There are over 3,000 manmade satellites currently operating around Earth, but what are they actually being used for? Today, we’re talking about satellite applications.
Earth observation satellites
Some satellites are launched into space in order to get a better view of Earth. These satellites are used for purposes such as monitoring weather, mapping terrain and aerial photography. They often help to monitor the environment; earth observation satellites can measure air quality, ocean salinity, ice thickness and crop health, and can track changes in vegetation or coastlines.
Google Earth is a particularly well-known illustration of the power of satellite imaging. It uses satellite photography to create an intricate aerial map of the world, with finer details becoming visible as the user zooms in. The images have been taken by different satellites at different times and are combined to create an impression of seamlessness. It’s possible to zoom out far enough to see the entire Earth suspended in space, from a distance of tens of thousands of kilometres.
Earth observation satellites can be used to aid in the response to disasters. Infrared sensors allow satellites to detect and track the spread of wildfires, enabling governments to respond swiftly and warn those who may be in harm’s way; NASA has an article on this. Satellites have also been used to monitor oil spills and volcanic eruptions.
As more powerful equipment would be required to photograph or monitor Earth from further away, satellites used for Earth observation purposes are often in low orbits (also called low Earth orbit, or LEO), although some are maintained at higher altitudes. For more on the advantages and disadvantages of having satellites at certain altitudes, take a look at our previous post on the different types of satellite orbit.
Satellite navigation systems have become a part of daily life. If you use satnav while driving, or pull out your phone to check your position on the map, it’s easy to forget that you’re receiving signals from space.
The US Space Force’s Global Positioning System (GPS) relies on a network of 31 satellites in medium Earth orbit (MEO), at an altitude of just over 20,000 kilometres. The first GPS satellite was launched in 1978. This system was originally created for the use of the US military, but it’s now so widely used that people often call any satellite navigation system a GPS.
How does GPS work? The satellites in the network are sending out radio signals constantly, telling us two things: where the satellite is, and what time the signal was sent.
A GPS receiver, such as the one in your phone, listens out for these signals. When it receives a signal, it checks the difference between the time the signal was sent and its own internal clock. Because the signals travel in a straight line at a predictable speed (the speed of light), this tells your phone exactly how far it is from the satellite.
Of course, knowing how far you are from one satellite doesn’t necessarily help much. If you’re 10,000 kilometres from Peru, you could be in the United Kingdom, New Zealand or Tunisia.
Because of this, GPS receivers narrow down their location by using signals from multiple satellites. If you’re about 10,000 kilometres from Peru and Cambodia and South Africa, there’s a good chance you’re in the United Kingdom. In order to pin down your location and altitude, your phone needs information from at least four satellites, so the orbits of the GPS satellites are calculated to make sure there are always at least four in view from any location on Earth.
There’s a strange and fascinating problem involved in the way GPS works. According to Einstein’s general theory of relativity, time moves more slowly in areas of high gravity. This means that time runs faster on the satellite than it does for your phone. Why don’t the satellites fall out of sync with Earth-based clocks, meaning that it’s impossible to calculate how much time has passed since a signal was sent, and therefore impossible to calculate how far you are from a satellite?
Fortunately, the system takes that into account. The clocks aboard the satellites are designed to run very slightly slower than normal clocks, making up for the time difference between Earth and their orbit.
If you’re interested in learning more about GPS and relativity, Professor Richard W Pogge of Ohio State University wrote a piece on the subject in 2017: ‘Real-World Relativity: The GPS Navigation System’.
We work with communications at Darwin, so naturally we’re interested in communications satellites. These are satellites that receive signals and transmit them elsewhere.
For example, how does satellite television work? An Earth-based station transmits signals – the visuals and sounds of a television programme – to a satellite in orbit. This satellite amplifies the signals and returns them to Earth, where your satellite dish picks them up and decodes them.
The specific area a satellite can serve depends on its orbit, but, in theory, satellites can send signals to anywhere on the Earth’s surface. This means that areas without the infrastructure for terrestrial or cable television may still be able to receive satellite television signals.
Similarly, 5G has remarkable potential, but you can’t make use of it if you’re in an area without any 5G towers, or if a disaster has destroyed the local communications infrastructure. A satellite phone, on the other hand, can be used almost anywhere on the planet. Mountaineers sometimes carry satellite phones or satellite-linked emergency beacons, meaning they can get in contact with rescue services if something goes wrong in a remote spot.
Satellite communications aren’t always the best tool for a task. To use a satellite phone effectively, you’ll need a clear line of sight to the satellite. This means that satellite phones tend to be less useful than traditional mobile phones indoors or in built-up areas, unless there’s a nearby antenna to relay the signal. Currently, satellite phones are also more expensive to buy and use than traditional mobile phones.
However, satellite signals can reach places that terrestrial networks can’t, meaning that satellites provide a valuable complement to terrestrial communications technology.
There are other types of artificial satellite we haven’t touched on here. For example, space stations are a type of satellite, and Earth observation satellites aren’t the only sort of observation-focused satellite; some satellites, such as the Hubble Space Telescope, exist to research or photograph space. Hopefully, though, this article has given an overview of why satellites are such a useful tool.
From their high-up vantage point, satellites have the ability to connect the world. At Darwin, we’re making use of that to achieve our goal of ubiquitous communications.
Darwin Innovation Group is a UK-based company that provides services related to autonomous vehicles and communications. If you’re interested in working with us, take a look at our careers page. If you’d like to know how we can help your organisation make use of autonomous vehicles, contact us. You can also follow us on LinkedIn or Twitter.