Robin Clark
No comment on the above, as I'm no navigator. However, thought you might be interested in this, from the (fabricated) CA Release and RTS, current at time of accident;
1. GPS has not yet been declared operational by the US DoD and therefore accuracy is not guaranteed to any level. Even when GPS is declared operational, accuracy of GPS could degrade substantially without any indications to the crew. For this reason SPS GPS should not be used as the sole navigation aid.
2. In addition to (1) above, the GPS is highly susceptible to jamming of which the only crew indication is loss of GPS. The "Err" (Error) figure displayed (on the RNS252), which has conventionally been taken as a measure of GPS performance is MEANINGLESS and so no indication of GPS accuracy is available to the user. The GPS suffers from an average 2 second processing delay on the displayed GPS position.
Call me pedantic, but "Err" must mean something, even if it is "This system is so bloody immature only a complete prat would release it to Service before we understand how it works and can train the aircrew". Which is effectively what Boscombe said, only more forcefully.
Lack of understanding constitutes a Human Factors hazard - in the same way the master caution warning relating to FADEC was not understood. In isolation they may, eventually, become understood and be deemed aceptable, but the cumulative effect at this time was, I believe, an unnacceptable risk.
Interestingly, at a time when the RAF were opertating a mixed fleet (Mk1 and Mk2) there was no equivalent GPS/RNS252 warnings for Mk1. Is it any wonder that Flt Lt Tapper thought it necessary to visit Racal in a vain attempt to work out how GPS/RNS252 worked and behaved? That shows a dedication completely missing in those who were meant to ensure he had a mature aircraft.
Is it not also an indication that the so-called "Mk2 conversion course" was highly premature? 4 months later the crew were still so uncertain of the aircraft maturity they asked for a Mk1?