Datum: Based upon a spheroid, a datum acts a starting point for making the most accurate measurements on Earth's surface. A datum is a local point of reference, and as such there is a different datum for almost every region of the planet.
Differential Correction: A mathematical method of correcting inaccuracies in GPS data during post-processing. Differential correction compares the positions recorded by a mobile GPS unit with those taken at the same time at a nearby base station with a fixed location. The idea is that whatever variations occur in the signals received by the mobile device will also have occurred at the base station. Since the base station knows its exact position, any variances in the positions recorded by the base station can be reduced mathematically, or corrected. Whenever we apply this same correction to positions recorded at the same moment by a mobile GPS unit, we increase the accuracy of the position, often on the sub-meter level.
FOV: Field of View. This is the area of sky that you can see--basically everything above the horizon. Your FOV will be reduced by anything that obstructs the horizon, e.g. buildings, trees, mountains, etc.
GPS: Global Positioning System.
LAT/LONG: Latitude/Longitude. A cartesian coordinate system that uses arc degrees, minutes, and seconds to specify location. Latitude is the distance north or south of the Equator. Longitude is the distance east or west of the Prime Meridian, which passes through Greenwich, England, U.K.
NAD27: North American Datum of 1927. This datum is one of the most common in use in North America, but is gradually being replaced by the updated NAD83 and the WGS84.
NAVSTAR: Navigation Satellite Timing and Ranging. This is the constellation of satellites that make up the space portion of GPS, maintained by the U.S. Department of Defense. There are 27 satellites total, 24 active and three spares.
PDOP: Position Dilution of Precision. This is a number that indicates the relative accuracy of the GPS positions calculated by the receiver in the field. It takes into account each satellite s location relative to the other satellites (the satellite geometry). The higher the number, the less accurate the reported position.
PDOP Mask: The upper limit of toleration at which a GPS receiving unit will cease to record readings. This is defaulted to 6 on most professional units, but the user can change this value according to need.
Projection: A way to represent the three-dimensional globe on a two-dimensional map. There are three basic types of projections--cylindrical, conic, and planar. Each type of projection has advantages and disadvantages as a result of the inherent distortion involved in going from 3D to 2D. Cylindrical projections are most often used for navigation, conic projections are mostly used for mid-latitude maps, and planar projects are commonly found represent the polar regions.
SNR: Signal-to-Noise Ratio. Noise is an effect of scattering and interference of the satellite signal. In this case, the higher the value of SNR, the stronger the signal and the greater the accuracy.
Spheroid: A detailed mathematical calculation of the shape of the earth. A spheroid takes into account the non-spherical shape of Earth, and offers a model that increases the accuracy of measurements on its irregular surface.
UTM: Universal Transverse Mercator. The UTM is a map projection system that divides the earth into 60 long strips (called zones) that are six degrees longitute in width. These are then subdivided into a grid that uses a central vertical line as its east-west reference (called Easting). Depending upon which hemisphere a location is in, the UTM uses either the Equator or the South Pole for its north-south reference (called Northing). The measuring units are in meters.
WGS84: World Geodetic System 1984. A datum developed in Europe as an effort to create a global stadardized model. It is primarily used by international civil aviation.