Introduction - Understanding GPS: Principles and Applications - page 3

charged with determining the viability of the DNSS and planning its development.
From this effort, the system concept for NAVSTAR GPS was formed. The
NAVSTAR GPS program was developed by the GPS Joint Program Office (JPO) in
El Segundo, California [5]. At the time of this writing, the GPS JPO continued to
oversee the development and production of new satellites, ground control equip-
ment, and the majority of U.S. military user receivers. Also, the system is now most
commonly referred to as simply GPS .

1.3 GPS Overview

Presently, GPS is fully operational and meets the criteria established in the 1960s for
an optimum positioning system. The system provides accurate, continuous, world-
wide, three-dimensional position and velocity information to users with the appro-
priate receiving equipment. GPS also disseminates a form of Coordinated Universal
Time (UTC). The satellite constellation nominally consists of 24 satellites arranged
in 6 orbital planes with 4 satellites per plane. A worldwide ground control/monitor-
ing network monitors the health and status of the satellites. This network also
uploads navigation and other data to the satellites. GPS can provide service to an
unlimited number of users since the user receivers operate passively (i.e., receive
only). The system utilizes the concept of one-way time of arrival (TOA) ranging.
Satellite transmissions are referenced to highly accurate atomic frequency standards
onboard the satellites, which are in synchronism with a GPS time base. The satellites
broadcast ranging codes and navigation data on two frequencies using a technique
called code division multiple access (CDMA); that is, there are only two frequencies
in use by the system, called L1 (1,575.42 MHz) and L2 (1,227.6 MHz). Each satel-
lite transmits on these frequencies, but with different ranging codes than those
employed by other satellites. These codes were selected because they have low
cross-correlation properties with respect to one another. Each satellite generates a
short code referred to as the coarse/acquisition or C/A code and a long code denoted
as the precision or P(Y) code. (Additional signals are forthcoming. Satellite signal
characteristics are discussed in Chapter 4.) The navigation data provides the means
for the receiver to determine the location of the satellite at the time of signal trans-
mission, whereas the ranging code enables the user's receiver to determine the tran-
sit (i.e., propagation) time of the signal and thereby determine the satellite-to-user
range. This technique requires that the user receiver also contain a clock. Utilizing
this technique to measure the receiver's three-dimensional location requires that
TOA ranging measurements be made to four satellites. If the receiver clock were
synchronized with the satellite clocks, only three range measurements would be
required. However, a crystal clock is usually employed in navigation receivers to
minimize the cost, complexity, and size of the receiver. Thus, four measurements
are required to determine user latitude, longitude, height, and receiver clock offset
from internal system time. If either system time or height is accurately known, less
than four satellites are required. Chapter 2 provides elaboration on TOA ranging as
well as user position, velocity, and time (PVT) determination.
GPS is a dual-use system. That is, it provides separate services for civil and mili-
tary users. These are called the Standard Positioning Service (SPS) and the Precise