# Analysis of 5G and LTE Signals for Opportunistic Navigation and Time Holdover
## 1 Introduction  
While global navigation satellite systems (GNSS) are the preferred options for many positioning navigation and timing (PNT) users and applications, the signals that reach terrestrial users have power levels of approximately \(-130\) dBm. Therefore, they are ill suited for penetrating deep into buildings and are susceptible to even unintentionally generated radio frequency interference (RFI) [1]. To provide alternative and fallback navigation capability to users who must continue to navigate or synchronize their systems when GNSS signals are unavailable, a large body of research has been dedicated in recent years to the exploitation of Signals of Opportunity (SoP) including, but not limited to, television, cellular [2; 3; 4; 5; 6; 7; 8], low earth orbit (LEO) satellite [9], and other emitters of opportunity [10] that can be used as orsatz PNT observables. Cellular signals in particular stand out as excellent candidates for three very important reasons. First, the signals have much higher received power levels due to the much shorter ranges between the transmitters and devices within their coverage area (even penetrating deep into buildings by design unlike most signals transmitted from navigation or communication satellites). Second, the infrastructure in urban areas and along roadways, where GNSS RFI is relatively common, is dense by virtue of the market pressures for multiple network operators to cover entire cities and transport routes. Third, the third-generation partnership project (3 GPP) standard organizations have proposed features within newer cellular standards, such as fifth generation (5G), which are directly intended to support navigation. While 5G navigation support might sound ideal, the primary operating mode foreseen is for the network operator to collect informationfrom the user and to return their position information as a service, such as in past network time difference of arrival (TDOA) methods [11]. This makes it unfeasible to combine observations from competing network operators, as well as makes it unclear whether the various operators will deploy dense enough networks to enable practical navigation when using only their infrastructure. As an additional complication, the use of the term 5G has become more of a marketing term than a technical one in that user devices may report 5G coverage while the underlying signal structure remains almost indistinguishable from the previous generation of long-term evolution (LTE) coverage in that area. To investigate the suitability of the locally available LTE and 5G signals for SoP and synchronization purposes, the authors undertook observation and analysis of the availability, stability, and compatibility of the signals transmitted by local network operators in the vicinity of the facilities of SINTEF and the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway. The results of this study indicate that, while the supposed 5G signals often implement only a small subset of the possible enhancements discussed in the context of 5G evolution, the signals transmitted by the base stations have characteristics that are very promising for use as both navigation and timing constraints. The novel contribution of this study to the existing body of literature involves the time scale-dependent analysis of the stability of the signal sources in both single-network-sourced and multi-operator modes, which helps inform what measurement and averaging intervals may be most appropriate for future PNT-focused users of 5G signals. While knowledge about cellular base station clock stability is of limited value taken by itself, especially since no explicit time reference is embedded in the signal structure, the core novelty of this work is in demonstrating that this information is in fact very valuable for exploiting these signals in the domains of PNT.