Trimble Navigation, 1999. All About GPS

Global Positioning Systems (GPS) is a satellite based navigation system for identifying position (X, Y, and Z) on the earth’s surface. Twenty-four satellites orbit the earth, ensuring that there are between four and eight satellites (four is the minimum) available at anytime or place on the earth's surface. Position is determined by measuring the distance from a point on the earth’s surface to the orbiting satellites.

 
Overview of GPS Technology

Introduction

GPS was developed, and is controlled, by the Department of Defense (DOD), primarily as a result of the missile buildup during the cold war. Over 12 billion dollars was spent developing the system, and now GPS is available for the general public to use. Indeed, GPS has become very popular in the public domain, particularly since the early 90s. The increase in public usage of GPS can be related to several factors, including a reduction in price and an increase in quality, the large demand for knowledge about our environment, and the DODs commitment to invest billions of dollars in a missile defense strategy. In fact, it was only a few years ago that the 24^th satellite was put into orbit by the Air Force, enabling GPS to be invoked at any time or place on the earth's surface. However, perhaps the single, all inclusive reason for the growth and proliferation of GPS has been a very fundamental issue that has always confronted human beings: the desire to know our location in space (we, as Geographers can appreciate this!). This is perhaps the manifest destiny of GPS technology: a single, burning question that has pushed technology in the direction of creating an affordable and reliable system. Thus, today the general public can obtain a quality GPS unit for a few hundred dollars!.
 

GPS Satellite System

The modern GPS system consists of twenty-four satellite vehicles (SV) in continuous orbit. Each SV was carefully put into orbit by the Air Force, approximately 20,200 km above the earth’s surface. Additionally, the satellites orbit within six equally spaced orbital planes, with four satellites within each plane. The high altitude of the satellites greatly reduces the amount of distortion and interference that would occur from the earth’s atmosphere. This insures the SVs stay on track, and follow very predictable orbital paths. A network of six ground-based control stations monitors the health of each SV. Each control station checks for errors in the SVs orbital paths, which are caused by fluxes in solar radiation and the gravitational pull of the sun and the moon. Twice daily the control stations transmit an ephemeris code to each SV, which is a model of their orbital path, insuring that the system of twenty-four SVs are correctly positioned. Each SV then broadcasts the ephemeris code to the ground-based receivers (your hand-held GPS units) with its unique Pseudo Random Noise (PRN) Code. The ephemeris code may then be used by the GPS receivers, or a PC, for correcting positional errors.

 
GPS Satellite Signals

The key to GPS is that each satellite transmits its own PRN, which allows for each satellite to be uniquely identified by the GPS receivers on earth. The PRN code has a low frequency (similar to radio waves) that does not send very much information. This allows the PRN to be received by relatively low power receivers, and is one of the features of modern GPS units that make them so portable. The PRN signal is divided into time periods, referred to as chips. There are two types of PRN codes: the coarse acquisition code (C/A), which is used by civilian receivers, and the P code, a slightly more powerful code that is reserved exclusively for military purposes. The fundamental distinction between the military and civilian codes is that the C/A code is interfered with by the military in a procedure referred to as Selective Availability (S/A). S/A creates a time uncertainty within the PRN, and is the single greatest source of error in civilian GPS. Effectively, it increases the error in position estimates from the order of centimeters to tens of meters. The main reason cited for S/A is one of defense: the military does not want our enemies to be able to utilize our GPS system to direct missiles at the U.S. The unique PRN code is transmitted as time-tagged data bit frames that enable the time of transmission of each PRN code to be exactly determined. Each SV repeatedly transmits its data code at precisely the same time that the other satellites in the system transmit their own unique PRN code. To insure that the SVs each transmit their satellites at exactly the same time, they are synchronized by a set of atomic clocks. Although only one atomic clock is necessary, each SV carries four atomic clocks to prevent against failure. In essence, if the atomic clocks were to go bad, the SV would be lost from the system, and that would jeopardize GPS. Atomic clocks are the most precise clocks ever built. They are not actually fueled by atomic energy; but instead rely on the known oscillation of a particular atom as a metronome to keep time. The atomic clocks are also very expensive, with each costing over $100,000. The timing of the atomic clocks are continuously monitored and adjusted by the ground-based control stations, which transmit corrections to insure that all of the SVs are synchronized.
 

Using GPS to Determine Position (X,Y, Z)

Obtaining a position on the earth’s surface with GPS is based on satellite ranging, which means that our position is determined by measuring the distance to a group of moving satellites. This is made possible because we know the exact altitude of each SV and the time that the PRN was transmitted. In essence, GPS works by measuring how long it takes a radio signal from a satellite traveling at a known rate (186,000 miles per second) to reach our GPS receiver, and then calculating the distance from the time. Thus, distance is equal to velocity x travel time. For example, if a car leaves a specific point traveling at 60 miles an hour and travels for two hours it has traveled a distance of 120 miles (60 x 2 = 120 mi). Therefore, because we know: 1. The exact orbital path of each SV, 2. The speed of the PRN (186,000 mph), and 3. Exactly when it was transmitted from the SV, the distance between each SV and a point on earth can easily be calculated for each unique PRN code received by a our ground-based GPS units. However, although knowing the distance from a point on the earth’s surface to a single satellite greatly reduces the possibility of where a point could be located, we need several SVs so that we can accurately pinpoint a location. Although theoretically knowing the distance to three SVs allows us to triangulate our position, in practice we need a fourth SV so that we can eliminate the chance for error.
 

Sources of Error

There are several sources of error common to most GPS. One of the most significant is from the earth’s ionosphere, a blanket of electrically charged particles 140-200 km above the earth’s surface. Ionospheric particles slow down the PRN signals transmitted from the SVs. Other sources of error include the atomic clocks (not believed to be a major problem), errors inherent within the GPS unit due to rounding of numbers, and multipath errors. Multipath errors refer to the deflection of the GPS signal as it travels through the lower atmosphere. Additional sources of error include Geometric Dilution of Precision (GDOP), which causes uncertainty due to the angle PRN code transmitted by the SVs. A good GPS unit will select the SVs that occur at the optimum angle. Finally, the most common and troublesome source of error is due to SA. Collectively, these errors can result in a great deal of uncertainty for GPS measurements, although their severity can be greatly reduced through a procedure known as Differential Correction.