Space as part of the observing grid

01Sensing, connectivity and time from orbit

For a long time, space was a separate world in the design of an observing network. Satellite images arrived from one direction, ground measurements from another, and the two met only in an analyst's workstation.

That separation no longer fits. A modern observing grid uses satellites in three ways at once: to sense the planet, to carry data from places no cable reaches, and to give every site a common clock.

Sensing is the oldest role. Radar satellites such as Sentinel-1 image the surface day and night, regardless of weather, with very short revisit times².

Connectivity is the fastest-growing role. Non-terrestrial networks use satellites or high-altitude platforms to carry a relay node or a base station, so that devices on the ground can join a mobile network from orbit¹.

Timing is the quietest role and one of the most important. The Global Positioning System provides positioning, navigation and timing services that networks and instruments around the world depend on³.

Each role brings its own constraints: orbit height, delay, spectrum, ground stations and the long-term health of the orbits themselves. The rest of this piece takes them in turn.

02Orbits and what they mean on the ground

Height above the Earth decides most of what a satellite can do for a site. Mobile standards describe low Earth orbits at typically 500 to 2,000 kilometres, medium orbits at 8,000 to 20,000 kilometres, and geostationary orbit at 35,786 kilometres¹.

A low orbit gives a short delay and a strong signal, but each satellite crosses the sky in minutes, so a constellation is needed for continuous service¹.

A geostationary satellite stays over one point and covers a vast area, at the cost of a long path. Terrestrial networks usually see delays below one millisecond, while geostationary links can reach a few hundred¹.

The simulator shows the trade for a single satellite. Set the altitude and the lowest elevation at which a terminal can still see it, and it computes the footprint, the slant range, the delays and the orbital period.

The elevation mask matters as much as the height. A site in a valley or among buildings may only see satellites high in the sky, which shrinks every footprint and lengthens the gaps between passes.

For an observing site, the right orbit depends on the job. Imagery favours the repeat geometry of a sun-synchronous orbit, while a backup link favours a low constellation with many satellites in view.

03Satellites inside the mobile standard

Mobile standards began to study satellite access for the new radio in a technical report on non-terrestrial networks, which summarised use cases, adapted channel models and described deployment scenarios⁴¹.

A follow-on report set out solutions for supporting non-terrestrial networks in the new radio⁵.

Release 17, with its protocol definitions frozen in June 2022, brought normative requirements for non-terrestrial networks into the mobile specifications¹.

Two kinds of payload are distinguished. A transparent, or bent-pipe, payload filters, shifts and amplifies the signal like a repeater, while a regenerative payload carries base station functions on board¹.

Low and medium orbit satellites move faster than the Earth turns, so the feeder link must eventually change to another gateway, and a terminal must be handed to another satellite to keep its service¹.

For an observing site, this means one device can use a ground network where one exists and a satellite where it does not, under the same standard and the same security.

04Ground segments join space to sites

Every satellite service ends on the ground. Imaging satellites downlink to ground stations, broadband constellations land traffic at gateways, and each observing site needs its own terminal or receiver.

Open standards govern how data crosses that boundary. Synchronisation and channel coding schemes are specified for space-to-ground, space-to-space and ground-to-space links, so that data arrives intact⁶.

At the site, a flat-panel terminal for a low orbit constellation turns satellite connectivity into an ordinary network link. It can serve as the main path at a remote site or as a backup where fibre or radio already exist.

The choice changes how the site behaves in a crisis. A site with only one terrestrial path goes silent when that path fails, while a site with a satellite backup can keep sending its most important data.

Terminals still need careful siting. They need a clear view of the sky above their elevation mask, power through outages and protection from snow, ice and wind.

Ground stations and gateways are sites in their own right. They need power, links, security and maintenance like any observing site, and their availability sets how quickly data from orbit reaches the people who need it.

05Sharing the radio spectrum

Satellites, ground networks, radars and radio telescopes all depend on the same radio spectrum. Sharing it without harmful interference is a matter of international agreement, not goodwill alone.

The Radio Regulations contain the texts adopted by World Radiocommunication Conferences, and the edition of 2024 includes the revisions made at the conference held in Dubai in 2023⁷.

For an observing site, spectrum shapes the design long before installation. The bands a terminal uses, the power it may radiate and the neighbours it must protect are all set by these agreements and by national rules.

Radio astronomy shows why care matters. A large dish listens for signals far weaker than any communication link, so terminals and transmitters near such sites must be planned with special attention.

The same discipline protects the observing site itself. Its own receivers, from satellite timing antennas to weather radars, depend on clean spectrum to do their work.

06Keeping orbits usable

Orbits are a finite resource. The more an observing grid relies on satellites, the more it depends on space remaining safe to use for decades to come.

About 40,000 objects are now tracked in orbit, of which about 11,000 are active payloads, and the number of debris objects larger than one centimetre is estimated at over 1.2 million⁸.

Intact satellites or rocket bodies are now re-entering the atmosphere on average more than three times a day, yet not enough satellites leave congested orbits at the end of their lives⁸.

International guidelines ask that spacecraft and launch stages that pass through low Earth orbit be removed in a controlled way after their mission, or placed in orbits that avoid long-term presence there⁹.

In June 2019 the space committee of the United Nations adopted guidelines for the long-term sustainability of outer space activities, covering policy, safety of operations, cooperation and research¹⁰.

Some regulators have tightened the rules. One national licensing authority shortened its benchmark for disposing of low orbit satellites after their mission from 25 years to five¹¹.

Users of satellite services share this responsibility. Choosing operators that plan for disposal and publish their practices is part of designing an observing grid that will still work in the future.

07Designing a site with space in mind

Start by listing what the site needs from space: imagery of its surroundings, a backup or primary link, timing for its clock, or all three. Each need brings its own requirements for antennas, power and view of the sky.

Plan the sky as carefully as the ground. Check where satellites will appear above the elevation mask, where terminals and timing antennas can stand, and what nearby structures or trees will block.

Treat satellite links as part of the network design, with their delay, capacity and outages known in advance. Decide which data uses them and which waits for a terrestrial path.

Choose services with their whole life in view. Operators that follow debris mitigation practice and plan for disposal help keep the orbits the site depends on usable.

Nexus Observatory Inc. designs observing sites that use space for sensing, connectivity and timing as part of one grid, with each path's quality recorded beside the data it carries.