Insight | Space explained: how do we fly our satellites?


Space explained: how do we fly our satellites?


Our geostationary satellites provide connectivity for hundreds of thousands of users across the world each day, while travelling in the vacuum of space at over 11,000kph. But they are not alone. 

Our space engineers look after them around the clock from our Satellite Control Centre in London. But what exactly does piloting a spacecraft involve? Alessandra Rossetti, Senior Operations Engineer, explains…

Experience and expertise

We have dozens of experienced teams, each responsible for different operational roles. All contribute to two overarching objectives: keeping our satellites operational and healthy, and avoiding potential collisions.

Some of the roles we employ include systems engineers, software engineers, software developers, IT infrastructure specialists, flight dynamicists, operations engineers, ground system engineers, spacecraft analysts and spacecraft controllers. Our aim is to provide world-leading communications services with better than 99.99% availability for our customers. We are proud to have kept that promise.

Using telemetry signals from our satellites – which work like a continuous heartbeat – Inmarsat teams routinely receive vast quantities of data from our satellites. This includes information about on-board temperatures, positioning, battery power, solar panel production, speed and communications performance.

With this data to hand, our teams can monitor and assess the status of each on board subsystem, send signals back to control each satellite and make electronic, software, or positioning adjustments as required using onboard computers. We operate antennas all over the world and can send and receive signals between our satellites and our Satellite Control Centre within about half a second.

Satellites can move due to gravitational or magnetic forces caused by Earth, the Sun and Moon, or solar winds – which are streams of particles pushed out from the Sun. As a result, satellites tend to naturally drift away from their orbital slots over time. Onboard propulsion systems are commanded from the ground to adjust their position and maintain pointing accuracy. This is more like a series of small, gentle ‘pushes’ every now and again, rather than a dramatic battle against explosive elements. 

Reading and reacting

Like many industries, over the years we have innovated continuously and automated our control systems. Our satellites are not actively ‘piloted’ by a person, like driving a car.

Instead, we rely on highly advanced ground control software - developed by L3 Harris Technologies and Inmarsat Satellite Operations Support Group - to keep our satellites in the correct position and make any adjustments in performance.

While automation is vital, we do operate a 24/7 rota of engineers to continuously monitor our satellites. Our team can then react to any unexpected events and call-in additional technical support if required.

Each new generation of satellite has brought radically advanced onboard software and hardware. I-6 F1, for example, weighs 5.5 tonnes and has a solar panel array measuring 47 metres across. It also features an electric propulsion system which is more efficient. Its dual ELERA and Global Xpress communications payloads will also offer wholly enhanced customer capabilities on Earth.

Avoiding collisions

Our satellites are far away from any asteroid belts; however, we do need to monitor for collisions with space debris. Based on statistical models produced by the European Space Agency’s (ESA) space debris office, there are an extraordinary 130 million objects between 1mm to 1cm orbiting the Earth. While most space debris is in low Earth orbit – much closer to the Earth than our geostationary satellites – our automated software and on-call engineers regularly monitor and react to any potential impacts.

For now, we need to take evasive manoeuvres relatively rarely. However, we believe that current unmanaged expansion and unsustainable practices in space raise deep concerns for the future. As on Earth, we must treat space sustainably and protect the extra-terrestrial environment for future generations. Inmarsat has published a list of recommendations to meet this challenge through international cooperation in our Space Sustainability Report.

Space Sustainability Report

An extra-terrestrial car park

Our fleet of 15 satellites are all ‘parked’ in specific locations approximately 36,000km above the Earth’s equator in geostationary orbit. Each generation of satellite has brought vast new capabilities to revolutionise the services we can provide on Earth. We are currently on our sixth generation of satellites. We recently launched two new Inmarsat-6 (I-6) satellites - I-6 F1 and I-6 F2 - in December 2021 and February 2023 respectively. They are the most sophisticated commercial communications satellites ever launched.

Once our satellites reach geostationary orbit – a journey which can take seven months from launch for those with electric propulsion orbit raising systems – they complete rigorous in-orbit technical testing. They are then moved to a specific slot orbiting the Earth’s equator. Think of this like a traditional car park slot outside a local supermarket.

Our satellites are highly tuned and custom-built to work within their specific slot. Although they are in reality travelling around the Earth at exceptional speeds, they stay fixed above the same location on Earth from the perspective of the person on the ground. This means our customers can easily fix onto the satellite to receive our connectivity services.

Being in space, these slots are slightly bigger than the average car park: essentially a box that can be between about 150 km to 75 km wide for L-band and Ka-band respectively. This is still a proportionately miniscule position given their distance from Earth and the vastness of space. This means we need highly trained experts to continuously watch over our technology to make sure our satellites remain in there designated slots.

Satellite slots are managed and approved by the International Telecommunications Union (ITU). The ITU is also responsible for coordinating which companies use which frequencies, or ‘bands’, to make sure the global telecoms industry works efficiently.

About the author

Alessandra Rossetti is Senior Operations Engineer in the Inmarsat Satellite Control Centre (SCC), which is a role that sees her planning, monitoring, and troubleshooting operations for Inmarsat’s fleet of satellites in real time. She is also heavily involved in thinking through the procedures and tools required for the spacecrafts’ intended activities, so that these can be catered for by the manufacturer.