Product specifications

11
P OSITIONING A CCURACY
The Crescent Vector is a sub-meter 95% accurate product under ideal
conditions horizontally (minimum error). To determine the positioning
performance of the Crescent Vector, Hemisphere GPS gathers a 24-hour data
set of positions in order to log the diurnal environmental effects and also to
log full GPS constellation changes. Shorter data sets than 24 hours tend to
provide more optimistic results.
Keeping in mind that this horizontal performance specification is a real world
but ideal scenario test, obstruction of satellites, multipath signals from
reflective objects, and operating with poor corrections will detract from the
module’s ability to provide accurate, reliable positions. Differential
performance can also be compromised if the Crescent Vector module is used
in a region without sufficient ionospheric coverage. If external corrections are
used, the baseline separation between the remote and base station antennas
can affect performance.
Since the Crescent Vector will be used in the real world, blockage of the line
of sight to SBAS satellites is often inevitable. The COAST function provides
solace from obstruction of SBAS services for up to 30 to 40 minutes,
depending on the amount of tolerable performance drift.
The estimated positioning precision is accessible through the use of NMEA
0183 command responses as described in Chapter 6 (The GST NMEA data
message). As the receiver is not able to determine accuracy with respect to a
known location in real time (this is traditionally performed in post-mission
analyses), the precision numbers are relative in nature and are only
approximates.
U PDATE R ATES
The update rate of each NMEA and binary message of the Crescent Vector can
be set independently with a maximum that is dependant upon the message
type. Some messages have a 1 Hz maximum, for example, while others are
20 Hz. The maximum default rates of the Crescent Vector are 5 Hz position,
and 20 Hz heading. Higher position update rates of 10 or 20 Hz are an option
and can be obtained at an additional cost.
SBAS
The following sections describe the general operation and performance monitoring of
the SBAS demodulator within the Crescent Vector module.
A UTOMATIC T RACKING
The SBAS demodulator featured within the Crescent Vector will automatically
scan and track two SBAS satellite signals, specified by the user by the
$JWAASPRN command (defaulted to both WAAS satellites). This automatic
tracking allows the user to focus on other aspects of their application rather
than ensuring the receiver is tracking SBAS correctly.