Building satellites for Earth observation

Satellites are built to perform specific functions. Expecting a GPS satellite to collect weather data is a bit like expecting your mountain bike to pull water skiers!

Satellite missions and payloads

Satellites are grouped by the jobs they are sent into space to do, also known as their missions. Missions include communications (internet, phones and television), weather, navigation and Earth observation. All satellites will have common components like batteries and solar panels to keep them powered, but the payload components are specific to the kind of mission data they are designed to collect.

Simply explained, the satellite’s payload consists of one or more communication antennas, transmitters and receivers. Antennas and receivers collect data using instruments like radar, GPS and cameras. These sensors receive electromagnetic radiation from what is being observed and convert it into signals – for example, numerical data or images. This information is transmitted (downloaded) to ground stations where it is often interpreted and analysed and made available for others to use.

The following information explores three situations in which Earth observation satellites are used to gather data.

Tracking ship movements

Aotearoa New Zealand’s exclusive economic zone (EEZ) is 4 million square kilometres – more than 20 times the size of our land area! New Zealand governs this area and decides who can use the resources. Our EEZ is far too large to monitor via marine patrol boats, so satellites are used to monitor ship movements. The Xerra Earth Observation Institute created ©Starboard Maritime Intelligence, a software platform that monitors the sea for activities like illegal fishing and dark vessels. Dark vessels either do not have automatic identification systems or they switch off their location-transmitting devices to avoid being detected.

Starboard uses satellites with a variety of sensors to track vessels. Sensors using synthetic aperture radar (SAR) send radar emissions that ping from the surface of a ship back to the satellite. Radar is more efficient than optical sensors that take images – clouds and big waves can block the satellite’s view.

The article How do we find dark vessels on the ocean? provides additional information about maritime tracking. The activity Analysing satellite data for finding dark vessels uses actual satellite data from ©Starboard Maritime Intelligence.

Monitoring endangered animals

Earth observation satellites can be very useful when monitoring animal populations. The sensors are not in direct contact with the animals so monitoring does not disturb them or interfere with migration activities or nesting sites. Although the scientists who do the on-the-ground monitoring find the work exciting, travel to these places is often expensive, sometimes dangerous and provides only one snapshot in time. High-resolution remote sensing is able to make these sorts of measurements quickly, precisely and often.

Scientists (and citizen scientists) examine these high-resolution images to gather evidence about population numbers and learn more about the day-to-day activities of the animals. They can use this evidence to make inferences about habitat, food webs and much more.

The article How are satellites helping albatross? provides additional information about species monitoring. The activity Analysing satellite data for albatross research uses actual satellite data of albatross monitoring in the Chatham Islands.

Monitoring slow slips

We usually know when an earthquake occurs – we feel the ground shaking or seismometers pick up the movements. Some of New Zealand’s land surface is gradually moving as a result of normal tectonic movement called slow slips. Sometimes the movements take days to move a few millimetres, while other slips take many months. Learning about slow slips helps scientists gain a better understanding of the relationship between slow slips and earthquakes.

One way to track these tiny movements is through the network of global navigation satellite system (commonly referred to as GPS) receivers and antennas operated by GNS Science and Land Information New Zealand. There are over 50 monitoring sites across the North Island, South Island and Chatham Islands. The receivers at these sites record very precise positions on the surface of the Earth so scientists are able to continually measure even the smallest of movements – and the bigger ones too.

The article How do we know about Earth movements? provides additional information about slow slip monitoring. The activity Analysing satellite data to track Earth movements uses actual Geonet satellite data of slow slip movements around Māhia Peninsula.

Satellites and orbits

The job a satellite is designed to do will determine the orbit the satellite occupies. These are three common orbits for Earth observation:

• Low Earth orbit – an orbit with an altitude from 200–2,000 km. This orbit is ideal for missions that require high-resolution optical or synthetic aperture radar images to provide detailed information.
• Medium Earth orbit – an orbit with an altitude that is commonly 20,000 km. This orbit is ideal for GPS satellite constellations.
• Geostationary orbit – an orbit with an altitude that is commonly 35,786 km. This orbit is ideal for tracking the weather.

Essential satellite components

These components are common in most satellites:

• Bus/frame – the frame and structure of the satellite. All other components are attached to it.
• Solar panels – to provide electrical energy for the satellite to operate.
• Batteries – to store energy from the solar panels so the satellite can operate.
• Heat control – to maintain an optimum working temperature. It covers the other components to protect them.
• Computer – to control the operation of the satellite and collect, store and send data back to Earth.
• Thrusters – to move and position the satellite so the payload component and the antenna are pointing in the required direction.
• Transmitter/receiver – to get information from Earth to control the satellite and also to send data back to Earth.
• Antenna – the external part of the transmitter/receiver that converts information to radio waves (transmitter) and from radio waves (receiver).

Payload components

The satellite’s payload consists of one or more communication antennas, transmitters and receivers. These are some common components:

• Radar – this works as a transmitter and a receiver. It transmits pulses of microwave radiation towards the Earth’s surface. Each pulse bounces off objects on the Earth’s surface back to the radar detector.
• Radio GPS – this precise atomic clock continually transmits its time value to the Earth via radio. GPS trackers placed near tectonic plate boundaries communicate with at least four GPS satellites and use their time values to calculate the trackers' positions on the Earth's surface to within a few millimetres.
• Camera – a camera detects visible light coming from the Earth's surface. It is more powerful than a standard digital camera – it has a larger and more accurate optical lens system and a larger electronic sensor chip.

Smaller satellites usually carry one type of payload due to the cost involved in building the satellite and getting it into orbit.