Transcript [Project Name]

Transitioning to Time of Transmission
Control in the U.S. Loran System
ILA 2003 Boulder, CO
Mr. Gene Schlechte
CAPT Curtis Dubay, P.E.
U. S. Coast Guard Navigation Center
CDR John Macaluso
U.S. Coast Guard Loran Support Unit
Disclaimer
The views expressed herein are those of the authors and are
not to be construed as official or reflecting the views of the
Commandant or of the U. S. Coast Guard.
The Central Question
What are the potential methodologies and
projected effects of transitioning the U.S.
Loran system to a Time of Transmission
(TOT) method of control?
Overview
Presentation will specifically address:
•Current method of using far-field System Area Monitors
•Potential methods to transition to TOT control
•Changes that might be needed to Loran Signal Spec
•Anticipated effects on legacy (hyperbolic) Loran receivers
North America Loran-C
Current Loran Control
Far-field System Area Monitors track (for hyperbolic
navigation):
•Time Difference (TD)
•Pulse Shape (ECD)
•Signal Strength (Gain)
Current Loran Control
Control Network has three main elements:
•Control Station w/ Loran Consolidated Control
System (LCCS)
•Transmitting Station
•Primary Chain Monitor Sites (SAM)
Acts as a real-time monitor of transmitting station data,
alarms, and physical condition
Current Loran Control
Normal mode is Alpha Control:
•TD (Master to Secondary) – standard sampling point
25 uSec into pulse (far field), based on CSTD
determined during calibration at 30 usec point (at
transmitting station)
•Far Field ECD
•Signal Strength
Current Loran Control
LCCS Time Difference Controller:
•Automatic insertion of LPAs
•Each LPA must be 40 nSec or less
•No more than 2 LPAs /baseline /hour
•Cum total of LPAs not to exceed 100 nSec /24 hours
Current Loran Control
Envelope to Cycle Difference (ECD) characterized by
relationship between:
•Phase of RF Carrier
•Time origin of Envelope Waveform
Needed for proper acquistion of signal
Current Loran Control
Transmitting Station:
•TINO
•Peak Volts
•Assigned/Nominal ECD
Current Loran Control
System Area Monitors (SAM)
•Long term monitoring and control
•Two Alpha monitor receiver sites (A-1 & A-2)
•Alarm tolerances based on operations and seasonal
variations
SAM to TOT Methodologies
Three switch-over methodologies possible:
•Station by Station
•Chain by Chain
•Whole System
SAM to TOT Methodologies
Legacy Receiver Performance Factors:
•Station geometry
•Control point for grid (SAM receiver)
•Distance from user to SAM
•Accuracy of USCG provided ASFs
•Accuracy of receiver TD measurement
SAM to TOT Methodologies
Modern Receiver Performance Factors:
•Station geometry
•Precision of TOT control to UTC
•Accuracy of updated ASFs
•Accuracy of receiver TOA measurement
SAM to TOT Methodologies
Two Important Points:
•Master Loran Stations are already under TOT
control (currently +/- 100 nSec)
•New Timing & Frequency Equipment (TFE)
designed for SAM or TOT, steering phase of signal
will be possible instead of 20 nSec jumps)
SAM to TOT Methodologies
Station by Station:
•Most gradual transition for legacy users
•Incremental improvement for modern receivers
•Increased complexity for control watchstander
•Lowest risk - facilitates longest performance
validation period
SAM to TOT Methodologies
Chain by Chain:
•Less confusing for legacy users
•Faster improvement for modern receivers (on
regional basis)
•Increased complexity for control watchstander
•Medium risk - facilitates regional performance
validation period
SAM to TOT Methodologies
Whole System:
•Longest delay in impact on legacy users
•Slowest improvement for modern receivers
•Least complexity for control watchstander
•Highest risk – does not facilitate performance
validation period
SAM to TOT Methodologies
Methodology of Choice: Chain by Chain
•Phase-in/out period for legacy/modern user
•Moderate complexity for control watchstander
•Facilitates performance validation in mid-2004
•Manageable transition
SAM will be used to monitor ECD and SS in far field
Changes to Signal Specification
Components of a modernized Loran:
•Supportable into the future
•TOT control
•Data channel for broadcast of temporal correctors
•Backwards compatible w/ legacy receivers
•Compatible w/current tri-state PPM
Changes to Signal Specification
What might be changed:
•Definition/implementation of “blink”
•Addition of new pulse for Loran data channel
•Data channel for broadcast of temporal correctors
•Standards for spatial & temporal correction data
•Quickened rise time to mitigate early skywave?
Anticipated Effect on Legacy Users
Studied using Double Range Difference Model, for:
•30 nSec Synchronzation
•100 nSec Synchronization
Repeatable accuracy predicted to degrade
30 nSec case better than 100 nSec case
Anticipated Effect on Legacy Users
New TFE will allow 5 – 10 nSec Synchronization:
•May mitigate some loss of repeatable accuracy
•Data needs to be collected
•Study needs to be re-visited
Emission Delays may need to be re-assigned
•Use of historical means may help
Questions?
Transitioning to Time of Transmission
Control in the U.S. Loran System
ILA 2003 Boulder, CO
Mr. Gene Schlechte
CAPT Curtis Dubay, P.E.
U. S. Coast Guard Navigation Center
CDR John Macaluso
U.S. Coast Guard Loran Support Unit