[{9}
`United States Patent
`4,983,980
`Jan. 8, 1991
`Ando
`[45] Date of Patent:
`
`[11] Patent Number:
`
`Assistant Exam1'ner—Tod Swarm
`Attorney, Agent, or Firm—Sughrue, Mion, Zinn,
`Macpeak & Seas
`
`ABSTRACI‘
`[57]
`A vehicle-mounted GPS receiver for positioning and
`reckoning being capable of tracking satellite signals
`without serious interruption when passing through a
`dead zone such as a turmel by quick recapturing of
`satellite signals after the vehicle has cleared the dead
`zone, wherein the GPS receiver determines, by calcula-
`tion, one GPS satellite which is at the maximum eleva-
`tion angle among the GPS satellites at the very moment »
`if the satellite signals have interrupted simultaneously
`over a specified period of time, and starts to track the
`determined GPS satellite as soon as the vehicle clears
`the dead zone, whereas the GPS receiver calculates
`orbital positions of other GPS satellites required for
`positioning,
`two dimensional or three dimensional,
`based on the orbital position of the GPS satellite which
`is at the highest elevation angle, further, when the GPS
`receiver has a plurality of receiving channels, a plurality
`of GPS satellites are assigned, one by one, to the plural-
`ity of receiving channels from the highest elevation
`angle in due order for reducing the time in recapture.
`
`[54] SATELLITE RADIO SIGNAL TRACKING
`METHOD FOR GPS RECEIVERS
`
`[75]
`
`Inventor: Hitoshi Ando, Saitama, Japan
`
`[73] Assignee:
`
`Pioneer Electronic Corporation,
`Tokyo, Japan
`
`[21] Appl. No.: 549,514
`
`[22] Filed:
`
`Jul. 6, 1990
`
`Foreign Application Priority Data
`[30]
`Nov. 2, 1939 [JP]
`Japan .................................. 1.234320
`
`Int. Cl.5 .............................................. .. G0lS 1/00
`[51]
`[52] US. Cl. .................................... 342/357; 364/449;
`342/457; 455/12
`[58] Field of Search ............... 364/449, 453, 450, 451,
`364/456, 459; 340/995; 342/356, 357, 457, 359,
`352, 420, 422; 455/12, 13
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`....................... 342/357
`4,445,118 4/1984 Taylor et al.
`4,468,793
`8/1984 Johnson et al.
`..
`342/357 X
`4,731,613
`3/1988 Endo et a1.
`......
`364/450 X
`4,928,107
`5/1990 Kuroda et al. .......
`364/449 X
`4,949,268
`8/1990 Nishikawa et al.
`..
`342/457 X
`
`
`
`Primary Examt'ner—Thomas H. Tarcza
`
`3 Claims, 3 Drawing Sheets
`
`
`
`NE OR
`MORE SATELL-
`ITES ARE CAPTURED
` [1]
`7
`
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`A
`SATNOTITE
` [2]
`: DlCA§’H]:RED P0 LA-
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`£5
`
`
`
`
`
`CEASE TRACKING FOR ALL
`THE SATELLITES WHICH HAVE
`BEEN CAPTURED
`
`[31
`
`ASSIGN SATELLITE5 FROM
`THE HIGHEST ELEVATION
`ANGLE TO A PLURALITY OF
`RECEIVING CHANNELS IN
`'1' R
`
`[4-1'
`
`
`
`
`
`
`
`
`COIVMENCE RECAPTURING TH
`SATELLITE
`
`[5]-
`
`No
`
`
`OT A
`SATELLITE HA
`BEEN RECAPTURED 0'> [6]
`LAST 5 MINU-
`TES
`
`_
`
`Yes
`
`PETITIONERS 1012-0001
`
`

`

` U.S. Patent
`
`Jan. 8, 1991
`
`Sheet 1 of 3
`
`4,983,980
`
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`PETITIONERS 1012-0002
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`

`

`US. Patent
`
`Jan. 8, 1991
`
`sheetz of3
`
`4,983,980
`
`F I G . 2
`
`
`
`[3]
`
`
`
`
`
` DETERMINE A SATELLITE AT
`THE MAXIMUM ELEVATION
`
`ANGLE
`
`CEASE TRACKING FOR ALL
`THE SATELLITES WHICH HAVE
`BEEN CAPTURED
`
`
`
`COMMENCE RECAPTURING THE
`SATELLITE
`
`
`
`SATELLITE HA
`BEEN RECAPTURED FO
`LAST 5 MINU-
`
`
`
`PETITIONERS 1012-0003
`
`

`

`US. Patent
`
`Jan. 8, 1991
`
`Sheet 3 of3
`
`4,983,980
`
`F I G. 3
`
`START
`
`[1 I
`
`
`
`MORE SATELL-
`
`ITES ARE CAPTURED
`
`
`T SMINUTES
`
` SATELLITE A
`
`BEEN CAPTURED O
`
`
`I3]
`
`
`
`CEASE TRACKING FOR ALL
`THE SATELLITES WHICH HAVE
`BEEN CAPTURED
`
`
`
`
`
`
`
`ASSIGN SATELLITES FROM
`THE HIGHEST ELEVATION
`ANGLE TO A PLURALITY OF
`RECEIVING CHANNELS IN
`ORDER
`
`
`
`COMMENCE RECAPTURING THE
`SATELLITE
`
`[5]I
`
`
`
`
`SATELLITE HA
`
`BEEN RECAPTURED FO
`LAST 5 MINU-
`
`
`
`
`
`PETITIONERS 1012-0004
`
`

`

`The present invention relates to a method of tracking
`satellite radio signals for Global Positioning System
`(GPS) receivers.
`2. Description of the Prior Art
`Positioning systems currently in service utilizing arti-
`ficial satellites include a so-called Global Positioning
`System (GPS). This positioning system, as the name
`implies, will cover the entire surface of the globe by a
`total of 24 geodetic satellites when all of them are oper-
`ational on six orbits, about 20,200 km above the Earth,
`which will accommodate four geodetic satellites each.
`Each geodetic satellite transmits a GPS signal contain-
`ing navigation data for reckoning to the Earth using a
`spread-spectrum system. The reckoning is carried out
`on the ground, on the sea as well as in the air by receiv-
`ing GPS signals from a plurality of geodetic satellites,
`for example, by receiving GPS signals from three geo-
`detic satellites for two-dimensional positioning and
`those from four of them for three-dimensional position-
`ing. In this way, based on the navigation data contained
`in the GPS signal from each geodetic satellite, position
`information on the receiving point such as a latitude,
`longitude and altitude thereof can be reckoned in real-
`time.
`This GPS system was originally developed for U.S.
`military use, however a part of the GPS signal (C/A
`code) has been made available for civil applications.
`Therefore, it is possible to build navigation systems for
`automobiles, ships and aircraft by using the GPS signal.
`The above GPS system, however, has a drawback.
`Since the GPS satellites are not geostationary satellites,
`' the frequency of the GPS signal may be shifted due to
`the Doppler effect when received by a GPS receiver.
`Thus, the GPS receiver must start receiving of the
`spread-spectrum signal from the satellite by locking a
`phase-locked loop (PLL) circuit of the receiver to a
`frequency of the GPS signal which may be shifted by
`the Doppler effect. Upon locking of the PLL circuit,
`the spread-spectrum signal
`is despread and demodu-
`lated to receive the GPS signal. For this reason, it is
`theoretically impossible to commence the receiving of
`the satellite signals immediately even when the signals
`have already arrived at the GPS receiver.
`When the GPS system is employed for vehicle navi-
`gation systems, the GPS receiver cannot discriminate
`between a situation wherein the GPS signal is not re-
`ceived due to the PLL circuit remaining unlocked or
`the GPS signal failing to be despread under the pres-
`ence of the satellite signal and a situation wherein the
`GPS signal is not received because the signal itself is not
`reaching the receiver as when the vehicle is in a tunnel.
`Therefore no appropriate steps can be taken on the part
`of the GPS receiver when the vehicle is in a tunnel
`where no satellite signals are received. This means that
`after the vehicle has passed through the tunnel, it takes
`time for the GPS receiver to reestablish the contact .
`with the satellite, resulting in no reckoning being per-
`formed for certain period of time and thus causing a
`problem for the driver.
`In case of adopting the GPS system in a vehicle-
`mounted navigation equipment, in view of balancing
`the reckoning accuracy and the production cost, a GPS
`
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`4,983,980
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`1
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`SATELLITE RADIO SIGNAL TRACKING
`METHOD FOR GPS RECEIVERS
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`
`2
`receiver having a small channel capacity of covering
`only one or two channels is utilized and thereby a time-
`division multiple-access system is employed to allow
`the GPS receiver to receive signals from a plurality of
`GPS satellites in a sequential matmer with a single chan-
`nel. However, in such a system, it takes considerable
`time for the GPS receiver to reestablish the contact
`with the satellite.
`
`It is an object of the invention to eliminate the afore-
`said problems and to provide a satellite signal tracking
`method for GPS receivers using a time-division multi-
`ple-access system, which enables the GPS receiver to
`pick up satellite signals as soon as the signal reception is
`recovered to the normal condition after the reception of
`the satellite signals has been lost, for example, within a
`tunnel and the like.
`
`SUMMARY OF THE INVENTION
`
`A satellite. radio signal tracking method for GPS
`receivers, in which a receiving channel is multiplexed
`by employing a time-division multiple-access system to
`receive satellite signals sequentially from a plurality of
`GPS satellites, comprising the steps of: releasing the
`GPS receiver from time-division multiple-access recep-
`tion when satellite signals from all the satellites in recep-
`tion are lost simultaneously over a specified duration of
`time; and changing the receiving mode so as to chase
`only one GPS- satellite which is at the maximum esti-
`mated angle of elevation at that moment.
`In case the GPS receiver has multiple receiving chan-
`nels, these channels are assigned, on a one-to-one basis,
`with satellites beginning with the highest elevation
`angle in descending order.
`The higher the angle of elevation" of the satellite, the
`less likely the GPS receiver is to be obstructed by obsta-
`cles such as buildings and trees and the better the sight
`to the satellite from the ground. That is, as the angle of
`elevation increases, it becomes easier to chase the satel-
`lite.
`If the reception of satellite signals is interrupted for
`more than a certain period of time, for example, when
`the vehicle enters a tunnel, it is decided that the GPS
`receiver has entered into a dead zone for signal recep-
`tion,_ and the GPS receiver predicts a satellite which
`may have the maximum angle of elevation at the very
`moment and tries to chase only the predicted satellite.
`This allows the GPS receiver to quickly receive satel-
`lite signals as soon as the vehicle comes out of the tun-
`nel, reestablishing the contact with the satellite.
`‘
`When one satellite is recaptured in the way men-
`tioned above, it is possible to know the accurate posi-
`tions of all other satellites in their orbits by using the
`almanac information of the recaptured satellite. This in
`turn permits the GPS receiver to locate other satellites
`required for the reckoning again.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a timing diagram showing an embodiment
`of this invention and illustrating an operation of recap-
`turing a GPS satellite with the GPS receiver having a
`single receiving channel;
`FIG. 2 is a flowchart showing a sequence of opera-
`tional steps performed by the GPS receiver of the type
`shown in FIG. 1; and
`FIG. 3 is a flowchart showing a sequence of opera-
`tions as performed by a GPS receiver having a plurality
`of receiving channels.
`
`PETITIONERS 1012-0005
`
`

`

`3
`
`4,983,980
`
`PREFERRED EMBODIMENTS OF THE
`INVENTION
`‘
`
`Some preferred embodiments of the invention will be
`described by referring to the attached drawings.
`FIG. 1 shows one embodiment of this invention illus-
`trating a satellite signal recaptured by a GPS receiver
`having a single receiving channel. FIG. 2 is a flowchart
`showing the operational steps performed by the GPS
`receiver having a single receiving channel.
`Assuming that the vehicle is traveling in an open area,
`the GPS receiver receives satellite signals from four
`satellites S1 to S4, which are needed for three-dimen-
`sional positioning, by time-division multiplexing within
`a single receiving channel at a predetermined sampling
`interval of At in a sequential manner, as shown in FIG.
`IA (step [1] in FIG. 2).
`When the vehicle enters a tunnel and the GPS re-
`ceiver receives no signal from any of the satellites for
`more than a specified period of time, for example, more
`than five minutes (step [2]). it is decided that the vehicle
`has entered a dead zone like a tunnel for satellite signal
`reception, and the operation for tracking the four satel-
`lites S1 to S4 is stopped (step [3]) in the GPS receiver.
`At the same time, the GPS receiver calculates which
`satellite is expected to be at the highest angle of eleva-
`tion at the very moment (step [4]).
`In order to determine the satellite at the maximum
`elevation angle, there needs to be: (1) a clock having a
`time with no substantial deviation from the GPS time
`(the deviation should be within .10 minutes); (2) approxi-
`mate position information not much different from the
`current position (if the position is within Japan, for
`example, the position of Tokyo may be appropriate);
`and (3) almanac information which can be used for
`estimating current positions of the GPS satellites. As for
`the time in item (1), a clock unit installed in the GPS
`receiver or a built-in clock of the vehicle can be used.
`Regarding the position information of item (2), it is
`enough to store the latitude and longitude of Tokyo, for
`instance, in a ROM (read-only memory) installed in the
`GPS receiver. As to the almanac information for each
`satellite of item (3), the almanac information of each
`GPS satellite that was received before losing the signal
`reception can be used. By utilizing these information, it
`is possible to predict the positions of all satellites by
`making use of the known satellite orbit calculating algo-
`rithm, and thereby to determine the satellite which has
`the maximum elevation angle at the very moment.
`If it is assumed that the satellite having the maximum
`elevation angle is the satellite S3 (step [4]), the GPS
`receiver changes the receiving mode so as to track only
`the satellite S3 at the maximum elevation angle (step
`[5])-
`When the satellite S3 having the maximum elevation
`angle is recaptured (step [6]), the processing proceeds to
`step [1]
`to perform the ordinary sequential tracking
`operation, that is, choosing, as the tracking targets, the
`satellite S3 at the maximum elevation angle and three
`other satellites, which are determined based on the
`almanac information of the satellites S3 as being most
`suited for three-dimensional positioning at
`the very
`moment, and then starting to receive signals from these
`four satellites in accordance with an ordinary sequential
`time-division multiple-access receiving mode.
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`On the other hand, if the GPS receiver fails to catch
`the signal from the satellite S3 for more than a predeter-
`mined period of time, five minutes in this embodiment
`(step [6]), the program returns to step [3] to repeat the
`above-mentioned satellite tracking operational steps.
`In the embodiment described above, the GPS re-
`ceiver has only one receiving channel. For a GPS re-
`ceiver having a plurality of receiving channels, how-
`ever, the operational steps shown in the flowchart of
`FIG. 3 apply. In this case, the same number of satellites
`as that of the receiving channels are selected beginning
`with the highest elevation angle in the descending order
`and are assigned, on a one-to-one basis, to the plurality
`of receiving channels at step [4’]. At step [6’], if none of
`the satellites assigned to these receiving channels is
`captured, the program returns to step [3]. The opera-
`tions performed in other steps are identical with those
`of FIG. 2.
`The advantages of this invention may be summarized
`as follows. When the signals from all of the satellites
`being tracked are lost from reception simultaneously for
`a specified length of time, the GPS receiver is released
`from the time-division multiple-access reception and the
`receiving mode is changed to make it possible to chase
`only one GPS satellite which is calculated to have the
`maximum elevation angle at the very moment. This
`enables the receiver to detect the GPS signal from the
`satellite as soon as it arrives, making it easier to track the
`satellites again after the vehicle has passed a dead zone
`like a tunnel where no satellite radio signal reaches. The
`positioning calculation therefore can be restarted more
`quickly than by the conventional apparatus of the prior
`art.
`
`If the GPS receiver has a plurality of receiving chan-
`nels, these channels are assigned, on a one-to-one basis,
`with satellites beginning with the one having the high-
`est elevation angle in the descending order, thus result-
`ing in more faster retracking of the satellites.
`What is claimed is:
`'
`1. In a GPS (Global Positioning System) receiver in
`which GPS signals from a plurality of GPS satellites are
`received sequentially through a receiving channel ac-
`cording to a time-division multiple-access system, a
`method of tracking the satellite signals for the GPS
`receiver comprising the steps of:
`releasing the GPS receiver from time-division multi-
`ple-access reception when satellite signals from all
`the satellites in reception are lost simultaneously
`over a specified duration of time; and
`changing the receiving mode so as to track only one
`GPS satellite which is at the maximum estimated
`angle of elevation at that moment.
`2. A method of tracking the satellite signals for the
`GPS receiver as claimed in claim 1, wherein the maxi-
`mum elevation angle of the GPS is calculated based on
`a present time, a present position of a satellite signal
`receiving point and an almanac information of each
`GPS satellite which was in contact.
`3. A method of tracking the satellite signals for the
`GPS receiver as claimed in claim 1, wherein the GPS
`receiver has a plurality of receiving channels and GPS
`satellites are assigned, one by one, to the plurality of
`receiving channels from the highest elevation angle in
`order.
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`PETITIONERS 1012-0006
`
`