AvMap GeoPilot II Plus - Manual (Page 47)

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• Cold Start (avg): < 45 sec.
• Warm Start (avg): < 40 sec.
• hot Start (avg): 8 sec.
7.3 hoW GPS WorKS
Each GPS satellite continuously broadcasts two signals: an SPS (Standard Positioning Service) signal
for worldwide civilian use and a PPS (Precise Positioning Service) signal for U.S. and Allied military
use. The SPS signal is a spread-spectrum signal broadcast at 155.42 MHz. The signal is virtually
resistant to multipath and nighttime interference and is unaffected by weather and electrical noise.
All commercial and consumer GPS receivers are SPS receivers.
The SPS signal contains two types of orbit data: almanac and ephemeris. Almanac data contains the
health 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
receiver’s location. Ephemeris data represents the precise orbital parameters of a specific satellite.
Receivers listen to signals from many satellites simultaneously and triangulate a position fix using
the interval between the transmission and reception of each satellite signal (a receiver tracks more
satellites than are actually needed for a position fix, so that if one satellite becomes unavailable, the
receiver knows exactly where to find the best possible replacement).
Three satellites are required for two-dimension positioning (which determines position only) and
four satellites are required for three-dimension positioning (to determine position and elevation).
7.3.1 Accuracy
In general, an SPS receiver can provide position information with an error of less than 25 Meters
and velocity information with an error of less than 5 Meters per second. For applications that require
much greater accuracy the effects of SA and environmentally produced errors can be overcome by
using a technique called Differential GPS (DGPS), which increases over-all accuracy.
7.3.2 WAAS
The Federal Aviation Administration (FAA), in cooperation with other DOT organizations and DOD,
is augmenting the GPS/SPS with a satellite-based Wide Area Augmentation System (WAAS). WAAS
will provide a signal-in-space to WAAS compatible receivers to support en route through precision
approach navigation. After achieving initial operational capability, WAAS will be incrementally
improved to expand the area of coverage, increase the availability of precision approaches, increase
signal redundancy and reduce operational restrictions.
7.3.3 differential GPS
Differential GPS (DGPS) uses a GPS receiver at a fixed point with a known position of sub-meter
accuracy. This is the control unit. The receiver collects data for all visible satellites and computes
predicted satellite ranges, which are compared with actual ranges. The difference is the satellite
range error, which is converted to correction signals for use by a roving receiver.
It is assumed that this correction will be the same for other GPS receivers that are in the same
area and are using the same satellites for positioning. If the correction is communicated to other
receivers in the area (usually by a beacon on the same site), the range error can be removed from
satellite signals and precise fixes calculated by these receivers.
It should be noted that not all data errors can be corrected in this way. Errors that are caused
by receiver noise (which is inherent in any GPS receiver) and multipath problems cannot be
eliminated with differential corrections. (Multipath errors occur when the receiver’s antenna “sees”
the reflections of signals that have bounced off of surrounding objects.)
Using DGPS to eliminate the effects of correctable errors requires that the user’s GPS receiver be
connected to a compatible differential beacon receiver and be within range of the broadcasting
beacon. The differential beacon receiver accepts and demodulates the broadcast corrections, which
are then relayed to the GPS receiver. The GPS receiver applies the corrections to the navigation
data it uses to compute a position solution and displays differentially corrected data. Care must be
taken to ensure that the DGPS receiver and the GPS receiver are compatible for this procedure to