Global Positioning System Reference
In-Depth Information
The system used for these experiments consists of a few (typically four) transmitters which
are located indoors and which transmit a signal provided by a GNSS-like signal generator
(we used both an AeroFlex GPS-101 and a Spirent GSS6560). Note that only one such signal
is required since the approach proposed is based on the transmission of the same signal
through the various transmitters deployed. Note also that in order to satisfy the ongoing
various regulations (both in the US and in Europe, briefly described in a following section)
the power transmitted is limited (from -80dBm to -65dBm). The principle of the approach is
given in the figure 17. The transmitting antennas had a radiating pattern with a maximal
gain of around 3dBi.
Signal generator
Transmitter 3
Transmitter 2
Transmitter 4
Transmitter 1
Electronic box
(carries out the amplification, the
cycling and the splitting of the
signals towards the transmitters)
GPS Receiver
(performs the pseudo-range measurements
at the instant of the transition from one
repeater to the next)
Indoor
receiving antenna
Fig. 17. The system as it was deployed
A summary of the results obtained, all environments included, is given in figures 18 and 19.
The first figure shows the results obtained in classrooms, an amphitheatre and an entrance
hall. About 20 different locations have been tested in these environments. The various
curves represent different ways to filter the resulting fixes obtained. The “unfiltered” curve
takes into account all the fixes, with no filtering at all. The other three curves, named “-xm”,
give the resulting fixes obtained once we remove the ones that are outside the largest
rectangle defined by the locations of the transmitters by more than x metres. Note that this is
achieved for two main reasons: outside this rectangle, the DOP values increase very rapidly
and the positioning algorithms sometimes do not converge.
Figure 19 is a summary of the results obtained in all experiments and with various receivers.
In red in the figure are the results obtained in the car park, and the two blue curves are the
results obtained in the other environments described. The two curves have been obtained
with -80dBm and -65dBm respectively.
The main conclusion is that the current performances are roughly in the range of 3 to 4
metres for 80% of the fixes. It is of uppermost importance to understand that this can be
considered as really raw fixes since calculations are carried out totally independently from
one fix to the next. It is highly probable that basic smoothing or filtering (applied on
pseudoranges or locations) would lead to a significant improvement. In addition, a complete
continuity with outdoor GNSS is achieved since velocity computations are also possible
(Samama and Vervisch-Picois 2005).
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