GPS System - PDF

GPS GPS (Global Positioning Systems) is a Satellite Navigation System also called the Navstar system (NAVigation Satellite Timing And Ranging system) - It is a GNSS (global satellite-based navigation system) system that covers the entire [earth](https://geolearn.in/planet-earth-general-i...

GPS GPS (Global Positioning Systems) is a Satellite Navigation System also called the Navstar system (NAVigation Satellite Timing And Ranging system) - It is a GNSS (global satellite-based navigation system) system that covers the entire [earth](https://geolearn.in/planet-earth-general-introduction/). GPS was originally developed for military purposes, but after some time it is being used by civilians for various purposes such as Marnie navigation, surveying, and car navigation. - There are different signals used by the GPS for different purpose applications, for the military perspective it uses precise positioning services (PPS), and for civilian uses, it uses the standard positioning services (SPS). - There are also some other available GNSS systems like GLONASS (an acronym for Globalnaya navigatsionnaya sputnikovaya Sistema) operated by Russian Aerospace defense forces, BeiDou Navigation System (BDS) operated by Chinese satellite navigation system which is operational since 2000, and Galilieo built by the European Union and Europe space agency. - The Global Positioning System (GPS) is a navigation system using satellites, a receiver and algorithms to synchronize location, velocity and time data for air, sea and land travel. Overview - How does it work? A diagram of a global positioning system Description automatically generated - GPS provides specially coded satellite signals (pseudo range) that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. - The satellite system consists of a constellation of 24 satellites in six Earth-centered orbital planes, each with four satellites, orbiting at 13,000 miles (20,000 km) above Earth and traveling at a speed of 8,700 mph (14,000 km/h). - While we only need three satellites to produce a location on earth's surface, a fourth satellite is often used to validate the information from the other three. The fourth satellite also moves us into the third-dimension and allows us to calculate the altitude of a device. GPS is made up of three different components, called segments, that work together to provide location information. The three segments of GPS are:  1. Space (Satellites) GPS Space Segment - Navipedia The satellites circling the Earth, transmitting signals to users on geographical position and time of day. It comprises 24 satellites orbiting the [earth](https://geolearn.in/planet-earth-general-introduction/) at approximately 20200km (altitude) every 12 hours. There are six orbital planes with nominally four satellites present in each orbit. - The Space Segment of the system consists of the GPS satellites. - These satellites also called space vehicles (SVs) send radio signals from space which are called pseudo ranges to the receiver. - There are 6 orbital planes with 4 SVs in each, equally spaced (60 degrees apart meaning that each satellite has a clear line of sight to the next in its plane.) and inclined at about 55 degrees to the equatorial plane (meaning that these satellites are tilted upward relative to the Earth\'s equator, which can enhance coverage in higher latitudes). - There will always be a minimum of at least four satellites visible above 15 degrees cut off/mask at any point on the surface of the [earth](https://geolearn.in/planet-earth-general-introduction/) at any time in any weather condition. - Each satellite carries a highly precise atomic clock onboard which operates at a fundamental frequency of 10.23 MHz. These clocks play a major role in generating signals which are broadcasted by the satellite. - New satellite are launched and put in reserve, ready to replace older satellites 2. Ground control  The Control Segment is made up of Earth-based monitor stations, master control stations and ground antenna. Control activities include tracking and operating the satellites in space and monitoring transmissions. There are monitoring stations on almost every continent in the world, including North and South America, Africa, Europe, Asia and Australia. - The Control Segment consists of a system of tracking stations located around the world. - The Master Control facility is located at Schriever AFB (Air Force Base) in Colorado Springs and an alternative MCS at Vandenberg AFB in California - Four other monitoring stations measure signals from the SVs which are incorporated into orbital models for each satellite - The signals generated by the GPS satellite onboard sensors are detected by the GPS receiver which later enables the exact position of each of the satellites. Signals from the satellites are recorded by the control stations which estimate the measurement errors. These errors are later transmitted to the master station in Colorado springs. Information from the master control station is resent to the monitoring stations which are later then uploaded to these satellites. - The models compute precise orbital data (ephemeris) and satellite clock corrections for each satellite. - The Master Control station uploads ephemeris and clock data to the satellite. - The satellite then send subsets of the orbital ephemeris data (satellites celestial location) to GPS receivers over radio signals. - The main responsibility of these stations is to track and control the orbital positions of the satellites. There are 1 master control and 5 monitoring networks. A diagram of a control segment Description automatically generated  3. User equipment GPS receivers and transmitters including items like watches, smartphones and telematic devices. - The User Segment consists of the GPS receivers and the user community - GPS receivers convert satellite signals into position, velocity and time estimates. - Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time. - GPS receivers are used for navigation, positioning, time dissemination, and other research. The GPS has some sort of positioning error where it is 3 to 10m accurate. - For RTK, you need two GNSS receivers: one is static and called a "base station," the other is moving and is called a "rover." While both receivers observe the same satellites simultaneously, the static base station is placed at a point with known coordinates (a benchmark or a point measured beforehand). Taking into account the known coordinates and receiving satellite signals, the base transmits corrections to the moving rover. This way, the rover can get sub-centimeter accurate positioning. - Navigation in three dimensions is the primary function of GPS. How GPS works GPS works through a technique called trilateration. Used to calculate location, velocity and elevation, trilateration collects signals from satellites to output location information. It is often mistaken for triangulation, which is used to measure angles, not distances. Satellites orbiting the earth send signals to be read and interpreted by a GPS device, situated on or near the earth's surface. To calculate location, a GPS device must be able to read the signal from at least four satellites. - Each satellite in the network circles the earth twice a day, and each satellite sends a unique signal, orbital parameters and time. At any given moment, a GPS device can read the signals from six or more satellites. - A single satellite broadcasts a microwave signal which is picked up by a GPS device and used to calculate the distance from the GPS device to the satellite. Since a GPS device only gives information about the distance from a satellite, a single satellite cannot provide much location information. Satellites do not give off information about angles, so the location of a GPS device could be anywhere on a sphere's surface area. - When a satellite sends a signal, it creates a circle with a radius measured from the GPS device to the satellite. - When we add a second satellite, it creates a second circle, and the location is narrowed down to one of two points where the circles intersect. - With a third satellite, the device's location can finally be determined, as the device is at the intersection of all three circles. Ranging That said, we live in a three-dimensional world, which means that each satellite produces a sphere, not a circle. The intersection of three spheres produces two points of intersection, so the point nearest Earth is chosen. As a GPS device moves, the radius (distance to the satellite) changes. When the radius changes, new spheres are produced, giving us a new position. We can use that data, combined with the time from the satellite, to determine velocity, calculate the distance to our destination and the ETA. Example: 1)  2) A diagram of a satellite Description automatically generated 3)  Determining differences in time - Each satellite has its own complex 'pseudo random' code that it transmits on a loop relative to its clock's time - The receiver also plays the same pseudo random codes relative to its clock's time - The difference, or offset, between the receiver's code and the received satellite code allows the calculation of time: A black and white screen with white text Description automatically generated Synchronisation of the clocks time Satellites have very accurate atomic clocks - A GPS receiver needs to synchronize its clock with the satellites' atomic clocks to accurately calculate position. It accomplishes this by continuously receiving time signals from the satellites. These signals contain the precise time when they were transmitted by the satellites. By comparing these time signals with its own clock, the receiver can determine the difference between the satellite's time and its own clock's time. This difference is used to adjust the receiver's clock and keep it in sync with the atomic clocks on the satellites. - Each GPS satellite contains multiple atomic clocks that contribute very precise time data to the GPS signals. GPS receivers decode these signals, effectively synchronizing each receiver to the atomic clocks. - An atomic clock is crucial for GPS because it provides extremely precise and consistent timekeeping. The signals transmitted by GPS satellites travel at the speed of light, covering a significant distance in a short amount of time. To accurately measure the signal's travel time and calculate distances, the receiver needs a highly accurate clock. Atomic clocks are capable of maintaining accurate timekeeping to within nanoseconds, ensuring precise calculations of the distances between the receiver and satellites. Receivers have very cheap clocks. The GPS makes a cheap clock become an atomic clock via a fourth satellite. - The clock error will be the same in all satellite ranges - So, the receiver adjusts its clock iteratively until a unique solution is reached - The receiver needs measurements from multiple satellites to determine its position accurately. Each satellite provides distance information based on signal travel time. By combining the measurements from multiple satellites, the receiver can triangulate its position using trilateration. This method relies on intersecting spheres or circles representing the distances from each satellite to pinpoint the receiver's location. Multiple measurements help to refine the position estimate and provide a more precise calculation. - Then the clock on the receiver MUST be correct, and effectively an atomic clock! To correct for the GPS receiver\'s clock error and find your precise position, a fourth satellite (satellite with the yellow sphere) must be used. With the fourth satellite, small timing errors from all four satellites to the point on the Earth have been adjusted, and your exact location on the Earth (the purple point) can be determined. The need for the fourth satellite: In GPS positioning, the fourth satellite serves two purposes. - Firstly, it allows the receiver to compute its latitude, longitude, and altitude in addition to time. - Secondly, it provides a reference for the receiver to synchronize its clock. - By comparing the satellite's time signal with its own clock, the receiver can adjust its clock's accuracy and maintain synchronization with the satellite's atomic clock. - By taking a measurement from a fourth satellite, the receiver can avoid the need for an atomic clock. The fourth satellite's signal provides an additional measurement that allows the receiver to calculate its position and synchronize its clock. This reduces the reliance on the receiver's own clock accuracy and ensures precise position determination. Measuring the distance How do we measure the distance to the satellite? By timing a radio signal from satellite to receiver: - Distance = Velocity x Time Pseudo-Range = Speed of Light x Travel Time **(186,000 mi/sec) x (signal travel time in seconds) = Distance of the satellite to the receiver in miles**. - knowing the speed of a radio signal and the difference between when it was sent and received, we can calculate the distance Example: Radio waves travel at speed of light at 186,000 miles/ second, approx. 299,338km So, if the difference between sent and received time is 0.1 second, distance to satellite = 18,600 miles If a satellite were directly overhead, travel time of radio waves would be 0.06 seconds Suggests the need for VERY accurate clock!