The present invention relates to roadside beacon systems, and more particularly to a roadside beacon system used for calibrating a vehicle position in a navigation system in which, after data representing the start point of the vehicle are inputted, the current position of the vehicle is calculated by measured vehicle speed and direction data to display its present position and also used for carrying out data communications between the vehicle and the roadside antenna.
A vehicular navigation system has been proposed in the art in which a small computer and a small display unit are installed in a vehicle; a road map stored in a memory device such as a compact disk is displayed on the display unit; speed data outputted by speed sensors and direction data outputted by direction sensors are utilized to calculate the present position of the vehicle and to determine the present direction of movement of the vehicle at all times, and according to the results of the calculations, a symbol representing the vehicle is indicated on the road map displayed on the display unit.
With such a navigation system, the operator of the vehicle can visually detect the present position and direction of movement of the vehicle with ease and thus can find a destination readily.
However, the above-described navigation system suffers from a potential for inaccuracies which are due to errors accumulated in speed sensors and direction sensors as the vehicle travels a long distance. Therefore, when the distance covered by the vehicle exceeds a certain value (which is not always a constant value and is dependent on the amount of errors accumulated in the speed sensors and the direction sensors and also environmental conditions), the vehicle position displayed on the display unit may be greatly shifted from a true position; that is, the navigation system becomes unreliable and the driver may lose his way.
In order to eliminate the errors in navigation systems, a so-called "roadside beacon system" has been proposed. In this system, roadside antennas are installed along roads at intervals shorter than those with which the errors thus accumulated reach serious values. Each of the antennas radiates signals including position data of antenna location and road direction data to cover a relatively small area. The signals thus transmitted are received by a mobile antenna installed on the vehicle and applied to the navigation equipment whereby, according to the signal thus received, the position and the moving direction of the vehicle are calibrated.
In the roadside beacon system described above, the accumulation of errors is attempted to be maintained at less than aggravating value, whereby an exact present position of the vehicle can always be displayed. Accordingly, the navigation system is reliable. Furthermore, the roadside beacon system has the advantage that, when a roadside antenna is installed, for instance, near a railroad or railroad crossing where the direction sensors are liable to produce a large error, errors attributed to external factors can be corrected effectively.
In the above-described roadside beacon system, however, the roadside antennas are of considerably high directivity, transmitting signals including position data and road direction data at all times, and the vehicle receives the beacon signals only when it passes through a covered area by the signals. If one attempts to cover large areas with each beacon antenna in order to get large communication area it becomes difficult to accurately detect the vehicle's position, because of wide spread field distribution which results in difficulties in exactly distinguishing the antenna position.
This will be described in more detail. The fundamental function of the roadside beacon system is to transmit signals including the position and road direction data to the navigation system in the vehicle. It may also be required for the roadside beacon system to further transmit data to perform the following functions in practical use:
(1) transmission of traffic information as to traffic congestion, construction work on road, detours, etc., in the area around the roadside antenna;
(2) information for a detailed map including the position of buildings or houses with names in the area around the roadside antenna;
(3) information for a road map of a relatively large area including the position of the roadside antenna to renew the road map on the display unit, and
(4) two-way communications between the roadside antenna and the vehicle.
Above mentioned applications can be fulfilled only with using an enlarged communication area covered by the roadside antenna in order to realize large volume data transmissions.
However, if the communication area covered by the roadside antenna is enlarged, the detection of the vehicle position which is the primary object of the navigation system, becomes inaccurate.
In addition to the spread field distribution in the enlarged communication area system, there are environmental obstacles to electro-magnetic waves such as vehicles moving nearby, buildings along a road and so on, causing the signal received by vehicle to fluctuate largely. An example of measured field distribution fluctuation caused by the above mentioned factor is shown in FIG. 14a. The fluctuation of signals is the results of receiving waves scattered by or reflected from such obstacles being different in amplitude and phase from one another. In other words, multi-path fading occurs in the signal. Accordingly, calibration of the vehicle position using the signal thus received may involve an unexpected error. That is, the signal received by the mobile antenna may have a high level at a position actually farther away from the roadside antenna, resulting in large positional errors.
The present applicant has filed a Japanese patent application for a roadside beacon system in which, to eliminate this difficulty, "split-beam" antennas having a radiation pattern whereby the electric field strength abruptly decreases directly in front of the antenna are employed for the roadside antennas. When the abrupt decrease of the received signal strength is detected, it is determined that the vehicle is directly confronting the roadside antenna, and the vehicle position data and the moving direction data are calibrated according to the data received just before the detection of the abrupt decrease point. Hence, the effects of multi-path fading on the received signal strength distribution are eliminated.
In this connection, a Japanese patent application has also been filed for a method in which roadside antennas are installed at elevated positions above the road, and the major lobe of the mobile antenna is extended upwardly. Also, a Japanese patent application has been filed for a method in which a roadside antenna is connected to the lower portion of a structure which extends over the road, and the major lobe of the mobile antenna is extended upwardly.
In these roadside beacon systems, a variety of data can be transmitted between the roadside antenna and the vehicle over a wide range with no difficulty, and the accuracy of detection of the vehicle position with respect to the roadside antenna can be considerably improved.
However, these roadside beacon systems may suffer from the following difficulty: In the case where large obstacles like truck and buses which are considerably larger than the vehicle installed with navigation system, are existing around the roadside antenna and the vehicle is in the communication area as shown in FIG. 2, or in the case where the roadside antenna is installed under the bridge structure which is constructed over the road illuminating the road from upside and a large obstructing vehicle or vehicles are moving nearby the vehicle installed with the navigation system as shown in FIG. 3, the large obstructing vehicle will shield or scatter the radio waves resulting in the deterioration of positioning accuracy of the vehicle.
Experimental results will be described in more detail. In the experiment the large obstructing vehicle traveling the central lane shown in FIG. 2 is 8.12 m long, 2.2 m wide and 3.5 m high, the vehicle with navigation system is 1.0 m high, the roadside antenna is 5.0 m in height, and the inclination angle of the main radiation beam of the roadside antenna is 30.degree.. In this case, the fluctuation in the signal level received by the mobile antenna is as shown in FIG. 4A. As is apparent from the comparison with the fluctuation of the signal level (as shown in FIG. 4B) in the case of clear environmental condition in which no substantial electromagnetic obstacles such as large trucks, buses and so on, the signal level variation (FIG. 4A) shows characteristics that an abrupt level decrease attributed to the shielding of the radio waves by the large obstacles and ripple components of up to 10 dB are superposed on the broader and smooth signal level distribution curve determined by the directivities of the roadside antenna and the mobile antenna. Therefore, the accuracy of detection of the vehicle position is somewhat deteriorated.
Another experimental data will now be considered where, in FIG. 3, a large vehicles is traveling along a center lane just below a roadside antenna which is 6 m in height, and the vehicle having the navigation system is moving along an adjacent lane. In this case, the variation in level of the signal received by the mobile antenna is as shown in FIG. 4C. As is apparent from a comparison with the variations the signal level received in the case no large vehicle is moving parallel to the vehicle with the navigation system (as shown in FIG. 4D), the signal level variation of FIG. 4C is such that ripple components of the order of 3 dB attributed to multi-path fading are superposed on the broader and smoother signal strength distribution curve determined by the directivities of the roadside antenna and the mobile antenna.
In both of the above-described experiments, the frequency of waves which carry transmitting data is set at about 2.5 GHz.
Thus, in both of the above-described cases, the signal received by the mobile antenna includes ripple components of the order of 3 to 10 dB. However, this causes no problem in data transmission; that is, the data can still be transmitted to the mobile antenna at high speed, because of enough C/N characteristics obtained at the bottom of signal fluctuations.
On the other hand, in the case where a split beam signal from the roadside antenna is used to detect the vehicle position, the following difficulties are involved: Since the split beam signal radiated from the roadside antenna sharply decreases just in front of the roadside antenna and at positions sufficiently far away from the roadside antenna, it is insufficient for detection of the vehicle position merely to detect the decrease in level of the received signal. That is, only sharp fall of the signal level which occurs when the vehicle goes just in front of the roadside antenna.
To satisfy the above requirement, a navigation device has been proposed which has been designed as follows: In the device, the received signal is applied to a low-pass filter so that abrupt variations are removed; i.e., the signal is converted into a signal having a more gently curved characteristic. The signal thus obtained is divided into two signals. From one of the two signals, it is detected that the vehicle is nearing the roadside antenna. When it has been detected that the vehicle has arrived at the region adjacent the roadside antenna, the other signal is applied through a gate circuit to a position detecting section so that the point where abrupt falling of the received signal level occurs can be detected. Thus, the navigator device can detect an abrupt decrease of the received signal level which corresponds to the position just in front of the roadside antenna. However, the device is still not sufficient in that, since the abrupt variations in the level of the received signal are removed by the low- pass filter, when the vehicle is driven at a high speed, the dip in the level of the received signal is not so deep, and therefore the position detecting section may not be able to detect the abrupt decrease in level of the received signal.
The above-described difficulty may be eliminated by removal of the low-pass filter. However, for reasons clear from the above discussion, doing so results in the difficultly that the received signal level decreases not only at the aforementioned abrupt decrease point P (cf. FIG. 4E) where the level should decrease, but also at other points P.sub.1, P.sub.2 and P.sub.3. Therefore, the position detection becomes inferior.
Moreover, the level of the signal received by the mobile antenna varies with the lane on which the vehicle moves; that is, it changes with the distance between the roadside antenna and the mobile antenna. Furthermore, the level depends on the inherent fluctuations in sensitivity of the roadside device and also the mobile device. Therefore, it is very difficult to set the fixed reference signal level at which the positioning of vehicle can be determined by detecting the falling of signal level to the reference point.
Experimental results will be described in more detail. For instance in the case where, on a three-lane highway, a split beam antenna is installed as a roadside antenna alongside the first lane, and an antenna of which main radiation lobe extends upwardly is used as the mobile antenna on a vehicle, the signal received by the mobile antenna on the vehicle traveling in the first lane is the highest in level (A in FIG. 5 and FIG. 6A), the signal received by the mobile antenna of the vehicle traveling in the third lane is lowest, and that received by the mobile antenna of the vehicle in the second lane falls between those two signals in level (B in FIG. 5 and FIG. 6B).
It is observed that there exists about 10 dB variations in receiving signal level according to the lane the vehicle is moving; however the sharp falling of the signal level just in front of the roadside antenna can be seen on any lanes.
This difference was observed in the experiment with only one set of roadside devices and the same mobile devices respectively. Considering a practical case in which a lot of beacon antennas are employed and a huge number of vehicles are involved, the variations in the signal level may actually become larger than 10 dB. Therefore, it is substantially impossible to set an absolute reference level for detecting the abrupt decrease the received signal.