Читать книгу Position, Navigation, and Timing Technologies in the 21st Century - Группа авторов - Страница 124

Subbu Meiyappan, Arun Raghupathy, and Ganesh Pattabiraman

NextNav LLC, United States

Satellite‐based systems can provide good‐quality positioning in clear‐sky outdoor environments and in some light indoor environments. A number of satellite systems have been developed for positioning and navigation such as GPS [1], Glonass [2], BeiDou [3], and Galileo [4], which all fall under the umbrella of global navigation satellite systems (GNSSs). However, all these satellite systems are limited in terms of their availability in deep‐indoor environments, due to their link budget, as well as in dense urban environments due to signal blockage. Terrestrial positioning systems can complement satellite‐based systems and work in environments where satellite‐based system performance is challenged.

In the following, we use the terminology “User Equipment (UE)” to refer to the entity whose position is to be computed. One form of terrestrial positioning systems is broadcast systems, where the signals used for positioning are broadcast from one or more transmitters of the positioning network to the UE. These can also be thought of as downlink systems. Examples of such systems include DTV, FM signals, OTDOA, Metropolitan Beacon System (MBS), and Locata. Another type of positioning system involves the UE transmitting a signal to one or more fixed receivers that comprise the positioning network. These systems can also be thought of as uplink systems. Examples of such systems include the UTDOA (Uplink Time Difference of Arrival) system developed by Trueposition. Other positioning systems can use bidirectional signaling between the UE and the network to compute position. An example of such a system is the IEEE 802.11n system that uses time‐of‐flight estimation from bidirectional transmissions for position estimation. Both uplink and bidirectional systems can have capacity limits in terms of the number of UEs that can be supported, since the UEs must transmit back to the network. Figure 39.1 illustrates the various terrestrial system architectures.

In this chapter, the transmitters of terrestrial broadcast systems are also referred to as beacons.

Terrestrial positioning systems can be classified based on their geographic scale:

1 Wide‐area terrestrial systems

2 Local‐area terrestrial systems

Wide‐area terrestrial systems have a wide coverage area extending beyond a building/venue, for example, to a metropolitan area. In contrast, local‐area terrestrial positioning systems such as WiFi and Bluetooth Low Energy (BT‐LE) are restricted in terms of their coverage. Among wide‐area terrestrial positioning systems, there are some signals such as TV, AM/FW radio, and cellular, whose primary application is different from positioning but can be used as signals of opportunity for positioning (for example, see Chapter 35 and [5]). Since these systems are not purpose‐built for positioning, they all have limitations with respect to position quality.

Terrestrial positioning systems can use a variety of metrics and methods to estimate the 2D position of the UE. Some systems may use signal strength metrics such as Received Signal Strength Indicator (RSSI) (e.g. WiFi 802.11a/g, BT‐LE, Polaris RFPM), whereas others may use pseudoranges or direct range measurements (e.g. UTDOA, OTDOA, MBS) to estimate position using some type of trilateration algorithm.

Among terrestrial systems that use ranging, some use transmissions that are by design synchronized, whereas some others may not be synchronized and need additional timing observations for synchronization. One example is a system that uses DTV signals for positioning, where additional timing monitoring units are required to be deployed to estimate the timing errors and provide them to the UE.

Figure 39.1 Broadcast, uplink, and bidirectional systems.

All terrestrial systems, and to a lesser extent GPS/GNSS, have a limitation with respect to estimating the height of the UE through trilateration. It is well known that GPS/GNSS systems have a limited vertical accuracy due to poor VDOP since satellites are only above Earth’s surface. Terrestrial systems have a similar limitation with respect to estimating the height of the UE through trilateration since the beacons are, roughly speaking, on the same plane. While some height differences in beacon deployment can help somewhat in improving the VDOP, the altitude accuracy is always limited for traditional terrestrial systems.

This chapter discusses the Metropolitan Beacon System in detail. We discuss the overall architecture of the system in Sections 39.1 and 39.1.1. A detailed overview of the MBS signal structure is provided in Section 39.1.2. Due to the similarity of the MBS signal structure to the GPS signal structure, various features of both signals are described in Section 39.1.3. An example receiver architecture and the associated challenges of implementing a receiver to process terrestrial‐based signals is described in Section 39.1.4. Section 39.1.5 describes the assisted mode of operation of an MBS receiver. The use of MBS signals for time and frequency synchronization is described in Section 39.1.6. Various industry standards that reference MBS signals are shown in Section 39.1.7. The chapter concludes with third‐party performance test results in Section 39.1.8 and a conclusion in Section 39.1.9.