Global Positioning System Reference
In-Depth Information
6.3 The case of indoor pseudolites
The GPS predicted the need for terrestrial generators when reserving the specific codes,
PRN 33 through 37, for ground transmitters. Galileo also included the possibility of using
such transmitters. Note that since codes are different from satellite's ones, the limitations are
a little bit relaxed in comparison to repeaters.
The following lines are based on report 168 of the ECC (ECC report 168), dated May 2011,
and relate to indoor pseudolites. Similar to the case of repeaters, computations were carried
out on the base on interference evaluations in the various GNSS associated frequency bands.
For the L1 band, the main conclusions are as follows:
•
The radiated power should not exceed -50dBm.
•
The antenna of the pseudolite should point at the ground and be directed towards the
inside of the building.
•
The radiated power for an elevation angle superior to 0 degree should be reduced by
more than 6dB.
•
The radiated power should be reduced to -59dBm in airport areas and specific
mitigation techniques implemented when aircraft are in their parking stands.
Note that the power level is rather high in comparison to repeaters and largely sufficient in
order to have all the techniques described in the chapter implemented in real conditions
with good performance. As a matter of fact, the estimated range with -60dBm is around one
hundred metres in real environments, i.e. including walls and multiple floor levels
(ceilings). The remaining 10dB margin could be used in order to provide the receiver with a
high SNR, required for the SMICL for instance. On the other hand, the interesting feature
that consists in positioning a pseudolite on the ground pointing at the top of the building (in
order to substantially increase the VDOP) will have to be implemented with a 6dB reduced
maximal power.
In addition, report 168 states the same as for repeaters concerning individual
authorisations, insertion of guidance instructions in order to help the applicant in the
deployment and interdiction of mobile pseudolites. It is also proposed that some
authorities (military, government and meteorological services) be allowed to apply for
specific site limitations.
Moreover, the report mentioned that longer codes could improve both the compatibility
with non-participative receivers and the performance of participative ones. Note that
research works are on-going in this direction.
7. Synthesis and future trends
The GNSS-like signal indoor positioning systems, either based on pseudolites, repeaters or
repealites are a real alternative in order to provide users with a continuous service, at the
cost of deploying a local infrastructure. This is now possible in particular thanks to
multipath and near-far effect mitigation techniques. Performance attainable is in the metre
range through rather good quality measurements and elementary computation algorithms.
In comparison, such solutions as WiFi based ones, are based on low quality measurements
(power level typically) and complex computation algorithms.
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