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.

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 max-

imum 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, longi-

tude, 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

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.

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 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)

Standard Positioning Service (SPS)

The SPS is a positioning and timing service that is available to all GPS users on a continuous,

worldwide basis with no direct charge. SPS is provided on one of the frequencies that the

GPS satellites use, called L1. It contains a coarse acquisition (C/A) code and a navigation

data message.

Precise Positioning Service (PPS)

The Precise Positioning Service (PPS) is a highly accurate military positioning, velocity, and

timing service that is available on a continuous, worldwide basis to users authorized by the U.S.

The P(Y ) code–capable military user equipment provides robust and predictable positioning

accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters vertically, and time

accuracy to within 200 nanoseconds (95 percent).

PPS is the data transmitted on both GPS frequencies: L1 and L2. PPS was designed primarily

for U.S. military use and access to it is controlled by encrypting the signal.

Anti-spoofing (A-S) measures guard against fake transmissions of satellite data by encrypting

the P-code to form the Y-code. This is only activated periodically when deemed necessary.

The basic principle behind GPS is straightforward: The GPS receiver picks up a signal from

three or more of the satellites and then uses this information to calculate the distance to the

satellites. This information is, in turn, used to determine a location on the globe where the

receiver is at that time. This whole process is based on a system called trilateration.

Trilateration is easy to visualize.

In principle, three-dimensional trilateration doesn’t differ much from two-dimensional trilater-

ation, but it is trickier to grasp. W hat you need to do is imagine the radii of the circles from the

preceding examples going off in all directions, so instead of a getting a series of circles, you get

a series of spheres.

If you know you are fifteen miles from Point A (or satellite A in the sky), you could be anywhere

on the surface of a huge, imaginary sphere with a fifteen-mile radius. If you also know you are

eighteen miles from satellite B, you can overlap the first sphere with second, larger sphere. These

spheres all intersect in a perfect circle. F inally, if you know the distance to satellite C, you get a

third sphere, which will intersect with the other circles at two points .

The Earth itself acts as another sphere. It is assumed that you are on the Earth, so you can

eliminate the other point in outer space,

Receivers generally use four or more satellites, however, to improve accuracy and provide pre-

cise altitude information.

In order to perform this simple calculation and determine where you are, the GPS receiver has

to know two things:

The location of at least three satellites above you

The distance between you and each of those satellites

The more accurate these values are, the more accurate your position will be.

The GPS receiver determines both the location of the satellites and your distance from them by

analyzing the high-frequency, low-power radio signals from the GPS satellites. Better units have

multiple (or parallel) receivers, so they can pick up signals from several satellites simultaneously.

If you buy a GPS receiver nowadays, it will be a multi-channel receiver (12 or even 14 channels

simultaneously).

The radio waves from the GPS SVs, like all other radio waves, are electromagnetic energy, which

means that they propagate at the speed of light (roughly 186,000 miles per second, or 300,000 km

per second, in a vacuum). A GPS receiver can figure out how far a signal has traveled by timing

how long it took the signal to arrive. The mechanism by which it does this is quite clever.

At a particular time (midnight, for example), the SVs begins transmitting a long digital pattern

called a pseudo-random code. This code consists of a carrier wave that transmits the digital “chips"

that make up the code. The receiver knows when this code starts (this information is transmitted

to the GPS along with the signal) and begins running the same digital pattern at exactly the same

time. When the satellite’s signal reaches the receiver, its transmission of the digital pattern will

lag slightly behind the digital pattern that the receiver would expect. This lag between the two

corresponds to the time delay from the satellite sending the signal and the receiver receiving it.

Speed

.

Time = Distance

Therefore, if it takes 0.08 of a second for a satellite’s signal to reach the GPS receiver, the

distance between the two must be 14,880 miles (186,000 miles per second

.

0.08 seconds = 14,880 miles). The GPS receiver must be located somewhere on an imaginary sphere that has a radius of 14,880 miles.

One thing that GPS relies on is accurate timing. To provide this, every GPS satellite carries

four atomic clocks on board that provide an extremely accurate timing signal. The GPS receiver,

however, doesn’t contain an atomic clock (this would make it preposterously expensive!). Instead,

it contains an inexpensive quartz clock, and the internal timekeeping is updated when the

receiver is capable of locking on and receiving a signal from four or more SVs.

The GPS system isn’t absolutely perfect, however, because the radio signal has to propagate

through the atmosphere to the receiver. The receiver isn’t perfect either. The following table

describes the sources of the possible errors present.

Error

Amount of Error (feet/meters)

Ionosphere

13.1/4.0

Clock

6.9/2.1

Ephemeris

6.9/2.1

Troposphere

1.3/0.7

Receiver

1.6/0.5

Multipath

3.3/1.0

TOTAL

33.1/10.4

The errors resulting from the ionosphere and the troposphere are due to the slowing down of

the signal (remember that the speed of light quoted is the speed of light in a vacuum). Clock

errors are due to inaccuracies in timing. Even using atomic clocks, the speed of light is so fast

and the distance that it has to travel is so short that small errors in timing add up. Ephemeris

errors are due to the fact that it is impossible to know absolutely the orbits of the satellites.

Slight variations cause small errors.

Receiver error is also significant; this is the error in the antenna and the delays in processing

caused by the internal circuitry (also due to temperature changes affecting the internal clock).

Finally, there are multipath errors. This error is caused by the signal being reflected or bounced

off things (such as a building or the ground).

There are other issues with the signal. Because it is a microwave signal, it is absorbed by water,

so a GPS receiver won’t work underwater. Fortunately, not many people need to use it under-

water, so this isn’t a huge problem, but water can affect the system in other ways:

Leaves contain water and can absorb the GPS signal, dramatically reducing signal strength

and quality. The denser the leaves, the more the signal is degraded. Worse than leaves are

pine needles. These not only contain water, but are the right size and shape to act as mini-

antennas, catching the signal. This usually means that if you are traveling though dense

forests (especially conifers), you will need to find a clearing to gain a good signal lock.

Humidity in the air, such as rain, snow, or fog, can also weaken the signal. In addition,

areas of high mist (such as near waterfalls) can be problematic because the high water

content of the air exacerbates the problem.

A layer of water on the antenna can dramatically reduce the signal’s quality.

This appendix provided a brief look at the GPS network and what makes it work. GPS is a

vastly complex multi-billion-dollar system, and certain key parts of it are classified. Nonetheless,

this appendix should give you a good working knowledge of the system and what can affect its

accuracy.