Hemisphere GPS A222, Руководство пользователя

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A222 User Guide Chapter 3 – Using A222 Page 13 of 20
Using A222
For your convenience, both the GNSS and differential correction of the A222 are preconfigured. The receiver will
work out-of-the-box, and for most applications, little user setup is necessary. When powered for the first time, the
A222 will perform a “cold start,” which involves acquiring the available GNSS satellites in view and the SBAS
differential service.
GNSS Operation
The GNSS receiver is always operating, regardless of the DGNSS mode of operation. The following sections
describe the general operation of the A222’s internal GNSS receiver.
Automatic Tracking
The A222’s internal GNSS receiver automatically searches for GNSS satellites, acquires the signals, and manages
the navigation information required for positioning and tracking.
Receiver Performance
The A222 works by finding four or more GNSS satellites in the visible sky and uses information from the satellites
to compute a position within 2.4 m. Since there is some error in the GNSS data calculations, the A222 also tracks a
differential correction. The A222 uses these corrections to improve its position accuracy to better than 0.6 m.
The two main aspects of GNSS receiver performance are 1) satellite acquisition, and 2) positioning and heading
calculation.
When the A222 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.
GNSS is essentially a timing system. The ranges are calculated by timing how long it takes for the signal to reach
the GNSS antenna. The GNSS 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 GNSS signals allows
the receiver to compute three-dimensional coordinates.
Differential Operation
The purpose of differential GNSS (DGNSS) is to remove the effects of atmospheric errors, timing errors, and
satellite orbit errors, while enhancing system integrity. Autonomous positioning capabilities of the A222 will result in
positioning accuracies of 2.4 m 95% of the time. To improve positioning quality to better than 0.6 m 95%, the A222
can use differential corrections received through the internal SBAS demodulator or through externally supplied
RTCM corrections.
Automatic SBAS Tracking
The A222 automatically scans and tracks SBAS signals without the need to tune the receiver. The A222 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.