CN1695069A - Directly acquiring precision code GPS signals - Google Patents

Abstract

In general, the invention is directed to techniques for directly acquiring P-codes without first acquiring C/A-codes. For example, in one embodiment, a system comprises an assist server to track a P-code signal from a Global Positioning System GPS satellite and generate acquisition assistance information from the signal. The system further comprises a mobile unit to receive the acquisition assistance data from the assist server, and to acquire the P-code signal from the satellite based on the acquisition assistance data. The acquisition assistance data may include time-of-week data indicating an initial time offset into a P-code pseudorandom code sequence for the satellite. The mobile unit may include a reference generator to locally generate a reference pseudorandom code sequence based on the time-of-week data.

Description

Direct capture of accurate code GPS signals

Related applications

This application claims priority to US Provisional Application No. 60/355,212, filed February 8, 2002, and US Provisional Application No. 60/362,476, filed March 6, 2002.

technical field

This invention relates to the Global Positioning System (GPS), and more particularly to the acquisition of GPS signals from satellites.

Background technique

The Global Positioning System (GPS) is a satellite navigation system designed to provide position, velocity, and time information almost anywhere in the world. GPS was developed by the US Department of Defense and now includes a constellation of 24 operational satellites.

GPS provides two levels of service: standard positioning service and precise positioning service. Standard Positioning Service (SPS) is a positioning and timing service available to all GPS users. SPS utilizes coarse capture pseudo-random codes (C/A codes) and navigation data messages. The SPS provides relative, predictable positioning accuracy to 100 meters horizontal (95%), vertical 156 meters (95%), and time transfer accuracy to within 340 nanoseconds (95%).

Precise Positioning Service (PPS) is a high-accuracy military positioning, velocity and timing service that is continuously available worldwide to authorized users. PPS utilizes a precise code (P-code), which is primarily designed for use by the US military. Military user equipment that can use P-codes provides a predictable positioning accuracy of at least 22 meters horizontally (95%) and 27.7 meters vertically, with a time accuracy converted to Universal Time (UTC) of no more than 200 nanoseconds (95%).

GPS satellites transmit three pseudorandom noise (PRN) ranging codes at two L-band frequencies (L1 = 1575.42 MHz and L2 = 1227.6 MHz) and use them. The C/A code transmitted by the satellite is a 1023-bit pseudo-random binary sequence modulated on the GPS L1 carrier signal with a code rate of 1.023MHz and a code repetition period of 1 millisecond.

The P code transmitted by the satellite is a very long pseudo-random binary sequence of biphase modulation, about 10 ¹⁴ bits, and the code rate is 10.23MHz. This sequence will not repeat within 267 days. Each GPS satellite transmits a unique "one week" segment of the P-code sequence, and the sequence is reset every week. The third code, called Y code, is an encrypted sequence used in conjunction with P code to improve security and anti-spoofing. Both the P code and the Y code can be on the L1 frequency and the L2 frequency.

Various receivers have been designed to decode signals transmitted by satellites to determine position, velocity or time. Typically, in order to decipher GPS signals and calculate a final position, a receiver must acquire GPS signals from one or more satellites in view, measure and track the received signals, and recover navigation data from these signals.

The process of searching for and acquiring GPS signals, and calculating the receiver's location can be time consuming. For example, to acquire a GPS signal carrying a C/A code, a mobile device typically performs a free-running correlation of the GPS signal with an internally stored C/A code sequence. Alternatively, the mobile device can collect snapshots of the GPS signal for a duration to capture the entire sequence, i.e. longer than 1 millisecond, and calculate a time offset for the satellite by correlating the buffered signal with the stored C/A code sequence (Pseudorange).

In addition to the code sequence, each satellite transmits a navigation message, which includes data called "ephemera", such as its orbital parameters, clock status, system time and status messages. After calculating the pseudoranges for the satellites, the mobile device extracts the ephemeris data and calculates the final position based on the calculated pseudoranges for the satellites and the ephemeris data received from the satellites.

Assisted location acquisition is a technique that has been used in commercial grade mobile devices to speed up the C/A code acquisition process. According to this method, the acquisition process of the C/A code is distributed between the mobile device and the network. The location assistance server operates continuously at a fixed location as a reference GPS receiver, such as a cellular base station. When a mobile device wishes to acquire a C/A code, the mobile device communicates with the location server and requests assistance information including ephemeris data. The mobile device utilizes this ephemeris data to speed up the correlation process for acquiring the C/A code. Specifically, the mobile device is able to estimate the time offset based on the ephemeris data received from the assistance server, thereby reducing the number of correlations that need to be performed.

Capturing P codes requires more computation than capturing C/A codes. For various practical purposes, the P-code sequence is non-repetitive. In other words, it is not feasible to buffer the entire P-code sequence, which is reset once a week for a given satellite. For this reason, military grade GPS receivers often utilize C/A codes to speed up acquisition of P codes. In other words, the mobile device first acquires the C/A code in order to extract the ephemeris data from the signal. The device utilizes the ephemeris data extracted from the C/A code to speed up the correlation process of capturing the P code.

However, even when capturing a signal using C/A codes, the process typically takes several minutes. In many cases, this long processing time is unacceptable, and further, the long processing time can greatly limit the battery life of portable applications. The acquisition process becomes more difficult in weak signal or interference environments.

Contents of the invention

In general, the present invention is directed to techniques for directly and efficiently acquiring P-code signals from GPS satellites. In other words, the P code signal can be acquired without first acquiring the C/A code signal from the satellite.

For example, in one embodiment, a system includes an assistance server for tracking signals from Global Positioning System (GPS) satellites and generating acquisition assistance data from the signals. The system further includes a mobile device for receiving acquisition assistance data from the assistance server, and acquiring a P code signal from the satellite according to the acquisition assistance data. The acquisition assistance data may include start-of-week data indicating an initial time offset of a P-code pseudo-random code sequence for the satellite. The mobile unit may include a reference signal generator for in-situ generation of a reference pseudo-random code sequence based on the origin clock data. The assistance server can track and generate capture assistance data from the C/A code signal or the P code signal. The mobile device and auxiliary server can be coupled by a wired or wireless communication link.

In another specific embodiment, a method includes receiving acquisition assistance data from an assistance server and determining a time offset of a P-code pseudo-random code sequence associated with a satellite signal based on the acquisition assistance data.

In another embodiment, an apparatus includes an antenna for receiving signals from global positioning system (GPS) satellites; and a wireless modem for receiving acquisition assistance data from an assistance server. The device further includes a reference signal generator for generating a reference pseudo-random code sequence based on the acquisition assistance data; and a processor for determining a P-code pseudo-random code sequence associated with the signal based on the acquisition assistance data A time offset.

In another embodiment, a computer-readable medium includes instructions that cause a programmable processor to receive capture assistance data from an assistance server, and cause the programmable processor to determine, based on the capture assistance data, a The time offset of the P-code pseudo-random code sequence related to satellite signals.

The details of one or more specific embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description, drawings and claims.

Description of drawings

FIG. 1 is a block diagram illustrating an example system in which a mobile device directly acquires P-code GPS signals using assistance information.

Figure 2 is a block diagram illustrating an example embodiment of the mobile device.

3 is a flow diagram illustrating an example mode of operation of the mobile device.

4 is a flow diagram further illustrating an exemplary process for computing pseudoranges for a given satellite according to the techniques.

Figure 5 illustrates an example alignment of digital data representing a P-code GPS signal.

FIG. 6 is a timing diagram illustrating example related operations performed by the mobile device using the overlap save method based on time uncertainty values.

FIG. 7 is a flowchart illustrating an example acquisition process when the temporal uncertainty value is relatively large.

Figure 8 is a flowchart illustrating an adaptive acquisition process for acquiring multiple satellites.

Detailed ways

1 is a block diagram illustrating an example system 2 in which a mobile device 4 directly acquires a P-code GPS signal 6 from a satellite 8 using acquisition assistance data. Specifically, an assistance server 10 continuously monitors and tracks GPS signals 6 from satellites 8 and prepares and maintains acquisition assistance data based on these signals. For example, the assistance server 10 may track a P-code GPS signal or a C/A-code signal and extract capture assistance data from the signal. This acquisition assistance data may include origin time of week (TOW) information for each of the satellites 8 . In addition, the acquisition assistance data may further include ephemeris data extracted from the signals 6, such as satellite (S.V.) identification of satellites 8 currently in view, clock status, system time, Doppler frequency of each satellite shift, status messages, etc.

The assistance server 10 communicates this acquisition assistance data to the mobile device 4, which, as described herein, utilizes the acquisition assistance data to directly and efficiently acquire the P-code signal without first acquiring the C/A-code signal. For example, mobile device 4 uses the TOW information to select an initial time offset within the P-code sequence for each of satellites 6 . The mobile device 4 uses this initial time offset as a starting position within the sequence during the correlation process. In this way, the mobile device 4 can generally shorten the acquisition process considerably, and can acquire the P code signals from the satellites 6 without first acquiring the C/A codes from those satellites.

After acquiring the P-code signal, the mobile device 4 calculates its position, velocity or other GPS data. Alternatively, the mobile device 4 may communicate the pseudorange data to the assistance server 10 . From these pseudorange data, together with knowledge about current ephemeris data, assistance server 10 calculates position, velocity or other GPS data for mobile device 4 and communicates the calculated GPS data to mobile device 4 .

Mobile device 4 may be any of a variety of mobile GPS receivers capable of receiving GPS signals and computing GPS data. Examples include handheld GPS receivers, GPS receivers mounted on a vehicle, including airplanes, cars, tanks, boats, and the like.

The secondary server 10 and the mobile device 4 can communicate over link 5, which can be a wireless link, a hardware interface (such as a serial port or a parallel port), Ethernet connection, etc. A common wireless communication protocol is Code Division Multiple Access (CDMA), in which multiple communications occur simultaneously over a radio frequency (RF) spectrum. Other examples include Global System for Mobile Communications (GSM), which uses narrowband time division multiple access to transmit data; and General Packet Radio Service (GPRS). In some embodiments, mobile device 4 may integrate a GPS receiver and wireless communication means for voice or data communication.

The assistance server 10 may comprise a high performance GPS receiver with a fixed location. For example, the assistance server 10 may be coupled to a wireless communication base station for seamless communication with the mobile device 4 .

FIG. 2 is a block diagram of an example mobile device 4 in more detail. In general, the mobile device 4 includes a GPS antenna 20, downconverter 22, frequency synthesizer 24, analog-to-digital converter (ADC) 26, memory 28, digital signal processor (DSP) 30 and reference signal generator 31 , for receiving and capturing GPS signals6. Furthermore, the mobile device 4 includes a wireless modem 32 and an RF antenna 34 for communicating with the auxiliary server 10 .

Downconverter 22 receives signals 6 from satellite 8 via GPS antenna 20, and mixes these signals with a signal generated by frequency synthesizer 24 to convert the signals from L-band frequency to L-band for processing. baseband frequency. Downconverter 22 may first convert the signals to an intermediate frequency for conditioning, and then convert the conditioned signals to baseband frequency. Alternatively, downconverter 22 may implement a zero-intermediate-frequency (ZIF) architecture for converting L-band frequencies directly to baseband.

ADC 26 samples the baseband signal to produce a digital representation of the signal, and then stores a snapshot of the digital data in memory 28 . For example, memory 28 may store a continuous set of digitized data, typically corresponding to a duration of 100 milliseconds to 1 second or more of the baseband signal, for use by digital signal processor 30 in the acquisition process.

A digital signal processor (DSP) 30 communicates with the auxiliary server 10 through a wireless modem 32 and an RF antenna 34 . Although not illustrated in FIG. 2 , wireless modem 32 typically includes a downconverter and an analog-to-digital converter for processing RF signals received from RF antenna 34 . Although GPS and cellular communications can share one antenna, separate antennas are preferred because cellular communications and GPS signals typically use different RF bands.

A digital signal processor (DSP) 30 receives capture assistance data including origin week and time information from the assistance server 10 via an RF antenna 34 and stores this information for use in the capture process. Digital signal processor (DSP) 30 may store this information in memory 28, internal on-chip memory, or other suitable computer-readable media. Additionally, digital signal processor (DSP) 30 typically operates according to executable instructions retrieved from a computer-readable medium. Examples of such media include random access memory (RAM), read only memory (ROM), nonvolatile random access memory (NVRAM), electrically erasable programmable read only memory (EEPROM), flash memory, and the like. Although described with reference to a digital signal processor, other forms of embedded processors or controllers may also be used in the mobile device 4 .

DSP 30 passes origin time of week (TOW) information received from assistance server 10 to reference signal generator 31 , which uses the TOW information to generate reference signal 33 for correlation with GPS signal data stored in memory 28 . The reference signal generator 31 may be implemented by a digital circuit, or take the form of a digital signal processor (DSP) 30 program feature.

FIG. 3 is a flowchart further illustrating the operation of the mobile device 4 . To speed up the capture process, mobile device 4 receives capture assistance data from assistance server 10 via wireless modem 32 and RF antenna 34 (42). Acquisition assistance data includes TOW information for each of the satellites 8, and may include additional ephemeris data such as satellite (S.V.) identification, clock status, system time, Doppler frequency for each satellite 8 that is currently in view. shift, status messages, etc.

To acquire P-code GPS signals, mobile device 4 receives GPS signals from satellites 8 and stores a snapshot of these signals in memory 28 (44). Next, the mobile unit 4 uses the TOW information of each transmitting satellite 8 to acquire each P-code signal. Specifically, the reference signal generator 31 locally generates the signal 33 according to the acquired current satellite 8TOW information to carry the reference P code sequence (46). In other words, signal 33 may carry a subset of P-code sequences based on acquired satellite 8TOW information. For example, the reference signal generator 31 can select an initial time offset in the P code sequence according to the TOW information, and generate a subset of the P code sequence, which is improved from the initial time offset or around the initial time Offset. The auxiliary server 10 may transmit the TOW information in a time-based format (such as hour:minute:second), or may transmit the TOW information as a chip offset within the P code sequence.

The reference sequence length used by the traditional system for capturing the C/A code corresponds to one frame of the sequence code, that is, 1023 chips of the C/A code. Unlike this traditional system, the P code sequence carried by the reference signal 33 The length may well exceed the amount of data stored in snapshot memory 28 . Additionally, DSP 30 may adjust the size of snapshots stored in memory 28 based on sensitivity requirements and an initial time uncertainty value associated with the TOW information received from secondary server 10 . For example, in some cases it may be sufficient to buffer 100 ms of data, but use a reference signal 33 of length 1 second or longer, where 1 second represents uncertainty in the TOW information.

Next, a digital signal processor (DSP) 30 calculates time offsets (pseudoranges) for the satellites by correlating the reference signal 33 with a digitized snapshot of the GPS signal stored in memory 28 (47). As described in more detail below, digital signal processor (DSP) 30 utilizes a Fast Fourier Transform (FFT) algorithm by performing a large number of correlation operations between reference signal 33 generated in situ and data stored in memory 28 to calculate pseudoranges very quickly. Specifically, the FFT algorithm allows to search all these positions simultaneously and in parallel, thus speeding up the required run by several orders of magnitude faster than conventional methods when the number of required correlations is large.

As noted above, memory 28 captures a stream of digitized data corresponding to a relatively long period of time. Efficient processing of this large block of data using fast convolution methods helps improve signal processing at low reception levels (for example, when reception is poor due to partial obstruction by buildings, trees, or other obstructions)6 Ability. Digital signal processor (DSP) 30 uses this same buffered data in memory 28 to calculate pseudoranges for visible GPS satellites 8, typically eight of the 24 satellites orbiting the Earth. In situations of rapidly changing signal amplitudes, such as urban blocking conditions, the method can provide improved performance compared to conventional continuous tracking GPS receivers.

Once the digital signal processor 30 has completed the pseudorange calculations (48) for each of the satellites 8 in view, it calculates the pseudoranges based on the calculated pseudoranges for each satellite 8 and the satellites provided by the auxiliary server 10 to the mobile device 4. The location of the mobile device 4 is calculated using the history information (49). Alternatively, digital signal processor (DSP) 30 may communicate the pseudoranges via modem 32 to auxiliary server 10, which provides a final position calculation.

Figure 4 is a flow diagram further illustrating an exemplary process for computing pseudoranges for a given satellite. Once the snapshot of the GPS signal and the acquisition assistance information from the assistance server 10 is obtained, the digital signal processor (DSP) synchronizes (50) the acquired data with the generated reference PN code sequence carried by the signal 33 . Unlike the C/A code acquisition technique, in order to directly acquire the P code GPS signal, the digital signal processor (DSP) 30 may be required to process a range of code phases beyond the length of one data bit. Accordingly, unlike conventional GPS receivers that first capture the C/A code and then the P code, the mobile device 4 establishes bit synchronization directly from the P code snapshot stored in memory 28 . In other words, because the C/A code sequence repeats multiple times within a single data bit, conventional GPS receivers do not need to be concerned with bit edge synchronization for acquisition purposes. However, one difficulty in directly capturing P-codes is that, for various practical reasons, P-code sequences are non-repetitive. Accordingly, the bit period of the transmitted GPS signal stored in memory 28 is repeated several times throughout the snapshot, but the code phase is not repeated.

To synchronize the digitized data stored in memory 28 with the code sequence generated by reference signal generator 31 , digital signal processor (DSP) 30 associates a time uncertainty value with the TOW information received from auxiliary server 10 . More specifically, the mobile device 4 associates some time uncertainty with the TOW information received from the assistance server 10, the digital signal processor (DSP) 30 representing this time uncertainty as a time interval. The TOW received from the assistance server 10 may differ from the actual TOW of the mobile device 4 due to factors such as communication delays between the assistance server 10 and the mobile device 4 . Also, it changes for different communication systems. For example, a CDMA system may have an inherent range of time delays, while other communication systems may have different time delays.

In general, a digital signal processor (DSP) 30 synchronizes the digitized GPS signal with a reference signal 33 based on the received TOW information and the time uncertainty. Specifically, digital signal processor (DSP) 30 first estimates a position for the first bit edge within the digitized data stored in memory 28 . The equivalent bit edges can be calculated with a data transmission of 50 bits/second. Accordingly, a bit edge occurs every 20 ms from the start of the week. Given a chip rate of 10.23MHz, a bit edge occurs every 204600 chips.

Digital Signal Processor (DSP) 30 ignores some chips at the beginning of the digitized data stored in memory 28 and some codes at the beginning of the PN code sequence produced by reference signal generator 31 according to the judgment and time uncertainty made. slice, start the correlation process on or before the first bit edge. Like this, the digital signal processor (DSP) 30 utilizes the TOW information and the relative time uncertainty transmitted from the auxiliary server 10 to generate a reference P code sequence in situ, and compare the generated sequence with the data stored in the memory 28 bit synchronization. In this way, the digital signal processor (DSP) 30 ensures that the correct polarity is maintained for the chips throughout the bit period during the correlation process.

5 illustrates an example alignment of data snapshots stored in memory 28, assuming TOW is within 0 to δ seconds from the routine current TOW _te , and assuming δ is relatively small (less than one data bit). In other words, δ represents the time uncertainty of the TOW received from the assistance server 10 relative to the actual TOW of the P-code GPS signal received by the mobile device 4 .

As shown, t _e + δ represents the code phase at the beginning of the snapshot, and t ₀ represents the first bit edge within the snapshot. Assuming that a PN sequence starting at time t _e + δ has been generated, as assumed in Fig. 5, the digital signal processor (DSP) 30 ignores some samples, which correspond to the time from t _e + δ to t ₀ . An equivalent approach is to start the PN sequence at time t ₀ (coinciding with and bounded by the initial bit boundaries of m), and by programming a number of samples equivalent to the time interval t _e + δ - t ₀ into the raw data buffer sequence to generate a new buffer. These techniques ensure that the PN sequence always has samples within one data bit.

To handle data with a delay of t _e + δ to t _e + 2 δ, these techniques drop some samples at the start of the snapshot and PN benchmark, which is equal to the time from t _e + 2 δ to t ₀ . Likewise, digital signal processor (DSP) 30 may start the PN reference at time t ₀ and program into memory 28 a number of samples equal to t _e +2 _δ -t ₀ . The next range of delays _te +2δ to _te +3δ, and subsequent ranges of size δ, can be processed in a similar manner.

Referring again to FIG. 4 , after synchronizing (50) the acquired data with the generated reference PN code sequence, the digital signal processor (DSP) 30 may perform multiplication on the carrier, for example by multiplying by a Doppler carrier correction index. Puller correction (52). Next, a digital signal processor (DSP) 30 implements matched filtering using FFT operations. During this process, digital signal processor (DSP) 30 may perform Doppler correction on the baseband signal by, for example, multiplying by a Doppler carrier correction index (54).

Typically, the correlation process digital signal processor (DSP) 30 performs a Doppler search on multiple Doppler hypotheses ("frequency receivers") to determine a peak. For example, the assistance server 10 may give a 1000 Hz Doppler indication of a satellite. Based on this Doppler indication, the digital signal processor (DSP) 30 assigns a frequency increment, say 50 Hz, which will effectively create many frequency receivers around the carrier frequency. A digital signal processor (DSP) 30 performs correlation operations on the frequency receivers until a peak is detected.

For each receiver, a digital signal processor (DSP) 30 divides the reference signal 33 and the snapshot data in memory 28 into L blocks, where the size of the largest block is less than the data bit period (204600 P-code chips) Add the estimated time uncertainty of the received TOW relative to the actual TOW of the P-code GPS signal. A digital signal processor (DSP) 30 performs a matched filter operation on each block to determine the relative timing between the snapshot data contained in the memory 28 and the PN code sequence carried by the reference signal 33 . At the same time, the digital signal processor (DSP) 30 can compensate for the effect of Doppler shift on the sampling time. The digital signal processor (DSP) 30 can greatly reduce the computation time of these operations by applying the fast convolution operations of the presently described FFT algorithm.

For each tile, a digital signal processor (DSP) 30 computes the FFT of the snapshot data and the FFT of the reference signal 33 . A digital signal processor (DSP) 30 multiplies the snapshot by the complex conjugate of the reference signal after Doppler correction. This results in a correlation peak when the time offset is equal to the relative delay of the GPS signal, which is assumed to occur at a time corresponding to the delay of the two sequences. A digital signal processor (DSP) 30 calculates the size of the data for the set of associated delays and sums the size of a number of integration blocks, also called post-detection sums.

As shown in FIG. 6 , a digital signal processor (DSP) 30 performs FFT by convolving data blocks of length N, where N is the length of snapshot data stored in memory 28, which is determined by the digital signal processor (DSP). ) 30 is set according to the time uncertainty value associated with the TOW information received from the auxiliary server 10 . A digital signal processor (DSP) 30 zero pads the block of the reference signal with N-M zeros, where M represents a bit period. Thus, the length of the zero padding of the reference block is a function of the time uncertainty associated with the TOW.

In addition, the digital signal processor (DSP) 30 adds data beyond each bit boundary to the previous block in an overlap-save method, as shown in FIG. 6, where M is equal to the number of non-zero reference P-code chips , N is equal to the FFT size, usually 2 ¹⁸ =262144 chips, and B is equal to the data bit size—204600 P code chips. Typically M≤B, as described below.

For example, if R(0) is the value of the PN code corresponding to time t ₀ , and D(0) is the first data word in the memory 28, when aligned with the first bit edge as previously described, the digital signal processor (DSP) 30 executes the following procedures:

Convolute the first block of data:

Ref Block 0＝[R(0,1,2,...,M-1)Zeros(1,N-M)], where M<N

Data Block 0＝D(0, 1, 2,..., N-1).

To increase sensitivity, the digital signal processor (DSP) 30 convolves the next L (overlapping) data blocks, calculates the sizes, and sums these sizes with the previous data blocks:

Ref Block 1＝[R(B+[0,1,2,...,M-1])Zeros(1,N-M)]

Data Block 1＝D(B+[0, 1, 2,..., N-1])

...

This operation is continued for L data blocks.

Ref Block L-1＝[R(j×B+[0, 1, 2,..., M-1]) Zeros(1, N-M)]

Data Block L-1＝D(j×B+[0, 1, 2,..., N-1]), where j＝2, 3,...L-1

The size of the output data is N, but because of the nature of the overlap-preserving convolution, only the first N-M samples (or "delays") do not include the alias term obtained by the circular convolution. Typically, digital signal processor (DSP) 30 only reserves these items. Repeating the entire process can determine other delays as the relative timing of the reference sequence and the data sequence is shifted.

It should be noted that some processing may be lost since only M/10.23×10 ⁶ s out of 20 ms are processed. This is because of the M non-zero samples of the reference signal, which correspond to the first M samples of each data bit (total data bit size 204600). The result may be a loss of sensitivity of about 10log(M/204600)dB relative to the case where all data bits are processed at a time.

This loss can be overcome in several ways. Additional energy can be obtained if two convolutions are performed before magnitude squaring (instead of one for each step above), and the results are summed. The second convolution can utilize the changed reference signal and data blocks. The processing of the first set of reference signal and data blocks begins by determining the M reference samples and N data samples on the left, as shown in FIG. 6 , referred to herein as the convolution OA. Convolution OB eliminates the above penalty and ends at the end of bit period 0 using reference data that starts at the end of the fade period of bit period 0.

In other words, 204600 data samples M (the total number is B-M, B=204600) will be supplemented with N+M-B zero-valued samples to produce N total reference samples. This data block will utilize buffered data starting at time M, and include N total samples. The result of convolution 0B is then added to the result of 0A. Using the full data bit period of 204600 samples, this result gives a full convolution. Note again that to avoid temporal aliasing, usually only the first N-M samples of this convolution are saved.

A potentially more efficient approach is to utilize an FFT of size 204600. In this technique, the shaded period M will be the same as the bit period B. This approach eliminates the need to perform two convolutions on all data within a bit period. However, implementing such non-standard dimensions can be more complex than performing a radix-2FFT.

Yet another approach could be to sample the data at a lower rate such that M data samples equal one bit period. However, this may result in some loss of sensitivity due to the narrowed bandwidth. Alternatively, for a size equal to one data bit, the data can be sampled at twice this rate before performing the FFT. However, this approach may require increased computing time and storage capacity.

Referring again to FIG. 4 , after applying the matched filter operation, digital signal processor (DSP) 30 performs peak detection ( 56 ) to determine the time offset of the acquired satellites ( 58 ). Specifically, a digital signal processor (DSP) 30 calculates the sizes of the M samples extracted from the FFT technique, and then adds these sizes to a running buffer. Digital signal processor (DSP) 30 continues this process a number of times equal to the number of post-detection sums. It should be noted that the FFT produces N output samples, but the processing gain is actually a function of M due to zero padding of the reference block. The digital signal processor (DSP) 30 then calculates the mean of the running buffer and subtracts it from its own buffer. Next, the digital signal processor (DSP) 30 calculates the RMS noise with a threshold equal to k times the RMS set in the running buffer. Digital signal processor (DSP) 30 may choose this threshold to avoid situations where noise spikes may be larger than the threshold, which is commonly referred to as a "false alarm".

Fig. 7 is a flowchart of an example acquisition process when the time uncertainty associated with the TOW received from the auxiliary server 10 is large, ie when the time uncertainty exceeds one data bit period M. As described above, the reference signal generator 31 uses the acquisition assistance data to generate in situ a signal 33 (60) containing a reference pseudorandom code and detects peaks (62) by correlating the reference signal 33 with a digitized snapshot of the GPS signal. As mentioned above, for each satellite, a digital signal processor (DSP) 30 performs a Doppler search on multiple Doppler "receivers" to determine a peak. For example, the assistance server 10 may give a 1000 Hz Doppler indication of a satellite. Based on this Doppler indication, the digital signal processor (DSP) 30 assigns a frequency increment, say 50 Hz, which will effectively build many frequency receivers around the carrier frequency.

If no peak is detected on all Doppler hypotheses (64), the digital signal processor (DSP) 30 will perform an additional search by "sliding" the reference P code by N-M chips relative to the stored snapshot data , where N represents the block size and M represents the bit period (70). After sliding the reference P code, digital signal processor (DSP) 30 repeats the set of convolution operations to determine if a peak can be found in any receiver (62). If no peak is detected in all Doppler hypotheses, the reference P code continues to be shifted until all temporal uncertainty ranges are exhausted (66). It should be noted that for each slip of the reference P-code, the digital signal processor (DSP) 30 resynchronizes the snapshot to the next data bit boundary as previously described. The digital signal processor (DSP) 30 continues this process until all satellites have been searched (74), and a final position is calculated (76).

Figure 8 is a flowchart illustrating an adaptive acquisition process for acquiring multiple satellites. As described above, the reference signal generator 31 generates the signal 33 in situ (80) and detects the peak value by correlating the reference signal 33 with a digitized snapshot of the GPS signal (82).

As mentioned above, for each satellite, the digital signal processor (DSP) 30 searches multiple Doppler "bins" to determine a peak. In order to reduce the number of searched receivers, the digital signal processor (DSP) 30 first searches for one of the satellites, for example the one indicated overhead by the assisted server 10, with a shorter integration time. For this satellite and integration time, digital signal processor (DSP) 30 performs a search for each of the set of Doppler receivers, as described above. For example, the digital signal processor (DSP) 30 may start searching at an intermediate receiver and proceed outward until all receivers are searched (e.g., receiver frequency offset 0, offset -50 Hz, offset amount+50Hz, offset-100Hz, offset+100Hz, etc.). Each search will produce a set of outputs from the FFT-based matched filter operation. If any output exceeds a detection threshold (84), the digital signal processor (DSP) 30 will indicate a preliminary detection and store the detection results, such as current Doppler shift, time offset, etc. (88 ). When a detection is determined, the digital signal processor (DSP) 30 examines measurements of adjacent Doppler shifts to determine whether detections of these offsets were found and whether the current Doppler's signal strength is relative to the adjacent The Doppler is at a maximum to determine whether the detection is valid (90). If such a maximum is found, and the measurement of the output signal-to-noise ratio (SNR) is high enough to produce good pseudorange measurements, e.g., the SNR is above a threshold, then the digital signal processor (DSP) 30 terminates the Handling of a given satellite.

If no detection is found (no branch of 90), then the digital signal processor (DSP) 30 uses a longer integration time for the given satellite, and repeats the process until a peak is detected, or until the maximum integration time is reached (92).

Once a satellite is acquired, the digital signal processor (DSP) 30 uses the actual Doppler information of that satellite to narrow down the search space for subsequent satellites. Specifically, after capturing a satellite, the digital signal processor (DSP) 30 uses the Doppler frequency shift of the captured satellite as an initial frequency offset. This takes advantage of the fact that a particular source of error (eg local oscillator drift) may be processed identically for multiple satellite signals. The second satellite's signal is then searched for in a similar manner to the first satellite's signal, and if a detection is made with a suitable output signal-to-noise ratio (SNR), then processing for this satellite is terminated. If multiple satellites are detected, the Doppler search for other satellites can be initiated using the mean or weighted value of their Doppler shifts (weighted by the signal-to-noise ratio (SNR)). The digital signal processor (DSP) 30 continues this process until all satellites are searched (94) and a final position is calculated (96).

For example, if the initial search time is 50 ms and the Doppler shift determined for the satellite is 0, then the digital signal processor (DSP) may only need to search for three Doppler hypotheses (receivers), namely the current receiver and two adjacent receivers to achieve this result. Comparing this result with a search performed with 1 second integration time and 9 Dopplers, the reduction in search time is then equal to 1 s/50 ms x 9 bins/3 bins, which equates to a 60:1 reduction in the number of calculations. Also, even for weaker satellite signals, properly initializing the Doppler shift can save processing time substantially.

As a variation, the digital signal processor (DSP) 30 can search for a given integration time for all Dopplers and all satellites before increasing the integration time. This can be very advantageous when there is no a priori knowledge of which satellite signals are better (eg signals corresponding to higher satellites in the sky). Alternatively, once a detection is made for a given combination of satellite, Doppler and integration time, the integration time can be increased for that given satellite and Doppler until the signal-to-noise ratio (SNR) exceeds a threshold, indicating Sufficient pseudorange accuracy. Then, the digital signal processor (DSP) 30 can similarly analyze the adjacent Doppler of the satellite by changing the integration time. This approach may be advantageous because the integration time can be increased without requiring additional storage and without loss of previously processed data.

Various specific embodiments of the invention have been described. These and other specific examples are within the scope of the following claims.

Claims (70)

1. A method comprising: receiving capture assistance data from an assistance server; and A time offset is determined for a P-code pseudo-random code sequence associated with a signal from a satellite based on the acquisition assistance data. 2. The method of claim 1, further comprising determining a location of the mobile device based on the time offset. 3. The method of claim 1, wherein the acquisition assistance data includes start-of-week data indicating an initial time offset of the P-code sequence. 4. The method of claim 3, wherein determining a time offset comprises generating a reference pseudo-random code sequence in situ based on origin week-of-week information received from the secondary server. 5. The method according to claim 4, wherein the generation of a reference pseudo-random code sequence comprises generating the reference pseudo-random code sequence, in order to include the P code sequence starting at the initial time offset in the P code sequence Continuous pseudorandom code. 6. The method of claim 4, further comprising: digitizing said signal to generate a digital data stream; buffer a certain amount of digital data; and The buffered digital data is correlated with the in situ generated reference code sequence. 7. The method of claim 6, further comprising setting the amount of digital data buffered based on a time uncertainty value associated with origin week-of-week information received from said secondary server. 8. The method of claim 7, wherein said uncertainty value comprises a predetermined uncertainty value. 9. The method of claim 6, wherein the step of correlating the buffered digital data with the in situ generated reference code sequence includes synchronizing the digital data with the reference pseudorandom code sequence. 10. The method of claim 9, wherein synchronizing the digital data with the reference pseudo-random code sequence comprises: evaluating a first bit edge in the digital data stored in the memory; and Based on the estimated first bit edge and a time uncertainty value associated with the origin week information received from the secondary server, ignoring chips within the digital data and within the reference pseudo-random code sequence Several chips. 11. The method of claim 6, wherein the step of correlating the buffered digital data with the in-situ generated reference code sequence comprises dividing the reference code sequence and digital data into blocks of size N , wherein the block size N is less than the data bit period M associated with said signal plus a time uncertainty value associated with the received start cycle information. 12. The method of claim 6, wherein dividing the reference code sequence and the digital data into blocks comprises forming blocks from the reference code sequence having M chips and N-M zero padding. 13. The method of claim 6, wherein correlating the buffered digital data with the in situ generated reference code sequence comprises applying a matched filter operation to each of the blocks to obtain A correlation peak is generated having a time offset relative to the reference code sequence. 14. The method of claim 13, further comprising: sliding the reference code by N-M chips relative to the digital data of the signal when the correlation peak is below a peak threshold, where N represents the block size and M represents the bit period; and The matched filtering operation is repeated. 15. The method of claim 13 , wherein sliding the reference code relative to the digital data comprises repeatedly sliding the reference code and performing the correlation process until the reference code is shifted by chips The total number exceeds a time uncertainty value associated with the origin time of week information received from said secondary server. 16. The method of claim 1, further comprising: performing a Doppler search on Doppler hypotheses to determine a Doppler shift for the signal; and The Doppler shift is used as an initial frequency offset for acquiring signals from subsequent satellites. 17. The method of claim 16, further comprising: performing a Doppler search on each of a set of Doppler bins using a first integration time to generate a set of outputs; and When no valid peaks are detected from the set of outputs, the integration time is increased and the process repeated. 18. The method of claim 17, further comprising: indicating a preliminary detection when any of said outputs exceeds a detection threshold; storing a current Doppler shift associated with said preliminary detection; and The preliminary detection is verified by detecting the output of neighboring Doppler receivers of the current Doppler receiver. 19. The method of claim 18, wherein verifying the preliminary detection includes examining the output of the adjacent Doppler shift to determine the relative signal strength of the current Doppler shift relative to the adjacent Doppler shift. Whether the shift is the maximum. 20. The method of claim 1, wherein said acquisition assistance data includes identification of satellites in view from a mobile device, origin weekly data for said satellites, Doppler information for said satellites, frequency synchronization information and the position information of the satellite. 21. The method of claim 1, further comprising: communicating the time offset to the secondary server; and Location data is received from the auxiliary server. 22. The method of claim 1, further comprising calculating a position based on the time offset. 23. A method comprising: Acquisition assistance data is received from an assistance server, wherein the acquisition assistance data identifies a group of satellites and includes origin week data indicating the initial time offset of the P-code pseudo-random code sequence for each of the satellites. displacement; receiving signals from said satellites; digitizing said signal to generate a digital data stream; buffer a certain amount of numeric data; generating a reference P-code pseudo-random code sequence for the satellite in situ based on the individual initial time offset of the satellite; correlating said digital data with said reference code sequence to determine correlation peaks with individual time offsets; and A location is determined for a mobile device based on the time offset. 24. The method of claim 23, wherein correlating the digital data with the in situ generated reference code sequence comprises: evaluating a first bit edge in the digital data stored in the memory; and Based on the estimated first bit edge and a time uncertainty value associated with the origin week information received from the secondary server, ignoring chips within the digital data and each of the reference pseudo-random code sequences A number of chips within a sequence. 25. The method of claim 22, wherein the step of correlating the buffered digital data with the in-situ generated reference code sequence comprises dividing the reference code sequence and digital data into blocks of size N , wherein the block size N is less than the data bit period M associated with the signal plus a time uncertainty value associated with the received start cycle information, and wherein the block of the reference code sequence has M chips and N-M zero padding. 26. The method of claim 23, wherein the step of correlating said buffered digital data with said in situ generated reference code sequence comprises performing a matched filter operation on each of said blocks to produce a correlation peak having a time offset relative to said reference code sequence. 27. The method of claim 26, further comprising: sliding the reference code for a given satellite relative to the digital data of said signal by N-M chips, where N represents the block size and M represents the bit period, when the correlation peak value for a given satellite is below a peak threshold; and The matched filter operation is repeated for the given satellite. 28. The method of claim 27 , wherein sliding the reference code relative to the digital data comprises repeatedly sliding the reference code and performing the matched filter operation until the reference code is shifted by a chip The total number exceeds a time uncertainty value associated with the origin time of week information received from said secondary server. 29. The method of claim 23, further comprising: performing a Doppler search on each of a set of Doppler hypotheses using a first integration time to produce a set of outputs for a first of said signals; and indicating a preliminary detection when any one of said outputs exceeds a detection threshold; storing a current Doppler shift associated with said preliminary detection; verifying the preliminary detection by detecting the output of the adjacent Doppler shift to determine whether the signal strength of the current Doppler shift is maximum relative to the adjacent Doppler shift; and The Doppler shift is used as an initial frequency offset for acquiring signals from subsequent satellites. 30. The method of claim 1, further comprising: passing the time offset to the server; and Location data is received from the server. 31. The method of claim 23, further comprising: performing a Doppler search on a plurality of Doppler hypotheses, determining a Doppler shift amount for a first of said signals; and The Doppler frequency shift is used as an initial frequency shift to perform a Doppler search on at least one of the other satellites. 32. A method comprising: receiving capture assistance data from the server; and A P-code signal from a satellite is directly acquired based on the acquisition assistance data without first acquiring a C-code signal from the satellite. 33. The method of claim 32, wherein the acquisition assistance data includes start-of-week data indicating an initial time offset of the P-code signal. 34. The method of claim 33, further comprising generating a reference pseudo-random code sequence in situ based on origin week data received from said auxiliary server. 35. The method of claim 34, wherein directly capturing the P-code signal comprises correlating the P-code signal with the in situ generated reference code sequence based on the origin cycle data. 36. A system comprising: an assistance server for tracking a signal from a Global Positioning System (GPS) satellite and generating acquisition assistance information from said signal; and A mobile device is used to receive acquisition assistance data from the assistance server, and acquire a P-code signal from the satellite according to the acquisition assistance data. 37. The system of claim 36, wherein said acquisition assistance data includes said satellite origin weekly time data indicating an initial time offset in a P-code pseudo-random code sequence. 38. The system of claim 37, wherein said mobile device includes a reference signal generator for in situ generating a reference pseudorandom code sequence based on said origin cycle data. 39. The system of claim 38, wherein the mobile unit correlates the signal with the reference pseudorandom code sequence based on the acquisition assist signal to determine a relative value of the signal to the code sequence. Actual time offset. 40. The system of claim 39, wherein said mobile device stores a time uncertainty value associated with said origin week information, and aligns said signal with said reference based on said time uncertainty value. Correlation of pseudorandom codes. 41. The system of claim 36, wherein said auxiliary server is coupled to a base station of a cellular communication system. 42. The system of claim 36, wherein said assistance server tracks a C/A code signal and generates said assistance information from said C/A code signal. 43. The system of claim 36, wherein said assistance server tracks a P-code signal and generates said assistance information from said P-code signal. 44. A device comprising: an antenna for receiving a signal from a satellite; a wireless modem for receiving capture assistance data from an assistance server; a reference signal generator, configured to generate a reference pseudo-random code sequence based on the acquisition assistance data; and A processor for determining a time offset for a P-code pseudo-random code sequence associated with said signal based on said acquisition assistance data. 45. The apparatus of claim 44, wherein the processor determines a location of a mobile device based on the time offset. 46. The apparatus of claim 44, wherein the acquisition assistance data includes start-of-week data indicating an initial time offset of the P-code sequence. 47. The device of claim 44, further comprising: an analog-to-digital converter for generating a digital data stream from said GPS signal; and A medium for buffering a certain amount of said digital data. 48. The apparatus of claim 47, wherein said processor sets the amount of buffered digital data based on a time uncertainty value associated with origin time of week information received from said secondary server. 49. The apparatus of claim 47, wherein said processor correlates said buffered digital data with said in situ generated reference code sequence to determine a correlation peak for said signal. 50. The apparatus of claim 47, wherein the processor synchronizes the digital data with the reference pseudorandom code sequence. 51. The apparatus of claim 50, wherein the processor synchronizes the digital data with the reference pseudo-random code by evaluating a first bit edge in the digital data stored in the memory, and according to the Estimated first bit edge and a time uncertainty value associated with the start-of-week signal received from said secondary server, ignoring chips in said digital data and codes in said reference pseudo-random code sequence piece. 52. The apparatus according to claim 47, wherein said processor divides said reference code sequence and said digital data into blocks of size N, wherein said block size N is smaller than A data bit period M is added to a time uncertainty value associated with the received start-of-week information, and said buffered digital data is correlated with said locally generated reference code sequence to determine a correlation peak. 53. The apparatus of claim 52, wherein the processor forms a block having M chips and N-M zero padding from the reference code sequence. 54. The apparatus according to claim 52, wherein said processor slides said reference code relative to digital data of said signal by N-M chips when said correlation peak is below a peak threshold, where N represents The block size, M representing the bit period; and repeating the matched filtering operation. 55. The apparatus of claim 54, wherein the processor repeatedly slides the reference code and performs the correlation process until the total number of chips moved by the reference code exceeds the starting point received from the auxiliary server A numerical value of time uncertainty associated with the time of week information. 56. The apparatus of claim 44 , wherein the processor performs a Doppler search on several Doppler hypotheses to determine a Doppler shift for the signal, and shifts the Doppler shift Used as an initial frequency offset to acquire signals from subsequent satellites. 57. The apparatus of claim 56, wherein said processor performs a Doppler search on each of a set of Doppler receivers using a first integration time to produce a set of outputs, and when based on the When the group output does not detect a valid peak, increase the integration time and repeat the process. 58. The apparatus of claim 57, wherein said processor indicates a preliminary detection when any one of said outputs is greater than a detection threshold, stores a current Doppler shift associated with said preliminary detection, and The validity of the preliminary detection is verified by examining the output of adjacent Doppler frequencies. 59. The apparatus of claim 44, wherein said acquisition assistance data comprises an identification of a satellite in view of a mobile device, origin weekly data for said satellite, Doppler information for said satellite, frequency synchronization information, and The location information of the satellite. 60. The device of claim 44, wherein said processor comprises a digital signal processor (DSP). 61. A computer readable medium comprising instructions for causing a programmable processor to: receiving capture assistance data from an assistance server; and A time offset is determined for a P-code pseudo-random code sequence associated with a signal from a satellite based on the acquisition assistance data. 62. The computer-readable medium of claim 61, wherein the instructions cause the processor to determine a location of a mobile device based on the time offset. 63. The computer-readable medium of claim 61, wherein the capture assistance data includes start-of-week data indicating an initial time offset of a P-code sequence. 64. The computer-readable medium of claim 63, wherein the instructions cause the processor to generate a reference pseudorandom code sequence in-situ based on origin week data received from the auxiliary server. 65. The computer-readable medium of claim 63, wherein the instructions cause the processor to set a value representing The amount of buffered digital data for the signal. 66. The computer-readable medium of claim 64, wherein the instructions cause the processor to: evaluating a first bit edge in the digital data stored in the memory; and Based on the estimated first bit edge and a time uncertainty value associated with the origin week information received from the secondary server, ignoring chips within the digital data and within the reference pseudo-random code sequence Several chips. 67. The computer-readable medium of claim 64 , wherein the instructions cause the processor to divide the reference code sequence and the digital data into blocks of size N, wherein the block size N less than a data bit period M associated with said signal plus a time uncertainty value associated with the received origin cycle information and correlating said buffered digital data with said locally generated reference code sequence to Identify a correlation peak. 68. The computer-readable medium of claim 67, wherein the instructions cause the processor to form a block having M chips and N-M zero padding from the reference code sequence. 69. The computer-readable medium of claim 67, the instructions cause the processor to slide the reference code relative to the digital data of the signal by N-M chips, where N represents the block size and M represents the bit period; and repeating the matched filtering operation. 70. The computer-readable medium of claim 69 , the instructions cause the processor to repeatedly slide the reference code and perform the correlation process until the total number of slide chips of the reference code exceeds the number of chips received from the reference code. A time uncertainty value related to the starting week information of the auxiliary server.

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