Satellite-based geolocation calibration system and method

A calibration system utilizing existing location techniques for a global radio telecommunication system 10 autonomously calibrates the communication system's location system by routing precision location data through system 10. Calibration reduces system errors by allowing time slots to remain narrow. The precision data is carried by existing commands throughout the system to measure location errors inserted by satellites 12. A gateway 16 compiles and evaluates received calibration data and determines when to update all gateways 16 with new satellite variance data tables 435. The calibration system provides early warning of degradation of system components and long-term analysis of system performance.

RELATED INVENTION 
This application is related to commonly assigned United States Patent 
Applications: 
"Geolocation Responsive Radio Telecommunication System and Method 
Therefor", having Ser. No. 8/105,730, filed Aug. 11, 1993, and assigned to 
the assignee of the present invention; and 
"Location System And Method With Acquisition Of Accurate Location 
Parameters", having Ser. No. 8/105,227, filed Aug. 11, 1993, and assigned 
to the assignee of the present invention. 
1. Field of the Invention 
The present invention relates generally to systems that determine the 
locations of subscriber units. More specifically, the present invention 
relates to calibration of non-coherent communication systems which 
determine the Doppler frequency and/or propagation delay of signals 
transmitted between subscriber units and satellites for use as location 
parameters. 
2. Background of the Invention 
A need exists for calibration of a substantially global radio 
telecommunications system that provides communication services to 
substantially any point on or near the surface of the earth. For such a 
system to achieve widespread acceptance, it should be capable of adapting 
to variable conditions present in the system such as aging and system 
degradation error. 
Such a global radio telecommunications system and other radio 
telecommunications systems must know the locations of subscriber units, at 
least for normal "paging" operations. It would be possible for subscriber 
units to initiate calls without a known location, but, at a minimum, in 
order to efficiently receive a call, the unit's location must be known to 
the system. For other non-technical reasons, it may also be necessary to 
establish the location of a subscriber unit before allowing access to the 
system. Performance of the location system may be critical to resolving 
billing, access, and other location-related issues. It is critical not 
only that the system be designed to provide sufficient location 
resolution, but that the system be monitored and calibrated to assure 
continued performance through its operational lifetime. It is desirable to 
have a calibration system that automatically and autonomously calibrates a 
geolocation system without intensive or extensive system intervention. 
It is also desirable to have a calibration system for a geolocation system 
capable of characterizing measurement error for the system satellites 
without requiring manual stimulation or other manual intervention. 
It is also desirable to have a calibration system providing early warning 
detection of degradation of system performance due to increased error in 
the measurement data for any particular satellite. 
It is also desirable for such a calibration system to be capable of 
providing the means and method for long-term analysis of geolocation 
functional performance including drift and system component degradation.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 is a layout diagram of an environment within which a radio 
telecommunications system 10 that incorporating a locating system and 
method may operate, in accordance with an embodiment of the present 
invention. System 10 includes a constellation 11 consisting of several 
satellites 12 placed in relatively low orbits 14 around the earth. Due to 
the configuration of constellation 11, at least one of satellites 12 is 
desirably within view of each point on the surface of the earth at all 
times. 
System 10 additionally includes one or more gateways (GW) 16. GW 16 reside 
on the surface of the earth and are in data communication with nearby ones 
of satellites 12 through GW communication channels 18. Satellites 12 are 
also in data communication with one another using crosslink communication 
channels 22. Hence, through constellation 11 of satellites 12, a GW 16 may 
control communications delivered to any region of the earth. However, the 
region controlled by each GW 16 is preferably associated with one or more 
specific geo-political jurisdictions, such as one or more countries. 
Gateways 16 couple to public switched telecommunication networks (not 
shown), from which calls directed toward subscribers of system 10 may be 
received and to which calls placed by subscribers of system 10 may be 
sent. 
System 10 also includes any number, potentially in the millions, of 
locatable subscriber units (SUs). SUs may be configured as conventional 
portable radio communication equipment. In other words, SUs may be battery 
powered, may consume relatively low power, and may include relatively 
small antennas. System 10 accommodates the movement of SUs anywhere on or 
near the surface of the earth. However, nothing requires SUs to move, and 
system 10 operates satisfactorily if a portion of the entire population of 
SUs remains stationary. SUs are configured to engage in communications 
with satellites 12 over portions of the electromagnetic spectrum allocated 
by governmental agencies associated with various geopolitical 
jurisdictions. SUs communicate with nearby satellites 12 through uplink 
communication channels 26. 
A number of GWs 16 maintain subscriber databases that are relevant to a 
discrete portion of the population of SUs. The database may include 
information describing features associated with SUs, rates to be 
associated with SUs, current locations for SUs, and other information. 
Each SU is assigned to one of GWs 16, and that GW 16 is considered the 
"home" GW for an SU. 
Due to the low earth orbits, satellites 12 constantly move relative to the 
earth. In the preferred embodiments, satellites 12 move in orbits at an 
altitude in the range of 500-1000 km above the earth. If, for example, 
satellites 12 are placed in orbits about 780 km above the earth, then 
satellite 12 travels at a speed of around 25,000 km/hr with respect to a 
point on the surface of the earth when overhead. Electromagnetic signals 
traveling at or near the speed of light between an SU positioned near the 
surface of the earth and a satellite 12 in such an orbit will require a 
propagation duration of 2-3 msec or more, depending on the satellite's 
angle of view. Moreover, electromagnetic signals traveling between an SU 
positioned near the surface of the earth and a satellite 12 in such an 
orbit may experience a considerable Doppler component of frequency shift, 
the precise value of which is dependent on a source frequency and velocity 
of the satellite relative to SU. 
SUs in system 10 require accurate location data to effectively route 
communications to SUs and to allow system access by SUs. Special purpose 
SUs are employed to perform calibration functions within system 10. Both 
SUs and calibration subscriber units CSU 24 communicate with system 10 
utilizing identical methods. CSU 24 is comprised of an independent 
precision location source coupled with a basic SU. The independent 
precision location source may be either a dynamic type such that location 
data is updated regularly, or it may be a static type such that precision 
location data is fixed according to the present location of CSU 24. 
FIG. 2 shows a typical communication path between a calibration subscriber 
unit 24 and a satellite 12, and also a typical communication path between 
satellite 12 and GW 16 for delivering calibration data to GW 16, in 
accordance with an embodiment of the present invention. System 10 (see 
FIG. 1) communicates through satellites 12 with CSUs 24 using a limited 
amount of the electromagnetic spectrum. In the preferred embodiment, a 
combined frequency division multiplex (FDM), time division multiplex (TDM) 
scheme is employed. For example, different channels may be assigned to 
different time slots at a common frequency, and different channels may be 
assigned to different frequencies at common time slots. Channels are 
specified or defined by identifying both a frequency and time slot. Each 
satellite 12 controls the implementation of the FDM/TDM scheme for its 
communications. CSUs 24 conform their operations to the FDM/TDM standards 
set by satellites 12. 
Channels may be grouped together into discrete channel sets. Desirably, 
each of these discrete channel sets is orthogonal to all other channel 
sets. In other words, simultaneous communications may take place at a 
common location over every channel in every channel set without 
significant interference. Conventional cellular communication techniques 
may be employed by assigning channel sets and employing a reuse scheme 
which prevents adjacent cells from using common channel sets. 
Utilizing this established communication channel, CSU 24 and satellite 12 
exchange multiple transmission bursts from which fundamental signal 
propagation parameters are extracted. These propagation parameters include 
time of arrival (TOA) and frequency of arrival (FOA) for signals arriving 
at each of CSU 24 and satellite 12. Generation of these parameters is not 
the subject of this invention, but is described in detail in the related 
applications as cited above. 
Upon completion of parameter extraction, CSU 24 forwards these parameters 
in an access request message to satellite 12 using the established uplink 
communication channel 26. Satellite 12 retrieves its own TOA and FOA 
parameters as established during uplink communication channel set-up and 
appends these parameters to an access request message as received from CSU 
24. Satellite 12 utilizes a previously established GW communication 
channel 18 to forward CSU 24 access request message containing TOA and FOA 
parameters from both the CSU 24 and satellite 12. 
Gateway 16 receives an access request as sent by satellite 12. Gateway 16 
stores TOA and FOA from both satellite 12 and CSU 24 for processing and 
analysis. Gateway 16 utilizes the stored TOA and FOA parameters to 
estimate the location of CSU 24. 
FIG. 3 is a flowchart of communication between a calibration subscriber 
unit CSU 24 and a satellite 12, and communication from satellite 12 to GW 
16, in accordance with an embodiment of the present invention. 
A method 300 performs parameter generation and collection for submission to 
GW processing. A task 305 establishes a dedicated communication channel 26 
between CSU 24 and satellite 12 or task 305 may identify an existing 
communication channel 26. This dedicated channel 26 may be established by 
either a CSU 24 or by satellite 12 as directed by GW 16. 
Following task 305, a task 310 utilizes uplink communication channel 26 for 
exchanging multiple messages between CSU 24 and satellite 12. The 
succession of messages allows both CSU 24 and satellite 12 to refine their 
individual TOA and FOA parameters. 
A task 315 receives precision location data from an independent source, 
which may include an independent location determination device such as GPS 
or a predetermined location value for fixed-location CSUs 24. 
A task 320 combines the precision location data with a CSU's 24 TOA and FOA 
as determined in task 310 into an access request message. CSU 24 transmits 
this access request message to satellite 12 using uplink communication 
channel 26. 
In a task 325, satellite 12 receives the access request message containing 
CSU's 24 TOA, FOA and precision location data. Satellite 12 then appends 
its specific TOA and FOA to that of CSU 24 to form a satellite access 
request message. 
A task 330 forwards this satellite access request to GW 16 using existing 
GW communication channel 18. Subscriber unit and satellite processing then 
complete at 335 and analysis then passes to GW processing. 
FIG. 4 is a flowchart for determining a location of CSU 24 by using both 
measured TOA and FOA values contained in an access request message, in 
accordance with an embodiment of the present invention. 
A task 405 utilizes both satellite TOA/FOA and CSU TOA/FOA values to 
calculate a CSU 24 location based upon existing satellite variance data 
parameters 435. This process is equivalent to a process used to locate a 
typical SU (non-CSU) in system 10. Existing satellite variance data table 
435 contains satellite identifiers with their respective calibrated time 
error (TE), and frequency error (FE) including bias parameters. A location 
error is also generated and stored in a calibration record when 
calibration is requested. 
A query task 410 analyzes the satellite access message to determine if the 
message received by satellite 12 originated from a CSU 24. If a CSU 24 
originated the message, then GW 16 retains all necessary information 
contained in the satellite access message as well as the calculated 
location derived from task 405. 
If the access message originated from a non-calibration SU, then a task 415 
performs a general registration of SU location to facilitate typical 
communications such as call setup, resource allocation, and proper 
billing. 
If task 410 determined that a CSU 24 originated the satellite access 
request message, then a task 420 creates a calibration record including 
all necessary information received in the satellite access request message 
as well as the calculated location derived from task 405. This completes 
the collection of calibration data 425 by GW 16. 
FIG. 5 shows a flowchart for analyzing calibration record information and 
for generating variance data for utilization in all subsequent GW location 
determination processes, in accordance with an embodiment of the present 
invention. 
A task 455 collects available stored calibration records compiled during GW 
processing in FIG. 4, including records from all accessible GW 16 
equipment or various GW 16 as specified by the system. Task 455 may 
actively request each GW 16 in system 10 to forward collected location 
calibration records, or, alternatively, may passively receive location 
calibration records forwarded from each GW 16. 
A task 460 processes the collected calibration records. This process 
selects representative records to be stored for later use in long-term 
trend analysis, and combines all records for further analyses. 
A task 465 utilizes the collected calibration records to calculate 
short-term variance coefficients representing short-term parameter 
variance values. These short-term changes typically represent sporadic 
deviations from nominal operations of system 10 including satellite 12 
short-term orbital anomalies and other transient equipment effects, as 
well as constellation changes represented by addition or deletion of 
individual satellites 12. 
A query task 470 determines if long-term coefficients need to be updated. 
This updating process occurs less frequently because these effects are 
generally less volatile because they relate generally to equipment aging. 
If necessary, a task 475 utilizes the collected long-term calibration 
records to calculate coefficients representing long-term variances in 
system 10. These variances typically represent system 10 evolutionary 
changes, e.g., satellite 12 equipment aging considerations. 
A task 482 utilizes both short-term coefficients and long-term coefficients 
to calculate satellite 12 variance values for use in future location 
determinations. This calculation produces variance values for TOA and FOA 
measurements unique to each satellite 12 but not unique to an individual 
SU. 
A task 484 compares the variance data table generated by task 482 with the 
variance data table which is currently in use at each GW 16. Task 484 
identifies significant differences between the newly computed variance 
data table and the currently used variance data table. 
A query task 486 then determines if the newly generated variance data table 
contains elements significantly different from elements of the current 
variance data table. This determination results from the comparison of 
each variance change to a predefined threshold value. 
If query task 486 determines that a significant change in variance data has 
occurred, a task 440 distributes the newly generated variance data table 
to each GW 16 for use in subsequent location processing. This distribution 
may be immediate or may be delayed to coincide with other system 
configuration deliveries to various GWs 16. 
FIG. 6 shows a data format of a satellite access control signal containing 
CSU 24 and satellite 12 TOA and FOA values, along with precision location 
data of CSU 24, in accordance with an embodiment of the present invention. 
FIG. 6 shows the configuration of a typical satellite access request 
message as compiled by satellite 12. 
Satellite access request message 500 comprises multiple data fields. Header 
505 contains message header information typical of messages in system 10. 
Header 505 may include information necessary to route the message to the 
intended destination GW 16. 
CSU TOA 510 contains TOA data measured by CSU 24. CSU FOA 515 contains FOA 
data measured by CSU 24. Precision location data 520 contains precision 
location data received from a location data source such as an alternate 
location source 620 or fixed-location data 720. CSU TOA, CSU FOA, and 
precision location data 520 are generated at CSU 24 as described above. 
Satellite TOA 525 contains TOA data measured by satellite 12. This datum is 
inserted into the message by satellite 12. Satellite FOA 525 contains FOA 
data measured by satellite 12. This data is inserted into the message by 
satellite 12. Time-stamp 535 contains time-stamp information representing 
the time at which the CSU 24 made its TOA and FOA measurements. Data for 
this field is provided by the CSU 24. 
FIG. 7 is a block diagram of a CSU 24, in accordance with an embodiment of 
the present invention. CSU 24 comprises an antenna 605 for providing 
transmission and reception signal gain. CSU 24 also includes a transceiver 
610 for transmitting and/or receiving signals on uplink communication 
channel 26 as discussed in connection with FIGS. 1 and 2. 
A Doppler and timing processor 615 directs both transmitting and receiving 
functions. Processor 615 during an acquisition of a communication channel, 
monitors a receive channel and performs estimations of both Doppler 
frequency and propagation timing. These estimations are then used in 
transmission to fall within the timeslot specifications of timing at 
satellite 12. Iterations of this process refine these Doppler frequency 
and propagation timing values. These values are then used as the TOA and 
FOA values as discussed above. 
Alternate location source 620 provides precision location data to Doppler 
and timing processor 615 for delivery to GW 16 via satellite 12. This 
precision location data may be derived from a number of location services 
or sources such as Global Positioning System (GPS), GLONASS, or Loran. 
Gateway processing utilizes precision location data for comparison with 
system-calculated location for derivation of variance data. 
FIG. 8 is a block diagram of an alternate embodiment of CSU 24, in 
accordance with an embodiment of the present invention. CSU 24 comprises 
antenna 605, transceiver 610, Doppler and timing processor 615, and 
fixed-location data source 720. Antenna 605, transceiver 610 and Doppler 
and timing processor 615 are the same as those in CSU 24 (FIG. 7). 
Fixed-location data source 720 provides precision location data to Doppler 
and timing processor 615 for delivery to GW 16 via satellite 12. CSU 24 
generally is located at a fixed site where location data remains constant 
and may therefore be stored for use by CSU 24 or provided external to CSU 
24 for use in GW calibration processing. FIG. 8 shows fixed-location data 
source 720 to be inclusive, however, one skilled in the art would 
recognize that fixed-location data source 720 could be any form of 
external storage as well such as floppy disk, or manual data entry such as 
from a keypad. 
In summary, the present invention provides an improved geolocation 
calibration system and method. The calibration system and method are 
compatible with the needs of a communication system. For example, 
satellites orbiting the earth provide different geometries with which 
subscriber units must compatibly interact. 
The geometries and more particularly the transmission characteristics of 
particular satellites require accurate timing and frequency adjustments by 
subscriber units. These frequency and timing adjustments or parameters are 
utilized by a GW to determine location of subscriber units for reliable 
call setup, access privileges, and proper billing of calling services. 
The present invention utilizes existing capabilities of satellites 12 and 
GWs 16 for automatic and transparent calibration of system 10. Those 
skilled in the art will recognize that changes and modification may be 
made in these embodiments without departing from the scope of the present 
invention. 
Moreover, those skilled in the art will appreciate that the flowcharts 
presented herein are intended to teach the present invention and that 
different techniques for implementing program flow that do not necessarily 
lend themselves to flow charting may be devised. In particular, each task 
discussed herein may be interrupted to permit program flow to perform 
background or other tasks. 
In addition, the specific order of tasks may be changed, and the specific 
techniques used to implement the tasks may differ from system to system. 
These and other changes and modification which are obvious to those 
skilled in the art are intended to be included within the scope of the 
present invention.