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by octoberfranklin 1557 days ago
In a major departure from previous GPS designs, the M-code is intended to be broadcast from a high-gain directional antenna, in addition to a wide angle (full Earth) antenna. The directional antenna's signal, termed a spot beam, is intended to be aimed at a specific region (i.e., several hundred kilometers in diameter) and increase the local signal strength by 20 dB (10× voltage field strength, 100× power). A side effect of having two antennas is that, for receivers inside the spot beam, the GPS satellite will appear as two GPS signals occupying the same position.

https://en.m.wikipedia.org/wiki/GPS_Block_III#Military_(M-co...
1 comments
T-A 1557 days ago
That's an ability to improve service for users of M-code receivers, which AFAIK are only available to the military (and it's new technology, so I'd expect it to be unsupported even by most military receivers currently in use).
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octoberfranklin 1556 days ago
The M-code is transmitted in the same L1 and L2 frequencies already in use

Nothing says that they have to transmit M-codes from the spot beam antenna.

In fact, since the M-codes are hardly used, it's pretty certain they're using that antenna for something else. How very convenient that those antennae are designed to transmit on the same frequency as the "ordinary" antenna.

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T-A 1551 days ago
Today I took the time to have a closer look. Turns out, the directional antenna for M-code is only present on Block III GPS satellites, and only four of those are currently in service:

https://en.wikipedia.org/wiki/GPS_Block_III

So this is definitely not the correct explanation.

Besides, even if there were enough Block III satellites in orbit to provide constant cover of the affected area, it would still make little sense to tie up their most advanced capabilities to jam their own legacy signal, when you could just turn off the legacy signal when transiting the target area and provide your own military with continued service using the directional antenna.

And of course, Russia has its own satellite navigation system

https://en.wikipedia.org/wiki/GLONASS

so you'd also have to explain what the US and NATO could hope to gain from turning off GPS.

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T-A 1552 days ago
> The M-code is transmitted in the same L1 and L2 frequencies already in use

Let's quote the whole thing, shall we?

The M-code is transmitted in the same L1 and L2 frequencies already in use by the previous military code, the P(Y) code. The new signal is shaped to place most of its energy at the edges (away from the existing P(Y) and C/A carriers).

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