How GPS works

In over-simplified terms, we triangulate using radio signals from the satellites to determine our position. Each satellite transmits a radio signal at 1575.42 megahertz; the signal, using Keplerian orbital elements and their rate of change, describes the location of the satellite. Keplerian orbital elements are a series of parameters that describe an object's orbit mathematically. The satellites also transmit a very accurate timing signal.

From a single satellite, we can determine where we are on a sphere because we know the location of the satellite, and by calculating how long it took the radio signals to reach us, we can determine our distance from the satellite.

Using a second satellite, we can determine where we are on a second sphere. The intersection of these two spheres defines a circle. Thus, we know where we are on a circle.

With a third satellite, we define a third sphere. The intersection of these three spheres defines two points. If we know we are on the surface of the earth, then we can assume one of the two points is wrong, and we can use the other. Usually, it is obvious which point is on the earth's surface.

If we are not on the surface of the earth, then we are at one of two points. Using a fourth satellite to define a fourth sphere, we define a single point in three-dimensional space, providing latitude, longitude, and altitude based on the World Geodetic Survey 1984 (WGS-84) Earth-Centered, Earth-Fixed (ECEF) Coordinate System.

Errors

Even with four satellites defining four spheres, DOD guarantees only 100-m horizontal and 156-m vertical accuracy, 95 percent of the time, for SPS. The difference between the horizontal and vertical accuracy is due to the geometry of the satellites. If the satellites were spaced equidistant in angle and range, then the horizontal and vertical accuracy would be equal. However, for this to be the case, at least one satellite would be below the horizon. Since we cannot receive signals from below the horizon, we use only the satellites above the horizon, and we accept an increase in vertical error.

Why isn't SPS more accurate? The primary reasons include intentional degradation by DOD, radio frequency (RF) propagation anomalies, and clock errors.

First, DOD intentionally degrades the accuracy of the Keplerian orbital elements in the interest of national security. The intentional degradation of the signal available for civilian use is called Selective Availability (SA). DOD uses an encrypted signal that provides much better accuracy and timing to ensure that the United States and its allies have a military advantage during times of conflict and tension.

The second error comes from the RF propagation of the radio signals. As the radio signals pass through the atmosphere, they slow down, depending on the density of the atmosphere. This occurs primarily in the troposphere, which extends from the surface of the Earth to approximately 15 km, and the ionosphere, approximately 100 km to 400 km above the Earth. The bent path, which takes into account the density of the ionosphere and the troposphere, appears longer than the straight path. Since the thickness and density of both of these regions of the atmosphere vary greatly from location to location and with the time of day and season of the year, it is very difficult to predict the errors with any certainty.

A third error can be found in the receiver's clock. To calculate the distance from a satellite, you need to know when the satellite transmitted the signal. The difference in the time of transmission and reception multiplied by the speed of light is the distance the radio waves traveled the distance from the satellite to the receiver. The more imprecisely this time difference is known, the greater the potential error. Clocks that are accurate enough to support the highest accuracy available from the system are very expensive and difficult to carry. Thus, accuracy is also a function of how much end users are willing to pay.

User Needs and Requirements

For many applications, SPS is adequate, but there are some applications that require better service. Depending on your application, budget, and available time, you can find a way to make GPS work for you. The federal government has numerous applications, including marine, aviation, railroads, space, and intelligent transportation systems (ITS).

Currently, many marine navigators rely on a complex system of buoys, navigational beacons, and landmarks. With GPS, they can reliably and accurately pinpoint their location on navigational charts.

As an additional aid, GPS can be used to place buoys and other channel markings. A recent example demonstrated GPS effectiveness. The Coast Guard had to reposition a number of channel markings after a hurricane. Normally, this task would take several weeks, but using GPS, the Coast Guard repositioned the buoys in a few days. This allowed commerce in those channels to resume full operation much sooner than previously possible, reducing the storm's impact on many industries that rely on waterborne commerce.

The aviation community may see the most dramatic impacts of GPS. The Federal Aviation Administration (FAA) is developing a system that will guide an aircraft from the time it takes off until it lands. FAA is currently allowing enroute

navigation by GPS and plans to implement a GPS complement that will allow the use of GPS as a sole-means navigational system for enroute operations through precision approach phases of flight. One projected use of GPS is for category III precision approaches. In this application, the aircraft is guided to within a few meters of the runway. Currently, several airports have implemented a special category I differential GPS system that enables specially equipped aircraft to land using GPS. A special category I landing allows an aircraft to approach the runway to not less than 65 m above the touchdown zone with a runway visual range of not less than 550 m.

The rail industry has several requirements, including safety. These range from tracking hazardous cargo to monitoring the location of entire trains on their track network to ensure that they will not collide.

When the rail industry implements their positive train control, which includes positive train separation, it will have an impact that extends beyond safety. With positive train control, the capacity of the existing rail infrastructure can be substantially increased, enabling the existing track to be used more efficiently by increasing the number of trains and the amount of goods transported.

While many partners in the development of ITS are not federal government organizations, ITS is a federally sponsored program with many GPS applications. These applications have a great diversity of requirements from the 1-m accuracy needed for crash-avoidance systems to the 100-m accuracy acceptable for some transit applications. The requirements of ITS, in some cases, require greater accuracy than SPS provides. Specific applications, with their required accuracy, include:

These ranges of performance are very achievable today at a relatively small price compared to just a few years ago.

Other highway applications include accident data collection, highway facility video log inventories, and controlled grading of highway construction sites.

GPS has many other uses. For example, the U.S. Forest Service uses it for photogrammetry to develop very accurate maps. GPS can assist in search and rescue operations by locating a victim or ensuring that an area has been adequately covered. GPS also has applications for surveying. It can be used to survey construction sites, automate site grading, and even monitor the flexion in a dam when the reservoir is filled with water.

Non-federal users also have applications. For example, natural resource companies use it for locating promising deposits of oil, gas, and other resources. In the agriculture industry, it can be used for monitoring crop yields to the square meter, allowing for more accurate applications of insecticides, herbicides, and fertilizers. In addition, it can be used for recreational purposes, such as finding a favorite fishing spot or tracking golf carts.

Augmented SPS

As noted before, many applications require enhancements in service beyond what SPS offers. The three main factors that characterize an improved service are:

To improve these three factors, either improvements to the GPS system need to be implemented or the system needs to be augmented. SPS can be augmented with accuracy corrections, additional ranging signals, and integrity information.

This can be achieved in real time or, for some very accurate survey applications, by using post-processing techniques.

The goal of almost every augmentation system is to improve the accuracy of SPS. This is achieved by broadcasting corrections for the satellite network that negate the effects of SA and compensate for local ionospheric and tropospheric errors. Many of these systems provide 3- to 10-m accuracy; some can provide sub -meter accuracy. Systems that improve accuracy are often referred to as differential systems because they provide the difference between the calculated position and the true position.

A few augmentations will improve availability of the system by providing another GPS-like signal either from another satellite or from a terrestrial location. These systems can improve the availability to 99.999 percent and allow an accuracy of 4 to 7 m.

Almost all augmentation systems provide an integrity or time-to-alarm function with a 2- to 10-second notification time. As mentioned earlier, there may be several hours of delay from the time a problem with the satellite network is first detected until all the GPS satellites can be informed and their databases updated to reflect the change. Most augmentation systems have an independent monitoring network that will detect an error and broadcast it on its own network within seconds after it is detected.

Augmentation Services

Several public and private organizations are currently operating or developing augmentation services. These include: the Department of Transportation (through the Coast Guard and FAA), the U.S. Forest Service, and private industry (through various companies).

The Coast Guard has implemented a series of local area differential stations to cover the harbors, harbor approaches, inland waterways, and coastal waterways of the United States. This coverage should be available by January 1996. There are approximately 50 stations currently in operation, and 11 more should become operational within the next several months. There have been studies that indicate as few as 19 more stations are needed to entirely cover the continental United States. The coverage is advertised as having better than 10-m accuracy, but in reality, it is better than 4-m accuracy. If the stations were networked together and a different ionosphere model was used, accuracy better than 0.5 m could easily be achieved.

The basic structure of the Coast Guard system is shown in figure 7. The reference station (represented by the large tower on the left) and the ship receive at least the same four satellites. The reference station antenna is at a precisely known and surveyed location. Corrections are calculated based on the satellite signals received at the reference station. These corrections are then broadcast to the ship. The ship uses the corrections to determine its location more accurately. Included with the corrections is information relating to the health of both the individual satellites and the satellite systems. This data, integrity or time-to-alarm, is specified to be no older than 6 seconds. The information is broadcast to a local area, and the system is referred to as a Local Area Differential GPS (LADGPS) system.

For their requirements, FAA needs at least one additional satellite signal to achieve the required availability. FAA covers a very large area because it must supply information to aircraft flying over international waters in U.S.controlled airspace.

Navigating the Future

FAA has chosen to implement their system by using geostationary satellites and transmitting the signal down to its users on the frequency used by the GPS satellites. FAA uses a series of ground stations to develop the corrections and uplink them to the satellite, which will then broadcast them to users on L1, the GPS downlink frequency. This system will provide 4- to 7-m accuracy with an availability of 99.999 percent. The geostationary satellite-based system is referred to as the Wide Area Augmentation System (WAAS).

This system will not meet all of FAA's requirements. For category III landings, the requirement is 1.6-m vertical and horizontal accuracy. To achieve this, FAA is planning to implement a network of local area differential GPS stations that will provide the information to aircraft. This system is referred to as the Local Area Augmentation System (LAAS).

DOT is currently working with the National Geodetic Survey to develop an integrated national augmentation system for federal users. The first step in identifying what this system could entail has been completed. For more information on this, please see reference 2.

As for private services, there are many. Some use a frequency modulation (FM) subcarrier to transmit corrections to a local area. To use their service, users need to purchase a receiver from them and pay an annual subscription fee. In many metropolitan areas, these companies provide good, reliable coverage, advertising sub-meter accuracy. Unfortunately, they cannot provide the reliability or coverage needed by many federal users, including FAA for aviation and the Coast Guard for marine navigation.

Another segment of the industry uses geostationary satellites as the communication link to users. These systems typically consist of a number of reference stations located around the country, networked through a centralized satellite uplink facility. The main difference between these systems and FAA's WAAS is the downlink frequency. These systems also advertise sub-meter accuracy.

Future Developments

There are several ongoing efforts within the Department of Transportation and other federal agencies that will improve the performance of SPS well beyond what is available today. These efforts include high-level technical and policy discussions concerning:

Summary

Technology is available today to meet many federal and private industry requirements.

The advances that are taking place within the GPS industry are nothing short of phenomenal. In just a few years, we have seen receivers drop from tens of thousands of dollars to just hundreds. It is difficult to say what will happen in the next few years, but the technology advancements do not seem to be slowing. It may be possible that in the near future we will have realtime, meter or even sub-meter accuracy everywhere in the world.

References

1. Report of the Joint DOD/DOT Task Force, The Global Positioning System: Management and Operation of a Dual Use System, a report to the Secretaries of Defense and Transportation, December 1993.

2. A Technical Report to the Secretary of Transportation on a National Approach to Augment GPS Service, U.S. Departments of Commerce and Transportation, Washington, D.C., December 1994.