Location of emergency service workers

Claims

We claim:

1. A method for monitoring the location of a general service worker or emergency service worker at a designated site, the method comprising the steps of:

selecting a designated site where the general or emergency service worker will perform services;

positioning a location-determining (LD) unit on the body or the garments of the worker, the LD unit including an antenna and receiver/processor for receiving a sequence of radiowave signals from three or more spaced apart electromagnetic signal transmitters whose transmitter locations are known with high accuracy, where these electromagnetic signals contain information that allows the present location of the LD unit to be determined, where the carrier frequencies of at least three of the electromagnetic signals are chosen so that these signals can be received, within a building-like structure having at least one aperture as well as outside such a structure, without substantial signal attenuation or distortion;

providing attachment means for attaching the LD unit to at least one of the worker's body and the worker's garments so that the LD unit does not interfere with performance of the worker's services;

providing a central station, having a signal receiver and processor, a signal transmitter, and an electronically sensible map of a selected portion of the Earth's surface that includes the coordinates of the designated site;

providing the LD unit with a sequence of two or more selected, spaced apart interrogation times;

causing the LD unit receiver/processor to determine the present location of the LD unit and to transmit information on the LD unit's present location to the central station receiver at the sequence of selected interrogation times;

causing at least one of the central station and the LD unit to determine the coordinates of the present location of the LD unit and to compare these coordinates with the coordinates of the designated site; and

causing the central station transmitter to communicate an alarm signal, which is perceptible by at least one person other than the worker at the designated site, if/at least one]any of the following conditions is present: (i) the present location of the LD unit is not within the designated site for at least one of the interrogation times; (ii) the central station does not receive transmitted information on the present location of the LD unit for at least K consecutive interrogation times, where K is a selected integer .gtoreq.1; (iii) the present location of the LD unit, as sensed by the central station, changes by less than a selected threshold amount during a time interval of selected time interval length .DELTA.t.sub.change that includes at least two consecutive interrogation times for the LD unit; (iv) the LD unit receives an interrogation signal requesting information on the present location of said LD unit; and (v) the accumulated time, during which the present location of the LD unit is within a selected sub-region of the designated site, exceeds a selected time .DELTA.t.sub.exposure.

2. The method of claim 1 further comprising the steps of:

choosing as said electromagnetic signal transmitters three or more FM subcarrier signal transmitters that each broadcasts an FM subcarrier signal having a preselected frequency;

providing said LD unit with information on a signal phase of each FM subcarrier signal relative to the phase of a selected one of the FM subcarrier signals; and

providing each of these subcarrier signals with a subcarrier source indicium contained therein that identifies which transmitter has transmitted a particular FM subcarrier signal.

3. The method of claim 2, wherein said step of causing said central station to determine said present location of said LD unit comprises the steps of:

determining an initial location of said LD unit with reference to said designated site;

determining initial relative phases of said FM subcarrier signals as these signals arrive at said LD unit at times near the interrogation times; and

subsequently determining changes in the relative phases of said subcarrier signals with reference to the initial relative phases, and determining the change in present location coordinates of said LD unit according to the changes in the relative phases.

4. The method of claim 2, wherein said step of causing said central station to determine said present location of said LD unit comprises the steps of:

providing a subcarrier signal receiver at a location that is known to said LD unit, and determining the relative phases of said three FM subcarrier signals;

providing this information on the relative phases of said subcarrier signals to said LD unit at one or more selected times; and

subsequently determining changes in the relative phases of said subcarrier signals with reference to an initial relative phase of each of said subcarrier signals, and determining the change in present location coordinates of the LD unit according to the changes in the relative phases.

5. The method of claim 2, wherein said step of providing said LD unit with information on the phase of each of said FM subcarrier signals comprises the steps of:

providing an FM signal monitor with known location that receives each of said FM subcarrier signals and determines the phase of each of said FM subcarrier signals relative to said selected FM subcarrier signal;

positioning the FM signal monitor at a location that is spaced apart from a plane defined by the locations of said three or more FM subcarrier signal transmitters; and

transmitting information on the relative phase of each of said FM subcarrier signals to said LD unit.

6. The method of claim 1, further comprising the steps of:

choosing as said electromagnetic signal transmitters a combination of (i) three or more FM subcarrier signal transmitters that each transmits an FM subcarrier signal having a subcarrier source indicium that identifies that transmitter and (ii) three or more outdoor LD signal transmitters that each transmits an outdoor LD signal having an LD source indicium that identifies that transmitter;

providing said LD unit with information on the phase of each FM subcarrier signal relative to the phase of a selected one of the FM subcarrier signals;

using the outdoor LD signals to determine the present location of the LD unit wherever the outdoor LD signals can be received without substantial attenuation or distortion; and

using the FM subcarrier signals to determine the present location of the LD unit wherever the outdoor LD signals cannot be received without substantial attenuation or distortion.

7. The method of claim 6, wherein said step of causing said central station to determine said present location of Said LD unit comprises the steps of:

determining at a selected phase determination time said present location of said LD unit on said designated site;

determining initial relative phases of said FM subcarrier signals as these signals arrive at said LD unit; and

subsequently determining changes in the relative phases of said subcarrier signals with reference to the initial relative phases, and determining the change in said present location of said LD unit according to the changes in the relative phases.

8. The method of claim 6, wherein said step of providing relative phase information on said FM subcarrier signals comprises the steps of:

providing an FM subcarrier signal receiver at a location that is known to said LD unit, and determining the relative phases of each FM subcarrier signal as this signal is transmitted;

providing this relative phase information to said LD unit at one or more selected times; and

subsequently determining changes in the relative phases of said FM subcarrier signals with reference to the initial relative phases, for at least one time subsequent to the time the relative phase information is provided to said LD unit.

9. The method of claim 6, further comprising the step of choosing said outdoor LD signals from a class consisting of GPS signals, GLONASS signals, Loran signals, Omega signals, Tacan signals, Decca signals, JTIDS Relnav signals and PLRS signals.

10. The method of claim 1, further comprising the step of causing said LD unit to monitor at least one physiological indicium of said emergency service worker, and

choosing said specified group of conditions to include the condition that this physiological indicium is within a predetermined danger zone for said worker.

11. The method of claim 1, further comprising the steps of:

choosing as said electromagnetic signal transmitters a combination of four or more FM subcarrier signal transmitters that each transmits an FM subcarrier signal having a subcarrier source indicium that identifies that transmitter, where one of these FM transmitters is located far from a plane passing through three other FM transmitters;

providing said LD unit with information on the phase of each FM subcarrier signal relative to the phase of a selected one of the FM subcarrier signals; and

using the FM subcarrier signals to determine said present location of said LD unit.

12. Apparatus for determining the present location, at a designated site, of a mobile user that carries the apparatus inside or outside buildings and structures, the apparatus comprising:

FM subcarrier means, carried by the user, for determination of the present location of the user, the FM means comprising:

an FM signal antenna and associated FM signal receiver/processor to receive FM subcarrier signals transmitted from at least three spaced apart FM subcarrier signal sources, with each of these FM subcarrier signals having a subcarrier source indicium that identifies the source for that FM subcarrier signal, to receive relative phase information on the FM signals received by the FM signal antenna, to determine the present location of the FM antenna from knowledge of the relative phases of signals received from the FM subcarrier sources, to determine an FM signal indicium that is a measure of at least one of the determined present location of the FM antenna, signal robustness and signal quality of these FM subcarrier signals, and to issue information on the FM antenna present location and the FM signal indicium as output signals; and

phase information means for receiving information on the relative phases of signals transmitted from the FM subcarrier signal sources and for passing this information to the FM receiver/processor;

outdoor location determination (LD) means, carried by the user, for determination of the present location of the user, the outdoor LD means comprising:

an outdoor LD signal antenna and associated outdoor LD signal receiver/processor to receive outdoor LD signals transmitted from at least three spaced apart outdoor LD signal sources, with each of these outdoor LD signals having an LD source indicium that identifies the source of that outdoor LD signal, to determine the location of the outdoor LD antenna from analysis of these LD signals, to determine an outdoor LD signal indicium that is a measure of at least one of the determined present location of the outdoor LD antenna, signal robustness and signal quality of these outdoor LD signals, and to issue the outdoor LD antenna present location information and the outdoor LD signal indicium as output signals;

controller means, for receiving the FM receiver/processor output signals and the outdoor LD receiver/processor output signals, for comparing the FM signal indicium with a selected FM signal indicium threshold, for comparing the outdoor LD signal indicium with a selected outdoor LD signal indicium threshold, for selecting from these comparisons at most one of the FM antenna present location information and the outdoor LD antenna present location information as user present location information, and for issuing the selected user present location information as a controller means output signal; and

transceiver means, connected to the controller means, for receiving the controller means output signal, for receiving at least two location interrogation signals, spaced apart in time, that command the transceiver means to transmit information on the present location of at least one of the FM signal antenna and the outdoor LD signal antenna, and for transmitting the controller means output signal to a selected receiver spaced apart from the user when at least one of a specified group of conditions is present.

13. The apparatus of claim 12, wherein said specified group of conditions includes at least one of the following conditions: (i) said present location of said LD unit is not within the designated site for at least one of the interrogation times; or (ii) said central station does not receive transmitted information on said present location of said LD unit for at least K consecutive interrogation times, where K is a selected integer >_ 1; (iii) said present location of said LD unit, as sensed by the central station, changes by less than a selected threshold amount during a time interval of selected time interval length .DELTA.t.sub.change that includes at least two consecutive interrogation times for said LD unit; and (iv) said LD unit receives an interrogation signal requesting information on said present location of said LD unit; and (v) the accumulated time, during which said present location of said LD unit is within a selected sub-region of the designated site, exceeds a selected time .DELTA.t.sub.exposure.

14. The apparatus of claim 12, further comprising physiological monitoring means for monitoring at least one physiological indicium of said mobile user, where

said specified group of conditions includes the condition that this physiological indicium is within a predetermined danger zone for said worker.

15. The apparatus of claim 12, wherein said transceiver means generates an alarm signal, which is perceptible by at least one person other than said mobile user, if at least one of the following conditions is present: (i) the present location of said subcarrier means or said outdoor LD means is not within the designated site for at least one of the interrogation times; (ii) the central station does not receive transmitted information on the present location of at least one of said subcarrier means or said outdoor LD means for at least K consecutive interrogation times, where K is a selected integer .gtoreq.1; (iii) the present location of said subcarrier means or said outdoor LD means, as sensed by the central station, changes by less than a selected threshold amount during a time interval of selected time interval length .DELTA.t.sub.change that includes at least two consecutive interrogation times for the LD unit; (iv) said subcarrier means or said outdoor LD means receives an interrogation signal requesting information on the present location of said LD unit; and. (v) the accumulated time, during which the present location of said subcarrier means or said outdoor LD means is within a selected sub-region of the designated site, exceeds a selected time .DELTA.t.sub.exposure.

16. A method for monitoring the location of a general service worker or emergency service worker at a designated site, the method comprising the steps of:

selecting a designated site where the general or emergency service worker will perform services;

positioning a first location-determining (LD) unit on the body or the garments of the worker, the first LD unit including an antenna and receiver/processor for receiving a sequence of radiowave signals from three or more electromagnetic signal transmitters whose transmitter locations are spaced apart from the designated site and are known with high accuracy, where these electromagnetic signals contain information that allows the present location of the first LD unit to be determined;

positioning a second LD unit on the body or the garments of the worker, the second LD unit operating independently of the first LD unit and including an antenna and receiver/processor for receiving a sequence of radiowave signals from three or more electromagnetic signal transmitters whose transmitter locations are spaced apart from the designated site and are known with high accuracy, where these electromagnetic signals contain information that allows the present location of the second LD unit to be determined, where the carrier frequencies for the electromagnetic signals used by the second LD unit are chosen so that these signals can be received, within a building-like structure having at least one aperture as well as outside such a structure, without substantial signal attenuation or distortion;

providing attachment .means for attaching the first LD unit and the second LD unit to at least one of the worker's body and the worker's garments so that the first LD unit and the second LD unit do not interfere with performance of the worker's services;

providing a central station, having a signal receiver and processor, a signal transmitter, and an electronically sensible map of a selected portion of the Earth's surface that includes the coordinates of the designated site;

providing the first LD unit and the second LD unit with a sequence of two or more selected, spaced apart interrogation times;

causing the first LD unit receiver/processor and the second LD unit receiver/processor to determine, at each interrogation time, the present location of the first LD unit and the second LD unit, respectively;

transmitting information on the present location of at least one of the first LD unit and the second LD unit to the central station receiver;

causing at least one of the central station processor, the first LD unit and the second LD unit to determine the coordinates of the present location of at least one of the first LD unit and the second LD unit from the information received and to compare these coordinates with the coordinates of the designated site; and

causing the central station transmitter to communicate an alarm signal, which is perceptible by at least one person other than the worker at the designated site, if one or more of a specified group of conditions is present, based on this comparison.

17. The method of claim 16, wherein said step of positioning said second LD unit on said body or garments of said worker comprises the steps of:

choosing as said electromagnetic signal transmitters three or more FM subcarrier signal transmitters that each broadcasts an FM subcarrier signal having a preselected frequency;

providing said second LD unit with information on a signal phase of each FM subcarrier signal relative to the phase of a selected one of the FM subcarrier signals; and

providing each of these subcarrier signals with a subcarrier source indicium contained therein that identifies which transmitter has transmitted a particular FM subcarrier signal.

18. The method of claim 17, further comprising the steps of:

providing as said first LD unit an outdoor LD unit that includes three or more outdoor LD signal transmitters that each transmits an outdoor LD signal having an LD source indicium that identifies that transmitter;

using the outdoor LD signals to determine the present location of the LD unit wherever the outdoor LD signals can be received without substantial attenuation or distortion; and

using said second LD unit to determine the present location of the LD unit wherever the outdoor LD signals cannot be received without substantial attenuation or distortion.

19. The method of claim 18, further comprising the step of choosing said outdoor LD signals from a class consisting of GPS signals, GLONASS signals, Loran signals, Omega signals, Tacan signals, Decca signals, JTIDS Relnav signals and PLRS signals.

20. The method of claim 16, further comprising the step of choosing said specified group of conditions to include at least one of the following conditions: (i) said present location of said first LD unit or of said second LD unit is not within the designated site for at least one of said interrogation times; or (ii) said central station does not receive transmitted information on said present location of said first LD unit or of said second LD unit for at least K consecutive interrogation times, where K is a selected integer .ltoreq.1; (iii) said present location of said first LD unit or of said second LD unit, as sensed by the central station, changes by less than a selected threshold amount during a time interval of selected time interval length .DELTA.t.sub.change that includes at least two consecutive interrogation times for said first LD unit or said second LD unit; (iv) said first LD unit or said second LD unit receives an interrogation signal requesting information on said present location of said first LD unit or said second LD unit; and (v) the accumulated time, during which said present location of said first LD unit or of said second LD unit is within a selected subregion of the designated site, exceeds a selected time .DELTA.t.sub.exposure.

21. The method of claim 16, further comprising the step of choosing said outdoor LD signals from a class consisting of GPS signals, GLONASS signals, Loran signals, Omega signals, Tacan signals, Decca signals, JTIDS Relnav signals and PLRS signals.

22. The method of claim 16, further comprising the step of choosing said specified group of conditions to include at least one of the following conditions: (i) the present location of the LD unit is not within the designated site for at least one of the interrogation times; (ii) the central station does not receive transmitted information on the present location of the LD unit for at least K consecutive interrogation times, where K is a selected integer .gtoreq.1; (iii) the present location of the LD unit, as sensed by the central station, changes by less than a selected threshold amount during a time interval of selected time interval length .DELTA.t.sub.change that includes at least two consecutive interrogation times for the LD unit; (iv) the LD unit receives an interrogation signal requesting information on the present location of said LD unit; and (v) the accumulated time, during which the present location of the LD unit is within a selected subregion of the designated site, exceeds a selected time .DELTA.t.sub.exposure.

FIELD OF THE INVENTION

This invention relates to use of electromagnetic signals to determine the present location of an emergency service worker, such as a firefighter or hazardous materials cleanup specialist, at the site of an emergency.

BACKGROUND OF THE INVENTION

After a firefighter has arrived at, and begun working at, a fire site, the present location of that firefighter may be difficult to determine, minute-by-minute. The firefighter may be working outside an enflamed structure but be hidden by the firefighting equipment or some other structure or by the local terrain. If the firefighter is working inside the structure, the problem of locating this person is doubly difficult, because line-of-sight location is usually impossible and because radio waves used for voice communication may not be transmitted past the structures walls. Visually perceptible markings have been developed for firefighters' out garments, and methods have been developed for locating the perimeter of a fire. However, methods for determining the present location of a firefighter or other emergency worker at the site of an emergency, second-by-second, no matter where the worker may be located, have not appeared yet.

Tung discloses a retroreflective protective helmet having a plurality of retroreflective stripes thereon that can be seen in darkened areas, if illuminated by light, in U.S. Pat. No. 3,885,246. The helmet requires line-of-sight visibility before the helmet can be illuminated and the retroreflected light can be visually perceived. Another protective and retroreflective helmet, with the same limitations on visual perception, is disclosed in U.S. Pat. No. 4,008,949, issued to Luna.

Bingham, in U.S. Pat. No. 4,533,592, discloses an upper body garment made of thermally stable, flame retardant material that includes a plurality of light-reflecting stripes thereon, for use in firefighters' coats. As in the Tung and Luna patents, use of this garment to locate a firefighter requires line-of-sight illumination of the stripes.

In U.S. Pat. No. 4,347,501, Akerberg discloses a portable alarm system useful for notifying others that the alarm sender requires assistance. The alarm signal carries a unique code that allows a central receiver to identify the sender. The alarm signal is relayed from the sender to the central station by intermediate retransmitters, positioned in or near the room where the alarm device wearer is located, that transmit the alarm signal with a code indicating the last known location of the wearer. The alarm device wearer would occasionally update the alarm system's knowledge of his/her location by moving to another room in the structure. This system requires that a one or more alarm signal retransmitters be located in each room of the structure and that the retransmitter perform its intended functions under all circumstances. Where a firefighter responds to a tire, these conditions will not often be present.

An out-of-range personnel monitor and alarm, useful for convalescent home residents and other monitored persons, is disclosed in U.S. Pat. Nos. 4,593,273 and 4,675,656, issued to Narcise. The monitored person carries a transceiver that receives a first signal and compares the first signal strength against a selected threshold that corresponds to a maximum distance the monitored person can move away from the first signal transmitter. If the first signal strength is below the selected threshold, the transceiver transmits a second signal that is received by a monitoring station, advising that the monitored person has moved outside the permitted range. This system requires that the region within which the monitored person moves is reasonable homogeneous in attenuating electromagnetic signals, and that the first signal generator can be located near the center of the permitted region of movement for the monitored person.

Engler et al disclose use of a high temperature resistant, retroreflective material for marking a firefighter's helmet, in U.S. Pat. No. 5,160,655. The helmet marking material reflects light directed at the helmet back toward the light source so that a firefighter's present location can be determined if (1) the firefighter is within a line of sight from the light source and is not concealed within a building and (2) the ambient gaseous medium at the fire site is not so smoke-filled that the light incident on, or reflected from, the helmet marking material is absorbed by the gas.

Treddenick, in U.S. Pat. No. 5,192,500, discloses a firefighter safety badge, having indicia on a first badge face regarding the medical history of the badge user, and having indicia on a second badge face noting the anticipated location of the badge user on the fire site. The second indicia can be removed to expose a plurality of indicator strips that are sensitive to different toxic gases, such as chlorinated hydrocarbons. The badge is intended to be secured to a post or other structure near where the badge user is working. However, if the present location of the badge user changes and the second badge face indicia is not changed to reflect this change, the badge user cannot be located using this indicia.

A personal alarm security apparatus that is worn on an arbitrary part of a person's body is disclosed by Young in U.S. Pat. No. 5,196,825. Normally, the apparatus transmits a first signal that is interpreted as indicating that no threatening event has occurred or is occurring. If an emergency or threatening event occurs, a second signals is transmitted. A redundant third signal is transmitted at the time the second signal should be transmitted, in case the second signal is not transmitted for whatever reason. The system uses two receivers to obtain some information on the wearer's present location when a second signal is received.

Several U.S. patents disclose sensing the approximate perimeter of a tire, using infrared or similar means to sense temperature level differences or other characteristics that distinguish enflamed from non-enflamed areas. These patents include U.S. Pat. No. 5,160,842, issued to Johnson, and a sequence of U.S. patents issued earlier to Brown de Colstoun et al (U.S. Pat. Nos. 4,567,367, 4,893,026 and 5,049,756). However, none of these approaches appears to allow determination of the present location of a firefighter or other emergency service worker within an enflamed region or other emergency site.

FM subcarrier signals and AM carder signals have been used for some types of radio wave communications. In U.S. Pat. No. 3,889,264, Fletcher discloses a vehicle location system in which the unsynchronized AM carrier signals from three or more AM radio stations form hyperbolic isophase grid lines that are used to determine location of a vehicle. The vehicle must be equipped with a three-channel, tunable receiver, and its location must be referenced to an initial known location by counting the number of isophase lines crossed after the vehicle leaves the initial location. Isophase drift is compensated for by subtraction from the count.

Dalabakis et al, in U.S. Pat. No. 4,054,880, disclose a radio navigation and vehicle location system employing three low frequency subcarrier signals received from three radio stations at a three-channel, tunable receiver located on the vehicle. Isophase lines crossed are counted after the vehicle leaves an initial known location. This system, like the Fletcher system, is a delta-position system that determines vehicle location only relative to an initially known location.

U.S. Pat. No. 4,646,290, issued to Hills, discloses use of F.C.C.-approved Subsidiary Communication Authorization (SCA) FM subcarrier signals for one way transmission. This patent discloses transmission of a plurality of messages, which may be delivered to the transmitter at a wide range of bit rates, to be transmitted at a single bit rate that is at least as large as the highest bit rate for message delivery. This method allows for downstream insertion of additional data.

An integrated radio location and communication system for a mobile station is disclosed by Martinez in U.S. Pat. No. 4,651,156. Each mobile station carries a transceiver that issues radio signals that are received by two or more signal transceiver reference sites having fixed, known locations. The transceivers at the mobile station and the reference stations are continuously phase locked to the RF carrier signal from a nearby commercial radio station. The radio station and the mobile station each transmit a brief, distinguishable range tone at a known sequence of times, and the range tone from each station is received by each reference station. From an analysis of the differences in arrival times of the range tones received from the radio station and from the mobile station, the reference stations determine the two-dimensional location of the mobile station. The mobile station uses the beat signal between two RF subcarrier frequencies to generate its range tone signal and to distinguish that mobile station transmissions from the transmissions of any other mobile station.

Young et al, in U.S. Pat. No. 4,660,193, discloses use of two SCA FM subcarrier signals, the first being amplitude modulated and the second being phase modulated, to provide a digital data transmission system. A subcarrier signal within this system may also be modulated to carry audio signals.

A multichannel FM subcarrier broadcast system that provides a sequence of relatively closely spaced channels, using independent sidebands of suppressed carriers, is disclosed by Karr et al in U.S. Pat. No. 4,782,531. The sideband signals are generated in pairs and are phase shifted before transmission. Upon receipt of the transmitted signals, the process is reversed. An earlier patent, U.S. Pat. No. 3,518,376, issued to Caymen and Walker, discloses a similar approach without use of signal phase shifting of pairs of sideband signals.

In U.S. Pat. No. 4,799,062, Sanderford et al disclose a radio location method that uses a central processing station, a plurality of signal repeater base stations with fixed, known locations, and a mobile station with a known location at any time. The central station transmits a master grid synchronization pulse, which serves as a time reference, to the other stations at a selected sequence of times. A roving station with unknown location transmits a pulse that is received by three or more base stations and is retransmitted to the central station. The central station determines the location of the roving station using the differences in time of arrival at each base station of the pulse transmitted by the roving station. The mobile station also transmits a pulse from time to time, and its known location is compared with its computed location by the central station to determine any multipath compensation required to reconcile the known and computed locations of the mobile station. The multipath compensation for a mobile station adjacent to the roving station is applied to correct the computed location of the roving station.

Ma, in U.S. Pat. No. 4,816,769, discloses receipt of SCA FM subcarrier signals for digital data paging at a radio receiver. The system measures signal-to-noise ratio of an output amplitude of a Costas loop, used to phase lock to the FM subcarrier frequency, to determine if the signal is sufficiently strong to be processed.

A system for detection of radio wave propagation time, disclosed by Ichiyoshi in U.S. Pat. No. 4,914,735, uses detection of phase differences for transmission of the signal over M (.gtoreq.2) different known signal paths to a target receiver. The transmitted signal includes a subcarrier signal, having a frequency that is higher than the transmitter clock frequency, modulated with a known modulation signal. The receiver has M demodulators for the signals received by the M different paths and has a phase comparator to compare the computed phases for each of these received signals. The phase differences are proportional to the signal path length differences, if compensation for transmission line distortions is included.

U.S. Pat. No. 5,023,934, issued to Wheeless, discloses a system for communication of graphic data using radio subcarrier frequencies. The data are broadcast on a subcarrier channel and received by a radio receiver that is connected to a computer. The computer receives the subcarrier signals, displays the graphic data on a computer screen, and performs other functions, such as transmission error checking and modification of the displayed graphic data. The system is intended for weather data communication and display.

Westfall, in U.S. Pat. No. 5,073,784, discloses a system for location of a transmitter ("unknown") at large distances, using a large network of pairs of spaced apart radio wave receivers whose locations are known and whose relative phases are synchronized. A signal, broadcast by the unknown transmitter at less than HF frequencies, is received at different time and space points by pairs of receivers. Simple geometrical computations allow determination of the location of the unknown transmitter by comparing times of arrival of the transmitted signal.

U.S. Pat. No. 5,170,487, issued to Peek, discloses use of FM sub-carrier signals for a pager system for mobile users. A plurality of transmitters are used, each of which transmits an FM subcarrier signal or a carrier signal modulated with a chosen message signal, slightly offset in time. Each page-receiving unit is assigned a time slot, during which the receiving unit dials through the set of frequencies corresponding to the FM subcarrier and modulated-carrier signals to determine if a page message has been sent for that mobile user.

A system that allows determination of an absolute location of a vehicle is disclosed by Kelley et al in U.S. Pat. No. 5,173,710. FM subcarrier signals are received from three radio stations with known locations but unknown relative phases by signal processors at the vehicle and at a fixed station with known location relative to the three radio stations. The fixed station processor determines the relative phases of the three radio stations FM subcarrier signals and broadcasts this relative phase information to the vehicle. The vehicle processor receives this relative phase data and determines its absolute location, using the phases of the FM signals it senses at its own location.

Chon, in U.S. Pat. No. 5,193,213, discloses an FM broadcast band system for receipt of relatively high frequency FM subcarrier signals. A tunable high pass receiver first circuit receives the carrier and a tunable low pass second circuit receives the subcarrier signal. Each signal can then be separately processed.

A navigation and tracking system using differential LORAN-C or differential Decca signalling is disclosed by Duffett-Smith in U.S. Pat. No. 5,045,861. A reference station transmits a reference signal to a mobile station and to three or more local LORAN-C or Decca (fixed) stations having known locations relative to the reference station. The fixed stations retransmit the reference signal to the mobile station, where the phase received signal differences are compared to determine the location of the mobile station.

Most of these systems use a single communication system, rather than integrating two or more communication systems to provide location or navi