V-1

A New Satellite Time Service

Enhancing and Extending LORAN-C

Al Gifford

National Institute of Standards and Technology

James Doherty

Institute for Defense Analysis

Tom Bartholomew

Northrop Grumman TASC

V-2

Overview

• The basic idea presented is the measurement of time differences between LORAN stations using a two-way-time-transfer device on NASA satellites

• A new LORAN S-band 2ns time service is proposed

• The technology, devices and development process will be discussed

• Projected performance goals of the enhanced and upgraded LORAN services are presented

V-3

Basic Idea

• LORAN-C is upgrading the three elements of timekeeping – Clocks upgraded to Agilent 5071s (approximately 100)

– Clock measurement capability upgraded to state-of-the-art; includes and GPS receiver for cross-site measurement

– Clock management will include ensembling of site clocks and a possible calculation of a system wide distributed time scale (steering to UTC(USNO) is the current plan)

• LORAN navigation and time services will be significantly enhanced

• This paper proposes an extension of these LORAN services

Utilizing the LORAN state-of-art distributed timekeeping system, an Ultra High Precision (UHP) global time service operation at S-band from NASA satellites could be realized

V-4

Enhanced and Extended Services

Regional Services • Navigation• Time (50-500ns)• GPS Augmentation

Global Services• Time (<2ns)• Ephemeris

Upgraded LORAN-C

Satellite Time Service

V-5

Why?

• UHP Time users with global applications are dependent solely on GPS

• USNO’s primary time transfer vehicle is GPS and its alternate to UHP users (Two-way Satellite Time & Frequency Transfer) is operated from only a single location

• A Backup to the DOD Positioning-Navigation-Timing (PNT) infrastructure is required

• DOD Instructions require backup for some applications (e.g. C4ISR)

• LORAN is a UHP user that would benefit from an alternate UHP time transfer service

V-6

Why Extend LORAN Services?

• LORAN has invested a significant amount in a distributed timekeeping system in order to provide a robust regional navigation and time service – Internally, time will be managed to the 15ns level via GPS direct broadcast

• There is a potential of utilizing GPS common view measurements to compute an independent time scale

– The broadcast LORAN signals will provide UTC <500ns– This service meets Stratum I frequency requirements but is not suitable for

UHP users– Time service will gradually degrade in the absence of GPS service

• The LORAN infrastructure could provide the basis for a UHP satellite service utilizing its distributed clock assets as a flywheel time scale– This LORAN capability coupled with recent technology developments in

communications based time-transfer devices could enable a 2ns global time transfer system

V-7

How it could work

• LORAN would compute a distributed time scale with the cross-station measurements of clocks

• Using the GPS timekeeping model, LORAN system time would be steered to UTC through USNO or directly to UTC(BIPM)

• NASA would provide a time-based-comms device on several satellites which would be accessible to multiple users

• LORAN operators would schedule the collection times for satellite and ground assets and upload these schedules

• The comm devices would initiate the measurements and provide these clock time differences to the operators in real-time

• A Low Earth Orbiting (LEO) satellite would require an atomic clock in order to flywheel between station measurements

• A Geosynchronous (GEO) satellite could provide continuous regional measurements between sites

V-8

Satellite Time Service

A LORAN/NASA Operated System

LEO

GEO

A LEO Implementation would require an atomic clock of the type that is

flown on GPS IIR. LEO coverage is global.

A GEO Implementation would not require an atomic

clock and could provide service continuously.

GEO coverage is regional

V-9

The Technologies

• Metrology: independent verification of time transfer– Flight verification of metrology in DARPA AT3 program

• Two-way Time Transfer measurement• Understanding and implementing physical principles

– Handbook and simulator for relativistic time transfer– Relativistic transformation of satellite proper time to coordinated time

• The hardware devices– NASA/Goddard Low Power Transceiver (LPT) (supporting manned

missions)– NASA/JPL BlackJack receiver (supporting science missions)

• The test and evaluation– Current aircraft testing underway

• Flight opportunities– LPT to fly on shuttle in early 03; time transfer mods to be complete in 04– BlackJack is currently flying on NASA science missions– Both devices will be utilized for time transfer on the Space Station

V-10

Clock offset data for the entire test period. 

The AT3 PVTF risk-reduction flight tests were conducted on a T-39 aircraft flown by the 412 Test Wing at Edwards AFB, CA.

Verification of Metrology

Aggregate Clock Offset

1450

1550

1650

1750

1850

1950

2050

3/20/010:00

3/25/010:00

3/30/010:00

4/4/010:00

4/9/010:00

4/14/010:00

4/19/010:00

4/24/010:00

Date/Time (local)

Off

se

t (n

s)

GroundTest

FlightTest 1

FlightTest 2

FlightTest 3

V-11

The estimated relativity effects during flight test 1 were:Gravity: 9.4 ns (fast)Velocity: -1.63 ns (slow)Sagnac: -0.1ns (slow)Total: 7.66 ns (fast)Measured: 5.97 ns (fast)Delta: 1.69 ns

The estimated relativity effects for flight test 3 are:Gravity: 5.83 ns (fast)Velocity: -1.19 ns (slow)Sagnac: 0.0 nsTotal: 4.64 ns (fast)Measured: 7.29 ns (fast)Delta: -2.65 ns

Looking across all three flight tests, the relativity prediction error statistics were:

Mean 0.35 nsMax 2.01 nsMin -2.65 nsMax abs 2.65 nsMin abs 1.69 nsStd dev 2.60 ns

The estimated relativity effects during flight test 2 were: Gravity: 8.96 ns (fast)Velocity: -1.64 ns (slow)Sagnac: -0.11 ns (slow)Total: 7.21 ns (fast)Measured: 5.2 ns (fast)Delta: 2.01 ns

V-12

Satellite orbital propertiesSatellite ISS TOPEX GPS Molniya GEO TundraSemimajor axis km 6766 7715 26 562 26 562 42 164 42 164Eccentricity 0.00 0.00 0.02 0.722 0.01 0.2684Inclination deg 51.6 66.0 55 63.4 0.05 63.4Argument of perigee deg 0 0 0 250 0 270Apogee altitude km 388 1337 20 715 39 362 36 208 47 103Perigee altitude km 388 1337 19 653 1006 35 364 24 469Ascending node altitude km 388 1337 19 653 10 507 35 364

32 749Period of revolution s 5539 6744 43 083 43 083 86 164 86 164Mean motion mrad/s 1.134 0.932 0.146 0.146

0.0729 0.0729rev/d 15.6 12.8 2.0 2.0 1.0 1.0Mean velocity km/s 7.675 7.188 3.874 3.874 3.075 3.075

Clock effectsSecular time dilation s/d -28 -25 -7 -7 -5 -5Secular redshift s/d 3 10 46 46 51 51Net secular effect s/d -25 -14 38 38 46 46Amplitude of periodic effect due to eccentricity ns 0 0 46 1653 29

774Peak-to-peak periodic effect due to eccentricity ns 0 0 92 3306 58

1549Secular oblateness contribution to redshift ns/d 23.0 27.6 0.5 2.5 -0.1

0.2Amplitude of periodic effect due to oblateness ps 256 286 38 167 0

27Peak-to-peak periodic effect due to oblateness ps 512 572 76 334 0

54Amplitude of periodic tidal effect of Moon ps 0.0 0.0 1.2 1.2 6.1

6.1Amplitude of periodic tidal effect of Sun ps 0.0 0.0 0.5 0.5 2.7

2.7

Signal propagationMaximum Sagnac effect ns 12 22 136 234 218

275Gravitational propagation delay along radius ps 0.8 2.5 -4.7 -4.7 -27.3

-27.3Amplitude of periodic fractional Doppler shift 1012 0.0 0.0 6.7 241.1 2.1

56.5

Excerpt from Handbook on Relativistic Time Transfer

V-13

NASA/Goddard LPT

MODEL WEIGHT SIZE POWER EDM ~ 4 kg 4.35” x 5.75” x 5.0” 7.5 W PPM ~ 3 kg 4.35” x 5.33” x 4.68” 7.5 W VFM* ~ 3 kg 4.35” x 5.33” x 4.68” 5 W

V-14

Enhanced and Extended LORAN Three Levels of Configuration and Performance

Core/GPS:

LORAN station timing systems interoperating with direct GPS and GPS common view between stations

Core/GPS/STS:

Interoperating via STS satellite(s) with TWTT and direct GPS and common view

Core/STS:

Operation using only LORAN system time-scale as reference input to STS; time-scale available via LORAN-C and STS

V-15

Predicted Performance ofEnhanced and Extended LORAN

System

Conf

Time Transfer

WRT UTC

Frequency Transfer

“UTC”

Recovery

without GPS

Flywheel (Fw)

Independent of GPS

Comments

CORE/

GPS

50-500ns 1x10-12 50-500ns

+ RSS Fw

Time <1 μs (days)

Freq<10-11 (forever)

CORE/

GPS/

STS

<5ns (STS)

<200ns (Loran)

2x10-14

1x10-12

<15ns (STS)

+ RSS Fw

<200ns (Loran)

+ RSS Fw

Time < 100ns (years)

Freq <1x10-13

(forever)

STS implemented

on LEO

CORE/

STS

(no GPS)

<5ns (STS)

<200ns (Loran)

2x10-14

1x10-12

<15ns (STS)

+ RSS Fw

<200ns (Loran)

+ RSS Fw

Time < 100ns (years)

Freq <1x10-13

(forever)

STS implemented

on LEO

V-16

Summary

• The basic idea presented in this presentation was the measurement of time differences between LORAN stations using a two-way-time-transfer device on NASA satellites

• A new LORAN time service would provide backup to GPS in UHP applications (including LORAN)

• The technology is mature enough to support this proposed Satellite Time Service

V-17

CLOCK 1Time = T1

+-

CLOCK 2Time=T2

+

-

MEAS2 = T2 - (T1+TD)

MEAS1 = T1 - (T2+TD)

Desired Measurement: T2 - T1 = .5*(MEAS2 - MEAS1)

Basic Two-Way Time Transfer Measurement

Measurement Requirements

1) Event (pulse) to measure2) Low noise measurement of event3) Mechanism to exchange data between locations4) Reciprocal Delay (over measurement interval)

Where:T1= Time of Clock 1T2= Time of Clock 2TD= Propagation Delay

Backup

V-18

Cross-Site Data via GEO

Daily data sets color coded

Standard Deviation of measurement noise is < 1ns

Long term variation in curve can be attributed to clock steering at RUNWAY and REGIME

Higher Noise due to lower bandwidth

REGIME Clock beginning to fail

REGIME Clock degraded

REGIME Clock replaced

REGIME timing deviation due to new clock

Backup

V-19

Two-way time transfer using Fiber

• Data collected in the lab from SONET fiber optic timing equipment (best case scenario) – 17 ps rms over 12 hours

-50

-40

-30

-20

-10

0

10

20

30

40

50

0 2 4 6 8 10 12

Hours

pic

ose

con

ds

Backup