There are two parts to most GNSS receivers; the antenna and the processing unit or receiver. The antenna is where the satellite signals are received, while the receiver makes sense of the information being received and turns it into measurements we understand, such as latitude and longitude. On dual antenna systems, they are usually called the ‘primary’ and ‘secondary’ antennas. The RT3000 unit shown has two GNSS receivers built into it.
Although the GNSS receivers do all the work, the actual measurements they produce relate to the position of the antennas themselves. This is important to bear in mind, because the length of the antenna cables means a receiver can sometimes be quite a distance from the output position measurements. On sat navs and everyday GPS products, this doesn’t matter as they are rarely capable of accuracy greater than several metres anyway.
It’s important to recognise that the calculations about position, speed and altitude relate to the antenna itself, and not the receiver. To understand how a GNSS works, we need to break the GNSS into parts and understand a little bit about each one. As GPS is the system people are most familiar with we’ll just look at that, and break it down into three parts:
- The space segment
- The control segment
- The user segment
The space segment
The space segment is concerned with the satellites in orbit. In 2015 the GPS constellation consists of 32 non-geostationary satellites in medium Earth orbit, although not all satellites are active. Each satellite orbits once every 11 hours, 58 minutes and 2 seconds at an average altitude of 20,200 km (that’s an orbital radius of 26,571 km).
The GPS satellite constellation is arranged into six equally-spaced orbital planes, with no fewer than four satellites in each plane. This arrangement ensures a minimum of four satellites can be seen 15° above the horizon at almost any time, from any point on the planet—although in reality there are generally more.
While satellites vary in age and design, their principle of operation remains the same. Each one contains four highly accurate clocks with a fundamental frequency of 10.23 MHz, and they and constantly transmit two carrier waves in the L-Band that travel back to earth at the speed of light. These carrier waves are referred to as L1 and L2.
- The L1 carrier has a frequency of 1575.42 MHz (10.23 MHz × 154 = 1575.42 MHz).
- The L2 carrier has a frequency of 1227.60 MHz (10.23 MHz × 120 = 1227.60 MHz).
The carrier waves are important because they bring the information from the satellite back to earth, and it’s that information that allows our receiver to work out where we are. Please see our GPS signal page for more details on this.
The control segment
The control segment refers to a number of ground stations situated around the globe (close to the equator) that are used to track, control and send information to each of the GPS satellites. This is an important role as it is vital the clocks in each satellite are synchronised—because the whole system relies on timing.
The orbit information that is sent up to each satellite is vital too, because we need it in order to work out where the satellite was when the information was sent. All of this information is sent up to the satellites, then carried to your GPS receiver within the L1 carrier wave navigation message.
The User segment
The user segment is the part most people are interested in. This segment includes anyone or anything with a GPS receiver; sat navs, mobile phones, UAVs, law enforcement. So how does it work?
As we’ve already seen, there is a constellation of satellites orbiting above our heads, sending a constant stream of information back to earth at the speed of light. Understanding how this helps pinpoint our location takes a bit of time, but it is based on a process called trilateration.
Before we get too involved, we should correct a common misconception. At no point does the GNSS receiver inside your sat nav or phone send any information up to the satellites. The receivers we use today are completely passive—they only receive information. When Europe’s Galileo system is operational, its receivers will be slightly different because there will be an emergency function, which will send information when activated, but this won’t apply in normal operation.
When you hear people talking about something being tracked by GPS, an armoured car for example, what is happening is this. The GNSS receiver on the vehicle is receiving signals from the satellites and working out where it is. Once it knows its position, it sends this information using some other system, a GSM data connection for example, back to some monitoring station.
So, for the time being, GNSS receivers work by receiving signals sent from the relevant satellites in orbit. The signals that are used depend on the type of receiver. A GPS receiver can only make use of signals from the GPS satellites, while a GLONASS receiver can only use signals from GLONASS satellites. There is another kind of receiver that can actually take signals from both types of satellites though (GPS and GLONASS), to augment its measurements.