Providing its user with location, distance and directional information, tracking, and route creation, GPS can seem a little bit like magic!
These little devices help you get from point A to point B without getting lost and can give your location very accurately to rescuers if the worst should happen.
But how does GPS work and just how reliable is it?
What is GPS and What Does GPS stand for?
GPS stands for Global Positioning System. It is a network of satellites that orbit our planet at an altitude of about 20,000 miles (32,000 km) above Earth's surface.
GPS is the US Global Navigation Satelite System (GNSS), other regions have their own systems. The GPS system was developed by the U.S. Department of Defense in 1974 as part of the Cold War space race between the United States and the Soviet Union.
How does GPS work?
GPS consists of three parts; satellites, receivers, and ground stations.
The GPS Satellites
Earth is currently orbited by a network of GPS satellites that are owned and operated by the US Government. This is often referred to as the "space segment" of the GPS infrastructure. The first satellite was launched into orbit on July 10th, 1978 and it became fully operational in 1993 when the 24th was launched.
These satellites are continually updated and replaced. There are currently 31 GPS satellites and many others in operation for competing GNSS. Each of these is called, rather poetically, a "satellite constellation"
Each satellite makes a full orbit of Earth every 12 hours in a tightly controlled orbit, so regardless of where you are on the planet, there will always be at least four GPS satellites in your line of sight.
GPS satellites transmit at least 2 low-power radio signals. These signals are unable to go through mountains or thick walls, but modern receivers can usually detect signals through obstructions like walls and trees.
This means that GPS works outside in all weather conditions, provided there is an unobstructed line of sight communication with 4 or more GPS satellites.
Each satellite continually broadcasts its exact position and current time using a high-powered transmitter.
What's in the GPS signal?
A GPS signal contains 3 different types of information:
A pseudorandom code identifies which satellite you are receiving the information from. If you are interested, you can see which satellites you're receiving signals from on your device’s satellite page.
Ephemeris data shows a satellite's position and contains information about the health of a satellite, current date, and time.
Almanac data tells the GPS receiver the exact location the satellite should be at any point in time throughout the day and shows the orbital information for that satellite.
Why Do GPS Satellites Have Atomic Clocks?
Even though these signals travel at the speed of light, there is still a delay between the time broadcast and the time received.
This tiny delay is what the receivers use to determine how far the signal has come, so you can see why they need to be incredibly accurate.
The accuracy in position is directly proportional to the time measurement, as speed * time = distance. This means that even a slight error in time broadcast can create a huge error in the distance calculation due to the immense speed of light.
To this end, GPS satellites carry atomic clocks to ensure the exact time is broadcast.
Receivers and Devices
Receivers are the devices you typically use such as your phone, handheld device, or your car's navigation system. These receivers constantly listen for the GPS satellite signal.
A GPS works independently of the user's Internet connection or phone signal, however, it can increase the accuracy of GPS location.
How Does A GPS Receiver Calculate its location?
In order for a GPS receiver to determine its exact location, you need at least four satellites in your line of sight.
The receiver looks at the time stamp received from each satellite and compares it to the time on its own clock, to determine how long the signal took to get from satellite to receiver.
The receiver then calculates its distance from these satellites based on the time taken for each signal to reach the receiver. The older the timestamp, the further a satellite is from the user
We also know the speed of light and that the formula distance * time = speed so the receiver can find the distance as it knows both the speed the signal traveled and the time taken to reach the receiver.
Since the receiver now knows the location of each satellite and how far it is from them, it can cross-reference and pinpoint your precise location. This process is called trilateration.
Why Does a Receiver Need a Signal from 4 Satellites?
As you may have guessed by the name trilateration (tri meaning three) if a receiver knows the ranges of three satellites and the location of the satellite when the signal was sent, the receiver can calculate its 2D position, ie the latitude and longitude on a map. In this case, the receiver usually assumes that it is at sea level.
With the addition of a 4th satellite a receiver can calculate its 3D position (latitude, longitude, and altitude), and the additional redundancy helps to compensate for atmospheric conditions.
Theoretically a receiver with an atomic clock synchronized to the GPS satellites would also be able to compute its 3D location from just three signals, but is a little impractical!
Thus, the receiver uses four satellites to compute latitude, longitude, altitude, and time.
How is Einstein's Theory of Relativity Used in GPS?
If this was not clever enough, the receiver must also account for propagation delays or decreases in the signal's speed caused by the ionosphere and the troposphere and even Einstein's law of relativity.
According to Einstein's theory of relativity, SpaceTime is warped by gravity. Since the satellites are further from the Earth's mass than you are on the surface, they experience time differently.
Einstein's theory also states that the faster one object moves compared to another, the more time is dilated and hence experienced differently. Since satellites move much faster compared to anything on the surface of Earth, this also needs to be accounted for.
So amazingly, built into each receiver is a bit of software that compensates for Relativity.
How Accurate is GPS?
A standard receiver can determine where you are within a few yards of your actual location.
The greater the number of satellites that are in the line of sight of your receiver, the better the accuracy in determining your location.
If you're working with a more high-tech DGPS receiver, it'll locate you within a few inches of your actual position.
Differential GPS (DGPS)
Differential GPS (DGPS)is an improved version of GPS that provides enhanced location precision.
DGPS uses a network of fixed, ground-based stations to determine the difference between the position indicated by the Global Positioning System (GPS) satellite system and the known position of each station. These stations broadcast the differences between the measured satellite ranges and actual ranges and the receivers may correct the range by the same amount.
GPS ground stations
GPS ground stations, sometimes termed the "ground segment" or "control segment", are part of the Global Positioning System (GPS), located on Earth, that is responsible for the operation of satellites and the entire system.
The ground segment includes a host of ground facilities including
- A master control station (MCS). This is where all of the satellite tracking takes place. It receives the signal from each satellite and calculates its position.
- An alternate master control station (AMCS). If something goes wrong with the main MCS, it can take over temporarily to maintain continuity of service.
- Command and control antennas These send out instructions to the satellites. There are currently 11 of these in the GPS system.
- Monitoring sites. These are located around the world and provide real-time status reports about the health of the satellites. They also relay important information to the MCS.
- Antennae. These are large dishes that capture the radio waves sent by the satellites, and convert them into electrical impulses that computers can understand.
The various components of the control segment work collectively to track satellites, consistently monitor their transmissions, perform analyses, and send data and commands to these satellites.
One thing that can cause problems with the satellite network is when atmospheric conditions delay the signals from satellites.
In the same way that if you put a pencil in a glass of water it appears to have a kink in it, changes in the atmosphere can lead to the signal from the GPS satellite being bent, which changes the distance and hence the time of arrival of the signal (not the speed of light as I have seen some people say!) causing position errors.
The ground segment monitors atmospheric conditions and runs tests and makes adjustments to the signals to compensate for the additional travel time of the signal.
There are many different types of global navigation satellite systems (GNSS), each with its own strengths and weaknesses. Some are better for certain uses than others. Here are some of the most common GNSS systems around today:
Global Positioning System (GPS):
This is the primary system used by most devices and smartphones, currently consisting of 31 satellites.
Glonass is a network of satellites operated by Russia. It provides positioning data for countries within Eurasia and parts of Africa and consists of 27 satellites.
BeiDou is a Chinese satellite navigation system designed to support civilian applications such as vehicle navigation and fleet management. It is made up of 27 satellites
Galileo is a European Union project for a global satellite navigation system, consisting of 30 satellites.
This is the system covering Japan, featuring 7 satellites
This is the Indian satellite system covering India and SE Asia, consisting of 7 satellites
This post should have answered the question of how GPS works and provided insight into why your GPS should form part of your kit checklist before every hike!
- 1 What is GPS and What Does GPS stand for?
- 2 How does GPS work?
- 3 GPS ground stations
- 4 Other Global Navigation Satelite System (GNSS) Systems
- 5 Final thoughts