JP1
Like most, I know little of the Nav System fitted to ZD576. What you describe is plausible, although refuted by the MoD who place 100% reliance on the unrepresentative, unvalidated and unverified post-accident testing of the SuperTANS, the alleged accuracy of which they use as absolute “proof” that the complete Nav System, including GPS, Radar Altimeter, Doppler, Air Data Computer, Gyro Magnetic Compass, Vertical Gyro and Height Encoding Altimeter, was serviceable and accurate. If you know anything about engineering, you’ll know how nonsensical this is.
From Racal report, “The (TANS) system will use Radar altitude if available”. MoD state (a) the complete nav system was serviceable and accurate (as above), yet (b) acknowledge faults in the Radar Altimeter. Please note the contradiction. In my opinion, this ridiculous position simply highlights the fact (acknowledged in far more recent, open source, BOI reports and other papers) that they often pay little or no attention to systems integration.
The GPS + base station system you describe is similar to Differential GPS. Boscombe got it that year, in a different aircraft, but I have no knowledge of it being fitted in-service. I may be wrong. Implementation was, in part, via a carry-on laptop (in lieu of an on-board data capture device) which would have to be subject to a Service Deviation – but a Chinook would have no need for this particular use of DGPS (which was to establish the precise position of the aircraft post-flight, for comparison with indicated position). I agree with your summary of the typical (and gross) errors sometimes experienced with GPS – they have been mentioned before on the thread by others. GPS is a timing system – positional data is simply a product – and it became apparent shortly after the accident that, with hindsight, GPS manufacturers had wrongly interpreted and implemented a Bit Timing Requirement in a GPS spec. Without going into detail (and I do not pretend to understand it all, especially as applied to the Mk2 nav system) this error of interpretation resulted in cumulative timing errors. Perhaps someone with detailed knowledge could say how this would affect Mk2.
Beyond all doubt? Don’t think so.