SX BLUE GPS iSXblue II+ GNSSTM, Руководство пользователя

Страница: 89 / 107

SXblue Series User Manual 88
The SXblue receiver also uses an RTK solution option which allows cm level field
data collection, precise guidance and stake-out applications.
Conventional Real-Time Differential
The majority of GPS navigation and positioning uses this form of positioning.
Conventional real-time differential techniques are more robust in their usage and
versatility than post-processing or RTK solutions. They are tolerant to errors in
communication of the real-time corrections from the base station or correction
network, provide a reasonable amount of accuracy (sub-meter accuracy is best-
case), and can be simply turned on and used without too much regard other than
ensuring a lock to GPS satellite signals and the correction communication link.
How it Works
Conventional DGPS involves setting up a reference GPS receiver at a point of
known coordinates. This receiver makes distance measurements, in real-time, to
each of the GPS satellites. The measured ranges include the errors present in the
system. The base station receiver calculates what the true range, without errors,
knowing its coordinates and those of each satellite. The difference between the
known and measured range for each satellite is the range error. This error is the
amount that needs to be removed from each satellite distance measurement in
order to correct for errors present in the system.
The base station transmits the range error corrections to remote receivers in real-
time. The remote receiver corrects its satellite range measurements using these
differential corrections, yielding a much more accurate position. This is the
predominant DGPS strategy used for a majority of real-time applications.
Positioning using corrections generated by DGPS radiobeacons will provide a
horizontal accuracy of 1 to 5 meters with a 95% confidence. More sophisticated,
short-range DGPS systems (10 to 15 km) can achieve centimeter-level accuracy,
but are expensive and often limited to precise survey applications due to technical
constraints on their use.
DGPS Format
For manufacturers of GPS equipment, commonality is essential to maximize the
utility and compatibility of a product. The governing standard associated with GPS
is the Interface Control Document, ICD-GPS-200, maintained by the US DoD. This
document provides the message and signal structure information required to
access GPS.
Like GPS, DGPS data and broadcast standards exist to ensure compatibility
between DGPS services and associated hardware and software. The Radio
Technical Commission for Maritime Services Special Committee 104 has
developed the primary DGPS standard associated with conventional DGPS,
designated RTCM SC-104 V2.2. This correction standard is used by many