[Goose]

Conversely, an antenna on a plane at 40,000 feet could impact GPS (jam or spoof) surface (and air) regions up to 400km.
And, of course, we're talking about the public GPS. 

This is indeed true. 

Against a device that can understand the military GPS (do we supply such devices to Ukraine?), the spoofing presumably won't work, but jamming still could.

https://en.wikipedia.org/wiki/Selective_...ing_module
The "old" selective availability is no longer physically present in the GPS satellites. We agreed not to ever use it, so it was decided there was no reason to support it any more. I doubt that the GPS supplied with the JDAMs to Ukraine (or the ones we use) are "M code" equipped. I could be wrong. Jamming could still work, depending on the exact "type" of jamming and many other factors.

 With a directional antenna array, a device can do better against jamming.

Indeed. It can render jamming ineffective. However, it requires a multi-channel receiver and multiple physical antennas. That does cost money and occupies space. On things like a JDAM, space and antenna location is a big problem.

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Related to this topic, GLSDBs which for sometime have not been very effective due to Russian jamming efforts and were not being used, seem to be back now.

It's been reported that their guidance systems have been reworked.

https://x.com/John_A_Ridge/status/1792238328214949895

There were several problems. One was with the launcher. This problem was the one that literally forced Ukraine to quit using them. That hopefully has been fixed. Reworking the software in the guidance module to resist jamming more effectively seems feasible. If it works well, that will be a "way ahead". We shall see.

[Goose]

Conversely, an antenna on a plane at 40,000 feet could impact GPS (jam or spoof) surface (and air) regions up to 400km.
And, of course, we're talking about the public GPS. 

This is indeed true. 

Against a device that can understand the military GPS (do we supply such devices to Ukraine?), the spoofing presumably won't work, but jamming still could.

https://en.wikipedia.org/wiki/Selective_...ing_module
The "old" selective availability is no longer physically present in the GPS satellites. We agreed not to ever use it, so it was decided there was no reason to support it any more. I doubt that the GPS supplied with the JDAMs to Ukraine (or the ones we use) are "M code" equipped. I could be wrong. Jamming could still work, depending on the exact "type" of jamming and many other factors.

 With a directional antenna array, a device can do better against jamming.

Indeed. It can render jamming ineffective. However, it requires a multi-channel receiver and multiple physical antennas. That does cost money and occupies space. On things like a JDAM, space and antenna location is a big problem.

---

Related to this topic, GLSDBs which for sometime have not been very effective due to Russian jamming efforts and were not being used, seem to be back now.

It's been reported that their guidance systems have been reworked.

https://x.com/John_A_Ridge/status/1792238328214949895

There were several problems. One was with the launcher. This problem was the one that literally forced Ukraine to quit using them. That hopefully has been fixed. Reworking the software in the guidance module to resist jamming more effectively seems feasible. If it works well, that will be a "way ahead". We shall see.