Radiometric Speed Sensor for Vehicles

Introduction

One of the most important tasks to increase traffic safety of vehicles and automating their movements is a high-precision measurement of their velocities. Research done recently in different countries has shown that the most effective way to solve the problem is the application of radio sensors which allow to estimate not only the speed of vehicles, but also their acceleration. One of main advantages of radio sensors in comparison, for example, with optical or infra-red sensors, is their continuous performance in any weather condition (rain, fog, snow, smoke, dust etc.), i.e. under conditions of limited optical visibility. They even work if there is no visibility at all.

Two basic radio engineering methods for speed measurement are known: utilization of the Doppler Effect and correlation of signals, whereby the correlation method can realize both active and a passive location of vehicles.

The essence of the correlation method (Borkus et al., 1973, Klyucharev et al., 1995) consists in measuring the time delay between signals received by antennas placed along the axis of the object movement (see Figure 1).

Figure 1.

Principle of operation of a speed sensor

These signals have identical form, but are displaced in time. The amount of time delay is equal to the time the moving object needs for a distance equal to half the distance between the receiver antennas. For the measurement of this shift the signal received by the first antenna is delayed and correlated with the signal of the second antenna until a maximum of the mutual correlation function (MCF) is observed. Thus, the amount of the delay corresponding to the maximum MCF unequivocally defines the traveling speed of the object.

For the realization of the correlation method in a speed sensor it is useful to dispose of some channels of the receiver. The necessary number depends on how many components of the velocity vector are to be determined. For the measurement of two vector components V_x and V_y it is necessary to have three channels. Thus, antennas should be placed both along the longitudinal axis OX, and along the perpendicular axis OY.

Depending on the demands, the speed sensor of correlation type (SSCT) can be used either in a stationary mode or on a mobile object. In the first case the SSCT can serve, for example, as speed sensor for various moving vehicles on roads with multi-line traffic. In the second case the application of the SSCT allows to supervise the speed of movement directly onboard a vehicle, for example, a car. In both cases application of the correlation method provides high accuracy in the determination of the speed.

Principles Of Designing A Radiometric Ssct

In this section the radiometric speed sensor of correlation type (RM SSCT) is considered (Klyucharev et al., 1995, Kukushkin et al., 1991).

Research on radiometric measuring instruments which determine the parameters of radio thermal radiation of a terrestrial surface from an airplane was carried out first in the laboratories of radio receiver devices of the Moscow Aviation Institute (MAI) in 1985. The research was supervised by Professor Alexey Petrovich Zhukovsky, a highly respected scientist of the USSR. As a result of the investigations the electrodynamic theory of radio thermal radiation of a terrestrial surface has been established (Zhukovsky, 1992). Taking into account this theory the methods of designing onboard radiometers of various types have been developed. As a result, modern radiometers have been designed for different frequency ranges. In the late 1990s in this laboratory were started research activities on radiometric measuring instruments of movement parameters of vehicles. Therefore, the results in the given chapter are based on a certain experience at the MAI in designing radiometric systems and measuring instruments.

This RM SSCT has a variety of essential advantages in comparison with known Doppler speed sensors. These advantages are: