Global Positioning System Reference
In-Depth Information
represents the remaining contributions whose first derivatives are not zero, for example the
clock acceleration) and 2/ a characteristic shape of the curve just after the skips, which is
due to the receiver's loop that tends to come back to the equilibrium after this
destabilisation. Note that the sum of the transitions, for a complete cycle should be zero. Of
course, due to measurement errors, this is usually not the case, and the choice of the best
transitions to be considered for positioning has to be carried out.
Time
Time
-43
0
-43
0
20
20
40
40
60
60
80
80
100
100
120
120
140
140
Cycle
Cycle
-43,5
-43,5
Rep 2
Rep 2
Rep 3
Rep 3
Rep 1
Rep 1
-44
-44
-44,5
-44,5
-45
-45
-45,5
-45,5
Transitions
Transitions
-46
-46
Pseudo-ranges differences
Pseudo-ranges differences
Fig. 12. Typical response of a receiver
The curve of figure 12 is a single difference of raw measurements. In order to extract the
differences of distances mentioned above, there is the need to carry out, at the precise
instant of transition, a second difference between two successive single differences. Thus, a
process of double differencing is the basis of this proposed approach to repeater positioning.
Following these measurement steps, the computations are similar to those described for the
clock bias based approach.
4.3 The performance achieved
The most often implemented approach is the second one because it is simply based on
classical measurements of GNSS receivers and that no additional computation errors affect
the positioning. Tests have been carried out in various environments: each time, the system
was deployed and positioning carried out with different receivers. Note that the receivers
used are so-called software defined radio (SDR) receivers since the method is affected by
multipath, in a similar way that pseudolite based systems are. Thus, a specific mitigation
technique was implemented (described in a following section) which required the tracking
loops to be slightly modified. Since proprietary receivers do not allow such modifications,
an SDR receiver was required. It should be pointed out that transmitters are located in such
a way that walls are included in the propagation path from the transmitters to the receiver.
These environments, together with their “Ergospace
23
” representations, are as given in
figures 13 to 16 below.
23
Ergospace is the electromagnetic propagation software used for the deployment phase. The main goal
is to evaluate the multipath related effects.
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