Abstract:
The present invention is a system and method for automated position location using RF signposting. The present invention comprises a plurality of transmitting units placed throughout a service area. Each transmitting unit repeatedly transmits a signal including position information related to a position associated with the transmitting unit. The position information may be an absolute position or an encoded position based on the structure of a building in which the transmitting unit is located. A receiving unit receives the signal transmitted from a transmitting unit and determines the position of the receiving unit, based on the received indication. The transmitting units are placed to provide uniform coverage of the service area, thus providing position location indoors and in urban areas where GPS does not function properly.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system and a method for determining position using terrestrial transmitters and mobile receivers. 
     BACKGROUND OF THE INVENTION 
     Automated position locating systems have become important components of many military and civilian systems and operations. The Global Positioning System (GPS) provides position location using satellites. However, a typical GPS receiver requires signals from four or more GPS satellites in order to function properly. A problem arises with GPS indoors or in urban areas where the signals from satellites are often blocked by buildings. A need arises for a system which provides automated position location indoors or in urban areas where GPS does not function properly. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and a method for automated position location using RF signposting. In that regard, the present invention provides accurate and available position location indoors and in urban areas where GPS does not function properly. 
     The present invention includes a plurality of transmitting units placed throughout a service area. Each transmitting unit repeatedly transmits a signal including position information related to a position associated with the transmitting unit. The position information may be an absolute position or an encoded position based on the structure of a building in which the transmitting unit is located. A receiving unit receives the signal transmitted from a transmitting unit and determines the position of the receiving unit, based on the received indication. The transmitting units are placed to provide uniform coverage of the service area, thus providing position location indoors and in urban areas where GPS does not function properly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The details of the present invention, both as to its structure and operation, can best be understood by referring to the accompanying drawings, in which like reference numbers and designations refer to like elements. 
         FIG. 1  is an exemplary block diagram of a radio frequency (RF) signposting system according to the present invention. 
         FIG. 2  is a block diagram of a transmitting unit according to the present invention. 
         FIG. 3  is a block diagram of a mobile receiving unit according to the present invention. 
         FIG. 4  is a block diagram showing more detail of a receiver according to the present invention. 
         FIG. 5   a  is an exemplary format of an absolute position signal transmitted by transmitting unit according to the present invention. 
         FIG. 5   b  is an exemplary format of an encoded position signal transmitted by transmitting unit according to the present invention. 
         FIG. 5   c  is an exemplary format of an absolute position output sentence generated by a receiver according to the present invention. 
         FIG. 5   d  is an exemplary format of an encoded position output sentence generated by a receiver according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is an exemplary block diagram of a radio frequency (RF) signposting system  100  installed in an urban area, such as, for example, New York City. A plurality of transmitting units  102   a–f  are deployed at various locations in the city. For example, transmitting units  102   e–f  are deployed on lampposts at selected street corners. Transmitting units  102   a–d  are deployed on different floors of a particular building. Transmitting units  102  may also be deployed at other locations, such as, for example, on traffic signals, utility poles, tops of buildings, etc. The locations are selected for ensuring complete coverage of the service area and for providing the desired position resolution. Each transmitting unit periodically transmits a radio frequency signal that includes information. All transmitting units transmit on a common frequency or RF channel. The repeat time of each transmitting unit is different so that packet collisions on the common RF channel are reduced. 
     Mobile receiving units  104   a–c  may be located anywhere within the service area. Each receiving unit  104  receives a signal transmitted from each transmitting unit which is within range and extracts the position information from each received signal. The position of the receiving unit is then determined from the respectively received position information. 
       FIG. 2  is a block diagram of a transmitting unit  102 , as shown in  FIG. 1 . Controller  202  generates a position information signal that is applied to radio frequency transmitter  204 , which modulates, in a well known manner, a radio frequency carrier in accordance with the input position information signal. Transmitter  204  outputs a radio frequency signal to antenna  206 , which broadcasts the signal. 
     Controller  202  includes a memory  210  which contains position data  212 . Position data  212  indicates the position of the transmitting unit within the service area. Signal generator and control circuit  208  accesses memory  210 , obtains position data  212  and generates the position information signal which is output from controller  202 . Circuit  208  also controls the timing of periodic or intermittent transmissions. Circuit  208  may be a microcontroller, a microprocessor, a programmed logic device, a custom integrated circuit, or other logic device. 
     Preferably, the transmitting unit used is a commonly available radio packet transmitter using a frequency of approximately 2.45 GHz. An antenna  206 , which matches the output impedance of the transmitter, is also commonly available. 
       FIG. 3  is a block diagram of a mobile receiving unit  104 , shown in  FIG. 1 . RF signals broadcast by transmitting units are received by antenna  306 . The signals are input to receiver  304 , which detects, amplifies and demodulates, in a well known manner, the signal and extracts the position information from the signal. Receiver  304  outputs a serial digital signal which is input through serial port  308  to computer system  302 . 
       FIG. 4  is a more detailed block diagram of receiver  304  of  FIG. 3 . The signal from antenna  306  is input to detector  402 , which demodulates, in a well known manner, the RF signal to a baseband signal. Amplifier  404  generates a digital signal from the baseband signal. Microcontroller  406  then extracts the position information from the digital signal output from amplifier  404 . Although a microcontroller is preferred, microcontroller  406  may be replaced by a programmed logic device, custom integrated circuit, or other logic device. The extracted position information is output to computer  302  through serial interface  408 , in a well known manner. 
     The position information modulated onto the RF signal depends upon the location of the transmitting unit generating the signal. Units located in outdoor or open areas transmit an absolute position, which includes up to three components.  FIG. 5   a  is an exemplary format  500  of an absolute position. Format  500  includes a latitude field  502 , a longitude field  504 , and may optionally include an altitude field  506 . Each field is four bytes long. 
     Units located indoors may transmit position information that is based on the structure of a building, for example, such as a section and an aisle. Position information in this form is denoted as an encoded position.  FIG. 5   b  is an exemplary format  508  of an encoded position. Format  508  includes a code field  510 , and between one and three parameter fields  512 – 516 . Code field  510  is four bytes long, starting with a flag character, such as, for example, “$”, followed by three alphanumeric characters. The flag character indicates that the position information is an encoded position. The format of the parameter fields  512 – 516  are indicated by the alphanumeric fields of code  510 . For example, the alphanumeric characters may indicate that the encoded position information identifies a particular building in which the transmitting unit is located. Parameter fields  512 – 516  are each four bytes long and represent a location within a building. However, the format of the location information may vary from building to building. For example, in some buildings parameter fields  512 – 516  may represent floor and room numbers, while in other buildings, parameter fields  512 – 516  may be coordinates in a grid system representing the building. 
     When position information is extracted by a receiving unit, it is further processed for generating data which is compatible with the output of GPS receivers. The GPS compatible data is sent to the computer system  302  over the serial port  308 . Use of GPS compatible output data allows existing GPS software to function transparently with the RF signposting system of the present invention. The receiving unit examines the first character of the extracted information to determine whether it is a special character, such as, for example, “$”. If the first character is not a special character the extracted information is determined to be an absolute position, and the receiving unit generates an absolute position output sentence  515  as shown in  FIG. 5   c . The absolute position output sentence includes a header  520 , for example, the character string “$SPOST”, followed by a space, and the extracted latitude, longitude and optional altitude fields  502 – 506 . 
     If the first character of the extracted information is a special character, such as “$”, the extracted information is determined to be an encoded position, and the receiving unit generates an encoded position output sentence  517  as shown in  FIG. 5   d . The encoded position output sentence includes a header  520 , which is the character string, such as “$SPOST”, followed by a space, code  510  and the extracted parameter fields  512 – 516 . 
     The computer system which receives the output sentence from the receiving unit determines the current position. Because the output sentence information is compatible with GPS information, well-known GPS software may be used to determine position. Such software typically maintains a history of the position information for more accurately estimating the current position. In addition, the software may also be enhanced to take advantage of the unique features of RF signposting. For example, the computer system may maintain a database of buildings in which it can look up encoded position output sentence information as indicated by the received building code, for more accurately estimating the current position. 
     RF signposting can be used for providing services which require position location in dense areas. For example, on a college campus, in a mall and a mall parking lot, an “emergency signaling service” can use RF signposting for determining a subscriber&#39;s location. With such a service, a subscriber needing emergency attention can send his current location information to an emergency control center. The control center then accurately and precisely determines the subscriber&#39;s location and dispatches emergency personnel immediately. Other location-based services include a map and navigation service, an electronic yellow pages service and electronic commerce. 
     Although a specific embodiment of the present invention has been described, it will be understood by those of skill in the art that there are other embodiments which are equivalent to the described embodiment. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.