Abstract:
Implementation of an improved matched filter system for despreading a PN code from a spread spectrum signal utilizes a matched filter system that may be broadly conceptualized as a system that optimizes the number of multipliers and adders utilized by the system in despreading a PN code from a spread spectrum signal. This lowers the power consumption of the improved matched filter system and increases the speed at which the system despreads the PN code from the spread spectrum signal.

Description:
REFERENCE TO EARLIER-FILED APPLICATIONS 
   This application is a divisional of Non-Provisional patent application Ser. No. 09/604,595, filed Jun. 27, 2000, titled “IMPROVED SYSTEM AND METHOD FOR DESPREADING IN A SPREAD SPECTRUM MATCHED FILTER,” which is a continuation-in-part of Non-Provisional patent application Ser. No. 09/498,893, filed Feb. 7, 2000 now U.S. Pat. No. 7,545,854, and titled “IMPROVED DOPPLER CORRECTED SPREAD SPECTRUM MATCHED FILTER,” which is a continuation-in-part of Non-Provisional patent application Ser. No. 09/145,055, filed Sep. 1, 1998, and titled “DOPPLER CORRECTED SPREAD SPECTRUM MATCHED FILTER,” now U.S. Pat. No. 6,044,105, all of which are incorporated herein by this reference. 

   TECHNICAL FIELD 
   This invention relates to electronic communication systems. In particular, the invention relates to an improved system and method for despreading a PN code from a spread spectrum signal in a matched filter. 
   BACKGROUND OF THE INVENTION 
   Conventionally, data communication systems use narrow band modulation techniques, such as amplitude modulation “AM,” frequency modulation “FM,” frequency shift keying “FSK,” binary phase shift keying “BPSK,” quadrature phase shift keying “QPSK,” and quadrature amplitude modulation “QAM.” With such systems, demodulation at the receiver may be achieved with a relatively small amount of circuitry. However, these types of systems suffer from several problems including multipath fading and narrow band noise. 
   In contrast, in spread spectrum communication systems, a data spectrum is spread by a pseudo-noise “PN” code at a transmitter while the PN code and the data are synchronized at a receiver. The PN code is composed of a binary sequence that is often referred to as the “chip sequence.” The binary symbols in the chip sequence are referred to as “chips” and it is appreciated by those skilled in the art that the transmitter and intended receiver both have available the same chip sequence. This technique reduces the adverse effects of multipath fading and narrow band noise. The military has employed spread spectrum communication systems to combat the intentional jamming and detection of radio and satellite communication links. Accordingly, spread spectrum communication systems have attracted increased attention as a promising technique for radio frequency transmission of binary data in the non-military sector. 
   One of the two most common spread spectrum techniques, referred to as frequency hopping spread spectrum “FH-DSS,” employs the chip sequence to shift, over a wide bandwidth, the carrier frequency of a conventional narrow band transmitter signal. The other common technique, referred to as direct sequence spread spectrum “DS-DSS,” directly multiplies a conventional narrow band signal by the chip sequence. The chip rate is typically much higher than the data rate of the conventional narrow band signal. In both of these common spread spectrum techniques, a conventional narrow band signal is viewed as a carrier that is either frequency modulated or directly multiplied by the chip sequence. It is appreciated that other types of spread spectrum systems include combinations of both FH-DSS and DS-DSS in one system. 
   Spread spectrum signals allow more than one transmission signal in the same frequency and time interval when each signal uses a different chip sequence. This technique is known as code division multiple access “CDMA.” An example application of Direct Sequence CDMA “DS-CDMA” is the Global Positioning System “GPS.” The GPS system uses DS-CDMA to broadcast time and position data to receivers, which use such data to determine position and navigation information. 
   In a spread spectrum system, the conventional narrow band signal is spread by a PN code signal that has a wider bandwidth than the conventional narrow band signal. In order to correctly restore the conventional narrow band signal, the demodulation PN code generated at the receiving side is synchronized to the modulation PN code generated at the transmitting side. Proper phase synchronization is typically achieved when the received spread spectrum signal is accurately timed in both its spreading PN code pattern position and its rate of chip generation. The phase synchronization process is preferably accomplished in two stages: an initial synchronization process to find a synchronous phase, and a process to track the detected phase. Known techniques for initial synchronization depend upon both analog and digital sliding correlators, matched filters and other equivalent devices. 
   In a conventional matched filter spread spectrum receiver, the receiver includes a radio frequency “RF” section that receives the spread spectrum signal having a PN code modulated therein. The receiver converts the received spread spectrum signal into an intermediate-frequency “IF” signal. An in-phase converter and a quadrature-phase converter convert the IF signal into an in-phase “I-channel” spread signal and a quadrature-phase “Q-channel” spread signal. A PN code sync device de-spreads the received PN code modulated from the spread spectrum signal by synchronizing a reference PN code with the received PN code and maintaining the two codes in fine synchronism using, for example, a pair of correlators or a tracking loop based on a matched filter. A data demodulator demodulates the spread spectrum signal into the original baseband “narrow band” signal. Utilizing a matched filter has the advantage that the transmitted spread spectrum signal may be acquired relatively quickly even with relatively large initial errors between the locally generated PN code and the received PN code. 
   Unfortunately, a problem associated with matched filters is that matched filters include register and summing circuits to despread a PN code from a spread spectrum signal. Conventional register and summing circuits use a significant number of multipliers and adders to perform the calculations necessary to despread the PN code. This increases the amount of power used by the circuit and slows down the calculations. Accordingly, there is a need for a register and summing circuit that avoids the limitations of the prior art, has low power consumption, and is fast. 
   SUMMARY 
   A number of technical advances are achieved in the art, by implementation of an improved matched filter system for despreading a PN code from a spread spectrum signal. The improved matched filter system may be broadly conceptualized as a system that optimizes the number of multipliers and adders utilized by the system in despreading a PN code from a spread spectrum signal; thus lowering the power consumption of the improved matched filter system and increasing the speed at which the system despreads the PN code from the spread spectrum signal. 
   For example, an improved matched filter system that receives a spread spectrum signal may utilize a system architecture that recognizes that the same calculations are performed for the in-phase portion and the quadrature-phase portion of each signal sample of the spread spectrum signal, as well as for alternating pairs of signal samples. An implementation of the system architecture may include a switch and a multiplier in signal communication with the switch. The switch selects either the in-phase portion or the quadrature-phase portion of the spread spectrum signal, and the multiplier multiplies the selected portion of one of the signal sample with one of the PN code chips to obtain a product. The implementation may further include another multiplier coupled to the switch and an adder coupled to the first multiplier and the second multiplier. The second multiplier multiplies the selected portion of a second signal sample (which succeeds the first signal sample) with the PN code chip to obtain a second product. The adder then adds the first product with the second product to obtain a sum. 
   The improved matched filter system may also utilize a system architecture that recognizes that the same calculations are performed for pairs of signal samples. However, the spread spectrum signal comprises a plurality of signal sample pairs. Each pair of signal samples includes an even signal sample and an odd signal sample, and each signal sample has an in-phase portion and a quadrature-phase portion. An implementation of the system architecture may include a number of switches and multipliers. In its simplest form, the implementation may include two switches and a multiplier. The first switch is coupled to the second switch and the multiplier is coupled to the second switch. The first switch selects either the in-phase portion or the quadrature-phase portion of the spread spectrum signal. The second switch selects either the even sample or the odd sample. The multiplier multiplies the selected portion of the selected sample of one signal sample pair with one PN code chip to obtain a product. 
   The system may also include a second multiplier coupled to the second switch and an adder coupled to the first multiplier and the second multiplier. The second multiplier multiplies the selected portion of the selected sample of a second signal sample pair (which succeeds the first signal sample pair) with a second PN code chip (which succeeds the first PN code chip) to obtain a second product. The adder then adds the first product with the second product to obtain a sum. 
   Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention can be better understood with reference to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
       FIG. 1  is a block diagram of an example implementation of a system utilizing a Doppler corrected matched filter in accordance with the invention. 
       FIG. 2  is a block diagram of an another example of system utilizing a Doppler corrected matched filter. 
       FIG. 3  is a block diagram of an example implementation of a matched filter shown in  FIG. 1 and 2 . 
       FIG. 4  is a block diagram of an example implementation of a Doppler generator shown in  FIG. 3 . 
       FIG. 5  is a state machine diagram for the state machine shown in  FIG. 4 . 
       FIG. 6  is a phasor representation of the state machine diagram shown in  FIG. 5 . 
       FIG. 7  is a logic diagram of one embodiment of the register and summing circuitry of the matched filter shown in  FIG. 3 . 
       FIG. 8  is a logic diagram of a second embodiment of the register and summing circuitry of the matched filter shown in  FIG. 3 . 
       FIG. 9  is a logic diagram of a third embodiment of the register and summing circuitry of the matched filter shown in  FIG. 3 . 
       FIG. 10  is a table comparing the calculations performed by the register and summing circuitry of  FIG. 8  and the register and summing circuitry of  FIG. 9 . 
       FIG. 11  illustrates an implementation of the matched filter system in a network assisted environment. 
   

   Reference will now be made in detail to the description of the invention as illustrated in the drawings. While the invention will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed therein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention is described for use with a spread spectrum matched filter with Doppler correction suitable for use in a radio frequency receiver. The Doppler corrected matched filter may serve a number of applications including use in spread spectrum or Global Positioning System “GPS” radio frequency receivers. In a normal GPS navigation application, the Doppler corrected matched filter system may selectively output code phase information that allows a signal processing tracking loop to directly acquire prepositioning data for a particular GPS satellite. The Doppler corrected matched filter system is then utilized in support of reacquisition and anti-multipath operations. Additionally, in a single fix application, the Doppler corrected matched filter system may provide the only data required because the signal processing channels are not employed. 
   1. System Architecture 
   Referring now in detail to the drawings in which the reference numerals indicate like parts throughout several views,  FIG. 1  is a block diagram showing an example implementation of a Doppler corrected matched filter system  100  utilizing a matched filter  102  in accordance with the invention. The Doppler corrected matched filter system  100  includes three sets of samplers  104 ,  106  and  108  (i.e., the samplers may selectively be latches), respectively, a limiter  110 , an optional filter  112 , the matched filter  102 , and a mixer  114 . The Doppler corrected matched filter system  100  also includes clock multiplier  116 , frequency generator  118 , carrier numerically controlled oscillator “NCO” and generator  120 , a code NCO and generator  122 , a code generator  124 , and a T20 offset measurement circuit  126 . 
   The Doppler corrected matched filter system  100  may be utilized in various contexts, such as in communications or navigation applications. The Doppler corrected matched filter system  100  is in signal communication with a conventional spread spectrum radio frequency “RF” receiver front end  128  and a signal processor  130 . The Doppler corrected matched filter system  100  has an interface to an external bus  132  over which the Doppler corrected matched filter system  100  may receive commands from a controller  134  (for example, in a carrier command register set  136  and/or a code command register set  138 ) and provide output results to the controller  134 . The external bus  132  is in signal communication with a controller  134 . The Doppler corrected matched filter system  100  may be integrated into a receiver system, or may constitute a separate device. 
   The controller  134  may selectively be any general-purpose processor such as an Intel XXX86, Motorola 68XXX or PowerPC, or other equivalent processor capable of running software instructions (not shown) resident on the controller  134 . Alternatively, a GPS-specific circuit or oriented device may selectively also be utilized. It is appreciated that the controller  134  may also be integrated into a signal semiconductor chip such as an Application Specific Integrated Chip “ASIC” or Reduced Instruction Set Computer “RISC,” or may be implemented via a Digital Signal Processor (DSP) chip. Examples of GPS-oriented devices include the “Scorpio 11577-11” digital integrated circuit produced by Conexant Systems, Inc., “GSP1 SiRFstar I” and “GSP2e SiRFstart II” architectures available from SiRF Technology, Inc., “MGPSCS-A1” and “MMC2003” from Motorola, Inc., and the “SAA1575HL” GPS baseband processor from Philips, Inc, or the equivalent. 
   The RF receiver front end  128  converts a received spread spectrum signal into an intermediate-frequency “IF” signal and then converts the IF signal into an in-phase I signal and a quadrature-phase Q signal  140  which correspond to the real and imaginary parts of the received signal. For example purposes, the I and Q signals are each represented as 2-bit binary values in the range of −1, 0, and +1. It is appreciated that the −1 value is preferably represented as the 2&#39;s complement of the value 1. Thus, the 2-bit binary values would be 00, 01, and 11. The RF receiver front end  128  also provides a clock signal  142  to the Doppler corrected matched filter system  100  via the clock multiplier  116 . In a GPS DS-CDMA system, the fundamental frequency F 0  is 10.23 MHz. This fundamental frequency is converted by the RF front end  128  to the clock signal  142  having a frequency 
           f   =       F   0     ⁢     137   128             
(approximately 11 MHz). The frequency
 
           f   =       F   0     ⁢     137   128             
is then coupled to the clock multiplier  116 . The signal processor  130  also receives the I and Q signals  140  and clock signal  142  from the RF receiver front end  128 , and may have control signals in signal communication with the RF receiver front end  128 , in known fashion.
 
   Examples of the RF front end  128  may selectively be a “Gemini/Pisces Monopack R6732-13” integrated circuit available from Conexant Systems, Inc., “GRF1 SiRFstarI” and “GRF2i SiRFstarIIe” architectures available from SiRF Technology, Inc., “MGPSCS-A1” GPS Chipset, “PSRF111A” RF module, and/or “MRFIC1502” Integrated GPS down converter from Motorola, Inc., and/or the “UAA1570HL” GPS front-end receiver circuit from Philips, Inc, or the equivalent. The signal processor  130  may be, for example, the “Scorpio 11577-11” digital integrated circuit produced by Conexant Systems, Inc., “GSP1 SiRFstar I” and “GSP2e SiRFstart II” architectures available from SiRF Technology, Inc., “MGPSCS-A1” and “MMC2003” from Motorola, Inc., and the “SAA1575HL” GPS baseband processor from Philips, Inc, or the equivalent. 
   In an example implementation of the Doppler corrected matched filter system  100 , input processing receives an input sample stream of data, reduces the sampling rate of the data, and stores 20 milliseconds (ms) worth of the resulting data samples. In this example, the Doppler corrected matched filter system  100  reduces the data sampling rate to about 2.046 MHz and stores complex data samples, each represented by a three level I sample (i.e., I=−1+j0, 0+j0and 1+j0) and a three level Q sample (i.e., I=0−j, 0+j0 and 0+j). As a part of the sample rate reduction, the received samples go through a complex mixing process to remove the 
           f   =       F   0     ⁢     1   8             
(approximately 1.28 MHz), IF frequency, plus any desired offset frequency due to an estimated error in the frequency standard or to average Doppler shift. The 20 ms period during which the samples are taken is related to a T20 by means of an offset counter (not shown). It is appreciated that this relationship allows the correlation values output by the Doppler corrected matched filter system  100  to be related in time to the code phase settings of the various signal processing channels in a signal processor such as signal processor  130 . In particular, the Doppler corrected matched filter system  100  utilizes the stored samples multiple times to calculate the cross correlation values between the received samples and several combinations of PN codes and Doppler shifts. The results of these operations are then supplied to the controller  134  (which may be part of the signal processor  130 ) via the external bus  132 .
 
   More particularly, in the Doppler corrected matched filter system  100 , the clock multiplier  116  generates a 
           f   =       F   0     ⁢     137   32             
(approximately 44 MHz) clock signal  144  from the input 11 MHz clock signal  142 . The 44 MHz clock signal  144  is applied to the code NCO  122 . The code NCO  122  generates a code clock  146  to re-sample the I and Q signals  140  in sampler  104 . The code clock  146  sample frequency has a nominal rate of f=2F 0  (approximately 20.46 MHz) that is modified by commands from the controller  134  via code command register  138  to account for estimated values of average Doppler and IF error. The re-sampled data is then applied to the second sampler  106  via signal path  148 . The second sampler  106  is also clocked by code clock  146 .
 
   The 44 MHz clock signal  144  is also applied to the frequency generator  118  which generates output signals of frequencies of f=2F 0  (approximately 20.46 MHz)  150 ; 
           f   =       F   0     ⁢     1   8             
(approximately 1.28 MHz)  152 ; and clock  154  at
 
           f   =       F   0     ⁢     1   30             
(approximately 341 kHz). The clocks  152  and  154  preferably have selectable rates of
 
           f   =       F   0     ⁢     1   2             
(approximately 5.115 MHz) and
 
           f   =       F   0     ⁢     1   60             
(approximately 171 kHz). These selectable rates accommodate a wider range of NCO frequencies which address systems with possibly less accurate fundamental frequency sources.
 
   The signal processor  130  provides a T20 sync input  156  that may be synchronized to a data capture start signal  158  of the matched filter  102 . When the controller  134  commands the matched filter  102  to capture data samples, the matched filter  102  provides a data capture start signal  150  to the T20 offset measurement circuit  126 . The T20 offset measurement circuit  126  counts the 44 MHz clock cycles  144  until there is a transition in the T20 sync input  156 . After the data capture is complete, the controller  134  reads the offset counter value from the T20 offset measurement circuit  126  via the external bus  132 . The offset measurement allows the controller  134  to use the subsequent matched filter  102  code correlation outputs to initialize tracking loops in the signal processor  130 . 
   The received samples in the second sampler  106  are re-sampled at f=2F 0  (approximately 20.46 MHz), via clock signal  146 , from the code NCO and generator  122  in a manner similar to a normal DS-CDMA signal processing channel. The mixer  114  performs a complex multiplication of the output signal  160  of the carrier frequency NCO and generator  122  and the output signal  165  of the second sampler  106 . The carrier frequency NCO and generator  122  produces the output signal  160  at the IF frequency, 
           f   =       F   0     ⁢     1   8             
(approximately 1.28 MHz), plus an estimated frequency and average Doppler error. The mixer  114  preferably utilizes the a 7 level (i.e., −3 to 3 and −3j to 3j for I and Q) mixing scheme (similar to a conventional signal processing channel), resulting in an output range for each of the I and Q signals of −6 to +6 and −6j to +6j [i.e., (1+j)(3−3j)=6, (−1−j)(3−3j)=−6, (1+j)(3 +3j)=6j, (1−j)(3−3j)=−6j, etc.], represented in 8 bits (i.e., 4 bits real and 4 bits imaginary). The output signal  164  of the mixer  114  stage may then be applied to the optional anti-aliasing filter  112  to limit the bandwidth of the output signal  164  to no more than twice the sampling rate of the third sampler  108  downstream from the filter  112 . The filter  112  preferably provides a sliding average of 20 of the input samples to the filter  112 .
 
   The processed signal  166 , from the either the optional filter  112  or the mixer  114  (same as output  164  if from mixer  114 ), is then applied to the limiter  110  which limits the range of binary values to −1, 0, and +1, represented in 2 bits each for the I and Q signals, to reduce the amount of data stored for 20 ms of the signal samples. The limited, mixed signal  168  is then re-sampled through the third sampler  108  at a frequency, 
             f   =       F   0     ⁢     1   5         ,         
(approximately 2.046 MHz) with a signal  170  from the code frequency NCO and generator  122  to reduce the amount of data stored for the 20 ms of signal samples. The output  172  of the third sampler  108  is input into the matched filter  102 . A separate code generator  124  provides a desired PN code  174  “chip sequence” to the matched filter  102  (described in further detail below). It is appreciated that the provision of both a carrier NCO/mixer and a code NCO/sampler allows independent correction for a pure frequency shift in the former and of a Doppler shift to both carrier and code in the latter.
 
     FIG. 2  is a block diagram of another example implementation of a Doppler corrected matched filter system  100 . In this example, the incoming I and Q signals  140  (from the RF front end  128  in  FIG. 1 ) are re-sampled through the first sampler  104  at F 0  and then subjected to 5 level (i.e., −2 to 2 and −2j to 2j for I and Q) mixing by mixer  200 . Limiting is shown occurring after the second sampler  202  at limiter  204 . An alternative approach is shown in dotted outline. In this alternative approach, a filtering operation (through two summing registers  206  for the I channel and  208  for the Q channel) reduces the sample rate by a factor of ten, rather than re-sampling by means of the second sampler  202 . It is appreciated that the net effect of the filtering is to reduce the sample rate and correspondingly the size of a sample storage device, while avoiding the loss due to sample rate reduction without the appropriate bandwidth limiting beforehand. This implementation may be preferable when code phase measurements at the output of the matched filter  102  are to be interpolated for improved resolution. 
   2. Matched Filter Processing 
   Once the received samples are re-sampled and limited, they are stored in the matched filter  102  for repeated processing.  FIG. 3  is an example block diagram of the matched filter  102  in accordance with the invention. In this example implementation, the sample signals  172 ,  FIG. 1 , are converted by a serial to parallel converter  300 ,  FIG. 3 , and stored in a sample storage device  302 . The sample storage device  302  may selectively be a 16-bit wide random access memory “RAM” device. With 4-bit complex (i.e., 4-bit for the real I channel and 4-bit for the imaginary Q channel) input samples, the data may selectively be stored as 4 samples for each 16-bit word. 
   It is appreciated that the 20 ms of data samples are stored in the sample storage device  302 . However, the data is processed in 1 ms segments, corresponding to the period of a GPS PN code. Each one millisecond period will have 2046 4-bit complex (i.e., 1023 4-bit real I channel and 1023 4-bit imaginary Q channel) signal samples and  1023  PN code chips, or 2 received samples per PN code chip. The PN chips may selectively be loaded serially into a PN code register  304  via signal path  306  (that may selectively be  174  from  FIG. 1 ) and the PN code is shifted through the PN code register  304  for each code phase via shift path  308 . As each one millisecond segment from the sample storage device  302  is about to be processed, it is pre-multiplied by a Doppler shifting circuit (not shown) and then stored in a signal sample register  310 . The Doppler shifting circuit comprises a Doppler generator  312  and a complex mixer  314 . 
   The complex mixing operation is similar to the mixing shown in  FIG. 1 . In  FIG. 1 , the mixing shifts the code spectrum to near zero frequency offset. In  FIG. 3 , however, the mixing shifts the spectrum by a relatively small amount, roughly over a range of about 8000 Hz. It is appreciated that there are no more than about 8 cycles of complex local oscillator error in a one millisecond segment of 1023 samples. 
   It is appreciated that the value of 8000 Hz of fine Doppler correction is not a hard limit. The particular frequency range depends on the basic signal-to-noise ratio of the input samples and the margin associated with this value. The matched filter Doppler correction is selectively applied to the complex phase of the stored samples. The Doppler of the PN code on the envelope of this phase is not corrected in the matched filter because it is corrected in the processing that is done before the samples are stored by means of the re-sampling from 
           f   =       F   0     ⁢     137   32             
(approximately 44 MHz) down to f=2F 0  (approximately 20.46 MHz) that is controlled by the NCO frequency setting.
 
   In this example, it is appreciated that both the PN code loading into the PN code register  304  and the Doppler mixing in mixer  314  occur faster than “real time.” Preferably, a clock such as f=2F 0  would be used. Thus, once data samples are loaded in “real time,” subsequent processing may selectively be faster than “real time.” 
   When both the PN code and the Doppler-shifted signal samples are loaded into the PN code register  304  and signal sample register  310 , respectively, the correlation process begins. Each complex signal sample is multiplied by its corresponding PN code chip. There are two samples for each code chip, or a pair of half chip spaced correlations for every two samples. All 2046 multiplications may selectively be done at once, since the process is a sign inversion. The resulting products are then input to an adder tree for summation and output as one millisecond of complex coherent integrations. A product/adder circuit  316  performs these functions. 
   The output of the product/adder circuit  316  is applied to a conventional envelope generator  318 , which in the present example calculates the square root of the sum of squares (i.e., the envelope generator  318  may selectively be a square root circuit). The conventional envelope generator  318  determines the envelope of the one millisecond waveform. It is appreciated that this calculation removes any problems associated with signal inversions due to the 50 Hz data modulation on the GPS signal. 
   The I and Q outputs  320  are then further processed by non-coherently combining their envelope through a mixer  322  with any prior one millisecond correlations for this same PN code phase, previously stored in a storage device  324 . A complete cycle of 20 summations is stored in the storage device  324  for each 20 ms PN code cycle. The storage device  324  may selectively be a RAM or other equivalent memory element. 
   After each such pair of half chip spaced correlations, the PN code register  304  is cyclically shifted by 1 chip, via shift path  308 , and the process is repeated. After 1023 such shifts, all 2046 correlations are computed for the current one millisecond segment. At that point, the next  1  ms segment from the sample storage device  302  is Doppler multiplied and loaded into the signal sample register  310 . Thus, the process is repeated for each of the 20 one millisecond segments stored in the sample storage device  302 . After 20 ms have been processed, the output storage device  324  contains 2046 correlation values. This procedure may selectively be repeated for as many combinations of PN code and Doppler shift as desired. 
   The final correlation values in the output storage device  324  may be selectively available over the external bus  132  for further processing by other systems. The Doppler corrected matched filter system  100 ,  FIG. 1 , may then generate an interrupt to an external signal processor, which may selectively read out the results of the correlation process. As an example, the desired result may be the maximum value and its code phase (located in the storage device  324 ). However, in other examples, additional information may be required that is different for various applications. For example, samples adjacent to the maximum value may be utilized to provide information about the likelihood of the peak being a cross correlation signal. Smaller peaks at earlier phases may indicate a partially blocked line-of-sight path. The three or four adjacent peaks of a maximum peak may be employed to better define the optimum location of the true correlation peak. In an another example, the Doppler corrected matched filter system  100  may provide a specialized search function  326  that automatically provides the largest correlation value or values (i.e., 8 largest peaks) and the corresponding location (code phase) in the array of 2046 non-coherent sum values after all twenty segments have been processed. 
     FIG. 4  is a block diagram of an example implementation of a Doppler generator  312  shown in  FIG. 3 . The Doppler generator  312  includes a register  400  coupled to an adder  402 , an accumulator register  404 , and a state machine  406 .  FIG. 5  is a state machine diagram for the state machine  406  shown in  FIG. 4 . The output of the Doppler generator  312  is a multi-level (i.e., 3 levels) quantized complex phasor (i.e., cos θ+j sin θ where θ is the phase of the phasor). The phase is quantized to 45 degrees.  FIG. 6  is a phasor representation of the state machine diagram shown in  FIG. 5 . 
   In operation, a desired Doppler value is written over the bus  408 ,  FIG. 4  (which may be in signal communication with complex mixer  314 ), into the register  400 . The magnitude is added to the accumulator  404  at the same rate that memory elements such as words containing four complex samples are read from the sample storage device  302 ,  FIG. 3 . Whenever the accumulator  404  has an overflow condition, the state machine  406  is advanced or retarded one state, depending on the value of the sign bit. The overflow condition represents 45 degrees of accumulated Doppler. In an another example implementation, the Doppler generator  312  may be a lookup table (i.e., in RAM or read only memory “ROM”) with stored pre-computed Doppler shift correction values. 
   3. Register and Summing Circuitry of the Matched Filter 
     FIG. 7  illustrates a logic diagram for a register and summing circuitry  700  in a matched filter. The register and summing circuitry  700  is used to despread a PN code from a spread spectrum signal in a matched filter. In this embodiment, a basic cell  702  is defined as including: two pairs  704 ,  706  of I and Q latches for storing input I and Q data samples each represented as two or three bit quantities that may have the values −1, 0 and +1 (or −2, −1, 0, 1, or 2 for three bits); corresponding one bit multipliers  708  for applying the PN chips as one bit quantities that have the values −1 and +1, hence, the multipliers  708  have the effect of changing the sign bit of the I and Q data values; a corresponding 1 bit shift register PN chip stage  710 ; and an odd-even switch  712  between cells  702  that allows each PN chip stage  710  to be applied to successive pairs of I and Q latches without shifting the PN code each time, thus saving power (i.e., this feature saves 1023 shift register stages and the power associated with making extra shifts). 
   The outputs of the multipliers  708  are coupled to an adder tree  714 , which hierarchically adds all of the 2046 stages until final I and Q sums are output. The example adder tree  714  shown has 14 bit adders as the final stage. The width of the adders may selectively be reduced after a few stages. The register and summing circuitry is implemented in relatively few gates because the data widths involved in the basic cell  702  are small. 
     FIG. 8  illustrates a second embodiment for a register and summing circuitry  800  in a matched filter. A basic cell  802  in this embodiment includes: four pairs  804 ,  806 ,  808 ,  810  of I and Q latches for storing input I and Q data samples; corresponding I-Q switches  812  between the I and Q latches  804 ,  806 ,  808  and  810 ; one bit multipliers  814  for applying the PN chips; a corresponding 1 bit shift register PN chip stage  816 ; and two odd-even switches  818  that allow each PN chip stage  816  to be applied to successive pairs of I and Q latches without shifting the PN code. The register and summing circuitry  800  of  FIG. 8  recognizes that the same calculations are performed for both the I and Q samples. Accordingly,  FIG. 8  includes I-Q switches  812  to reduce the number of multipliers  814  to half the number of multipliers  708  shown in  FIG. 7 . 
     FIG. 9  illustrates a logic diagram for a third embodiment of the register and summing circuitry  900  of the matched filter  102  shown in  FIG. 3 . The basic cell  902  includes: two pairs  904 ,  906  of I and Q latches for storing input I and Q data samples; corresponding I-Q switches  908  between the I and Q latches  904 ,  906 ; an odd-even switch  910  between the I-Q switches  908 ; a multiplier  912 ; and a I bit shift register PN chip stage  914 . The outputs of the multipliers  435  are coupled to an adder tree  916 . 
   As shown in  FIG. 9 , the in-phase/quadrature-phase portion of the spread spectrum signal and the odd/even signal sample are selected prior to reaching the multiplier  912 . In the circuitry  800  of  FIG. 8 , the odd/even signal sample is selected after the in-phase/quadrature-phase portion is multiplied by the PN code chip. Accordingly, the circuitry  900  of  FIG. 9  uses half the number of multipliers as the circuitry  800  of  FIG. 8 . 
     FIG. 10  illustrates a table comparing the output A n    1000  of the basic cell of  FIG. 9  with the output A n    1002  of the basic cell of  FIG. 8 . “C”  1004  represents the PN code chip currently being processed in the basic cell. There are two types of switches in each of the basic cells. In the table, an “O” indicates that the odd-even switch is in the odd state, an “E” indicates that the odd-even switch is in the even state, an “I” indicates that the I-Q switch is in the in-phase mode and a “Q” indicates that the I-Q switch is in the quadrature-phase mode. Thus, “EQ” in column  1006  identifies switches set to process “even” and “Q samples.” Column  1008  identifies the combination of the outputs  1000  of the cells of  FIG. 9  by row numbers  1010  which is equivalent to the output  1002  of the cells of  FIG. 8 . Accordingly, the adder tree  916  is designed to equate the register and summing circuitry  900  of  FIG. 9  to the register and summing circuitry  800  of  FIG. 8 . Because the circuitry  900  in  FIG. 9  uses half the number of multipliers as the circuitry  800  in  FIG. 8 , circuitry  900  in  FIG. 9  requires less power than the circuitry  800  of  FIG. 8 . 
   4. Programming and Control 
   The Doppler corrected matched filter system  100  is envisioned as an independent subsystem that is controlled by a processor such as the controller  134  or other equivalent processor. The degree of autonomy required of the Doppler corrected matched filter system  100  depends primarily on its speed of operation. If the time required to search one code/Doppler bin is on the order of 10-20 ms, then the most likely mode of operation would be for the controller  134  to issue a command to do a load or search, have the Doppler corrected matched filter system  100  execute that command, and then have the controller  134  collect the results and issue a command for the next operation, if necessary. Alternatively, if the Doppler corrected matched filter system  100  is operated at very high speed, such as the 5000 one-millisecond segment searches per second mentioned above, it would likely be required that a sequence of commands be issued and that the Doppler corrected matched filter system  100  have an automated method, such as a direct memory access “DMA” capability, of delivering results to the controller  134 . 
   After the controller  134  has read out all of the data required for the particular operation it is performing with the Doppler corrected matched filter system  100 , the controller  134  may issue the next command, if necessary. Thus, the Doppler corrected matched filter system  100  may operate asynchronously, as rapidly as the controller  134  may process its output results. 
   When the invention is used to acquire the signal of a GPS satellite, the objective for the Doppler corrected matched filter system  100  during initial acquisition is to get enough satellites in track to support navigation within one second. Because of the inexpensive frequency standard technology used, this may require searching 10-20 Doppler bins until the first satellite is acquired. In cold start conditions (where some system initialization information is lacking), it may be necessary to search for 2-3 satellites before a visible satellite is selected. Therefore, it is necessary to be able to search 50 to 100 Doppler bins per second to meet this objective. This search rate is consistent with allowing a current-technology microprocessor to control the Doppler corrected matched filter system  100  one command at a time. 
   Accordingly, under these assumptions, the command set for the Doppler corrected matched filter system  100  may be relatively simple. The following example commands may be implemented: 
   
     
       
             
             
           
         
             
                 
             
           
           
             
               X 
               LOAD SIGNAL SAMPLES: At the beginning of the next T20 
             
             
                 
               interval, the Doppler corrected matched filter system 100 would 
             
             
                 
               begin loading 20 ms of signal samples. 
             
             
               X 
               INPUT FREQUENCY: A register is required to specify the nominal 
             
             
                 
               frequency shift to use while the signal samples are loaded. Assuming 
             
             
                 
               reuse of the carrier and code NCO and generation circuits 120, 122 
             
             
                 
               and 124, FIG 1, 
             
             
               X 
               EXECUTE SEARCH: The Doppler corrected matched filter system 
             
             
                 
               100 would immediately execute a correlation search based on the 
             
             
                 
               programmed search parameters. 
             
             
                 
             
           
        
       
     
   
   Search parameter registers could be programmed to store the following values: 
   
     
       
             
             
           
         
             
                 
             
           
           
             
               X 
               DOPPLER SHIFT: This is a Doppler shift applied to the stored 
             
             
                 
               signal as each 1 ms segment is loaded into the signal sample 
             
             
                 
               register 310. For example, there may be on the order of 16 or 
             
             
                 
               so Doppler values, representing shifts of 750 * n Hz, where 
             
             
                 
               n = 0 though 15. 
             
             
                 
             
           
        
       
     
   
   PN CODE: This register would likely be used in association with the existing code generator  124 . The new function would be to inform the Doppler corrected matched filter system  100  that a new PN code is required, so that the code would be generated and loaded into the code register  304  for use in the next, and subsequent, correlation searches. 
   5. Program Implementation 
   For performance purposes with current technology, the invention is preferably implemented in dedicated circuitry. However, the functions performed by the invention may also be implemented in generalized programmable circuitry capable of generating the following steps:
     1) Receive and store an input sample of a complex signal to be Doppler shift corrected and correlated to a code signal; optionally limit the received complex signal in value range and/or sampling frequency.   (2) Complex mix at least a portion of the stored input sample with a Doppler shift correction value.   (3) Compute the complex products of the mixed portion of the stored input sample with a current code phase of the code signal.   (4) Sum the computed complex products as a current complex integration value.   (5) Compute, such as generating an envelope, (and normally store) the square root of the sum of the squares of the current complex integration value.   (6) Shift the code signal to a next current code phase.   (7) Repeat steps (3) through (6) for the current code phase of the code signal.   (8) Output the code phase and magnitude of the stored computed square root value having the largest magnitude as an indication of correlation between the input sample and the code signal, corrected for Doppler shift.   

   The following is an example pseudo-code version of one example implementation of a matched filter  102  in accordance with the invention: 
   
     
       
             
           
             
             
           
             
           
         
             
                 
             
           
           
             
               // Matched Filter Algorithm Pseudo-code 
             
             
               // This version assumes that the samples are loaded with the 
             
             
               // digital IF Fo/8 plus the nominal estimated frequency error 
             
             
               // already removed. The complex spectrum is then centered 
             
             
               // near zero Hz. Further complex frequency shifts will be made 
             
             
               // to search other Doppler bins. The IF has to be removed before 
             
             
               // the sample rate is reduced from 20/Tca to 2/Tca. The analog 
             
             
               // bandwidth is a little too wide for this sampling rate. It has 
             
             
               // to be verified that the PN waveform is as tolerant as expected 
             
             
               // to the resulting aliasing. 
             
             
               // ****** Load the matched filter ******** 
             
             
               // This operation is real time, in the sense that the samples are 
             
             
               // being created by a clock that has to sample uniformly in time. 
             
             
               // So, these samples have to be stored as fast as they are received. 
             
             
               // Subsequent processing in the MF can be non-real time (either 
             
             
               // faster or slower). 
             
             
               #define SAMPLES_PER_MS (2046) // Sampling at 2/Tca = 2.046 MHz 
             
             
               #define NUMBER_OF_MS (20) 
             
             
               // The signal samples are complex values (I,Q). Each component, 
             
             
               // in-phase or quadrature, can take on three values: −1, 0, +1. In the 
             
             
               // hardware before the MF, these samples are represented by two bits each 
             
             
               // for I and Q 
             
             
               int s[NUMBER_OF_MS][SAMPLES_PER_MS][2]; 
             
             
               int i,j; 
             
             
               for( i = 0 ; i &lt; NUMBER_OF_MS ; i++ )   // each one ms segment 
             
             
               { 
             
             
                for( j = 0 ; j &lt; SAMPLES_PER_MS ; j++ )  // each complex sample in the ms 
             
             
                { 
             
             
                 s[i][j][0] = Get_I_Phase_Sample( ); // three-valued, two bit I sample 
             
             
                 s[i][j][1] = Get_Q_Phase_Sample( ); //three-valued, two bit Q sample 
             
             
                } 
             
             
               } 
             
             
               // ******* Load GPS PN Code **************** 
             
             
               // GPS PN codes could be generated as needed by code generator (re-use design 
             
             
               // from signal processor channels). Alternatively, they could be stored in a ROM. 
             
             
               // Each chip is one bit representing the values −1 and +1. 
             
             
               #define CHIPS_IN_CODE (1023)   // periodic length of GPS PN codes 
             
           
        
         
             
               #define NUMER_OF_CODES (32) 
               // number of GPS codes (ignoring 
             
             
               pseudolites 
             
             
                 
               // and WAAS codes) 
             
           
        
         
             
               int c[NUMER_OF_CODES][CHIPS_IN_CODE];  // Assume ROM storage 
             
             
               notation. 
             
             
               // ******* Load Doppler Signal ************* 
             
             
               // The notation here assumes that the complex signals to mix the various Doppler 
             
             
               values 
             
             
               // to dc are stored in a ROM as complex values. That is not a likely implementation, 
             
             
               // but makes the math easier to visualize. The values are assumed to be 5 level signals 
             
             
               // like the normal signal processing channels use. Most likely, the generator used 
             
             
               // in the normal signal processing channels could be used as a starting point. Simpler 
             
             
               // implementations using two level quantization (−1,+1) are likely. 
             
             
               #define NUMBER_OF_DopplerS (16) 
             
             
               int d[NUMBER_OF_DopplerS][SAMPLES_PER_MS][2]; 
             
             
               for( code = 0 ; code &lt; NUMER_OF_CODES ; code++ ) // process each code 
             
             
               { 
             
             
                for( dop = 0 ; dop &lt; NUMBER_OF_DopplerS ; dop++ ) // process each Doppler 
             
             
               shift 
             
             
                { 
             
             
                 for( seg = 0 ; seg &lt; NUMBER_OF_MS ; seg++ ) // process each ms 
             
             
                 { 
             
             
                  for( sample = 0 ; sample &lt; SAMPLES_PER_MS ; sample++ ) // do Doppler 
             
             
               shift while loading 1 ms signal register 
             
             
                  { 
             
             
                   s_1ms[sample][RE] = d[dop][sample][RE] * s[seg][sample][RE] − 
             
             
               d[dop][sample][IM] * 
             
             
                        s[seg][sample][IM]; 
             
             
                   s_1ms[sample][IM] = d[dop][sample][IM] * s[seg][sample][RE] + 
             
             
               d[dop][sample][RE] * 
             
             
                        s[seg][sample][IM]; 
             
             
                  } 
             
             
                  for( shift = 0; shift &lt; CHIPS_IN_CODE ; shift++ ) // process each phase shift 
             
             
               of the code 
             
             
                  { 
             
             
               half_shift = 2 * shift; 
             
             
               for( half_chip = 0 ; half_chip &lt; 2 ; half_chip++ ) // two samples per chip 
             
             
               { 
             
             
               // ******* Run the Matched Filter ********** 
             
             
               // This version assumes that as each segment is loaded from RAM into a 1 ms long 
             
             
               latch 
             
             
               // for correlation with the PN code, the Doppler signal is applied. 
             
             
               int code;  // which pn code is being processed 
             
             
               int sample;  // which sample is being processed 
             
             
               int shift;  // which of the code phases is being processed 
             
             
               int half_shift;// because there are two samples per chip 
             
             
               int seg;  // which one ms signal segment is being processed 
             
             
               int dop;  // which Doppler bin is being processed 
             
             
               int chip;  // which chip is being processed 
             
             
               int r_ms;  // ms accumulator, real part 
             
             
               int i_ms;  // ms accumulator, imaginary part 
             
             
               int sum_20ms[SAMPLES_PER_MS];  // 20 ms non-coherent accumulators 
             
             
               int s_1ms[SAMPLES_PER_MS][2];  // Current 1 ms of Doppler shifted signal 
             
             
               #define RE (0) 
             
             
               #define IM (1) 
             
             
               // here we are showing that all codes, Dopplers and phase shifts are processed. 
             
             
               // We may want to be able to select a sub-range, list, or single one of these. 
             
             
               // There are probably some command registers to specify ranges &amp; modes of 
             
             
               operation. 
             
             
                   if(seq == 0)  // zero the 2046 non-coherent 20 ms integrator on first 
             
             
               segment 
             
             
                   { 
             
             
                    sum_20ms[half_shift + half_chip] = 0; 
             
             
                   } 
             
             
                   r_20ms = 0;  // zero the coherent 1 ms integrators 
             
             
                   i_20ms = 0; 
             
             
                   for( sample = 0 ; sample &lt; SAMPLES_PER_MS ; sample++ ) // form 1 ms 
             
             
               sums 
             
             
                   { 
             
             
                    chip = (sample + half_chip) / 2; // stays the same for two consecutive 
             
             
               samples 
             
             
                    // The sum can change only by −2, −1, 0, +1, or +2 at each pass 
             
             
                    // There is a way to use 5 level (rather than 3 level) Doppler signals and 
             
             
               still have only 
             
             
                    // these change values. This results in less implementation loss of C/No. 
             
             
                      // The last half chip of the last sample might just be skipped, if it helps. 
             
             
                  r_ms += c[code][(shift+chip)%CHIPS_IN_CODE] * ( s_1ms[sample][RE] ); 
             
             
                      i_ms += c[code][(shift+chip)%CHIPS_IN_CODE] * ( 
             
             
                  s_1ms[sample][IM] ); 
             
             
                     } 
             
             
                  // The sqrt of the sum of squares (envelope) can be approximated by: 
             
             
                     // max_abs = MAX( |r_ms|, |i_ms| ); 
             
             
                     // min_abs = MIN( |r_ms|, |i_ms| ); 
             
             
                     // sum_20ms = max_abs + min_abs/2; 
             
             
                  sum_20ms[half_shift + half_chip] += sqrt( r_ms * r_ms + i_ms * i_ms ); // exact 
             
             
                  value of envelope 
             
             
                  } // end of half chip 
             
             
                  } // end of code shift 
             
             
                  } // end of 1 ms segment 
             
             
               // The 20 ms second sums for this code and Doppler are complete. 
             
             
               // 
             
             
               // Alternatives at this point include 
             
             
               // 1. Interrupt processor to inspect 20 ms sums 
             
             
               //  (a lot of throughput, but might want in case of limited search). 
             
             
               // 2. Hardware search for N largest values and their locations; store for processor. 
             
             
                  // 3. Hardware search for largest value and its location. Store for processor. May not 
             
             
               be adequate. 
             
             
                  // 4. Store every value and location above a threshold. Store for processor. 
             
             
                  // 5. Hardware search for largest value. Store value and location for processor. 
             
             
               Interrupt processor and 
             
             
                  //  halt the MF until re-enabled by processor. Then continues with next search. 
             
             
               // Below shows illustration for largest value. 
             
             
                  MaxLoc = −1; 
             
             
                  MaxVal = −1; 
             
             
               for( sample = 0 ; sample &lt; SAMPLES_PER_MS ; sample++ ) 
             
             
                  { 
             
             
                   if( sum_20ms[sample] &gt; MaxVal ) 
             
             
                   { 
             
             
                    MaxVal = sum_20ms[sample]; 
             
             
                    MaxLoc = sample; 
             
             
                   } 
             
             
                  } 
             
             
               // Interrupt processor. Processor has until next start of search to get results. 
             
             
                  // Might want to have a mode that suspends until interrupt is acknowledged. 
             
             
                  // This would be useful if the processor were going to search the 20 ms sums itself or 
             
             
                  // extract some other data. 
             
             
               Interrupt(MF_Complete); 
             
             
               } // end of one Doppler bin 
             
             
               } // end of one pn code choice 
             
             
               MF_Idle( );  // Go to sleep. No clocks, no/low power. 
             
             
                 
             
           
        
       
     
   
   The software, which comprises an ordered listing of executable instructions for implementing logical functions, may selectively be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that may selectively fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” is any means that may contain, store, communicate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-readable medium may selectively be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device. More specific examples, i.e.,“a non-exhaustive list” of computer-readable media, would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (“RAM”) (electronic), a read-only memory (“ROM”) (electronic), an erasable programmable read-only memory (“EPROM” or Flash memory) (electronic), an optical fiber (optical), and a portable compact disc read-only memory (“CDROM”) (optical). Note that the computer-readable medium may even be paper punch cards, paper tape, or other suitable media, as the program can be electronically captured, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. 
   6. Implementation in Network Assisted Environment 
   The Doppler corrected matched filter system  100  of the present invention may be incorporated in a positioning system that includes satellites and/or pseudolites (base stations) to triangulate the position of a receiver.  FIG. 11  illustrates an example implementation of the matched filter system  100  in a network assisted environment.  FIG. 11  shows a radio frequency receiver, in the form of a wireless device  1100 , in signal communication with a stationary communication device  1102  (such as a base station or other network communication device) via signal path  1104 . The wireless device  1100  receives signals from a number of satellites  1104 ,  1106  and  1108  via signal paths  1110 ,  1112  and  1114 , respectively, and the stationary communication device  1102  receives signals from the satellites  1104 ,  1106  and  1108  via signal paths  1116 ,  1118  and  1120 , respectively. The wireless device  1100  includes an RF front end  1122  (similar to RF front end  128  in  FIG. 1 ), the matched filter system  100 ,  FIG. 11 , and a radio module  1124 . 
   In  FIG. 11 , a network  1126  is used to assist the wireless device  1100  to find the signals from the satellites  1104 ,  1106  and  1108 . The stationary communication device  1102  is part of the network  1126  and includes a satellite positioning system receiver  1128  that receives satellite data messages or signals from the satellites  1104 ,  1106  and  1108 . The stationary communication device  1102  is assumed to receive the signal approximately error free because it will have a clear view of the sky and will not have any obstructions blocking the signals. In the network  1126 , there may be numerous devices similar to the stationary communication device  1102 . For convenience only, one stationary communication device  1102  is shown; however, it is appreciated that the following description is equally valid with multiple stationary communication devices. Additionally, only three of the approximately 12 available satellites are shown. 
   The geographic position of a wireless device may be calculated once a sufficient number of satellite signals are received at the wireless device. U.S. Pat. No. 5,812,087 entitled “Method And Apparatus For Satellite Positioning System Based Time Measurement,” issued to Norman F. Krasner on Sep. 22, 1998, which is incorporated by reference, describes a method and apparatus for measuring time related to satellite data signals in satellite positioning systems. The measured time is used to calculate the position of the wireless device. U.S. Pat. No. 5,812,087 discloses the method and apparatus for establishing receiver timing at the wireless device by having a GPS receiver of the wireless device form an estimate of a portion of the satellite data message and transmit the estimate to a base station. The base station compares this estimate with a record of the satellite data signals received from another GPS receiver at the base station. The comparison determines which portion of the base station&#39;s data most closely matches the data transmitted by the wireless device and the result of the comparison is transmitted back to the wireless device for reference. The time measurements may also be implemented in the system described in U.S. Pat. No. 5,945,944, entitled “Method And Apparatus For Determining Time For GPS Receivers,” issued to Norman F. Krasner on Aug. 31, 1999, which is incorporated by reference. 
   U.S. Pat. No. 5,945,944 discloses a method and apparatus of determining the time for a global positioning system receiver. Timing signals derived from a communication system, such as cellular phone transmission signals, are received by a GPS receiver and decoded to provide accurate time information. The timing signals may be in the form of synchronized events marked by timing indicators, or as system time information. The timing signals in combination with satellite position signals received by the GPS receiver are used to determine the position of the GPS receiver. The time measurements may also be implemented in an obstructive environment by the system described in U.S. Pat. No. 5,831,574, entitled “Method And Apparatus For Determining The Location Of An Object Which May Have An Obstructed View Of The Sky,” issued to Norman F. Krasner on Nov. 3, 1998 and U.S. Pat. No. 6,016,119, entitled “Method And Apparatus For Determining The Location Of An Object Which May Have An Obstructed View Of The Sky,” issued to Norman F. Krasner on Jan. 18, 2000, which are both incorporated by reference. 
   U.S. Pat. No. 5,831,574 discloses a positioning sensor that receives and stores a predetermined record length of positioning signals while in a fix position located such that the positioning sensor can receive positioning signals. Thereafter, the stored positioning signals are processed to determine the geographic location of the fix position. The fix position may correspond to a location of an object of interest or it may be in a known location relative to the position of the object, in which case once the geographic location of the fix position has been computed, the geographic location of the object can be derived. The positioning sensor includes a Snapshot GPS receiver which may collect and process GPS signals transmitted by GPS satellites using fast convolution operations to compute pseudoranges from the GPS satellites to the fix position. Alternatively, these computations may be performed at a basestation. The computed pseudoranges may then be used to determine the geographic location of the fix position. The positioning sensor may be equipped with a depth sensing element, such as a pressure sensor, which allows a determination of the depth of a submerged object to be made. The positioning sensor may further be equipped with signal detecting means for determining when the positioning sensor is in the fix position. 
   U.S. Pat. No. 6,016,119 discloses a positioning sensor which receives and stores a predetermined record length of positioning signals while in a fix position located such that the positioning sensor can receive positioning signals. Thereafter, the stored positioning signals are processed to determine the geographic location of the fix position. The fix position may correspond to a location of an object of interest or it may be in a known location relative to the position of the object, in which case once the geographic location of the fix position has been computed, the geographic location of the object can be derived. The positioning sensor includes a Snapshot GPS receiver which may collect and process GPS signals transmitted by GPS satellites using fast convolution operations to compute pseudoranges from the GPS satellites to the fix position. Alternatively, these computations may be performed at a basestation. The computed pseudoranges may then be used to determine the geographic location of the fix position. The positioning sensor may be equipped with depth sensing means, such as a pressure sensor, which allows a determination of the depth of a submerged object to be made. The positioning sensor may further be equipped with signal detecting means for determining when the positioning sensor is in the fix position. 
   After processing by the cancellation system, the wireless device may make the time measurements based on the system described in U.S. Pat. No. 5,884,214, entitled “GPS Receiver And Method For Processing GPS Signals,” issued to Norman F. Krasner on Mar. 16, 1999, which is incorporated by reference. U.S. Pat. No. 5,884,214 discloses a global positioning system (GPS) receiver having first circuitry for receiving and processing pseudorandom sequences transmitted by a number of GPS satellites. The first circuitry is configured to perform conventional correlation operations on the received pseudorandom sequences to determine pseudoranges from the GPS receiver to the GPS satellites. The GPS receiver also includes second circuitry coupled to the first circuitry. The second circuitry is configured to receive and process the pseudorandom sequences during blockage conditions. The second circuitry processes the pseudorandom sequences by digitizing and storing a predetermined record length of the received sequences and then performing fast convolution operations on the stored data to determine the pseudoranges. The GPS receiver may have a common circuitry for receiving GPS signals from in view satellites and downconverting the RF frequency of the received GPS signals to an intermediate frequency (IF). The IF signals are split into two signal path, a first of which provides the conventional correlation processing to calculate the pseudoranges. During blockage conditions, the IF signal is passed to the second signal path wherein the IF signals are digitized and stored in memory and later processed using the fast convolution operations to provide the pseudoranges. Alternative arrangements for the two signal paths include separate downconverters or shared digitizers. One embodiment provides both signal paths on a single integrated circuit with shared circuitry executing computer readable instructions to perform GPS signal processing appropriate to the reception conditions. 
   Additionally, the wireless device may make the time measurements based on the system described in U.S. Pat. No. 5,781,156, entitled “GPS Receiver And Method For Processing GPS Signals,” which is incorporated by reference. U.S. Pat. No. 5,781,156 discloses a GPS receiver in one embodiment which includes an antenna which receives GPS signals at an RF frequency from in view satellites; a downconverter coupled to the antenna for reducing the RF frequency of the received GPS signals to an intermediate frequency (IF); a digitizer coupled to the downconverter and sampling the IF GPS signals at a predetermined rate to produce sampled IF GPS signals; a memory coupled to the digitizer storing the sampled IF GPS signals (a snapshot of GPS signals); and a digital signal processor (DPS) coupled to the memory and operating under stored instructions thereby performing Fast Fourier Transform (FFT) operations on the sampled IF GPS signals to provide pseudorange information. These operations typically also include preprocessing and post processing of the GPS signals. After a snapshot of data is taken, the receiver front end is powered down. The GPS receiver in one embodiment also includes other power management features and includes, in another embodiment, the capability to correct for errors in its local oscillator which is used to sample the GPS signals. The calculation speed of pseudoranges, and sensitivity of operation, is enhanced by the transmission of the Doppler frequency shifts of in view satellites to the receiver from an external source, such as a basestation in one embodiment of the invention. 
   Additionally, an example implementation of the time measurement utilizing the assistance of a base station may be implemented in the system described in U.S. Pat. No. 5,874,914, entitled “GPS Receiver Utilizing A Communication Link,” issued to Norman F. Krasner on Feb. 23, 1999, which is incorporated by reference. U.S. Pat. No. 5,874,914 discloses a GPS receiver in one embodiment which includes an antenna which receives GPS signals at an RF frequency from in view satellites; a downconverter coupled to the antenna for reducing the RF frequency of the received GPS signals to an intermediate frequency (IF); a digitizer coupled to the downconverter and sampling the IF GPS signals at a predetermined rate to produce sampled IF GPS signals; a memory coupled to the digitizer storing the sampled IF GPS signals (a snapshot of GPS signals); and a digital signal processor (DPS) coupled to the memory and operating under stored instructions thereby performing Fast Fourier Transform (FFT) operations on the sampled IF GPS signals to provide pseudorange information. These operations typically also include preprocessing and post processing of the GPS signals. After a snapshot of data is taken, the receiver front end is powered down. The GPS receiver in one embodiment also includes other power management features and includes, in another embodiment, the capability to correct for errors in its local oscillator which is used to sample the GPS signals. The calculation speed of pseudoranges, and sensitivity of operation, is enhanced by the transmission of the Doppler frequency shifts of in view satellites to the receiver from an external source, such as a basestation in one embodiment of the invention. 
   Additionally, another example implementation of the time measurement utilizing the assistance of a base station may be implemented in the system described in U.S. Pat. No. 5,841,396, entitled “GPS Receiver Utilizing A Communication Link,” issued to Norman F. Krasner on Nov. 24, 1998, which is incorporated by reference. U.S. Pat. No. 5,841,396 discloses a precision carrier frequency signal for calibrating a local oscillator of a GPS receiver which is used to acquire GPS signals. The precision carrier frequency signal is used to calibrate the local oscillator such that the output of the local oscillator, which is used to acquire GPS signals, is modified by a reference signal generated from the precision carrier frequency signal. The GPS receiver locks to this precision carrier frequency signal and generates the reference signal. In another aspect of the invention, satellite almanac data is transmitted to a remote GPS receiver unit from a base station via a communication link. The remote GPS receiver unit uses this satellite almanac data to determine approximate Doppler data for satellites in view of the remote GPS receiver unit. 
   Still another example implementation of the time measurement utilizing the assistance of a base station may be implemented in the system described in U.S. Pat. No. 5,999,124, entitled “Satellite Positioning System Augmentation With Wireless Communication Signals,” issued to Leonid Sheynblant on Dec. 7, 1999, which is incorporated by reference. U.S. Pat. No. 5,999,124 discloses a method and apparatus for processing position information from satellite positioning system satellites and from cellular based communication signals. In one example of a method according to the invention, a SPS receiver receives SPS signals from at least one SPS satellite. This SPS receiver is coupled to and typically integrated with a communication system which receives and transmits messages in a cell based communication system. In this method, a message is transmitted in the cell based communication signals between a communication system and a first cell based transceiver. A time measurement which represents a time of travel of a message in the cell based communication signals between the cell based transceiver and the communication system is determined. Another time measurement that represents a time of travel of the SPS signals is also determined. A position of the SPS receiver is determined from a combination of at least the time measurement which represents the time of travel of a message in the cell based communication signals and from a time measurement which represents a time travel of the SPS signals. The cell based communication signals are capable of communicating data messages in a two-way direction in one embodiment between the cell based transceiver and the communication system. 
   Another example implementation of the time measurement utilizing the assistance of a base station may be implemented in the system described in U.S. Pat. No. 6,002,363, entitled “Combined GPS Positioning System And Communications System Utilizing Shared Circuitry,” issued to Norman F. Krasner and is incorporated by reference. U.S. Pat. No. 6,002,363 discloses a combined GPS and communication system having shared circuitry. The combined system includes an antenna for receiving data representative of GPS signals, a frequency converter coupled to the antenna, a frequency synthesizer coupled to the frequency converter, an analog to digital converter coupled to the frequency converter and a processor coupled to the frequency converter. The processor processes the data representative of GPS signals to determine a pseudorange based on the data representative of GPS signals to determine a pseudorange based on the data representative of GPS signals. The integrated communication receiver includes a shared component which is at least one of the antenna, the frequency converter, the frequency synthesizer and the analog to digital converter. Typically, in certain embodiments, the processor also demodulates communication signals received as well as controls the modulation of data to be transmitted as a communication signal through a communication link. 
   While various embodiments of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.