V-1 TFE: The New Heartbeat of Loran T. P. Celano, Timing Solutions Corporation LT Kevin Carroll,...

V-1

TFE: The New Heartbeat of Loran

T. P. Celano, Timing Solutions CorporationLT Kevin Carroll, Loran Support Unit

V-2

Introduction

The USCG is leading an effort to modernize the LORAN-C network of transmitting stations in the U.S.

Part of the modernization effort is to replace the time and frequency generation system at transmitting LORAN-C stations

— Existing hardware is outdated and requires replacement The new design integrates all of the current LORAN-C timer

functionality with new timing technology into a single system— New timing component designed to maximize the benefits of co-located

cesium standards System is designed to satisfy the current operational requirements

as well as accommodate future requirements with minimum modifications

System functions aid in meeting the FAA’s RNP 0.3, Coast Guard’s Harbor Entrance and delivering Stratum I timing performance to LORAN timing users

V-3

Existing System

V-4

Existing System

The existing time and frequency equipment is a conglomerate of equipment designed and installed over the last 40+ years

Each station has a different list of equipment depending of master/secondary and dual/single rated

The system is based primarily on hardware technology that is outdated

Communication capability differs depending of component— Different hardware architecture forces communication capability to lowest level

System does not capitalize on the three cesium clocks for timing accuracy

— Cesiums are independently operated No automatic control of cesiums to USNO-UTC

V-5

Existing System Components

The current timing and frequency suite is collection of up to 30 separate components.

1960’s Vintage

Emergency Stop

Distributions Amplifiers

Frequency Patch Panel

Signal Alarm Unit

Time Counter

Electronic Pulse Analysis

Cycle Compensation Circuit

1970’s Vintage

Timer Units

Timer Set Control

Alternating Blanking Unit

Remote Control Interface

Communication Adapter

Waveform Panel

SSX IF

LSM IF

1980’s Vintage

Phase Micro-stepper

Time Counter

Multi-programmer

1990’s Vintage

Time of Transmission Patch Panel *

Timer Counter *

GPS Timing Receiver *

Time Reference Generator *

Automatic Blink System* Master Stations only

V-6

New TFE Design

V-7

TFE System Level

TFE consists of two redundant signal paths that generate transmitter drive signals with a known relationship to UTC(USNO)

— Each redundant half of the system operates independently to control primary frequency standards, generate transmitter drive signals and measure time differences

— Single (non-redundant) unit for distributing frequency signals from clocks for diagnostic use

TFE Standby Path

TFE Operate PathPFS 1

PFS 2

Primary Freq StdSignal Distribution

PFS 3

Switch

Switch

Transmitter

Transmitter

Output Monitor Signals

Ancillary Equipment Drive Signals



RS-232 Ethernet

RS-232 Ethernet

5 MHz, 1 PPS

5 MHz, 1 PPS

Command and Status

Command and Status

100 kHz, 1 MHz, 5 MHz, 10 MHz



User SelectedSignals

UTC via GPS

UTC via GPS

Status

TFE Standby Path

TFE Operate PathPFS 1

PFS 2

Primary Freq StdSignal Distribution

PFS 3

Switch

Switch

Transmitter

Transmitter





RS-232 Ethernet

RS-232 Ethernet

5 MHz, 1 PPS

5 MHz, 1 PPS

Command and Status

Command and Status




User SelectedSignals

UTC via GPSUTC via GPS

UTC via GPSUTC via GPS

Status

V-8

Local TFE User Interface

V-9

TFE Functional Diagram

Loran Signal Generation• PCI/TOC Generation• LPA implementation• Transmitter Drive Signals• Diagnostic Outputs

Clock Ensemble/UTC Recovery• GPS measurements• Inter-clock measurements• Timescale algorithm• Clock steers

TD Measurements• UTC Recovery TD• Loran Recovery TD• TOT TD’s• Additional Measurements

Closed Loop Control• Signal Phase Control

ABS• Signal Phase Control

V-10

Timescale Computation and Clock Steering

System utilizes three cesiums to compute a local timescale that is steered to UTC(USNO) via GPS

— 15 ns (RMS) UTC time recovery performance

Kalman filter models clocks and predicts clock performance when measurement data isn’t available

— System can flywheel through GPS system outages

Three clock timescale provides real-time clock fault monitoring as well as superior stability

— System designed to maximize the benefit of three atomic standards at each LORSTA

— Timescale reduces to two or one clock if three clocks are not available

V-11







V-12

LORAN Signal Generation

LORAN signal generation is accomplished in programmable firmware in the Loran Integrated Timer and Signals (LITS) chassis

— Unit receives 5 MHz from PFS and generates PCI, TOC, LI, and transmitter drive signals for two independent rates

» All LORAN signals originate from a single 5 MHz input (all signals are coherent)

— Command and control accomplished via RS-232— ABS control accomplished using direct digital lines

All system outputs on rear of chassis— Transmitter drive signals output on multi-pin connector— Copies of all signals for monitoring/diagnostic purposes— Spare connectors for future use

V-13

LORAN Signal Generation (cont’d)

Local phase adjustments (LPAs) inserted via a direct digital synthesizer (DDS) that creates a phase change by changing the frequency of the 5 MHz signal over a fixed time interval

— 5 MHz in, 5 MHz out tunable synthesizer— Control method results in phase changes without discontinuities in transmitted

data» Smooth transition reduces transmitter jitter during LPA

— LPAs can be completed over settable time period (time interval for frequency change is a settable parameter)

DDS5 MHz Rate

Counter

PCI

5 MHz(adjustable)

SignalCounters

5 MHz

LI

EMPT

MPT

PCSET

PCRESET

100 kHz

PCI

DDS5 MHz Rate

Counter

PCI

5 MHz(adjustable)

SignalCounters

5 MHz

LI

EMPT

MPT

PCSET

PCRESET

100 kHz

PCI

V-14







V-15

Signal Measurements

Six channel, sub-nanosecond event timer used to time tag the rising edge of the signals of interest

— PCI, TOC, 1 PPS, RF Gate, RF Pulse (detected pulse from transmitter)

— One timer per rate

Timer allows computation of the time differences by subtracting time tags from any pair of inputs

— Six simultaneous channels is more efficient than standard two-channel time interval counter

— For example TOC is compared to 1 PPS to verify timing and also processed with RF Pulse and RF Gate as time of transmission TDs

— Unit allows for future time differences of interest to be added to system output

V-16







V-17

Closed Loop Transmitter Control

Automatic Phase Adjustments (APAs) are inserted based on a proportional control loop closed around the transmitter

— RF feedback from transmitter drives a detection circuit TTL pulse produced based on SZC pulse measured in timer against TOC or RF Gate

Loop parameters control system’s response to transmitter delay changes— Time constant: how quickly the system responds to delay changes— Minimum Steer: APA not inserted unless error is large enough— Steer Interval: how often transmitted phase can be adjusted

APAs inserted using identical method as LPAs (DDS freq change) APAs can be computed based on UTC or LORAN data

— System can function without GPS using casualty receiver

Average

SavedTOT TD

- GainThreseholdCompare

DDSAdjust

MeasureTOT TD

steer interval

min steer

Gain = steer interval

time constant

VerifyNew TD

Average

SavedTOT TD

- GainThreseholdCompare

DDSAdjust

MeasureTOT TD

steer interval

min steer

Gain = steer interval

time constantGain = steer interval

time constant

VerifyNew TD

V-18

Closed Loop Transmitter Control Data

APA Pull-in

-1085

-1080

-1075

-1070

-1065

-1060

-1055

-1050

-1045

0.00 0.10 0.20 0.30 0.40 0.50 0.60

Hours

TO

T T

D (

mic

rose

con

ds)

0

2

4

6

8

10

12

14

16

18

AP

A (

mic

rose

con

ds)

TOT Control TD

APAs

System deliberately set off target time to show performance of proportional loop

V-19

Closed Loop Transmitter Control Data

APA Performance

-1050.25

-1050.2

-1050.15

-1050.1

-1050.05

-1050

-1049.95

-1049.9

0.8000 1.0000 1.2000 1.4000 1.6000 1.8000 2.0000 2.2000 2.4000 2.6000

Hours

TO

T T

D (

mic

rose

con

ds)

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

AP

A (

mic

rose

con

ds)

TOT Control TD

APAs

APAs required occasionally to compensate for timing events

V-20







V-21

Integrated Automatic Blink System (ABS)

ABS is critical for LORAN’s HMI performance for integrity protection — Monitors the transmitted signal for out-of-tolerance conditions through RF feedback

— Three selectable patterns for notifying the user

System utilizes direct digital lines to allow fast transition to blink — Blink control not affected by comms or OS latency

— System transitions to blink in less than one second after problem detection

Blink is initiated in hardware based on programmable rule set— Phase of transmitted signal vs local TOC estimate

— Phase error in transmitted pulses

— Lack of RF return from transmitter

— Time step in cesium standard

All of the ABS parameters can be adjusted to suit the performance of the transmitter

— As future transmitter jitter improves, the tolerances can be set tighter and/or averaging time in the ABS algorithm can be reduced

V-22

Technical Comparison of Systems

Function Legacy TFE

Local Phase Adjustments (LPA)

Minimum 20 ns Steps—20 ns

Minimum is settable down to 1 ns Step < 1 ns

Cesium Clock Control Independent clocks Manual control to UTC

Clock Ensemble Automatic cesium control to UTC

Parameter flexibility Not easily changed in hardware Software/Firmware allows easy adjustment

Loran Control No ability to control to Loran received signals

Control to selected Loran station

Time of Transmission Capability

Monitoring only 20 ns (rms) accuracy

Monitoring and Control <15 ns (rms) accuracy

There are significant performance advantages to the new system

V-23

Support Comparison

Improved support aspects of the new system provide some of the greatest benefits

Support Aspect Legacy TFE

Configuration Management

Different equipment at —Master vs. Secondary

—Dual vs. Single rate

Same configuration regardless of station

Preventative Maintenance 9 hours per quarter 30 minutes per quarter

Trouble Shooting Some components have minimal self-diagnosis

Complete system health monitoring at all times.

Causality Repair

Each component requires different procedures Not all modular design

Same procedure for entire system Modular design facilitates easy repair.

V-24

Future Requirements

TFE has been designed with the forethought that LORAN-C will be evolving over next 10 years

— Modular design that isolates timing to the one chassis and LORAN signals in LITS chassis Timing chassis consists of independent modular slots that isolate

functionality to specific components LITS chassis has been designed using field programmable hardware to

facilitate easy modifications— FPGA implementation for signal generation— Microprocessor for external interface and control

LITS design also includes empty expansion slot and spare connectors to facilitate addition of future capability to LORAN signal structure

System utilizes an IP socket interface that enables remote control— All command, control and status achieved using standard IP interface with ASCII

command set

V-25

Conclusions

The new LORAN-C transmitter timing system provides a single integrated system to bridge the gap between the cesium standards and the transmitter

— Old functions are replaced with higher performance equipment that is more reliable and flexible

— Logistics are simplified with respect to Configuration Management, maintenance, and repair

New timing architecture provides system flexibility to address future requirements and performance enhancements

— System can support a large range of possible architectures including new rates, data transfer capability and higher HMI responsibility

V-1 TFE: The New Heartbeat of Loran T. P. Celano, Timing Solutions Corporation LT Kevin Carroll,...

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