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

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297 297.5 298 298.5 299 299.5 300 300.5 301 301.5 302

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UT

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ffse

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TSC (Boulder)

Passive L1 GPS Data from Two Sites

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297 297.5 298 298.5 299 299.5 300 300.5 301 301.5 302

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Gillette Data

TSC (Boulder)

GPS Common View

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297 297.5 298 298.5 299 299.5 300 300.5 301 301.5 302

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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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52150 52200 52250 52300 52350 52400 52450 52500 52550 52600

mjd

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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*

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219 229 239 249 259 269 279 289

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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*

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280 285 290 295 300 305 310

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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

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282 284 286 288 290 292 294 296 298

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CV

Off

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(ns)

GPS CV

LORAN CV (9610V)

Comparison of GPS Common View and LORAN Common View*Zoom on Clock Cold Start at TSC

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287.5 287.6 287.7 287.8 287.9 288 288.1 288.2 288.3 288.4 288.5

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CV

Off

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(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

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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

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282 284 286 288 290 292 294 296 298

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CV

Off

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(ns)

LORAN CV (9610X)

GPS CV

Comparison of GPS Common View and LORAN Common View* Short Common View Baseline - TSC-NIST - Std Dev = 25ns

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282 284 286 288 290 292 294 296 298

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Off

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(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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282 284 286 288 290 292 294 296

Day of Year

Co

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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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282 284 286 288 290 292 294 296

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CV

off

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(ns)

GPS CV

9610M CV

GPS Common View vs LORAN Common View*Long Common-View Baseline - TSC (Boulder) to Gillette (WY)

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282 284 286 288 290 292 294 296

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off

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(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