Introduction to the Global Positioning System GPS

What is GPS?

The Global Positioning System (GPS) is a location system based on a constellation of 24 satellites orbiting

the earth at altitudes of approximately 17,000 km. GPS was developed by the United States Department of Defense (DOD), for its tremendous application as a military locating utility.

Over the past several years, GPS has proven to be a useful tool in non-military mapping applications as well. GPS satellites are orbited high enough to avoid the problems associated with land based systems, yet can provide accurate positioning 24 hours a day, anywhere in the world. Uncorrected positions determined from

GPS satellite signals produce accuracies in the range of 50 to 100 meters.

Trilateration - How GPS Determines a Location

In a nutshell, GPS is based on satellite ranging - calculating the distances between the receiver and the position of 3 or more satellites (4 or more if elevation is desired) and then applying some mathematics. Assuming the positions of the satellites are known, the location of the receiver can be calculated by determining the distance from each of the satellites to the receiver. GPS takes these 3 or more known references and measured distances and “triangulates” an additional position.

As an example, assume that I have asked you to find me at a stationary position based upon a few clues

which I am willing to give you. First, I tell you that I am exactly 10 km away from your house. You would know I am somewhere on the perimeter of a sphere that has an origin as your house and a radius of 10 km. With this information alone, you would have a difficult time to find me since there are an infinite number of locations on the perimeter of that sphere.

Second, I tell you that I am also exactly 12 km away from the office building. Now you can define a second sphere with its origin at the store and a radius of 12 km. You know that I am located somewhere in the space where the perimeters of these two spheres intersect - but there are still many possibilities to define my location.

Adding additional spheres will further reduce the number of possible locations. In fact, a third origin and distance (I tell you am 8 km away from the City Clock) narrows my position down to just 2 points. By adding one more sphere, you can pinpoint my exact location. Actually, the 4th sphere may not be necessary. One of the possibilities may not make sense, and therefore can be eliminated.

For example, if you know I am above sea level, you can reject a point that has negative elevation. Mathematics and computers allow us to determine the correct point with only 3 satellites.

Based on this example, you can see that you need to know the following information in order to compute your position:

A) What is the precise location of three or more known points (GPS satellites)?

B) What is the distance between the known points and the position of the GPS receiver?

How the Current Locations of GPS Satellites are Determined

GPS satellites are orbiting the Earth at an altitude of 17,000 km. The DOD can predict the paths of the satellites vs. time with great accuracy. Furthermore, the satellites can be periodically adjusted by huge landbased radar systems. Therefore, the orbits, and thus the locations of the satellites, are known in advance. Today's GPS receivers store this orbit information for all of the GPS satellites in what is known as an almanac. Think of the almanac as a "bus schedule" advising you of where each satellite will be at a particular time. Each GPS satellite continually broadcasts the almanac. Your GPS receiver will automatically collect this information and store it for future reference.

The Department of Defense constantly monitors the orbit of the satellites looking for deviations from predicted values. Any deviations (caused by natural atmospheric phenomenon such as gravity), are known as ephemeris errors. When ephemeris errors are determined to exist for a satellite, the errors are sent back up to that satellite, which in turn broadcasts the errors as part of the standard message, supplying this information to the GPS receivers.

By using the information from the almanac in conjuction with the ephemeris error data, the position of a GPS satellite can be very precisely determined for a given time.

Computing the Distance Between Your Position and the GPS Satellites

GPS determines distance between a GPS satellite and a GPS receiver by measuring the amount of time it

takes a radio signal (the GPS signal) to travel from the satellite to the receiver. Radio waves travel at the

speed of light, which is about 300,000 km per second. So, if the amount of time it takes for the signal to

travel from the satellite to the receiver is known, the distance from the satellite to the receiver (distance = speed x time) can be determined. If the exact time when the signal was transmitted and the exact time when it was received are known, the signal's travel time can be determined.

In order to do this, the satellites and the receivers use very accurate clocks which are synchronized so that they generate the same code at exactly the same time. The code received from the satellite can be compared with the code generated by the receiver. By comparing the codes, the time difference between when the satellite generated the code and when the receiver generated the code can be determined. This interval is the travel time of the code. Multiplying this travel time, in seconds, by 300,000 km per second gives the distance from the receiver position to the satellite in km.

Four (4) Satellites to give a 3D position

In the previous example, it took only 3 measurements to "triangulate" a 3D position. However, GPS needs a 4th satellite to provide a 3D position. Why??

Three measurements can be used to locate a point, assuming the GPS receiver and satellite clocks are precisely and continually synchronized, thereby allowing the distance calculations to be accurately determined. Unfortunately, it is impossible to synchronize these two clocks, since the clocks in GPS receivers are not as accurate as the very precise and expensive atomic clocks in the satellites. The GPS signals travel from the satellite to the receiver very fast, so if the two clocks are off by only a small fraction, the determined position data may be considerably distorted.

 The atomic clocks aboard the satellites maintain their time to a very high degree of accuracy. However, there will always be a slight variation in clock rates from satellite to satellite. Close monitoring of the clock of each satellite from the ground permits the control station to insert a message in the signal of each satellite which precisely describes the drift rate of that satellite's clock. The insertion of the drift rate effectively synchronizes all of the GPS satellite clocks.

The same procedure cannot be applied to the clock in a GPS receiver. Therefore, a fourth variable (in addition to x, y and z), time, must be determined in order to calculate a precise location. Mathematically, to solve for four unknowns (x,y,z, and t), there must be four equations. In determining GPS positions, the four equations are represented by signals from four different satellites.

The GPS Error Budget

The GPS system has been designed to be as nearly accurate as possible. However, there are still errors.

Added together, these errors can cause a deviation of 50 -100 meters from the actual GPS receiver position. There are several sources for these errors, the most significant of which are discussed below:

Atmospheric Conditions

The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in

the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from "Signal Speed x Time" will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space.

Ephemeris Errors/Clock Drift/Measurement Noise

As mentioned earlier, GPS signals contain information about ephemeris (orbital position) errors, and about the rate of clock drift for the broadcasting satellite. The data concerning ephemeris errors may not exactly model the true satellite motion or the exact rate of clock drift. Distortion of the signal by measurement noise can further increase positional error. The disparity in ephemeris data can introduce 1-5 meters of positional error, clock drift disparity can introduce 0-1.5 meters of positional error and measurement noise can introduce0-10 meters of positional error.

Selective Availability

Ephemeris errors should not be confused with Selective Availability (SA), which is the intentional alteration of the time and epherimis signal by the Department of Defense. SA can introduce 0-70 meters of positional error. Fortunately, positional errors caused by SA can be removed by differential correction.

Multipath

A GPS signal bouncing off a reflectilve surface prior to reaching the GPS receiver antenna is referred to as multipath. Because it is difficult to completely correct multipath error, even in high precision GPS units, multipath error is a serious concern to the GPS user.

The chart below lists the most common sources of error in GPS positions. This chart is commonly known as the GPS Error Budget:

GPS Error Budget

Source                         Uncorrected Error Level

Ionosphere                   0-30 meters

Troposphere                 0-30 meters

Measurement                Noise 0-10 meters

Ephemeris Data            1-5 meters

Clock Drift                    0-1.5 meters

Multipath                       0-1 meter

Selective Availability       0-70 meters

Measuring GPS Accuracy

As discussed above, there are several external sources which introduce errors into a GPS position. While

the errors discussed above always affect accuracy, another major factor in determining positional accuracy is the alignment, or geometry, of the group of satellites (constellation) from which signals are being received. The geometry of the constellation is evaluated for several factors, all of which fall into the category of Dilution of Precision, or DOP.

DOP

DOP is an indicator of the quality of the geometry of the satellite constellation. Your computed position can vary depending on which satellites you use for the measurement. Different satellite geometries can magnify or lessen the errors in the error budget described above. A greater angle between the satellites lowers the DOP, and provides a better measurement. A higher DOP indicates poor satellite geometry, and an inferior measurement cofiguration.

Some GPS receivers can analyze the positions of the satellites available, based upon the almanac, and

choose those satellites with the best geometry in order to make the DOP as low as possible. Another important GPS receiver feature is to be able to ignore or eliminate GPS readings with DOP values that exceed user-defined limits. Other GPS receivers may have the ability to use all of the satellites in view, thus minimizing the DOP as much as possible.

Levels of GPS Accuracy

There are three types of GPS receivers which are available in today's marketplace. Each of the three types offers different levels of accuracy, and has different requirements to obtain those accuracies. To this point, the discussion in this book has focused on Coarse Acquisition (C/A code) GPS receivers. The two remaining types of GPS receiver are Carrier Phase receivers and Dual Frequency receivers.

C/A Code receivers

C/A Code receivers typically provide 1-5 meter GPS position accuracy with differential correction. C/A Code GPS receivers provide a sufficient degree of accuracy to make them useful in most GIS applications. C/A Code receivers can provide 1-5 meter GPS position accuracy with an occupation time of 1 second. Longer occupation times (up to 3 minutes) will provide GPS position accuracies consistently within 1-3 meters. Recent advances in GPS receiver design will now allow a C/A Code receiver to provide sub-meter accuracy, down to 30 cm.

Carrier Phase receivers

Carrier Phase receivers typically provide 10-30 cm GPS position accuracy with differential correction. Carrier Phase receivers provide the higher level of accuracy demanded by certain GIS applications.

Carrier Phase receivers measure the distance from the receiver to the satellites by counting the number of waves that carry the C/A Code signal. This method of determining position is much more accurate; however, it does require a substatially higher occupation time to attain 10-30 cm accuracy. Initializing a Carrier Phase GPS job on a known point requires an occupation time of about 5 minutes. Initializing a Carrier Phase GPS job on an unknown point requires an occupation time of about 30-40 minutes.

Additional requirements, such as maintaining the same satellite constellation throughout the job, performance under canopy and the need to be very close to a base station, limit the applicability of Carrier Phase GPS receivers to many GIS applications.

Dual-Frequency receivers

Dual-Frequency receivers are capable of providing sub-centimeter GPS position accuracy with differential correction. Dual-Frequency receivers provide "survey grade" accuracies not often required for GIS applications.

Dual-Frequency receivers receive signals from the satellites on two frequencies simultaneously. Receiving GPS signals on two frequencies simultaneously allows the receiver to determine very precise positions.

GPS and Canopy

GPS receivers require a line of sight to the satellites in order to obtain a signal representative of the true

distance from the satellite to the receiver. Therefore, any object in the path of the signal has the potential to interfere with the reception of that signal. Objects which can block a GPS signal include tree canopy, buildings and terrain features.

Further, reflective surfaces can cause the GPS signals to bounce before arriving at a receiver, thus causing an error in the distance calculation. This problem, known as multipath, can be caused by a variety of materials including water, glass and metal. The water contained in the leaves of vegatation can produce multipath error. In some instances, operating under heavy, wet forest canopy can degrade the ability of a GPS receiver to track satellites.

There are several data collection techniques which can mitigate the effects of signal blockage by tree canopy or other objects. For example, many GPS receivers can be instructed to track only the highest satellites in the sky, as opposed to those satellites which provide the best DOP. Increasing the elevation of the GPS antenna can also dramatically increase the ability of the receiver to track satellites. Unfortunately, there will be locations where GPS signals simply are not available due to obstruction. In these cases, there are additional techniques which can help to solve the problem. Some GPS receivers have the ability to collect an offset point, which involves recording a GPS position at a location where GPS signals are available while also recording the distance, bearing and slope from the GPS antenna to the position of interest where the GPS signals are not available. This technique is useful for avoiding a dense timber stand or building.

Further, a traditional traverse program can be used to manually enter a series of bearings and ranges to

generate positions until satellite signals can again be received. This position data can then be used to augment position data collected with the GPS receiver.

Glossary of GPS Terms

- A -

Almanac - the Almanac is a file which contains positional information for all of the GPS satellites. The Almanac is used by the GPS receiver to determine which satellites to track, and can also be used for mission planning.

- C -

C/A Code - the standard (Course/Acquisition) GPS code used by most GIS level GPS receivers. Also known as the civilian code.

Carrier - the signal that carries the C/A Code from the satellite to the GPS receiver.

Carrier-aided Tracking - a signal processing technique that uses the GPS carrier signal to achieve an exact

lock on the pseudo random code generated by the GPS satellite. Carrier-aided tracking is more accurate

than standard C/A Code tracking.

Channel - a channel of a GPS receiver consists of the circuitry necessary to track the signal from a single

GPS satellite.

Cycle Slip - a loss of continuity in the measured carrier beat phase which results from a temporary loss of

lock on a GPS satellite.

- D -

Differential Correction - the technique of comparing GPS data collected in the field to GPS data collected at a known point. By collecting GPS data at a known point, a correction factor can be determined and applied to the field GPS data.

Dilution of Precision (DOP) - an indicator of satellite geometry for a unique constellation of satellites used to determine a position. Positions tagged with a higher DOP value generally constitute poorer measurement results than those tagged with lower DOP.

Dynamic Positioning - the process of collecting GPS data while the GPS antenna is in motion. Often associated with Line or Area Features.

- E -

Ephemeris - the predicted changes in the orbit of a satellite that are transmitted to the GPS receiver from the individual satellites.

Ephemeris Errors - errors which originate in the ephemeris data transmitted by a GPS satellite. Ephemeris errors are removed by differential correction.

- G -

Geographic Information System (GIS) - a mapping system which combines positional data with descriptive information to form a layered map.

Global Positioning System (GPS) - a system for providing precise location which is based on data transmitted from a constellation of 24 satellites

- L -

L-band - the group of radio frequencies which carry the GPS data from the satellites to the GPS receivers.

- M -

Multipath - the interference to a signal that has reached the receiver antenna by multiple paths; usually

caused by the signal being bounced or reflected. Signals from satellites low on the horizon will have high

multipath error. Receivers that can be configured to "mask out" signals from such satellites can help minimize multi-path.

- P -

Pseudorange - an uncorrected measurement of the distance between a GPS satellite and a GPS receiver

determined by comparing a code transmitted by the satellite to a code generated by the receiver.

- R -

Residual - a quality indicator for a GPS position that is determined during the differential correction process. Indicates uncorrectable error. High residuals are not desirable.

- S -

Satellite Constellation - the group of GPS satellites from which data is used to determine a position.

Static Positioning - the process of averaging GPS positions taken successively over a period of time with a stationary antenna to increase accuracy.