Peter wrote:
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.
RAIM does not require barometric input, but barometric input can facilitate RAIM by reducing by one the number of satellites that are required for RAIM. As you know, you are required to use a RAIM prediction tool before starting a flight where you will need RAIM (e.g. the Eurocontrol AUGUR system) and it will ask if you have baro-aiding or not. Also, I used to fly with a non-baro-aided GNS430 which certainly did provide RAIM. Then of course, there is the the Wikipedia article on RAIM.
If the number of available satellites is small enough that the barometric input has to be used for RAIM (only 4 for fault detection or 5 for fault exclusion), then you are probably right about horizontal shift. On the other hand, if you can spoof several satellites, why not all of them and then any fault detection function will be pointless, baro-aided or not.
I would think the fix for this is to make the GPS receiver directional and point it upwards?
directional and point it upwards
To which direction exactly?
Peter wrote:
Yeah, we fly there all the time…
Who cares? the point is, if they can do such criminal acts there, they can do them where we fly as well. So relying on sat nav (not only GPS) may bite us in the ass pretty badly if i.e. things deteriorate further in Eastern Europe.
At the same time, more and more countries dismantle all their VOR DME capability. That is not helpful at all.
To which direction exactly?
Towards the blue side obviously 
More seriously I would think to shield the receiver so as to create an inverted “cone of silence” starting at the aircraft pointing down. Perhaps only even a backup receiver. Make the cone shadow big enough so that a spoofing station capable of overcoming it would have to be so powerful that it would be easily spotted by anyone with enough interest. But I suppose it will never work because how are you going to fly those LPVs inverted 
?
For jam resistance of the receiver, mount it on a ground plane, and then the jammer needs to be above the receiver level – more or less.
Does that work for extremely weak signals (below the noise level)?
It will help with all signals.
Whether it does the job, depends on the jammer power and the other usual factors. There is a good reason why e.g. the ATACMS has the antenna on top and not on the bottom
I don’t doubt that having the antenna at the top helps with reception.
But AIUI the GPS signal is extremely weak- around -125 dBm compared to the general radio noise level of around -75 dBm; 5 orders of magnitude weaker.
So it is not very hard for a spoofer to “overpower” the real GPS signal; but by how much does a ground plate below the anteanna reduce the signal reception from below? Or are there even more sophisticated ways to make the GPS antenna directional?
I would expect an antenna mounted of top of a metal (or other conductive) surface to fairly effectively reject signals radiated from down below.
But there will also be signal processing technologies used in mil grade GPS receivers which we don’t know about and which are not available to civilian GPS units.
One day we will hear about the use of GPS in Ukraine (during the Russian invasion) and that will be super interesting. Clearly attempts are made to jam GPS but not very effectively otherwise HIMARS and ATACMS would be useless.
