OK465:
For any RNP AR below RNP 0.3 GPS is required. Even some 0.3 procedures do not allow DME/DME only solutions.
In airspace regulated by the FAA, no RNAV IAPs are permitted to use DME/DME updating. GPS is mandatory, whether conventional RNAV, RNP AR, or emerging advanced RNAV. DME/DME updating is permitted in Level 1 SIDs and STARS provided DME masking is demonstrated to be sufficent. In the case of RNP AR most, if not all, ICAO states have elected to use FAA's RNP AR criteria, as set forth in FAA Order 8260.52.
Though highly unlikely to lose GPS due to 'loss' of sats, as long as ANP is below RNP you can continue. (You probably wouldn't notice a sat loss unless you were observing that page in the MCDU.)
Indeed, a loss of GPS is unlikely but it is nonethess the assumption made for either minimums predicated on RNP of less than 0.30 and/or a missed approach that requires RNP of less than 1.0.
And in the 738, the first indication of FMS GPS positional problems (sats or receivers) will probably be an amber TERRAIN POS message on the ND as the ANP builds followed eventually by the amber UNABLE REQD RNP. With GPS probs, the IRU's will not prevent this from occurring even if they are perfect and you can't continue to the MAP.
Agree, and thus a loss of GPS on a dual-string RNP AR IAP will likely require aborting the IAP prior to the MAP.
But, the assumptions in criteria are worst case; loss of GPS at DA.
In the '73 the IR pos is third in the hierarchy as far as FMS weighting and the IRU's themselves (att or pos) are not in anyway updated by the GPS.
Which is they way most, but not all systems, function. The point is, the output of the IRUs (or IRU) is updated so the apparent IRU position has no significant errors in the event GPS is lost.
There are three types of missed approach procedures permitted by Order 8260.52:
RNP of less than 1.0, also known as "telescoping RNP," which requires at least one IRU, and because the worst case assumption is loss of GPS at the MAP, positive course guidance is then required from the IRU or IRUs, which position is updated in the FMS continuously until/if a loss of GPS occurs. The telescoping increasing size of the missed approach procedure is calculated upon agreed upon assumptions for IRU drift once GPS is lost.
Following are the two RNP missed approach containment areas from Order 8260.52. The first one requires at least one IRU and turns are permitted either with TF of RF legs (RF legs highly preferred.) The second illustrated RNP missed approach containment area does not require updated IRU position (unless the minimums are predicated on less than RNP 0.30, in which case at least one updated IRU position is required). In this second case no turns are permitted until the splay to Level 1 RNP is competed.
If the terrain is benign the third option is a conventional TERPs (or PANS-OPS) missed approach procedure to any nav aid or even DR. And, a turn is permitted (if necessary) as low as 400 feet AFE.
(The RNP 0.10 final approach segment in Figure 4-1B is an example of maximum application of criteria. the FAS could be as high as 0.30 and, if this type of missed approach is dictated by obstacles or airspace, then at least one IRU is required.)
