H

Hand-Over Word (HOW)

The second word in each subframe of the navigation message. It contains the Z-count at the leading edge of the next subframe and is used by a GPS receiver to determine where in its generated P-code to start the correlation search process

Hardover word

The word in the GPS message that contains synchronization information for the transfer of tracking from the C/A to P code.

HDOP

HDOP = Horizontal Dilution of Precision (Latitude, Longitude).

Height Aiding

An accuracy enhancement technique in which the known height of the receiver is entered into the navigation solution, which in effect provides another satellite's range

Hertz

A unit used to measure a wave's frequency, one cycle per second

High Frequency (HF)

Radio frequencies in the band from 3 to 30 MHz

Horizontal Dilution of Precision (HDOP).

The effects on accuracy of the combined errors in a two-dimensional fix obtained from crossing two lines of position

HTDP

Horizontal Time Dependent Positioning model A computer database and interpolation program to predict horizontal displacements between coordinate points over time.

I

I/O

Abbreviation for Input/Output.

Illumination

The signal coming from the GPS Satellites

Inclination

One of the orbital parameters that describes the orientation of an orbit. It is the angle between the orbital plane and a reference plane, the plane of the celestial equator for geocentric orbits and the ecliptic for heliocentric orbits

Initialization

Entering such data as time, time off-set, approximate position, and antenna height into a receiver (cold start with no almanac) to help the unit find and track satellites

Inmarsat

An international consortium chartered in the mid-1970s to provide improved maritime public correspondence and radio determination capabilities

Integrity

The ability of a system to supply timely warnings in the event of a loss of navigation solution, excessive noise, or other factors affecting measured position

Interference

Any distortion of the transmitted signal that impedes the reception of the signal at the receiver

Interferometric Differencing

A variation of phase differencing in which two different antennas sample the GPS signal wavefront at two locations and then feed the information into a single amplifier and interferometer (mixer) in which phase difference observations are made

Ionization

The process by which atoms form electrically charged particles called ions

Ionosphere

That layer of the atmosphere approximately 30-300 miles above the earth's surface that contains electrically charged particles (ions). These charged particles interfere with or distort transmissions of electromagnetic signals through the layer

Ionospheric Delay

Signal delay or acceleration as a wave propagates through the ionosphere. Phase delay depends upon the electron content and affects the carrier signal. Group delay depends upon the dispersion in the ionosphere as well, and affects the code signal

Ionospheric Refraction

The change in the propagation speed of a signal as it passes through the ionosphere

Isogonic Lines

A line on a map or chart where the magnetic deviation is the same

ITU

International Telecommunication Union

J

JPALS

Joint Precision Approach and Landing System, is a DGPS system pro rapid deployment

K

Kalman Filter

An optimum mathematical procedure for recursively estimating dynamically changing parameters, such as the position and velocity of a vessel, from noise-contaminated observations

Keplerian Elements

A set of six parameters that describe the position and velocity of a satellite in a purely elliptical (Keplerian) orbit. These parameters are the semimajor axis and eccentricity of the ellipse, the inclination of the orbit plane to the celestial equator, the right ascension of the ascending node of the orbit, the argument of perigee, and the time the satellite passes through the perigee

Kilohertz

A radio signal that has 1,000 cycles per second

Kinematic GPS

Observations while a receiver is in motion. In surveying applications, kinematic refers to uninterrupted carrier

Kinematic Positioning

Positioning a continuously moving platform by using GPS carrier-phase data while operating in a differential mode

Kinematic Surveying

A precision differential GPS surveying technique in which the roving user does not need to stop to collect precision information. Meter to centimeter-level accuracy is available using mode, dual-frequency, carrier-phase measurement techniques

Knot

A speed of one nautical mile per hour