AvMap ekp iii c ユーザーズマニュアル

EKP III USER MANUAL

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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).

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 applica-

tions 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.

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.

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 communi-

cated 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 can-

not 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 correc-

tions to the navigation data it uses to compute a position solution and displays differentially