I 1111111111111111 11111 111111111111111 111111111111111 1111111111111111111 IIII
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`5,781,150
`United States Patent c191
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`[111 Patent Number:
`
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`US00578 I I 50A
`
`
`Norris
`
`
`
`[451 Date of Patent: Jul. 14, 1998
`
`[54]GPS RELATIVE POSITION DETECTION
`SYSTEM
`
`X. A Marriage Made In Orbit: GPS and PCS by Francis
`
`
`
`
`Kane.
`
`[75]Inventor:
`Elwood G. Norris. Poway, Calif.
`
`
`
`A Sampling Of Global Positioning System Receivers by
`Don Herskovitz.
`Corporation.[73] Assignee: American Technology
`
`
`
`
`Poway. Calif.
`
`Gerald Houston.
`
`How Mobile Computers Can Help You Find Yourself by
`
`[21]Appl. No.: 542,799
`
`This 'Remote' Shows Its Users Exactly Where Here Is by
`
`
`
`Liz Mullen.
`
`(22] Filed: Oct. 13, 1995
`
`
`
`Related U .S. Application D ata
`
`[56]
`
`
`
`References Cited
`
`U.S. PATENf OOCUMENfS
`
`or Finn-Thorpe. North & Western. LLP
`
`CAR 54. Where Are You? By Michael Puttre.
`
`
`
`
`
`United States Securities And Exchange Commission Form
`
`10-K FRO Trimble Navigational Limited.
`[63]Continuation-in-part of Ser. No. 377,973, Jan. 25, I 995, Pat.
`
`
`No. 5,689,269.
`M. Blum
`Primary Examiner-Theodore
`[51]Int. Cl.6 ............
`
`
`................... H04B 7/185; GOlS 5/02
`
`
`Attome); Agent,
`
`[52]U.S. Cl .............................................. 342/357; 342/419
`
`ABSTRACT
`[57]
`
`
`
`[58]Field of S earch ..................................... 342/357. 419;
`455/12.1
`A system of GPS devices which receive civilian GPS signals
`
`
`
`
`
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`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
`
`11/1962 Lehan et al ..
`3,063,048
`
`
`
`
`between GPS devices. A first GPS device with the person or
`5/1977 Culpepper et al ..
`4,021,807
`
`
`
`to its GPS determined location object to be located transmits
`6/1986 Narcisse.
`4,593,273
`
`
`a second GPS device. This second GPS device includes a
`4,675,656
`
`6/1987 Narcisse .
`
`6/1991 Lawrence .
`5,021,794
`
`
`
`means for receiving the GPS determined position of the first
`5,119,504
`6/1992 Durboraw, III .
`
`
`
`GPS device. and also includes means for calculating the
`5,146,231
`
`9/1992 Ghaem et al ........................... 342/419
`
`
`
`relative position of the first GPS device relative to the
`5,172,110
`
`12/1992 Tiefengraber .
`
`
`second GPS device based on a comparison of the received
`5,245,314
`9/1993 Kah, Jr ..
`
`telemetry of the first GPS device and its own GPS deter­
`
`
`2/1994 Inoue .
`5,289,195
`
`
`mined position. The relative position of the first device is
`4/1994 Hirano.
`5,307,277
`
`
`
`
`then graphically displayed on an interface of the second GPS
`
`7 /1995 Fraker et al. ........................... 364/460
`5,434,789
`the need for a map in
`device in a manner which eliminates
`
`4/1996 Lans ........................................ 342/357
`5,506,587
`
`
`
`order to travel to the location of the first GPS device. While
`
`7 /1996 Hall et al. ............................... 340/907
`5,539,398
`
`
`
`
`pr�iding 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
`GPS Technology and Opportunities by Clyde Haris and Roy
`
`
`
`
`changes with respect to a compass.
`Sikorski.
`
`
`Utah Meeting Shows Amazing World of Navigation Satel­
`
`lites By Lee Siegel.
`
`OTHER PUBLICATIONS
`
`
`
`13 Claims, 8 Drawing S heets
`
`GPSCore
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`Transmits GPS Location
`
`Coordinates
`
`475,
`
`430
`
`LCD Dis.play
`
`48P
`
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`U.S. Patent
`Jul. 14, 1998 Sheet 1 of 8
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`5,781,150
`
`FRONT
`
`BACK
`
`Fig .. 1
`(PRIOR ART)
`
`100
`
`Fig. 2A
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`U.S. Patent Jul. 14, 1998 Sheet 2 of 8 5,781,150
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`Fig. 2B
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`U.S. Patent
`Jul. 14, 1998 Sheet 3 of 8
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`5,781,150
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`U.S. Patent Jul. 14, 1998
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`Sheet 4 of 8
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`5,781,150
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`5,781,150
`U.S. Patent Jul. 14, 1998 Sheet 5 of 8
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`U.S. Patent Jul. 14, 1998 Sheet 7 of 8 5,781,150
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`U.S. Patent Jul. 14, 1998 Sheet 8 of 8 5,781,150
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`1
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`5.781.150
`
`RELATED INVENTION
`
`BACKGROUND OF THE INVENTION
`
`2
`sufficient number of satellites such that a person with a GPS
`
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`GPS RELATIVE POSITION DETECTION
`
`
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`receiver is able to determine their own longitude and latitude
`SYSTEM
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`to within several meters. as well as their elevation. However.
`
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`
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`knowing your own position in longitude and latitude does
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`precise topo­extremely 5 not help others find you without
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`This patent application is a continuation-in-part of U.S.
`
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`graphical or geophysical maps which also show longitude
`
`
`
`
`patent application Ser. No. 08/377.973. filed Jan. 25. 1995.
`
`
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`and latitude. Furthermore. the degree of precision in position
`now U.S. Pat. No. 5,689.269.
`
`
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`determination is then only accurate to the resolution of the
`
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`maps on hand. Nevertheless. the elements for a novel search
`
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`are purpose locator. and rescue system. as well as a general
`1.Field of the Invention
`10
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`
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`made possible by the present invention utilizing GPS tech­
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`This invention pertains to position determining devices.
`
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`nology. Before the invention can be explained. however. a
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`and in particular to devices that enable the position of an
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`potential problem with GPS signals must first be explained.
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`object or person to be determined relative to another person
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`In navigation. a method of guiding ships commonly used
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`seeking said object. wherein a global positioning system
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`and direc­15 is dead-reckoning. whereby the known velocity
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`receiver is used to determine the distance. direction and
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`tion of travel of a ship from a known position such as a port
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`possible elevation difference between another global posi­
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`is used to calculate the present position. The drawback is that
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`tioning system receiver.
`
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`the further a ship moves away from the known position. the
`2.Prior Art
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`
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`less accurate the dead-reckoning position becomes. lnclem-
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`Being able to determine the precise whereabouts of some­
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`ent weather can further erode the accuracy of a ship's
`20
`
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`one or something on or above the surface of the earth has
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`navigation. and endanger lives and property when traveling
`
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`long held promise for many purposes. Missing person
`
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`in close proximity to land. However. using a GPS receiver
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`searches would be much simpler if people who were lost had
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`and a very accurate map with a sufficient degree of
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`a transmitting device with them which constantly broadcast
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`resolution. the movements of even a large vessel can be
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`their precise position. Such a transmitter would be better
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`The problemdegree of precision. 25 guided with a satisfactory
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`than just a voice transmitter because the age of the people or
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`with GPS signals. surprisingly. arises from the high degree
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`their medical condition might prevent people from
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`of precision that the system is able to provide.
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`responding. or from responding in a helpful manner.
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`It is the potential application of GPS technology to
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`However. numerous difficulties arise when actually search­
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`military uses which is responsible for the concern over GPS
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`ing for a transmitter which severely undermines the useful­
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`of targets 30 receiver accuracy. Specifically. precise positioning
`ness of such systems.
`
`
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`can enable pinpoint accuracy in the delivery of highly
`For example. U.S. Pat. No. 4.021.807 teaches how a
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`destructive military payloads. Therefore. the possibility
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`transmitter hidden among stolen money could be used to
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`exists that our own satellite network could be used against
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`locate those responsible for the theft and the money. A UHF
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`the United States. For this reason. the GPS timing signals
`homing device hidden among the money is capable of
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`use are network for commercial 35 broadcast by the satellite
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`transmitting a signal which can be tracked by UHF tracking
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`intentionally made less accurate than the encoded military
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`devices. Such a tracking device indicates whether the UHF
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`signals. These timing and position errors are called Selective
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`horning signal is being transmitted from the front or rear. and
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`Availability (SA) and reduce the accuracy of civilian users
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`from the left or right of a current position and orientation of
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`to roughly 100 meters. While this inaccuracy is irrelevant on
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`the tracking device. Signal strength can also be used to give
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`applications or land-based 40 the high seas. coastal navigation
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`a crude estimation of distance between the tracking and
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`such as search and rescue suffer. and potentially destroy the
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`horning devices if the signal is not too distorted by inter­
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`benefits of GPS technology.
`vening structures.
`To overcome the intentional errors introduced in the GPS
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`The UHF homing signal and tracking devices comprise
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`timing signals. a system known as differential GPS (DGPS)
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`the same principle taught in U.S. Pat. No. 5.021.794. This
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`civilian users in a 45 was developed to reestablish accuracy for
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`patent teaches how a miniaturized transceiver carried by a
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`small. localized area such as coastal navigation. The system
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`child can be remotely activated by a parent to enable the
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`requires that a permanent GPS receiving and broadcasting
`child to be located by police cars with UHF trackers.
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`station be established. and that the precise position of the
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`One of the drawbacks of such locator systems is that the
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`station be determined. Using the fact that the errors intro-
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`position of the person or object is never known with any
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`50 duced by a system of satellites will be the same errors
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`great degree of accuracy. A related issue is that the reliability
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`transmitted to all receivers in a localized area. a mobile GPS
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`of the signal received is also suspect. and can not be
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`receiver in range of the permanent station can determine its
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`confirmed. Furthermore. a vehicle with a tracking device
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`position and achieve the same degree of accuracy enjoyed
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`might circle a homing beacon many times before finding it
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`by the military. This is accomplished by having the perma­
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`due to the crude distance and direction indications of the 55
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`nent station calculate the error introduced by the GPS
`technology.
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`satellites by comparing the signal received with the actual
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`Fortunately. a boon to precise location
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`known position. determining This error factor can be transmitted to and
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`used by all mobile receivers within the vicinity of the
`occurred when the United States saw fit to invest over $12
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`permanent station to determine their position accurately to
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`Billion in creating a network of 24 satellites in low earth
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`within several meters instead of 100 meters. Of course. the
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`orbit. each broadcasting precise timing signals from two 60
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`accuracy of this DGPS determined position decreases the
`on-board atomic clocks. Using precise and well-developed
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`further away that a GPS receiver is from the permanent GPS
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`triangulation and quadrangulation formulas. a receiver that
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`receiving and broadcasting station.
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`picks up signals from several satellites simultaneously can
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`determine its position in global coordinates. namely latitude
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`Another form of differential GPS position determination
`and longitude.
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`65 has also substantially increased the usefulness of GPS
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`receivers. As taught in Smith. U.S. Pat. No. 5.408.238. a
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`With this network orbiting overhead. a person anywhere
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`comparison of absolute GPS determined locations can be
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`on the earth has a 24 hour a day line-of-sight view to a
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`5.781, 150
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`35
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`45
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`3
`4
`of the GPS n or location used to determine the relative positio
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`desired GPS device. a distance of, direction to and elevation
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`eliminates devices relative This comparison to each other.
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`variance of a plurality of different GPS devices is possible.
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`the need for a permanent base station which transmits an
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`Also disclosed is a method for determining the distance,
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`error correction factor because the absolute position of the
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`direction and elevation to a GPS device, and includes the
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`GPS receivers is relevant only so far in that they are
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`5 steps of (i) determining a location of a first GPS device
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`compared to each other to provide a relative position dif­
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`including a Selective Availability (SA) induced longitude
`ference.
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`a location of a second and latitude error. (ii) determining
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`GPS including the approximately same SA induced longi­
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`Returning now to our problem of locating a missing
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`the location of the tude and latitude error. (iii) transmitting
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`person, the exact longitude and latitude provided by DGPS
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`IO first GPS device to the second GPS device, (iv) enabling the
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`is not often useful without very precise maps of sufficient
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`second GPS device to receive the first GPS device's telem­
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`resolution and of the area in question. Elevation may also
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`etry signal including the location of the first GPS device, (v)
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`play a very important factor if someone is lost in mountain­
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`comparing the telemetry of the first GPS device to that of the
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`ous terrain. Therefore. it would be an advance over the prior
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`second GPS device. and using the comparison of absolute
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`art if a graphical interface could be provided for a differen­
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`15 longitudes and latitudes to determine a relative distance to,
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`tial or relative position GPS position detection system which
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`direction of and elevation variance between said GPS
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`would intuitively provide searchers a distance measurement
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`devices. and (vi) displaying the relative position of the first
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`and direction. It would also be an advantage if the graphical
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`interlace provided position information accurate to several
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`GPS device on an interface of the second GPS device in a
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`meters using only GPS signals and positions determined by graphical manner so as to intuitively provide the relative
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`20 location of. the distance to and the elevation variance of the
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`the systems GPS receivers. regardless of whether a perma­
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`first GPS device relative to the second GPS device.
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`nent station is nearby providing GPS SA error compensation
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`information. It would also be an advance over the prior art
`DESCRIPTION OF THE DRAWINGS
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`if the difference in elevation between the searchers and the
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`FIG. 1 is an illustration of the components in a UHF
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`lost person could be provided to that same degree of
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`position tracking display 25 tracking device with the associated
`accuracy.
`of the prior art.
`OBJECTS AND SUMMARY OF THE
`
`
`FIG. 2A is a perspective view of the components of a
`INVENTION
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`Global Positioning System (GPS).
`
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`FIG. 2B is an illustration of a GPS receiver and its
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`It is therefore an object of the present invention to provide
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`30 associated display as found in the prior
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`a method and apparatus for locating the relative position of
`art.
`FIG. 3 is a perspective view of the components of a
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`a first GPS receiver with respect to a second GPS receiver.
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`Differential GPS (DGPS) system which provides absolute
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`It is another object to provide a method and apparatus for
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`longitude and latitude while eliminating the Selective Avail-
`graphically representing the relative position above. such
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`ability induced error.
`that the information is displayed in an intuitive manner.
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`in a view of the components It is yet another object of the present invention to provide FIG. 4 is a perspective
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`relative GPS system made in accordance with the principles
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`a method and apparatus for determining the difference in
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`of the present invention.
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`elevation between the GPS receivers.
`of an interlace FIG. SA is the preferred embodiment
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`It is still another object to provide a method and apparatus
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`providing a graphical display for the relative position deter-
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`for providing the precise distance. direction and elevation to
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`40 mining GPS device system illustrated in FIG. 4.
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`a GPS receiver that broadcasts a predetermined signal by
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`FIG. SB is a variation of the preferred embodiment shown
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`
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`selectively tuning the apparatus to the signal.
`in FIG. SA.
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`
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`These and other objects not specifically recited are real­
`FIG. SC shows how the arrow of a graphical display
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`ized in a system of GPS devices which receive civilian GPS
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`remains stationary relative to a fixed reference point (a
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`signals and provide an intuitive graphical interlace for
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`compass) when the GPS device is rotated relative to the
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`displaying the relative position of GPS devices in relation to
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`compass.
`each other. the relative position being accurate to several
`
`
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`FIG. SD illustrates a modification to the preferred graphi­
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`meters and defined as the distance to. direction of and height
`
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`cal display embodiment of FIG. SA.
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`variance between GPS devices. A first GPS device with the
`FIG. 6 is an alternative embodiment of an interlace
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`so person or object to be located transmits its GPS determined
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`providing a graphical display for the system of GPS devices
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`location to a second GPS device. This second GPS device
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`illustrated in FIG. 4.
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`includes a means for receiving the GPS determined position
`FIG. 7A is an alternative embodiment of an interface
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`of the first GPS device. and also includes means for calcu­
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`providing a graphical display of variance in elevation for the
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`lating the relative position of the first GPS device relative to
`the second GPS device based on a comparison of the 55
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`system of GPS devices illustrated in FIG. 4.
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`received telemetry of the first GPS device and its own GPS
`FIG. 7B is a variation of the embodiment of FIG. 7 A.
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`determined position. The relative position of the first device
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`FIG. 8 is a block diagram of the components of the
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`is then graphically displayed on an interface of the second
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`relative GPS system used in FIG. 4.
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`GPS device in a manner which eliminates the need for a map
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`in order to travel to the location of the first GPS device. 60
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`present invention.
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`While providing an interface which displays a relative
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`position of the first GPS device. this information remains
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`the present invention.
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`accurate no matter how the orientation of the second GPS
`DETAILED DESCRIPTION OF THE
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`device changes with respect to a compass.
`INVENTION
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`The system would further include the ability of the second
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`GPS device to tune to a signal broadcast by different GPS
`FIG. 1 illustrates the components and a typical display of
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`transceiver devices. By selectively tuning to the signal of a
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`a UHF tracking system. As shown. a transmitter 10 is at
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`FIG. 10 is a perspective view of another embodiment of
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`FIG. 9 is a perspective view of another embodiment of the
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`65
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`5.781.150
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`6
`5
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`FIG. 4 illustrates the preferred embodiment of the present
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`some unknown location some distance from the tracking
`device
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`20. The tracking device is typically mounted inside a
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`invention which overcomes the need for detailed maps when
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`vehicle, such as a police car. When the transmitter 10 is
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`locating a GPS receiver made in accordance with the prin­
`activated, the tracking device "homes in" on the transmitter.
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`ciples of the present invention. The same number of satel-
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`This is accomplished by a display 30 indicating whether the
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`determin­as in the normal GPS position 5 lites are necessary
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`transmitter 10 is in front 40 or in back 50. to the left 60 or
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`ing system of FIG. 1. Three satellites 300. 310 and 320
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`the right 70 of the tracking device 20. A distance indicator
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`provide sufficient information to determine a position. and a
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`75 also shows a relative distance to the transmitter 10 by
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`fourth satellite 330 can provide altitude information. What
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`indicating the strength of the signal received.
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`should also be explained before discussing the operation of
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`invention is that while the
`10 the GPS devices of the present
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`Such a system only provides vague references to the
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`location of the transmitter 10 at best. For example. the
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`term "receiver" is accurate or GPS device of the prior art. the
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`direction of the transmitter 10 can only be known to within
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`GPS devices of the present invention can be receivers or
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`90 degrees. This is because the front\back and left\right
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`transceivers. depending upon the particular application of
`indicators 40. 50. 60 and 70 only define four quadrants, 80.
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`the present invention. Therefore. the specification will now
`82.84 and 86 in which the transmitter 10 can be found. In
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`refer to GPS devices which implies that they can be either
`15
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`addition. because the distance indicator 75 relies only on a
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`receivers or transceivers. A last convention to note is that the
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`measure of the signal strength received. distortion or inter­
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`"first GPS device" is always assumed to be the GPS device
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`ference with the transmitted signal can give a false indica­
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`being tracked. and the "second GPS device" will always be
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`tion of actual distance to the transmitter 10. There is also no
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`assumed to be the GPS device which is receiving telemetry
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`way to know whether there is interference until a UHF
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`noted.
`20 so as to track the first GPS device. unless otherwise
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`transmitter 10 is tracked down. Furthermore. the UHF signal
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`As stated previously. the differential or relative location
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`tracker 20 cannot indicate a height variance between the
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`determining method used in the present invention is different
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`transmitter 10 and the tracking device 20. A tracker using a
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`from that described in FIG. 3. This method eliminates the
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`UHF signal tracker 20 mounted in a car might arrive at a
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`need for permanent GPS stations which provide error
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`mountain and still show substantial distance to the trans­
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`correction. because the location of the GPS device defined
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`mitter 10. and yet the distance might be vertical and impass-25
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`by the actual longitude and latitude is relevant only insofar
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`able. Forewarning of great altitude variations is helpful in
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`as they are used to calculate the distance between a first or
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`planning the method and supplies required for tracking.
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`tracked GPS device and a second or tracking GPS device.
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`FIG. 2A illustrates the original concept of the Global
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`The only limitation is that the induced SA error be nearly the
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`Positioning System (GPS). A GPS receiver 100 receives
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`timing signals from at least three. and preferably four low
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`same for both receivers to achieve a distance calculation
`30
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`earth orbiting satellites 110. 120. 130 and 140. The timing
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`accurate to less than 100 meters. This requirement is easily
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`signals are provided by extremely accurate atomic clocks in
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`satisfied because the induced SA position error will be
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`the satellites. two redundant clocks aboard each satellite
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`nearly the same for GPS devices within one hundred miles
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`providing backup. Three satellites provide sufficient infor­
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`of each other and therefore substantially insignificant. In
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`mation for a GPS receiver 100 to calculate a longitude and
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`addition. as the GPS receivers get closer. the error becomes
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`latitude using triangulation formulas well known to those 35
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`negligible. What should be obvious. therefore. is that dis­
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`skilled in the art. If a signal can be received from four
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`tance is always accurate to at lease 100 meters.
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`satellites. the altitude of the GPS receiver 100 can also be
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`The first and second GPS devices are capable of deter­
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`determined using a modified formula.
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`mining their location in terms of longitude and latitude
`FIG. 2B illustrates a typical display of a GPS device 100
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`according to the methods well known to those skilled in the
`40
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`as found in the prior art which provides location information
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`art through triangulation (location) and quadrangulation
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`to the user in longitude 142 and latitude 144 coordinates.
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`(location and elevation) formulas. The innovation of the
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`This is because the GPS was originally intended for use as
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`present invention begins with the first GPS device 340 being
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`an absolute location determining device and had only an
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`modified to be a transceiver so as to transmit this location or
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`antenna 146 for receiving GPS signals. In this configuration.
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`location and elevation as telemetry data. Another point of
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`the only useful information the GPS device can provide is 45
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`novelty is that the second GPS device 350 is modified not
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`coordinates which can be used to find a location on a map.
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`only to receive GPS signals. but also to receive this telem­
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`FIG. 3 illustrates the differential GPS (DGPS) concept
`etry data from the first GPS receiver.
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`that was made necessary by the rnilitary's introduction of an
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`error into the GPS signals broadcast by the GPS satellites.
`A further modification is that the second GPS device 350
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`For coastal navigation. a series of permanent GPS stations
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`is advantageously and selectively tuneable to receive telem­
`50
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`200 such as the one shown broadcast an error correction
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`etry from a desired frequency. This enables the second GPS
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`code which enables mobile GPS receivers 210 in the vicinity
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`device 350 to be be able to track multiple GPS devices. It is
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`of the permanent GPS station 200 to determine their location
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`also possible to provide a tuner such that a plurality of GPS
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`to the same level of accuracy enjoyed by military systems.
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`devices can be simultaneously tracked and displayed on the
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`The Selective Availability (SA) error is corrected by using
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`second GPS device 350 interface. These features also imply
`55
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`the previously determined accurate location of the perma­
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`that the first GPS device 340 can advantageously selectively
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`nent station 200, receiving the GPS signals to calculate a
`transmit
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`telemetry on a desired freq uency.
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`location. determining the error between the broadcast posi­
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`After receiving the telemetry transmission of the first GPS
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`tion and the known position. and then broadcasting the error
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`device 340. device 350 calculates a relative distance
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`correction factor to mobile GPS receivers. GPS receivers
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`between the GPS receivers 340 and 350 by comparing
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`210 then correct their own GPS calculated position using the 60
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`absolute longitudes and latitudes. The interface of the sec­
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`broadcast correction factor. The error correction factor is
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`ond GPS device 350 then graphically displays the position
`thus only accurate for GPS receivers near the permanent
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`of the first GPS device 340 relative to the second GPS device
`station.
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`immediate travel 350 in an intuitive manner which facilitates
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`While the DGPS system does restore accuracy to the GPS
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`to the first GPS device 340 without consulting a map.
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`location calculations. the system is only useful for search
`65
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`Specifically, the interface 352 of the second GPS receiver is
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`and rescue or location determination if very detailed maps
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`shown in FIG. SA and is comprised of an LCD screen 352.
`are available.
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`5,781.150
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`7
`8
`GPS device 350 is an arrow 370 as shown in figure SD.
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`such as the type used in portable notebook computers but
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`smaller. The interface 352 consists of an arrow 354. an end
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`Instead of being anchored at an end point 356. this arrow 370
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`rotates about a midpoint of the arrow 370. The advantage of
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`352 and 356 of the arrow 354 generally fixed on the display
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`this design is that it provides a larger arrow 370 within the
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`an opposite pointing end 358 of the arrow 354 which
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`continuously points in the direction of the first GPS device
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`screen 352 of the second GPS 5 relatively small LCD display
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`device 350.
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`340. This is accomplished by pivoting or rotating the arrow
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`354 about the fixed end 356. The circle 360 defines the limit
`FlG. 6 illustrates an alternative embodiment of the graphi­
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`of travel of the arrow 354 on the interface 352 and does not
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`cal screen display of FlGS. SA and SD. The displayed
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`need to be shown. However. if left on the display. the circle
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`information can be modified to present different and advan­
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`360 can be conveniently divided by tick marks 362. as 10
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`tageously more useful and intuitive information to the user.
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`shown in close-up view FlG. SB. The tick marks 362
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`at a cost to the user of more circuitry and sophistication of
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`represent the 360 degrees of a compass.
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`the GPS devices. More intuitively useful information is
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`displayed on the interface 352 by replacing the direction
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`Returing now to the system of GPS devices, the second
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`GPS device 350 is constantly receiving updated telemetry
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`arrows 354 or 370 with a grid. Centered on the location of
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`the user or second GPS device 350. represented by some
`data from the first GPS device 340 and from the GPS 15
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`satellites 300. 310. 320. 330 overhead. This allows the
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`type of mark 372. are a plurality of increasingly larger
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`concentric circles 374. The circles 374 are scaled so as to
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`second GPS device 350 to continuously update the direction
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`represent uniformly spaced distances. Finally. some type of
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`This ability is crucil!l in which the arrow 354 is pointing.
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`because the orientation of the second GPS device 350
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`mark 378 such as a small circle. square or other designation
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`relative to a compass may be changing constantly. 20
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`which is easily visible on the screen represents the first GPS
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`Therefore, the present invention envisions that a user will be
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`device 340 which is being tracked.
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`able to hold the second GPS device 350 and turn in a circle.
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`The significant advantage of this display is that not only
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`and the arrow 354 will always point toward the first GPS
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`does it show the direction to travel. but at a single glance
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`device 340. This implies that the circle 360. if shown. also
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`gives the user some easily discernible and graphical repre­
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`remains fixed relative to the compass. This ability is a result 25
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`sentation of the distance to the first GPS device 340. A scale
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`of an internal compass of the second GPS device 350. The
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`also appears on the display so that the user is able to quickly
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`internal compass provides a fixed reference point relative to
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`on the uniform distance between
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`calculate the distance based
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`which the continuously displayed arrow 354 will use to
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`each concentric circle. This is done by counting the number
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`always point toward the first GPS device 340.
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`of circles from the center 372 out to the relative position 378
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`The feature described above is illustrated. for example. in 30
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`of the first GPS device 340. then multiplying this number by
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`FIG. SC. For this drawing. the direction north of the fixed
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`the scale of the