Global Positioning System Reference
In-Depth Information
Another interesting proposition concerns the potential use of maximal sequences that have
the advantage of providing us with a unique value of auto-correlation outside the main
peak (Vervisch-Picois and Samama 2009). Thus, it is possible to carry out differences
without the need for a half chip delay for the additional signal. The implementation is then
quite easy and can be applied to an almost unlimited number of repealites.
In these cases, interference with outdoors is a very interesting and challenging topic since
regulations are appearing in order to “preserve” the GNSS bands. The fact of using similar
codes to those used outdoors is a real concern which could find an elegant solution
through the use of originally designed sequences. Of course, a frequency shifted approach
would definitively solve the interference problem with outdoors, but would require new
frequency resources in the case of the modern Code Division Multiple Access (CDMA)
GNSS systems.
5.5 A few preliminary estimated performances
The smoothing of the code with the carrier phase is a very classical operation in GNSS. It
consists of using the low noise carrier phase measurements in order to smooth the pseudo-
range measurements. It is very efficient in order to reduce thermal noise but not really for
multipath. Thus, the coupling of the SMICL with this smoothing technique is a very nice
combination. The Kalman filter implemented is then nearly optimal. Note that indoors, the
main error source comes from multipath, since no atmospheric contributions or clock bias
errors of transmitters (in this repealite based configuration) are present. In the present case,
the filter uses the carrier phase measurement in order to carry out its estimation of the
future state.
Simulations have been carried out considering a circular displacement of a pedestrian in a
place where a severe multipath (only one) is present, sometimes of even greater amplitude
than the direct path from a transmitter. This is achieved through a perfect reflector located
in the close vicinity of the trajectory. As can be seen in figure 25, the repealites are located in
R4
R3
h3 = +3 m
h4 = -3 m
20 m
Receiver trajectory
h1 = +3 m
h2 = -3 m
R1
R2
Fig. 25. Considered trajectory and repealite distribution
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