GPS 101: How GPS works
Introduction
Have you ever wondered how a utility provider can map the location of their network or a surveyor pinpoint a property boundary with incredible accuracy? The answer lies in the Global Navigation Satellite System (GNSS), a technology that has revolutionized navigation and positioning across the globe.
In this blog, we'll explore the basics of how GNSS works, including how receivers calculate their position by measuring the time it takes for signals to travel from satellites to the receiver. We’ll also discuss the various sources of error that can affect GNSS accuracy and how these errors can be mitigated.
Whether you're new to GNSS or just curious to learn more, this blog will provide you with a foundational understanding of this incredible technology.
The Global Positioning System
The Global Positioning System (GPS) was developed by the US Department of Defense as a worldwide navigation resource for military and civilian use. It was originally based on a system, or constellation, of 24 satellites orbiting the Earth acting as reference points from which GPS receivers on the ground can use to compute their position. Today, GPS is joined by other satellite constellations, including GLONASS, Galileo, and BeiDou, greatly expanding the number of positioning satellites in orbit. Collectively, these constellations are referred to as GNSS—Global Navigation Satellite Systems. For the rest of this blog, we will use the term GNSS in place of GPS, as all modern Trimble receivers are capable of tracking multiple constellations.
GNSS receivers work by listening to a series of specially coded messages transmitted by each satellite; a GNSS receiver on the ground can calculate how long it took the signal to get from the satellite to its own antenna. Then, to calculate the distance from the antenna to the satellite, the receiver multiplies that travel time by the speed of light. A GNSS receiver is able to make this calculation with multiple satellites simultaneously. If the GNSS receiver is receiving messages from at least four satellites, it can then triangulate its location anywhere on the surface of the Earth.
GNSS receivers work by calculating the distance between the receiver antenna and the satellite. By tracking signals from at least four satellites, the receiver can calculate its location anywhere on Earth.
What causes errors in GNSS location?
GNSS receivers need to receive messages or track at least four different satellites in order to calculate a location, but this location will not be perfectly accurate. Various sources of error can reduce the accuracy of the calculated location.
The first step in improving location accuracy and reducing errors is ensuring the receiver is able to track as many GNSS satellites as possible. Tracking four satellites is the minimum, but each additional satellite the receiver can track provides more messages that can be used to improve the accuracy of the calculated location. Tracking more satellites also enables the receiver to prioritize using messages from satellites that are of the highest quality, or are the least impacted by errors, further improving the accuracy of the calculated location. When working with GNSS, it is important to try to maintain a clear view of the sky so the receiver has the best opportunity to track as many satellites as possible.
You won’t always be able to maintain a clear view of the sky when capturing data in the field, especially when working in environments with trees and buildings. If you find yourself in an environment where you can’t see the sky at all, like a tunnel, you might need to look for a different positioning solution!
There are four main sources of error that can impact the location calculated by the receiver:
- Receiver and antenna error
- Satellite error
- Atmospheric error
- Multipath error
1. Receiver error
Receivers and antennas can introduce errors into the location calculation. Electromagnetic interference from other components on a GNSS receiver (like a modem or display) can reduce the quality of the messages before they are received by the antenna, while the quality of the GNSS receiver and a larger antenna will have a positive impact on location accuracy. This is why a Trimble GNSS receiver is able to calculate a more accurate location than a smartphone.
2. Satellite error
The timing of when a message is broadcast from a satellite is critical to GNSS, and so GNSS satellites are equipped with very accurate atomic clocks. Unfortunately, these clocks are not perfect, and slight inaccuracies in the timing of messages broadcast from the satellite can lead to errors in the location calculated at the receiver. Also, the satellite’s position in space is important, but satellites can drift slightly out of their predicted orbit. This can introduce errors in the receiver when it calculates the distance between the antenna and the satellite.
3. Atmospheric error
GNSS satellites transmit their messages by radio, and since radio signals in the Earth’s atmosphere do not always behave predictably, this is another source of error. The receiver makes an assumption that radio signals travel at the speed of light and that the speed of light is a constant, but this is only true in a vacuum.
In the real world, light slows down, depending on what it is traveling through. As a GNSS signal travels to the surface of the Earth, it gets delayed a little. Because the calculation of distance assumes the signals travel at a constant speed, this delay leads to a miscalculation of the satellite’s distance, which in turn creates an error in location.
Trimble receivers use a correction factor for the signal’s trip through the atmosphere, but given the number of variables, no correction factor or atmospheric model can compensate exactly for the delays that actually occur.
4. Multipath error
When the signal arrives at the surface of the Earth, it can reflect off obstructions such as buildings and trees before making it to the antenna. The signal arrives at the antenna by ‘multiple paths’ which is why this type of error is called multipath error. The antenna receives the direct signal first because the direct route is always fastest, and then the reflected signals arrive later, interfering with the direct signal and giving ‘noisy’ results. Trimble antennas are designed to reduce multipath interference, but it is not possible to eliminate it completely.
Eliminating errors with GNSS corrections
A GNSS receiver needs signals from at least four satellites to establish its location, and each of these signals has its own set of errors as described above. The satellite clocks might be inaccurate, or the satellites might not be in their predicted orbits. As the signals travel down through the atmosphere, they are deflected and delayed because the Earth’s atmosphere is not a vacuum. The signals are bounced around when they meet local obstructions (creating multipath error) before finally reaching the receiver
Fortunately, satellites are so far out in space that distances on Earth are small by comparison. If multiple GNSS receivers are within a few hundred kilometers of each other, the signals that reach them will have traveled through virtually the same piece of atmosphere and will have virtually the same delays. This means receivers will typically be impacted by the same errors, excluding multipath and receiver errors. So, one receiver can measure the errors and provide corrections for these errors to the other. This process of eliminating errors is called GNSS corrections.
What are GNSS corrections & how do they work?
If multiple GNSS receivers are located within the same area, the GNSS signals that reach them have traveled through the same atmosphere, and so they have the same satellite and atmospheric errors.
If one of these receivers is located on a control point that has been accurately surveyed, and the location of GNSS satellites in space is known, then it is possible to compute an accurate distance between the receiver and each satellite. This “reference” receiver then divides that distance by how quickly the signal travels (the speed of light) and calculates the expected time the signal should have taken to reach it. This theoretical time is compared with the time the signals actually took to reach it, and this difference represents the satellite error. This measured error in the satellite's signal can then be applied to other GNSS receivers working in the same region to reduce errors those receivers are experiencing and help improve their accuracy.
Types of GNSS corrections
The source of the GNSS corrections is always from a reference station as described above, but the technology used to transmit the correction information varies. The error value is computed as the GNSS signal is received at the reference receiver, or base station, and then this correction is made available to the other receivers being used in the field. Radios, satellites and internet connections are all used to stream corrections in real time in the field. If you are using real-time GNSS corrections in the field, your receiver is sent a complete list of errors for all satellites visible at the reference receiver, and it will then apply the corrections for the same satellites it is using.
Alternatively, GNSS corrections can be applied to the signals after they have been received in the field by processing them in the office to remove errors and improve accuracy. This method uses technology known as post-processing or baseline processing. With post-processed corrections, the post-processing software calculates the error in each GNSS measurement logged by the reference station receiver and applies the error corrections to the measurements collected in the field by the roving receiver. New, more accurate positions are then calculated from the corrected measurements.
The key difference between real-time and post-processed corrections is that real-time corrections require a connection to the correction source in the field, and if this connection is lost, high-accuracy positioning is also lost. With post-processing, you can always log GNSS measurements in the field, and the correction will happen after the data has been captured.
There are a number of different real-time correction services (SBAS, Trimble RTX, VRS) that sit alongside post-processing. The choice of correction service best suited for your field data collection will depend on the level of accuracy you require, the environment you are working in, and what correction services are available in your region.
Satellite-based correction services like SBAS or Trimble RTX are generally available in most locations but require a consistent and clear view of the sky. Internet-based correction services like VRS require a consistent internet connection in the field and may have limited areas of coverage.
To learn more about what correction service is best for your project, visit our website or contact your local Trimble Dealer.