Global Positioning System (GPS) Receivers

Global Positioning System

The Planning staff in a wildfire operation use GPS to create accurate maps of a fire. Staff members in the Operations and Logistics sections of the wildfire operation use the maps to decide where to place other personnel and equipment.  People who work in the Operations section of a wildfire use GPS to communicate with people who handle the aerial fire control.  GPS is more efficient method of communication between ground  and air personnel; it replaces the use of mirrors and bright objects to signal pilots.

GPS receivers receive signals from either 3 or 4 satellites at a time and triangulate a position using the interval between the transmission and reception of the satellite signal. Any given receiver tracks more satellites than are actually needed for a position fix. The reason for this is that if one satellite becomes unavailable, the receiver knows exactly where to find the best possible replacement. Two dimension positioning, which reports position only, requires 2 satellites.  Three-dimension positioning requires 4 satellites to calculate position and elevation.

Each GPS satellite broadcasts two signals, PPS (Precise Positioning Service) and SPS (Standard Positioning Service). The PPS signal is an encrypted military-access code. The SPS signal is an unencrypted, spread-spectrum signal broadcast at 1575.42 MHz. The SPS signal is virtually resistant to multipath and night-time interference, and is unaffected by weather and electrical noise.  A SPS receiver can provide position information with an error of less than 25 meters.

The SPS signal contains two types of orbit data: almanac and ephemeris. Almanac data contains the operating condition and approximate location of every satellite in the system. A GPS receiver collects almanac data from any available satellite, then uses it to locate the satellites that should be visible at the receivers location. Ephemeris data contains the precise orbital parameters of a specific satellite.