Ground ↔ Satellite
A ground station sees a LEO satellite only when it is above the local horizon. The elevation angle — the angle between the horizon and the satellite as seen from the station — determines link quality: higher elevation means shorter path through the atmosphere and better signal. A minimum elevation of 5–10° is typical; below that, atmospheric attenuation and multipath make the link unreliable.
Contact Windows
A single pass lasts 5–15 minutes, depending on the satellite's altitude and the pass geometry (a pass directly overhead lasts longer than one near the horizon). At 550 km altitude, a satellite completes one orbit in ~96 minutes, so a ground station sees it roughly once every 1.5 hours — but not every orbit, because Earth rotates and the satellite's ground track shifts westward with each pass.
High-latitude ground stations (Kiruna, Svalbard, Fairbanks) see more passes per day for near-polar orbits because the orbital planes converge near the poles.
Ground Link Throughput
The ground-to-satellite link is the bottleneck in the system. Unlike ISLs which operate in vacuum, the ground link must traverse Earth's atmosphere, which introduces losses that ISLs avoid:
- Atmospheric attenuation — the atmosphere absorbs and scatters the signal. The effect depends on frequency, weather, and the path length through the atmosphere (longer at low elevation angles).
- Rain fade — at higher RF frequencies (Ka-band and above), rain can severely attenuate the signal. Heavy rain can reduce throughput by an order of magnitude or cause link outages.
- Spectrum regulation — RF downlink frequencies are shared and regulated. Satellites cannot transmit at arbitrary power or bandwidth. Available spectrum is limited.
Typical ground link data rates for Earth observation missions:
- X-band (8 GHz) — 150–800 Mbps. The traditional workhorse for Earth observation downlink. Moderate bandwidth, relatively tolerant of weather.
- Ka-band (26 GHz) — 1–4 Gbps. Higher throughput but more susceptible to rain fade. Used by newer missions needing higher data rates.
- Optical ground link — 1–10+ Gbps. Highest throughput but requires clear skies — clouds block the laser entirely. Only viable at sites with high clear-sky availability.
For comparison, a single ISL achieves 10–100+ Gbps in vacuum with no weather dependence. The ground link is typically 10–100× slower than the ISL mesh. This asymmetry is the core of the downlink wall: the constellation can move data internally much faster than it can get data to the ground.
Cross-Orbit Relay
LEO satellites can communicate with relay satellites in GEO to maintain near-continuous ground connectivity, bypassing the contact window limitation.
The relay path has two segments that use different technologies:
- LEO → GEO (inter-orbit uplink) — optical laser. EDRS uses a 1064 nm laser link at 1.8 Gbps between the LEO satellite and the GEO relay. The link operates in vacuum, so it has the same characteristics as an ISL — no atmospheric effects, but requires precise pointing between orbits 35,000 km apart.
- GEO → Ground (relay downlink) — Ka-band RF (~26 GHz). Even systems that use laser for the inter-orbit leg switch to RF for the final hop to ground, because laser links through the atmosphere are blocked by clouds. Ka-band is reliable enough for continuous service.
| System | Agency | LEO ↔ GEO link | GEO ↔ Ground link | Data rate | Status |
|---|---|---|---|---|---|
| TDRSS | NASA | S-band and Ka-band RF | S-band and Ka-band RF | ~300 Mbps | Operational since 1983, 9 satellites |
| EDRS | ESA / Airbus | Optical laser (1064 nm) | Ka-band RF | 1.8 Gbps | Operational since 2016, 40 TB/day capacity |
| LCRD | NASA | Optical laser | Optical laser | 1.2 Gbps | Demonstration since 2021, testing optical ground links |
A GEO relay adds ~250 ms round-trip latency (LEO → GEO → ground → GEO → LEO) but eliminates the ground contact gap. This creates two communication planes:
- Control plane — through GEO relay: commands, status, workflow uploads. Low bandwidth, near-continuous.
- Data plane — through ISL mesh and direct ground passes: sensor data, file transfers, results. High bandwidth, intermittent.
The downlink wall — the fundamental mismatch between data generation rate and downlink capacity — is not solved by a GEO relay. Even EDRS at 1.8 Gbps cannot match the 1–2 TB/day that an Earth observation satellite generates. Bulk data still needs onboard processing and selective downlink.