Abstract:
A method of recalibrating a Global Positioning System (GPS) receiver includes applying a first control signal to an oscillator for generation of an oscillator signal having an initial frequency and determining if an indicator of correct oscillator output frequency is present at the initial frequency. If an indicator of correct oscillator output frequency is not present, then iterative steps are followed until an indicator of correct oscillator output frequency is found. The iterative steps include: applying a new adjusted control signal to the oscillator for generation of a new frequency; determining whether an indicator of correct oscillator frequency is present; and if an indicator of correct oscillator output frequency is present with the oscillator having the new frequency, storing said adjusted control signal. If an indicator of correct oscillator frequency is present with the oscillator output signal having said initial frequency, then storing said initial control signal.

Description:
TECHNICAL FIELD 
     The present invention generally relates to a Global Positioning System (GPS) receiver and more particularly to recalibration of a GPS receiver. 
     BACKGROUND 
     GPS receivers have local oscillators or oscillators that are used in a heterodyne or superheterodyne configuration for acquiring GPS satellite signals. The center frequency of the oscillator is typically determined by a controller, which uses a control signal calibrated to produce an oscillator output signal with an appropriate frequency for acquisition of one or more GPS satellite signals. GPS receivers generally search a window (e.g., +/−80 Hertz (Hz)) or band of frequencies centered on the center frequency. 
     Typically, a GPS receiver recalibrates the relationship between the control signal and the oscillator output signal each time the receiver successfully acquires a three-dimensional (3-D) position fix. Acquiring a 3-D position fix generally includes acquiring GPS satellite signals from four (4) GPS satellites. Therefore, acquisition of a 3-D position fix is an acceptable indicator that the frequency of the oscillator output signal is correct and also that the control signal producing the frequency of the oscillator output signal is correct. 
     Some Global Positioning System (GPS) receivers are unused for substantial periods. For example, GPS receivers used in search and rescue operations, humanitarian missions, or stored in warehouses or on store shelves can go months or even years before use. During these idle periods, aging of the oscillator can cause the frequency of the oscillator to drift. When the oscillator drifts, the control signal that once produced the correct oscillator center frequency will produce a different oscillator center frequency and the window may no longer include the frequencies for acquiring GPS satellite signals. Consequently, the GPS receiver may be unable to acquire one or more of the GPS satellite signals. To correct this problem, GPS receivers must be taken out of service to undergo a calibration procedure at a depot or factory. Also, GPS receivers have a limited shelf-life because of oscillator drift. The limited shelf-life further increases costs, complicates logistics, and degrades reliability of GPS receivers. 
     Accordingly, it is desirable to provide a GPS receiver with recalibration that addresses one or more of the foregoing problems and other problems not expressly discussed in this background. In addition, it is desirable to provide methods of recalibrating a GPS receiver and methods for reconfiguring a GPS receiver for recalibration. Furthermore, it is desirable to provide a program product that can be used by a GPS receiver for recalibration or uploaded during the reconfiguring of a GPS receiver. Moreover, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     SUMMARY 
     A method for recalibrating a global positioning system (GPS) receiver is provided in accordance with an exemplary embodiment of the present invention. The method includes applying a first control signal to an oscillator for generation of an oscillator signal having an initial frequency and determining if an indicator of correct oscillator output frequency is present at the initial frequency. If an indicator of correct oscillator output frequency is not present, then iterative steps are followed until an indicator of correct oscillator output frequency is found. The iterative steps include: applying a new adjusted control signal to the oscillator for generation of a new frequency; determining whether an indicator of correct oscillator frequency is present; and if an indicator of correct oscillator output frequency is present with the oscillator having the new frequency, storing said adjusted control signal. If an indicator of correct oscillator frequency is present with the oscillator output signal having said initial frequency, then the initial control signal is stored. 
     A GPS receiver is also provided in accordance with the present invention. The GPS receiver includes an oscillator configured to generate an oscillator output signal, a mixer that is configured to receive an RF signal and convert that RF signal to at least one acquired GPS satellite signal using the oscillator output signal; and a controller coupled to said oscillator and to said mixer, said controller configured to implement a method including applying a first control signal to an oscillator for generation of an oscillator signal having an initial frequency and determining if an indicator of correct oscillator output frequency is present at the initial frequency. If an indicator of correct oscillator output frequency is not present, then iterative steps are followed until an indicator of correct oscillator output frequency is found. The iterative steps include: applying a new adjusted control signal to the oscillator for generation of a new frequency; determining whether an indicator of correct oscillator frequency is present; and if an indicator of correct oscillator output frequency is present with the oscillator having the new frequency, storing said adjusted control signal. If an indicator of correct oscillator frequency is present with the oscillator output signal having said initial frequency, then said initial control signal is stored. 
     In addition to the GPS receiver and the method for recalibrating a GPS receiver, a program product is provided in accordance with an exemplary embodiment of the present invention. The program product includes recalibration software executable in the processor of a GPS receiver for applying a first control signal to an oscillator for generation of an oscillator signal having an initial frequency and determining if an indicator of correct oscillator output frequency is present at the initial frequency. If an indicator of correct oscillator output frequency is not present, then iterative steps are followed until an indicator of correct oscillator output frequency is found. The iterative steps include: applying a new adjusted control signal to the oscillator for generation of a new frequency; determining whether an indicator of correct oscillator frequency is present; and if an indicator of correct oscillator output frequency is present with the oscillator having the new frequency, storing said adjusted control signal. If an indicator of correct oscillator frequency is present with the oscillator output signal having said initial frequency, then the initial control signal is stored. The program product also includes signal-bearing media bearing said recalibration software. 
     Furthermore, a method of reconfiguring a GPS receiver for recalibration is provided in accordance with an exemplary embodiment of the present invention. The method includes uploading a program into a memory of the GPS receiver that is executable by a processor of the GPS receiver. The program comprises recalibration software executable in the processor for applying a first control signal to an oscillator for generation of an oscillator signal having an initial frequency and determining if an indicator of correct oscillator output frequency is present at the initial frequency. If an indicator of correct oscillator output frequency is not present, then iterative steps are followed until an indicator of correct oscillator output frequency is found. The iterative steps include: applying a new adjusted control signal to the oscillator for generation of a new frequency; determining whether an indicator of correct oscillator frequency is present; and if an indicator of correct oscillator output frequency is present with the oscillator having the new frequency, storing said adjusted control signal. If an indicator of correct oscillator frequency is present with the oscillator output signal having said initial frequency, then the initial control signal is stored. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
     FIG. 1 is a flowchart of an exemplary method for recalibrating a GPS receiver in accordance with an exemplary embodiment of the present invention; 
     FIG. 2 is a block diagram of an exemplary OPS receiver in accordance with an exemplary embodiment of the present invention; 
     FIG. 3 is an indexed control signal table in accordance with an exemplary embodiment of the present invention; 
     FIG. 4 illustrates overlapping frequency windows covering a bandwidth of an oscillator in accordance with an exemplary embodiment of the present invention; and 
     FIG. 5 is a flowchart of an exemplary method for reconfiguring a GPS receiver for recalibration in accordance with an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     FIG. 2 shows a block diagram of an exemplary embodiment of a GPS receiver  206  configured for recalibration in accordance with an exemplary embodiment of the present invention. The GPS receiver  205  has an antenna  208  that is configured to receive GPS satellite output signals  204  from one or more GPS satellites  202 . The antenna  208  is coupled to an amplifier  210  for increasing received signal strength. The output of the amplifier  210  includes amplified GPS satellite output signals  204  and the amplified GPS satellite output signals  204  are provided to a mixer  212  for mixing with the output signal of a local oscillator or oscillator  214 , which produces one or more acquired GPS satellite signals  213 . The GPS receiver  206  further includes a controller  216  coupled to the local oscillator  214  and configured to a supply control signal along a signal path  221  to the oscillator  214  that dictates the output frequency of the oscillator  214 . The controller  216  is also coupled to the output of the mixer  212  for monitoring the presence or absence of acquired GPS signals  213 . 
     The controller  216  may include a processor  220  coupled to a memory  230  configured to store field calibration software  234 , a factory default control signal  232 , and the last calibrated control signal  233 . In one exemplary embodiment, the memory  230  may also be configured to store other data  237  that can be used to select control signals during recalibration, such as date of last calibration  237  and oscillator aging data  237 . Those of skill in the art will appreciate still other data  237  that may be useful in field calibration, which may also be stored in memory  230 . 
     In an exemplary embodiment, the memory  230  is configured to store an indexed table of control signals  235  including a minimum set of control signals  232 ,  304 ,  306 ,  308 , and  310  that span the capability of the oscillator as shown in FIG.  3 . The indexed table  235  preferably includes a factory default control signal  232  that produces an initial frequency, a second control signal  304  that originally produced a frequency equal to the initial frequency, a predetermined frequency increment, a third control signal  306  that originally produced a frequency equal to the initial frequency minus the frequency increment, a fourth control signal  308  that originally produced a frequency equal to the initial frequency plus twice the frequency increment, and a fifth control signal  30  that originally produced a frequency equal to the initial frequency minus twice the frequency increment. The frequency increment shifts the center frequency of the oscillator and the window centered upon the center frequency. 
     The windows  0 - 4  preferably overlap as shown in FIG. 4 as a compensation for uncertainty in the difference between originally produced frequency increments and currently produced frequency increments. FIG. 4 shows a bandwidth  402  of an oscillator  214  and windows  0 - 4  corresponding to the windows created by the control signals  232 ,  304 ,  306 ,  308 , or  310  having the same index number  301 , 303 ,  305 ,  307 , or  309 , respectively. A frequency increment of 128 Hz has been found useful in GPS receivers having +/−80 Hz windows. However, other values can be used in accordance with the present invention. The index values  301 ,  303 ,  305 ,  307 , and  309  allow the field calibration software  234  to step through each control signal  232 ,  304 ,  306 ,  308 , and  310  in sequence while searching for the correct control signal. Various search sequences, as understood by those of skill in the art, are contemplated within the invention. 
     In accordance with one exemplary embodiment of the present invention, the controller  216  is configured to perform the method of recalibration of a GPS receiver as shown in FIG.  1 . For example, the processor  220  of the controller  216  can be configured to access and execute the software  234  stored the memory  230  of the GPS receiver. Alternatively, any number of hardware, software or combination of hardware and software configurations of the controller  216  can be used to perform an exemplary method of recalibration of a GPS receiver such as that shown in FIG.  1 . 
     Referring to FIG. 1, the process  100  is initiated in step  102 . In an exemplary embodiment, process  100  may be initiated by an operator-supplied signal. For example, the operator may push a button or select an item from a displayed menu to initiate process  100  in step  102 . Alternatively, the process  100  may be initiated automatically (i.e., without human intervention) in response to a particular event. For example, failure to acquire GPS signals  213  or a background timer reaching a predetermined time indicating an oscillator age of concern may initiate process  100  in step  102 . 
     In step  104 , an index in the field calibration software  234  is initialized to the value of the first-index  301  in the indexed table of control signals  235 . For example, if the first table index  301  is zero, then the index in the field calibration software  234  is initialized to zero in step  104 . Step  106  halts the running of GPS core process software  236 , which inevitably is supplying an unsuccessful control signal  221  to the oscillator  214 . Halting the GPS core process software  236  and later restarting it in step  114  allows a change in the control signal. Step  108  imposes a delay after halting the GPS core process software  236  in step  104 . The delay provides deconstruction time for GPS core process software  236  objects. For example, a delay of about two (2) seconds may be imposed in step  108 . 
     Step  110  sets the control signal on signal path  221  to the indexed value  232 ,  304 ,  306 ,  308 , or  310 . The first control signal that is set in step  110  is the first control signal in a tabulated search pattern of control signals. In an alternate embodiment, one or more of the control signals  232 ,  304 ,  306 ,  308 , and  310 , may be calculated in real time. Step  112  sets a timer that may later allow a time-out exit from the search loop  116 - 122 - 116  for each searched control signal  232 ,  304 ,  306 ,  308 , and  310 . The timer should be set for a period that will provide a favorable opportunity to acquire four GPS satellite signals given the known coverage pattern of the GPS satellite constellation. For example, the period of the timer set in step  112  may be about nine (9) minutes. Step  114  restarts the GPS core process software  236  with the new control signal  232 ,  304 ,  306 ,  308 , or  310 , thereby enabling the GPS receiver  206  to search a frequency window  0 - 4  of FIG. 4 centered on the new oscillator frequency produced in response to the new control signal. Step  116  determines if a GPS 3-D position fix (i.e., acquisition of four (4) GPS satellite signals) has occurred. Step  116  uses the 3-D position fix as the indicator that the oscillator output frequency is correct. However, other indicators may be used in accordance with the present invention. For example, a 2-D position fix or a single GPS satellite signal acquisition may be used as the indicator of a correct oscillator output frequency. In some embodiments, signals from software-accessible self-test points in the mixer may be suitable as indicators that the oscillator has the correct frequency. In yet other embodiments, a frequency meter may be coupled to the oscillator and the output of the meter read and compared to a stored correct value. If step  116  determines that no position fix has been acquired, then step  122  determines whether the timer set in step  112  has expired. If not, step  116  again determines whether a position fix has been acquired. The loop  116 - 122 - 116  continues until the timer set in step  112  expires or a position fix is acquired. Loop  116 - 122 - 116  may have a delay or dwell step (not shown) such that step  116  executes periodically. For example, step  116  may execute every sixteen (16) seconds during a timer period of nine (9) minutes. 
     If step  122  determines that the timer set in step  112  has expired, step  124  increments the index and step  126  determines if the new index  304 ,  306 ,  308 , or  310  exceeds the bounds of the indexed table of control signals  235 . If step  126  determines that the newly incremented index is out of bounds, a calibration failure exists and a user is preferably notified in step  120  of such a failure. The user may be further prompted, in step  120 , to seek better sky access and retry the field calibration. If step  126  determines that the newly incremented index  304 ,  306 ,  308 , or  310  is not out of bounds, then the GPS core process software  236  is halted in step  106  and the control signal testing loop  106 - 108 - 110 - 112 - 114 - 116 - 122 - 124 - 126 - 106  is reiterated to test the next newly indexed control signal  306 ,  308 , or  310 . 
     If step  116  detects a 3-D position fix, step  118  saves the currently indexed control signal  232 ,  304 ,  306 ,  308 , or  310  in memory as the last calibration control signal  133 . Step  120  notifies the user that the field calibration completed successfully. It will be understood that after the 3-D position fix, the automatic calibration that typically accompanies a 3-D position fix may further refine the control signal and save the refined version as the last calibration control signal  233  in memory  230 . 
     Certain GPS receivers are reprogrammable and therefore upgradeable. Reprogrammable GPS receivers typically have one or more I/O ports useful for uploading software and data. FIG. 5 shows a flowchart of an exemplary method  500  of reconfiguring a reprogrammable GPS receiver for recalibration in accordance with an exemplary embodiment of the present invention. The method  500  begins with step  502 , which may include preparing the GPS receiver for reprogramming. For example, in a reprogrammable GPS receiver having a reprogramming mode, setting the mode to reprogramming mode would be included in step  502 . Other preparatory steps for each specific GPS receiver are also preferably included in step  502 . In step  504 , the field calibration software  234  is uploaded into memory  230 . The field calibration software  234  may contain a user interface portion for prompting the user to initiate calibration (step  102 ) and for informing the user of calibration progress (step  120 ). In step  506 , field calibration data, such as a table of indexed control signals  235 , a factory default control signal  232 , a last calibration control signal  233 , or other data supporting field calibration  237  is preferably uploaded into memory  230 . In step  508 , the reprogramming ends. Step  508  may include compiling the uploaded code and resetting the mode for normal operations. In some alternate embodiments, either of steps  504  and  506  may be omitted from process  500  and executed separately. 
     It should be understood that while the present invention is described here in the context of a fully functioning GPS receiver, those skilled in the art will recognize that the mechanisms of the present invention are capable of being distributed as a program product in a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media used to carry out the distribution. Examples of signal bearing media include: recordable media such as floppy disks, hard drives, memory cards, installed memory, and optical disks, and transmission media such as digital and analog communication links, including wireless communication links. Accordingly, field calibration software  234  may be made available for upload as a program product on any type of signal bearing media. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. For example, the method of the exemplary embodiment uses five discrete control signals to produce five discrete center frequencies in a preferred search sequence, but other search sequences, starting points, and different numbers of control signals are contemplated within the invention. For example, starting at the highest frequency of which the oscillator is capable and working a monotonic sequence of seven frequencies to the lowest frequency of which the oscillator is capable is included in the invention. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. For example, any method of determining that the control signal is producing the correct oscillator output frequency is contemplated within the present invention. The foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.