# Analysis of 5G and LTE Signals for Opportunistic Navigation and Time Holdover
## 3 Results
### Differential Stability Measurement
#### 3.3.2 Evaluation of the Differential Tower-to-Tower Stability  
The main motivation for extending the analysis to include tower-to-tower differential measurements was to further isolate and eliminate any potential contributions of the local clock used to drive the SDRs, which, despite being a GNSS-disciplined atomic clock, will also have instabilities. While the results in Figure 2a show that the differences between the transmitter and receiver time scales were limited to less than 5 ns over this hour of observation, the specification provided by the manufacturer of the GNSS-disciplined Rb clock only stated a performance guarantee of 50 ns, thus causing this uncertainty to dominate the results. To remove this uncertainty, we resorted to observing two cells simultaneously via radios driven by a common oscillator, two pairs of which are plotted in Figure 3.  
Figure 3: **(a)** Phase measurement evolution of two Telenor cells; (b) phase measurement evolution of one Telenor cell and one Telia cell.  
To eliminate the contribution of our local oscillator error, we needed only to differentiate the measurement series, which resulted in the traces shown in Figure 4, where the behavior of the differential cell site clocks was isolated.  
Once the impact of the local clock instability was eliminated through the differential measurement process, the relative phase shift between the two measured cells exposed clear behavioral differences between the first and second combinations. The first combination of the two sources that were operated by the same service provider showed a nearly three times lower peak-to-peak relative variation in the TDOA than that which was formed between the two cells that were operated by different service providers. This is easily explicable as being a consequence of the expectation that two proximate cells operated by the same service provider are likely to be tightly steered to a common reference time scale when compared to infrastructure that is operated by different service providers. In the former case, the transmitters are potentially steered to a common physical timing device, while in the latter case the networks may or may not even be steered to the same realization of a common time scale, such as in the case with equipment that steers to the GPS realization of UTC (USNO) versus the European Galileo realization UTC (GST).  
The visible step changes in the delta between the two towers operated by the same network operator could indicate a phase multipath reflection effect, or they could be real shifts in the phase of one or both transmitters. Meanwhile, by contrast, the process between the two dissimilar service providers appeared dominated by ramp behaviors with a small range of slew rates. On cursory investigation, the nearly three times better (smaller) peak-to-peak delta variation shown in Figure 4a compared to that shown in Figure 4b suggests it is the better combination for use as a PNT measurement pair. However, a deeper analysis showed that this was not actually the case for most measurement rates, as will now be shown.