downlink

A downlink is a radio signal transmitted from a satellite orbiting earth to one or more receiving stations on the ground. The satellite acts as a transponder, receiving an uplink signal from earth, amplifying it, shifting its frequency, and retransmitting it back down. The receiving station uses a parabolic antenna to capture this weak signal, which has been attenuated by free-space path loss over distances typically ranging from 400 km for low-earth orbit to 36,000 km for geostationary orbit.

Downlink frequency bands are assigned by international agreement and vary by application. Communications satellites typically use C-band (4 to 8 GHz), Ku-band (11 to 18 GHz), or Ka-band (27 to 40 GHz), with higher bands offering greater bandwidth but suffering more atmospheric absorption, especially in rain. Weather satellites use X-band and S-band. GPS satellites broadcast L-band signals. Each frequency window requires different antenna designs and receiver sensitivities.

The strength of a downlink signal at the receiver depends on the satellite's transmitted power, antenna gain on both satellite and ground, path loss, and atmospheric conditions. A typical geostationary communications satellite might transmit at 40 to 100 watts per carrier, achieving received power levels of negative 100 decibels or weaker. Ground receivers must use low-noise amplifiers (LNAs) with noise figures below 1 dB to achieve usable signal-to-noise ratios, and must account for rain fade in tropical regions.

Downlink versus uplink

The uplink carries user data or commands from earth to the satellite; the downlink returns processed or relayed information to earth. Modern satellite systems are asymmetrical: downlink bandwidth may be much larger than uplink, especially for broadcast or data distribution applications. In bent-pipe (transparent) satellites, downlink frequency is simply shifted from uplink. In regenerative satellites, the signal is demodulated, decoded, and re-encoded before transmission, allowing format conversion and more complex routing.

Downlink quality degrades from Doppler shift, multipath fading near the ground station, and interference from other satellites sharing the same frequency band. Geostationary satellites suffer minimal Doppler, while polar-orbiting satellites exhibit Doppler shifts of several kilohertz over a pass. Ground stations must track satellites and adjust receiver frequency accordingly, and must maintain sufficient antenna directivity to reject off-axis interference.