SATELLITE SYSTEMS - GPS Satellite Surveying - page 75

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3.2.2 Satellite Transmissions at 2002

Th e ICD-GPS-200C (2000) is the authoritative source for details on the GPS signal
str uctures, usage of these signals, and other information broadcasts by the satellites.
Th e document can be downloaded from GPS (2002). All satellite transmissions are
co herently derived from the fundamental frequency of 10.23 MHz, made available
by onboard atomic clocks. This is also true for the new signals discussed further
be low. Multiplying the fundamental frequency by 154 gives the frequency for the L1
ca rrier, f 1 = 1575 . 42 MHz, and multiplying by 120 gives the frequency of the L2

ca rrier, f 2 1227 . 60 MHz. The chipping (code) rate of the P(Y)-code is that of

th e fundamental frequency, i.e., 10.23 MHz, whereas the chipping rate of the C/A-
co de is 1.023 MHz (one-tenth of the fundamental frequency). The navigation message
(te lemetry) is modulated on both the L1 and the L2 carriers at a chipping rate of 50
bp s. It contains information on the ephemerides of the satellites, GPS time, clock
be havior, and system status messages.
The space vehicle time is defined by the onboard atomic clocks of each satellite.
Th e satellite operates on its own time system, i.e., all satellite transmissions such as
th e C/A-code, the P(Y)-codes, and the navigation message are initiated by satellite
ti me. The data in the navigation message, however, are relative to GPS time. Time is
m aintained by the control segment and follows UTC(USNO) within specified limits.
GP S time is a continuous time scale and is not adjusted for leap seconds. The last com-
m on epoch between GPS time and UTC(USNO) was midnight January 5-6, 1980.
Th e navigation message contains the necessary corrections to convert space vehicle
ti me to GPS time. The largest unit of GPS time is one week, defined as 604,800 sec.
Ad ditional details on the satellite clock correction are given in Section 5.3.1.
The atomic clocks in the satellites are affected by both special relativity (the
sa tellite's velocity) and general relativity (the difference in the gravitational potential
at the satellite's position relative to the potential at the earth's surface). Jorgensen
(1 986) gives a discussion in lay terms of these effects and identifies two distinct parts
in the relativity correction. The predominant portion is common to all satellites and
is independent of the orbital eccentricity. The respective relative frequency offset
is

=

[75

Lin
—
6.0
——
Lon
PgE

[75

10 − 10 . This offset corresponds to an increase in time of ∆ f/f =− 4 . 4647 ×

38 . 3 s per day; the clocks in orbit appear to run faster. The apparent change in

fre quency is

µ

0 . 0045674 Hz at the fundamental frequency of 10.23 MHz. The
fre quency is corrected by adjusting the frequency of the satellite clocks in the factory
be fore launch to 10.22999999543 MHz. The second portion of the relativistic effect
is proportional to the eccentricity of the satellite's orbit. For exact circular orbits,
th is correction is zero. For GPS orbits with an eccentricity of 0.02 this effect can be
as large as 45 ns, corresponding to a ranging error of about 14 m. This relativistic
effect can be computed from a simple mathematical expression that is a function of
the semimajor axis, the eccentricity, and the eccentric anomaly (see Section 5.3.1). In
relative positioning as typically carried out in surveying, the relativistic effects cancel
for all practical purposes.
The precision P(Y)-code is the principal code used for military navigation. It is
a pseudorandom noise (PRN) code which itself is the modulo-2 sum of two other
pseudorandom codes. The P(Y)-code does not repeat itself for thirty-seven weeks.

∆ f =