SXBlue GPS Reference Manual
79
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 SXBlue GPS 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 SXBlue GPS includes a primary source of conventional real-time corrections –
Space-Based Augmentation System (SBAS). External corrections may also be input to
the SXBlue GPS 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 SXBlue GPS also has a documented binary raw measurement protocol. A RINEX
translator is available from Geneq 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 SXBlue GPS 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.
