GPS-glossary-d

Map Datum's are the reference system used between the Lat/Longs and the map being used as a reference. A vertical datum is a level surface to which heights are referred. The horizontal datum is used as a reference for position. Common f. ex. default to WGS84

Datum, geodetic

A set of constants specifying the coordinate system used for geodetic control. A complete geodetic datum provides, as a minimum, definition for orientation, scale and dimensions for the reference ellipsoid. The concept is generally expanded to include the published coordinates of control stations within the system

DBR

Differential Beacon Receivers tune to the United States Coast Guard's high differential correction beacon stations for improved position accuracy. RTCM 104 input signal is supplied to a DGPS unit to correct for SA and Atmospheric errors. The operating frequency range is 285 kilohertz to 325 kilohertz

DDE

Dynamic Data Exchange. A Windows protocol that allows communication between applications. When information is updated in one application, related information will be updated in other applications

Dead Reckoning

A very simple method of using time and distance to navigate. Distance traveled is determined by multiplying speed by elapsed time

Deflection of the vertical

The angle at a point on the surface of the earth between the vertical at that point (the line normal to the geoid) and the line through the point which is normal to the reference ellipse.

Degrees of freedom

The number of observations minus the minimum number required to uniquely define the figure

Delay-Lock Loop

Another term for a code-tracking loop

Demodulation

Separating coded data from the carrier signal

Deviation

Errors from your course. Either built in or unintentional.One type of deviation is Magnetic, the difference between a true course and what a compass shows

DGPS

Differential GPS. A local transmitter is used for greater accuracy.

Differential GPS (DGPS)

A technique to improve GPS accuracy that uses pseudorange errors measured at a known location to improve the measurements made by other GPS receivers within the same general geographic area. One to ten meter accuracy is typical, possible: <1 cm

Differential positioning

Precise measurement of the relative positions of two receivers tracking the same GPS signals

Digital Raster Graphic (DRG)

The United States Geological Survey is releasing digital versions of all of the topographic maps. These files are called Digital Raster Graphics (DRG) maps. The images themselves are stored in a format called GeoTiff

Dilution of Precision (DOP)

A dimensionless number that accounts for the contribution of relative satellite geometry to errors in position determination. DOP has a multiplicative effect on the User Equivalent Range Error. Generally, the wider the spacing between the satellites being tracked by a GPS receiver, the smaller the position error. The most common quantification of DOP is through the position dilution of precision (PDOP) parameter. PDOP is the number that, when multiplied by the root mean square (rms) UERE, gives the rms position error. Other DOPs include the geometric dilution of precision (GDOP), horizontal dilution of precision (HDOP), and vertical dilution of precision (VDOP)

Distance Root Mean Square (DRMS)

A measurement used to describe the accuracy of a fix. It is twice the square root of the sum of the squares of all radial errors surrounding a true point divided by the total number of measurements

Dithering

The introduction of digital noise. This is the process the US Department of Defense (DoD) uses to add inaccuracy to GPS signals to induce Selective Availability

DMA

Defence Mapping Agency

DOP

Dilution of Precision. Errors caused by bad geometry of the Satellites. The higher the number, the more "noise" in the position reading

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

Doppler shift

The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver

Double difference

A GPS observable formed by arithmetically differencing carrier phases simultaneously measured by a pair of receivers tracking the same pair of satellites. First, the phases obtained by each receiver from the first satellite are differenced. Second, the phases obtained by each receiver from the second satellite are differenced. And third, those differences are differenced. This procedure removes essentially all of the satellite and receiver clock errors. Although primarily used with carrier phases, the procedure can also be applied to pseudoranges

DRG

Digital Raster Graphic. The United States Geological Survey is releasing digital versions of all of the topographic maps. These files are called Digital Raster Graphics (DRG) maps. The images themselves are stored in a format called GeoTiff

Drift

The rate of a vessel's departure from a given course as measured in knots

DTK

Desired Track. The course between to and from

Dynamic positioning

Dynamic positioning or kinematic positioning refers to applications in which the position of a moving object is determined