Geoscience Reference
In-Depth Information
part of the carrier-phase measurement due to the receiver incorrectly predicting the elapsed
number of cycles between signal loss and reacquisition.
GPS: Data Message
A message included in the GPS signal that reports the satellite location, clock
corrections, and health. Included is approximate information about the other satellites in the
system as well.
GPS: Datum
The coordinate system used to define position on the Earth surface.
GPS: Differential GPS (DGPS)
An extension of the GPS system that uses land-based radio bea-
cons to transmit position corrections to GPS receivers. DGPS reduces the effect of selective
availability, propagation delay, and so forth, and can improve position accuracy to better
than 10 meters.
GPS: Dilution of Precision (DOP)
A measure of the GPS receiver and satellite geometry, A dimen-
sionless number that accounts for the contribution of relative satellite geometry to errors in
position determination. A low DOP value indicates better relative geometry and higher cor-
responding accuracy. The DOP indicators are GDOP (geometric DOP), PDOP (position DOP),
HDOP (horizontal DOP), VDOP (vertical DOP), and TDOP (time clock offset).
GPS: Doppler Effect
The shift in the frequency of a received radio signal due to the relative
motion of the transmitter and receiver.
GPS: P-Code
The precise code of the GPS signal typically used only by the U.S. military. It is
encrypted and reset every seven days to prevent use from unauthorized persons. The code
consists of about 2.35 × 1014 chips and is sent at a rate of 10.23 megabits per second. At
this rate, it would take 266 days to transmit the complete code. Each satellite is assigned a
unique one-week segment of the code that is reset at Saturday/Sunday midnight. The P-code
is currently transmitted on both the L1 and L2 frequencies.
GPS: Pseudo-Random Code
Pseudorandom Noise (PRN) Code. Deterministic binary sequences
with noise-like properties. These codes are used in spread-spectrum communications sys-
tems and in ranging systems such as GPS. Two PRN codes are transmitted by GPS satel-
lites: the C/A-code and P-code. The identifying signature signal transmitted by each GPS
satellite and mirrored by the GPS receiver in order to separate and retrieve the signal from
background noise.
GPS: Pseudorange
The measured distance between the GPS receiver and the GPS satellite using
uncorrected time comparisons from satellite-transmitted code and the local receiver's refer-
ence code.
GPS: Real-Time Kinematic (RTK)
The DGPS procedure whereby carrier-phase corrections are
transmitted in real time from a reference receiver to the user receiver. RTK is often used for
the carrier-phase integer ambiguity resolution approach.
GPS: Relative Accuracy
The accuracy with which a user can measure position relative to that of
another user on the same navigation system at the same time.
GPS: Selective Availability (SA)
The random error, which the government can intentionally add
to GPS signals, so that their accuracy for civilian use is degraded. SA is not currently in
use.
GPS: Wide Area Augmentation System (WAAS)
A system of satellites and ground stations that
provide GPS signal corrections for better position accuracy. A form of DGPS in which the
user's GPS receiver receives corrections determined from a network of reference stations
distributed over a wide geographical area. A WAAS-capable receiver can give a position
accuracy of better than three meters, 95 percent of the time. WAAS consists of approxi-
mately twenty-five ground reference stations positioned across the United States that moni-
tor GPS satellite data. Two master stations, located on either coast, collect data from the
reference stations and create a GPS correction message.
GPS: WGS-84
World Geodetic System, 1984. The primary map datum used by GPS. Secondary
datums are computed as differences from the WGS 84 standard.
GPS: Y-Code
The encrypted P-Code.
