3 ms·

If line of sight to the jamming antenna is required to be jammed, why do aircraft not have a downwards shield so that they only receive GPS signal from the sky

If line of sight to the jamming antenna is required to be jammed, why do aircraft not have a downwards shield so that they only receive GPS signal from the sky (satellites) and not from jammers (coming from the bottom hemisphere)? Or is the jamming signal so many orders of magnitudes stronger than the satellites that there's always going to be some gain no matter how good the shield is? Ok it exists, but shielding is (only) about 20dB looking downwards, which may not be enough: https://safran-navigation-timing.com/product/8230aj-gps-gnss-anti-jam-outdoor-antenna/ https://safran-navigation-timing.com/product/8230aj-gps-gnss...

0cf8612b2e1e 3y ago
I thought GPS signals from space were incredibly weak. Limited power budget + 100km in the sky. Seems trivial for a ground based system to crank up the watts to whatever arbitrary limit they desire.
- croemer 3y ago
  True, signal per satellite is only around 150-160dBW on earth despite them radiating at 25W. Satellites are ~20000km away. If a jammer is 100 times closer (200km), they need to use only 1/10,000 (1/100^2) the power, so it's very easy to jam sadly.
morcheeba 3y ago
Two issues to consider: - GPS positioning is more accurate if the satellites it sees come from a variety of angles (GDOP), so the satellites near the horizon are valuable. - Aircraft pitch and roll, so a fixed antenna like this would lose precision as it turns to make an approach - just about the worst possible time. It's difficult to make an antenna with a sharp cutoff to limit the ground vs. above-ground. So, most anti-jammers will use beamforming to cancel out interference in one or more specific directions. So, the null in the antenna moves to follow the interference. GDOP: https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation) https://en.wikipedia.org/wiki/Dilution_of_precision_(navigat...

0cf8612b2e1e 3y ago

I thought GPS signals from space were incredibly weak. Limited power budget + 100km in the sky. Seems trivial for a ground based system to crank up the watts to whatever arbitrary limit they desire.

croemer 3y ago
True, signal per satellite is only around 150-160dBW on earth despite them radiating at 25W. Satellites are ~20000km away. If a jammer is 100 times closer (200km), they need to use only 1/10,000 (1/100^2) the power, so it's very easy to jam sadly.

morcheeba 3y ago

Two issues to consider: - GPS positioning is more accurate if the satellites it sees come from a variety of angles (GDOP), so the satellites near the horizon are valuable. - Aircraft pitch and roll, so a fixed antenna like this would lose precision as it turns to make an approach - just about the worst possible time. It's difficult to make an antenna with a sharp cutoff to limit the ground vs. above-ground. So, most anti-jammers will use beamforming to cancel out interference in one or more specific directions. So, the null in the antenna moves to follow the interference. GDOP: https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation) https://en.wikipedia.org/wiki/Dilution_of_precision_(navigat...