|Quoting akberc (Reply 80):|
so the pings may even originate from the ground, bounce off Inmarsat, to the Satcom modem, and back up through Inmarsat to the ground station.
Of course they would.
There was much discussion about the ping measurement process.
Don't expect to see clocks on board communication satellites. Think of them as a bidirectional mirror with very little to no processing on board.
Pinging would be a purely ground process running in the Earth station computers based on profiles in databases and the realtime status of the aircraft modem .
For an accurate measurement you need in principle:
- precise time stamping upon generating the ping request
- estimating any delays that may occur if messages are queued according to priority before being transmitted
- knowing the position of the satellite as accurately as possible. Yes, geostationary satellites are not absolutely stationary. They oscillate in longitude and latitude around their nominal station. Their fuel on-board is used to keep them within their "box" so that they don't go mess up their neighbors.. 3F1 is an old satellite operating beyond its nominal life but it only has a small orbital inclination (only 1.6 Deg), I am sure Inmarsat tracks its real-time position very well as that is essential for station keeping and now for RTD measurements (for instance apogee-perigee difference around 40kms )
- qualified software of the satcom modem on-board the aircraft as to latency in response to a message ping. It does not matter how long the modem takes to respond as long as it always answers with the same latency
Now the answer goes back through the satellite and reaches the earth station. Several operations have to occur to decode the message including error correction and assess its time of arrival. That time has to be measured AND
recorded with enough precision (I hope no worse then 1msec). Just like for the aircraft modem, the receive chain has to operate with constant latency.
The problem I see is that a precise RTD operation was probably never envisioned and therefore never specified for that satellite, aircraft modem and earth station.
There are payloads on other GEO satellites that have been designed specifically to allow for precise ranging using appropriate waveforms and algorithms. For a ground target i.e. the altitude of which can be inferred from a DEM (Digital Elevation Model), positioning to a few tens of meters can be obtained.
In our case, only Inmarsat knows from their analysis and calibration data what can be expected in practice.