How do GNSS receivers work?

Most GNSS (Global Navigation Satellite System) receivers have two parts: an antenna and a processing unit or receiver. The antenna is where the satellite signals are received, while the receiver makes sense of the information received and turns it into measurements we understand, such as latitude and longitude. In dual antenna systems, these are often referred to as 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 are relative to the location of the antennas themselves. This is important to remember because the length of the antenna cables means that the receiver can sometimes be quite far away from the output position measurement. This doesn't matter much for satellite navigation and everyday GPS products, as they are rarely accurate beyond a few meters.

It is important to realize that the calculations regarding position, velocity, and altitude are relative to the antennas themselves, not the receiver. To understand how GNSS works, we need to break GNSS into its parts and understand a little bit about each part. Since GPS is the most familiar system, we will only look at it and divide it into three parts:

Space Segment

Control Segment

User Segment

Space Segment

The space segment involves the satellites in orbit. In 2015, the GPS constellation consisted of 32 non-geostationary satellites in medium Earth orbit, but not all of them are active. Each satellite orbits once every 11 hours, 58 minutes and 2 seconds at an average altitude of 20,200 kilometers (i.e. an orbital radius of 26,571 kilometers).

The GPS satellite constellation is arranged in six equally spaced orbital planes, with no fewer than four satellites in each plane. This arrangement ensures that at almost any time, from any point on Earth, at least four satellites can be seen 15° above the horizon, although in practice there are usually more satellites.

Although the age and design of the satellites vary, their operating principle remains the same. Each clock contains four high-precision clocks with a base frequency of 10.23 MHz, which continuously transmit two carrier waves in the L-band that are transmitted back to Earth at the speed of light. These carriers are called L1 and L2.

The frequency of the L1 carrier is 1575.42 MHz (10.23 MHz × 154 = 1575.42 MHz).

The frequency of the L2 carrier is 1227.60 MHz (10.23 MHz × 120 = 1227.60 MHz).

The carriers are important because they carry information back to Earth from the satellites, and it is this information that allows our receivers to determine where we are. See our GPS Signals page or more details on this.

Control Segment

The control segment refers to the multiple ground stations located around the globe (near the equator) that track, control, and send information to each GPS satellite. This is an important role because it is critical that the clocks of each satellite are synchronized - since the entire system relies on timing.

The orbital information sent to each satellite is also critical because we need it to determine where the satellite was when the information was sent. All of this information is sent to the satellite and then delivered to the GPS receiver via the L1 carrier navigation message.

User Segment

The user segment is the segment that most people are interested in. This segment includes anyone or anything that has a GPS receiver; satnav, mobile phones, drones, law enforcement. So how does it work?

As we’ve already seen, there’s a constellation of satellites orbiting overhead, sending a constant stream of information back to Earth at the speed of light. It takes a little while to understand how this helps determine our location, but it’s based on a process called trilateration.

Before we jump in, we should correct a common misconception. The GNSS receiver inside a satnav or mobile phone never sends any information to satellites. The receivers we use today are completely passive – they only receive information. When Europe’s Galileo system was operational, its receivers were slightly different in that there was an emergency feature that would send information when activated, but this wasn’t intended for normal operation.

When you hear people talking about GPS tracking something, such as an armored vehicle, this is what’s happening. The China GNSS receiver on the vehicle is receiving signals from satellites and determining where it is. Once it knows where it is, it uses other systems, such as a GSM data connection, to send this information back to some monitoring station.