United States Patent 19
`Norris
`
`(54). GPS RELATIVE POSITION DETECTION
`SYSTEM
`
`75) Inventor: Elwood G. Norris. Poway, Calif.
`73) Assignee: American Technology Corporation,
`Poway, Calif.
`
`(21) Appl. No.: 542,799
`22 Filed:
`Oct. 13, 1995
`Related U.S. Application Data
`63 Continuation-in-part of Ser. No. 377,973, Jan. 25, 1995, Pat.
`No. 5,689.269.
`(51) Int. Cl. ...................... H04B 7/185: G01S 5/02
`52 U.S. Cl. ........................................... 342/357; 342/419
`(58) Field of Search ..................................... 342/357,419;
`455/12.1
`
`56
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,063.048
`11/1962 Lehan et al. .
`5/1977 Culpepper et al..
`4,021,807
`6/1986 Narcisse.
`4.593,273
`4.675,656 6/1987 Narcisse.
`5,021,794 6/1991 Lawrence.
`5,119,504 6/1992 Durboraw, III .
`5.146,231
`9/1992 Ghaem et al. .......................... 342/419
`5,172.110 12/1992 Tiefengraber.
`5.245,314
`9/1993 Kah, Jr. .
`5,289,195
`2/1994 Inoue.
`5,307,277 4/1994 Hirano .
`5,434,789
`7/1995 Fraker et al. ........................... 364/460
`5,506.587 4/1996 Lans ........................................ 342/357
`5,539,398
`7/1996 Hall et al. ............................... 340/907
`OTHER PUBLICATIONS
`GPS Technology and Opportunities by Clyde Haris and Roy
`Sikorski.
`Utah Meeting Shows Amazing World of Navigation Satel
`lites By Lee Siegel.
`
`III IIII
`5,781,150
`Jul. 14, 1998
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`US005781 150A
`Patent Number:
`11
`45) Date of Patent:
`
`A Marriage Made In Orbit: GPS and PCS by Francis X.
`Kane.
`A Sampling Of Global Positioning System Receivers by
`Don Herskowitz.
`How Mobile Computers Can Help You Find Yourself by
`Gerald Houston.
`This 'Remote' Shows. Its Users Exactly Where Here Is by
`Liz Mullen.
`CAR 54, Where Are You? By Michael Puttre.
`United States Securities And Exchange Commission Form
`10-K FROTrimble Navigational Limited.
`
`Primary Examiner. Theodore M. Blum
`Attorney, Agent, or Firm-Thorpe, North & Western, LLP
`57
`ABSTRACT
`A system of GPS devices which receive civilian GPS signals
`and provide an intuitive graphical interface for displaying
`the relative position of GPS devices in relation to each other,
`the relative position being accurate to several meters and
`defined as the distance to, direction of and height variance
`between GPS devices. A first GPS device with the person or
`object to be located transmits its GPS determined location to
`a second GPS device. This second GPS device includes a
`means for receiving the GPS determined position of the first
`GPS device, and also includes means for calculating the
`relative position of the first GPS device relative to the
`second GPS device based on a comparison of the received
`telemetry of the first GPS device and its own GPS deter
`mined position. The relative position of the first device is
`then graphically displayed on an interface of the second GPS
`device in a manner which eliminates the need for a map in
`order to travel to the location of the first GPS device. While
`protiding an interface which displays a relative position of
`the first GPS device, this information remains accurate no
`matter how the orientation of the second GPS device
`changes with respect to a compass.
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`13 Claims, 8 Drawing Sheets
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`GPS Satelliteransfers
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`Receiver
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`Embedded Controller 25
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`- m --- m we ... ... ... RF Transfter
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`Transmits GPS location
`Coordinates
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`3.
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`lfcd. Display
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`1
`GPS RELATIVE POSITON DETECTION
`SYSTEM
`RELATED INVENTION
`This patent application is a continuation-in-part of U.S.
`patent application Ser. No. 08/377,973, filed Jan. 25, 1995,
`now U.S. Pat. No. 5,689,269.
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention pertains to position determining devices.
`and in particular to devices that enable the position of an
`object or person to be determined relative to another person
`seeking said object, wherein a global positioning system
`receiver is used to determine the distance, direction and
`possible elevation difference between another global posi
`tioning system receiver.
`2. Prior Art
`Being able to determine the precise whereabouts of some
`one or something on or above the surface of the earth has
`long held promise for many purposes. Missing person
`searches would be much simpler if people who were lost had
`a transmitting device with them which constantly broadcast
`their precise position. Such a transmitter would be better
`than just a voice transmitter because the age of the people or
`their medical condition might prevent people from
`responding, or from responding in a helpful manner.
`However, numerous difficulties arise when actually search
`ing for a transmitter which severely undermines the useful
`ness of such systems.
`For example, U.S. Pat. No. 4,021,807 teaches how a
`transmitter hidden among stolen money could be used to
`locate those responsible for the theft and the money. A UHF
`homing device hidden among the money is capable of
`35
`transmitting a signal which can be tracked by UHF tracking
`devices. Such a tracking device indicates whether the UHF
`homing signal is being transmitted from the front or rear, and
`from the left or right of a current position and orientation of
`the tracking device. Signal strength can also be used to give
`a crude estimation of distance between the tracking and
`homing devices if the signal is not too distorted by inter
`vening structures.
`The UHF homing signal and tracking devices comprise
`the same principle taught in U.S. Pat. No. 5,021,794. This
`patent teaches how a miniaturized transceiver carried by a
`child can be remotely activated by a parent to enable the
`child to be located by police cars with UHF trackers.
`One of the drawbacks of such locator systems is that the
`position of the person or object is never known with any
`great degree of accuracy. A related issue is that the reliability
`of the signal received is also suspect, and can not be
`confirmed. Furthermore, a vehicle with a tracking device
`might circle a homing beacon many times before finding it
`due to the crude distance and direction indications of the
`technology.
`Fortunately, a boon to precise location determining
`occurred when the United States saw fit to invest over $12
`Billion in creating a network of 24 satellites in low earth
`orbit, each broadcasting precise timing signals from two
`on-board atomic clocks. Using precise and well-developed
`triangulation and quadrangulation formulas, a receiver that
`picks up signals from several satellites simultaneously can
`determine its position in global coordinates, namely latitude
`and longitude.
`With this network orbiting overhead. a person anywhere
`on the earth has a 24 hour a day line-of-sight view to a
`
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`2
`sufficient number of satellites such that a person with a GPS
`receiver is able to determine their own longitude and latitude
`to within several meters, as well as their elevation. However.
`knowing your own position in longitude and latitude does
`not help others find you without extremely precise topo
`graphical or geophysical maps which also show longitude
`and latitude. Furthermore, the degree of precision in position
`determination is then only accurate to the resolution of the
`maps on hand. Nevertheless, the elements for a novel search
`and rescue system. as well as a general purpose locator, are
`made possible by the present invention utilizing GPS tech
`nology. Before the invention can be explained, however, a
`potential problem with GPS signals must first be explained.
`In navigation, a method of guiding ships commonly used
`is dead-reckoning, whereby the known velocity and direc
`tion of travel of a ship from a known position such as a port
`is used to calculate the present position. The drawback is that
`the further a ship moves away from the known position, the
`less accurate the dead-reckoning position becomes. Inclem
`ent weather can further erode the accuracy of a ship's
`navigation, and endanger lives and property when traveling
`in close proximity to land. However, using a GPS receiver
`and a very accurate map with a sufficient degree of
`resolution, the movements of even a large vessel can be
`guided with a satisfactory degree of precision. The problem
`with GPS signals, surprisingly, arises from the high degree
`of precision that the system is able to provide.
`It is the potential application of GPS technology to
`military uses which is responsible for the concern over GPS
`receiver accuracy. Specifically, precise positioning of targets
`can enable pinpoint accuracy in the delivery of highly
`destructive military payloads. Therefore, the possibility
`exists that our own satellite network could be used against
`the United States. For this reason, the GPS timing signals
`broadcast by the satellite network for commercial use are
`intentionally made less accurate than the encoded military
`signals. These timing and position errors are called Selective
`Availability (SA) and reduce the accuracy of civilian users
`to roughly 100 meters. While this inaccuracy is irrelevant on
`the high seas, coastal navigation or land-based applications
`such as search and rescue suffer, and potentially destroy the
`benefits of GPS technology.
`To overcome the intentional errors introduced in the GPS
`timing signals, a system known as differential GPS (DGPS)
`was developed to reestablish accuracy for civilian users in a
`small. localized area such as coastal navigation. The system
`requires that a permanent GPS receiving and broadcasting
`station be established, and that the precise position of the
`station be determined. Using the fact that the errors intro
`duced by a system of satellites will be the same errors
`transmitted to all receivers in a localized area, a mobile GPS
`receiver in range of the permanent station can determine its
`position and achieve the same degree of accuracy enjoyed
`by the military. This is accomplished by having the perma
`nent station calculate the error introduced by the GPS
`satellites by comparing the signal received with the actual
`known position. This error factor can be transmitted to and
`used by all mobile receivers within the vicinity of the
`permanent station to determine their position accurately to
`within several meters instead of 100 meters. Of course, the
`accuracy of this DGPS determined position decreases the
`further away that a GPS receiver is from the permanent GPS
`receiving and broadcasting station.
`Another form of differential GPS position determination
`has also substantially increased the usefulness of GPS
`receivers. As taught in Smith, U.S. Pat. No. 5.408.238, a
`comparison of absolute GPS determined locations can be
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`used to determine the relative position or location of the GPS
`devices relative to each other. This comparison eliminates
`the need for a permanent base station which transmits an
`error correction factor because the absolute position of the
`GPS receivers is relevant only so far in that they are
`compared to each other to provide a relative position dif
`ference.
`Returning now to our problem of locating a missing
`person, the exact longitude and latitude provided by DGPS
`is not often useful without very precise maps of sufficient
`resolution and of the area in question. Elevation may also
`play a very important factor if someone is lost in mountain
`ous terrain. Therefore, it would be an advance over the prior
`art if a graphical interface could be provided for a differen
`tial or relative position GPS position detection system which
`would intuitively provide searchers a distance measurement
`and direction. It would also be an advantage if the graphical
`interface provided position information accurate to several
`meters using only GPS signals and positions determined by
`the systems GPS receivers, regardless of whether a perma
`nent station is nearby providing GPSSA error compensation
`information. It would also be an advance over the prior art
`if the difference in elevation between the searchers and the
`lost person could be provided to that same degree of
`accuracy.
`OBJECTS AND SUMMARY OF THE
`INVENTION
`It is therefore an object of the present invention to provide
`a method and apparatus for locating the relative position of
`a first GPS receiver with respect to a second GPS receiver.
`It is another object to provide a method and apparatus for
`graphically representing the relative position above, such
`that the information is displayed in an intuitive manner.
`It is yet another object of the present invention to provide
`a method and apparatus for determining the difference in
`elevation between the GPS receivers.
`It is still another object to provide a method and apparatus
`for providing the precise distance, direction and elevation to
`a GPS receiver that broadcasts a predetermined signal by
`selectively tuning the apparatus to the signal.
`These and other objects not specifically recited are real
`ized in a system of GPS devices which receive civilian GPS
`signals and provide an intuitive graphical interface for
`45
`displaying the relative position of GPS devices in relation to
`each other, the relative position being accurate to several
`meters and defined as the distance to, direction of and height
`variance between GPS devices. A first GPS device with the
`person or object to be located transmits its GPS determined
`location to a second GPS device. This second GPS device
`includes a means for receiving the GPS determined position
`of the first GPS device, and also includes means for calcu
`lating the relative position of the first GPS device relative to
`the second GPS device based on a comparison of the
`55
`received telemetry of the first GPS device and its own GPS
`determined position. The relative position of the first device
`is then graphically displayed on an interface of the second
`GPS device in a manner which eliminates the need for a map
`in order to travel to the location of the first GPS device.
`While providing an interface which displays a relative
`position of the first GPS device, this information remains
`accurate no matter how the orientation of the second GPS
`device changes with respect to a compass.
`The system would further include the ability of the second
`65
`GPS device to tune to a signal broadcast by different GPS
`transceiver devices. By selectively tuning to the signal of a
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`4
`desired GPS device, a distance of, direction to and elevation
`variance of a plurality of different GPS devices is possible.
`Also disclosed is a method for determining the distance.
`direction and elevation to a GPS device, and includes the
`steps of (i) determining a location of a first GPS device
`including a Selective Availability (SA) induced longitude
`and latitude error, (ii) determining a location of a second
`GPS including the approximately same SA induced longi
`tude and latitude error, (iii) transmitting the location of the
`first GPS device to the second GPS device, (iv) enabling the
`second GPS device to receive the first GPS device's telem
`etry signal including the location of the first GPS device, (v)
`comparing the telemetry of the first GPS device to that of the
`second GPS device, and using the comparison of absolute
`longitudes and latitudes to determine a relative distance to,
`direction of and elevation variance between said GPS
`devices, and (vi) displaying the relative position of the first
`GPS device on an interface of the second GPS device in a
`graphical manner so as to intuitively provide the relative
`location of, the distance to and the elevation variance of the
`first GPS device relative to the second GPS device.
`DESCRIPTION OF THE DRAWINGS
`FIG. 1 is an illustration of the components in a UHF
`tracking device with the associated position tracking display
`of the prior art.
`FIG. 2A is a perspective view of the components of a
`Global Positioning System (GPS).
`FIG. 2B is an illustration of a GPS receiver and its
`associated display as found in the prior art.
`FIG. 3 is a perspective view of the components of a
`Differential GPS (DGPS) system which provides absolute
`longitude and latitude while eliminating the Selective Avail
`ability induced error.
`FIG. 4 is a perspective view of the components in a
`relative GPS system made in accordance with the principles
`of the present invention.
`FIG. 5A is the preferred embodiment of an interface
`providing a graphical display for the relative position deter
`mining GPS device system illustrated in FIG. 4.
`FIG. 5B is a variation of the preferred embodiment shown
`in FIG. 5A.
`FIG. 5C shows how the arrow of a graphical display
`remains stationary relative to a fixed reference point (a
`compass) when the GPS device is rotated relative to the
`compass.
`FIG.SD illustrates a modification to the preferred graphi
`cal display embodiment of FIG. SA.
`FIG. 6 is an alternative embodiment of an interface
`providing a graphical display for the system of GPS devices
`illustrated in FIG. 4.
`FIG. 7A is an alternative embodiment of an interface
`providing a graphical display of variance in elevation for the
`system of GPS devices illustrated in FIG. 4.
`FIG. 7B is a variation of the embodiment of FIG. 7A.
`FIG. 8 is a block diagram of the components of the
`relative GPS system used in FIG. 4.
`FIG. 9 is a perspective view of another embodiment of the
`present invention.
`FIG. 10 is a perspective view of another embodiment of
`the present invention.
`DETALED DESCRIPTION OF THE
`INVENTION
`FIG. 1 illustrates the components and a typical display of
`a UHF tracking system. As shown, a transmitter 10 is at
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`some unknown location some distance from the tracking
`device 20. The tracking device is typically mounted inside a
`vehicle, such as a police car. When the transmitter 10 is
`activated, the tracking device "homes in" on the transmitter.
`This is accomplished by a display 30 indicating whether the
`transmitter 10 is in front 40 or in back 50, to the left 60 or
`the right 70 of the tracking device 20. A distance indicator
`75 also shows a relative distance to the transmitter 10 by
`indicating the strength of the signal received.
`Such a system only provides vague references to the
`location of the transmitter 10 at best. For example, the
`direction of the transmitter 10 can only be known to within
`90 degrees. This is because the front\back and left\right
`indicators 40, 50, 60 and 70 only define four quadrants, 80.
`82, 84 and 86 in which the transmitter 10 can be found. In
`addition, because the distance indicator 75 relies only on a
`measure of the signal strength received. distortion or inter
`ference with the transmitted signal can give a false indica
`tion of actual distance to the transmitter 10. There is also no
`way to know whether there is interference until a UHF
`transmitter 10 is tracked down. Furthermore, the UHF signal
`20
`tracker 20 cannot indicate a height variance between the
`transmitter 10 and the tracking device 20. A tracker using a
`UHF signal tracker 20 mounted in a car might arrive at a
`mountain and still show substantial distance to the trans
`mitter 10, and yet the distance might be vertical and impass
`able. Forewarning of great altitude variations is helpful in
`planning the method and supplies required for tracking.
`FIG. 2A illustrates the original concept of the Global
`Positioning System (GPS). A GPS receiver 100 receives
`timing signals from at least three, and preferably four low
`earth orbiting satellites 110. 120. 130 and 140. The timing
`signals are provided by extremely accurate atomic clocks in
`the satellites, two redundant clocks aboard each satellite
`providing backup. Three satellites provide sufficient infor
`mation for a GPS receiver 100 to calculate a longitude and
`latitude using triangulation formulas well known to those
`skilled in the art. If a signal can be received from four
`satellites, the altitude of the GPS receiver 100 can also be
`determined using a modified formula.
`FIG. 2B illustrates a typical display of a GPS device 100
`as found in the prior art which provides location information
`to the user in longitude 142 and latitude 144 coordinates.
`This is because the GPS was originally intended for use as
`an absolute location determining device and had only an
`antenna 146 for receiving GPS signals. In this configuration,
`the only useful information the GPS device can provide is
`coordinates which can be used to find a location on a map.
`FIG. 3 illustrates the differential GPS (DGPS) concept
`that was made necessary by the military's introduction of an
`error into the GPS signals broadcast by the GPS satellites.
`For coastal navigation, a series of permanent GPS stations
`200 such as the one shown broadcast an error correction
`code which enables mobile GPS receivers 210 in the vicinity
`of the permanent GPS station 200 to determine their location
`to the same level of accuracy enjoyed by military systems.
`The Selective Availability (SA) error is corrected by using
`the previously determined accurate location of the perma
`nent station 200, receiving the GPS signals to calculate a
`location, determining the error between the broadcast posi
`tion and the known position, and then broadcasting the error
`correction factor to mobile GPS receivers. GPS receivers
`210 then correct their own GPS calculated position using the
`broadcast correction factor. The error correction factor is
`thus only accurate for GPS receivers near the permanent
`station.
`While the DGPS system does restore accuracy to the GPS
`location calculations, the system is only useful for search
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`and rescue or location determination if very detailed maps
`are available.
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`FIG. 4 illustrates the preferred embodiment of the present
`invention which overcomes the need for detailed maps when
`locating a GPS receiver made in accordance with the prin
`ciples of the present invention. The same number of satel
`lites are necessary as in the normal GPS position determin
`ing system of FIG. 1. Three satellites 300, 310 and 320
`provide sufficient information to determine a position. and a
`fourth satellite 330 can provide altitude information. What
`should also be explained before discussing the operation of
`the GPS devices of the present invention is that while the
`term “receiver" is accurate or GPS device of the prior art, the
`GPS devices of the present invention can be receivers or
`transceivers, depending upon the particular application of
`the present invention. Therefore, the specification will now
`refer to GPS devices which implies that they can be either
`receivers or transceivers. A last convention to note is that the
`“first GPS device” is always assumed to be the GPS device
`being tracked, and the "second GPS device" will always be
`assumed to be the GPS device which is receiving telemetry
`so as to track the first GPS device, unless otherwise noted.
`As stated previously, the differential or relative location
`determining method used in the present invention is different
`from that described in FIG. 3. This method eliminates the
`need for permanent GPS stations which provide error
`correction, because the location of the GPS device defined
`by the actual longitude and latitude is relevant only insofar
`as they are used to calculate the distance between a first or
`tracked GPS device and a second or tracking GPS device.
`The only limitation is that the induced SA error be nearly the
`same for both receivers to achieve a distance calculation
`accurate to less than 100 meters. This requirement is easily
`satisfied because the induced SA position error will be
`nearly the same for GPS devices within one hundred miles
`of each other and therefore substantially insignificant. In
`addition, as the GPS receivers get closer, the error becomes
`negligible. What should be obvious, therefore, is that dis
`tance is always accurate to at lease 100 meters.
`The first and second GPS devices are capable of deter
`mining their location in terms of longitude and latitude
`according to the methods well known to those skilled in the
`art through triangulation (location) and quadrangulation
`(location and elevation) formulas. The innovation of the
`present invention begins with the first GPS device 340 being
`modified to be a transceiver so as to transmit this location or
`location and elevation as telemetry data. Another point of
`novelty is that the second GPS device 350 is modified not
`only to receive GPS signals, but also to receive this telem
`etry data from the first GPS receiver.
`A further modification is that the second GPS device 350
`is advantageously and selectively tuneable to receive telem
`etry from a desired frequency. This enables the second GPS
`device 350 to be be able to track multiple GPS devices. It is
`also possible to provide a tuner such that a plurality of GPS
`devices can be simultaneously tracked and displayed on the
`second GPS device 350 interface. These features also imply
`that the first GPS device 340 can advantageously selectively
`transmit telemetry on a desired frequency.
`After receiving the telemetry transmission of the first GPS
`device 340, device 350 calculates a relative distance
`between the GPS receivers 340 and 350 by comparing
`absolute longitudes and latitudes. The interface of the sec
`ond GPS device 350 then graphically displays the position
`of the first GPS device 340 relative to the second GPS device
`350 in an intuitive manner which facilitates immediate travel
`to the first GPS device 340 without consulting a map.
`Specifically, the interface 352 of the second GPS receiver is
`shown in FIG. 5A and is comprised of an LCD screen 352.
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`such as the type used in portable notebook computers but
`Smaller. The interface 352 consists of an arrow 354, an end
`356 of the arrow 354 generally fixed on the display 352 and
`an opposite pointing end 358 of the arrow 354 which
`continuously points in the direction of the first GPS device
`340. This is accomplished by pivoting or rotating the arrow
`354 about the fixed end 356. The circle 360 defines the limit
`of travel of the arrow 354 on the interface 352 and does not
`need to be shown. However, if left on the display, the circle
`360 can be conveniently divided by tick marks 362, as
`shown in close-up view FIG. 5B. The tick marks 362
`represent the 360 degrees of a compass.
`Returing now to the system of GPS devices, the second
`GPS device 350 is constantly receiving updated telemetry
`data from the first GPS device 340 and from the GPS
`satellites 300, 310, 320, 330 overhead. This allows the
`second GPS device 350 to continuously update the direction
`in which the arrow 354 is pointing. This ability is crucial
`because the orientation of the second GPS device 350
`relative to a compass may be changing constantly.
`Therefore, the present invention envisions that a user will be
`able to hold the second GPS device 350 and turn in a circle,
`and the arrow 354 will always point toward the first GPS
`device 340. This implies that the circle 360, if shown, also
`remains fixed relative to the compass. This ability is a result
`of an internal compass of the second GPS device 350. The
`internal compass provides a fixed reference point relative to
`which the continuously displayed arrow 354 will use to
`always point toward the first GPS device 340.
`The feature described above is illustrated, for example, in
`FIG. 5C. For this drawing, the direction north of the fixed
`compass 368 is toward the top of the paper. The direction
`"north" might be true north or magnetic north. The two GPS
`devices illustrated are the same GPS device 366, but shown
`35
`in two different positions or orientations relative to the fixed
`compass 368. What remains constant (as long as the object
`being tracked does not move) is that the arrow 354 always
`points due east to some tracked GPS device whose telemetry
`data has been received by the pictured GPS device 366. Not
`shown because of the scale of the drawing is the fact that the
`arrow 354 also points to the same tick mark 362 at approxi
`mately 90 degrees, the circle 360 and tick marks 362 also
`remain fixed relative to the compass 368.
`With respect to the intuitive nature of this preferred
`embodiment shown in FIG.5A, it should be noted that while
`the direction to travel is displayed graphically on this
`particular display, distance is not. Distance, as well as other
`useful but presently nongraphically displayed information is
`displayed as text in an unused portion of the LCD screen
`352. This information includes but is not limited to the
`selected telemetry frequency or frequencies of remote first
`GPS devices 340. It is also possible to choose a units of
`distance for the displayed distance measurement shown as
`text so as to conform to user preferences for the U.S. or
`metric system.
`While the preferred embodiment has discussed a first GPS
`device 350 which does not receive but only transmits
`telemetry data, and a second GPS device 350 which does the
`reverse, it should be obvious that the second GPS device 350
`can be modified to transmit as well as to receive telemetry
`data, and that more than one of these modified second GPS
`350 type devices can be used. This enables the users of a
`system of two second GPS type devices 350 to simulta
`neously move toward each other as depicted in FIG. 10.
`A variation of the arrow 354 with an end 356 fixed at a
`center of a circle 360 representing the location of the second
`
`50
`
`55
`
`65
`
`8
`GPS device 350 is an arrow 370 as shown in figure SD.
`Instead of being anchored at an endpoint 356, this arrow 370
`rotates about a midpoint of the arrow 370. The advantage of
`this design is that it provides a larger arrow 370 within the
`relatively small LCD display screen 352 of the second GPS
`device 350.
`FIG. 6 illustrates an alternative embodiment of the graphi
`cal screen display of FIGS. 5A and SD. The displayed
`information can be modified to present different and advan
`tageously more useful and intuitive information to the user,
`at a cost to the user of more circuitry and sophistication of
`the GPS devices. More intuitively useful information is
`displayed on the interface 352 by replacing the direction
`arrows 354 or 370 with a grid. Centered on the location of
`the user or second GPS device 350, represented by some
`type of mark 372, are a plurality of increasingly larger
`concentric circles 374. The circles 374 are scaled so as to
`represent uniformly spaced distances. Finally, some type of
`mark 378 such as a small circle, square or other designation
`which is easily visible on the screen represents the first GPS
`device 340 which is being tracked.
`The significant advantage of this display is that not only
`does it show the direction to travel, but at a single glance
`gives the user some easily discernible and graphical repre
`sentation of the distance to the first GPS device 340. A scale
`also appears on the display so that the user is able to quickly
`calculate the distance based on the uniform distance between
`each concentric circle. This is done by counting the number
`of circles from the center 372 out to the relative position 378
`of the first GPS device 340,