ipexSR
- a GNSS Software Receiver
The ipexSR
(Institute of Geodesy and Navigation
PC-based
Experimental
Software
Receiver) is a GNSS receiver realized completely
in software (Visual C++/assembler) capable of tracking GPS and GNSS signals
in real-time or post-processing. It implements various tracking and positioning
techniques, including the CRUSR (Cramer-Rao Under Sampling Receiver) technology
and has been developed since
2002.
A screenshot showing the real-time data input, the tracking,
the single point positioning and the signal monitor is shown below (click
to enlarge):
The basic data flow is illustrated in the following figure
(click to enlarge):
The receiver accepts as input signals IF samples taken
from hard disc (post-processing mode) or from a National Instruments ADC
card (real-time). The ipexSR performs all tasks
of a conventional hardware receiver, like acquisition, tracking and positioning.
It also includes a signal monitor which analyzes the GNSS signals present
in the IF data stream. At the moment the ipexSR
has been validated for GPS C/A code signals and
will be updated in 2004 to track the GPS L2 civil signal and Galileo type
signals.
ipexSR:
Signal Monitor
ipexSR:
Tracking
ipexSR:
Positioning
ipexSR:
Applications and Publications
Processing Performance
An important
issue of a software receiver is the required processing power to process
GNSS signals in real-time. A measure, called MCOPS (Mega Correlations per
Second), has been established to quantify the processing power of a GNSS
receiver. One correlation in this context means processing of one
IF sample in one channel of a receiver. It includes generation of a reference
sample and multiplication of the reference signals with the IF sample. The
MCOPS performance of various software and hardware receivers is shown in
the figure below.
For
example the a low bandwidth (2 MHz) 12 channel GPS receiver, has a processing
performance of 48 MCOPS. For the ipexSR an optimized code has been developed
making use of the Intel Pentium IV SSE2 multi media extensions. This code
achieves nearly 200 MCOPS on a single P4 CPU running at 3 GHz.
The
CRUSR technology implemented in the ipexSR makes also use of a special technique
to reduce processing power at the cost of a decreased signal-to-noise ratio.
Only every n'th IF sample is processed and the value of n can be chosen be
the user. For an explanation of the under sampling technique click
here.
Point of contact: Thomas
Pany |
