ashtech DG14, Справочное руководство

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54 DG14 and DG16 Board & Sensor Reference Manual
meters with SA on. In differential mode, a DG16 in rover mode can achieve
sub-meter accuracy.
Real-time differential operation requires a communication link between the
base and rover receivers. A wireless link, such as a radio-modem link or
cellular/modem link is typically used, although any other medium that can
transfer digital data can be used.
Sources of Error
The major sources of error affecting the accuracy of GPS range
measurements are satellite orbit estimation, satellite clock estimation,
ionosphere, troposphere, multipath, and receiver noise in measuring range.
The first four sources of error are almost totally removed by differential
corrections. The residual error is in the order of one millimeter for every
kilometer of separation between base and remote receivers.
Receiver noise is not correlated between the base and the remote receiver
and is not canceled by differential GPS. However, in the DG16, integrated
doppler measurements are used to smooth the range measurements and
reduce the errors resulting from receiver noise.
At the instant an satellite is locked, there is also RMS noise affecting the
range measurement. RMS noise is reduced over time by the square root of
the number of measurements computed by the receiver. For example, after
100 seconds of locking to a satellite, the rms noise in range measurement is
reduced by a factor of 10 (one meter of noise is reduced to 0.1 meter). The
noise is further reduced with each additional measurement.
If the lock to a satellite is lost, the noise goes back to one meter and
smoothing starts from the one-meter level. The loss of lock to a satellite is
rare, and typically happens only when the DG16 antenna’s line of sight to the
satellite is blocked by an object, or when the satellite goes below the horizon.
Total position error (or error-in-position) is a function of the range errors (or
errors-in-range) multiplied by the PDOP (three-coordinate position dilution of
precision). PDOP is a function of satellite geometry; that is, the positions of
the satellites in relation to one another.
RTCM Messages
The DG16 accepts differential correction messages in the RTCM format (refer
to
RTCM Recommended Standards for Differential GNSS , version 3.0). The
DG16 can be set to output or receive RTCM messages using either of its two
ports by issuing the set command
$PASHS,RTC,s1,c2 where s1 is either
BAS (base station mode) or REM (remote mode) and c2 is the port
designator for the input or output of differential corrections. The DG16