Global Positioning System – A Mini Review
If we are ever been lost and wished there was an easy way to find out which way we needed to go and ever find that perfect fishing or hunting spot and not been able to remember how to get back to it easily? How about finding ourselves out hiking and not known which direction we should go to get back? Ever been flying along and needed to locate the nearest airport or identify the type of air space we were in? With GPS (Global Positioning System) unit we could know where we are located on the planet at all times. Whether it be for fun, saving lives, or whatever use you can ever dream of, GPS navigation is becoming more common every day. In this paper, we have elucidated the segments of GPS, the working principle, and its receiver technology. Besides these we also elaborated the sources of errors in GPS positioning, working of DGPS (Differential GPS), and WAAS (Wide Area Augmentation System). The concepts of Mapping and Navigation are briefed. A brief description of Mapping, Navigation is dealt and finally the important applications of GPS are discussed.
What Is GPS
The Global Positioning System (GPS) is a space-based radio-navigation system. It consists of 24 satellites, which orbit the Earth at an altitude of approximately 11,000 miles, and ground stations. GPS provides users with accurate information on position, velocity, and time anywhere in the world and in all weather conditions. GPS, formally known as the Navstar Global Positioning System, was initiated in 1973 .Concisely it is a network of satellites that continuously transmit coded information which makes it possible to precisely identify locations on earth by measuring distances from the satellites. It refers to a group of US Department of Defense Satellites constantly circling the earth. The satellites transmit very low power radio signals allowing any one with a GPS receiver to determine their location on earth. GPS actually predates introduction of the personal computer. The designers originally had military application in mind. GPS receivers would aid Navigation, troop deployment and artillery fire. Later GPS was made available for civilian use also.
Satellite Position In GPS
Stand-alone GPS uses 24 satellites placed asymmetrically in six orbital planes, where 550 relative to the equatorial plane incline each plane. The satellites are approximately 26,560 km above the center of the earth, with the orbital period of 12hr and repeating ground tracks. The orbital period is measured in sidereal time, where a sidereal day is the period of earth’s rotation relative to the fixed stars (not the sun) and is approximately four minutes shorter than a solar day. Hence the noontime subsatellite point lowly drifts in its fixed ground track from day to day.
The Three Segments Of GPS
The Space Segment: The space segment, which consists of at least 24 satellites, is the heart of the system. The satellites are in what’s called a “high orbit” about 12000 miles above the Earth’s surface. Operating at a high altitude allows the signals to cover a greater area. The satellites are arranged in their orbits. So a GPS receiver on Earth can always receive from at least four of them at any given time. The satellites are traveling at speeds of 7000 miles an hour, which allows them to circle the earth once every 12 hours. They are powered by solar energy and are built to last about 10 years. If the solar energy fails they have backup batteries on board to keep them running. They also have small rocket boosters to keep them flying in the correct path. Each satellite transmits a low power radio signals on several frequencies, Civilian GPS receivers “listen” on the L1 frequency of 1575.42 MHz in the UHF band. The signal travels “line of sight”, meaning it will pass through clouds, glass and plastic, but will not go through most solid objects such as buildings and mountains. L1 contains 2 “pseudorandom”(a complex pattern of
digital code) signals, the Protected (P) code and the Coarse/Acquisition (C/A) code. Each satellite transmits a unique code, allowing the GPS receiver to identify the signals. ”Anti Spoofing” refers to the scrambling of the P code in order to prevent its unauthorized access. The P code is also called the “P(Y)” or “Y” code. The main purpose of these coded signals is to allow for calculating the travel time from the satellite to the GPS receiver on the Earth. This travel time is also called the Time of Arrival. The travel time multiplied by the speed of light equals the
satellite range (distance from the satellite to the GPS receiver). The Navigation message contains the satellite orbital and clock information and general system status messages and an ionospheric delay model. The satellite signals are timed using highly accurate atomic clocks.
The Control Segment: The control segment “controls” the GPS satellites by tracking them and then providing them with corrected orbital and clock (time) information. There are 5 control stations located around the world-4 unmanned monitoring stations and 1”master control station“. The 4 unmanned receiving stations constantly receive data from the satellites and then send that information to the master control station. The master control station “corrects” the satellite data and, together with 2 other antenna sites uplinks the information to the GPS satellites.
The User Segment: The user segment simply consists of you and your GPS receiver. The user segment consists of boaters, pilots, hikers, hunters, the military and anyone else who wants to know where they are, where they have been or where they are going.
Working Of GPS
Location is everything: The GPS receiver has to know 2 things if it is going to do its job. It has to know where the satellites are (location) and how far away they are (distance) .The GPS receiver picks up 2 kinds of coded informat6in from the satellites. One type of information, called “almanac” data, contains the appropriate positions of the satellites. This data is continuously transmitted and stored in the memory of GPS receiver so it knows the orbits of satellites and where each satellite is supposed to be. The almanac data is periodically updated with new information as satellites move around. Any satellite can travel slightly out of orbit, so the ground monitor stations keep track of the satellite orbits, altitude, location and speed. The ground station sends the orbital data to master control station, which in turn sends the corrected data up to the satellites. This corrected and exact position data is called the “ephemeris” data which is valid for about 4 to 6 hours, and is transmitted in the coded information to the GPS receiver. So having received the almanac and ephemeris data the GPS receiver knows the location of the satellites at all times.
Time is of the Essence: Even though GPS receiver knows the precise location of the satellites in space, it still needs to know how far the satellites are so it can determine its position on Earth. GPS works on the principle “Time of Arrival”. Using the basic formula velocity x time travel = distance, we can find the distance from the satellite. The receiver already knows the velocity. Its speed of a radio wave is 186,000miles per second (speed of light), less any delay as the signal travels through the Earth’s atmosphere. Now GPS receiver needs to determine the time part of the formula. The transmitted code is called “Pseudo-random code” because it looks like a noise signal. When a satellite is generating the pseudo-random code, the GPS receiver is generating the same code and tries to match it up to the satellite core. This delay shift (time) is multiplied by the speed of light to get the distance. GPS receiver clock does not keep the time as precisely as the satellite clocks. Putting an automatic clock in your GPS receiver would make it much larger and far too expensive! So each distance measurement needs to be corrected to account for the GPS internal clock error. Hence the range measurement is referred to as “pseudo range”. To determine the position using pseudo-range data, a minimum of 4 satellites must be tracked and 4 fixes must be recomputed until the clock error disappears.
Coming Full Circle: Now that we have satellite location and distance, the receiver can determine a position. Let’s say we are 11000 miles from 1 satellite. Our location would be somewhere on an imaginary sphere that has the satellite in the center with a radius of 11000 miles. Then let’s say we are 12000 miles from another satellite. The second sphere would intersect the first sphere to create a common circle. If we add a third satellite, at a distance of 13000 miles, we now have 2 common points where the 3 spheres intersect. Even though there are 2 possible positions, they differ greatly in latitude/longitude position and altitude. To determine which of the two common points is your actual position; you will need to enter your approximate altitude into the GPS receiver. This will allow the receiver to calculate a 2 dimensional position (latitude, longitude). However, by adding a fourth satellite, the receiver can determine your 3-dimensional position (latitude, longitude, and altitude). Let’s say our distance from a fourth satellite is 10000 miles. We now have a fourth sphere intersecting the first three spheres at one common point.
Sources Of Errors
Civilian GPS receivers have potential position errors due to the result of the accumulated errors due to some of the following sources:
Ionosphere and Troposphere delays
As the ionosphere is dispersive in nature, the delay in the GPS signals is proportional to the inverse of the squared frequency and directly proportional to the refractive index of Total Electron Content (TEC) i.e. the free electrons in a column of 1 m2 cross sectional area centered on the signal path. The TEC in turn depends on the geographic latitude, longitude, local time, season and geomagnetic activity.
Signal multi-path: This occurs when the GPS signal is reflected of objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal there by causing errors.
Receiving Clock errors: Since it is not practical to have an atomic clock in your GPS receiver, the built-in clock can have very slight timing errors.
Number of satellites visible: The more satellites the receiver can “see”, the better the accuracy. Buildings, terrain, electronic interference or sometimes even dense foliage can block the signal reception, causing position errors or possibly no position reading at all. The clearer the view, the better the reception. GPS units will not work indoors (typically), under water, or underground.
Satellite geometry/shading: This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.
Intentional Degradation of the satellite signal: The US military’s intentional degradation of the signal is known as “Selective Ability” (SA) and is intended to prevent military adversaries from using the highly accurate GPS signals. SA accounts for the majority of the error in the range. SA was turned of on May 2nd 2000, and is currently not active. This means you can expect typical GPS accuracies in the range of 6 to 12 meters. However, accuracy can be improved by combining the GPS receiver with a differential GPS (or DGPS) receiver, which can operate from several possible sources to help reduce some of the sources of errors described above.
Differential GPS works by placing a GPS receiver (called a Reference Station) at a known location. Since the reference station knows its exact location, it can determine the errors in the satellite signals. It does this by measuring the ranges to each satellite using the signals received and comparing these measured ranges to the actual ranges calculated from its known position. The difference between the measured and calculated range for each satellite in view becomes a “differential correction “. The differential corrections for each tracked satellite are formatted into a correction message and transmitted to DGPS receivers. These differential corrections are then applied to the GPS receiver’s calculations, removing many of the common errors and improving accuracy. The level of accuracy obtained is a function of the GPS receiver and the similarity of its “environment” to that of the reference station, especially its proximity to the station. The reference station receiver determines the error components and provides corrections to the GPS receiver in real time. Corrections can be transmitted over GM radio frequencies, by satellite, or by beacon transmitters maintained by the U.S. Coast Guard. Typical DGPS accuracy is 1-5 meters (about 3-16 feet).
When we fly, there is one thing we all desire: SAFETY, exceptional positioning information is the key to flight safety. In deterioration weather conditions, when visual navigation is difficult or not possible, we need the best position accuracy possible. Enter the “Wide Area Augmentation System” or simply WAAS. “Wide Area” refers to a network of 25-ground reference stations that cover the entire U.S and some of Canada and Mexico. Implemented by the FAA (Federal Aviation Administration) for aviation users, these 25 reference stations are located at precisely surveyed spots and compare GPS distance measurements to known values. Each reference station is linked to a master station, which puts together a correction message and broadcasts it via satellite. WAAS capable receivers typically have accuracies of 3-5 meters horizontally and 3-7 meters in altitude.
A GPS unit may be just what you need to know where you are and where you are going. GPS units are available with different types of map data. Models vary from having no map, to a base map, to a highly detailed map.
Non mapping Units: GPS units with no map detail have a plotter screen that can show an overhead view of your location relative to any way points, routes, or track logs you have created. The plotter screen will aid kin determining you position in relation to these items. Most GPS receivers will have the ability to show this basic information. Some models have an additional city point database that displays city locations.
Base map Units: A GPS unit with a base map will typically show interstates, U.S. and state highways, and major thoroughfares in metro areas, lakes, rivers, railroads, coastlines, cities, airport locations, and exit information for the federal interstate highway system.
Mapping Units: By stepping up to a unit with the ability to download detailed map data from CD-ROMs, on-screen information really takes a leap forward. Map data may include business and residential streets, restaurants, banks, gas stations, tourist attractions, marine navigational data, boat ramps, topographic detail, off-road trails and much, much more. Using a data cartridge can incorporate map data into the unit either or by downloading the information directly form a CD to the GPS unit. Some units utilize GPS’s pre-programmed G-chart CD, allowing you to select an area of detail to program into the data cartridge. Yet other units can have the data loaded directly into internal memory without the need for data cartridges.
Waypoints: The main purpose of navigation is to be able to get from point A to point B as easily as possible. GPS units can store several hundred points, or locations, called “way points” your house, dock, airport, parked car, a great fishing/hunting spot or even some scenic spots you would like to revisit are just a few examples of the locations you could store and navigate back to later. With GPS receivers, you can even create waypoints of places you have never been to and navigate your way (or GOTO) to that spot.
Track Logs: As you travel along, your GPS unit will automatically record your journey in a “track log”. As you twist and turn along a forest path or through a group of islands, your every movement is being stored in the GPS. If you want to travel back along the same path you came; you can simply activate GPS Track Back feature. When activated, the unit will look at your track log and automatically create a reverse route along your same path, taking you back to where you started. You can even store this information to use over and over again. So you’ll know that you are heading in the right direction!
True and Magnetic North: With direction in mind, you’ll need to determine if you want to use true north or magnetic north references. True north uses the North Pole as a 00 reference, whereas magnetic north uses the Magnetic North pole, which is actually in northern Canada. If you are using your GPS along with a standard compass, you will normally set the GPS to magnetic north. The difference between true and magnetic north at your current location is known as “magnetic variation” (or magnetic declination). GPS receivers have a built-in model or the earth’s magnetic variation and can automatically set the variation for your location anywhere on the planet. You may also choose to set the variation manually using a user-defined north setting.
Position Formats and Grids: Your current location can be viewed in the GPS in the form of coordinates. The most common format is latitude and longitude. On most models, you may choose to change the position format to use other coordinate systems. UTM/UPS (Universal Transverse Mercator/Universal Polar Stereographic) are easy-to-use metric grids that are found on most USGS topographic quadrangle maps. MGRS (Military Grid Reference System) are very similar to UTM/UPS and are used mainly with military maps. Several other grids, including a user-definable grid (for the advanced user), may also be selected on most units.
Map Datum: Maps and charts are essentially grids created from a starting reference point called a datum. Many maps still being used today were originally created decades ago. Over time, technology has allowed us to improve our surveying skills and crate more accurate maps. However, there is still a need to adapt GPS receivers to use with those older maps. Most receivers include over 100 available map datum, which allow you to switch to a setting that matches your map. Using a map datum that does not match the chart you are using can result in significant differences in position information.
Uses Of GPS
GPS has a variety of applications on land, at sea and in the air. Basically GPS allows recording or creating locations from places on the earth and helping you navigate to and from those spots. GPS can be used everywhere except where it is impossible to receive the signal such as inside buildings, in caves, parking garages, and other subterranean locations and under water. The most common air borne applications include Navigation by general aviation and commercial aircraft. Land based applications are more diverse. The scientific community uses GPS for its precision timing capability and a myriad of other applications. Surveyors use GPS for an increasing portion of their work. It offers an incredible cost savings by drastically reducing set up time at the survey site. It also provides amazing accuracy. GPS is becoming increasing popular among snow mobilers, mountain bikers, and cross-country skiers. GPS is rapidly becoming commonplace in automobiles as well. Some systems are already in place providing emergency roadside assistances at the push of a button. More sophisticated systems can show the vehicle’s position on an electronic map display, allowing drivers to keep track of where they are and look up street address, restaurants, hotels and other destinations. Some systems can even automatically create a route and give turn-by-turn directions to a designated location.
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