Two-Way Time Transfer (TWTT)Jeremy Warriner, Symmetricom

TWTT Overview

Phase offset between two clocks can be determined using Two-Way Time Transfer (TWTT) technique A pulse is transmitted by each clock at the top of the second Time difference transmitting a pulse and receiving a pulse from the

remote clock is measured The difference of the two measurements is twice the clock offset Requires that the path delay be symmetric and requires a mechanism for

sharing the measurements between the two sites

CH1CH2

Time Interval Counter

Local Clock

MEASLOCAL = A – (B + dBA)

CH2CH1

Time Interval Counter

Remote Clock

dAB

dBA

MEASREMOTE = B – (A + dAB)

A

B

dAB = dBA

(MEASREMOTE - MEASLOCAL) / 2 = B – A = Remote Clock Delay

Time Transfer Modem

Time Transfer Modem (TSC 4402) technology development completed using program funding

Time Transfer Modem accepts clock inputs and generates a coherent waveform at an IF that can then be transmitted over RF links Time difference measurements are made between the transmitted waveform and the

received waveform Measurements are shared between the two sites using the established RF link Modem calculates the relative clock offset between the two sites and provides the

information to the user

T1 T2

ATS 6502

Original modem has been replaced with the ATS 6502 Technology and specialized hardware

remains unchanged but the power and timing distribution is improved

– Integrated power control allows hardware to be powered on sequentially to eliminate race conditions

– Front panel interface added to ease setup Calibration data stored on the hardware so

software can be updated with no impact to the calibration

Improved thermal management allows for more air flow and cooler operating temperatures

Capability to integrate Rb oscillator internal to the system and provide local time recovery

Capability to integrate GPS receiver internal to the system and provide positioning information as well as backup to TWTT timing

Form factor is extended to full rack width of a 2U instrument instead of the half rack TSC 4402 version

ATS 6502 - Time Transfer Modem

TSC 4402 - Time Transfer Modem

Block DiagramP

CI

BU

S

ATS 6502

Time-based Transmitter

10 MHz IN

1 PPS IN

10 MHz OUT

IF OUT

Single-board Computer

NTP

Digital Processor

HSI

RF Motherboard

10 MHz IN

NTP 10 MHz 1 PPS

IF IN Tx Filter

Rx Filter / AGC

IF OUT

IF INRx IF

CH 1 (Tx)

CH 2 (Rx)HSI

Two-Channel Digitizer

Sample CLK

50 MHz OUT

Tx IF

Block Diagram Explained

Clock inputs are accepted by the ATS 6502 and used to generate an IF waveform within the time based transmitter

Transmitted and received IF waveforms are digitally sampled by the two-channel digitizer

Sampled waveform data is passed to the digital processor over a high speed interface (HSI)

Digital processor calculates the time difference between the transmitted signal and received signal Measurement passed to the computer using the PCI bus

Measurement information is passed to the time-based transmitter via the PCI bus and transmitted to the remote site via the RF link

Once the local and remote measurement is received by the computer it calculates the relative clock offset between the two sites

IF Waveform

Waveform generated by the Time Transfer Modem must fulfill two requirements Must be coherent with the reference clock being measured

– If the waveform is not coherent with the reference clock then the clock offset provided by the system will be that of the internal oscillator and not the reference

Must include a unique event in time that can be measured– Without a unique event the system has no way of ensuring that the event

being measured locally is the same event that was measured by the remote site

Synchronous data link structure developed to provide timing markers at regular intervals Timing markers are unique and provide an event in time that the system

can measure Unused data bandwidth is available to users for data transfer

Also used for transferring the timing measurement to the remote system

Signal Structure

Frame Structure – 16 bits (12 bits fixed (1’s), 4 bit counter)

Data Structure, Size = 2368 data bits / second on a 2.5 kbit/sec link = 276 bytes / second

0 250 500 750

Time (ms)

1000

GPS Second Counter, 32 bits

Station Identifier, 32 bits

Fixed bit (0)

Totals: 625 bits / frame, 4 frames / second, 276 bytes / second

Measurement Event

0 250 500 750

Local Time (ms)

1000

MEASLOCAL

TX

RX

The time difference between transmitting the first data bit of the framing pattern and receiving the first data bit of the framing pattern from the remote modem is the desired measurement event Measurement process is repeated for all 2500 bits within the second Measurements are then averaged to improve the system performance

Clock Measurement

Clock offset is obtained by differencing the measurements from the two modems and dividing by two Process is completed automatically by the ATS 6502 modem

Clock offset data collected from two time transfer modems co-located in a laboratory Both modems running off of the same input reference

2

LocalRemote MEASMEAStClockOffse

ATS 6502 Measurement Precision

Time (0.2 Hours / div)

Clo

ck O

ffs

et (

2 p

s /

div

) σ = 2.36 ps

Tx IFRx IF

ATS 6502 (Local)

Tx IFRx IF

ATS 6502 (Remote)

LocalMEAS

RemoteMEAS

Extension of TWTT to Dynamic Scenarios

Static TWTT

Static two-way time transfer involves making simultaneous time difference measurements between two fixed points on the earth

In the static case, propagation delay to the satellite cancels and two-way equation reduces to:

Where SAGNAC is a time-of-flight measurement effect and is a constant

MEASLocal = T1 – (T2 + delay3 + delay4 + Sagnac12)

MEASRemote = T2 – (T1 + delay2 + delay1 + Sagnac21)

In Static Two-Way Time Transfer, all of the data corrections are constants and can be computed ahead of time

delay3

delay2delay1

delay4

delay1 + delay2 = delay3 + delay4

SAGNAC

MEASMEASkDelayRemoteCloc LocalRemote

2

T1 T2

Dynamic TWTT

2

Dynamic TWTT is identical to static TWTT except that one or both of the platforms may be moving during the measurement interval Propagation delay of signals no longer cancels Sagnac effect is time varying instead of being a constant

TWTT equation must be modified to use corrected TWTT measurements instead of raw TWTT measurements

delay1

delay2

delay4 delay3

delay1 + delay2 ≠ delay3 + delay4 2

LocalRemote CorrectedCorrectedkDelayRemoteCloc

Corrected TWTT Measurements

2

Corrected TWTT measurements are comprised of the raw measurement and two correction terms First term corrects for platform motion during the measurement interval Second term corrects for changes in path delay due to Sagnac effect

Raw measurement can be corrected before transmitting the measurement to the opposite side of the link Reduces the amount of data that needs to be transmitted

2

LocalRemote CorrectedCorrectedkDelayRemoteCloc

LocalLocalLocalLocal SAGNACMOTIONMEASCorrected

RemoteRemoteRemoteRemote SAGNACMOTIONMEASCorrected

Where,

Correction for Platform Motion

Correction for platform motion given by the above equation First term corrects the TWTT measurement for the motion of the platform when

both timing signals are transmitted simultaneously Second term corrects the TWTT measurement for non-coincident times of

transmission Speed of light terms in the denominator reduces the effect of position errors on

the TWTT algorithm Remote platform refers to the satellite in the case of a satellite relay

c

cremloc

c

c

c

ctt

ccMOTION

lorem

loremloc

lorem

loremloc

loremloc

xx

xxv

xxxxv

xxv

2

xloc = position vector (ECEF datum) of the local platform

xrem = position vector (ECEF datum) of the remote platform

vloc = average velocity vector (ECEF datum) of the local platform during the measurement interval

tloc = time the local timing signal is transmitted

trem = time the remote timing signal is transmitted

c = speed of light.

Correction for Sagnac Effect

Correction for the Sagnac effect on the TWTT measurement given by above equation Sagnac does not change appreciably over the measurement interval

but the effect is seen when larger distances have been traveled by one of the platforms

The speed of light term in the denominator reduces the effect of position errors on the correction

Remote platform refers to the satellite in the case of a satellite relay

= angular velocity of the Earth

xloc = position vector (ECEF datum) of the local platform

xrem = position vector (ECEF datum) of the remote platform

c = speed of light.

2

ˆˆˆˆˆˆˆˆˆ2

cSAGNAC

zyyxxxxyyxxxx remremlocloc

Flight Test

Test Plan

Characterize the offset between two Cesium clocks (HP 5071A) prior to the flight test Clocks are co-located at Kirtland AFB Time offset measured using a time interval counter (TIC)

Transport the flight clock to the aircraft Clock remained operational on battery power

Measure the offset between the flight clock and the ground clock using TWTT Two independent TWTT systems operated throughout the flight

– Both systems measure the time offset between the flight clock and the ground clock– Measurement between the two systems should agree because they both measure the

same physical quantity Using different satellites for the communication link creates significantly different

dynamics for the system– Difference appears in the raw TWTT measurements (presented later)– Real-time TWTT corrections must be correct or the clock offset calculated by the two

systems will not agree Transport the flight clock from the aircraft to the hangar

Clock remained operational on battery power Characterize the offset between the ground clock and the aircraft clock

using a TIC

Flight TestIntelsat 707 (53º West)

Intelsat Americas 7 (129º West)

Kirtland AFB, NM

Big Crow, C-135

Side A Side B

Ground Setup

Ground clock was a Cesium reference (HP 5071A) and provided timing signals (10 MHz, 1 PPS) to both TWTT systems

Each ground terminal comprised of a 0.75 meter dish and 25 Watt Transceiver

TSC 4400 (GPS Time & Frequency Reference) used to provide NTP to the TSC 4402 (Time Transfer Modem)

Side A Side B

GPS Antenna

HP 5071A

Aircraft Setup

Flight clock was a Cesium reference (HP 5071A) and provided timing signals (10 MHz, 1 PPS) to both TWTT systems

RF equipment and timing equipment separated into two co-located racks on the aircraft

Two independent Ku-band antennas (24 inch) installed under radome Each antenna tracks a geostationary satellite in

azimuth, elevation, and polarization

Side ASide B

Timing RackRF Rack

Flight Path

Take Off: 0740 MST, Wednesday April 5th, 2006 Landing: 1350 MST, Wednesday April 5th, 2006

TWTT Data (Raw) Traditional TWTT

calculation performed on flight data No corrections made

for platform motion or Sagnac effect

Dominant effect seen in raw data is a result of aircraft motion during the measurement interval Only motion in the

direction of signal propagation causes error

140 ns of variation seen during this test

Raw TWTT measurements are considerably different because two different satellites are used

Time (1.2 hours/division)

Off

se

t (2

0 n

s/d

ivis

ion

)

2Re Re Localmote MEASMEAS

elaymoteClockD

Raw TWTT Measurements

TWTT Data (Corrected for Motion)

Data corrected for platform motion Discrete jumps in

measurement no longer present

No corrections have been made for the Sagnac effect

Time (1.2 hours/division)

Off

se

t (1

0 n

s/d

ivis

ion

)

2

)()(Re ReRe LocalLocalmotemote MOTIONMEASMOTIONMEAS

elaymoteClockD

TWTT Measurements (Motion Corrected)

TWTT Data (All Corrections Applied)

TWTT data corrected by the terms for platform motion and the Sagnac effect

Measurements from both TWTT systems agree Validates the real-

time corrections

Time (1.2 hours/division)

Off

se

t (2

ns

/div

isio

n)

2

)()( LocalLocalLocalRemoteRemoteRemote SAGNACMOTIONMEASSAGNACMOTIONMEASkDelayRemoteCloc

Real-time TWTT Data (60s Avg)

TWTT Accuracy (60s Avg) Real-time TWTT data is consistent with the “truth” data from before and

after the flight TWTT data provides a record of what happened to the clock during the flight

Measurements from two independent TWTT systems are consistent with each other Mean offset between systems was 0 ns RMS difference between systems was 860 ps over the duration of the flight

Time (1.2 hours/division)Time (1.2 hours/division)

Off

se

t (2

ns

/div

isio

n)

Off

se

t (2

ns

/div

isio

n)

Difference Between Measurements

RMS = 860 ps

Real-time TWTT Data (60s Avg)

Real-time TWTT Data

Real-time data collected on 1 second measurement intervals Side A system had difficulty maintaining lock during the first half of the

flight Appeared to be related to the look angle through the radome

Side B system noisy due to two satellites being close enough in orbit that the timing signal passed through both satellites creating a multi-path scenario

Time (1.2 hours/division)Time (1.2 hours/division)

Off

se

t (5

ns

/div

isio

n)

Off

se

t (5

ns

/div

isio

n)

Real-time Measurements – Side BReal-time Measurements – Side A

Real-time Performance

Residual between two systems used to estimate real-time performance 1 second measurements from side A compared to the 60 second average of

side B (served as truth) Exploded view of “good” section shows the expected performance of the

system This is what the system performance would have looked like if there weren’t

issues with the RF link during the first part of the flight

Time (15 minutes/division)Time (1.2 hours/division)

Re

sid

ua

l (2

ns

/div

isio

n)

Re

sid

ua

l (2

ns

/div

isio

n)

Expected PerformanceReal-time Performance

σ = 1.34 ns σ = 600 ps

Static TWTT Performance

Static Testing

Objective Characterize performance of dynamic TWTT system (TSC 4402)

operating over a static baseline Description

Compare TWTT measurements of the same event using two independent systems

– Event: Clock offset between U.S. Naval Observatory and Symmetricom (Boulder)

– System 1: TSC 4402 (Device under test)– System 2: Timetech SATRE TWSTFT Modem (Baseline)

Differences in the measurements represent the combined measurement error of the two systems

Experimental setup (diagram on next slide) Each site clock used as a common reference for both TWTT systems Both TWTT systems utilize same type of Ku-band transceiver

– Minimize performance differences due to ancillary equipment Each TWTT system operated over a different satellite

– Effects from satellite motion will be different for the two systems

Experimental Setup

Size: 1 meterBW: 2.5 MHzData Rate: 10 kbpsCode: 27

Size: 1.8 meterBW: 3.0 MHzData Rate: 0.25 kbpsCode: 7

Size: 1 meterBW: 2.5 MHzData Rate: 10 kbpsCode: 11

Size: 1.8 meterBW: 3.0 MHzData Rate: 0.25 kbpsCode: 6

Symmetricom (Boulder) U.S. Naval Observatory

Intelsat 707 (53º West) AMC-1 (129º West)

Antenna Configuration

SATRE TWTT system 1.8 meter dish 4 Watt Ku-band

Transceiver– Anasat 4Ku

Symmetricom TWTT system 1.0 meter dish 4 Watt Ku-band

Transceiver– Anasat 4Ku

Symmetricom (Boulder)

1.8 meter

1.0 meter

Performance Comparison Comparison of the data between the systems is as expected

Short-term noise is comparable Long-term agreement is very good Medium-term variations dominate the measurement noise

– Result mostly from using short code lengths (127 bits) on TSC 4402 (discussed later)

Only significant disagreement occurred during a snow storm in Boulder System returned to normal when the sun came out and began melting the

snow

σ = 1.25 ns

TWTT Data (60 sec average)

Time (0.5 days / division) Time (0.5 days / division)

Off

set

(2

ns

/ d

ivis

ion

)

Res

idu

al (

1 n

s /

div

isio

n)

Difference (60 sec average)

Snow Storm

Medium Term Variations

Medium term variations are the result of an effect referred to as code-contamination

The TSC 4402 uses Gold codes as the chipping code for generating a direct-sequence spread spectrum signal Gold codes are families of codes that exhibit well behaved cross-

correlation properties Gold codes from the same family look like pseudo-random noise to

each other when they are summed together The longer the Gold code sequence the lower the “noise” floor

appears to other codes in the family– TSC 4402 uses 127 bit Gold codes whereas systems such as GPS use

1023 bits

Noise from other codes is not truly random and can affect the timing measurement differently depending on how the codes are summed together

Effect of Satellite Motion

Symmetricom (Boulder) U.S. Naval Observatory

Satellite motion causes changes in path delay from the ground stations to the satellite

Changes in path delay cause the timing signals from the ground station to sum together differently over time (represented by the changing colors in the summed signal)

Code 1

Code 2

Sum

0 1 1 1 1 10 0 0

0 0 0 0 01 1 1 1

Evidence of Code Contamination

If code contamination is a significant portion of the medium term noise than the variations should repeat on a diurnal cycle

Plots show data from the same time on two different days Strong correlation between the

major noise components of both plots

Supports claim that major noise component of the system is from code contamination

Time (1.2 hours / division)

Off

set

(1

ns

/ d

ivis

ion

)

TWTT Data (TSC 4402)

Off

set

(1

ns

/ d

ivis

ion

)

TWTT Data (TSC 4402)

Time (1.2 hours / division)

Static vs Dynamic

Why does the TSC 4402 perform better in dynamic applications than it does in static ones? In dynamic applications the path delay to the satellite changes much

quicker and thus the measurement variations occur on a much shorter timescale

Significant portions of the variation are averaged out by the 1-second measurement interval

Expected Performanceσ = 1.3 ns

Time (0.5 days / division)

Res

idu

al (

1 n

s /

div

isio

n)

Static Test Result (1 sec) Flight Test Result (1 sec)

σ = 0.6 ns

Time (15 min / division)

Res

idu

al (

1 n

s /

div

isio

n)

Improving Static TWTT Performance

Increasing Gold code length Increasing the code length provides better cross-correlation properties with other

codes and reduces the amount of “contamination” between codes Increasing the code length in the TSC 4402 requires that the data rate be

reduced in order to occupy the same signal bandwidth– Data rate will be reduced from 10 kbps to 2.5 kbps when the code is extended from

127 bits to 511 bits Work performed under current contract

Changing the point of coincidence for the system Currently the system transmits the timing signals so that they leave the Earth

stations at the same time Transmission time can be adjusted so that the timing signals arrive at the satellite

at the same time– Ensures that signals always sum with the same phase relationship and thereby

removes the time varying component of code contamination– Loopback signal from the satellite serves to range the satellite and continually adjust

the time of transmission– Adjustments to the time of transmission must be corrected in the TWTT measurement

data To be investigated under new contract (discussed later)

Extended Gold Code

Gold code length extended from 127 bits to 511 bits to improve timing performance in static scenarios

Characterization of performance improvement completed using identical test setup as before between Symmetricom and USNO

Standard deviation improved from 1.3 ns to 0.95 ns

Time (0.5 day / division)

Res

idu

al (

1 n

s /

div

isio

n)

ATS 6502 (black) vs SATRE (red) TWTT Comparison

Time (1.2 hours / division)

Off

set

(1

ns

/ d

ivis

ion

)

σ = 0.952 ns

Improving TWTT Performance

Improving performance

FY08 funding Characterize the fundamental limits of the current TWTT

technology Identify potential improvements to the TWTT technology that will

enable precise TWTT (sub-100ps) Prototype the precise TWTT system and characterize the

performance improvement

Primary areas of interest Error contributions of dynamic TWTT corrections Alternative signal structures that may yield improved measurement

performance Systemic errors induced by hardware and temperature variations System architecture changes that can improve TWTT performance

TWTT Error Budget Current TWTT error budget identifies

multiple areas requiring enhancements in order to break the 100 ps barrier

Error contribution for static scenarios and dynamic scenarios may be different but the budget incorporates the error from the worse of the two scenarios

Primary error sources Architecture: Variations due to the

overall system architecture design RF link: Variations introduced by the

details of the RF link budget Hardware: Variations introduced by

systemic hardware issues such as temperature coefficients

Corrections: Variations introduced by uncertainty in the correction terms applied to TWTT measurements

Atmosphere: Variations introduced by non-symmetric delays through the atmosphere

Architecture 650 ps

RF Link 550 ps

Hardware 400 ps

Corrections 50 ps

Atmosphere 10 ps

Total 950 ps

SOURCE ERROR

System Architecture

System architecture for TWTT design focused on maximizing scalability of the system and portability of TWTT terminals while achieving sub-ns real-time performance TWTT terminals were to be easily added to a network as well as be able

to operate from small ground stations and airborne platforms Code Division Multiple Access (CDMA) technique chosen to allow

multiple terminals to operate simultaneously in the same frequency band

Geostationary Satellite

Master 1 Master 2 Master Standby 1

Master Standby 2

Slave 1 Slave 2 Slave N-1

LANLAN

Master ChannelMeasurement Channel 1

Measurement Channel 2

Distress Channel

Slave N

System Architecture (cont.)

CDMA architecture implemented using Gold codes to generate a direct sequence spread spectrum BPSK signal Low data rate signal (2.5 kbps) is

spread across a wider frequency bandwidth (2.5 MHz) by chipping the data signal with a Gold code

Use of Gold codes allows all stations to transmit at the same frequency

– Stations are distinguished by their code Spread spectrum communication signal

provides a wide bandwidth signal with a relatively low data rate Wider bandwidth signals improve the

measurement precision of the system Lower data rate (2.5 kbps) enables the

use of small 1-meter antennas

2.5 MHz Signal

Current TWTT Ground Terminal

Architecture Limitations

Transmitting all signals on the same frequency channel introduces error due to non-zero cross-correlation values between the signals Increasing the Gold code length improves the cross-correlation

properties but reduces the overall data rate that can be transmitted in the same frequency bandwidth

Error introduced by this effect is on the order of 650 ps for the 511 bit Gold code operating at a 2.5 kbps data rate

Two options for reducing error contribution of the system architecture Move the point of coincidence for the system from the ground

terminals to the satellite Change architecture to use frequency division of signals so that

they no longer occupy the same spectrum and interfere with one another

Architecture Enhancement

Symmetricom (Boulder) U.S. Naval Observatory

Changing the point of coincidence involves measuring the range to the satellite and adjusting the time at which the signal is transmitted so that it always arrives at the satellite at the top of the second Causes the Gold codes from the various ground terminals to always sum in the

same way and removes the time-varying portion of the error Error bias still exists but it no longer varies in time so it can be calibrated

Code 1

Code 2

Sum

0 1 1 1 1 10 0 0

0 0 0 0 01 1 1 1

Architecture Change

Changing system from a code division architecture to a frequency division architecture reduces interference between signals from different TWTT terminals Eliminates errors due to non-zero cross-correlation properties Easier to support larger data rates because it is no longer necessary to

maintain cross-correlation properties by utilizing higher order Gold codes Easier to support asymmetric data rates on the forward and reverse data

links because there are no cross-correlation properties to maintain As with any trade-space, compromises must be made in other areas to

achieve the increased performance Requires more EIRP for same performance because bandwidth and SNR

are not equivalent contributors to Cramer-Rao inequality Introduces additional asymmetry in group delay between the TWTT

terminals because they no longer occupy the same frequency space and therefore go through different parts of the satellite transponder

– Placing signals close to each other in frequency helps mitigate this effect

Architecture Change (cont.) Primary benefit of architecture change is

that it allows the investigation of alternative signal structures that may improve measurement performance

Offset Frequency Division Multiplexing (OFDM) allows the use of carrier phase measurement techniques in addition to the code phase measurements currently being made OFDM signal is comprised of multiple

sub-carriers which each contain their own modulated data stream

Code phase of the individual carriers is used in making a coarse timing measurement

Precise timing measurement is performed by measuring the differential phase of the individual sub-carriers within the OFDM signal

Allows the use of low-noise phase measurement technologies developed under other projects

OFDM Signal

vd = dΦ / df

RF Link RF link budget affects performance

according to the Cramer-Rao inequality and is bounded by the signal bandwidth, SNR, and integration period Portable ground terminals used in current

TWTT architecture limit system performance to around 550 ps

Performance threshold can be improved by increasing the dish size or transmitted power

Changing the system architecture from a CDMA scheme to a frequency division scheme decreases measurement accuracy versus EIRP ratio Signal bandwidth has larger effect of

measurement performance than does SNR

System performance can be enhanced as necessary by increasing the transmitted data rate and EIRP

Current TWTT Ground Terminal

Hardware Constraints Measurement errors introduced by

hardware at the TWTT ground terminals is around 400 ps Primary measurement error is due to

temperature variations within the modem and Ku-band transceiver

Decreasing error contribution of hardware can be accomplished by either decreasing the temperature coefficient of the system or calibrating out temperature effects Both methods are to be investigated

L-band BUC is to be analyzed for delay variation with respect to temperature Currently use a 70 MHz IF input to the

transceiver but L-band BUC does not include as much signal filtering and may have a better temperature coefficient

Requires incorporating a L-band IF signal as part of the time transfer modem

Calibration of time transfer modem is to be performed with respect to temperature Allows the removal of a significant portion of

the delay variation within the modem

Modem Delay Variations Due to Temperature

Time (2 Hours/Division)

Del

ay (

100

ps/d

ivis

ion

)

σ = 175 ps

Dynamic Corrections

Corrections to the TWTT measurement are required when a system is operating on a mobile platform Corrections account for non-symmetric changes in path delay as well as time-varying

Sagnac effects Uncertainty in the position and velocity of the platforms manifests itself as error in

the correction terms Current errors are on the order of 50 ps depending upon the accuracy of the

positioning system Total impact of position errors is not fully understood at this point

Position of a satellite relay includes a substantial portion of cancelling error Detailed analysis is to be completed to determine how accurately platform positions

must be known in order to provide the desired level of time transfer performance

c

cremloc

c

c

c

ctt

ccMOTION

lorem

loremloc

lorem

loremloc

loremloc

xx

xxv

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2

2

ˆˆˆˆˆˆˆˆˆ2

cSAGNAC

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Atmospheric Constraints

TWTT requires symmetric path delay between the two ground sites Differences in the path delay result from

ray path refraction due to water vapor in the atmosphere

Changes in path delay are not necessarily an issue, only non-symmetric changes between the uplink and downlink introduce error

Non-symmetric path delay through the atmosphere is due to the different uplink/downlink frequencies of commercial Ku-band Uplink: 14.5 GHz, Downlink: 12.0 GHz Net error due to this effect is less than 10

ps and will not be investigated further under this contract

delay3

delay2delay1

delay4

delay1 + delay2 = delay3 + delay4

T1 T2

Plan of Action

Improving TWTT to sub-100ps level requires effort in multiple areas Changing system architecture to reduce signal interference Analyzing alternative waveforms to improve measurement performance Improving hardware stability to mitigate effects of temperature variations Analyzing effect of position error on TWTT to determine the accuracy needed from a

positioning system Prioritizing tasks is based upon availability of resources and other issue

Analysis of position error effect can begin immediately Measurements and characterization of the hardware stability can begin with the

current hardware and continue as hardware improvements are made Analysis of various TWTT waveforms will begin in the laboratory using X-MIDAS

system so that data can be analyzed without significant hardware changes being made

– Once an acceptable waveform is identified then it will be implemented and tested in hardware

– Changing the waveform also requires significant changes to the system architecture to be completed before it can be realized in hardware

Changing system architecture requires some initial analysis to determine the most efficient approach for changing the system architecture