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NAVIGATION INFORMATION SERVICE BULLETIN BOARD

703-313-5910

A.2.8 GPS

GPS is a space-based positioning, velocity, and time system that has three major

segments: space, control, and user. The GPS Space Segment, when fully

operational, will be composed of 24 satellites in six orbital planes. The satellites

operate in circular 20,200 km (10,900 nm) orbits at an inclination angle of 55

degrees and with a 12-hour period. The spacing of satellites in orbit will be

arranged so that a minimum of five satellites will be in view to users worldwide,

with a position dilution of precision (PDOP) of six or less. Each satellite

transmits on two L band frequencies, L1 (1575.42 MHz) and L2 (1227.6 MHz). L1

carries a precise (P) code and a coarse/acquisition (C/A) code. L2 carries the

P code. A navigation data message is superimposed on these codes. The same

navigation data message is carried on both frequencies.

The Control Segment has five monitor stations, three of which have uplink

capabilities. The monitor stations use a GPS receiver to passively track all

satellites in view and thus accumulate ranging data from the satellite signals.

The information from the monitor stations is processed at the Master Control

Station (MCS) to determine satellite orbits and to update the navigation message

of each satellite. This updated information is transmitted to the satellites via the

ground antennas, which are also used for transmitting and receiving satellite

control information.

The user segment consists of antennas and receiver-processors that provide

positioning, velocity, and precise timing to the user.

A. Signal Characteristics

The GPS concept is predicated upon accurate and continuous knowledge of the

spatial position of each satellite in the system with respect to time and distance

from a transmitting satellite to the user. Each satellite transmits its unique

ephemeris data. This data is periodically updated by the Master Control Station

based upon information obtained from five widely dispersed monitor stations.

Each satellite continuously transmits a composite spread spectrum signal at 1227.6

and 1575.42 MHz. The GPS receiver makes time-of-arrival measurements of the

satellite signals to obtain the distance between the user and the satellites. These

distance calculations, together with range rate information, are combined to yield

system time and the user's three-dimensional position and velocity with respect

to the satellite system. A time coordination factor then relates the satellite

system to Earth coordinates. The characteristics of GPS are summarized in Table

A-10.

Table A-10. GPS Characteristics (Signal-In-Space)

/--------------------- ACCURACY (METERS)* --------------------/

/ PREDICTABLE / REPEATABLE / RELATIVE /

/--------------------/--------------------/--------------------/

/ PPS / / /

/ / / ** /

/ Horz - 21 / Horz - 21 / Horz - 1 /

/ Vert - 29 / Vert - 29 / Vert - 1.5 /

/ Time - 200 ns / / /

/--------------------/--------------------/--------------------/

/ SPS / / /

/ / / ** /

/ Horz - 100 / Horz - 100 / Horz - 1 /

/ Vert - 140 / Vert - 140 / Vert - 1.5 /

/ Time - 340 ns / / /

/--------------------------------------------------------------/

/--------------------------- AVAILABILITY ---------------------/

/ /

/ Expected to approach 100% /

/--------------------------------------------------------------/

/--------------------------- COVERAGE -------------------------/

/ /

/ Worldwide continuous (PDOP < = 6) /

/--------------------------------------------------------------/

/--------------------------- RELIABILITY ----------------------/

/ /

/ 99% probability that a 21-satellite constellation /

/ will be operating /

/--------------------------------------------------------------/

/----------------------------- FIX RATE -----------------------/

/ /

/ Essentially continuous /

/--------------------------------------------------------------/

/--------------------------- FIX DIMENSION --------------------/

/ /

/ 3D + Velocity (***) + Time /

/--------------------------------------------------------------/

/------------------------- SYSTEM CAPACITY --------------------/

/ /

/ Unlimited /

/--------------------------------------------------------------/

/------------------------ AMBIGUITY POTENTIAL -----------------/

/ /

/ None /

/--------------------------------------------------------------/

* Horizontal (2 drms); Vertical (95%); Time (95%).

** Preliminary estimates.

*** System specifications not defined for SPS.

SYSTEM DESCRIPTION: GPS is a space-based radio positioning system

that will provide three-dimensional position, velocity and time

information to suitably equipped users anywhere on or near the

surface of the Earth. The space segment will consist of 24 satellites

in 6 orbital planes of 12 hour periods. Each satellite will transmit

navigation data and time signals on 1575.42 and 1227.6 MHz.

B. Accuracy

Accuracy projections for the operational satellite constellations are based upon

computer simulations. At a specifed time of day, the programs calculate the

positions of the GPS satellites and determine which ones are visible at a given

location on earth. They select four of the visible satellites and calculate the

location solution that a GPS receiver would provide. Since a GPS receiver

determines location by estimating the user's range to each of the four satellites,

the simulations mimic the real errors in this process by introducing a range

error for each of the simulated satellites, using Monte Carlo techniques. The

range data are used to solve for the user's location, and the instantaneous

position error is determined by subtracting the true position from the calculated

position.

By repeating this process at many locations around the Earth, and over a

24-hour period, the simulations produce a composite view of system performance.

These results are dependent upon several program inputs:

o The number of satellites in the GPS constellation.

o The orbits chosen for the satellites.

o The locations of the simulated users.

o The local visibility contraints on receiving signals from satellites.

o The criteria for selecting four satellites from among the visible ones.

o The magnitude of the User Range Errors (URE) experienced by users.

URE is an aggregate of all the range measurement uncertainties, including the

GPS receiver itself. It can be expressed as a zero-mean Gaussian distribution

with a specified standard deviation.

The position errors calculated by the simulations are normalized by dividing them

by the standard deviation of the URE originally used to generate the Monte Carlo

range errors. Normalized error curves are often confused with Dilution of

Precision (DOP) curves. DOP is a geometric quantity that depends upon the

relative positions of the user and the selected satellites. Statistically, high

values of DOP cause small range measurement errors to be amplified into large

position errors. GPS constellations are selected to minimize these high-DOP areas

of reduced accuracy. Normalized position error distributions are preferable to

DOP distributions; the position error distributions implicitly include not only the

overall error amplification of the DOP curves, but also the fact that north-south,

east-west, and vertical location errors all have different trends.

All accuracy projections are based upon a fully operational system: 24 healthy

satellites, normal uploads by the Control Segment, etc. The accuracy simulations

use the 24 satellites. Satellite visibility depends upon local conditions. Some

users may be able to track satellites less than 5 degrees above the horizon, while

other users may have difficulty even at 10 degrees. DOD accuracy simulations

use 5 degrees.

Accuracy simulations use the four-satellite combination that minimizes

three-dimensional position DOP (PDOP). In some applications, a user receiver may

have access to additional information, such as being at a known altitude (relative

to mean sea level), or may have a more accurate atomic clock in place of the

usual crystal clock. In general, such information improves location accuracy

substantially. When discussing horizontal accuracy it is important to differentiate

between a user whose horizontal errors are based upon the use of four satellites

that minimize DOP, and one based upon a known altitude and the use of three

satellites that minimize horizontal DOP (HDOP). As noted above, the GPS accuracy

simulations are usually based solely upon the four satellites that minimize PDOP.

GPS provides two services for position determination, SPS and PPS. Accuracy

of a GPS fix varies with the capability of the user equipment.

1. Standard Positioning Service (SPS)

SPS is the standard specified level of positioning and timing accuracy that is

available, without qualification or restrictions, to any user on a continuous

worldwide basis. The accuracy of this service will be established by the U.S.

Department of Defense based on U.S. security interests. When GPS is declared

operational, the DOD plans to provide, on a daily basis at any position worldwide,

horizontal positioning accuracy within 100 meters (2 drms) and 300 meters with

99.99 percent probability.

2. Precise Positioning Service (PPS)

PPS is the most accurate positioning, velocity, and timing information

continuously available, worldwide, from the basic GPS. This service will be

limited to authorized U.S. and allied Federal Governments; authorized foreign and

military users; and eligible civil users. Unauthorized users will be denied access

to PPS through the use of cryptography. P code capable military user equipment

will provide a predictable positioning accuracy of at least 22 meters (2 drms)

horizontally and 27.7 meters (2 sigma) vertically and timing/time interval accuracy

within 90 nanoseconds (95 percent probability).

C. Availability

GPS will provide availability approaching 100 percent to be refined based on

orbital experience. This is based upon a 24 satellite constellation with at least

four satellites in view above a 5o masking angle.

D. Coverage

A 24-satellite constellation will provide worldwide three-dimensional coverage.

E. Reliability

GPS operational (Block II) satellites have a design life of 7.5 years. Reliability

figures can only be determined after satellites are launched and data are

collected and evaluated. With the planned replenishment strategy, a constellation

of 24 satellites will provide a 98 percent probability of having 21 or more

satellites operational at any time.

F. Fix Rate

The fix rate is essentially continuous. Actual time to a first fix depends on user

equipment capability and initialization with current satellite almanac data.

G. Fix Dimensions

GPS provides three-dimensional positioning and velocity fixes, as well as

extremely accurate time information.

H. System Capacity

The capacity is unlimited.

I. Ambiguity

There is no ambiguity.

J. Integrity

According to DOD's concept of operation, GPS satellites are monitored more than

95 percent of the time by a network of five monitoring stations spread around

the world. The information collected by the monitoring stations is processed by

the Master Control Station at Colorado Springs, Colorado, and used to periodically

update the navigation message (including a health message) transmitted by each

satellite. The satellite health message, which is not changed between satellite

navigation message updates, is transmitted as part of the GPS navigation message

for reception by both PPS and SPS users. Additionally, satellite operating

parameters such as navigation data errors, signal availability/anti-spoof failures,

and certain types of satellite clock failures are monitored internally within the

satellite. If such internal failures are detected, users are notified within six

seconds. Other failures detectable only by the control segment may take from

15 minutes to several hours to rectify.

DOD GPS receivers use the information contained in the navigation and health

messages, as well as self-contained satellite geometry algorithms and internal

navigation solution convergence monitors, to compute an estimated figure of merit.

This number is continuously displayed to the operator, indicating the estimated

overall confidence level of the position information.

Both DOT and DOD have recognized the requirement for additional integrity for

aviation and all other users of GPS. The development of integrity capabilities to

meet flight safety requirements is underway.