pick it up. > Plus an OCXO will add weight and require extra power for the heater. > Another idea would be to send pulses at a very fast rate, say 1 kHz to stay > within the TDC window. But then I need to worry about what happens if the > pulses get too close to the edge of the TDC window. One other variable is > the delay through the RF chain on the receive end but I figure I could > calibrate that out. > > The other idea, and I'm not sure exactly how to implement it, would be to > transmit a continuous tone (1 kHz for example) and perform some kind of > phase measurement at each ground station against a reference. I could use > a GPSDO to divide down the 10 MHz to 1 kHz to compare with the received > signal but how can I assure the divided down 1 kHz clocks are synchronized > between ground stations? Are the 10 MHz outputs from GPSDOs necessarily > aligned to each other? I let two Thunderbolts sit for a couple of hours > and the 10 MHz outputs seemed to stabilize with an offset of about 1/4 of a > cycle, too much for this application. Another related idea would be to use > the 10 MHz output to clock an ADC and then sample several thousand points > using curve fitting, interpolation, and averaging to get a more accurate > zero crossing than you could get based on the sample rate alone. Adding a > TDC would allow the use of RIS (random interleaved sampling) for repetitive > signals which could generate an effective sample rate of 1 GS/s. > > Does anybody have advice or practical experience on which method would > work better? > > Thanks, > -Bob > _______________________________________________ > 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. > _______________________________________________ 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.
