common-view loran-c for precision time and frequency recovery
DESCRIPTION
Common-View LORAN-C for Precision Time and Frequency Recovery. Tom Celano, Timing Solutions Corp LT Kevin Carroll, USCG Loran Support Unit Michael Lombardi, NIST. Introduction. Timing Solutions is investigating the potential performance of time recovery using common-view LORAN-C - PowerPoint PPT PresentationTRANSCRIPT
V-1
Common-View LORAN-Cfor Precision Time and Frequency Recovery
Tom Celano, Timing Solutions Corp
LT Kevin Carroll, USCG Loran Support Unit
Michael Lombardi, NIST
V-2
Introduction
Timing Solutions is investigating the potential performance of time recovery using common-view LORAN-C
— Study is funded by LSU as part of the LORAN Accuracy Panel (LORAPP) that is chaired by the USCG
Common-View time recovery has been used for years in the GPS community with good results
— Common-View GPS has repeatedly demonstrated < 10 ns timing performance
Common-View LORAN-C provides a differential solution that will help to reduce the dominating propagation delay errors that drive LORAN-C timing performance
LORAPP study to determine feasibility and projected performance of common-view approach for Enhanced LORAN era
— Can Enhanced LORAN be a viable backup to GPS?
V-3
Common View GPS Common View GPS involves computing a relative time difference between two
clocks by subtracting the GPS measurements collected at each site— Each site collects passive GPS data from the individual GPS SV’s
— The common view difference is computed by subtracting the GPS data sets by SV» Common mode GPS noise (like ionospheric delay) cancels
If one of the two clocks has a known relationship to UTC, absolute time can be transferred
GPS SV
Clock 1
Site 1
GPSRx
LocalClock
Site 2
GPSRx
Clock 1 - GPS Clock 2 - GPS-
Clock 1 – Clock 2
V-4
Passive L1 GPS Data from One Site
-50
-40
-30
-20
-10
0
10
20
30
40
50
297 297.5 298 298.5 299 299.5 300 300.5 301 301.5 302
DOY
UT
C O
ffse
t (n
s)
TSC (Boulder)
Passive L1 GPS Data from Two Sites
-50
-40
-30
-20
-10
0
10
20
30
40
50
297 297.5 298 298.5 299 299.5 300 300.5 301 301.5 302
DOY
UT
C O
ffse
t (n
s)
Gillette Data
TSC (Boulder)
GPS Common View
-50
-40
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-10
0
10
20
30
40
50
297 297.5 298 298.5 299 299.5 300 300.5 301 301.5 302
DOY
CV
Off
set
(ns)
Common View GPS (cont’d)
Ionospheric noise (and other systematic GPS noise) that is common to both sites cancels in the calculation, resulting in a lower noise, relative measurement between the two clocks
— Absolute reference of measurement is lost if one of the clocks is not related to UTC
Precision of common view measurement is significantly better than passive measurement
V-5
Common View LORAN-C
Common view LORAN-C is the same idea as common view GPS with GPS satellites replaced by LORAN-C transmitters
— The data from each LORAN-C transmitter is treated independently and is corrected using TOA monitor data
— Data from all transmitters can be combined after common view differences are calculated
LORAPP timing study initiated in July 2003 in order to examine common view technique LORAN-C for time recovery
Primary goal of LORAPP timing study is to determine if common view LORAN-C can be considered as a backup to GPS for precision timing users
— Performance to be analyzed using real-world data collection» Bound best case using short baseline
» Gauge expected performance using long baseline
— Determine requirements and candidate architecture for precision time recovery in Enhanced LORAN era
V-6
Experiment Set-up
Dual GPS/LORAN data collection systems are installed at three locations
— TSC (Boulder, CO), NIST (Boulder, CO), LORSTA Gillette (Gillette, WY)
LORAN data and GPS data logged continuously against local UTC source
Common-view differences computed using both LORAN and GPS data
LORAN-C common-view performance compared to GPS common-view data
Clock
GPS Rx
LORAN Rx
LocalUTCEstimate
Log Data
Log Data
NIST (Boulder)
Clock
GPS Rx
LORAN Rx
LocalUTCEstimate
Log Data
Log Data
Timing Solutions (Boulder)
Clock
GPS Rx
LORAN Rx
LocalUTCEstimate
Log Data
Log Data
LORSTA (Gillette)
Common View LORAN-C &
Common View GPS Calculated between
each site pair300 Miles
V-7
Hardware Configuration Systems installed at each location consist of separate hardware for GPS and
LORAN-C processing— Independent methods for timing computation
GPS data collected using NIST common-view service (TSC and NIST) and ONCORE based common-view computation (TSC and Gillette)
LORAN-C data produced by Peterson Integrated Geopositioning (PIG) software using data from a LOCUS LORAN-C receiver
— External time interval counter and local UTC estimate used to compute TOA referenced to local time
HP 5071
GPS Rx
TimescaleSoftware
Steer
1 PPS
Time IntervalCounter
LORAN-CRx
1 PPS
PCI
PIG Software LORAN-CTOA Data
GPS SV Data
GPS data
MeasurementData
V-8
“Passive” LORAN-C Timing Data Uncorrected LORAN-C data is subject to diurnal and seasonal variations in
propagation delay— Seasonal term dominates the time of arrival data and limits time recovery
performance
Without differential correction, LORAN-C timing is limited to microsecond level performance
Correlation in propagation delay over different transmission paths allows for common view principles to be applied
— Same concept as single frequency GPS with ground propagation replacing ionospheric delay as the common mode noise source
LORAN-C Boise City - NIST
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0
100
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300
400
52150 52200 52250 52300 52350 52400 52450 52500 52550 52600
mjd
24 h
ou
r p
has
e o
ffse
t (n
s)
V-9
Correlated Effects in LORAN-C Data Degree of correlation for propagation delay between monitor site and user
site will drive performance— This will also drive the required number and density of monitor sites
Comparison of Uncorrected LORAN-C Data*
5800
5900
6000
6100
6200
6300
6400
6500
6600
6700
6800
219 229 239 249 259 269 279 289
DOY
TO
A (
ns)
9610M (NIST)
9610M (TSC)
*Data has been externally calibrated by applying a bias to each LORAN transmitter
V-10
Passive LORAN-C Data*
8950
9000
9050
9100
9150
9200
9250
9300
9350
280 285 290 295 300 305 310
DOY
TO
A 9610M (Gillette)
9610M (TSC)
Common-View LORAN-C Data Common-View LORAN computation removes a significant portion of the long term
propagation delay variations— Like GPS case, common noise is apparent in passive LORAN-C data
Noise level of common-view data is related to proximity to monitor station— Short baseline data is slightly noisier than common view GPS
— Longer baseline data shows higher noise level but is still considerably better than passive case
*Data has been externally calibrated by applying a bias to each LORAN transmitter
V-11
Comparison of GPS Common View and LORAN Common View*Short Common View Baseline - TSC-NIST - Std Dev = 8ns
-40
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0
10
20
30
40
50
60
282 284 286 288 290 292 294 296 298
DOY
CV
Off
set
(ns)
GPS CV
LORAN CV (9610V)
Comparison of GPS Common View and LORAN Common View*Zoom on Clock Cold Start at TSC
-40
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0
10
20
30
40
50
60
287.5 287.6 287.7 287.8 287.9 288 288.1 288.2 288.3 288.4 288.5
DOY
CV
Off
set
(ns)
GPS CV
LORAN CV (9610V)
Clock at TSC cold started
Common View LORAN-C Short Baseline Short baseline between TSC and NIST provides the best case scenario for
common view LORAN-C — TSC and NIST only 5 miles apart
Data collected between TSC and NIST shows excellent precision and can be compared favorably to GPS common view
— Data clearly shows cold start of TSC timing system (50 ns effect)
V-12
Common View LORAN-C Short Baseline (cont’d) Best case scenario data ranges from 8 ns (RMS) to 25 ns (RMS)
depending on distance from transmitter— Higher noise stations would not be used in real solution
— Low noise stations can be combined to increase robustness
Comparison of GPS Common View and LORAN Common View*Short Common View Baseline - TSC-NIST - Std Dev = 8ns
-40
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0
10
20
30
40
50
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282 284 286 288 290 292 294 296 298
DOY
CV
Off
set
(ns)
GPS CV
LORAN CV (9610V)
Comparison of GPS Common View and LORAN Common View* Short Common View Baseline - TSC-NIST - Std Dev = 13ns
-40
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0
10
20
30
40
50
60
282 284 286 288 290 292 294 296 298
DOY
CV
Off
set
(ns)
LORAN CV (9610X)
GPS CV
Comparison of GPS Common View and LORAN Common View* Short Common View Baseline - TSC-NIST - Std Dev = 25ns
-40
-30
-20
-10
0
10
20
30
40
50
60
282 284 286 288 290 292 294 296 298
DOY
CV
Off
set
(ns)
LORAN CV (9960Z)
GPS CV
Comparison of GPS Common View and LORAN Common View*Short Common View Baseline - TSC-NIST
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0
20
40
60
80
282 284 286 288 290 292 294 296
Day of Year
Co
mm
on
-Vie
w O
ffse
t (n
s)
8290 M
8290 X
9610 V
NIST CV
V-13
GPS Common View vs LORAN Common View*Long Common-View Baseline - TSC (Boulder) to Gillette (WY)
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10
30
50
70
282 284 286 288 290 292 294 296
DOY
CV
off
set
(ns)
GPS CV
9610M CV
GPS Common View vs LORAN Common View*Long Common-View Baseline - TSC (Boulder) to Gillette (WY)
-70
-50
-30
-10
10
30
50
70
282 284 286 288 290 292 294 296
DOY
CV
off
set
(ns)
8290M CV
GPS CV
Common View LORAN-C Long Baseline Long baseline between TSC and Gillette provides a more realistic
operational scenario for common view LORAN-C— 300 mile baseline
Data collected between TSC and Gillette still shows excellent precision and a significant reduction in propagation delay effects
V-14
Timing User Spectrum
0.1 ns 1 ns 10 ns 100 ns 1 µs 10 µs 100 µs 1 ms 10 ms 100 ms 1 s
PTTI/R&D- NIF
Scientific/Experimental
High Precision Military- GPS Monitor Stations- GPS Weapons- AT3 Airborne Geolocation Demo- Bistatic Radar- Various Classified
Advanced Comms
Power Systems- Fault Location- Phasor Meas- Data Sharing
CDMA2000- Base Stations
Low Precision Military- Ground Terminals- VHF Special Comms
Astronomy
Financial Transactions
National Timing Labs
Wide Area Data Logging- Seismic monitoring- Nuclear Blast Detection
Digital Time Servers- NTP, etc Authentication
- Internet loginCould be served by Enhanced LORANTiming user survey not intendend to be a complete representation of all users. Requirements have been generalized and averaged over user groups
High Speed Photometry
V-15
LORAN-C for Frequency Recovery Long baseline common view LORAN-C frequency recovery data shows
Stratum I performance with less than 1 hour of averaging time— No significant difference from the passive case over the short term
Common view technique not as beneficial for frequency recovery in short term— Long term performance expected to show performance benefit but we don’t have
the data
V-16
Common View Requirements
Common view LORAN-C time recovery will be enabled by Enhanced LORAN assuming that TOA operations become the norm
— TOA monitoring, TOA receivers
SAM sites will require precision timing in order to compute TOA corrections
— TD monitoring is not sufficient
A calibrated and delay stable TOA LORAN-C receiver is also required in order to recover and maintain absolute time
— TOA receiver required to get an absolute measurement from the LORAN-C receiver at the <10 ns level
— Delay stable over time, power cycling and temperature
V-17
On-going Work
Work is continuing on LORAPP timing experiment— Hardware to be left in place to continue to collect data
Goal is to collect long term data so that full seasonal effect can be seen in data and processed using common view
Current data collection procedures is not robust enough to facilitate long term data collection
— Data logging is too easy to disrupt
— Too much data being lost due to unattended operation
Attempt will be made to provide better visibility to data collection operations
— Need to enable long, continuous data runs
V-18
Summary
Data collection continues on a common-view LORAN-C timing experiment between three sites
Preliminary results indicate that precision time recovery is possible using common view LORAN-C
— Results over 300 mile baseline with initial data set show that 25-50 ns (RMS) time recovery is possible
— In order to bound performance, long term data is required
Preliminary data points to common view LORAN-C is a viable method for precision time recovery backup to GPS
— Only method to provide < 50 ns timing to anywhere in US
In order to realize precision timing performance, Enhanced LORAN-C baseline must include TOA based monitoring with timed monitor sites
— Requires a change to the current SAM hardware configuration
— GPS dependence can be addressed if required
Experiment will continue and results will be presented at upcoming conferences