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

40 Navigation with Terrestrial Digital Broadcasting Signals

       Chun Yang

       Sigtem Technology Inc., United States

      This chapter describes the available opportunistic terrestrial signals suitable for navigation, particularly digital television, requirements for their use, and implementations. General methods and practical constraints for obtaining position, navigation, and timing (PNT) solutions with broadcasting signals are first discussed in Section 40.1. Representative broadcasting signals are then presented in Section 40.2 with software receivers. The conversion of acquired broadcasting signals into pseudorange measurements is formulated in Section 40.3 with test data illustrations. Finally, an example of radio dead reckoning with mixed signals of opportunity (SOOP) is presented in Section 40.4 together with a discussion of important issues such as directions for future research.

40.1 PNT Mechanisms with Broadcasting Signals

      There is a growing demand for high‐quality PNT solutions for mobile devices due, on the one hand, to FCC’s E911, which requires mobile telecommunication operators to locate their subscribers in case of emergency [1], and, on the other hand, to the emerging market of location‐based services (LBSs) [2]. Both E911 and LBS inevitably involve positioning in urban and indoor environments, where, unfortunately, GNSS alone cannot provide continuous, reliable, and accurate position solutions to its users. Consequently, the search for alternative technologies to supplement or replace GNSS in these circumstances marches on, as evidenced by the many chapters devoted to this topic in this book. In this regard, this chapter addresses broadcasting signals, often referred to as SOOP, which are not designed for but contain useful information that can help find PNT solutions.

      The use of broadcast radio signals for PNT is not new. Since the 1960s, the National Institute of Standards and Technology (NIST) radio station WWVB in Fort Collins, Colorado, has been providing precise time and frequency reference nationwide and is still in use today by many clock radios, wall clocks, and other devices [3]. WWVB continuously broadcasts time and frequency signals at 60 kHz, in the radio spectrum band known as low frequency (LF). The time code of the WWVB signal contains all the information necessary to synchronize radio‐controlled clocks in the United States and surrounding areas. In addition, the carrier frequency of 60 kHz is often used as a reference for calibration of electronic equipment.

      An early example of using analog television (TV) signals for navigation is TELENAV [4]. TELENAV receives TV signals from a triad of stations or two pairs of stations at known locations and generates the time differences of arrival (TDOAs) to establish hyperbolic lines of position (LOP). The intersection of LOPs provides the user position [5]. Repetitive waveforms such as the horizontal and vertical sync pulses may be used as reference points for the determination of time delays, but the color burst signal is preferred for the purpose. Deploying TELENAV requires synchronization among received TV signals. It can be achieved when a number of stations across a large area simultaneously transmit the same program in a network broadcasting mode, when one station takes the feed off the air from another station, or when an inter‐station cross time synchronization is established through either a common carrier signal, an off‐the‐air signal, or other means.

Radio signals are broadcast not only by terrestrial transmitters but also by low‐earth orbit (LEO) or geosynchronous satellites as well as from such airborne platforms as blimps. This chapter will focus on terrestrial transmissions of digital television (DTV) or digital video broadcast (DVB) signals. Such radio SOOP are available in populated areas as they are designed primarily for indoor reception, where GNSS often fails. Indeed, DTV and DVB signals, now deployed worldwide [6], offer many advantages over their analog predecessors, which can be exploited for precise timing, positioning, and navigation [7–11].

      As discussed in [12], terrestrial transmissions can have high signal power on the order of hundreds to thousands kilowatts (kW), thus covering a large area. The transmission frequency is in the VHF and UHF bands (300–900 MHz), thus resulting in better urban propagation and building penetration than GNSS signals in the L‐band around 1.5 GHz. Besides, DTV transmitters are typically sited on the highest land near inhabited areas with antenna towers stretching several hundred feet above the natural elevation. The line‐of‐sight (LOS) transmissions are mostly horizontal, reaching indoors via windows or through walls, with much easier access than for GNSS signals, which come down across roofs. Therefore, good reception is expected indoors. Since the DTV transmitters are fixed on the ground, DTV signals experience less Doppler frequency shift than GNSS signals from orbiting satellites. Tracking loop bandwidth can be tuned down in favor of noise performance over dynamics. The bandwidth of terrestrial DTV signals is between 6 and 8 MHz, wider than that of GPS C/A‐code and comparable to the chipping rate of GPS P(Y)‐code. Wider bandwidth leads to more accurate timing and ranging, which in turn leads to better positioning. Annual, seasonal, and diurnal changes result in variations in propagation delay over the radio path length. However, at DTV frequencies and over a short distance of several hundred kilometers, the delay is much less than that of ionosphere and troposphere propagation delay experienced by GNSS signals. The last – but not the least – point is that broadcasting signals can be used for free as far as the purpose of PNT is concerned since the broadcasting infrastructures already exist, except for the add‐on PNT capability in user devices.

      Commonly used mechanisms for obtaining a PNT solution with SOOP [5] include (i) signal power pattern matching (fingerprinting), which requires a pre‐established database or a map containing location‐dependent signal signatures; (ii) triangulation, which requires a means of measuring the angles of arrival (AOAs) of radio signals; (iii) trilateration, which measures the ranges to signal sources either from the received signal strength (RSS) via a propagation loss model or through the time of flight (TOF); (iv) multilateration, which measures differential ranges to a pair of signal sources;

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