Peter wrote:
since there is no SBAS coverage, you get no verification of the GPS solution.
When there is no SBAS, you get verification of the GPS solution using RAIM.
RAIM protection is an interesting point – why did that not work in the incidents described?
Cobalt wrote:
RAIM protection is an interesting point – why did that not work in the incidents described?
Possibly there was no RAIM availability — it requires that additional satellites are available. Or the FMS doesn’t implement RAIM because the designers thought that comparing with INS and VOR/DME would provide sufficient integrity checking.
RAIM is no good if the shift is only horizontal.
I doubt that RAIM was unavailable, in theory 24 satellites mean that it is available everywhere all the time (for en-route precision at least), there are 31 operational sats at this point.
RAIM detects “odd one out” satellite signals where the pseudoramge is inconsistent with the other sats and either excludes it (FDE) or packs it in (FD only). So either as you say RAIM was not implemented (unlikely but some airliner avionics are hopelessly out of date) or the spoofing was done in a way that it was beyond the capability of the RAIM alhgorithm to detect. For example, spoofing multiple satellites at the same time so all received data is consistent could do that.
An IRS is normally updated using GPS and/or DME/DME location data when available, so if GPS jumps and updates the IRS position it somwhat ruins the day.
However, VOR/DME raw data should still be available.
e.g. by Iranians trying to learn how to play games with US drones
Civilian GPS doesn’t use same code as military, so hacking civilian L1 C/A code doesn’t influence P code on L1 and L2 frequencies.
Peter wrote:
RAIM is no good if the shift is only horizontal.
Certainly it is! The direction of the shift doesn’t matter. In fact when making an LNAV approach with a non-SBAS navigator you only get horizontal guidance and RAIM is required in that case.
Or the FMS doesn’t implement RAIM because the designers thought that comparing with INS and VOR/DME would provide sufficient integrity checking.
That would be a real surprise because RAIM check is mandatory AFAIK.
Emir wrote:
That would be a real surprise because RAIM check is mandatory AFAIK.
It is mandatory with an independent GPS receiver. It is not obvious that it is mandatory for an integrated nav system which also has INS and VOR/DME/DME. In any case, the aircraft flying over Iraq would most likely use RNAV 5 which doesn’t require RAIM. RAIM is needed only for the RNP navigation specifications.
RAIM is a barometric input only. So like I said it won’t pick up clever spoofing where the GPS solution matches the baro level well enough i.e. a horizontal shift only.
That level of spoofing was developed many years ago for local jamming for military purposes.
And it is entirely possible that the coders of the rest of the kit never tested it without GPS input. Dumb, yes, but all sorts of things happen.
