(12)
`
`United States Patent
`Gronemeyer
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,985,811 B2
`Jan. 10, 2006
`
`USOO698,5811B2
`
`(54) METHOD AND APPARATUS FOR REAL
`TIME CLOCK (RTC) BROWNOUT
`DETECTION
`
`(75) Inventor: Steven A. Gronemeyer, Cedar Rapids,
`IA (US)
`
`(73) Assignee: SIRF Technology, Inc., San Jose, CA
`(US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 10/600,174
`(22) Filed:
`Jun. 20, 2003
`
`(65)
`
`Prior Publication Data
`
`US 2004/0225439 A1 Nov. 11, 2004
`Related U.S. Application Data
`(63) Continuation-in-part of application No. 10/021,119, filed on
`Oct. 30, 2001, now Pat. No. 6,662,107.
`(51) Int. Cl.
`G0IS 5/02
`
`(2006.01)
`
`(52) U.S. Cl. ....................... 701/213; 340/988; 340/990;
`342/357.06; 342/357.15; 342/.457; 375/136
`(58) Field of Classification Search ................. 375/324,
`375/149, 152, 140, 150,343, 136, 1; 368/47;
`701/213, 214, 226, 215, 216; 342/357.06,
`342/357.15,357,12,357,358, 5. 340/990,
`See application file for compi2. 12.1
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`5,398,263 A * 3/1995 Vanderspool et al. ....... 375/376
`
`1/1997 Lau et al.
`5,592,173 A
`1/1997 Rodal et al.
`5,594,453 A
`5,629,708 A 5/1997 Rodal et al.
`5,654,718 A 8/1997 Beason et al.
`5,864,315 A 1/1999 Welles et al.
`5,893,044 A
`4/1999 King et al.
`5,940,027 A 8/1999 Forseth et al.
`5,982,322 A * 11/1999 Bickley et al. ........ 342/357.08
`6,225,945 B1
`5/2001 Loomis ................. 342/357. 12
`6,297,771 B1 10/2001 Gronemeyer
`6,373,429 B1 * 4/2002 Eschenbach ........... 342/357.03
`2001/001931.6 A1 * 9/2001 Hasegawa .............. 342/357. 12
`2002/0136094 A1
`9/2002 Kawai ......................... 368/47
`2003/OO1616.6 A1
`1/2003 Jandrell ................... 342/357.1
`
`FOREIGN PATENT DOCUMENTS
`
`JP
`JP
`JP
`
`O9005419 A * 1/1997
`O928.1209 A * 10/1997
`2000075070 A * 3/2000
`
`* cited by examiner
`
`Black
`intain the
`(57)
`ABSTRACT
`
`A method and apparatus for real time clock brownout
`detection. A low power real time clock (RTC) operates
`continuously to keep time in a global positioning System
`(GPS) receiver while some receiver components are pow
`ered down. In various embodiments, a brownout detector
`circuit detects a loss of RTC clock cycles. If a loss of RTC
`clock cycles exceeds a predetermined threshold Such that the
`RTC is not reliable for GPS navigation, an RTC status signal
`So indicates.
`
`28 Claims, 9 Drawing Sheets
`
`
`
`TEMPERATURE
`SENSOR
`206
`
`OWPOWER
`TNAEKEEPNG
`CIRCUIT
`200
`
`GPSSGMA
`
`PROCESSORS
`
`.
`
`NAWIGATION
`PROCESSOR
`
`5. 20
`
`MATCHED FELTER
`22
`
`AD
`CONVERTER
`24
`
`GPSCOCK8.
`PTK CIRCUIT
`EMPffREO
`ERROR
`TABLE
`224
`
`NEMORY
`WAKE-UP
`ALARM
`OGIC
`222
`
`GPS RECEIVER UNIT
`
`100
`
`EGEALIGNED
`R1 RAID COUNTER
`218
`
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`U.S. Patent
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`Jan. 10, 2006
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`Sheet 1 of 9
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`US 6,985,811 B2
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`F.G.
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`U.S. Patent
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`Jan. 10, 2006
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`Sheet 2 of 9
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`US 6,985,811 B2
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`oo.7
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`U.S. Patent
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`Jan. 10, 2006
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`Sheet 3 of 9
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`US 6,985,811 B2
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`
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`Z
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`º Bolyaeº
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`X0010 S?5) TWO
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`G?
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`
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`130 1/10MM088
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`U.S. Patent
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`Jan. 10, 2006
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`Sheet 4 of 9
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`US 6,985,811 B2
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`088
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`U.S. Patent
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`Jan. 10, 2006
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`Sheet 5 of 9
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`US 6,985,811 B2
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`
`
`FIG. 5
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`U.S. Patent
`
`Jan. 10, 2006
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`Sheet 6 of 9
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`US 6,985,811 B2
`
`READ RTC
`602
`
`TRANSFERRTC TIME
`TO EARC
`640
`
`TRANSFER RTCTIME
`TO GPS CLOCK
`USINGEARC
`606
`
`RTC GOOD
`608
`
`USE TRANSFERRED
`TIME FOR ACO
`60
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`
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`PERFORM COLD
`START
`612
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`PRODUCE TIME AND
`POSITION SOLUTION
`64
`
`SET RTC
`616
`
`VERIFY RTC CLOCK
`618
`
`SERTC STATUS
`GOOO
`620
`
`F.G. 6
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`U.S. Patent
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`Jan. 10, 2006
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`Sheet 7 of 9
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`US 6,985,811 B2
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`BEGIN WITH WAKE-UP COMMAND OR
`POSITION OUERY FROM USER
`402.
`
`
`
`FG. 7A
`400
`1-1
`
`GA)
`
`
`
`
`
`
`
`NAVIGATION
`WHAT WAS
`11E REASONRS rRN UPDATE
`POWER-UP
`404
`K32 CLOCKUPDATE
`POWER UP SELECTED COMPONENTS
`406
`
`MEASURE K32 OSCILLATOR TEMPERATURE
`408
`
`DETERMINEAVERAGE TEMPERATURE
`410
`
`ACCESS TIME FROM LOWPOWER (LP)
`(K32) TIME KEEPING CIRCUIT 412
`
`DETERMINE TIME (K32) ERROR FROM
`TEMP/FREQTABLE 414
`
`CORRECT TIME (K32) FOR LPTIME
`KEEPING CIRCUT 416
`
`UPDATE ALARM FOR NEXT WAKE-UP
`418
`
`POWER DOWN SEESTED COMPONENTS
`420
`
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`U.S. Patent
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`Jan. 10, 2006
`
`Sheet 8 of 9
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`US 6,985,811 B2
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`
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`POWER UP SELECTED AND
`ADDITIONAL COMPONENTS 422
`
`MEASURE K32OSCILLATOR
`EMPERATURE 7 CORRECT TIME
`FOR LPTIME KEEPNG CIRCUIT
`(SEE BLOCKS 408-416) 424
`TRANSFER LPTIME (K32) TO
`M11 TIME USINGEARC 426
`
`
`
`MEASURE GPS OSCILLATOR
`TEMPERATURE 428
`
`DETERMINE CURRENT GPS
`OSCILLATOR TEMPERATURE 430
`
`DETERMINE M11 FROERROR FROM
`TEMP/FREO TABLE 432
`
`USET2D TO ESTMATE POSITION OF
`VISIBLESATELLITES 436
`
`USEMF OR SP CHANNELS TO
`MEASURE MODULO 1 MSPN
`CODE PHASE 438.
`
`USE T20 GPS TIME TO
`CALCULATE EXPECTED CURRENT
`FULLPN CODE PHASE (TOW) 440
`
`
`
`
`
`
`
`CORRECTEST FULL CODE PHASE
`(TOW) TOMATCH WITH MEASURED
`MODULO 1 MSPN CODE PHASE 442
`
`COMPLETE NAVIGATION SOLUTION
`INSECONDS 444
`
`DETERMINE DIFFERENCE IN GPS
`ESTMATED TIME AND TIME FROM
`PREVIOUS GPS POSITION 446
`
`
`
`S
`DETERMINED
`CHANGE (0.5 MS2
`448
`
`
`
`COLLECT 6 SECOND SUBFRAME TO
`ESTABLISH GPS TIME 450
`
`UPDATE NAVIGATION SOLUTION
`USINGA CONVENTIONAL METHOD
`452
`
`
`
`FIG. 7B
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`U.S. Patent
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`Jan. 10, 2006
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`Sheet 9 of 9
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`US 6,985,811 B2
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`USE CORRECTED GPS TIME TO
`UPDATE LPTIME KEEPING
`CIRCUIT TIME (M11) USINGEARC
`454
`
`UPDATE M11 & K32 TEMP/FREQ ERROR
`TABLE A CURRENT TEMPERATURES
`456
`
`
`
`
`
`
`
`REASON
`TO STAY ON?
`458
`
`YES
`
`
`
`
`
`
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`PERFORM OTHER
`FUNCTION
`460
`
`F.G. 7C
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`US 6,985,811 B2
`
`1
`METHOD AND APPARATUS FOR REAL
`TIME CLOCK (RTC) BROWNOUT
`DETECTION
`
`2
`If Some information is known prior to acquisition, the
`time to acquire sufficient information from the GPS satellites
`for navigation can typically be reduced. For example, a
`“warn Start” process may be used if almanac data, approxi
`mate GPS time and approximate receiver position allow
`approximate Satellite locations and Doppler shifts to be
`calculated. A “hot start” process may be used if the
`ephemeris, approximate GPS time and approximate receiver
`position are known So that approximate Satellite locations
`and Doppler shifts can be calculated and the time to collect
`ephemeris data can be avoided. However, a complete Six
`Second Sub-frame of data from at least one Satellite is
`required in order to establish time with Sufficient accuracy to
`compute a navigation Solution.
`The GPS receiver unit determines its distance from each
`Satellite by determining the code phase of the transmission
`from each satellite. The code phase (CP) is the delay, in
`terms of chips or fractions of chips, that a Satellite trans
`mission experiences as it travels the approximately 11,000
`mile distance from the Satellite to the receiver. At each
`satellite, the time of transmission of each PN chip is con
`trolled down to a few nanoSeconds. Consequently, knowl
`edge of precise GPS time allows the GPS receiver unit to
`know exactly what chip of a Satellite's waveform is being
`transmitted at any given time. If the arrival of a given chip
`at a receiver is measured relative to a local timing epoch,
`Such as the T20 epoch, then the propagation time of that chip
`from the satellite to the GPS receiver unit can be measured
`as accurately as GPS time at that T20 epoch is known. If the
`propagation times from each of four Satellites are measured
`relative to the same T20 epoch, then the GPS receiver unit
`can Solve for the location of the receiver in three
`dimensional space, along with the error in the value of GPS
`time at the reference T20 epoch.
`The GPS receiver unit precisely determines the distance
`to the satellite by multiplying the time delay by the velocity
`of the transmission from the satellite. The GPS receiver unit
`also knows the precise orbits of each of the Satellites.
`Updates of the locations of the Satellites are transmitted to
`the receiver by each of the satellites. This is accomplished by
`modulating a low frequency (50 Hz) data signal onto the PN
`code transmission from the Satellite. The data Signal encodes
`the time-dependent positional information for the Satellite
`and the time errors in its on-board clock in the ephemeris
`data Subframes. Precise time of each Satellite's transmission
`is given in each Six-Second data Sub-frame relative to a
`reference chip at the Start of the next Sub-frame.
`Conceptually, the receiver uses the estimated range from
`a Satellite to define a sphere around the Satellite upon which
`the receiver must be located. The radius of the sphere is
`equal to the range to the Satellite the receiver has determined
`from the code phase. The receiver performs this proceSS for
`at least three Satellites. The receiver derives its precise
`location from the points of interSection between the at least
`three spheres it has defined. Measurements from three
`satellites are Sufficient if the receiver knows the altitude at its
`location. When the altitude is unknown, measurements from
`four Satellites are required So that altitude can also be Solved
`for, along with latitude, longitude and the error in the local
`clock measurement epoch (e.g., GPS time at the T20 epoch).
`The detection of the Signals from each Satellite can be
`accomplished in accordance with a GPS Signal detector that
`is disclosed in, for example, but not limited to, U.S. patent
`application Ser. No. 09/281,566, entitled “Signal Detector
`Employing Coherent Integration', filed on Mar. 30, 1999,
`which is incorporated herein by reference. A signal detector
`as disclosed therein may use a correlation mechanism, for
`
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`RELATED APPLICATIONS
`This application is a continuation-in-part of U.S. patent
`application Ser. No. 10/021,119, entitled “Calibrated Real
`Time Clock for Acquisition of GPS Signals. During Low
`Operation”, filed Oct. 30, 2001, now U.S. Pat. No. 6,662,
`107, which is hereby incorporated by reference.
`FIELD OF THE INVENTION
`This invention is generally related to Global Positioning
`System (GPS) receivers. More particularly, it is related to
`accurately detecting when a real time clock has become
`inaccurate due to power brownout.
`BACKGROUND
`The Global Positioning system (GPS) is a collection of
`twenty-four earth-orbiting satellites. Each of the GPS satel
`lites travels in a precise orbit about 11,000 miles above the
`earth's Surface. A GPS receiver locks onto at least three of
`the Satellites to determine its precise location. Each Satellite
`transmits a signal modulated with a unique pseudo-noise
`(PN) code. Each PN code is a sequence of 1023 chips that
`are repeated every ms consistent with a chip rate of 1.023
`megahertz (MHz). Each Satellite transmits at the same
`frequency. For civil applications, the frequency is known as
`L1 and is 1575.42 MHz. The GPS receiver receives a signal
`that is a mixture of the transmissions of the Satellites that are
`Visible to the receiver. The receiver detects the transmission
`of a particular Satellite by correlating the received signal
`with shifted versions of the PN code for the satellite. If the
`level of correlation is Sufficiently high So that there is a peak
`in the level of correlation achieved for a particular shift and
`PN code, the receiver detects the transmission of the satellite
`corresponding to the particular PN code. The receiver then
`uses the shifted PN code to achieve synchronization with
`Subsequent transmissions for the Satellite.
`GPS employs a unique time keeping system. GPS time is
`kept in terms of seconds and weeks since Jan. 6, 1980. There
`are 604.800 seconds per week. Therefore, GPS time is stated
`in terms of a time of week (TOW) and a week number. TOW
`ranges from 0 to 604800, corresponding to the number of
`45
`Seconds in a week. The week number Started with week Zero
`on Jan. 6, 1980 and is currently in excess of one thousand
`weeks. The TOW can have a fractional part, particularly
`when oscillators provide a resolution of 1/32,768" of a
`Second (an oscillation frequency of 32 kilohertz, or kHz), or
`when the GPS time is computed from range measurements
`relative to a specific clock epoch. GPS time can have
`accuracy on the order of a few tens of nanoseconds. GPS
`time is fundamental to the GPS system.
`During the initial determination of position of the GPS
`receiver unit, a “cold Start process is initiated. For a cold
`Start, the GPS receiver begins the acquisition process with
`out knowledge of GPS time, GPS position or ephemeris data
`for the GPS satellite orbits. Therefore, the GPS receiver unit
`Searches for all Satellites over a wide range of possible
`frequencies. In Some situations, almanac data is also
`unknown for the GPS satellites. Eventually, after many
`Seconds, at least four Satellite Signals are acquired. The
`satellites PN encoded signals identify each of the satellites
`and each Satellite transmits ephemeris data. Ephemeris data
`includes precise orbital information, for example orbital
`location as a function of GPS time, for that satellite.
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`example a matched filter, and a coherent integration Scheme
`to detect the appropriate Satellite signals.
`Once the Satellite signals are detected, the low frequency
`50 Hz, data that is modulated onto the PN code signal
`received from the satellite is decoded to determine the
`precise location of the GPS receiver unit. Conventional
`location determination processes require Several Seconds to
`complete. These conventional Schemes typically run
`continually, thus consuming valuable processor resources.
`This is especially disadvantageous in the case of GPS
`receiver unit with very limited power resources, Such as a
`portable GPS receiver unit. Portable GPS receiver units may
`be designed Such that Selected components may be shut off,
`or powered down, during periods when the user is not
`querying the GPS receiver unit for location information.
`When the user (or an automated process) queries the GPS
`receiver unit, the GPS receiver unit reactivates the powered
`down components and reacquires Satellite data to determine
`the current location. If the user's location has not changed
`Significantly, and/or if the shut down period has been Suf
`ficiently short, it may be possible to reacquire the previous
`Satellite Signals and achieve nearly immediate correlation of
`the code phase data (rather than the Several Seconds to
`minutes associated with the hot, warm or cold Start
`procedures). Nearly immediate correlation of the code phase
`data Saves Several Seconds, thereby Saving a Substantial
`amount of the limited power available in a portable GPS
`receiver unit.
`However, reacquisition of the Satellite Signals with nearly
`immediate correlation of the code phase data requires pre
`cise time keeping during the period the receiver is off. More
`particularly, the GPS oscillator and timing system must
`maintain accuracy of the various clocking Signals in the GPS
`receiver unit to within +0.5 ms to avoid losing track of which
`PN code period within the overall GPS signal structure the
`receiver expects to receive at reacquisition. This 0.5 mS
`criterion corresponds to one half of a 1 mS code period. In
`addition, movement of the GPS receiver unit introduces
`error that may be associated with timing of the PN code
`Signals. If the accuracy of the clocking Signals plus the error
`introduced by movement of the GPS receiver unit can be
`maintained to within +0.5 ms of the incoming PN code
`Signals, the time consuming and power consuming proceSS
`of determining location using the hot, warm or cold Start
`procedures may be avoided because the GPS receiver unit
`matching filters can immediately lock onto the four previ
`ously acquired satellite PN code signals and know which PN
`code period of the Signal Structure has been acquired.
`Otherwise, the hot, warm or cold Start procedures must be
`used, depending on the prior information (e.g., almanac,
`ephemeris, GPS time, and receiver position) that was pre
`Served while Selected receiver components, or the entire
`receiver, were powered down.
`Typically, a conventional real time clock (RTC) circuit
`may be used to maintain rough GPS time while the rest of
`the GPS circuitry is off. Typical RTC circuits may maintain
`accuracy of a few Seconds over extended periods. Such
`accuracy is adequate for hot and warm starts. However, the
`accuracy of a conventional real time clock degrades rapidly
`below the +/-0.5 ms level due to poor stability and tem
`perature characteristics of typical low cost, low power RTC
`circuits. Therefore, even after a very brief time, a hot start is
`required.
`Maintaining accuracy of the various clocking Signals in
`the GPS receiver unit to within +0.5 ms (one half of a 1 ms
`code period) is not possible with a conventional GPS
`oscillator and timing System if the oscillator is powered
`
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`4
`down between navigation updates. However, since the GPS
`oscillator and the associated timing System consume signifi
`cant power, powering down these components is very desir
`able in a portable GPS receiver unit to conserve power
`CSOUCCS.
`Under Some circumstances, the real time clock may stop
`altogether due to partial or total loSS of the local power
`Source. If the RTC is not operating at all, it is evident on
`Start-up that a cold Start procedure should be used to acquire
`Satellites. Under other circumstances, the RTC may seem
`normally operational on Start-up, but may be inaccurate
`because it has experienced partial power loSS, or a brownout
`condition that caused the RTC to miss cycles. For example,
`a battery used to power the RTC may provide inadequate
`power levels because it is near the end of its life or because
`it is Subjected to temperatures beyond its operating range.
`This is especially problematic when the time from the RTC
`is transferred to the GPS clocking Scheme to Support rapid
`acquisition. If incorrect RTC time is relied on, incorrect
`range measurements result. Using incorrect range measure
`ments in a navigation Solution results in an incorrect position
`calculation.
`One prior RTC failure detector includes a circuit correctly
`Sets a Status flip-flop when the RTC backup power is applied.
`Usually, this backup power is a Small battery. Hence, the
`circuit can accurately detect when the backup battery is
`replaced. This is a relatively useleSS feature. The user knows
`the battery is being replaced. A Set up routine may be
`invoked after battery replacement to Set the time.
`Prior art methods of RTC failure detection that essentially
`monitor Voltage levels are particularly inadequate when the
`battery is near its end of life or when the battery is Subjected
`to wide variations in temperature. For example, the GPS
`receiver may be placed in a car in a cold environment. The
`battery Voltage and current capability may decline in this
`condition so that the RTC oscillator stops. The user may then
`take the receiver, place it in an inside jacket pocket and take
`a hike. The receiver warms up enough that the battery
`recovers its capacity and the oscillator restarts. When the
`user attempts to use the receiver, the receiver makes the
`usual checks. The RTC appears to be running, because time
`is incrementing. The battery backed RAM (usually on the
`same battery as the RTC) has good checksums because the
`RAM retains its contents to much lower voltages than the
`RTC oscillator needs for operation. The RTC oscillator
`failure FF indicates good Status, because the Voltage did not
`fall below the reset threshold and because the logic may
`retain its valid State at lower Voltages than the oscillator
`requires for operation. Hence, the receiver tries to use the
`RTC value, assuming it is good, and produces an incorrect
`Solution because the time was in fact in error. The receiver
`takes longer to produce a Solution, or worse yet, continues
`to produce bad Solutions.
`In theory, if the Status flip-flop failure detection Voltage
`threshold could be set accurately, the failure would be
`detected. This is difficult for a number of reasons. One wants
`to set the threshold as low as possible so that the battery life
`is maximized. This means the threshold must be precise and
`that it must respond to different oscillator requirements for
`oscillation. These different conditions can be a function of
`the particular crystal, the temperature and circuit parameter
`variations over manufacturing proceSS Variations and So on.
`Hence, Some margin in the threshold has to be provided,
`Shortening the useful battery life. Even with a margin, Some
`failure events may occur on a Statistical basis.
`It is desirable to have a method and apparatus for GPS
`navigation can be operated to conserve power resources by
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`US 6,985,811 B2
`
`S
`powering down Selected components when they are not in
`use, yet can quickly acquire Satellites on Start-up using a real
`time clock that operates continuously. It is further desirable
`to have an RTC clock failure detection circuit that reliably
`detects oscillator failure without compromising battery life.
`
`SUMMARY OF THE DISCLOSURE
`A low power real time clock (RTC) is operated continu
`ously in a Global Positioning System (GPS) receiver unit.
`Power is conserved in the. GPS receiver unit by shutting
`down selected components during periods when the GPS
`receiver unit is not actively acquiring Satellite information
`used to calculate the location of the GPS receiver unit. A
`K32 (typically a nominal 32,768 Hz) oscillator residing in a
`low power time keeping circuit accurately preserves GPS
`time when the selected components are shut off. The K32
`oscillator generates the RTC or low power clock. The terms
`low power clock and RTC are used interchangeably herein.
`A method and apparatus for determining whether the RTC
`is accurate enough to be used on Start-up is disclosed. In one
`embodiment, actual loSS of RTC clock cycles, Such as during
`a brownout episode, is detected. In one embodiment, an
`output of an RTC clock oscillator is half-wave rectified and
`placed on the input to a resistor-capacitor (RC) circuit with
`a calculated RC time constant. The output of the RC circuit
`is placed on one input of a Voltage comparator. A reference
`Voltage is placed on the other input of the Voltage compara
`tor. If the RTC oscillator misses a predetermined number of
`cycles, the output Voltage of the RC circuit on the Voltage
`comparator decays and the comparator detects the loSS of
`clock cycles, which is reflected on the Voltage comparator
`output.
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`BRIEF DESCRIPTION OF THE DRAWINGS
`The components in the figures are not necessarily to Scale,
`emphasis instead being placed upon illustrating the prin
`ciples of the invention.
`FIG. 1 is a block diagram of an exemplary environment
`for operation of a global positioning System (GPS) receiver,
`FIG. 2 is a block diagram of an embodiment of a GPS
`receiver;
`FIG. 3 is another block diagram of the embodiment of the
`GPS receiver;
`FIG. 4 is another block diagram of the embodiment of the
`GPS receiver;
`FIG. 5 is a block diagram of an embodiment of a
`brownout detection circuit;
`FIG. 6 is a flow diagram illustrating a brownout detection
`process of one embodiment, and
`FIGS. 7A, 7B, and 7C are a flow chart illustrating one
`embodiment of a process that includes using an RTC clock
`Signal to update a GPS clock Signal, and determining
`whether or not an estimated GPS time is sufficiently accurate
`to acquire position of a GPS receiver.
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`DETAILED DESCRIPTION
`FIG. 1 illustrates an example environment for operation
`of a global positioning system (GPS) receiver. FIG. 1 shows
`a GPS receiver unit 100 and four GPS satellites 102, 104,
`106 and 108. Each satellite 102, 104, 106 and 108 is
`transmitting to the GPS receiver unit 100. Satellite 102 is
`moving towards the GPS receiver unit 100 along the line of
`sight (LOS) 110 at a velocity v"; satellite 104 is moving
`away from the GPS receiver unit 100 along the LOS 112 at
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`a velocity v, and satellite 106 is moving away from the
`GPS receiver 100 along the LOS 106 at a velocity v.
`Consequently, assuming a carrier wavelength of W, the
`transmission from Satellite 102 experiences a positive Dop
`pler shift of v"/w; the transmission from satellite 104
`experiences a negative Doppler shift of v/w; and the
`transmission from Satellite 106 experiences a negative Dop
`pler shift of v/.
`Satellite 108 is similarly moving away from the GPS
`receiver unit 100 along the LOS 116 at a velocity v.
`Information provided by the fourth satellite 116 may be used
`in Some applications to determine the error in the altitude
`value of the receiver if it is not known beforehand. The four
`Satellites must have adequate geometry in order to provide
`measurements capable of Solving for latitude, longitude,
`altitude and time error. Range measurements from more than
`the minimum quantity of four visible satellites may be
`required to Solve for the four unknown quantities when
`Satellite geometry is poor.
`FIG. 2 is a block diagram of a GPS receiver unit 100
`according to one embodiment. The GPS receiver 100
`includes radio frequency (RF) functionality shown here
`residing on an RF chip 103. The GPS receiver unit 100
`further includes baseband functionality shown here residing
`on baseband chip 105. Various components that perform
`various functions will be described in certain arrangements
`herein, but the invention as disclosed contemplates alterna
`tive arrangements. For example, the baseband chip 105 may
`include a navigation processor 210 and a memory device
`220, as shown. In other embodiments, the navigation pro
`ceSSor and the memory device may not reside on the
`baseband chip 220, but may communicate with the baseband
`chip 220 through, for example, a peripheral interface. In yet
`other embodiments, all of the components shown and func
`tionalities described reside on one chip.
`The RF chip 103 includes a GPS oscillator 204, which is
`a high accuracy oscillator used to keep GPS time. The
`following is an overview of general operation of the GPS
`receiver unit 100 according to one embodiment. Compo
`nents named in the following overview will be shown and
`described below. Power is conserved in GPS receiver unit
`100 by shutting down selected components, including the
`GPS oscillator 204, during periods when the GPS receiver
`unit is not actively acquiring Satellite information used to
`calculate the location of the GPS receiver unit. A K32
`(typically a nominal 32,768 Hz) oscillator residing in a low
`power time keeping circuit accurately preserves GPS time
`when the Selected components are shut off.
`The GPS oscillator 204 generates a clock signal, referred
`to as the M11 clock signal, that is used to accurately
`determine GPS time based upon signals detected from the
`plurality of Satellites. An edge aligned ratio counter con
`tinuously monitors the K32 and M11 clock signals with free
`running counters, and when an edge of the K32 clock signal
`aligns with an edge of the M11 clock Signal within a
`predetermined small tolerance, the K32 and M11 counter
`values are latched. Since the GPS timing generator that
`produces the T20 epochs is driven by the M11 clock, the free
`running M11 counter can also be latched at a T20 epoch to
`establish a relationship between that counter and the T20
`epoch. Thus, the GPS receiver unit 100 is able to correlate
`the timing and the rates of the K32 clock signal and the GPS
`M11 clock signal with the T20 timing epoch. The correlated
`timing and rates of the K32 clock signal, the GPS M11 clock
`Signal and the T20 epoch are provided to the navigation
`processor 210 so that a sufficiently accurate estimate of GPS
`time at a T20 epoch is calculated to allow determination of
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`7
`the PN code periods in the Signal Structures of acquired
`satellite PN code signals.
`During operation of the GPS receiver unit, frequencies of
`the local GPS oscillator and the K32 oscillator are detected
`at various operating temperatures Such that a temperature/
`frequency is defined for both oscillators. The data for both
`temperature/frequency tables are Stored in a memory.
`Selected components residing on the GPS receiver unit,
`including the GPS oscillator, are then shut down
`(deactivated) to conserve power. The low power time keep
`ing circuit remains on. Periodically, after a predetermined
`period of time, the System is powered up in response to a
`wake-up command generated by an alarm unit. The K32
`clock signal from the low power time keeping circuit is
`recalibrated based upon the actual operating temperature of
`the K32 oscillator and data from the K32 clock temperature/
`frequency table. Thus, the K32 clock rate is periodically
`updated to more accurately track GPS time.
`At a particular point in time, a navigation update is
`performed in accordance with the requirements of the par
`ticular System application. The periodically recalibrated K32
`clock Signal and data from the GPS clock temperature/
`frequency table are used to Set the M11 clock Signal rate and
`GPS time. Positions of the GPS satellites are then estimated
`such that the real GPS time can be quickly determined from
`the received satellite signals. Once the precise GPS time is
`determined from the detected satellite signals, the M11 and
`K32 Signals are latched together and correlated with the real
`GPS time at a T120 epoch, as described above, to further
`improve and update their temperature calibration tables. The
`Selected components are then shut off once again to conserve
`power.
`The process described above is repeated as necessary So
`that accurate GPS time is maintained by the low power time
`keeping circuit. When a user of the GPS receiver unit
`requests position information, the GPS receiver unit deter
`mines position from the GPS satellites more quickly,
`because the GPS Satellite positions and ranges arc estimated
`with a high degree of precision based on more accurate time
`keeping. That is, the power consuming and time consuming
`process of detecting Sub-frame data and determining Sub
`frame timing to Set the GPS time accurately enough to
`estimate the ranges to the GPS Satellites using conventional
`processes is avoided.
`Referring again to FIG. 2, the RF chip 103 and the
`baseband chip 105 communicate through a system interface
`109. In one embodiment, the system interface 109 is a serial
`peripheral (SPI) interface, but in other embodiments, the
`System interface could be any adequate messaging Scheme.
`The RF chip 103 receives signals from satellites in view via
`an antenna 107. The Satellite Signals are Sampled and Sent to
`the navigation processor as a Serial Stream on the SIGN/
`MAG line. The baseband chip 105 and its components
`operate with an ACQCLK Signal that is generated from a
`GPS OScillator crystal, and typically has a frequency that is
`a multiple of F. Various other Signals are exchanged via the
`system interface as show. A power up (PWRUP) signal is
`sent to the RF chip 103 to power up the powered down
`components of the RF chip 103 prior to acquisition and
`navigation. An SPI CLK signal is sent to the RF chip 103
`from the baseband chip 105 for synchronization. Data lines
`SPI DI and SPI DO carry data back and forth. A chip
`enable signal (RFRST) is sent to the RF chip 103 on the
`RFRST line and a reset signal (SRESET N) is sent to the
`baseband chip 105 on the RFRST line. In other
`embodiments, various different protocols are used to
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`exchange information between the RF chip 103 and the
`baseband chip 105.
`FIG. 3 is a block diagram illustrating Selected components
`of the GPS receiver unit 100, including a low power time
`keeping circuit 200. GPS receiver unit 100 includes at least
`a radio 202, the