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| Data message |
A 1500 bit message included in the GPS signal which reports
the satellite's location, clock corrections, and health. Included
is rough information on the other satellites in the constellation |
| Datalogger |
Also called a data recorder. A handheld, lightweight data
entry computer. It can be used to store additional data obtained
by a GPS reciever. |
| Deflection of the vertical |
The angle between the normal to the ellipsoid and the normal
to the Geoid (true plumbline). Since this angle has both a magnitude
and a direction, it is usually resolved into two components:
one in the meridian and the other perpendicular to it in the
prime vertical. |
| Delay Lock |
The technique whereby the received code (generated by the
satellite clock) is compared with the internal code (generated
by the receiver clock) and the latter is shifted in time until
the two codes match. |
| Delta pseudorange |
See reconstructed
carrier phase |
| Differential positioning (DGPS) |
Determination of the relative coordinates of one or more
receivers with respect to the position of a Receiver located
at a known position (referred to as the Base, Monitor Reference
station. Dynamic differential positioning is the real time application
of the technique. In practice, most references to DGPS involve
Dynamic DGPS which entails the real time correction of measurements
made by each roving receiver with data received by radio transmission
from a reference station. The corrections are usually transmitted
in RTCM format. Static Differential GPS involves determining
baseline vectors between pairs of receivers. For most DGPS,
applications only the code measurements are considered and it
is sometimes referred to as Code Differential. Although this
application was initially confined to Navigation at sea, an
increasing number of applications are "land based"
such as GIS, Resource Management and Vehicle Location. |
| Differential processing (carrier phase) |
GPS measurements for Land Surveying are the digital values
given to the carrier phase signal received by the Geotracer.
The various differential processing methods use the carrier
phase data to calculate a difference in the position of two
or more receivers. The data used in the calculation must be
recorded simultaneously. Various methods are used to calculate
the integer ambiguity, but the final result is calculated by
comparing various combinations of the phase data. These are
called differences. All combinations have advantages during
processing and typically all are used at some stage during static
processing. The triple difference is useful in correcting cycle
slips, the double difference is generally used to calculate
the final result for both static and kinematic. Differential
processing eliminates most of the errors found in single point
positioning.
A single difference between receivers is the instantaneous
arithmetic difference between the carrier phase measurement
to satellite, measured by two receivers simultaneously.
A double difference is the instantaneous arithmetic difference
between the single difference for one satellite and a single
difference for another satellite. Normally, one satellite
is chosen to act as a reference satellite and all double differences
are derived by comparing the various single differences with
those of reference satellite.
A triple difference is
the difference between a double difference at one epoch of
time and the same double difference at the previous epoch
of time. |
| Digital |
Generally, information is expressed, stored and transmitted
by either analog or digital means. In a digital form, this information
is seen in a binary state as either a one or a zero, a plus
or a minus. The computer uses digital technology for most actions. |
| Dilution
of Precision (DOP) |
A description of the purely geometrical contribution to the
uncertainty in a position fix. It is given by the expression
DOP = SQRT TRACE (A A ) where A A is the matrix of data used
to calculate the instantaneous position solution. A low number
of DOP (2) is good, a high number (>7) is considered to be
bad.
The DOP factor depends on the parameters
of the position fix solution. Standard terms for the GPS application
are:
GDOP: General (three dimensional coordinates
plus clock offset errors in the solution)
PDOP: Position (three dimensional coordinates
E,N Ht.)
HDOP: Horizontal (two dimensional coordinates
E,N,Ht.)
VDOP: Vertical (height only Ht.)
TDOP: Time (clock offset only)
RDOP: Relative (normalized to 60 seconds
and based on a change in geometry)
|
| Distance root mean square (drms) |
The root-mean-square value of the distances from the true
location point of the position fixes in a collection of measurements.
As typically used in GPS positioning, 2 drms is the radius of
a circle that contains at least 95 percent of all possible fixes
that can be obtained with a system at any one place. |
| Distance
To Go (DTG): |
Digital readout (selectable in miles, nautical miles or kilometers)
displayed only when navigating to a waypoint. It simply indicates
the remaining distance from your present position to the next
waypoint. |
| Dithering |
The introduction of digital noise. This is the process the
Department of Defense (DoD) uses to add inaccuracy to GPS signals
to induce Selective Availability. |
| DME |
Distance Measuring Equipment |
| DOD |
Department of Defense |
| DOP |
See dilution of precision |
| Doppler shift |
The apparent change in frequency of a received signal due
to movement of a satellite towards or away from a receiver.
See reconstructed carrier phase |
| Doppler-aiding |
A signal processing strategy that uses a measured doppler
shift to help the receiver smoothly track the GPS signal. Allows
more precise velocity and position measurement. |
| DOT |
Department of Transportation |
| Double difference method |
A processing method used to determine the difference in phase
of GPS observations. When the integer ambiguities have been
fixed, it is the method used to give the best determination
of baseline vector components. |
| drms |
See Distance Root Mean Squared. |
| Dry Nitrogen Filled: |
A special process in which a unit is sealed and filled with
dry nitrogen to help prevent fogging and internal corrosion. |
| Dynamic positioning |
Determination of coordinates for a moving receiver, each
set of coordinates being determined from a single data sample,
and usually computed in real time. See also Kinematic
Positioning. |
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