Position, Navigation, and Timing Technologies in the 21st Century. Группа авторов

       No Timing and Location Coded on Signals. In GNSS, the time of transmit (TOT) and the satellite orbit at transmission can be decoded from the navigation message embedded in the received signals. With SOOP, however, there may be no explicit information about timing and location of transmitter (LOT) modulated on broadcast signals. In fact, one of the major challenges in positioning with SOOP is how to cope with the unknown TOT and LOT.

       Source Locations. As stated above, a critical aspect of positioning with SOOP is how to obtain accurate and up‐to‐date knowledge of the locations of signal sources or a database of location‐dependent signatures. Typically, the source locations such as DTV transmitters can be found from a regulatory registry or by a means of intelligence. A location‐dependent signature database, however, is only available for those regions that have been surveyed beforehand. Although the general characteristics of a SOOP can be learned from the standards to which the signal design adheres or estimated with SIGINT, there are circumstances in which the locations of signal sources are difficult to obtain. Some sources may be easily moved around. It is costly to build and maintain such a database. An alternative is to apply simultaneous localization and mapping of emitting radio sources (SLAMERS) [15, 16].

       Clock Errors. The clocks of SOOP transmitters are initially unknown to a user, each subject to a different bias and drift. Although there are synchronous transmitters such as the single‐frequency network (SFN) for DVB, most of SOOPs are not synchronized. For synchronous transmitters, there is only one clock error between the network and the user, which can be estimated as part of the navigation solution. However, for asynchronous transmitters, there is a clock error term for each transmitter. To solve for such clock unknowns, self‐calibration can be applied when external information is available [17]. A reference station at a known location can estimate the transmitter clock errors and pass it along to the users [18, 19]. In the same way, two collaborative users can form a single spatial difference between their timing measurements with respect to a signal source, thus eliminating the common clock error [20, 21]. Both schemes, however, require a data link and synchronization between them. Another way to remove the clock error relative to a source is to form a temporal difference, leading to differential or relative range measurements for radio dead reckoning [22–25], which can be further combined with other sensors [26–27].

       Number of SOOP and poor geometric dilution of precision (GDOP). The number of “independent” SOOP sources of the same kind in a region is typically not enough for robust and precise position location. Several antennas may be mounted on the same transmission tower. This tends to produce a rather poor GDOP. The problem may be alleviated by using different types of (mixed) SOOP such as TV and cellular signals [29, 30]. Making known displacements, a way to rotate the LOS vectors, is equivalent to adding fictitious sources at different locations to improve GDOP [21, 22]. Taking orthogonal measurements (e.g. obtain AOA in addition to TOA) is another way to enable positioning with limited signal sources.

       2D versus 3D. Due to the limited height of terrestrial transmitters, the position solution is most likely to be two‐dimensional (2D) rather than three‐dimensional (3D). Compensation for slant ranges is required if significant height differences among transmitters and/or users are expected. Such compensation is facilitated when a digital terrain elevation database (DTED) is available. The height can also be solved with a radar or barometric altimeter.

       Multipath at Reception. At reception, severe multipath is expected, particularly in an urban environment [31]. Multipath may create deep fading. Frequency diversity coding (OFDM) and spatial diversity combining (multiple‐input multiple‐output or MIMO) are techniques for better channel equalization. Rapid fading due to motion requires agile and robust tracking of code and carrier. While non‐line‐of‐sight (NLOS) signals are desirable for communications to reach shadowed areas, it is problematic for ranging. NLOS signals can be excluded, de‐weighted, or estimated as bias with robust estimation techniques. On the other hand, multipath can be exploited for constructive use [32, 33]. It can improve the positioning geometry when NLOS paths can be resolved from an environment map. In fact, multipath is troublesome only for geometric positioning methods, but it is rather a blessing for feature‐matching based (non‐geometric) methods, where multipath makes each location rather unique in terms of a rich set of features.

       Signal Integrity/Authenticity. By its very nature, the use of SOOP faces the issues of integrity and authenticity in the context of navigation warfare. The signal physical characteristics and the information content carried on the signals can be used for authentication and assurance. Mixed SOOP and other types of sensors may be used for cross‐checking to ensure source authenticity, measurement integrity, and solution viability.