Radio is the only option
Nothing physical moves between a spacecraft and Earth except electromagnetic waves, so every command, every image and every byte of telemetry rides on radio (or, increasingly, laser). Which frequency you use is the first decision in any communications design, and it is rarely free: bands are allocated internationally by the ITU and licensed nationally (the FCC in the US), so you get what you can coordinate, not what you want.
The bands you will actually meet
| Band | Range (approx.) | Typical use in space | Character |
|---|---|---|---|
| UHF | 300 MHz – 1 GHz | CubeSat telemetry, amateur, some military | Cheap omni antennas; very low data rates; crowded |
| S | 2 – 4 GHz | TT&C (tracking, telemetry & command) for most satellites; NASA Near Space Network | Robust, forgiving of pointing, modest rates |
| X | 8 – 12 GHz | Earth-observation downlinks, deep space, government | Good rates, mature ground networks, little rain fade |
| Ku | 12 – 18 GHz | Broadcast TV, VSAT, Starlink user links | High rates; some rain fade |
| Ka | 26 – 40 GHz | High-throughput GEO, Starlink gateways, EO downlinks (hundreds of Mbps–Gbps) | Very high rates; serious rain fade; tight pointing |
| Optical | ~200 THz (1550 nm) | Inter-satellite links, NASA LCRD/DSOC, commercial terminals | Gbps+, no spectrum licensing; clouds kill ground links |
Three things change as frequency goes up
- Bandwidth grows. A 1 % fractional bandwidth is 20 MHz at S-band but 300 MHz at Ka. More bandwidth is more bits per second — that is the whole reason to climb.
- Antennas focus better. For a fixed dish size, gain rises with the square of frequency (next lesson). A 1 m dish at Ka has about 16 dB more gain than at S-band.
- The atmosphere fights back. Above ~10 GHz, rain, clouds and water vapour absorb signal. Ka-band links routinely carry 5–10 dB of rain margin or accept outages; X-band barely notices weather.
The trade in one sentence
Low bands are robust and easy to point but slow; high bands are fast but demand precise pointing, bigger link margins, and more careful licensing. Most missions use two: a low-band TT&C link that always works, and a high-band payload downlink for the data that pays the bills.
Where the rules come from
The ITU Radio Regulations divide spectrum by service (Fixed-Satellite, Mobile-Satellite, Earth Exploration-Satellite, Space Research…) and by region. An operator files through its national administration, coordinates with anyone already using the band, and then licences the ground stations too. Skipping this step is how you get a cease-and-desist instead of a downlink — the Space Law track covers the process in detail.