GB2415558A

GB2415558A - Locating a transmitter within a building

Info

Publication number: GB2415558A
Application number: GB0414042A
Authority: GB
Inventors: Philip Williams
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-06-23
Filing date: 2004-06-23
Publication date: 2005-12-28
Also published as: GB0414042D0

Abstract

A means for determining the location of a transmitter A within a building is disclosed by use of a number of strategically located receivers B whereby the received signal strength is used to determine the position of the transmitter A. Each receiver B may be coded to identify which of the receivers B have detected a signal from the transmitter A. Each receiver B may communicate receipt of a valid signal from the transmitter A to a central point C thru the mains wiring within the building. Communication to the central point C is delayed by an amount inversely proportional to the signal strength received from the transmitter A such that the receiver B nearest to the transmitter A sends its identity first to the central point C. The transmitter A may be a low power key-fob transmitter and may utilise radio, infrared or ultrasonic transmission.

Description

241 5558 Tracking Sec ritv Personnel The object of the application is to

allow the holder of a small key-fob type transmitter to be located within a building using simple techniques. It is appreciated that a transmitter may be located using direction-finding receivers but these require the use of elaborate directional aerials and signals are adversely affected by buildings and metalwork particularly at VHF and UHF frequencies. Direction finding is also made more complicated in multi storey buildings where the third dimension of height is involved.

The technique to be described uses any number of receivers suitably located at strategic positions one or more of which may pick up the signal transmitted from the low-power key-fete sender. It should be remembered that the sending device is not restricted to VHF/UHF Radio and Induction loop, Infra Red or Ultra-Sonic methods could be adopted.

It will be appreciated that the sending transmitter may either be manually operated by pushing a button or it could be arranged to automatically send on a regular basis, say every minute or so. For the purposes of the description however it is assumed that manual sending is the preferred method In the example the sender and receiver are of the type fitted with codes that are typically found in the low-power radio world so that the receivers only respond when the received code is valid and intended for use by the system. Such coding systems are well known and are not the subject of this application.

The receivers provide two outputs one for the code and another analogue value corresponding to the received signal strength and it is the signal strength which provides the position-indicating element in the system described here.

In order to provide an indication of the location ofthe transmitter it is necessary for the receivers to convey their information to a central point such as a security office or a reception desk or part of a more elaborate alarm system as required by the particular end user. This is achieved by each receiver having a unique identity code and that this code is conveyed to the central point either by wire, by second radio or by the use of the buildings mains wiring as in the case to be described.

It is likely that in a building where a number of receivers are strategically located that several of them will receive that transmitted signal. However it is equally likely that one receiver will be nearest to the sender and receiver the strongest signal.

We therefore have one sender, a number of receivers and a means for the receivers to send their identity to a central point via the mains wiring. The specific point of this application is that the received signal strength is used to identify the receiver nearest to the key-fob sender, and that value is used to accelerate the timing of the transmission of the receivers identity to the remote central display receiver. The weaker the received signal, the longer is the delay in sending the receivers identity to the central point.

In addition, to prevent mutual interference in the mains wiring path, all receivers are provided with additional circuitry to detect the presence of a transmission in progress on the mains wiring a Line busy' condition and further delay their own transmission until the path is clear. This is similar to a squelch circuit where a call in progress.

This also means that the first transmission gets through unimpeded. It is a C Loudest Signal First Served' arrangement. To summarise, If more than one receiver picks up the call from the key-fob sender then the receiver will send its identity after a period dependent on the signal strength of the original received signal. The stronger the signal, the shorter the period. Other receivers will be prevented from sending as the line busy feature will be active.

The remote display receivers which may be plugged into any mains socket connected to the same mains circuitry, will respond to the first signal it receives and displays the identity of the receiver which has picked up the strongest signal from the key-fate sender and thus the nearest point to the origin has been identified. It will display the identity number and sound alarms as appropriate.

In practice it is important to position the receivers in such a way as to maximise the potential to accurately track the senders position this may be significantly improved by paying attention to the receiver aerials and the use of external dipole or It4 wave vertical aerials is highly recommended.

With reference to the block diagram Fig I AND Fig 2 Fig I is the overall layout the key-fob sender is item (A) Bl to B5 are receivers each with their individual identification codes they are all connected to the buildings mains wiring represented by the lines and joints and also connected is the display receiver (C). Any number of receivers may be connected but 5 are shown Fig2 represents the block diagram of one of the receivers.

The signal from the VElF/UHF transmitter is picked up by the aerial (D) and fed to the Receiver (J) this provides 2 outputs. The first is a signal which indicates the reception of a valid code from the Receiver and this is fed to the logic electronics (G) The second is an analogue signal which reflects the received signal strength. This is fed to the delay circuitry ( F). The output of this block is delayed in time depending upon the strength of the incoming signal. For example a Strong signal might provide a delay of only IOmS but the weaker the signal the longer will be the delay of the output of block (F) up to say a delay of 1 Second. These are practical values but not in any way restrictive. The output of Block F is also sent to the logic unit (G).

The receivers also monitors activity on the mains wiring line, a function provided by block (H) if another receiver unit is sending to the remote display receiver (Fig I block (C)) then Block H sends a busy signal to the logic Unit (G). Block (1) is the mains sender circuit which sends the identification of the individual receiver to the remote display receiver (Figl (C) ) . The method of signalling via the mains wiring is not the subject of this application and has previously been filed as GB0322785.7 The logic Unit (G) therefore needs to determine that the incoming radio signal is valid, and that there is no activity on the line and it will provide an output to the mains sender after a time predetermined by the incoming signal level.

Claims

Claims 1) A method of determining the location of a transmitter by He use

of a network of receivers the output of which is delayed by an amount inversely proportional to signal strength received from the said transmitter.
2) A system as in 1) where the receivers are gated together by the presence of a line busy element such Cat the receiver receiving the strongest signal may send its identity without secondary receivers sending theirs. s