Global Positioning System Reference
In-Depth Information
4.1 Introduction to the basic idea
The first simplification concerns the synchronisation. In a similar way that outdoor
pseudolites can synchronise themselves using GNSS signals, the idea here is to put an
outdoor antenna on the roof of the building in order to obtain the constellation signals.
Note, that in this case the antenna is probably (certainly indeed) receiving several satellite
signals. Here is taken into account the second new idea that consists in forwarding this
signal to the transmitters: the innovation lies in the fact that the same signal (which is
probably made up of many satellite signals, as mentioned) will then be transmitted from the
various transmitters, now called repeaters 20 . In this way, an obvious problem appears: if the
repeaters are transmitting simultaneously, the same signals will be transmitted from
different locations and, once received by the terminal, will certainly be considered as
reflected paths. Since the principle is to carry out time measurements, and thus distance
measurements, it is clearly not acceptable. Thus, the transmission is now achieved in a
sequential manner with always only one repeater transmitting at a given time. This presents
another interesting advantage: the near-far effect is now removed 21 .
4.2 The systems proposed
Two measurement systems are then possible.
The first one carries out the computation of the location, at the receiver's end, for each
transmitter successively. At each corresponding time the fourth coordinate (the so-
called clock bias 22 ) of the navigation solution vector is recorded. As soon as four
successive computations have been obtained, it is possible to compute the indoor
distances through the calculations of the differences between the fourth coordinates
considered at different time. These differences give a new system of three independent
equations that can be solved classically. The resolution gives the indoor location of the
receiver. A short demonstration of this principle is given below.
The second one carries out some differences of pseudo-range measurements at the
precise instant of the transitions from one repeater to the next (Fluerasu et al. 2009,
Fluearsu and Samama 2009). At these instants, the difference of the pseudo-ranges that
are measured just before and just after the transition shows the value of the difference
of distances between the two repeaters and the receiver. In order to obtain the indoor
distances, a second difference is needed, as briefly explained below. Note that this
approach also removes all the effects whose second derivative is zero, including
20 Please note that if the term is appropriate since transmitters are just “repeating” the outdoor received
signal, it should not be confused with the classical repeater that is used for demonstration purposes or
just for having outdoor signals available indoors. Here repeaters represent a new approach for indoor
positioning and should be seen more as a means of improving some aspects of pseudolite rather than
just forwarding signals. It is so true that all the sections could have been written with a signal generator
instead of the outdoor antenna.
21 Only the dynamic range is now a limitation when the receiver is processing signals from two
successive repeaters.
22 This is clearly not the clock bias, but indeed the sum of all contributions that are common to all the
satellites that are considered for the resolution: thus, this included the free space indoor distance that
we want to obtain.
Search WWH ::




Custom Search