Global Positioning System Reference
In-Depth Information
6.1 General introduction
The problem is due to the inter correlation functions (ICF) of the various code sequences
that are used. As a matter of fact, these ICF have small peaks, comparable to the secondary
peaks of the ACF. If the number of the ground based transmitters is too high or if the total
power is too high, then the addition of these secondary peaks is likely to generate
interferences to an unacceptable level for outdoor receivers.
Two different cases have been considered by the regulatory authorities: the repeaters and
the pseudolites. The repeater case corresponds to a transmitter which uses the outdoor
available signals and, after amplification, retransmits them indoors. The ICF between indoor
signals and outdoor ones can been considered as being indeed ACF, thus leading to
potentially higher interferences. Thus, the maximal acceptable power associated with
repeaters is lower than for pseudolites
29
.
6.2 The case of the repeaters
In the United States it is not legal to sell GPS repeaters and only the Federal government or
agencies operating under its direction, parties that would have received either a Special
Temporary Authority (STA) or an Experimental License, or parties operating in an anechoic
chamber are authorised to use such devices.
In Europe, things are a little bit different and regulations are based on the Electronic
Communications Committee (ECC) report 145 (ECC report 145), dated May 2010. Studies
were carried out on the base on interference evaluations in the various GNSS associated
frequency bands. Let us concentrate on the L1 band (1559 to 1610 MHz). The global
conclusions are as follows:
•
The maximum gain of the repeater, from outdoor antenna to indoor antenna should be
limited to 45dB.
•
The radiated power
30
should not exceed -77dBm.
•
The maximum power re-radiated that are not GNSS signals should be less that -20dBm.
•
The repeater should include filtering.
Some experimental results presented in previous sections were carried out with -80dBm and
have shown acceptable performance within a typical range of 20 metres.
In addition to the above technical recommendations, report 145 states that any
authorisations should include guidance instructions in order to help the applicant in the
deployment phase of the repeaters. Also, particular attention is recommended for
installations close to airports or to military sites.
Finally, the report proposes that any uses of repeaters should be subject to individual
authorisation and that no mobile use should be permitted.
29
Please note that the various indoor positioning systems proposed in this chapter have to be
considered as « pseudolite based » for regulation purposes, although the so-called « repeater based »
approach could also be implemented using repeaters (in the sense of the regulations), and then fall into
the corresponding regulation, of course.
30
The so-called eirp (Equivalent isotropically radiated power).
Search WWH ::
Custom Search