Seiwa Si-Tex Vector 3D GPS - Manual (Page 22)

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PS Overview
Vector 3D User Guide
For your convenience, both the GPS and SBAS operation of the Vector 3D features automatic operational
algorithms. When powered for the first time, the Vector 3D performs a "cold start," which involves
acquiring the available GPS satellites in view and the SBAS differential service.
If SBAS is not available in your area, an external source of RTCM SC-104 differential corrections may be
used. If you use an external source of correction data, it must support an eight data bit, no parity, one stop bit
configuration (8-N-1).
GPS Operation
The GPS receiver is always operating, regardless of the DGPS mode of operation. The following sections
describe the general operation of the 3D’s internal GPS receiver.
Note: Differential source and status have no impact on heading, pitch, or roll. They only have an impact on
positioning.
Automatic Tracking
The 3D’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 3D works by finding four or more GPS satellites in the visible sky. It uses information from the
satellites to compute a position within 4.0 m (8.2 ft). Since there is some error in the GPS data calculations,
the 3D also tracks a differential correction. The 3D uses these corrections to improve its position accuracy to
better than 1.0 m (1.97 ft).
There are two main aspects of GPS receiver performance:
• Satellite acquisition
• Positioning and heading calculation
When the 3D is properly positioned, the satellites transmit coded information to the antennas on a specific
frequency. This allows the receiver to calculate a range to each satellite from both antennas. 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 and a valid heading.
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 3D will result in positioning accuracies of 4.0 m (8.2 ft) 95% of the time. In order to
improve positioning quality to better than 1.0 m (1.97 ft), the 3D is able to use differential corrections
received through the internal SBAS demodulator or externally-supplied RTCM corrections
.
Automatic SBAS Tracking
The 3D automatically scans and tracks SBAS signals without the need to tune the receiver. The 3D features
two-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.