Vehicle location unit

A Vehicle Location Unit (VLU) device includes in functional relationship an RF receiver, a transmitter, a receive/transmit switch, a microcontroller to control the receiving/transmitting path, a digital Large Scale Integration (LSI), having a digital signal processing device, a crystal oscillator, and a filtering device for an outcoming signal.

FIELD OF THE INVENTION 
The present invention relates to a vehicle location system. More 
particularly, the invention relates to an improved unit to be incorporated 
in a vehicle. 
BACKGROUND OF THE INVENTION 
Vehicle Location Systems (VLS) have been used for some time. Such systems 
comprise transmitting/receiving stations which are capable of transmitting 
signals specific to a given vehicle, which signals are received by the 
Vehicle Location Unit (VLU) of the vehicle, which in turn emits signals 
which are received by the various receiving stations. Receipt of a signal 
by four or more receiving stations makes it possible to determine the 
exact location of the vehicle. 
These systems have a variety of applications. For instance, they are used 
in controlling the movement of commercial vehicles, such as lorries and 
taxis, busses or other vehicles the operation of which requires 
coordination and control. Another interesting application is the location 
of stolen property, particularly motorcars, or of vehicles in which an 
emergency situation has occurred. A motorcar incorporating a VLU can be 
located at all times by its VLS, through the various receiving stations 
positioned at different locations. Vehicle locating systems of various 
types are described, e.g., in U.S. Pat. No. 4,905,271, JP 02-57450, JP 
01-177724 and JP 63-235877. 
THE PRIOR ART 
The use of these systems, however, has been limited so far due to a number 
of problems related to the VLU. First of all, because of their design, 
which utilizes conventional off the shelf radio components, the VLUs sold 
today are prohibitively expensive, and cannot be normally afforded by 
private users. This, as will be appreciated, considerably limits the 
applicability of these systems which could be conveniently exploited 
virtually in every vehicle if this problem were solved. 
Another considerable drawback of existing VLUs resides in their high power 
consumption. Prior art VLUs have a power consumption of more than 100 mA 
at 12 V, and because of such high power consumption, the VLU cannot 
operate continuously and must be switched off when the motor is turned 
off, or shortly thereafter. Commercially available VLUs are normally 
automatically switched off about 4 hours after the motor is turned off. 
This means that during certain periods of time the location of the vehicle 
cannot be verified through this system. 
An additional, and severe, drawback of existing VLUs is their relatively 
low reliability, due to the large number of components which must operate 
in a difficult environment, under mechanical and thermal shocks. Because 
of these factors, a high Failure rate of the VLUs is seen. 
SUMMARY OF THE INVENTION 
It is therefore clear that it would be highly desirable to be able to 
overcome the aforesaid drawbacks, and to permit thereby a widespread and 
everyday use of VLUs to the general public. 
It is an object of the present invention to provide a VLU which overcomes 
the aforesaid drawbacks, which is inexpensive, highly reliable and which 
does not require high power consumption. The VLU hereinafter described has 
an average power consumption of less than 40 mA at 13.6 VDC. 
It is another object of the invention to provide improved functional 
components which permit to carry out the operations required by the VLU in 
an effective and inexpensive manner. 
The VLU device according to the invention comprises, in functional 
relationship, the following elements: 
1. an RF receiver; 
2. a transmitter; 
3. a receive/transmit switch; 
4. a microcontroller to control the receiving/transmitting path; 
5. a digital LSI, (Large Scale Integration) comprising digital signal 
processing means; and 
6. crystal oscillator and filtering means for the outcoming signal; coupled 
to conventional circuit elements, such as memory means, power supply 
means, etc. 
According to a preferred embodiment of the invention, the crystal 
oscillator and filtering means are comprised in an analog LSI. In another 
preferred embodiment of the invention, the crystal oscillator and 
filtering means are provided as separate circuits. 
As will be detailed hereinafter, the invention is not only directed to the 
VLU described above, but also encompasses novel elements and combinations 
of elements, which can be exploited in manufacturing VLUs according to 
preferred embodiments of the invention. All the above and other advantages 
and characteristics of the invention will be better understood through the 
following illustrative and non-limitative description of a preferred 
embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, numeral 1 is an antenna which is connected, through 
receive/transmit switch 2, either to the receiving or the transmitting 
path. The operation of the switch 2 is controlled by the microcontroller 
3, which signals whether the transmitting or the receiving path are to be 
activated at any given time. 
The signals normally employed in VLSs are 925 MHz FSK modulated signals. 
Looking now at the receiving path, the RF receiver 4 converts the received 
signals into 2400 bits/sec baseband signals, and transmits them to the 
digital LSI 5. In the digital LSI 5 the incoming signal data are first 
decoded by the processor 6. The operation of the processor 6 is as 
follows. The data is first processed by a bit synchronizer circuit which 
samples the incoming bits and corrects for the difference between the 
system PAGER clock and the VLU clock. The output of the bit synchronizer 
circuit is fed to a SYNC/ADDRESS decoder, which looks for a correlation 
between the detected Sync word and the known Sync word, and does the same 
for the VLU specific address (ID) versus the detected one. This permits to 
make sure that proper identification of the VLU is effected. The various 
circuits, such as the synchronizer circuit and the SYNC/ADDRESS decoder, 
are conventional and providing them is within the scope of the skilled 
engineer. Therefore, for the sake of brevity and simplicity, such 
conventional circuits and means, when referred to herein, are not 
described in detail, it being understood that the general teachings of the 
art in this respect are incorporated herein by reference. 
Once the decoder has detected the SYNC word, the VLU is synchronized and 
address detection can be carried out. The positive output of the address 
detection circuit, indicating that detection has been successfully carried 
out, interrupts the microcontroller 3, which activates the transmitter 7 
at the predetermined timing. 
The timing and control circuit 8 initializes and stops the various 
procedures which take place during reception and transmission, by 
providing the necessary timing and control signals. It is an advantageous 
feature of the invention that the circuit parameters are set by the 
microcontroller, and therefore it is possible to employ the timing circuit 
outputs for each specific transmission/reception phase of operation of the 
VLU, since its operation can be easily programmed. This, as will be 
apparent to the skilled person, provides for an enhanced performance of 
the unit. 
As will be appreciated, the output of the VLU must have a predetermined 
shape, according to the accepted standards. This is achieved by providing 
a PN Generator circuit 9, which comprises a 10 bit register with feedback, 
which generates the required bit pattern. The PN data so generated are 
input to the Digital Shaper 10, which rotates them to provide the 
necessary shaping control signals. 
The ECM (Emergency Channel Message) Generator 11 circuit comprises digital 
logic, dataloading register and a parallel load shift register. The 
microcontroller 3 loads the register at each "load" command with a byte. 
The register feeds the shift register while the ECM digital logic controls 
the timing and the sequence of the synchronous load commands. The 
procedure is repeated 16 times, so that the shift register outputs the 128 
ECM bits which are then received by the digital shaper. 
The analog LSI 12 is novel in design, and incorporates the analog section 
of a Digital Temperature Compensated Crystal Oscillator (DTCXO), 
designated by numeral 13. The DTCXO, as seen in FIG. 2, comprises a 
digital-to-analog (D/A) converter 20, and analog-to-digital (A/D) 
converter 21, a voltage controlled crystal oscillator (VCXO) 22, and a 
temperature sensor 23 coupled to a sensing diode 24. The sensed 
temperature value is translated into a voltage by the temperature sensing 
circuit 23, and the circuit output voltage is changed according to every 
sensed temperature. The output of the temperature sensor circuit is fed to 
the A/D 21, which translates the voltage levels into bytes that are read 
by the data bus of the microcontroller 3. 
As stated, the analog LSI of FIG. 1 can be replaced by appropriate circuits 
for the. DTCXO and the transversal filter, according to another preferred 
embodiment of the invention. A separate figure is not provided, for the 
sake of brevity, as this substitution is clear to the skilled person. 
Returning now to FIG. 1, the microcontroller 3 is coupled to an erasable 
programmable read only memory, which according to a preferred embodiment 
of the invention is EEPROM 14. EEPROM 14 contains a table in which the 
compensation values for the VCXO 22 (FIG. 2) are stored. These values are 
read by microcontroller 3, which writes the appropriate values into the 
input of the D/A convertor 20. The D/A output voltage is fed into the VCXO 
compensation voltage input so as to maintain its frequency within .+-.1 
ppm through the full operating range. 
The Analog LSI 12 also contains a transversal filter 15. This filter and 
its use, within the context of this invention, are novel and as such also 
form a part of the invention. The filter 15 provides the transmitter 7 
with a sine-shaped bit stream on its real (I) and imaginary (Q) outputs. 
This is further illustrated in FIG. 3, in which the functional diagram of 
filter 15 is shown. As seen in this figure, the (I) data bit stream input 
to shift register 30 is converted into an analog shaped data output which 
is the sum of the shift register outputs divided by the respective 
resistors values. The (Q.sub.0) and (Q.sub.1) data bit streams inputs to 
shift registers 31 and 32 respectively, and are converted to an analog 
shaped data output which is the sum of the output of the shift registers 
divided by the respective resistor values. 
The transversal filter, if properly designed, specifically by the proper 
selection of resistor values, together with the shaping function design 
implemented in the digital shaper 10 (FIG. 1), optimizes the tradeoff 
required by the system to meet both the FCC and the location accuracy 
requirements. 
In FIG. 1, additional functional elements can be seen. The 908M synthesizer 
16, which is conventional in construction, is interfaced with the Analog 
LSI 12. The synthesizer is based on a frequency multiplier which 
multiplies the reference DTCXO frequency and provides local oscillator for 
both the receiver and the transmitter sections. Also a power supply means, 
which can be supplied by the battery of the vehicle, or by an external 
battery, is of conventional type, and is therefore not discussed herein in 
detail. 
FIG. 4 illustrates an additional improvement of the DTCXO circuit intended 
to solve drift problems caused by crystal aging. This is done, according 
to one embodiment of the invention, using the receive channel signal as a 
reference frequency. 
As in the analog LSI, the TCXO system is based on a standard VCO (voltage 
controlled oscillator) controlled by the system. The compensation value 
from the L.U.T (look-up table) driven by DAC (Digital to Analog Converter) 
is used to control the VCO. The address to the L.U.T. is based on a T.S. 
(Temperature Sensor) driven by ADC. 
The L.U.T. is filled during a calibration procedure and updated (for aging) 
during the tracking procedure. As reference to the calibration and 
tracking loop, the system uses the received RF signal of the FCM (Forward 
Channel Messages). The VCO frequency is multiplied to generate REF1 
frequency for down-converting the FCM RF signal to IF level. The IF 
frequency is counted in the calibration counter and compared to a 
reference count. The deviation of the measured IF frequency from a known 
reference IF frequency generates an offset value added to the L.U.T value 
for the appropriate address according to the temperature sensor output. 
As will be apparent to the skilled engineer, the device according to the 
invention can be provided in many different embodiments. For instance, the 
Digital LSI 5 can be engineered to contain a more limited number of 
functional elements, e.g., the signal processor 6 or the digital shaper 10 
could be conventional units, external to, and interfaced with, LSI 5. 
While this arrangement would diminish some of the advantages of the 
invention, by increasing the cost of the unit, decreasing it reliability, 
due to the increased number of elements, and being in general less 
desirable, it would by no means exceed the scope and spirit of the present 
invention.