Originally Posted by
RichardC10
3. Changing the assumed speed would cause problems for the Doppler model that INMARSAT used. This is very sensitive to the aircraft speed which has to be removed to reveal the spacecraft Doppler signal used to decide it was on the Southern route.
If (if) the interim ping arcs are being used and are still constraining the choice of route and hence search area, I would guess that the constant magnetic heading option is now being used, which allows a shorter required range.
Consider the following.
Doppler shifts are measurements of "radial speed": speed with which the aircraft is getting closer to or further away from the satellite.
If you have a bunch of speed values and you assume that the trajectory is reasonably smooth (no zigzagging), you can calculate the change in radial distance (ie arc) by summation of speed*time.
In other words, if the aircraft is moving in a smooth manner, Doppler shifts and arcs are not independent sources of information. You can fit a number of trajectories to the Doppler shift curve, but, as long as they fit, all points always end up on the same arcs. The unknown is the transverse speed and the transverse distance traveled: how far along each arc your points end up.
So, if we take the previous search point and recalculate the trajectory using lower speeds, all points move northeast and the point of last transmission moves northeast quite a bit, while staying on the ~40 degree arc.
If you look at my spreadsheet a couple of pages ago, I had possible Southern routes worked out for 400 knots and 450 knots, and the results were as expected: going from 450 to 400 moved the endpoint 4 degrees north and 5.5 degrees east.