T

TDOP

Time Dilution of Precision

Telemetry

Transmission of radio signals and coded data from a space vehicle

Temporal Decorrelation

The age or time lapse in corrections used in differential GPS. The longer the time lapse between the corrections, the less accurate they become

Terminal Node Controller (TNC)

A device that converts and controls the radio transmission and reception of digital signals from a computer

Topography

Relief of the land surface; the graphic portrayal of that relief in map form by the use of contour lines

Track

Your current direction of travel relative to a ground position (same as COG, Course Over Ground)

Track (TRK)

The direction of movement relative to a ground position.

Transit

The satellite-based system that measured successive Doppler (frequency) shifts of signals transmitted from satellites in polar orbits to determine position

Triple Difference

The arithmetic difference of sequential, doubly differenced carrier-phase observations. The triple-difference observable is free of integer ambiguities. It is a useful observable for determining initial, ap- proximate coordinates of a site in relative GPS positioning and for detecting cycle slips in carrier-phase data

Troposphere

The portion of the atmosphere from the earth's surface to the stratosphere; that is, the lowest 10 to 20 kilometers of the atmosphere

Tropospheric Delay

Retardation of GPS signals caused by elements in the troposphere such as temperature, air pressure, and water vapor

True North

The direction to the geographic North Pole. It is different from Magnetic North, by the value of the Magnetic Variation

U

Ultra High Frequency (UHF)

Radio frequencies in the band from 300 to 3,000 MHz

Universal Time (UT)

see UT0, UT1 and UT2

Universal Transverse Mercator (UTM)

A grid coordinate system that projects global sections onto a flat surface to measure position in specific zones

UPS

Universal Polar Stereographic, a version of UTM

URA

User Range Accuracy The contribution to the range measurement error from an individual error source

User Equivalent Range Error (UERE)

Any error contributing to the error budget of stand-alone GPS receiver positioning, expressed as an equivalent error in the range between a user's receiver and a satellite. Also known as user range error (URE). UERE errors originate from different sources and thus are independent of each other. The total UERE is the square root of the sum of the squares of the individual errors. A prediction of maximum anticipated total UERE (minus ionospheric error) is provided in each satellite's navigation message as the user range accuracy (URA)

User interface

The way a receiver conveys information to the person using it. The controls and displays

User segment

The part of the whole GPS system that includes the receivers of GPS signals

USNG

U.S. National Grid

UT0

Universal Time as deduced directly from observations of stars (based on the rotation of the earth) and the fixed numerical relationship between Universal and Sidereal Time; 3 minutes 56.555 seconds. When UT0 is corrected for the shift in longitude of the observing station caused by polar motion, the time scale UT1 is obtained

UT1

Is UT0 corrected for the effects of small movements of the earth relative to the axis of rotation (polar motion). Because Earth does not spin at exactly a constant rate, UT1 is not a uniform time scale

UT2

Is UT1 corrected for seasonal variation in the earth's rotation rate

UTC

Universal Time Coordinated. The standard time common to every place in the world. Uniform atomic time system kept very closely to UT2 by offsets. Maintained by the U.S. Naval Observatory. GPS time is directly relatable to UTC. UTC-GPS = 9 seconds (in 1994)

UTM

Universal Transverse Mercator, A type of map projection. The grid lines you see on a USGS Quads represent a map projection known as Universal Transverse Mercator (UTM), and are 1000 meters apart. See map projections