receiver discussed in Section 38.5.2. The receivers were tuned to the cellular carrier frequency 882.75 MHz, which is a channel allocated to the US cellular provider Verizon Wireless. The mapper was stationary and was estimating the clock biases of the 3 BTSs via a WLS estimator as discussed in Section 38.4.1. The BTSs’ positions were known to the mapper, and the position states were expressed in a local 3D frame whose horizontal plane passes through the three BTSs and is centered at the mean of the BTSs’ positions. The height of the navigator was known and constant in the local 3D frame over the trajectory driven and was passed as a constant parameter to the estimator. Hence, only the navigator’s 2D position and its clock bias were estimated through the WNLS described in Section 38.4.1. The weights of the WNLS were calculated using Eq. (38.17) with Figure 38.22 BTS environment, true trajectory, and experimental hardware setup for the ground vehicle experiment. Map data: Google Earth (Khalife et al. [12]).
Source: Reproduced with permission of IEEE.
Figure 38.23 Variation in pseudoranges and the variation in distances between the receiver and two cellular CDMA BTSs for the ground vehicle experiment (Khalife et al. [12]).
Source: Reproduced with permission of IEEE.
38.5.4.3 Aerial Vehicle Navigation
Two identical UAVs (mapper and navigator) were equipped with the cellular CDMA navigation receiver discussed in Section 38.5.2. Here, both the mapper and navigator were mobile. The receivers were tuned to the cellular carrier frequency 882.75 MHz used by the US cellular provider Verizon Wireless. The mapper and navigator were listening to the same four BTSs with known positions. The mapper was estimating the BTSs’ clock biases via a WLS estimator as discussed in Section 38.4.1. Similar to the ground vehicle navigation setup, the height of the navigator was a known constant in the local 3D frame, and only the navigator’s 2D position and its clock bias were estimated through the WNLS, whose weights and initialization were calculated in a similar way as the ground vehicle navigation setup. The ground‐truth references for the mapper and navigator trajectories were taken from the UAVs’ onboard navigation systems, which use GPS, INS, and other sensors. Figure 38.25 shows the BTS environment in which the mapper and navigator were present as well as the experimental hardware setup. The navigator’s true trajectory and estimated trajectory from cellular CDMA pseudoranges are shown in Figure 38.26.
38.6 Navigation with Cellular LTE Signals
Two different techniques can be employed to use LTE signals for PNT: network‐based and UE‐based. The network‐based technique was enabled in LTE Release 9 by introducing a broadcast positioning reference signal (PRS). The expected positioning accuracy with PRS is on the order of 50 m [59]. Network‐based positioning suffers from a number of drawbacks:
Figure 38.24 Experimental hardware setup, navigator trajectory, and mapper and BTS locations for ground experiment. Map data: Google Earth (Khalife et al. [18]; Khalife and Kassas [25]).
Source: Reproduced with permission of IEEE.
The user’s privacy is compromised, since the user’s location is revealed to the network [60].Figure 38.25 BTS environment and experimental hardware setup with a mobile mapper. Map data: Google Earth (Khalife and Kassas [25]).Source: Reproduced with permission of IEEE.Figure 38.26 Navigating UAV’s true and estimated trajectory.Map data: Google Earth.
Localization services are limited to paying subscribers and from a particular cellular provider.
Ambient LTE signals transmitted by other cellular providers are not exploited.
Additional bandwidth is required to accommodate the PRS, which caused the majority of cellular providers to choose not to transmit the PRS in favor of dedicating more bandwidth for traffic channels.
To circumvent these drawbacks, UE‐based PNT techniques, which exploit the existing reference signals in the transmitted LTE signals, have been developed. This section focuses on UE‐based PNT techniques. When a UE enters an unknown LTE environment, the first step it performs to establish communication with the network is synchronizing with the surrounding LTE BTSs, also referred to as Evolved Node Bs (eNodeBs). This is achieved by acquiring the PSS and the SSS transmitted by the eNodeB. The PSS and SSS can be directly exploited for navigation. Another LTE signal that can be exploited for navigation is the CRS; however, exploiting CRS is not as straightforward due to its scattered nature in time and frequency. Table 38.1 compares the salient navigation properties of PSS, SSS, and CRS.
This section is organized as follows. Section 38.6.1 discusses the LTE frame structure and reference signals that could be exploited for navigation. Section 38.6.2 presents a receiver architecture for producing navigation observables from received LTE signals. Section 38.6.3 analyzes the code phase error of SSS signals with coherent and non‐coherent DLL tracking.
