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A ground station for Earth observation: a reference design

An Earth observation ground station has minutes to catch each satellite pass and the time until the next one to turn it into products. This reference design shows how the antenna, radio, clock and edge work together, and what happens when a link fails1.

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By Nexus Observatory desk

A ground station on a flat pad: a large tracking antenna in the centre, an edge compute shelter beside it, a lattice tower for terrestrial links and a field truck.

Figure 1 The reference station: antenna, edge shelter, tower and service vehicle

Key points

  1. A ground station observes satellites, not weather: its job is to catch each pass cleanly and hand on what it received.
  2. International space data standards define how the radio link and the data on it are coded, so stations and missions from different agencies can work together.
  3. Each pass is turned into catalogued, analysis-ready products at the station before the next pass arrives.
  4. When the link to users fails, the station keeps receiving, stores what it made and delivers it later in order.

What a ground station needs to do

An Earth observation ground station watches the sky rather than the land. Its job is to point at each satellite as it rises, receive everything it sends during the pass, send commands up when asked, and hand the result on cleanly.

Large agency networks show the shape of the work. The European tracking network runs its stations from a central operations centre, acquiring satellites on a precise pointing schedule and handing passes from one mission to the next1.

The same network notes that its stations also provide frequency and timing control using atomic clocks1. Every received frame is stamped with the station's time, and that time is part of the record.

Radio is the medium. Downlinks from low-orbit Earth observation satellites share spectrum with other services, and international recommendations set sharing criteria for them and design guidelines for the band from 8 025 to 8 400 MHz23.

Siting the antenna

The antenna needs an open horizon. Every degree of hills, trees or buildings above the horizon shortens each pass, because the satellite cannot be received until it clears them. The site survey records the horizon all the way round, and that mask is stored with the station.

It also needs a quiet radio neighbourhood. A downlink from orbit arrives weak, so the station is placed away from strong local transmitters, and its own tower is kept out of the main antenna's view of the sky.

A closer view of the same pad, showing the antenna with open sky around it, the edge shelter beside it and the tower set back from the antenna.

Figure 2 An open horizon for the antenna, a short path to the edge

The edge shelter sits close to the antenna, so the received signal and data travel a short, protected path. The pad is fenced, the antenna has room to sweep in every direction, and there is access for the service vehicle in any weather.

Power and links are planned as carefully as the sky view. The antenna, receiver and edge run from a supply that rides through short outages, and the terrestrial link leaves the pad by a route that does not cross the antenna's view.

From pass to product

The antenna follows the satellite across the sky and passes the signal to the receiver. How the carrier is modulated is described by international recommendations for radio frequency and modulation systems at Earth stations and spacecraft4.

The receiver then finds the frames in the bit stream and corrects errors. The synchronization and channel coding used for this are specified in a recommended standard for space links, written so that one agency's station can support another agency's mission5.

A systems diagram in four tiers: the satellite in space, the antenna, receiver and clock below it, the station edge with processing and a local archive, and mission control and product users at the top.

Figure 3 From pass to product: the tiers of the reference station

At the edge, the frames become products. Analysis-ready data is satellite data processed to a minimum set of requirements and organised so that it can be analysed immediately and combined with other datasets6.

Each product is then described in a common catalogue language for geospatial data, so users can search by place, time and sensor and find what the station made7. Mission control receives housekeeping data straight from the edge.

Every product carries its own history: the pass it came from, the station time at which each frame arrived, the frames that failed decoding, the processing steps applied and the versions of the software that applied them. A user can then trace any pixel back to the pass that produced it.

Pass geometry is the constraint

A low-orbit satellite is in view for minutes at a time. How long depends on its altitude, on how high it rises above the station and on the elevation mask, the lowest angle at which the antenna can usefully receive.

Raising the mask clears obstacles and noise near the horizon, but it shrinks the footprint and shortens every pass. Lowering it gains time but asks more of the site. The simulator shows the footprint, the slant range and the delay for a given altitude and mask.

The edge is sized against the same geometry. A pass delivers a burst of data, and the station must finish processing and cataloguing it before the next pass begins, or the backlog grows with every orbit.

An interactive orbit calculator. Set the satellite altitude and the elevation mask; it shows the footprint radius, the slant range at the mask, the one-way delay and the orbital period.

Figure 4 Footprint, slant range and period by altitude and mask

Questions and answers

Is this a station that exists?

No. It is a reference design that shows how the parts fit together. A real station is designed from its own site survey, frequency coordination and mission requirements.

Why process at the station instead of sending raw data on?

Processing close to the antenna means users receive catalogued products rather than raw frames, and the station can keep working when its onward link is slow or lost.

Glossary

Pass
The period during which a satellite is above a station's elevation mask and can be received.
Elevation mask
The lowest angle above the horizon at which a station tracks and receives a satellite.
Analysis-ready data
Satellite data processed and organised so that it can be analysed immediately and combined with other datasets.

References

  1. Government or regulator: Estrack: ESA's global ground station network. European Space Agency. esa.int (accessed )
  2. Standard or official statistic: Recommendation ITU-R SA.1027: Sharing criteria for space-to-Earth data transmission systems in the Earth exploration-satellite and meteorological-satellite services using satellites in low-Earth orbit. International Telecommunication Union. itu.int (accessed )
  3. Standard or official statistic: Recommendation ITU-R SA.1810: System design guidelines for Earth exploration-satellites operating in the band 8 025-8 400 MHz. International Telecommunication Union. itu.int (accessed )
  4. Standard or official statistic: CCSDS 401.0-B-32: Radio Frequency and Modulation Systems, Part 1: Earth Stations and Spacecraft. Consultative Committee for Space Data Systems. . public.ccsds.org (accessed )
  5. Standard or official statistic: CCSDS 131.0-B-5: TM Synchronization and Channel Coding. Consultative Committee for Space Data Systems. . public.ccsds.org (accessed )
  6. Peer-reviewed or intergovernmental: CEOS Analysis Ready Data. Committee on Earth Observation Satellites. ceos.org (accessed )
  7. Consortium or technical document: STAC: SpatioTemporal Asset Catalogs. STAC community. stacspec.org (accessed )
  8. Standard or official statistic: CCSDS 727.0-B-5: CCSDS File Delivery Protocol (CFDP). Consultative Committee for Space Data Systems. . public.ccsds.org (accessed )

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