Certainly, if it's a bent-pipe repeater, that makes extracting the Dopplar a whole lot easier. Furthermore, since it's unlikely that the missing plane was the only signal, you can essentially do a differential Dopplar measurement against other sorces, stationary or moving in a know trajectory.
-John ============== > On Mon, Mar 24, 2014 at 06:15:57PM -0700, Chris Albertson wrote: >> Yes, word is that they were able to determine the Doppler shift in the >> plane's signal. I'm surprised this was even recorded but it must have >> been >> in the satellite's telemetry downlink. Projecting radial velocity and >> constraining it to be close to the earth's surface, I guess determines >> one >> path and the direction on it. > > > Perhaps some of the readers here are unaware that the INMARSAT > F3 in question is a bent pipe repeater in both directions. It takes a > C band uplink from the ground and turns it around to L band, and turns L > band uplinks around to a C band downlink. > > It has 8 spot beams, and one regional beam. Channelization of > the uplink and downlink bandwidth and an board switch matrix allows > various allocations of frequencies and bandwidth to the 9 beams varying > with load and demand. > > There is no on satellite signal demodulation/modulation or > protocol processing for the classic AERO signals to/from the plane ... > that is ALL done on ground at the GES (in Perth Australia AFAIK). > > This would make it possible for INMARSAT (and others in the > region tasked with monitoring such things) to capture the actual > repeated RF from the plane and digitize it - this happens in the ground > equipment as part of the normal processing anyway - and dumping it to a > disk array somewhere is certain to be going on, either both inside > INMARSAT at the GES or at least at other (less public) sites such as > Alice Springs. The C band downlinks are global beams BTW and can > be received anywhere that sees the satellite. > > As such the quality of the recovered Doppler and other signal > parameters is very much a function of the stability of the various LOs > (and sample clocks) involved, which I believe can correctly be presumed > to be really high grade both in space and certainly on the ground. AES > (plane) timing and frequency may be less good, but it is more or less > locked to the L band downlink timing and frequency signals as reference. > > The newer INMARSAT F4 birds do have DSP processing on the > satellite, but apparently NOT used for demodulating and processing the > various control channel signals on the satellite - but just for doing > beam forming and power allocation for the 120 spot beams these birds > support. This of course would impact delay through the satellite > for precision timing and ranging. > > But so far there are no reports that the F4 POR satellite was > involved. The high gain antennas on the AES (plane) are fairly > directional and if they were in use there might not be a lot of signal > seen on the POR bird. Not sure if those pings would have been sent > via a low gain antenna on the AES, but I suspect normally not. > > -- > Dave Emery N1PRE/AE, d...@dieconsulting.com DIE Consulting, Weston, Mass > 02493 > "An empty zombie mind with a forlorn barely readable weatherbeaten > 'For Rent' sign still vainly flapping outside on the weed encrusted pole - > in > celebration of what could have been, but wasn't and is not to be now > either." > > _______________________________________________ > 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.