[sennight]

This has been a solved problem for a very long time. The nice thing about GPS satellites is that you know where they should be, and where they shouldn't, so long as your RTC is properly set and you've got a compass.

A GPS Digital Phased Array Antenna and Receiver (2000): https://apps.dtic.mil/sti/tr/pdf/ADA452307.pdf

[KaiserPro]

yes, phased arrays, especially if you have a tight beam, can give you an extra 40db of SNR (see figure 9 https://www.analog.com/en/analog-dialogue/articles/phased-ar..., also in the charts on your link)

but thats still like -70dbm at best. That's about the level of being a few meters away from a wifi AP level of power.

a 1 watt(RF) jammer is going to cast a large cone of non-GPS even with a phased array antenna.

[sennight]

You are forgetting about the phased nulling. Military GPS has been including jammer detection in the firmware for a long time. I remember my handheld unit constantly annoying me anytime the IED jammer (which was waaaay more than 1 watt of rf) went active... 2005ish - it would simply mask the signal coming from that direction and continue to provide positioning data.

[londons_explore]

Add in coding gain over 1 second (a code 10 million chips long), and that gives you an extra 70dB. Now it's getting hard to jam.

Downside: This only works if your movement over the 1 second period is predictable, to a small part of a wavelength. If you're moving unpredictably, you'll have to give up a few db's of coding gain.