New Look at the evidence....
Further thoughts...........
My understanding of Doppler radar navigation is that it has to be initialized in the same manner as INS ie. told the local time and location in latitude and longitude , and it tracks the motion of the A/C from that point .......in the case of the SuperTANS without the built in 6 channel GPS receiver ..
( a later model than that installed in ZD576 ) this would mean manually entering the position unless there is another external source , which in this case there was .... the Trimble GPS receiver ............
So the starting location would be derived from the GPS source and any comparison early in the flight would show the two sources ....GPS and Doppler , to be in close agreement......................................but we know from the data later recovered from the SuperTANS that the two sources were NOT in agreement just before the crash.....
The Doppler source info. can be expected to drift as discussed previously by many contributors , due to poor returns from a flat water surface and/or wave and tide motion......
The GPS source accuracy can vary wildly depending on the motion of the satellites , the application of the encrypted algorithm to remove the deliberate corruption of the signal ( selective availability ) for military use , but mostly from the accuracy of the frequency standard inside the SuperTANS ......
Any drift on the internal clock would have a dramatic and unpredictable effect on the position error and extremes of temperature is one factor to effect the drift . Normally on powerup there will be a period when the equipment is slowly reaching a working temperature inside .......
If we assume that the GPS error changed in a linear fashion during this short flight , unlikely in practice but as an example..............
and the crew had the opportunity to compare the Doppler and GPS sources halfway across the sea , they would still have seen little difference between the two sources , ie. crosschecking would have still shown agreement........
...but if the GPS error apparent at the crash site , were present when they were two miles from the Mull , a crosscheck would have
shown a 2 or 3 degree difference , and a crosscheck at the position where the waypoints were changed would display a 10 degree difference................
Tragically , if they were navigating purely from the Doppler source it would have led them to a waypoint 'A' which was on the shoreline.....
just a little south of the light house.......they would have safely overflown this part of the coast and not come close to any high ground till a little further north.............giving them much longer to assess the flying conditions along the edge of the localized cloud formation.......
and quite possibly continuing the journey without incidence ............
From what I have read , the crew would have had low confidence in the SuperTANS and may have decided to check the accuracy whenever possible .
One easy way to do this during pre-flight would be to pretend they wished to fly to waypoint 'H'........which was programmed into the SuperTANS.......
......using a location a few miles away from Aldergrove and at roughly right angles to their intended departure would give them a good
reference to a known location whilst they were stationary.... .
From the centre of Aldergrove airfield this SHOULD be a course of 331 degrees true and distance of 9.8nm .......................... but the
starting location which appears to have been recorded in the SuperTANS would have given them a course of 320 degrees true and
a distance of 8.7 nm ......( in my entry #6155 I pointed out the starting Lat.and Long. were halfway to the town of Antrim)
....thus it would have been possible to determine that in that present condition the SuperTANS would guide them to a waypoint 'A'
which was some distance offshore of the lighthouse ..................
I do NOT find it at all strange that there was information stored inside the SuperTANS................ as with a large number of devices there would be a second power source or internal batteries to save any important information in the event of a power interuption ......
.......the name for this varies with manufacturers but Non-Volatile-Storage , or NVS is a common term..............in this way the
equipment can continue to function after power is restored ..........
I entirely agree about the references to an immature a/c by other contibutors , in fact you could almost claim it was an experimental a/c.!.
....I found one document which says that the first 20 Chinooks converted by Boeing were all delivered with different faults present........ .........varying from loose nuts and bolts to old wiring partly disconnected but left in situ...... ...........?????????...