# Background and Recent Advances in the Locata Terrestrial Positioning and Timing Technology
## 5 Challenging Issues for Locata
### Raw Data Analysis of a LocataNet  
Locata technology can deliver metre-level positioning accuracy using code pseudorange measurements, and centimetre-level accuracy using carrier phase measurements. For the test THAT was conducted on Sydney Harbour in 2012, eight LocataLites (LL) were deployed in the LocataNet, with six installed along the shore (LL1-LL6), one on the Harbour Bridge (LL7), and another at Kirribilli (LL8). Figure 5 shows the location of the LocataLites and the Rover trajectory during the three hours experiment. To investigate the Locata signal quality, the trajectory was divided into five sections or areas, which are denoted by different colours in Figure 5. The Locata Rover trajectory was calculated from the GPS-RTK solution, and is thus accurate to the centimetre-level. Five types of measurements are made on a Locata Signal: code pseudorange in metres (pr), carrier phase in cycles (icp), Doppler frequency in Hz, received signal strength indicator in dBm (rssi), and signal-to-noise ratio (S/N) in dB. Figure 6 is a plot of the five types of raw measurements from LL1 (a) and LL8 (b), as well as the corresponding distance from the Rover to the LL (top panel). In each panel, four values from two antennas (Tx1, Tx2) at two frequencies (S1, S6) are plotted. Both of the subfigures show that there were frequent signal blockages or interruptions, both for LL1 and LL8 during the test, and signals within the areas 3, 4, and 5 are, in general, unavailable. Furthermore, the subfigure (b) shows that signals from LL8 are unavailable during the first 75 minutes, when the vessel travelled in areas 1 and 2. Figure 5: Location of LocataLites and experiment trajectory. Figure 6: \(Cont\). To further analyse the signal availability for each LL, the probability of available measurements within each area for each LL are shown in Figure 7, where the solid line denotes the probability of availability, and the dash line denotes the probability of completeness, as defined by Equation (4). The probability of availability accounts for epochs when at least one measurement from the two antennas at the two frequencies is available, and the probability of completeness only counts epochs when the four measurements are all available. Figure 7 generally shows that using dual-frequency signals and dual antennas significantly improves the signal availability, especially within areas 2, 3, and 4, where the probability of availability is obviously higher than the probability of completeness for most LLs. Within area 1, the probabilities of availability is very similar to the probability of completeness for LL1, 2, 3, 5, and 8, and are nearly 10% higher for LL4, 6, and 7. In general, the LocataNet configuration provides optimal navigation performance within area 1 other than within area 2 to area 5. \[\left\{ \begin{array}{l}\text{probability of availability}=\dfrac{\text{ epochs of at least 1 measurement valid}}{\text{total number of epochs}}\\ \text{probability of completeness}=\dfrac{\text{epochs of four measurements all valid}}{\text{total number of epochs}}\end{array} \right. \tag{4}\]  
Figure 6: Raw measurements from LL1 (**a**) and LL8 (**b**). Figure 8 shows the Received Signal Strength Indicator (rssi) changes for each LL with respect to distance. Figure 9 shows the corresponding change trends for S/N (in dB). Based on a standard wireless signal propagation model, the rssi is a log function of the reciprocal of signal propagating distance. From Figure 8, it can be seen that the rssi values for LL1 to LL5 are consistent with this model, however not in the case of LL6 to LL8. Figure 9 shows similar change trends for the S/N, where the value of S/N for LL1 to LL5 linearly reduces as the distance increases. However, the change trend for LL6 to LL8 is more disordered due to significant signal interferences and blockages. Furthermore, the changes in S/N depend on the frequency, since the slopes for the S1 signal are steeper than for S6 signals, as shown in the panels for LL1, LL2, and LL5.