The manual makes it clear that the multiple DME position solution is considered the most accurate, when receiving sufficient DME signals.
A DME signal has an accuracy that is rated at 3% of distance, with a maximum predictable accuracy of 90 meters. When you have multiple signals, the software can balance them. As the ac is moving, each DME source is being updated. Update rates and distance affect each accuracy level, so that is why multiple are better than one.
The accuracy is also based on the DME itself, as the RNAV system balances the ranges, it has to take into account multi-path errors, looking at signal in space accuracy tolerances along the procedure path. The ANSP is supposed to continuously validate the signals, but that requires a flight validation, so well, there you are, put up a new building and all bets are off. Even then, the ICAO standards are to maintain accuracy at 95%.
In designing procedures, I frequently have to do a DME coverage analysis, which will fry brain cells for sure...
Along with multi-path errors, the RNAV system will identify errors resulting from field strength below min standards.
All of this is why DME/DME accuracy is at best around 90 meters.
Now about that GPS....
in regards to jammers, the hand held units only have very limited ranges, and in reality, are only used to either block the GPS signal from your phone, or the vehicles GPS. There used to be higher powered units, that would be used around public and secure building, that had ranges of up to 300m, but those had significant power requirements.
There are units used by the military, but have very significant power and signal requirements.
What I believe most of you are experiencing, especially with the the messages such GPS L/R INVALID, GPS INTEGRITY PRESERVED, are the result of the HAL of your unit, passing the HAL. Given some of the locations that were talked about, as referencing the RAIM map below, one can also surmise the reason for some of the error messages.
I guess I would have to ask about the process for determining the RAIM prediction for your company as well, and how that information gets to each flight deck. I know that many Operational SOP's require the RAIM prediction along the flightpath, and this will determine if the flight may use RNP, or specifically, which RNP levels can be used.
From the RAIM prediction map, one can always see issues along the equatorial regions in general, but site specific and route prediction is required.
While this was posted on another thread, it is applicable here as well. What I suspect is either you are not looking at the RAIM predictions, and/or dont understand what they mean. In the text when I speak of the Alarm, that is that the unit is returning messages such as GPS INVALID, and can also show certain RNP levels not available.
RAIM is built into the GPS system on the aircraft, and is part of what is balancing all the the different sat connections and detecting faults the pseudorange measurements.
(in regards to Gimpo, this is a GBAS test setup, but GBAS only broadcasts the corrected algorithm, so I have not heard of a GBAS confusing a non GBAS system, and the Airspace around Korea is a difficult one anyways)
When you look at RAIM predictions,they are configured with 3 modes, Fault Detection, FD(5 sats req'd), Fault detection and exclusion, FDE (6 sats req'd), and if Selective Avail, SA is turned on.
For 3D navigation, you need a min of 4 sats, but will have no RAIM fault detection.
The RAIM function that you will see in the box is represented with the HIL number. Depending on your box, and RNP level set, the box will alarm when the HIL number reaches a certain threshold, the HAL, telling you that you horizontal integrity is getting near the limit Horizontal Alarm Limit of the RNP level.
The HIL, HAL, and HPL numbers are some voodo combination of the secret sauce inside each box, and given the potential combinations of algorithms and factors, I have no idea how alarm limits are decided.
As an example, during flight validation testing, with the Smiths box, RNP level 0.3, I had the box alarm at HIL of .51 in a 737, while in a 320,with a Honeywell box on RNP 0.1 it alarmed at .67....
Hope that helps!
GPS/WAAS RAIM prediction tools