Position, Navigation, and Timing Technologies in the 21st Century. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Физика
Год издания: 0
isbn: 9781119458517
Скачать книгу
after preamble detection."/> Graphs depict (a) a correlation function for a scenario with detectable LoS path, (b) shows a correlation function for the NLoS scenario with a strong early NLoS path, (c) shows a correlation function for the NLoS scenario with a weak early NLoS path.

      Note that there are cases where the earliest path is weaker than the multipath. In order to retain the channel information, a simple two/three tap early‐late‐prompt correlation will not suffice. A multi‐delay correlation function, as shown in Figure 39.15, is required from the receiver to facilitate accurate ranging.

      In order to get the best performance in a positioning system, the ranges should correspond to the LoS or the earliest arriving detectable path in the channel response to minimize range bias errors. The MBS system link budget and beacon network plan facilitate high‐resolution range determination to determine the earliest detectable path since the signals are designed to have higher SNRs as compared to GPS systems.

      39.1.4.3 Position Calculation

      The MBS system facilitates accurate 3D position computation. Since the MBS is a network of tightly synchronized beacons, trilateration can be done using pseudoranges determined from time‐stamped TOA measurements from the beacons and the beacon coordinates available from the beacon data.

      The range equation in 3D space from the receiver to the transmitter is given by

      (39.1)equation

      The location of the transmitters is given by (xi, yi, zi), and the unknown location of the mobile units is given by (X, Y, Z) in some local coordinate frame. The pseudorange measurement has a receiver time bias additive term as well, so that the usual pseudorange measurement equation can be written as

      (39.2)equation

      where c is the speed of light, and Δt corresponds to the receiver time bias. Traditionally, a minimum of four pseudorange measurements would be required for 3D trilateration to solve for the four variables: X, Y, Z, and receiver time bias. In a terrestrial network, estimating the Z coordinate through trilateration is error prone due to limited VDOP. When the z‐axis is available through barometric techniques, a minimum of three pseudorange measurements is sufficient for 3D trilateration.

Graphs depict sample channel responses from MBS beacons. Bar chart depicts the channel spread statistics.

      (39.3)equation

      39.1.5 Assisted Mode of MBS

      The Assisted GPS (A‐GPS) concept was developed to improve the sensitivity and time to fix of a GPS receiver when compared to a stand‐alone GPS receiver. See [10] for an explanation of the A‐GPS concept. A‐GPS facilitates improved sensitivity by providing assistance in the form of ephemeris/almanac information to the receiver so that decoding is no longer required. In addition, assistance, in the form of a coarse/fine GPS time estimate and rough receiver position, enables the receiver to compute the list of visible satellites, rough satellite Doppler frequency, and code phase, thereby significantly reducing its acquisition search space.

      Analogous to the assisted mode of GPS, an assisted mode can be considered for MBS as well. In A‐MBS mode, beacon information such as the almanac, any corrections, and atmospheric information may be transmitted over a cellular/other side channel. A list of visible beacons (based on the rough user position) can also be provided to the receiver to help reduce the PRN search space.

      39.1.6 MBS System for Time and Frequency Synchronization