I always wonder how much (relevant) sigcom satellites can nowadays collect given the ever-growing increase in encrypted traffic, especially from the most interesting targets like foreign militaries.
And of course, as always, the easiest way to break a code is always the 5 dollar wrench attack (something like the way the Soviets paid the Walker spy ring a few thousand dollars a month to just give them the decryption keys (2)).
Finally, there is Radio MASINT that also gathers information from radio receivers, even if they are encrypted (3).
1: https://en.wikipedia.org/wiki/Operation_Ivy_Bells 2: https://en.wikipedia.org/wiki/John_Anthony_Walker 3: https://www.dni.gov/files/ODNI/documents/21-113_MASINT_Prime...
And when you are trying to actually collect communications, sometimes you can get a lot just from the metadata. Even if you don't know the content of the communications, you can know that at time T, emitter A sent a message on frequency F using protocol P, lasting for duration D and transmitting B bytes. Depending on the communication mechanism, you could also determine the intended destination as well. Even if that data is perfectly encrypted, there's a lot you can do with just traffic analysis, especially when you look at patterns over time and try to correlate this with known events.
Imagine if the US tested some classified missile over the ocean, and some unknown ship nearby broadcast something, and minutes later a bunch of encrypted transmissions are detected at various places in China, places that don't usually send communications at that particular time or that particular volume. That would be a clue that those locations are related to Chinese military intelligence. Comparing exact timestamps, and looking at what happened during similar tests in the past, you can narrow that down further.
And that's just in peacetime. In an active war, this information becomes far more useful. Anything transmitting a lot of data near the frontlines is probably something worth targetting. Maybe just an information relay point, or maybe the headquarters of an armored division. You can also look at the volume of traffic and try to discern intentions. A big spike in the amount of communications in a particular region, followed by near radio silence? That's probably the start of an offensive.