David, While I can easily see how you can do closed loop correctioin for Dopplar from the transmission point for a 'bent pipe' repeater, at any other location that correction would not be valid, because the paths are not parallel.
-John ============= > On Wed, Jul 10, 2013 at 08:10:45PM +0200, Magnus Danielson wrote: >> On 07/09/2013 04:25 AM, Joseph Gwinn wrote: >> >Yes, of course, but I don't think I explained very well. The issue was >> >more economic than technical. >> > >> >There isn't much spare space, weight, or power in the birds, technology >> >moves rapidly, and the satellite companies don't want to have expensive >> >satellites that no longer generate rental income because something >> >became obsolete. So they ruthlessly simplify. A bent pipe will handle >> >any possible band-limited modulation, no matter if currently known or >> >not, and so is the safest solution. >> > >> >Now WAAS may have become important enough to command dedicated >> >hardware, but that came later, to the degree it came at all. >> >> A bent pipe is more generic, but there are limits to how much you can >> alter the output frequency too. > > It seems completely inconceivable to me that either the antenna > system (particularly feeds) or transponder RF hardware on any commercial > Ku or C or Ka or X band satellite could possibly be frequency agile > enough to tune to 1575.42 MHz unless it was purpose designed to radiate > on that frequency from the start. > > So any hosted WAAS payload is completely application specific. > > What is not clear from anything I have read so far is whether > the UPLINK of the modulated WAAS signal is somewhere in the normal > (usually 6 GHz for C band satellites) uplink frequency band (probably > off one end or the other of the frequency range used). Seems rather > likely that the ability to reuse the UPLINK common RF hardware > (reflector, feeds, filters, plumbing, maybe transponder front ends and > preamps) would make this a very natural design. > > It also seems clear that doppler and bent pipe conversion > oscillator correction is done closed loop by having the ground station > that generates the uplinked WAAS signal monitor the downlink from the > bird. Obviously correcting for the uplink doppler is a matter of > computation from knowing the bird's orbit orbit precisely, something > that would certainly be aided by constantly monitoring the range to the > bird from that WAAS uplink ground station and maybe another couple (for > ionospheric corrections). Apparently the newer stuff uses two L band > frequencies to improve this (correct for plasma delay). And the WAAS > signal of course allows continuous measurement of range accurately. > > Correcting for a generally stable but slowly aging conversion > oscillator should be pretty straightforward as well, and presumably such > a closed loop system could hold the downlink frequency to rather tight > tolerances given a reasonably predictable stable oscillator on the bird. > The 240 ms up and back delay does make the loop a bit more complex, but > the bandwidth is very low I would think since the major perturbation is > probably thermal (satellite going into eclipse once a day at certain > times, and changes in sun angle over a day). > > For an observer on the ground it is of course necessary to > correct for the satellite orbit induced doppler... which can be up to a > couple of hundred Hz or more at 6 GHz - especially with inclined orbit > birds such as the INMARSATs. The downlink carrier, while more stable > in frequency than GPS bird downlinks is hardly a highly accurate > frequency reference on its own. But knowing the geo bird ephemeris > (which is broadcast on the WAAS) should allow single signal time and > frequency solution for an observer at an accurately known location - by > correcting for bird movement. > > How good the closed loops are relative to the precision clocks > on GPS satellites is an interesting question, there seems to be no > obvious design need to reach that level of stability... but it does not > seem impossible to get pretty close. And much of what has been > achieved here seems related to a cost/power trade off in the hosted > payload in regards to its reference oscillator. > > > -- > Dave Emery N1PRE/AE, [email protected] 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 -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
