Sunday, June 9, 2013

An Overview of Global Positioning System: GPS

Muhammad Nisar Yousuf
Divisional Engineer (Codevtel)
Telecommunication Staff College(TSC), BTCL

GPS is a constellation of satellites owned and managed by the US DoD. There are over 30 SVs (space vehicles) in 55 degree orbits on 6 different planes (spaced 30 degrees apart). They are constantly broadcasting a number of different signals, only one of which is used by consumer GPS receivers (the L1). Your GPS receiver does its best to determine how long it took that signal to travel to it from the satellite. By determining time of travel it then determines how far away it is from the satellite. By doing this for multiple SVs it can trilaterate (not triangulate) your location.

--- GPS:  Global Positioning System is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.
---Uses the principle of triangulation and time- of-arrival of signals to determine the location  of a GPS receiver.
Components of the System of GPS :

(1)   Control Segment:  five ground stations located on earth.
(2)   Space Segment:  satellite constellation (24 active satellites in space).
(3)   User Segment: GPS receiver units that receive satellite signals and determine receiver location from them.
GPS Applications

(1)   Location - determining a basic position (2)  Navigation - getting from one location to another (3) Tracking - monitoring the movement of people and things. (4)  Mapping - creating maps of the world  (5)  Timing - bringing precise timing to the world


The Global Positioning System consists of a network of 24 broadcasting satellites orbiting the earth at a height of 20,200km. GPS also consists of receivers on the ground, which listen to and interpret the transmissions of the satellites. Stations (the Ascension Island, Diego Garcia and Kwajalein monitor stations) on the earth carefully monitor the orbit of each satellite, maintaining a highly accurate record of the satellites instantaneous position. The knowledge of the precise position of the satellites allows them to be used as reference points, from which GPS receivers on earth can determine their position. This technique of determining the position of an object is called ranging.

RANGING principles:
The concept of ranging is best illustrated by example. Consider one satellite that is a distance of 25,000 kilometers from a person holding a GPS receiver. Then the person's position is known to be somewhere on a sphere 25,000 km in radius, centered on the satellite. However, the exact location of the person on that sphere is yet unknown. If, at the same time, the distance from the person to a second satellite can be discovered to be 20,000 km, then a second sphere of radius 20,000 km on which the person is positioned can be determined. Thus the person must be on the circle formed by the intersection of the two spheres of position. A third satellite provides yet a third sphere, which narrows down the location of the person to exactly two points. One of these points is often an impossible solution, frequently several thousand kilometers off in space, thus three satellite ranges can determine the precise position of the person. Three satellites provide enough information to find the x, y, and z coordinates (measured from the center of mass of the earth). However, in practice, four satellites are required to pinpoint a position, for reasons that will soon become clear.
Resecting places using Satellites
Intersection of three imaginary spheres: sphere concept
In the above model of ranging, the distance between one satellite and the person on earth is given to be 20,000 km. However, no mention was made as to how the distance was determined. The Global Positioning System works by having each of the 21 active satellites constantly radiate microwaves. These microwaves are received by the GPS receiver, which can use the method of ranging to locate its position. The distance from the receiver to one satellite is measured in the following way. The satellite and receiver are controlled by separate clocks. The satellites are set as accurately as possible with an atomic clock, and are assumed to be synchronized with one another. At some known time a satellite emits a signal in the form of microwaves. This signal reaches the receiver after a certain interval has passed. Since microwaves travel at the speed of light, a known velocity and a known time allows the receiver to determine the distance to the satellite. Thus it is important that the time be measured precisely in order to accurately measure distance, as an error of the synchronization of the two clocks of one microsecond creates an error of 300 meters. This requires a fourth satellite, since a fourth variable, time, has been added to the unknowns who previously included only the x, y, and z distances.
Basic Functions of Monitor Stations
l  These stations are the eyes and ears of GPS, monitoring satellites as they pass overhead by measuring distances to them every 1.5 seconds
l   This data is then smoothed using ionospheric and  meteorological information and sent to Master Control Station at Colorado Springs. 
l  The ionospheric and  meteorological data is needed to get more accurate delay measurements, which in turn improve location estimation.
l  Master control station estimates parameters describing satellites' orbit and clock performance,.  It also assesses health status of the satellites and determines if any re-positioning may be required.
l    This information is then returned to three uplink stations (collocated at the Ascension Island, Diego Garcia and Kwajalein monitor stations) which transmit the information to satellites.
Triangulation Requirements
l  To triangulate, a GPS receiver measures distance using the travel time of radio signals.
l  To measure travel time, GPS receiver needs very accurate timing.
l  Along with distance, receiver needs accurate data on where satellites are in space.
l  System will also need to correct for any delays the signal experiences as it travels through atmosphere.
Space Segment
l  Space segment is the satellite constellation.
l  24 satellites with a minimum of 21 operating 98% of the time
l  6 Orbital planes
l  Circular orbits
l  20-200 km above the Earth's surface
l  11 hours 58 minute orbital period
l  Visible for approximately 5 hours above the horizon
l  Orbits of GPS satellites need to be updated every once in a while because orbit does not stay circular without adjustments.
l  Adjustments needed because:
l  Other objects exert gravitational force on each satellite (e.g. sun, moon)
l  Effect of  gravity is non-uniform during orbit.
l  Radiation pressure (due to solar radiation).
l  Atmospheric drag
l  Other effects
User Segment
l  User segment comprises receivers that have been designed to decode signals transmitted from satellites for purposes of determining position, velocity or time.
l  Receiver must perform the following tasks: 
(a) select one or more satellites in view (b) acquire GPS signals (c) measure and track signal  (d) recover navigational data
Important Terminology
l  Satellite transmits Ephemeris and Almanac Data to GPS receivers.
l  Ephemeris data contains important information about status of satellite (healthy or unhealthy), current date and time. This part of signal is essential for determining a position.
l  Almanac data tells GPS receiver where each GPS satellite should be at any time throughout day. Each satellite transmits almanac data showing orbital information for that satellite and for every other satellite in the system.
Measuring Time Of Arrival (TOA) in GPS
TOA Concept
l  GPS uses concept of time of arrival (TOA) of signals to determine user position.
l  This involves measuring time it takes for a signal transmitted by an emitter (satellite) at a known location to reach a user receiver.
l   Time interval is basically signal propagation time. 
l  Time interval (signal propagation time) is multiplied by speed of signal (speed of light) to obtain satellite to receiver distance.

l   By measuring propagation time of signals broadcast from multiple satellites at known locations, receiver can determine its position.
l  Assuming we have precise clocks, how do we measure signal travel time?
Measuring Distance using a PRC Signal
l  At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code (PRC).
l  The receiver begins running the same digital pattern also exactly at midnight.
l  When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.
Measuring Distance
l  The length of the delay is equal to the signal's travel time.
l  The receiver multiplies this time by the speed of light to determine how far the signal traveled.
l  Assuming the signal traveled in a straight line, this is the distance from receiver to
Synchronizing Clocks
l  In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond.
l  Accurate time measurements are required.  If we are off by a thousandth of a second, at the speed of light, that translates into almost 200 miles of error.
l  To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself.
l  But atomic clocks cost somewhere between $50,000 and $100,000, which makes them a just a bit too expensive for everyday consumer use.
l  The Global Positioning System has a clever, solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets.
l  The Global Positioning System has a clever, effective solution to this problem.
l  Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets.
l  In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy.
l  When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point.
l  Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly.
l  Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.
l  The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point.
l  Based on this, it resets its clock to be in sync with the satellite's atomic clock.
l  The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.
l  The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point.
l  Based on this, it resets its clock to be in sync with the satellite's atomic clock.
l  The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.
Knowing Satellite Locations
l  In order to properly synchronize clocks and figure out which PRC signal to listen to, the receiver has to know where the satellites actually are.
l  This isn't particularly difficult because the satellites travel in very high and predictable orbits.
Using Almanac Information
l  The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time.
l  Things like the pull of the moon and the sun do change the satellites' orbits very slightly.
l  However, the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.
2 Types of Errors
l  Errors can be categorized as intentional and unintentional.
l  (1) Intentional errors:  government can and does degrade accuracy of GPS measurements.  This is done to prevent hostile forces from using GPS to full accuracy. (2) Policy of inserting inaccuracies in GPS signals is called Selective Ability (SA). SA was single biggest source of inaccuracy in GPS.  SA was deactivated in 2000.
Sources of Unintentional Timing Errors : (Typical Errors )
Source of Error
Satellite Clocks
Orbit Errors          
Receiver Noise    
Typical Error in Meters
(per satellite)
Differential GPS
l  Technique called differential correction can yield accuracies  within 1-5 meters, or even better, with advanced equipment.
l  Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely  known point.
l   Because physical location of base station is known, a correction factor can be computed by comparing known location with GPS location determined by using satellites.
Improved Offered by Differential GPS
Source             Uncorrected                 With Differential
Ionosphere                   0-30 meters                  Mostly Removed
Troposphere                0-30 meters                  All Removed
Signal Noise                 0-10 meters                  All Removed
Orbit Data                    1-5 meters                    All Removed
Clock Drift                  0-1.5 meters                 All Removed
Multipath                     0-1 meters                    Not Removed
Receiver Noise               ~1 meter                    Not Removed
SA                               0-70 meters                  All Removed
Using GPS Data
l  A GPS receiver essentially determines the receiver's position on Earth.
l  Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude of its current position. To make the navigation- more userfriendly, most  receivers   plug this raw data into map files stored in  memory.
l  You can
l  use maps stored in the receiver's memory,
l  connect the receiver to a computer that can hold more detailed maps in its memory, or
l  simply buy a detailed map of your area and find your way using the receiver's latitude and longitude readouts.
l  Some receivers let you download detailed maps into memory or supply detailed maps with plug-in map cartridges.
l  A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move.
l  If you leave your receiver on, it can stay in constant communication with GPS satellites to see how your location is changing.
l  This is what happens in cars equipped with GPS.
    With this information and its built-in clock, the receiver can give you several pieces of valuable information:   
(A) How far you've traveled (odometer) ,  (B) How long you've been traveling, (C)  Your current speed (speedometer) , (D)  Your average speed  , (E)  A "bread crumb" trail showing you exactly where you have traveled on the map , (F)  The estimated time of arrival at your destination if you maintain your current speed .