Some ranges are set aside for GPS/GNSS: https://gssc.esa.int/navipedia/index.php?title=GNSS_signal

Some are monitored by the SMAP satellite: https://en.wikipedia.org/wiki/Soil_Moisture_Active_Passive

(Also in the L-band are all sorts of other things. A lot of cellular bands, known in the US as PCS and AWS, and more. Several different sat-phone systems. ADSB in 1090 is juuuust inside L-band. And more...)

All transmitters produce a bit of out-of-band interference, beyond where they meant to transmit. This is filtered to reduce it to a certain level below the intentional frequency, but filters aren't perfect. So when someone tries to jam GPS or other services, they inevitably bleed some energy into neighboring allocations too, some of which seems to be being picked up by SMAP.

Note that SMAP's passive radiometer doesn't have pinpoint spatial resolution, 36km is stated. This means it's listening to a pretty significant patch of ground at any given time, so for a source to be picked up among all that, it's got to be pretty loud. It also means that attributing the source is limited in precision, you can get to city level but not city-block level from this data.

There’s a reason that microwaves (the cooking instrument, named after the rough band of RF frequencies it uses) interfere with WiFi when improperly shielded: they run at approximately 2.4GHz. That’s a part of the band that’s free to use without licensing (ISM band, runs at 900MHz, 2.4GHz, 5Ghz, and so on). That’s because microwave manufacturers don’t want to license spectrum, and it’s the part of the ISM band closest to the ideal absorption frequency of water. The point of microwaves is to push energy into water molecules and make them move around faster, aka heat them. Water molecules do that well at 2.4GHz. This also means that water blocks RF at that frequency very well, because it absorbs it. Trying to shoot 2.4GHz WiFi through trees sucks because it gets blocked by the leaves.

So you want to control drones. You can’t be around 2.4GHz because rain would screw you up. You can’t go above that so you can get enough distance. You can’t go as low as 900MHz or other stuff on the ISM band might interfere with you.

Not that much left, given that 1.7GHz is a popular cell phone frequency, and so on - you can look at publicly available frequency charts to see what’s assigned for what where.

This glosses over a lot and is heavily simplified.

https://web.archive.org/web/20230517192717/https://aquarius....

for occupying armed forces.

Normally ground transmitters in this band are using just a couple watts or less, so they don’t significantly impact the readings of a satellite looking at a large area on the earth, but a jammer uses a lot more power and can be noticed.

https://www.youtube.com/watch?v=hlbjUvkoyBA

At a very high level handheld satellite communications devices work just like regular cellphones, except they also function in the middle of the ocean.