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Radio propagation in urban environments is known to be subject to two types of fading [42, 83]: large‐scale fading, which is range dependent (this range dependency is the main idea behind the RSS‐based positioning mentioned earlier), and small‐scale fading, which occurs over short distance (several wavelengths) and a short period of time (seconds). In small‐scale fading, the received signal is subject to rapid changes, mainly caused by multipath signals with constructive and destructive additions. It causes signal dispersions in time, frequency, and angle of arrival, which are known as delay spread, Doppler spread, and angular spread, respectively. It is this small‐scale fading that severely hampers mobile tracking of SOOP such as cellular and DTV signals. An agile radio receiver is therefore desired to implement fast acquisition and reacquisition schemes to coast through the “holes” of deep fading with instantaneous recovery after a complete signal loss, a correlator structure commensurate with the variable delay experienced by the receiver, and tracking loop parameters optimized to balance out dynamic tracking versus noise performance requirements.

In urban environments, the direct signal may be blocked or severely attenuated while strong multipath components are omnipresent. The conventional strategy of picking up the peak among a limited number of early and late correlation values, though still valid for communications purposes to demodulate the data bits, is error prone for LOS timing, which may create bias in range measurements and, even worse, outliers. Holistic processing of the entire CIR, as described in Section 40.2.2 [51, 52], is more promising for dealing with multipath and NLOS signals [27, 94, 95]. Outliers can be treated with robust estimation methods [96–98].

Mixed SOOP are capable of offering a stand‐alone solution in a rather poor geometry, as demonstrated in the examples presented. It can be further integrated with other sensors and used to assist GNSS for more robust operations. The GNSS and SOOP aiding is mutual. All missions start from a known initial condition just like an inertial navigation system that is initialized with the position, velocity, and attitude at the starting point. When GNSS is available, its solution can help SOOP determine the hearable transmitter location (akin to mapping and intelligence gathering) and calibrate the time offset and drift rate. As part of an integrated navigation system, the GNSS‐based solution can relax the stringent requirement otherwise placed on the number of independent SOOP and their geometric distribution. The continuity and availability of a hybrid solution can be ensured based on one to two GNSS signals, one to two DTV signals, and/or one to two cellular signals with a reasonably good geometry and signal quality.

When GNSS is not available due to blockage or jamming, mixed SOOP can augment GNSS for indoor positioning [18, 19] and complement GNSS to maintain aiding to an integrated inertial solution [27, 28]. When GNSS is challenged, the internal clock starts to drift even under aiding from other non‐GNSS sensors. To maintain a stable timing source and enable fast reacquisition, it is possible to perform time – and in particular frequency – transfer with SOOP [8, 99]. Early studies showed such a possibility with DTV and CDMA signals to enable fast reacquisition when GNSS becomes available again.

SOOP can also be used for information assurance against spoofing. Cellular networks are synchronized to GPS, and new ATSC standards recommend the same for DTV transmitters. Because spoofing has only a local effect, widely distributed cell towers and DTV transmitters and their GPS timing sources are unlikely to be affected simultaneously by a spoofing attack. As a result, the time and frequency information derived from SOOP can be used as an independent source to mount countermeasures against spoofing.

For public broadcasting stations, information about the transmission characteristics and transmitter location can be found from the regulatory agency’s database. However, there is a need to determine the locations of private and commercial radio and TV broadcasting transmitters, which may not be known to the public, for opportunistic use as a reference for PNT. Such approaches as simultaneous localization and mapping of emitting radio sources (SLAMERS) [16] are valuable for solving the problem. However, from the PNT perspective, the most desirable approach is for broadcasting sources to encode their transmitter location and in particular clock error parameters onto their data stream in a way similar to radio beacons [100]. Transmission of the transmitter’s location and clock data offers an add‐on PNT service to the main business of broadcasting at the expense of intermittent overhead. As part of the accelerated convergence of broadcasting, communications, and Internet, it serves as an enabler of ubiquitous positioning and seamless transition of outdoor and indoor positioning for the burgeoning LBS market.