A325 GNSS Smart Antenna User Guide
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PN 875-0318-000 Rev D2
Chapter 3: GPS Overview
This chapter describes the various modes of operation and features of your A325 and internal
sensors.
For your convenience, both the GPS and differential correction of the A325 are pre- configured. The
receiver will work out of the box, and for most applications, little user setup is necessary. When
powered for the first time, the A325 will perform a “cold start,” which involves acquiring the
available GPS satellites in view and the SBAS differential service.
GPS Operation
The GPS receiver is always operating, regardless of the DGPS mode of operation. The following
sections describe the general operation of the A325’s internal GPS receiver.
Automatic Tracking
The A325’s internal GPS receiver automatically searches for GPS satellites, acquires the signals, and
manages the navigation information required for positioning and tracking.
Receiver Performance
The A325 works by finding four or more GPS satellites in the visible sky and uses information from the
satellites to compute a position within 2.5 m. Since there is some error in the GPS data calculations,
the A325 also tracks a differential correction. The A325 uses these corrections to improve its position
accuracy to better than 0.6 m.
The two main aspects of GPS receiver performance are 1) satellite acquisition, and 2) positioning
and heading calculation.
When the A325 is properly positioned, the satellites transmit coded information to the antenna on a
specific frequency. This allows the receiver to calculate a range to each satellite. GPS is essentially a
timing system. The ranges are calculated by timing how long it takes for the signal to reach the GPS
antenna. The GPS receiver uses a complex algorithm incorporating satellite locations and ranges to
each satellite to calculate the geographic location and heading. Reception of any four or more GPS
signals allows the receiver to compute three-dimensional coordinates.
Differential Operation
The purpose of differential GPS (DGPS) is to remove the effects of selective availability (SA),
atmospheric errors, timing errors, and satellite orbit errors, while enhancing system integrity.
Autonomous positioning capabilities of the A325 will result in positioning accuracies of 2.5 m 95%
of the time. In order to improve positioning quality to better than 0.6 m 95%, the A325 is able to
use differential corrections received through the internal SBAS demodulator or through externally-
supplied RTCM corrections.
Automatic SBAS Tracking
The A325 automatically scans and tracks SBAS signals without the need to tune the receiver. The
A325 features three-channel tracking that provides an enhanced ability to maintain a lock on an
SBAS satellite when more than one satellite is in view. This redundant tracking approach results in
more consistent tracking of an SBAS signal in areas where signal blockage of a satellite is possible.
