Global Positioning System Reference
In-Depth Information
can reach a maximum of less than 2dB, but is a real problem indoors. In terms of
phenomenon, the problem is related to the fact that if secondary peaks of a transmitter are
greater than the main peak of another, then this second one will appear as noise and will not
be detectable. This 24dB margin in power is reached as soon as the ratio between the closer
and the farther transmitters reaches four: this is not an unusual situation indoors. Thus, a
few solutions have been proposed, among which: 1/ pulsed transmissions consisting in
allocating between 10 and 20 percent of the time to a particular pseudolite (this has shown
to provide an additional margin of about 10dB corresponding to nearly an additional factor
of two in distance), 2/ frequency shifts in order to almost eliminate the near-far effect, but at
the cost of a substantial increase in the terminal complexity or 3/ in sophisticated mitigation
algorithms that successively suppress the more powerful signals to finally extract the lowest
one.
Interferences
with outdoor signals. Another advantage of pseudolites is the ability to decide
the power level to be transmitted, depending on the required coverage and performance,
and of course on the environments. This advantage becomes a major drawback when
thinking in terms of cohabitation with the outdoor world (Glennon et al. 2007, Yang and
Morton 2009). If one takes the case of GPS (but this is true whatever the system considered),
using GPS-like signals for indoor transmission is susceptible to create interference with the
signals that could be received by an outdoor receiver receiving signals from the satellites. As
a matter of fact, the same phenomenon as described indoors for the near-far may occur.
Thanks to GPS project management, some specific PRN codes have been reserved for
pseudolite operation
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at the early stages and this interference problem is slightly relaxed,
but is still a real concern for GPS authorities. A specific section, at the end of this chapter, is
dedicated to the regulations restricting the power levels allowed to be transmitted for
indoor operations.
Finally,
multipath
are a major issue. Mitigation techniques must be found in order to imagine
a proper operation of the code phase pseudolite system. This topic is such a challenge that
the next section is dedicated to it.
3.5 The specific problem of multipath in indoor environments
As already discussed in previous sections, indoor environments are characterized by the
presence of many reflectors in the path from the transmitter and the receiver. All these
reflected signals are going to combine at the receiver end and produce the really received
signal on the receiver antenna. This signal is the one that the receiver is going to deal with
since this is the real physical received signal. As this is not only the direct signal from the
transmitter, and depending on the signal processing techniques used, the distance finally
measured can be erroneous (remember that as a matter of fact, this is a time that is measured
and not a distance).
From a physical point of view, the situation can be seen as follows: the physical quantity
that is transmitted is indeed an electric field, given in V/m. It is furthermore characterized
by a frequency, an amplitude, a phase and a delay, in comparison, say, with the first
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PRN 1 to 32 are reserved for so-called space vehicles, the satellites, and PRN 33, 34, 35, 36 and 37 are
reserved for terrestrial transmitters.
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