iSXblue/SXBlue II GNSS Series Technical Reference Manual
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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 iSXBlue/SXBlue II
GNSS can be configured to output raw measurement data at rates of up to 20 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 iSXBlue/SXBlue II GNSS includes a primary source of conventional real-time corrections –Space-
Based Augmentation System (SBAS). External corrections may also be input to the iSXBlue/SXBlue II
GNSS 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
iSXBlue/SXBlue II GNSS 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.
The iSXBlue/SXBlue II GNSS also uses an RTK solution option in two versions: fixed and floating-only
solution. This allows for 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.
