Global Positioning System Reference
In-Depth Information
⎧
⎪
PR
=
d
+
Δ
ΔΔ
ΔΔΔ
Δ
1
1
cable
PR
=+ +
d
⎨
2
2
cable
cable
12
(6)
PR
=+ + +
d
⎪
⎪
3
3
12
23
PR
=+ + + +
d
ΔΔΔ
⎩
4
4
cable
12
23
34
Where the PR
k
are the indoor pseudo-ranges measured by the receiver, Δ
cable
is the common
part of the delay in the cable between the generator and the first repealite (including error
and clock bias between the generator clock and the clock of the receiver), the Δ
uw
are the
delays between repealite R
u
and R
w
and the d
k
are the indoor geometric distances between
repealite R
k
and the indoor receiver.
The locations of the transmitters have to be known
27
, as usual, and the indoor position is
computed in a local referential with a classical GNSS algorithms. Note that the velocity can
also be calculated in the local referential, just like GNSS outdoors, since the contribution of
the clock drift of the generator to the Doppler is common to the 4 repealites and that the
only contribution of the signal to Doppler is the relative velocity between the antenna of the
indoor receiver and the antenna of repealite R
i
.
5.3 The main advantages
The fact that repealites are transmitting in a continuous way allows us to follow the carrier
phase of the signal, a source of potential improvements in the positioning accuracy. This
feature could lead to a similar operating mode to carrier phase pseudolites, but this is not
the main objective here. Another interesting improvement compared to repeaters is the
ability to carry out dynamic positioning with no restriction since instantaneous
measurements and calculations are carried out. It is also noticeable that dynamic positioning
is bound to be of better quality since the receiver movement will have a direct impact on the
average multipath distribution, leading to a more efficient averaging of their effects.
The continuity with outdoor GNSS is even simplified in comparison with a repeater where a
switch between the outdoor mode and the indoor mode and its cycling scheme was
required. With repealites, this switch only concerns the PRN number used which should be
characteristic of indoors: the same apply to pseudolites.
The last main advantage is associated to synchronisation. The fact of using a single signal is
an advantage in comparison to pseudolites, but does not allow the synchronisation problem
to be completely removed since transmitters still have to be synchronised. This is currently
achieved through wire connections, either by coaxial cables or by the way of optical fibres
28
.
The synchronisation of the system is obtained once several measurements are carried out at
known locations.
5.4 The remaining limitations and the ways they are dealt with
The two most important remaining limitations are respectively the multipath and the near-
far effect. Multipath effects are dealt with through the use of the SMICL. Note that good
27
Some works are under consideration in order to propose methods for auto-positioning the
transmitters.
28
Optical fibres are also considered for the physical realization of the time delays between repealites.
Search WWH ::
Custom Search