Method and apparatus for satellite positioning system based time measurement

A method and apparatus for measuring time related to satellite data messages which are used with satellite positioning systems (SPS). In one method, a first record of at least a portion of a satellite data message is received at an entity, which is typically a basestation. The first record is compared with a second record of the satellite data message, where the first record and the second record overlap at least partially in time. Then a time is determined from this comparison, and this time indicates when the first record (or the source from which the first record was obtained) was received at a remote entity which is typically a mobile SPS receiver. Various other methods of the invention are described and various apparatuses of the invention are also described. The methods and apparatuses measure time of day using SPS signals without reading the satellite data messages which are transmitted as data within these signals. The methods and apparatuses are suitable for situations in which the received signal level is too weak to allow reading of the satellite data messages.

BACKGROUND OF THE INVENTION 
This invention relates to systems which utilize received signals from 
satellite positioning systems (SPS) to locate themselves or to determine 
time-of-day. 
SPS receivers such as GPS (Global Positioning System) receivers normally 
determine their position by computing relative times of arrival of signals 
transmitted simultaneously from a multiplicity of satellites such as GPS 
(or NAVSTAR) satellites. These satellites transmit, as part of their 
satellite data message, both satellite positioning data as well as data on 
clock timing, so-called "ephemeris" data. In addition they transmit 
time-of-week (TOW) information that allows the receiver to determine 
unambiguously local time. Each received GPS signal (in C/A mode) is 
constructed from a high rate (1.023 MHz) repetitive pseudorandom (PN) 
pattern of 1023 symbols, commonly called "chips." Further imposed on this 
pattern is low rate data at a 50 Hz rate. This data is the source of the 
above mentioned time-of-week information. The process of searching for and 
acquiring GPS signals, reading the ephemeris data and other data for a 
multiplicity of satellites and computing the location of the receiver (and 
accurate time-of day) from this data is time consuming, often requiring 
several minutes of time. In many cases, this lengthy processing time is 
unacceptable and, furthermore, greatly limits battery life in 
micro-miniaturized portable applications. 
In addition, in many situations, where there is blockage of the satellite 
signals, the received signal level from the GPS satellites is too low to 
demodulate and read the satellite data signals without error. Such 
situations may arise in personal tracking and other highly mobile 
applications. Under these situations it is possible for a receiver to 
still acquire and track the GPS signals. However, performing location and 
unambiguous time measurement without such data requires alternative 
methods. 
Tracking the GPS signals without reading the data messages may result in 1 
millisecond ambiguities in time, as explained below. Such ambiguities are 
normally resolved in a conventional GPS receiver by reading the satellite 
data message, as previously described. At very low received signal levels, 
the pseudorandom pattern may be tracked, or otherwise used to provide 
ambiguous system timing by processing many repetitions of this signal 
(e.g. 1000 repetitions over 1 second). However, unless the signal-to-noise 
ratio measured over one data period (20 milliseconds) is above about 12 
dB, there will be many errors present when attempting to demodulate this 
signal. The current invention provides an alternative approach for 
resolving ambiguities in time when such reading is impossible or 
impractical. 
SUMMARY OF THE INVENTION 
The present invention provides methods and apparatuses for measuring time 
related to satellite data messages which are used with satellite position 
systems, such as GPS or Glonass. A method in one embodiment comprises the 
steps of: (1) receiving, at an entity, a first record of at least a 
portion of a satellite data message; (2) comparing the first record with a 
second record of the satellite data message, where the first record and 
the second record overlap at least partially in time; and (3) determining 
a time from the comparing step, where the time indicates when the first 
record (e.g., the source of the first record) was received at a remote 
entity. In one example of this embodiment, the remote entity is a mobile 
SPS receiver and the entity is a basestation which communicates with the 
mobile SPS receiver through a wireless (and perhaps also wired) link. A 
method of the present invention may be performed exclusively at the 
basestation. 
An embodiment of the present invention for establishing receiver timing is 
for the receiver to form an estimate of a portion of the satellite data 
message and transmit this estimate to the basestation. At the basestation 
this estimate is compared to a record of the satellite data message 
received from another GPS receiver or source of GPS information. This 
record is assumed to be error free. This comparison then determines which 
portion of the basestation's message most closely matches the data 
transmitted by the remote unit. Since the basestation has read the 
satellite data message without error it can associate each data bit of 
that message with an absolute time stamp, as seen by the transmitting 
satellite. Hence the comparison results in the basestation assigning an 
appropriate time to the estimated data transmitted by the remote. This 
time information may be transmitted back to the remote, if desired. 
A variation on the above approach is to have the basestation send a clean 
record of the satellite data message to the remote plus the absolute time 
associated with the beginning of this message. In this case the remote 
compares this record to the estimate of this data which it forms by 
processing a GPS signal which it receives. This comparison will provide 
the offset in time between the two records and thereby establish an 
absolute time for the locally collected data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Various methods and apparatuses for measuring time related to satellite 
data messages for use with satellite positioning systems are described 
below. The discussion of the invention focuses upon the United States 
Global Positioning Satellite (GPS) system. However, it should be evident 
that these methods are equally applicable to similar satellite positioning 
systems, such as the Russian Glonass system. Moreover, it will be 
appreciated that the teachings of the present invention are equally 
applicable to positioning systems which utilize pseudolites or a 
combination of satellites and pseudolites. Moreover, the various 
architectures for basestations and mobile SPS receivers are provided for 
illustrative purposes rather than to be construed as limitations of the 
present invention. 
FIG. 2 shows a generalized method of the present invention which may be 
utilized with a mobile SPS receiver which is combined with a mobile 
communication receiver and transmitter, such as that shown in FIG. 1A. The 
mobile GPS receiver 100 shown in FIG. 1A samples the satellite data 
message, such as ephemeris, and creates a record of the message in step 
201. Next in this method 200, the remote or mobile GPS receiver transmits 
this record to a basestation, such as the basestation shown in FIGS. 5A or 
5B in step 203. This record is typically some representation of the 
message received by the mobile SPS receiver. In step 205, the basestation 
compares the record transmitted from the mobile SPS receiver to another 
record which may be considered a reference record of the satellite data 
message. This reference record has associated time values wherein various 
segments of the satellite data message have specified "reference" times 
associated therewith. In step 207, the basestation determines the time of 
sampling by the mobile GPS receiver of the satellite data message. This 
determination is based upon a time value which is associated with the 
reference record and this determination will indicate the time when the 
record or the source of the record was received by the mobile GPS 
receiver. 
FIG. 7 illustrates in a simplified way the comparison operation in step 205 
of FIG. 2. In particular, FIG. 7 shows the attempted comparison between 
the mobile receiver's record and the basestation's reference record shown 
respectively as records 491 and 495. The horizontal axes for both records 
indicate time. There is a portion 493 of the mobile's record which 
represents the portion transmitted to the basestation for purposes of 
comparison. Typically, the basestation will have a corresponding portion 
497 which will overlap at least partially in time with the record received 
from the mobile receiver. In FIG. 7, this overlap is complete in that the 
reference record provides the satellite data message throughout the entire 
interval of the mobile receiver's record. However, this is only one 
example and the overlap may be such that only a portion of the mobile 
receiver's record overlaps with the reference record from the basestation. 
FIG. 3 illustrates in further detail a method 220 of the present invention 
for measuring time related to satellite data messages for use with a 
satellite positioning system. The mobile or remote GPS receiver acquires 
in step 221 GPS signals and determines pseudoranges from those acquired 
GPS signals. In step 223, the mobile GPS receiver removes the PN data and 
creates a record of the satellite data message from the acquired GPS 
signals used to create or determine the pseudoranges. This record is 
typically some representation of the ephemeris data in the acquired GPS 
signals and typically represents an estimate of the data. In step 225, the 
mobile GPS receiver transmits the record and the determined pseudoranges 
to a basestation, such as the basestation shown in FIGS. 5A or 5B. 
In step 227, the basestation performs a cross-correlation of the record 
transmitted from the mobile GPS receiver to a reference record of 
ephemeris of the satellites. This reference record typically includes an 
accurate time stamp associated with the data in the reference record (e.g. 
each bit of data in the reference record has an associated time value or 
"stamp"), and it is this time stamp which will be used to determine the 
time of receipt by the mobile GPS receiver of the originally acquired GPS 
signals. In step 229, the basestation determines from the 
cross-correlation operation the time of acquiring by the remote GPS 
receiver of the acquired GPS signals. The basestation then uses in step 
231 the time of the acquiring by the remote GPS receiver of the GPS 
signals and uses the determined pseudoranges to determine a position 
information, which may be a latitude and longitude of the remote/mobile 
GPS receiver. The basestation, in step 233, may communicate this position 
information of the remote GPS receiver to another entity, such as a 
computer system coupled through a network, such as the Internet, or an 
intranet, to the basestation. This will be described further below in 
conjunction with FIGS. 5B and 6. 
Below we explain in further detail several methods for estimating the 
satellite data at the remote SPS receiver. The methods fall into two 
classes: one which performs differential demodulation and soft decision of 
the data (after PN is removed) and the other which samples the raw I/Q 
data after PN is removed. The first method is shown diagramatically in 
FIGS. 4A and 4B and the second is indicated in FIGS. 8A and 8B. Note that 
the object here is to determine the difference in times of arrival between 
the reception of the signal at the remote and at the basestation. Since 
the basestation is presumed to have the precise time, this difference in 
time will determine the precise time of reception of data at the remote. 
As explained below, the two approaches differ by the amount of processing 
that must be done by the remote (mobile SPS receiver) and the amount of 
information that must be transferred from the remote to the basestation 
over a communication link. In essence, there is a tradeoff in the 
processing burden at the remote versus the quantity of data that must be 
passed over the link. 
Before describing the details of the procedures in FIGS. 4A and 4B and 
FIGS. 8A and 8B, a review of conventional GPS operation is provided to 
provide a contrast to the methods of this invention. A simplified version 
of a conventional GPS receiver 601 is shown in FIG. 9. 
This conventional receiver 601 receives digitized I/Q input signals 603 
from a GPS RF front end (e.g. downconverter and digitizer) and mixes in 
mixer 605 these input signals 603 with oscillator signals from digital 
oscillator 607. The output from mixer 605 is then mixed in mixer 609 with 
the output of a PN generator 611 which is controlled for chip advance by 
signals 619 from the microcontroller 617. The microcontroller 617 also 
controls the digital oscillator 607 in order to translate the signal to 
near baseband. 
In the operation of a conventional GPS receiver, a signal received from a 
GPS satellite in the absence of noise has the form 
EQU y(t)=A P(t) D(t) exp(j2.pi.f.sub.0 t+.phi.), (eq. 1) 
where P(t) is a 1023 length repeating binary phase-shift keyed pseudorandom 
sequence (chip rate 1.023 Mchips/sec) having values .+-.1 and D(t) is a 50 
baud data signal aligned with the beginning of the PN framing, again 
assuming values .+-.1. After translating the signal to near baseband (e.g. 
by the mixer 605), the PN code is normally removed by using a correlator 
(which may be considered to include elements 609, 611, 613, 615 and 617 of 
FIG. 9). This device locally reproduces the code P(t) (for the given 
satellite) and determines the relative phasing of the received PN with the 
locally generated PN. When phase aligned, the correlator multiplies this 
signal by the locally generated reference resulting in the signal form: 
EQU P(t)x y(t)=P(t)A P(t) D(t) exp(j2.pi.f.sub.0 t+.phi.)=A D(t) 
exp(j2.pi.f.sub.0 t+.phi.) (eq. 2) 
At this point the signal is narrowband filtered (e.g. in filter 613) to 
remove noise outside the band of the data signal D(t). The sample rate may 
then be reduced to a small multiple of the data rate by sampler 615. Thus, 
the time variable t in the right hand side of equation (2) takes on values 
of the form mT/K, m=0,1,2, . . . where K is a small integer (e.g. 2) and T 
is the bit period. 
The samples of data at this point then are used for performing the PN 
tracking operations, carrier tracking and data demodulation. This is 
normally done by software algorithms in a microcontroller, but may 
alternatively be done in hardware. In FIG. 9 the microcontroller 617 feeds 
back correction signals 621 and 619 to the digital oscillator and PN 
generator respectively in order to keep the locally generated carrier 
signals and PN signals in phase synchronism with the received signal. This 
operation is normally done in parallel for a multiplicity of 
simultaneously received GPS signals (typically 4 or more GPS signals from 
4 or more GPS satellites). 
Now, in some circumstances (e.g. a low signal to noise ratio ("SNR")) the 
GPS signal may be so weak that the data D(t) cannot be extracted with high 
reliability. As described previously, a conventional GPS receiver needs to 
read this data in order to determine a universal time as well as provide a 
position fix. An alternative approach, provided by the present invention, 
in this low SNR situation is for the remote to work together with a 
basestation, the latter of which has access to this satellite data 
information. The remote sends information to the basestation that allows 
it to compute the time associated with the original reception of such data 
by the remote. An alternative configuration exists in which the 
basestation sends information to the remote in order for it to compute 
this time of reception. We mainly consider the first case. 
It should be noted that time coordination between the base and the remote 
may, in some cases, be achieved by sending accurate timing signals (e.g. 
pulses or specialized waveforms) across a communication link and 
accounting for any transit time by either a priori knowledge of the link 
latencies or measuring a round trip delay (assuming a two-way symmetric 
link). However, there are many circumstances where this approach is 
impractical or impossible. For example, many links include packetized 
protocols in which latencies may be variable from one transmission to 
another and span many seconds. 
The approach of this invention is for the remote to form an estimate of a 
portion of the data sequence D(t) or an estimate of a processed version of 
it, and transmit this data to the basestation. This data sequence can be 
compared against a similar but much higher fidelity signal generated at 
the basestation. The two sequences are slid in time relative to one 
another until the best match occurs, according to a given metric, such as 
minimum mean-squared error. This "correlation" procedure is very similar 
to that used by GPS receivers in order to synchronize to the PN spreading 
sequences; here, however, the operation is done on much lower rate data 
signals and, furthermore, the pattern of such signals is constantly 
changing and may be unknown a priori. 
Since the basestation presumably knows the precise time associated with 
each element of the message, it may utilize this knowledge plus the 
aforementioned comparison to ascertain the original time associated with 
the signal received at the remote. 
Thus, the main problem lies in the estimation at the remote of the data 
sequence D(t) or a derivative thereof. 
One particular embodiment of the invention, shown in FIGS. 8A and 8B, for 
estimating the data sequence is to simply sample and store a record of the 
signal after the PN is removed, e.g. as shown in equation (2). Here the 
signal is assumed to be sampled at a small multiple of the data rate; a 
100 sample per second rate may be suitable for this purpose. Note that 
both I and Q tributaries must be sampled. Also, a record of length of 
around 25 or more data symbols (0.5 seconds) should be taken in order to 
make it likely that the data pattern is unique for the purpose of 
identification at the basestation. Note from equation (2) that a small 
residual carrier f.sub.0 and unknown carrier phase .phi. may still be 
present. It is highly beneficial that the carrier frequency be known to an 
accuracy better than .+-.one-half the sample rate of the data signal; 
otherwise the carrier may effectively introduce phase reversals of the 
data signal and so corrupt the data. 
FIG. 8A illustrates the method performed in the mobile GPS receiver 
according to this particular embodiment. The receiver acquires the first 
(or next if not the first) PN code for the particular GPS signal and 
removes the PN code from the signal in step 503. Then, the receiver 
performs an accurate estimate of carrier frequency in step 505 and them 
removes the carrier from the inputted signal in step 507. Then the I and Q 
data is sampled and quantized in steps 509 and 511, and this quantized 
result is saved as a record of the corresponding satellite data message 
and then transmitted to the basestation (perhaps also with the 
corresponding pseudorange from the GPS satellite transmitting the 
particular GPS signal). In step 513, the receiver determines whether the 
receiver has performed steps 503, 505, 507, 509, and 511 (and hence 
determined a record) for all satellites of interest (e.g. all satellites 
in view of the mobile GPS receiver or at least four satellites in view). 
If a record of the satellite data message has been determined from each 
satellite of interest, then the GPS receiver transmits (in step 515) the 
records with an elapsed time tag to the basestation. The elapsed time tag 
may be used by the basestation to estimate and/or select the "reference" 
record at the basestation which will be compared (e.g. by correlation) to 
the record. If the receiver has not determined a record from each 
satellite of interest, then the mobile GPS receiver proceeds from step 513 
back to step 503, and repeats steps 503, 505, 507, 509, and 511 in order 
to determine a record of the satellite data message received from the next 
satellite of interest. An example of a GPS receiver (and communication 
receiver/transmitter) which may perform the method of FIG. 8A is shown in 
FIG. 1A, and this GPS receiver is described in further detail below. 
The basestation when receiving this information can refine the frequency 
estimate and remove the carrier and then determine relative timing by 
cross-correlating this data against similar data extracted from a high 
fidelity signal received from a GPS receiver with a clear view of the sky 
(or received from some other source of high fidelity GPS signals, such as 
from the Internet or from a GPS ground control station). 
FIG. 8B shows a method 521 performed by the basestation upon receiving the 
record of the satellite data message transmitted from the remote. In step 
523, the basestation receives a record corresponding to a satellite data 
message, and then in step 525 phaselocks to the record and removes any 
residual phase error/roll in step 525. Contemporaneously with steps 523 
and 525, the basestation will typically be tracking and demodulating GPS 
data messages and applying time tags to these data messages in order to 
provide an accurate time value in association with various intervals of 
the satellite data message which has been demodulated. This is shown in 
step 527. Typically, the basestation will be performing the tracking and 
demodulation of satellite data messages on an ongoing basis such that a 
continuous reference record is being generated and a running sample of 
this "reference" record is stored at the basestation. It will be 
appreciated that this running record of the reference may be maintained 
for a time period of up to perhaps 10 to 30 minutes prior to the current 
time. That is, the basestation may maintain a copy of the reference record 
for as long as 30 minutes before discarding the oldest portion of the 
reference record and in effect replacing it with the newest portion in 
time. 
In step 529, the basestation correlates the base's reference record against 
the reference record from the remote for the first (or the next, if not 
the first) satellite data message from the first (or next) satellite. This 
correlation is effectively a comparison between the two records in order 
to match the patterns such that the basestation may determine the time 
accurately when the remote received the record (which is typically, in 
effect, the time when the source of that record was received by the remote 
since the record is itself an estimate of the source). It will be 
appreciated that as used to describe the present invention, the time of 
receipt of the record by the remote effectively is the time of receipt of 
the source of the record at the remote. At step 531, the basestation finds 
and interpolates the peak location which indicates the time at which the 
remote received the record for the current satellite and its corresponding 
satellite data message. In step 533, the basestation determines whether 
all times associated with all corresponding records have been determined 
for all satellites of interest. If not, the processing proceeds back to 
step 529 and the process is repeated for each record received from the 
remote. If all records have been processed in order to determine 
corresponding times for all satellites of interest and their corresponding 
satellite data messages, then processing proceeds from step 533 to step 
535, wherein the times are compared for the different satellites of 
interest. In step 537, majority logic is used to discard erroneous or 
ambiguous data and then in step 539 it is determined whether all data is 
ambiguous. If all data is ambiguous, the basestation commands the mobile 
GPS receiver to take further data by transmitting a command to the 
communication receiver in the mobile GPS unit. If all data is not 
ambiguous, then in step 543 the basestation performs a weighted average of 
the times to determine an average time of receipt of the satellite data 
messages at the mobile GPS receiver. It will be appreciated that in 
certain circumstances such as those when a sample of GPS signals is 
digitized and stored in a digital memory for further processing that there 
will be in effect one time of receipt as long as that sample is of a short 
duration. In other instances, such as those involving serial correlation 
where one satellite at a time is processed and signals from that satellite 
are acquired and a record is made of that signal and then next in time 
another satellite signal is acquired, in this case, there may be multiple 
times of receipt and the basestation may determine each of those times and 
use them in the manner described below. 
It will be appreciated that the time of receipt of the record in 
conjunction with the pseudoranges which are typically transmitted from the 
mobile GPS receiver, at least in some embodiments, will be used by the 
basestation to determine a position information, such as a latitude and 
longitude and/or an altitude of the mobile GPS receiver. 
In some cases it may be difficult to determine the residual carrier 
frequency (in step 525) to sufficient accuracy and then a differential 
demodulation of the data from the remote and the locally received data may 
precede the cross-correlation. This differential demodulation is further 
described below in conjunction with FIGS. 4A and 4B. 
If the communication link capacity (between the mobile GPS receiver and the 
basestation) is low, it is advantageous for the remote to perform 
additional processing on the despread signal (the signal with PN removed). 
A good approach toward this end, as illustrated in FIGS. 4A and 4B, is for 
the remote to differentially detect this signal by performing a 
delay-multiply operation on the data signal, with delay set to a bit 
period (20 milliseconds) or a multiple thereof. Thus, if the baseband 
signal of equation (2) is represented as 
EQU z(t)=A D(t) exp(j2.pi.f.sub.0 t+.phi.) (eq. 3) 
then the appropriate operation would be: 
EQU z(t) z(t-T)*=A.sup.2 D(t) D(t-T) exp(j2.pi.f.sub.0 T)=A.sup.2 D.sub.1 (t) 
exp(j2.pi.f.sub.0 T) (eq. 4) 
where the asterisk represents complex conjugate, T is the bit period (20 
msec) and D.sub.1 (t) is a new 50 baud sequence formed by differentially 
decoding the original data sequence (e.g. mapping a transition to a-1 and 
no transition to a+1). Now if the carrier frequency error is small 
compared to the reciprocal of the symbol period, then the latter 
exponential term has a real component that dominates the imaginary 
component and only the real component may be retained yielding the result 
A.sup.2 D.sub.1 (t). Thus, the operation of equation (4) produces a real 
signal stream instead of the complex signal stream of the method shown in 
FIG. 8A. This, by itself, halves the required transmission message length 
when the record is transmitted across the communication link. Since the 
signal A.sup.2 D.sub.1 (t) is at baseband, it may be sampled at a somewhat 
smaller rate than that of the method shown in FIG. 8A. It is possible, 
also, to retain only the sign of this data, thereby reducing the amount of 
data to be transmitted. However, this approach will reduce the ability of 
the basestation to resolve time much better than one symbol period (20 
msec). Here we should note that the PN code repeats at a 1 msec interval 
and hence will not be useful by itself for further resolving this 
measurement error. 
FIG. 4A illustrates the processing steps performed in the mobile GPS 
receiver, and FIG. 4B illustrates the processing steps performed at the 
basestation according to this particular embodiment of the present 
invention. The mobile GPS receiver receives in step 301 a request for 
position information from a basestation. It will be appreciated that in a 
typical embodiment, this reception will occur by a communication receiver 
such as that shown within the mobile GPS receiver 100 in FIG. 1A. In 
response to that request for position information, the mobile GPS receiver 
in step 303 acquires the first (or the next, if not the first) PN code 
from a GPS signal and removes that PN code from the received GPS signal. 
In step 305, the remote performs an accurate estimate of the carrier 
frequency; the accuracy of this estimate should be better than the sample 
rate of the GPS data message, which is typically 100 Hz in the case of 50 
baud GPS data. Step 305 may be performed by using conventional frequency 
measurement systems in GPS receivers; these frequency measurement systems 
typically use carrier tracking loops which often include phaselock loops 
to extract the carrier and then a frequency measurement circuit or 
alternatively, a frequency tracking loop with a phaselock loop. In step 
307, the carrier frequency is removed by the mobile GPS receiver from the 
remaining signal, leaving the 50 baud data. Then in step 309, the 
remaining data is differentially detected by sampling the data at 
typically twice the rate of the data itself. It will be appreciated that 
rather than differentially detect the data as in step 309, the remote GPS 
receiver may transmit the data itself to the basestation and allow the 
basestation to perform the differential detection and quantization steps 
of steps 309 and 311. The mobile GPS receiver continues, in step 311, by 
quantizing and storing the result which is a record of the satellite data 
message typically having a duration in time of from one-half to one 
second. Then in step 313, the mobile GPS receiver determines if a record 
of satellite data message has been created for each satellite of interest, 
which may be all satellites in view or at least four satellites in view. 
If a record has not been created for each satellite of interest and its 
corresponding satellite data message, then processing proceeds from step 
313 back to step 303 and this loop continues until a record has been 
created for each of the satellite data messages for each satellite of 
interest. If all records for all satellites of interest have been 
determined and created, then processing proceeds from step 313 to step 315 
in which the mobile GPS receiver transmits through its communication 
transmitter the records for all satellites of interest with a coarse 
(elapsed) time tag which is used by the basestation in the manner 
described above. 
The basestation receives these records from the mobile GPS receiver in step 
327 shown in FIG. 4B. Contemporaneously with the operation of the mobile 
GPS receiver, the basestation is typically tracking and demodulating GPS 
data messages and applying time tags to those data messages in order to in 
effect time stamp these data messages; this is performed in step 321 as 
shown in FIG. 4B. Then in step 323, the basestation differentially decodes 
the data to provide the base's data which will be used in the correlation 
operation in step 325. The received data from the mobile GPS receiver will 
typically be stored for the correlation operation and compared against the 
stored differentially decoded data from step 323. In step 325, the 
basestation correlates the base's data against the record from the mobile 
GPS receiver for the first (or the next, if not the first) satellite. In 
step 327, the basestation finds and interpolates the peak location which 
indicates the time of arrival at the mobile receiver of the satellite data 
message from the current satellite being processed. In step 329, the 
basestation determines if correlation has been performed for all records 
received from the mobile receiver. If not, then processing proceeds back 
to step 325 in which the next record for the next satellite data message 
is processed in steps 325 and 327. If in step 329, it has been determined 
that correlation has been performed for all records received from the 
mobile GPS receiver, then in step 331, a comparison is made between the 
determined times for different satellites of interest. In step 333, the 
basestation uses majority logic to discard erroneous or ambiguous data. 
Then in step 335, the basestation determines if all data is ambiguous or 
erroneous. If so, the basestation commands the mobile receiver in step 337 
to acquire more data and the entire process will be repeated starting from 
the method shown in FIG. 4A and continuing to the method shown in FIG. 4B. 
If all data is not ambiguous as determined in step 335, then the 
basestation performs a weighted average of the times in step 339 and uses 
this weighted average with the pseudoranges transmitted from the mobile 
GPS receiver, at least in some embodiments, in order to determine a 
position information of the mobile GPS receiver. 
In order to illustrate the processing steps just described a live GPS 
signal was sampled, collected into a record, despread and sampled at a 
rate of 4 samples per symbol period. FIG. 10A shows a 1 second record of 
the real portion of the despread waveform with carrier partially removed. 
The symbol pattern is evident, but a small residual carrier offset of 
about 1 Hz is obviously still present. FIG. 10B shows the signal 
differentially detected by multiplying it by a conjugated and delayed 
version of itself with delay equal to 20 milliseconds. The symbol pattern 
is clearly evident. FIG. 10C shows the ideal data signal and FIG. 10D 
shows the cross-correlation of the ideal signal (e.g. produced at the 
basestation) and the signal of 10B. Note the glitches in 10B that result 
from sampling effects and the nonideal nature of the signal due to noise, 
etc. 
FIG. 11A shows the demodulated data when noise was added to the signal so 
that the SNR of the demodulated signal is approximately 0 dB. This models 
the situation when the received GPS signal is reduced in power by over 15 
dB relative to its nominal level, e.g. by blockage conditions. FIG. 11B 
shows the differentially demodulated data. The bit pattern is 
undetectable. Finally FIG. 11C shows the cross-correlation of this noise 
signal with the clean reference. Obviously the peak is still strong, with 
peak to RMS level in excess of 5.33 (14.5 dB), allowing accurate 
time-of-arrival estimation. In fact, an interpolation routine applied 
about the peak of this signal indicated an accuracy of less than 1/16 
sample spacing, i.e. less than 0.3 msec. 
As mentioned previously, the basestation can send to the remote the data 
sequence together with the time associated with the beginning of this 
message. The remote can then estimate the time-of-arrival of the data 
message via the same cross-correlation methods described above except that 
these correlation methods are performed at the remote. This is useful if 
the remote computes its own position location. In this situation the 
remote may also obtain satellite ephemeris data by a transmission of such 
data from the basestation. 
FIG. 1A shows an example of a combined mobile GPS receiver and 
communication system which may be used with the present invention. This 
combined mobile GPS receiver and communication system 100 has been 
described in detail in copending application Ser. No. 08/652,833 filed May 
23, 1996 and entitled "Combined GPS Positioning System and Communication 
System Utilizing Shared Circuitry" which is hereby incorporated herein by 
reference. FIG. 1B illustrates in further detail the RF to IF converter 7 
and the frequency synthesizer 16 of FIG. 1A. These components shown in 
FIG. 1B are also described in copending application Ser. No. 08/652,833. 
The mobile GPS receiver and communication system 100 shown in FIG. 1A may 
be configured to perform a particular form of digital signal processing on 
stored GPS signals in such a manner that the receiver has very high 
sensitivity. This is further described in copending U.S. patent 
application Ser. No. 08/612,669, which was filed Mar. 8, 1996, and is 
entitled "An Improved GPS Receiver and Method for Processing GPS Signals", 
and this application is hereby incorporated herein by reference. This 
processing operation described in application Ser. No. 08/612,669 computes 
a plurality of intermediate convolutions typically using fast Fourier 
transformations and stores these intermediate convolutions in the digital 
memory and then uses these intermediate convolutions to provide at least 
one pseudorange. The combined GPS and communication system 100 shown in 
FIG. 1A also may incorporate certain frequency stabilization or 
calibration techniques in order to further improve the sensitivity and 
accuracy of the GPS receiver. These techniques are described in copending 
application Ser. No. P003X, which was filed Dec. 4, 1996, and is entitled 
"An Improved GPS Receiver Utilizing a Communication Link", and which 
application is hereby incorporated herein by reference. 
Rather than describing in detail the operation of the combined mobile GPS 
receiver and communication system 100 shown in FIG. 1A, a brief summary 
will be provided here. In a typical embodiment, the mobile GPS receiver 
and communication system 100 will receive a command from a basestation, 
such as basestation 17, which may be either one of the basestations shown 
in either FIG. 5A or FIG. 5B. This command is received on the 
communication antenna 2 and the command is processed as a digital message 
after stored in the memory 9 by the processor 10. The processor 10 
determines that the message is a command to provide a position information 
to the basestation, and this causes the processor 10 to activate the GPS 
portion of the system at least some of which may be shared with the 
communication system. This includes, for example, setting the switch 6 
such that the RF to IF converter 7 receives GPS signals from GPS antenna 1 
rather than communication signals from the communication antenna 2. Then 
the GPS signals are received, digitized, and stored in the digital memory 
9 and then processed in accordance with the digital signal processing 
techniques described in the aforementioned application Ser. No. 
08/612,669. The result of this processing typically includes a plurality 
of pseudoranges for the plurality of satellites in view and these 
pseudoranges are then transmitted back to the basestation by the 
processing component 10 activating the transmitter portion and 
transmitting the pseudoranges back to the basestation to the communication 
antenna 2. 
The basestation 17 shown in FIG. 1A may be coupled directly to the remote 
through a radio communication link or may be, as shown in FIG. 6, coupled 
to the remote through a cellular telephone site which provides a wired 
communication link between the telephone site and the basestation. FIGS. 
5A and 5B illustrate these two possible basestations. 
The basestation 401 illustrated in FIG. 5A may function as an autonomous 
unit by providing a radio link to and from mobile GPS receivers and by 
processing received pseudoranges and the corresponding time records 
according to the present invention. This basestation 401 may find use 
where the basestation is located in a metropolitan area and all mobile GPS 
receivers to be tracked are similarly located in the same metropolitan 
area. For example, this basestation 401 may be employed by police forces 
or rescue services in order to track individuals wearing or using the 
mobile GPS receivers. Typically, the transmitter and receiver elements 409 
and 411, respectively, will be merged into a single transceiver unit and 
have a single antenna. However, these components have been shown 
separately as they may also exist separately. The transmitter 409 
functions to provide commands to the mobile GPS receivers through 
transmitter antenna 410; this transmitter 409 is typically under control 
of the data processing unit 405 which may receive a request from a user of 
the processing unit to determine the location of a particular mobile GPS 
receiver. Consequently, the data processing unit 405 would cause the 
command to be transmitted by the transmitter 409 to the mobile GPS 
receiver. In response, the mobile GPS receiver would transmit back to the 
receiver 411 pseudoranges and corresponding records in one embodiment of 
the present invention to be received by the receiving antenna 412. The 
receiver 411 receives these messages from the mobile GPS receiver and 
provides them to the data processing unit 405 which then performs the 
operations which derive the position information from the pseudoranges 
from the mobile GRS receiver and the satellite data messages received from 
the GPS receiver 403 or other source of reference quality satellite data 
messages. This is further described in the above-noted copending patent 
applications. The GPS receiver 403 provides the satellite ephemeris data 
which is used with the pseudoranges and the determined time in order to 
calculate a position information for the mobile GPS receiver. The mass 
storage 407 includes a stored version of the reference record of the 
satellite data messages which is used to compare against the records 
received from the mobile GPS receiver. The data processing unit 405 may be 
coupled to an optional display 415 and may be also coupled to a mass 
storage 413 with GIS software which is optional. It will be appreciated 
that the mass storage 413 may by the same as the mass storage 407 in that 
they may be contained in the same hard disk or other mass storage device. 
FIG. 5B illustrates an alternative basestation of the present invention. 
This basestation 425 is intended to be coupled to remote transmitting and 
receiving sites such as a cellular telephone site 455 shown in FIG. 6. 
This basestation 425 may also be coupled to client systems through a 
network, such as the Internet or an intranet, or other types of computer 
networking systems. The use of the basestation in this manner is further 
described in copending application Ser No. 08/708,176, which was filed 
Sep. 6, 1996 and which is entitled "Client-Server Based Remote Locator 
Device" and which is hereby incorporated herein by reference. The 
basestation 425 communicates with a mobile GPS unit, such as the combined 
mobile GPS receiver and communication system 453 shown in FIG. 6 through 
the cellular telephone site 455 and its corresponding antenna or antennas 
457 as shown in FIG. 6. It will be appreciated that the combined GPS 
receiver and communication system 453 may be similar to the system 100 
shown in FIG. 1A. 
The basestation 425, as shown in FIG. 5B, includes a processor 427 which 
may be a conventional microprocessor coupled by a bus 430 to main memory 
429 which may be random access memory (RAM). The basestation 425 further 
includes other input and output devices, such as keyboards, mice, and 
displays 435 and associated I/O controllers coupled via bus 430 to the 
processor 427 and to the memory 429. A mass storage device 433, such as a 
hard disk or CD ROM or other mass storage devices, is coupled to various 
components of the system, such as processor 427 through the bus 430. An 
I/O controller 431 which serves to provide I/O control between the GPS 
receiver or other source of satellite data messages, is also coupled to 
the bus 430. This I/O controller 431 receives satellite data messages from 
the GPS receiver 430 and provides them through the bus 430 to the 
processor which causes the time stamp to be applied to them and then 
stored in the mass storage device 433 for use later in comparing to 
records received from mobile GPS receivers. Two modems 439 and 437 are 
shown in FIG. 5B as interfaces to other systems remotely located from the 
basestation 425. In the case of modem or network interface 439, this 
device is coupled to a client computer, for example, through the Internet 
or some other computer network. The modem or other interface 437 provides 
an interface to the cellular telephone site, such as the site 455 shown in 
FIG. 6 which illustrates a system 451. 
The basestation 425 may be implemented with other computer architectures as 
will be appreciated by those skilled in the art. For example, there may be 
multiple busses or a main bus and a peripheral bus or there may be 
multiple computer systems and/or multiple processors. It may be 
advantageous, for example, to have a dedicated processor to receive the 
satellite data message from the GPS receiver 403 and process that message 
in order to provide a reference record in a dedicated manner such that 
there will be no interruption in the process of preparing the reference 
record and storing it and managing the amount of stored data in accordance 
with the present invention. 
The system 451 shown in FIG. 6 will typically operate, in one embodiment, 
in the following manner. A client computer system 463 will transmit a 
message through a network, such as the Internet 461 to the basestation 
425. It will be appreciated that there may be intervening routers or 
computer systems in the network or Internet 461 which pass along the 
request for position of a particular mobile GPS receiver. The basestation 
425 will then transmit a message through a link, which is typically a 
wired telephone link 459, to the cellular telephone site 455. This 
cellular telephone site 455 then transmits a command using its antenna or 
antennas 457 to the combined mobile GPS receiver and communication system 
453. In response, the system 453 transmits back pseudoranges and records 
of the satellite data messages in accordance with the present invention. 
These records and pseudoranges are then received by the cellular telephone 
site 455 and communicated back to the basestation through link 459. The 
basestation then performs the operations as described according to the 
present invention using the records to determine the time of receipt of 
the satellite data messages and using pseudoranges from the remote GPS 
system 453 and utilizing the satellite ephemeris data from the GPS 
receiver at the basestation or from other sources of GPS data. The 
basestation then determines a position information and communicates this 
position information through a network, such as the Internet 461, to the 
client computer system 453 which may itself have mapping software at the 
client computer system, allowing the user of this system to see on a map 
the exact position of the mobile GPS system 453. 
The present invention has been described with reference to various figures 
which have been provided for purposes of illustration and are not intended 
to limit in any way the present invention. Moreover, various examples have 
been described of the methods and apparatuses of the present invention, 
and it will be appreciated that these examples may be modified in 
accordance with the present invention and yet fall within the scope of the 
following claims.