Communicating with MethaneSAT

Chris Jackson, an engineer at the University of Auckland’s Te Pūnaha Ātea – Space Institute, explains how data is transferred from MethaneSAT to ground stations.

Jargon alert

• Sun-synchronous orbit: an orbit that matches the rate at which the Earth goes around the Sun.

• Polar orbit: an orbit in which the satellite crosses over both poles on each revolution.

Questions for discussion:

• What are the advantages of using a Sun-synchronous orbit?

• Why can the satellite only communicate with a ground station for a few minutes at a time?

• Why does MethaneSAT need to communicate with ground stations spread around the world?

Transcript

Chris Jackson

Head of Space Operations and Ground Segment, Te Pūnaha Ātea – Space Institute, University of Auckland

MethaneSAT will actually be around about 600 kilometres above the Earth. And it’s in what’s called a Sun-synchronous polar orbit. The satellite effectively goes over the North Pole and then over the South Pole, doing one orbit of the Earth roughly every 100 minutes at around about 7.5 kilometres a second.

And at the same time that it’s going north to south, the Earth is rotating underneath the satellite. Obviously, it does one rotation a day, so we have day and night every 24 hours. But the satellite is actually seeing day and night once every 100 minutes.

The reason people tend to use Sun-synchronous orbits is it’s very good from a spacecraft engineering perspective because you tend to have a stable thermal and also power environment. But additionally, the lighting on the ground tends to be largely the same from one orbit to the next and throughout the year. This means that it’s much easier to compare information that’s captured across the different times of the year.

So this is one of the unique things is that it’s able to take global data basically every day. So we need to send the data down from the spacecraft using radio transmitters on board. To do that, we have a number of ground stations around the world to get down the amount of data that MethaneSAT will be generating, and these are the big satellite dish antennas. So we have very high data rate transmitters and four or five ground stations around the world to get around about 20 odd contacts per day from the spacecraft.

Typically, a satellite orbiting around the world at a great altitude, we only therefore will be able to see it for a very short period of time as it passes over a ground station, and this is what we call a contact. And for MethaneSAT, the contact time that the ground station will be able to communicate with the spacecraft will be round about 10 minutes per orbit per ground station. Because we’ve got such a vast amount of data coming off the sensors on board the spacecraft, we can’t typically get that down in a single contact. We use a number of different ground stations spread around the world so that we’re able to get the data in a timely way.

Acknowledgements Chris Jackson, Te Pūnaha Ātea – Space Institute, University of Auckland Awarua Ground Station, MBIE Near Earth Network Ground Stations in Norway and Chile, NASA