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Another thing that is being looked at are antennas (CRPA: Controlled Reception Pattern Antennas) which filter signals more, so that "GPS signals" that come from

Another thing that is being looked at are antennas (CRPA: Controlled Reception Pattern Antennas) which filter signals more, so that "GPS signals" that come from the ground and the sides are more likely to be rejected: * https://rntfnd.org/2025/02/26/faa-moving-toward-crpa-on-aircraft-rfi-for-study-released-by-nawc/ https://rntfnd.org/2025/02/26/faa-moving-toward-crpa-on-airc... One tripping point is that in the US, CRPAs are on ITAR, so exports are difficult: * https://www.gpsworld.com/first-fix-freeing-crpas/ https://www.gpsworld.com/first-fix-freeing-crpas/ Given that GPS/GNSS comes from satellites, ignoring signals from not-from-the-sky seems like a quick win.

burnerthrow008 2y ago
> Given that GPS/GNSS comes from satellites, ignoring signals from not-from-the-sky seems like a quick win. You're right, but GPS antennas already have some rejection from the "bottom" hemisphere. So they're already rejecting not-sky. CRPAs (of the type contemplated by ITAR) are electronically steerable antennas (phased arrays), that allow you to steer one or more nulls to the direction of the noise source(s). That gives much better rejection of point-source noise.
jpm_sd 2y ago
They're taking CRPAs off the ITAR list later this year, supposedly. https://insidegnss.com/crpas-to-be-removed-from-itar-list-opening-market-for-u-s-manufacturers/ https://insidegnss.com/crpas-to-be-removed-from-itar-list-op...
KennyBlanken 2y ago
GPS signals from low attitudes improve accuracy (to a point) because they provide much better triangulation. You want low attitude GPS satellites. You also don't want to lose signal every time the receiver tips, like when going up/down slight hills.
- Aloisius 2y ago
  They don't lose signal when the receiver tips (which would make it useless for planes). They use antenna arrays to filter signals coming from directions they don't expect, too strong, etc. These systems have been used used in military aircraft for a long time.
- GJim 2y ago
  > triangulation *Trilateration One measures the time of flight of the satellites signal, not the angle it arrives from! https://en.wikipedia.org/wiki/Trilateration https://en.wikipedia.org/wiki/Trilateration
Scoundreller 2y ago
Also, the Doppler shift of jamming/spoofing will be all wrong, unless they’re specifically targeting your vehicle and accounting for its speed and direction in their attack. … if you can get the precision to filter that out…

burnerthrow008 2y ago

> Given that GPS/GNSS comes from satellites, ignoring signals from not-from-the-sky seems like a quick win. You're right, but GPS antennas already have some rejection from the "bottom" hemisphere. So they're already rejecting not-sky. CRPAs (of the type contemplated by ITAR) are electronically steerable antennas (phased arrays), that allow you to steer one or more nulls to the direction of the noise source(s). That gives much better rejection of point-source noise.

jpm_sd 2y ago

They're taking CRPAs off the ITAR list later this year, supposedly. https://insidegnss.com/crpas-to-be-removed-from-itar-list-opening-market-for-u-s-manufacturers/ https://insidegnss.com/crpas-to-be-removed-from-itar-list-op...

KennyBlanken 2y ago

GPS signals from low attitudes improve accuracy (to a point) because they provide much better triangulation. You want low attitude GPS satellites. You also don't want to lose signal every time the receiver tips, like when going up/down slight hills.

Aloisius 2y ago
They don't lose signal when the receiver tips (which would make it useless for planes). They use antenna arrays to filter signals coming from directions they don't expect, too strong, etc. These systems have been used used in military aircraft for a long time.
GJim 2y ago
> triangulation *Trilateration One measures the time of flight of the satellites signal, not the angle it arrives from! https://en.wikipedia.org/wiki/Trilateration https://en.wikipedia.org/wiki/Trilateration

Scoundreller 2y ago

Also, the Doppler shift of jamming/spoofing will be all wrong, unless they’re specifically targeting your vehicle and accounting for its speed and direction in their attack. … if you can get the precision to filter that out…