GPS Primer

This appendix is a quick GPS primer for anyone who wants to know a little more about the GPS network and how it works; and how the

little plastic box they have in their hand is capable of giving them such a tremendously accurate positional fix anywhere on the globe.

The GPS Network

The GPS network consists of three distinct levels, or segments:

Space segment

Control segment

User segment

The Space Segment

The space segment consists of around 30 NAVSTAR satellites (also known as Space Vehicles, or SVs for short). The exact number varies, but is normally between 27 and 30. These satellites are the property of the U.S. Department of Defense and are operated and controlled by the 50th Space Wing, located at Schriever Air Force Base, Colorado.

NAVSTAR is an acronym for NAVigation Satellite Timing And

Ranging.

Of these 30 SVs, about 24 are active, and three are kept as spares in case of problems with any of the others.

The spare satellites are positioned so that they can be quickly moved to the appropriate orbit in the event of a failure of one of the operational satellites. Satellites that are not working properly are considered sick, and you may occasionally notice such a satellite oddly labeled on your GPS screen (its icon might appear gray or the lock-on bar may show a good signal but no lock). This is likely to be during testing when the Department of Defense deliberately marks a “healthy” satellite as “sick” to see how the system copes.

The 24 operational satellites are arranged in six orbital planes around the Earth, with four satellites in each plane. The satellites have a circular orbit of 20,200 km (10,900 nm), and these orbits are arranged at an inclination angle of 55 degrees to each other.

appendix

in this appendix

˛The GPS Network

˛How GPS Works

304 Appendix A — GPS Primer

Several incarnations of GPS satellites have been put into orbit. The first set, called Block I, were launched between 1978 and 1985, none of which are now operational. Replacements for these were called the Block II and Block IIA. Additional replacements are called Block IIR, and the latest satellites are called IIF.

The 27 satellites currently in use are a combination of Block II, Block IIA, Block IIR, and Block IIF satellites.

The satellites were built by a variety of U.S. defense contractors:

Block II/IIA: Rockwell International (Boeing North American)

Block IIR: Lockheed Martin

Block IIF: Boeing North American

The orbital period (the time it takes for a satellite to orbit around the Earth) is twelve hours. This means that at any given location, each satellite appears in the sky four minutes earlier each day. The apparent groundtrack of the satellites (the path that their orbits would draw on the surface of the Earth) is not the same each day because it is shifted westward slightly with each orbit (a drift of 0.03 degrees each day).

The orbits of the satellites form a birdcage around the Earth such that there should always be four or more satellites above the horizon at any one time. Two places on the globe, however, do not fully benefit from the way in which the GPS satellite orbits are orientated: the north and south poles. The orbital coverage here is not as good (for example, satellites are never overhead at the poles), but this was considered a good compromise given the limited use that GPS would see at these locations.

Why 30 satellites? This is the number considered sufficient to ensure that at least four (and a maximum of twelve) satellites are always visible, at all sites on the Earth, at all times.

The GPS space segment was supposed to be activated in the late 1980s, but several incidents (one of which, sadly, was the Challenger Space Shuttle disaster on January 28, 1986) caused significant delays, and the full system of 24 SVs wasn’t deployed until 1994.

Some of the SVs that you will be using are now well over a decade old. This exceeds their initial design life span of around 8 years!

The job of the satellites is multifold:

To provide extremely accurate, three-dimensional location information (latitude, longitude, and altitude), velocity, and a precise time signal

To provide a worldwide common grid reference system that is easily converted to any local grid in use

Appendix A — GPS Primer 305

To be capable of passive all-weather operations

To provide continuous real-time information

To provide support for an unlimited number of users and areas

To provide high-precision information for military and government use

To provide support to civilian users at a slightly less accurate level Here are some interesting facts about the GPS SVs:

Power plant:

■The SVs are powered by solar panels generating 800 watts.

■The panels on the newer Block IIFs have been upgraded to generate 2,450 watts.

Weight:

■Block IIA: 3,670 pounds (1,816 kilograms)

■Block IIR: 4,480 pounds (2,217 kilograms)

■Block IIF: 3,758 pounds (1,705 kilograms)

Height:

■Block IIA: 136 inches (3.4 meters)

■Block IIR: 70 inches (1.7 meters)

■Block IIF: 98 inches (2.4 meters)

Width (includes wingspan)

■Block IIA: 208.6 inches (5.3 meters)

■Block IIR: 449 inches (11.4 meters)

■Block IIF: approximately 116 feet (35.5 meters)

Design life:

■Block II/IIA: 7.5 years

■Block IIR: 10 years

■Block IIR-M (modernized): 8.57 years

■Block IIF: 11 years

Date of first launch: 1978

Launch vehicle: Originally, the Delta II rocket was used; but for the bigger Block IIF SVs, the EELV launch vehicle was used.

306 Appendix A — GPS Primer

The Control Segment

The control segment, just like the space segment, is U.S. Department of Defense property. Just as we have no direct access to the space segment, the same is true of the control segment. The control segment is made up of a worldwide network of monitoring stations, ground antennas, and a master control station.

There are five monitoring stations:

Hawaii

Kwajalein (on the Marshall Islands in the Pacific Ocean)

Ascension Island (South Atlantic Ocean)

Diego Garcia (Indian Ocean)

Colorado Springs, Colorado

There are three ground antennas:

Kwajalein (on the Marshall Islands in the Pacific Ocean)

Ascension Island (South Atlantic Ocean)

Diego Garcia (Indian Ocean)

There is also one master control station located at Schriever Air Force Base in Colorado.

This vast array of systems is used to passively track all satellites in view and gather ranging data. This information is passed on to the master control station where it is processed in order to determine precise satellite orbits and update each satellite’s navigation message so that they are as accurate as possible. Updated information is transmitted to each satellite via the ground antennas.

The User Segment

The user segment is the part of the system to which you and I have access. This is where all the GPS receivers come in. There are many types of receivers in the user section:

Handheld systems

Car navigation systems

Professional commercial systems used for navigation and surveying

Military receivers

The satellites transmit two types of signal that can be received by the user segment:

Standard Positioning Service (SPS)

Precise Positioning Service (PPS)