The Global Positioning System
THE GLOBAL POSITIONING SYSTEM
INTRODUCTION
The Global Positioning System is a space-based triangulation system using satellites and computers to measure positions anywhere on earth. It is first and foremost a defense system currently under development by the United States Department of Defense, and is referred to as the 'Navigation Satellite Timing and Ranging Global Positioning System' or NAVSTAR GPS.
The uniqueness of this navigational system, once it is fully operational, is that it avoids the limitations of other land- based systems such as limited geographic coverage, lack of continuous 24-hour coverage, and the limited accuracies of other related navigational instruments. The high accuracies obtainable with the Global Positioning System also make it a precision survey instrument.
HISTORICAL BACKGROUND
A space-based positioning system has been pursued by the United States Department of Defence and the United States National Aeronautics and Space Administration (NASA) since the early 1960s. One of the early, successful satellite positioning systems was TRANSIT also known as SATNAV. This system was released for commercial use in 1967 and found many applications in surveying and geodesy. An important accomplishment during this early period was the establishment of modern geocentric datums, and the connection of various (local) national datums to a geocentric reference plane.
However, there were limitations to the TRANSIT system which included: the long observation times required to achieve high accuracy; the low earth orbits of the six satellites were affected by local gravity disturbances; radio frequency transmissions were more susceptible to atmospheric delay and disturbances; and the clock technology was not as good as the current NAVSTAR GPS. The TRANSIT system was also very susceptible to small movements at the receiving end resulting in poorer accuracies and limited use in navigation.
The current NAVSTAR Global Positioning System was initiated in 1974 in order to satisfy anticipated military navigation and timing needs. The United States Federal Radio Navigation Plan , developed jointly by the Department of Defense and the Department of Transportation, indicates that the NAVSTAR Global Positioning System will fall under civilian authority once the satellite constellation is complete in 1993.
GLOBAL POSITIONING SYSTEM COMPONENTS
The Global Positioning System Consists of three components: the Space Segment, the User Segment, and the Control Segment (Figure 1, Table 1).
THE SPACE SEGMENT
The Space Segment in its final form will consists of a constellation of 21 satellites and 3 spare satellites, orbiting in 6 planes oriented at 55o to the equator (figure 2). Each orbital plane will contain 4 satellites at an elevation of 26,000 kilometers above the earth.
The 24 satellites form part of the Block II satellite constellation and are produced by Rockwell International, Satellite Systems Division. The Block I constellation, the predecessor to the current GPS Block II constellation, was transitory and experimental in nature with a focus on military applications.
GPS Satellites
Each Block II satellite weighs 844 kilograms and has a design life of 7.5 years (Figure 3). Each satellite is about the size of a large van with each solar panel covering a surface area of 7.2 m2. The satellites contain two rubidium and two cesium atomic clocks, and three nickel-cadmium batteries which provide energy during eclipse periods.
Orbital Configuration
The current GPS satellite constellation consists of 18 satellites. By the end of 1993 the full constellation is expected to be complete with 24 satellites including three spares. With an orbital elevation of 26,000 kilometres, the satellites will have an orbital period of 12 hours such that they complete 2 orbital revolutions within a 24 hour period while the earth rotates 3600. This results in a trace of the satellite orbit on the earth's surface which will repeat itself daily. Thus, the positions of the satellites in the sky at any location can be defined for any particular period of time (figure 4).
Currently the orbits of the satellites are under adjustment and the trace of the satellite orbit on the earth's surface is not fixed. This is caused by a slightly faster orbital period than 12 hours, thus satellites rise 4 minutes earlier each day. In the final constellation the orbital elevation will be increased by 50 kilometres which will synchronize the earth's rotation with the 12 hour orbital period.
Influences on the GPS Satellite Constellation
Other influences which have an effect on the GPS satellites are their relative position to the sun and the moon as the earth orbits the sun (figure 5 and 6). There are two periods each year when each satellite will pass through the earth's shadow region resulting in an eclipse of the sun. This occurs whenever the sun is in or near the satellite's orbital plane. The satellite passes through the shadow region in less than 60 minutes, at which time it is affected by a change in the solar radiation force acting on the satellite, and has to resort to battery power.
Satellite Transmissions
There are two carrier frequencies which carry the GPS signal:
L1: 1575.42 MHz
L2: 1227.60 MHz
There are three types of codes which are carried by the carrier frequencies (figure 7); these are:
P-Code (Precision Code)
C/A Code (Coarse Acquisition Code)
Navigational Code
The precision P-code is the principal code used for navigation by the U.S. military and forms part of the Precise Positioning Service (PPS). It is referred to as the Pseudo-Random Noise (PRN) Code which is generated mathematically. The code consists of a series of digital pulses (figure 7) which are pseudo-random and repeat thenselves over time. Each satellite uses a portion of the pseudo-random code and is identified by a PRN number (ie. PRN 13). Most commercial GPS receivers do not use the P-code.
The coarse-acquisition (C/A) code (also referred to as Clear/Access code) is synchronised with the P-code and forms part of the Standard Positioning Service (SPS). It is used as a medium accuracy navigational signal. The C/A code is also a mathematically generated code consisting of a series of digital pulses (figure 7 and 8). Each satellite transmits mutually exclusive C/A-codes.
The navigational code is a data message which contains information on the position of the satellites (also referred to as the satellite ephemeris), GPS time, clock behavior, and a system status message.
The C/A code is available on the L1 carrier; the P-code is provided on the L1 and L2 carriers; and the navigational code is provided on both the L1 and L2 carriers.
Signals from the satellites are usually observed once the satellite rises 10o above the horizon; this is generally referred to a the 'mask' angle. A 10o mask angle is commonly used, even in areas with good visibility, because the atmospheric effects on the GPS signal within this region are unpredictable. In effect, the GPS signals must travel through a rapidly changing thickness of atmosphere at this shallow angle. This is similar to the colours of sunset and sunrise as sunlight traverses a varying thickness of atmosphere containing dust particles.
CONTROL SEGMENT
The Control Segment is responsible for operating the Global Positioning System. The GPS Master Control Station (also known as the Consolidated Satellite Operations Center - CSOC) is located near Colorado Springs, Colorado. The primary mission of the control segment is to update the navigational message of the satellites. In order to achieve this the Master Control Station has monitoring stations distributed around the world which continuously track the satellites in view. Information on the satellites is then transmitted to the Master Control Station where computations are made and an up-to-date navigation message is uploaded to the satellites; there may be several uploads per day per satellite.
The Master Control Station operates under the specifications of a 3-D positional accuracy of 16 metres with full access to the P- code (for military uses), and 2-D civilian accuracy of 100 metres with degradation of the C/A code by Selective Availability.
Selective Availability
As soon as satellites are added to the operational constellation, the Master Control Station implements a method of control which limits civilian access to the system's full capabilities. This method of control is referred to as 'Selective Availability'. The method involves introducing slowly varying time errors by "dithering" the satellite clock, and by altering the navigational message (the satellite ephemeris).
GPS Services
Two services are provided through the Global Positioning System: for civilian use there is the Standard Positioning Service (SPS) which allows access to the coarse-acquisition (C/A) code and carrier phase frequencies; for military use there is the Precise Positioning Service (PPS) which allows access to the precision (P) code as well as C/A code and carrier frequencies.
USER SEGMENT
GPS Useage
Projections on use of the Global Positioning System are summarized in figure 9 and Table 2. It is expected that there will be widespread national and international civil use of the GPS Standard Positioning Service (SPS). Participation in the development and deployment of the Global Positioning System involve the 10 NATO countries and Australia.
GPS Receivers
GPS receivers are subdivided into the two main areas of usage:
Military Use: Precise Positioning Service (PPS) Receivers are for military use. They are military equipment and require encryption keys to operate at their full level of accuracy. PPS receivers track the precise (P) code on two frequencies (L1/L2) in order to correct for delay of the signal through the upper atmosphere (ionosphere).
Civilian Use: Standard Positioning Service (SPS) Receivers are for civilian use. they are designed to track the coarse- acquisition (C/A) code broadcast by the satellites. The SPS receivers provide 100 metre accuracy under good geometric conditions.
GPS receivers for civilian use can be subdivided into three classes:
Codeless Receivers are designed to measure only the carrier phase (L1 and L2). By making dual frequency measurements of the carrier phase, the delay to the signal caused by the ionosphere can be corrected thereby achieving greater accuracy. These receivers require external information on the satellite ephemerides and an external time synchronization. They are commonly used for precision surveying (1 centimeter, static; <10 centimetres, kinematic).
Hybrid Receivers consist of codeless and code-correlating technology, and are designed to track the C/A code on the L1 carrier frequency, and measure the carrier frequency on L1 or L2. The combination of code-correlating and codeless technology allows the user to obtain moderate to survey precision accuracies.
Code-correlating Receivers track the coarse-acquisition (C/A) code broadcast by the satellites. These receivers vary in the number of 'channels' they have. A separate channel is required for each GPS satellite tracked. In order to compute a navigational fix (latitude, longitude, and elevation) at least four satellites must be tracked. The more channels, the more satellites that can be tracked and the more options available for satellite selection. The receiver cost, weight, and power requirements usually vary in proportion with the number of channels.
The code-correlating GPS receivers can be grouped into two types:
1) Continuous Tracking Receivers
2) Sequencing Receivers
Continuous Tracking GPS Receivers can monitor four or more satellites simultaneously and can give instantaneous position and velocity. Although generally more expensive than sequencing receivers, they have better accuracy.
Sequencing GPS Receivers use a single channel and move it from one satellite to the next to obtain the data necessary to calculate the position.
User Community
The GPS constellation and control segments are scheduled to be fully operational in 1993. When GPS becomes fully operational it is expected that common-use radio-navigation systems within the U.S. military and NATO forces will be phased-out. For both civil aviation and marine radio-navigation it is expected that there will be an integration of GPS with other radio-navigation systems in the near-term. Civilian useage will continue to be limited by the inherrent inaccurracies due to selective availability, however, the use of differential GPS is overcoming these limitations.
How GPS Works
There are five basic principles to understanding how satellites are used to calculated a position on earth (figure 10):
1) The basis for calculating a position on the earth is by triangulation using the location of satellites in orbit.
The principle of triangulation is illustrated in figure 10. One distance measurement would place the GPS receiver somewhere on an imaginary sphere with having an 18,000 kilometre radius; two distance measurements would narrow the location to somewhere on a circle (intersection of the two spheres); three distance measurements would place the GPS receiver in one of two points - one which is on the earth and another which is incorrect and located in outer space.
2) In order to triangulate, the distance between a receiver and a satellite is measured using the travel time of a radio message.
The travel time is determined by measuring the displacement in the digital code arriving from the satellite compared with the same code produced by the GPS receiver. The displacement in the digital code is the time it takes the signal to go from the satellite to the receiver. Multiplying the time difference by the speed of light equals the distance to the satellite. This distance is referred to as 'pseudo-range' because it is not corrected for any asynchronization errors between the receiver clock and GPS time.
3) In order to measure the time it takes a radio message to travel from a satellite to a GPS receiver requires very accurate clocks (atomic clocks).
Since the GPS receiver clocks are not as accurate as those on the satellite, a fourth satellite measurement is required to synchronize the receiver clocks to GPS time. By using information from a fourth satellite it is possible, using trigonometry, to correct for the receiver time offset. The GPS receivers can calculate this error and cancel-out any consistent clock error.
4) The ionosphere and the earth's atmosphere (troposphere) causes the radio message to be delayed, which affects travel time of the GPS signal.
For the ionosphere, the delay of the GPS signal can be measured and modelled accurately, thus, the time delay can be compensated. In the troposphere, the time delay of the GPS signal can be accurately modelled for most gases except water vapour; the tropspheric model is particularly unstable between 0 and 10o above the horizon, which is commonly referred to as the mask angle.
5) Once you know the distance to a satellite, you need to know where the satellite is in space.
This information is provided by the navigation message which contains the satellite ephemerides. A satellite ephemeris is the predicted satellite position as it orbits within the orbital plane above the earth. Due to the high elevation of the satellites, their orbits are clear of the earth's atmosphere and are not strongly influenced by variations in the earth's gravitational field. This means that the orbital path can be easily modelled mathematically. The navigation message sent out by each satellite contains this information and any corrections provided by the monitoring stations. The satellite ephemeris is captured by the GPS receiver as an 'almanac' of satellite positions.
GPS Navigation
GPS receivers can determine a position using two methods (or modes):
3D Mode - 3D (3-dimensional) mode calculates the latitude, longitude and elevation of a position. 3D requires GPS signals to be acquired from four satellites.
2D Mode - 2D (2-dimensional) mode calculates the latitude and longitude of a position, but the elevation has to be known and entered into the GPS receiver to complete the calculation. If the elevation is not accurate, then the latitude and longitude of the position will be inaccurate to a similar order of magnitude. 2D requires GPS signals from three satellites.
GPS Accuracy
Inaccuracies are introduced due to satellite clock error, ephemeris error, GPS receiver error, and atmospheric/ionospheric errors. These errors may result in up to +/-5 metres inaccuracy. More severe errors in accuracy are caused artificially by Selective Availability which can be up to +/-30 metres.
In addition, an important source of error in GPS measurements is the geometric configuration of the satellites in the sky. This source of error is expressed as 'Dilution of Precision' (DOP); Dilution of Precision can be subdivided into five types:
GDOP: Geometric Dilution of Precision
PDOP: Position Dilution of Precision
HDOP: Horizontal Dilution of Precision
VDOP: Vertical Dilution of Precision
TDOP: Time Dilution of Precision
The PDOP is the most commonly displayed in GPS receivers. The basis for this error lies in the fact that the initial measurement errors are compounded in the calculation of position when the satellites are closer together in the sky. If the satellites are spread apart, the geometry they provide is enhanced and the navigational accuracy improves. As a common guideline, GPS positions with PDOP values less than 5 are acceptable, between 5 and 10 should be used with caution, and above 10 will have increasingly poorer and unacceptable accuracies.