Vector Sensor Reference Manual 126
intentional degradation is called Selective Availability (SA). The effect of SA has been turned to
zero since mid-2000, however, it has not been officially ‘turned off’.
Currently, autonomous GPS is able to provide accuracy on the order of 10 meters, depending on
the sophistication of the GPS engine. For many positioning and navigation applications, this level of
accuracy is not sufficient, and differential techniques must be employed.
Differential GPS
The primary sources of errors that degrade GPS performance include SA (currently set to a zero
effect), atmospheric errors, timing errors, satellite orbit errors, and multipath. Differential GPS
(DGPS) is essentially a differencing process that removes sources of error from the GPS position
solution and improves the integrity of the GPS position solution.
There are a number of methods of differential measurement correction.
• Conventional real-time differential - This is the most common form of correcting GPS errors in real-time
with corrections sent to the rover GPS receiver by some form of communications equipment.
Conventional real-time differential uses C/A code range measurements and their associated corrections.
Carrier phase corrections are not used with this form of differential technique.
• Post processing - This method is often used when either higher accuracy than achievable through
conventional differential is needed, or a conventional form of real-time corrections is not available in the
region where the rover receiver is being operated. Depending on receiver hardware and the
methodology used for post process, performance can be from many centimeters to millimeter precision.
A variety of 3
rd
party software packages are available to post process GPS raw measurement data. The
Vector Sensor can be configured to output raw measurement data at rates of up to 5 Hz in a proprietary
format. This data can be converted to an industry standard RINEX format if needed.
• Real-Time Kinematic - This method uses more sophisticated techniques to resolve the number of
wavelengths between the satellite and the user, to provide centimeter-level positioning (or better) in real-
time. This technique uses high-end receiver hardware, antennas, and internal operating software to
compute accurate position solutions. The compromise with this method of differential correction is
increased system complexity, cost, and operating constraints.
The Vector Sensor includes two primary sources of conventional real-time corrections - beacon
DGPS and Space-Based Augmentation System (SBAS). External corrections may also be input to
the Vector Sensor for situations where either internal correction services is not available or an
external source is preferential.
In addition to the conventional differential positioning with internal sources of corrections, the
Vector Sensor also has a documented binary raw measurement protocol. A RINEX translator is
available from CSI Wireless in the event that this may be useful, in addition to some C code
snippets to aid in integrating the binary format itself into your own application.
For heading determination, the Vector Sensor uses a moving base station RTK solution. This
allows for a very precise computation of heading regardless of whether or not the receiver is in
differential mode using either internal source of corrections or those from an external source.
Conventional Real-Time Differential
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