Budget is a concern but not an overriding concern. I'd like to keep the whole system around $1k. I was planning on making it as portable as possible with each ground station being self contained and sending their data to the launch site over a serial RF modem at 9600 baud. I agree though - fiber connections would make it a lot easier.
-Bob > On Mar 26, 2015, at 08:41, Anders Wallin <anders.e.e.wal...@gmail.com> wrote: > > 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. > _______________________________________________ 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.
