The GPS receiver also knows the exact position in the sky of the satellites, at the moment they sent their signals. So given the travel time of the GPS signals from three satellites and their exact position in the sky, the GPS receiver can determine your position in three dimensions - east, north and altitude.
There is a complication. The GPS satellites have atomic clocks that keep very precise time, but it's not feasible to equip a GPS receiver with an atomic clock. However, if the GPS receiver uses the signal from a fourth satellite it can solve an equation that lets it determine the exact time, without needing an atomic clock. If the GPS receiver is only able to get signals from 3 satellites, you can still get your position, but it will be less accurate.
As we noted above, the GPS receiver needs 4 satellites to work out your position in 3-dimensions. If only 3 satellites are available, the GPS receiver can get an approximate position by making the assumption that you are at mean sea level.
If you really are at mean sea level, the position will be reasonably accurate. However if you are in the mountains, the 2-D fix could be hundreds of metres off. A modern GPS receiver will typically track all of the available satellites simultaneously, but only a selection of them will be used to calculate your position.
To determine the location of the GPS satellites two types of data are required by the GPS receiver: the almanac and the ephemeris. The almanac contains information about the status of the satellites and approximate orbital information.
The GPS receiver uses the almanac to calculate which satellites are currently visible. The almanac is not accurate enough to let the GPS receiver get a fix. If the GPS receiver is new, or has not been used for some time, it may need 15 minutes or so to receive a current almanac.
In older GPS receivers, an almanac is required to acquire the satellites, but many newer models are able to acquire the satellites without waiting for the almanac. Then, if a fourth satellite signal is received and a fourth distance is measured, it will also be possible to determine with high precision this time offset and then to find the correct space coordinates.
Said in other words, the four distances to the four satellites will only fit and determine one particular point in space, if the time offset has a certain value. This calculation is done automatically by the software in the GPS receiver.
This is the reason that the acquisition of three GPS satellites does not give a very high precision, and that at least four are needed for a satisfactory measurement. They are received by the GPS receiver and contain much detailed information.
In addition to the timing signal, there are also data for identification of the satellite by its number , about the status of the satellite clock, the satellite orbit, the current status of the satellite health and various correction data. The data is divided into frames of bits; one frame is transmitted in about 30 seconds. These data are stored in the receiver and updated regularly. The approximate directions and distances to individual GPS satellites that are momentarily above the horizon are calculated from the orbital data.
There are of course many other uncertainties involved in a GPS measurement. In other words, the GPS module receives a timestamp from each of the visible satellites, along with data on where in the sky each one is located among other pieces of data. From this information, the GPS receiver now knows the distance to each satellite in view. This is also called a lock or a fix. Did you catch all of that? If not or if you want more, check out a much more detailed explanation, in volume 1 of GPS Fundamentals by Dan Doberstein.
Volume 1 has been released for free, but you must support the author to read volume 2. An artist's rendition of the control segment. Along with satellites and GPS receivers, there are ground based stations that can communicate with the satellite network and some GPS receivers.
This system is formally called the control segment and increases the accuracy of your GPS receiver. DGPS units are also expensive and tend to be larger because they require an additional antenna.
GPS Accuracy depends on a number of variables, most notably signal to noise ratio noisy reception , satellite position, weather and obstructions such as buildings and mountains.
These factors can create errors in your perceived location. Signal noise usually creates an error from around one to ten meters. Mountains, buildings and other things that might obstruct the path between the receiver and the satellite can cause three times as much error as signal noise. A GPS receiver must be able to get a lock on 4 satellites to be able to solve for a position. The first lock it gets allows the receiver to obtain the almanac information and thus what other satellites it should listen for.
Although it is possible to get a position from less than 4 satellites, the margin of error of this position can be rather large. Your most accurate read of your location comes when you have a clear view of a clear sky away from any obstructions and under more than four satellites.
To combat these errors, a couple of different assistants have been created. This method uses wireless ground-based networks to help relay between the satellite and the receiver when the GPS signal is weak or not able to be picked up. There are two ways AGPS can help out. The first is to provide the receiver with the proper almanac data and the precise time.
The second utilizes the higher computing power and good satellite signal of the ground base to interpret the broken or fragmented information the receiver is receiving to provide a more accurate position reading to the receiver.
When communicating with these receivers, the GPS can acquire a lock on the satellite more quickly as well as receive more accurate information.
But AGPS is present in more devices than just cellphones; it's even available in cameras and some vehicles. DGPS also uses ground or fixed GPS stations to determine the location, but differs in that it finds the difference between both the satellite and the ground location reading. These ground stations may be up to nautical miles from the receiver, and it is important to note that accuracy deteriorates the further you are from the ground station.
DGPS is accomplished by a ground station broadcasting a signal which dictates the error between the actual pseudorange and the measured pseudorange. This value is calculated by multiplying the speed of light by the time it takes the signal to travel from the satellite to the receiver. WAAS holds a specific set of accuracy standards that ground station measurements must meet.
Laterally and vertically, WAAS must be accurate to within 7. These ground stations send their measurements to master stations which send the corrections to WAAS satellites every 5 seconds or quicker. From the Satellite, a signal is broadcast back to the receivers on earth where the corrections are used to improve the GPS accuracy.
In some locations, WAAS is able to provide an accuracy of 1 meter lateral and 1. GPS data is displayed in different message formats over a serial interface. There are standard and non-standard proprietary message formats.
The NMEA standard is formatted in lines of data called sentences. Each sentence contains various bits of data organized in comma delimited format i. The data is separated by commas to make it easier to read and parse by computers and microcontrollers. This data is sent out on the serial port at an interval called the update rate.
Most receivers update this information once per second 1Hz , but more advanced receivers are capable multiple updates per second.
Most GPS modules have a serial port , which makes them perfect to connect to a microcontroller or computer. Once a GPS module is powered, NMEA data or another message format is sent out of a serial transmit pin TX at a specific baud rate and update rate , even if there is no lock. It is common for the microcontroller to parse the NMEA data.
Parsing is simply removing the chunks of data from the NMEA sentence so the microcontroller can do something useful with the data. Instead of dealing with all of this text, the microcontroller can parse the GPGGA sentence and end up with only the altitude in meters. Once the microcontroller can grab the data needed, the information can be manipulated to create other interactions on the microcontroller.
Next, open a serial terminal program at the same baud rate of your GPS module. The GPS chipset contains a powerful processor that is responsible for the user interface, all of the calculations, as well as analog circuitry for the antenna. Instead of stars, we use satellites. Over 30 navigation satellites are zipping around high above Earth. These satellites can tell us exactly where we are. Satellites act like the stars in constellations—we know where they are supposed to be at any given time.
A receiver, like you might find in your phone or in your parents car, is constantly listening for a signal from these satellites. The receiver figures out how far away they are from some of them. Once the receiver calculates its distance from four or more satellites, it knows exactly where you are.
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