U.S. patent application number 09/788753 was filed with the patent office on 2002-09-26 for time synchronization of a satellite positioning system enabled mobile receiver and base station.
Invention is credited to Wang, Hugh, Zhao, Yilin.
Application Number | 20020135511 09/788753 |
Document ID | / |
Family ID | 25145439 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020135511 |
Kind Code |
A1 |
Zhao, Yilin ; et
al. |
September 26, 2002 |
Time synchronization of a satellite positioning system enabled
mobile receiver and base station
Abstract
Satellite positioning system enabled mobile receivers (310) and
cellular communication network base stations (330) synchronized
with satellite positioning system clocks and method therefore. In a
network-assisted embodiment, a variable propagation delay for
transmission of an assistance message (232) from the base station
to the mobile receiver is determined for correcting the handset
clock (318). In others embodiments, local clock drift of mobile
receivers (310) and/or base stations (330) are determined by a
ratio of local and satellite time differences, based on sequential
time snapshots, for use in correcting the local clocks.
Inventors: |
Zhao, Yilin; (Northbrook,
IL) ; Wang, Hugh; (San Diego, CA) |
Correspondence
Address: |
Motorola, Inc.
Intellectual Property Dept. (RKB)
600 North U.S. Highway 45, AN475
Libertyville
IL
60048
US
|
Family ID: |
25145439 |
Appl. No.: |
09/788753 |
Filed: |
February 20, 2001 |
Current U.S.
Class: |
342/357.62 |
Current CPC
Class: |
H04W 56/0015 20130101;
G01S 19/235 20130101 |
Class at
Publication: |
342/357.02 |
International
Class: |
G01S 005/14 |
Claims
What is claimed is:
1. A method for synchronizing a satellite positioning system
enabled mobile receiver having a local clock with a satellite
positioning system, comprising: sampling first and second satellite
signals at the mobile receiver; sampling first and second local
clock signals at the mobile receiver, the first local clock signal
having the same relationship to the first satellite signal as the
second local clock signal has to the second satellite signal;
determining a local clock drift proportional to a difference
between the first and second sampled satellite signals divided by a
difference between the first and second local clock signals;
correcting the local clock based upon the local clock drift.
2. The method of claim 1, sampling the first satellite signal and
the first local clock signal at substantially the same time,
sampling the second satellite signal and the second local clock
signal at substantially the same time.
3. The method of claim 1, the mobile receiver is a cellular handset
for use in a cellular communication network, periodically
correcting the local clock, updating the local clock drift less
frequently than the local clock is corrected.
4. The method of claim 3, sampling the first satellite signal and
the first local clock signal at substantially the same time,
sampling the second satellite signal and the second local clock
signal at substantially the same time.
5. A method for synchronizing a cellular communications network
base station local clock with a satellite positioning system clock,
comprising: sampling first and second satellite signals having
satellite time at the base station; sampling first and second base
station local clock signals, the first local clock signal having
the same relationship to the first satellite signal as the second
local clock signal has to the second satellite signal; determining
a local clock drift proportional to a difference between satellite
times of the first and second satellite signals divided by a
difference between the first and second local clock signals;
correcting the local clock based upon the local clock drift.
6. The method of claim 5, re-correcting the local clock.
7. The method of claim 5, sampling the first satellite signal and
the first local clock signal at substantially the same time,
sampling the second satellite signal and the second local clock
signal at substantially the same time.
8. A satellite positioning system enabled mobile receiver,
comprising: a satellite positioning system interface for receiving
satellite signals having satellite time from a satellite
positioning system; a local clock; means for determining a local
clock drift (T.sub.DRIFTMOBILE) proportional to
[T.sub.MS1-T.sub.MS2]/[T.sub.GPS1-T.sub.GPS2], where T.sub.MS1 and
T.sub.MS2 are first and second sampled local clock times and
T.sub.GPS1 and T.sub.GPS2 are first and second sampled satellite
times, the first satellite time having the same relationship to the
first local clock time as the second satellite time having to the
second local clock time.
9. The mobile receiver of claim 8, means for correcting the local
clock based on the local clock drift.
10. The mobile receiver of claim 8 is a satellite positioning
system enabled cellular handset comprising a wireless
communications interface for communicating in a cellular
communication network.
11. A cellular communication network base station, comprising: a
satellite positioning system interface for receiving satellite
signals having satellite time from a satellite positioning system;
a local clock; means for determining a local clock drift
(T.sub.DRIFTBS) proportional to
[T.sub.BS1-T.sub.BS2]/[T.sub.GPS1-T.sub.GPS2], where T.sub.BS1 and
T.sub.BS2 are first and second sampled local clock times and
T.sub.GPS1 and T.sub.GPS2 are first and second sampled satellite
times, the first satellite time having the same relationship to the
first local clock time as the second satellite time having to the
second local clock time.
12. The base station of claim 11, means for correcting the local
clock based on the local clock drift.
13. A method for synchronizing a satellite positioning system
enabled mobile receiver in a network having a base station that
periodically determines a round trip delay (RTD) between the mobile
receiver and a base station based on a known bit duration (BD) and
that transmits an assistance message with satellite time between
synchronization signals transmitted at a known synchronization
interval, comprising: determining a time factor that compensates
for movement of the mobile receiver relative to the base station;
determining an estimated round trip delay (eRTD) based on the RTD
and the time factor that compensates for movement of the mobile
receiver; determining an estimated propagation delay between the
base station and the mobile receiver proportional to a product of
the eRTD and the BD; setting a clock in the mobile receiver based
on the estimated propagation delay.
14. The method of claim 13, the assistance message transmitted at a
known assistance message time offset relative to the transmission
of a synchronization signal, determining a time interval between
sequential synchronization bursts received at the mobile receiver;
determining a time difference between the time interval and the
known synchronization interval; determining the time factor
proportional to a product between the time difference and a ratio
of the assistance message offset divided by the known
synchronization interval.
15. The method of claim 13, generating the assistance message at a
reference node, transmitting the assistance message from the
reference node to the base station, determining a total propagation
delay between the reference node and the mobile receiver by adding
the estimated propagation delay to a propagation delay between the
reference node and the base station, setting the clock in the
mobile receiver based on the total propagation delay.
16. The method of claim 13, determining the estimated round trip
delay (eRTD) by calculating
eRTD=RTD+(T'.sub.SCH/T.sub.SCH-1)*(T.sub.OFFSET), where T.sub.SCH
is the synchronization interval and T'.sub.SCH is an interval
between sequential synchronization signal received at the mobile
receiver; determining the estimated propagation delay (T.sub.PROP)
between the base station and the mobile receiver by calculating
T.sub.PROP=[1/2]*[eRTD]*[BD].
17. A satellite positioning system enabled mobile receiver in a
network having a base station that periodically determines a round
trip delay (RTD) between the mobile receiver and a base station
based on a known bit duration (BD) and that transmits an assistance
message with satellite time between synchronization signals
transmitted at a known synchronization interval (T.sub.SCH),
comprising: means for determining an estimated round trip delay
(eRTD=RTD+(T'.sub.SCH/T.sub.SCH-1)*(T.sub.O- FFSET) between the
mobile receiver and the base station, where T.sub.OFFSET is a time
interval between a synchronization signal and the assistance
message, and T'.sub.SCH is a time interval between sequential
synchronization bursts received at the mobile receiver; means for
determining an estimated propagation delay
(T.sub.PROP=[1/2]*[eRTD]*[BD]) between the base station and the
mobile receiver; means for synchronizing a clock in the mobile
receiver based on the estimated propagation delay.
18. A method for synchronizing a satellite positioning system
enabled mobile receiver in a network having a base station that
periodically determines a round trip delay (RTD) between the mobile
receiver and a base station based on a known bit duration and that
transmits an assistance message with satellite time between
synchronization signals transmitted at a known synchronization
interval, comprising: determining a time correction component
proportional to a product of a resolution of the bit duration and
an average of two or more time intervals between sequential
synchronization signals received at the mobile receiver;
determining an estimated propagation delay between the base station
and the mobile receiver proportional to a summation of RTD and the
time correction component. setting a clock in the mobile receiver
based on the estimated propagation delay.
19. The method of claim 18, determining a time correction,
T.sub.CORRECTION, component by calculating: 3 T CORRECTION = [ [ i
= 1 n T SCH ' ] / n ] [ BDR ] , where T'.sub.SCH is an average of
an interval between two or more synchronization signals T.sub.SCH
received at the mobile receiver and n is the number of intervals;
determining the estimated propagation delay, T.sub.DELAY, by
calculating: T.sub.DELAY=[1/2]*[RTD]*[BD]+[T.sub.CORRECTI- ON],
where BD is the bit duration.
20. A satellite positioning system enabled mobile receiver in a
network having a base station that periodically determines a round
trip delay (RTD) between the mobile receiver an a base station
based on a known bit duration (BD) having a bit duration resolution
(BDR) and that transmits an assistance message with satellite time
between synchronization signals (T.sub.SCH) transmitted at a known
synchronization interval, comprising: means for determining a time
correction component 4 ( T CORRECTION = [ [ i = 1 n T SCH ' ] / n ]
[ BDR ] )where T'.sub.SCH is an average of an interval between two
or more synchronization signals T.sub.SCH received at the mobile
receiver; means for determining an estimated propagation delay
(T.sub.DELAY=[1/2]*[RTD]*[- BD]+[T.sub.CORRECTION]) between the
base station and the mobile receiver; means for synchronizing a
clock in the mobile receiver based on the estimated propagation
delay.
Description
FIELD OF THE INVENTIONS
[0001] The present inventions relate generally to locating mobile
receivers, and more particularly to time-synchronizing network base
stations and satellite positioning system enabled mobile receivers,
for example GPS enabled cellular handsets in cellular
communications networks.
BACKGROUND OF THE INVENTIONS
[0002] Satellite positioning system enabled mobile receivers are
known generally, and include for example navigational and two-way
radio communication devices.
[0003] Known satellite positioning systems include the Navigation
System with Time and Range (NAVSTAR) Global Positioning System
(GPS) in the United States of America, the Global Navigation
Satellite System (GLONASS) in Russia, and the proposed European
satellite navigation system (Galileo).
[0004] Regulatory and market driven forces are motivating
communications equipment manufacturers and service providers to
more quickly and accurately locate these and other mobile
receivers, for example, to support enhanced emergency 911 (E-911)
services, to provide promotional and fee based value-added
services, for navigation, etc.
[0005] In the near term, network-assisted satellite based
positioning schemes will likely support some of the demand for
mobile receiver location services, especially for cellular handsets
in cellular communication networks. The positioning of mobile
receivers may also be made autonomously, in other words without
network assistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The various aspects, features and advantages of the present
inventions will become more fully apparent to those having ordinary
skill in the art upon careful consideration of the following
detailed description thereof in conjunction with the accompanying
drawings, which are described below.
[0007] FIG. 1 illustrates a cellular communication network
supporting assisted satellite positioning system location of a
satellite positioning system enabled mobile receiver.
[0008] FIG. 2 is an exemplary cellular communication network
synchronization signal timing diagram.
[0009] FIG. 3 is an exemplary cellular communication
network-assisted satellite positioning system.
[0010] FIG. 4 is an exemplary time signal sampling diagram.
DETAILED DESCRIPTION OF THE INVENTIONS
[0011] The internal or local clocks of mobile receivers are
generally not as accurate as those of satellite positioning
systems. The local clocks of cellular communication network base
stations, used in network-assisted satellite positioning systems,
are also comparatively imprecise.
[0012] Improved timing and synchronization in mobile receivers and
in network base stations will provide improved positioning
performance in both autonomous and network-assisted satellite
positioning system based location schemes.
[0013] FIG. 1 is a network-assisted satellite positioning system
100 including generally a reference receiver at a surveyed location
having an unobstructed skyward view of satellites 120 in a
constellation, and a server, or reference node, 130 coupled
thereto. In some networks, the reference receiver is a part of the
server or vice versa, and the combination thereof constitutes the
reference node. In other networks, the server is at another
location. The reference node is generally coupled to several
network base stations directly or indirectly via other network
nodes, only one of which, base station 140, is identified in FIG.
1.
[0014] The reference receiver receives satellite signals, and the
reference node generates assistance messages based on the received
satellite signals in a format suitable for transmission over the
network to one or more mobile receivers. The assistance messages
are generally modulated on a cellular carrier signal 101, which is
transmitted in a point-to-point mode to a particular cellular
handset 104, or in a point-to-multipoint, or broadcast, mode to
multiple mobile receivers.
[0015] The assistance message includes, for example, reference
location, reference time, GPS time, GPS time of Week (TOW), TOW
assist, Doppler, code phase as well as its search windows,
ephemeris and clock corrections, ionospheric delay elements,
Universal Time Coordinate (UTC) offsets, Almanac, real-time
integrity data, among other information. GPS time is generally
stamped on the assistance message at the server, or more generally
at the reference node. In Differential Global Positioning Systems
(DGPS), the assistance message may include differential correction
information.
[0016] In assisted satellite positioning schemes where an
assistance message having GPS time is transmitted from the
reference node to the mobile receiver via the network, there is a
delay, referred to herein as a propagations delay, between the time
the GPS time is applied to the assistance message and the time the
assistance message is received at the mobile receiver.
[0017] The propagation delay generally has one or more fixed and
variable components, T.sub.FIXED DELAY and T.sub.VARIABLE DELAY,
which when summed constitute the total propagation delay,
T.sub.PROPTOTAL. In FIG. 1, for example, the distance between the
reference node and base station is fixed, and thus the propagation
delay therebetween is generally known in advance or is at least
reasonably predictable by virtue of the fixed distance therebtween.
The propagation delay between the base station 140 and the mobile
receiver 104, however, is generally variable since the location of
the mobile receiver relative to the base station changes as the
mobile receiver moves about.
[0018] Many cellular communication networks periodically determine
a round trip delay (RTD) between the base and a mobile station for
hand-offs or time slot synchronization, etc. In Global Systems for
Mobile (GSM) communication networks and other networks, the RTD is
known as timing advance (TA). In 3G W-CDMA based networks, RTD is
known as round trip time (RTT). Other communication networks also
determine a round trip delay (RTD), which generally provides an
estimate of the distance between the mobile receiver and the base
station.
[0019] In networks that generate RTD measurements in bits having a
corresponding bit duration (BD), an estimated propagation delay
between the base station and the mobile receiver is proportional to
a product of the RTD and the BD as follows:
T.sub.VARIABLEDELAY=[1/2]*[RTD]*[BD]. (1)
[0020] The BD and bit duration resolution (BDR) for a particular
network are generally specified in the corresponding cellular
communication standards. In GSM networks, for example, the
estimated propagation delay between a base station and a mobile
receiver is:
T.sub.VARIBLEDELAY=[1/2]*[TA]*[3.692 ms], (2)
[0021] where TA is the timing advance and 3.692 is the bit duration
(BD) in a GSM network.
[0022] In GSM networks, the timing advance (TA) is determined at
the base station approximately every 480 ms. Thus a handset
traveling at a speed of 100 km per hour during the time interval
between subsequent TA determinations may move as far as about 13
meters.
[0023] In FIG. 2, a more accurate estimate of the propagation delay
between the base station and the mobile receiver may be determined
by using an estimated round trip delay (eRTD) determined as
follows:
eRTD=RTD+(T'.sub.SCH/T.sub.SCH-1)*(T.sub.OFFSET), (3)
[0024] where T.sub.SCH is the time interval between subsequent
synchronization bursts, or pilot signals, SCH.sub.i 230 and
SCH.sub.i+1 234 transmitted from the base station. T'.sub.SCH is
the time interval between the reception of sequential
synchronization signals, SCH.sub.i and SCH.sub.i+1, at the mobile
receiver. T'.sub.SCH is generally different than T.sub.SCH,
depending on whether the mobile receiver is moving toward or away
from the base station. T.sub.OFFSET is the interval measured
between the transmission of a synchronization signal, for example
SCH.sub.i, and the transmission of an Assistance Message 232. In
GSM and other networks, T.sub.SCH and T.sub.OFFSET or analogous
quantities are also known.
[0025] An estimated variable propagation delay between the mobile
receiver and the base station may be determined by substituting the
estimated round trip delay (eRTD) of equation (3) for RTD in
equation (1) as follows:
T.sub.VARIABLEDELAY=[1/2]*[RTD+(T'.sub.SCH/T.sub.SCH-1)*(T.sub.OFFSET)]*[B-
D]. (4)
[0026] In a GSM network, equation (4) may be expressed as
follows:
T.sub.VARIABLEDELAY=[1/2]*[TA+(T'.sub.SCH/T.sub.SCH-1)*(T.sub.OFFSET)]*[3.-
692]. (5)
[0027] The propagation delay determined according to equations (4)
and (5) compensates for movement of the mobile receiver relative to
the base station during the interval between periodic RTD
determinations.
[0028] Another approach to determining the propagation delay
between the mobile station and the base station may be determined
as follows:
T.sub.VARABLEDELAY=[1/2]*[RTD]*[BD]+[T.sub.CORCTION]. (.sub.6)
[0029] The correction time component, T.sub.CORRCTION, is
proportional to a product of the bit duration resolution (BDR) and
an average of two or more T'.sub.SCH measurements at the mobile
receiver as follows: 1 T CORRECTION = [ [ i = 1 n T SCH ' ] / n ] *
[ BDR ] . ( 7 )
[0030] In GSM networks, for example, the BD has quarter-bit
resolution at the mobile receiver, i.e. BDR=0.923 ms where BD=3.692
ms, and equation (6) is expressed as follows: 2 T VARIABLDELAY = [
1 / 2 ] * [ TA ] * [ 3.692 ms ] + [ [ i = 1 n T SCH ' ] / n ] * [
0.923 ms ] . ( 8 )
[0031] The T.sub.CORRECTION component provides a higher degree of
resolution, dependent upon the resolution of the bit duration, BDR,
which is specified in the cellular communication standards for the
particular network. Equations (6) and (8) are better suited for
determining the variable propagation delay when the mobile receiver
is stationary, whereas equations (4) and (5) are better suited for
determining the variable propagation delay when the mobile receiver
is moving relative to the base station.
[0032] As noted, the total propagation delay may be determined by
summing the fixed propagation delay with the variable propagation
delay as determined by one of the general equations (4) and (6)
discussed above. The total propagation delay is used in the handset
to compensate for the time required to propagate the assistance
message to the mobile receiver, for example the total propagation
delay time may be added to the GPS time stamped onto the assistance
message.
[0033] It is desirable generally to periodically synchronize the
local clock of the handset with satellite positioning system time
directly, provided that the handset has an unobstructed skyward
view of a satellite in the constellation.
[0034] FIG. 3 illustrates a mobile receiver 310 in the exemplary
form of a cellular handset comprising a communications network
interface 314, for example a transmitter/receiver (Tx/Rx), for
communicating with a cellular communication network base station
330. Alternatively, the mobile receiver 310 may be a handheld or
mounted GPS navigation or tracking device, with or without a
communication network interface.
[0035] The mobile receiver 310 further comprises a satellite signal
reception interface 312, for example a GPS measurement sensor, for
receiving satellite signals 322 from satellites in an overhead
constellation 320. The mobile receiver also includes a processor
316 having memory associated therewith coupled to the satellite
signal reception interface, and a local clock 318. The exemplary
cellular handset may be configured for autonomous or
network-assisted positioning. The sensor 312 can be a fully
functional GPS receiver. Alternatively, this fully functioned GPS
receiver can be an independent device connected with the cellular
phone, such as an accessory.
[0036] FIG. 4 illustrates the sampling of satellite time and local
clock time at the mobile receiver. More particularly, the mobile
receiver samples first and second satellite times T.sub.GPS1 410
and T.sub.GPS2 420 and first and second local clock times T.sub.MS1
412 and T.sub.MS2 422.
[0037] The first satellite signal preferably has the same
relationship to the first clock signal as the second satellite
signal has to the second clock signal. In one embodiment, the first
satellite signal is sampled concurrently with the first local clock
signal, and the second satellite signal is sampled concurrently
with the second local clock signal. If there is a delay between the
sampling of the first satellite and local clock signals, the same
delay exists between the sampling of the second satellite and local
clock signals.
[0038] The drift of the local clock in the mobile receiver, or
mobile station, may be determined as follows:
T.sub.DRIFTMOBILE=[T.sub.MS1-T.sub.MS2]/[T.sub.GPS1-T.sub.GPS2].
(9)
[0039] The local clock drift is calculated in the handset by the
processor 316, for example under control by a software program. The
calculated local clock drift may thus be used to correct the local
clock, for example by adding or subtracting the calculated drift to
or from the local clock time, depending on whether the local slow
or fast.
[0040] In cellular communications networks that provide location
assistance, the base station 330 may have associated therewith a
GPS receiver, for example a GPS receiver 342 which is part of a
location measurement unit (LMU) 340 used to provide measurements
for location services. The LMU may be part of the base station or
alternatively may be connected to the base station either directly
or via an air interface.
[0041] In applications where an assistance message is transmitted
to the mobile receiver from a cellular communication network, the
local clocks 332 in the base stations may be corrected. A processor
334 with memory in the base station may sample GPS time, for
example that derived from the GPS receiver 342 in the LMU, and the
local clock 332 to calculate local drift as follows:
T.sub.DRIFTBS=[T.sub.BST1+T.sub.BST2]/[T.sub.GPS1-T.sub.GPS2].
(10)
[0042] Alternatively, the processor in the LMU 340 can perform the
sampling and drift calculation. Those of ordinary skill in the art
will also realize that the same technique can be applied to the
base station that has a similar architecture as the mobile receiver
310.
[0043] The local clocks in the mobile receiver and the base
stations are preferably corrected periodically. The local clock
drift may also be updated periodically as discussed above, although
the drift rate is substantially constant over relatively short time
intervals, and thus need not be updated as frequently as the local
clock is corrected. Assuming the local clock oscillator has a drift
rate of approximately 50 nanoseconds per second, a 1 ms clock
precision may be maintained by correcting the local clock
approximately every 5.5 hours.
[0044] While the present inventions have been described hereinabove
to enable those of ordinary skill in the art to make and use what
is presently considered to be the best modes thereof, those of
ordinary skill will understand and appreciate that equivalents,
modifications and variations may be made thereto without departing
from the scope and spirit of the invention, which is to be limited
not by the exemplary embodiments but by the appended claims.
* * * * *