| ipexSR - a GNSS Software
Receiver
The
ipexSR is designed to implement and test different
acquisition
and tracking algorithms. Currently a standard correlation
technique and the CRUSR
technology is implemented.
CRUSR
Tracking
The CRUSR
(Cramer-Rao
Under Sampling Receiver) implements a novel tracking
channel structure exploiting the flexibility of a software receiver. It consists
of three reference signal generators, one for Doppler removal,
one punctual code generator and one for
code tracking (usually generating an early minus
late replica of the code sequence). The code generators
make use of user defined and precomputed lookup tables such that the same
tracking channel can be used to implement an early minus
late tracker, a strobe correlator or more sophisticated
structures like a Cramer-Rao correlator or a third derivative
correlator. NCO, integrate and dump registers
and loop filters are implemented in a standard way.
Exemplary reference signals for code tracking
are shown in the figure
below.
The black
line is an early minus late difference of the punctual code sequence (a
correlator spacing of d=0.2 and a BOC(2,2) modulation scheme is used). The
green line is the first derivative of the band limited BOC(2,2) signal at
baseband and is used for a Cramer-Rao correlator, which has optimal thermal
noise performance.
Standard
Tracking
The standard correlation technique is based on textbook
knowledge. An early-minus-late non-coherent dot-product correlator with a
correlator spacing of d=1 is used. For phase tracking
an arc tan phase discriminator is used to
steer the PLL.
Parameters
The tracking
channel allows to configure the coherent integration time (for GPS C/A: 1-20
ms), the output rate of the pseudorange measurements (highest rate can be
1 kHz), the DLL, PLL and FLL parameters and the tracking threshold values.
Also the under sampling ratio can be configured.
Under
sampling is used when the code and carrier lock is achieved and the
signal-to-noise ratio is high enough.
Display
The receiver
displays the nominal receiver time and for each tracked signal the Doppler
frequency, the code phase, the lock status, the SNR, and parts of the navigation
message. Below the code and phase pseudorange are displayed.
Channel
logic
Currently
the receiver implements a serial search technique. It will be upgraded to
a FFT based technique used in the signal monitor. The serial search is visualized
in the following two figures:
The left
image show code and phase pseudorange before and after signal acquisition.
The right figure shows
the Doppler frequency during signal acquisition and
while FLL and PLL tracking.
The output of the prompt I and Q channel are shown in the next
figure. The plot shows three periods: before acquisition, FLL tracking and
PLL tracking.
Output
All possible data can be
retrieved from the software receiver. Currently
the data is written in a ASCII file and Matlab is used for visualization.
Two exemplary outputs are shown here. The first one is the power spectrum
of the prompt I channel for a GPS C/A code signal. One sees the power
spectrum of the navigation data message. The second one
is difference in [m] of the code pseudorange minus
carrier phase pseudorange. It is usually used to assess the code tracking
performance, e.g. to detect code
multipath.
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