heading - The direction one is traveling.hillock - A small hill.
HTDP - Horizontal Time Dependent Positioning - A computer database and interpolation program to predict horizontal displacements between coordinate points over time.
IGEB - Interagency GPS Executive Board - established by Presidential directive in 1996 to manage the Global Positioning System (GPS) and its U.S. Government augmentations as a national asset.
illumination - The signals coming from GPS satellites.
index contour - A contour line on a topographic map that is labeled as to elevation; used as a reference.
initialization - The first time a GPS receiver orients itself to its current location.
INMARSAT - International Mobile Satellite Organization - Established in 1979 to serve the maritime industry by developing satellite communications for ship management and distress and safety applications.
ionosphere - A region of the Earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of radio waves. It extends from a height of 43 miles (70 kilometers) to 250 miles (450 kilometers) above the surface.
isobath - A line on a map connecting two points of equal water depth.
isogloss - A line on a map between linguistically varied areas.
isogone - A line on a map used to show characteristics of the earth's magnetic field.
isogonic lines - A line on a map where the magnetic deviation is the same.
isogram - A line on a map connecting points of equal value.
isograph - A line on a map indicating areas that are linguistically similar.
isogriv - A line drawn on a map such that all points have equal grid variation.
isohel - A line on a map connecting points receiving equal sunshine.
isohyet - A line on a map connecting points having equal rainfall.
isotach - A line on a map connecting points of equal wind velocity.
isotherm - A line on a map connecting points of equal mean temperature.
JPALS - Joint Precision Approach and Landing System - A differential GPS that will provide a rapidly deployable mobile, adverse weather, adverse terrain, day-night, and survivable precision approach and landing capability that supports joint service, civil, and multi-national interoperability.
