Vector Sensor Reference Manual 127
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
correction services, including many private reference stations and DGPS beacon systems. The
Vector Sensor smart antenna supports this correction protocol via either of its two serial ports.
In addition to the RTCM standard, the Radio Technical Commission for Aeronautics has a
differential service intended for wide area correction services, designated RTCM SC-159. The
United States Federal Aviation Administration’s Wide Area Augmentation System (WAAS) and
other compatible Space Based Augmentation Systems (SBAS) such as the European Geostationary
Navigation Overlay System (EGNOS) and the Japanese MT-SAT Satellite-based Augmentation
System (MSAS) use this data format. The Vector Sensor system is compatible with each of these
differential services.
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