Loran of the 21st Century– One perspective on how to define the changes in the U.S. Loran system

Presented by Tom GuntherBooz Allen Hamilton

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The Questions to Address

What was the Loran of the 1990’s?

What are the new requirements for Loran of the 21st Century?

What must we consider and address in defining Loran of the 21st Century?

What is Loran of the 21st Century?

What does Loran of the 21st Century mean to the user communities?

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Loran System—1990’s and before

Loran System Performance characteristics -- they depend on your Terms of Reference but here is what is in the Federal Radionavigation plan.

Many users include aviation (terminal and enroute), Marine (required carriage of RTCM compliant system), Time and Frequency users, multimodal surveillance.

– Acknowledge that many users were shifting to GPS… Worldwide availability, accuracy, and the all important bells and whistles.

Loran of the 21st Century builds and improves on this performance and both systems’ user base.

FIX FIX SYSTEM AMBIGUITYPREDICTABLE REPEATABLE AVAILABILITY COVERAGE RELIABILITY RATE DIMENSION CAPACITY POTENTIAL

0.25nm 60-300 ft. 99.7% Continental US, 99.7%* 10-20 2D Unlimited Yes, easily(460m) (18-90m) Alaska fix/sec. + Resolved

Selected Timeoverseas Areas

ACCURACY (2drms)

*Triad reliability SOURCE: 2001 FRP

Current Loran-C System Performance Characteristics (Signal-in-Space)

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“New” Factors & A Few Revised Definitions

GPS vulnerability mitigation– redundant, back-up or contingency systems. Redundant: Seamless transition in process and procedures – no change in Ops Tempo Back-up: Some change in process and procedures due to difference performance capabilities of systems – Ops Tempo reduced. Contingency: Cease/Complete operation in a safe manner– Ops severely reduced

New requirements and definitions for the Performance Capabilities – four key elements: Accuracy: The degree of conformance between the estimated, measured, or desired position and/or the velocity of a platform at a given time and its true position or velocity. Integrity: The ability of a system to provide timely warnings to users when the system should not be used for navigation. Availability: The ability of the system to provide usable service within the specified coverage area. Signal availability is the percentage of time that navigational signals transmitted from external sources are available for use. Availability is a function of both the physical characteristics of the environment and the technical capabilities of the transmitter facilities. Continuity: The capability of the total system [to] be available for the duration of a phase of operation, presuming that the system was available at the beginning of that phase of operation.

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New Aviation Requirements: RNP 0.3 (target) RNP 0.5 (minimum)Performance Requirement Value

Accuracy (target) 307 metersAccuracy (minimum) 802 meters

Monitor Limit (target) 556 metersMonitor Limit (minimum) 926 meters

Integrity 10-7/hour

Time-to-alert 10 seconds

Availability (minimum) 99.9%Availability (target) 99.99%

Continuity (minimum) 99.9%Continuity (target) 99.99%

(Source: FAA Loran Evaluation Report, June 2002)

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New Marine HEA Requirements (Primary)Performance Requirement Value

Accuracy (target) 10 meters, 95%Accuracy (threshold) 8-20 meters, 95%

Monitor/Alert Limit (target) 25 metersMonitor/Alert Limit (threshold) 50 meters

Integrity (target) 3x10-5

Time-to-alert 10 seconds

Availability (threshold) 99.7%Availability (target/VTS) 99.8%

Continuity (threshold) 99.85% over 3 hoursContinuity (target) 99.97% over 3 hours

(Sources: FRP, DOT Task Force, TASC DGPS Mission Needs Analysis: Harbor Entrance and Approach, IMO Resolutions A.815(19) and draft revisions to A.860(20))

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New Marine HEA Requirements (Back-up)Performance Requirement Value

Accuracy (backup) 20 meters, 95%

Monitor/Alert Limit (backup) 50 meters

Integrity (target) 3x10-5

Time-to-alert 10 seconds

Availability (minimum) 99.7%

Continuity (minimum) 99.85% over 3 hours

(Sources: FRP, DOT Task Force, TASC DGPS Mission Needs Analysis: Harbor Entrance and Approach,IMO Resolutions A.815(19) and draft revisions to A.860(20))

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New and Old Timing Requirements

Performance Requirement Value

Frequency Accuracy (target) 1 in 1013 averaged over 24 hrsFrequency Accuracy (minimum) 1 in 1012 averaged over 24 hrs

No External Antenna (desired)

Backward Compatibility (desired)

Integrity Data Minimum of USE/NO USE Flag

Higher Accuracy Time of Day Time Tag Leap Second information

Timing Accuracy <100nsec (RMS?)

(Source for frequency requirements: DOT Task Force, T1X1 letter of Oct 2002)

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What must we consider and address? Federal Policy support and encouragement of Loran for critical applications to mitigate the

vulnerabilities of GPS (e.g., safety of life or vital economic areas)

– Volpe GPS vulnerability study identified the need to have redundant, ground-based navigation and timing/frequency capability in place during times of GPS outages.

The Department of Transportation prefers a single, cross-modal system rather than unique systems for each transportation mode.

Loran theoretically provides independent, redundant capabilities to GPS for aviation, marine, timing/frequency, and terrestrial users… but this must be proved.

If at all possible Legacy users should be minimally affected.

International acceptance and use is highly desired

– coexist with EUROFIX used in Europe and the Far East.

Costs should be minimized while benefits and useable time maximized.

Spectrum won’t be increased (easily).

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What must we consider and address? (continued) Integrity is the major factor affecting suitability of Loran for Aviation applications.

Accuracy is the major factor affecting a suitability of for Loran for Marine applications

Signal phase stability is the major factor affecting suitability of Loran for Frequency applications.

Seasonal changes in propagation delay is the major factor affecting suitability of Loran for Precise Time applications.

Continuity of the service is equally important to all navigation applications but not ascritical to Precise Time and Frequency applications.

Loran is a “redundant” system if RNP 0.3 RNAV and “Back-up” if HEA requirements are met.

Funding agreements must in place to allow these “things” to happen

Build on the existing infrastructure (develop prototype and concepts).

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What must we consider and address? (continued) Precise Time and Frequency users of Loran are important and may be the largest

(if least understood and visible) user group.

Canadian stations (if needed) must use the same equipment and policies as US sites.

Many more airport (1000’s ) in US will be served by GPS procedures.– Larger, more diverse aviation user community

Navigation integrity hazards associated with transmitting stations, propagation (including ECD and phase distortion), receivers, and intentional errors (jamming/spoofing).

System certifications and options.

Must now define signal integrity at skin of the aircraft vice base of the antenna.

Developing new receiver MOPS for Loran equipment (aviation and marine).– Incorporate complementary aspects of GPS MOPS such as database, user interface, etc.– Incorporate study findings for receiver processing needed to make Loran an RNP 0.3 system.

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A Framework for Loran 21st Century -- Trade Spaces. Trade spaces are used to identify ways to meet the same results which allows for the “best” way

(e.g., cost effective or technically complex) or as place holders for areas of uncertainty.

– They can help in breaking down work requirements and who has responsibility for the work.

For Loran, integrity, accuracy, availability and continuity are common treads to consider in the Trade Spaces.

What must change in Loran is a combination of modernized Loran-C infrastructure, new policies, and new technology user receivers.

Trade Spaces aren’t a Signal Specification, Ops Doctrine, or MOPS but help define each of them.

The four we’ve picked for describing Loran of the 2000’s are: * Federal Policy -- High level statement of performance, certification, funding, etc. * Operations Doctrine --OOT limits, control parameters, off-air planning, ASF tables, etc. * Transmitting and Control Equipment --signal generation, monitoring , etc. * User Equipment -- MOPS, antenna types, algorithms, define requirements.

THIS IS WORK IN PROGRESS!

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Federal Policy Expectations Federal policy commitment* to continue Loran-C operations as a part of the mix of Federally

provided radionavigation systems until 2025**

– Agency’s should support (endorse, encourage, or require) their respective user groups to use Loran for critical applications -- infrastructure protection and maintaining normal operations in the event of a GPS outage.

-*Provided the data show that Loran-C meets requirements.

-**This will be driven by the VOLPE Benefit Cost Study and may change.

The FRP should discuss systems using common terms of reference with validated requirements for the applications.

Each airport using LORAN for NPA must be periodically flight checked by the FAA (not a unique requirement). If the airport can not support LORAN NPA then it shall be identified as such.

– FAA to collect airport data to refine ASF correction model.

Differential Loran correction will be required for the HEA applications.

– Each harbor that will use Loran as a back-up system will need an ASF survey.

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Federal Policy Expectations (continued)

Determine if any stations will move, if additional stations are necessary or if stations are no longer needed.

Create or modify agreements for operations and cost sharing between agencies that are reflect and implement Loran of the 21st Century.

Coordinate agreements for the international stations.

Promote a common specification for the new Loran, internationally.

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Operations Doctrine Expectations

The basic response time for system repairs remain as previously identified or be improved.

The basic operational structure of the system remains the same except as noted.

There will be a scheme that will allow for a unique station identifier, for transmission of time information, and for other data (TBD).

For Out-of-Tolerance (OOT) of the Loran system (longer than 3 secs) the signal will go off-air vice the historic method of BLINK.

If the signal is off-air for longer than 10 seconds. The signal will come back on-air and BLINK (new method) until it is in tolerance; this period of BLINK will be no less than 30 seconds in duration.

– Currently this is not automatic but can be remoted. The operator has to take the system out of off-air mode and put it into pulse blink.

Loran will use Time of Transmission Control vice the historic System Area Monitor Control.

A signal is considered usable with a SNR down to 40 db/uv/m.

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Operations Doctrine Expectations (continued)

Maintenance periods will be limited and controlled so as to minimize the system sensitivity to continuity and availability performance parameters. For example, off-airs of the two secondaries in the 9940 chain will be minimized.

Stations will be unstaffed to the maximum extent.

Transmitter performance data will be maintained and analyzed to ensure that performance requirements are being met by the system.

Modulation of the transmitted signal will not adversely affect navigation performance.

User notification of planned outages will remain as it is now.

The ABS tolerance will be set at somewhere between 50 and 100nsec for timing.

UTC synchronization at a station will be steered by GPS (1PPS) and be established at somewhere within 10 to 15 nsec.

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Operations Doctrine Expectations (continued) There will be no drift in the synchronization of Loran to UTC under normal operations (30-45ns

transmitted, 10-15ns on board the station).

If the primary means of UTC synchronization fails the drift with respect to UTC shall not be drift more than a few (<7) nsec per day.

Operational plans will be developed and implemented to transfer time between Loran transmitting station for periods when long term loss of GPS occurs.

– This upgrade maintains performance while making the system GPS independent.

Phase adjustment process will be changed (noted in transmitter section) to enable the frequency application of LORAN.

Routine transmitter “switches” for maintenance must be minimized.

Stations visits and maintenance schedules may change.

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Operations Doctrine Expectations (continued) Creation of a “LORAN Clock”.

– Use common-view GPS techniques to timescale all the LORAN clocks together.

– Can monitor the LORAN network of transmitting stations too… great diagnostics on the performance of any individual station and powerful tools to manage performance.

Differential Loran correction for the HEA applications may be part of the DGPS radiobeacon broadcast (although this is now unlikely).

There will be a Loran software model that will be used for planning maintenance, off-air.– This model will be based on the performance parameter and generic receiver performance

requirements. (We do need to address the parameters of the model)

System Calibration process and equipment.– Recalibrate the timing system, as well as the delay path from the transmitter to determine if

there are any aging effects, effects of maintenance performed, as well as how well one can calibrate at 100 kHz period.

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Transmitting and Monitor Equipment Expectations Keep the basic transmitted signal shape the same, as well as the tolerance within a pulse.

However, recent discussions may require rise time change.

Phase coding will not change.

The LORAN-C spectrum performance will not change.

Blanking will not change.

All stations may be dual rated (still a subject of discussion).

All station will have the new SSX switch cabinets.– Will allow them to switch O/C units in 3 seconds or less.

New TFE will be installed.

New Cesium's will be installed.

All Stations will have upgraded SSX’s.

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Transmitting and Monitor Equipment Expectations

(continued) New TCS & RAIL units installed.

No interruption in power in the event of commercial power failure.– UPS in the Ops Room (approx 40 minutes) and Transmitting Room (approx 10+ minutes)– Power will be provided by the UPS until the emergency power is online or the commercial power

is restored.

ABS and/or RAIL will address phase, ECD and signal strength OOT vice only phase.

SAM sites retained as fixed monitors for monitoring propagation in the far-field.– There is also benefit in making the SAM sites time-based in order to provide timing

corrections for common-view LORAN timing.

All station data will be remotely accessible by NAVCEN.

There will be no node-of-failure that does not have a some form of back-up.– Will allow for the continuous operation of the signal.

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Transmitting and Monitor Equipment Expectations (continued)

Output power fluctuations will not exceed (notional 1%).

ECD fluctuations will not exceed (notional .25 usec)

Phase jitter will be minimized so as not to exceed (notional 30-50 nsec)

The difference in transmitter O/C unit transmitted signal performance characteristics at a station must be less than (notional .1%-10%) .

The pulse (s) will have to be modulated so as to provide the required time reference for the time and frequency applications, as well as a data channel for differential corrections.

TFE will support three different kinds of phase adjustments. The time period for APAs and LPAs will be inserted over 5-20 minutes.– LPAs: entered by operator, take place over long period of time.– ILPAs: entered by operator, take place instantaneously..intended for fault clearing or

initializing station.– APAs: entered by control loop, take place over long period of time.

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User Equipment Expectations Must be an all-in-view receivers capable of cross rate interference removal.

Must have ASF map for corrections and the ability to download or modify ASF data in the map/table.

Must be able to detect new form of blink (station goes off-air first and then when it back comes on-air it is blinking).

Must be able to perform HMI calculation.

Must be able to meet MOPS certification for aviation (dynamics, SNR, sky wave etc). There may also be a need for standard test scenarios for receivers.

Must be able to meet some type of certification for marine (dynamics, SNR, sky wave etc).

Must use H-field antennas for aviation. (likely)

The receiver must be able to coast through a 3 sec off-air of single station.

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User Equipment Expectations (continued)

The receiver must be able to give a cockpit indication of the Loran system being unusable within 10 sec of the onset of the unusable situation.

Must have interfaces and the requisite bells and whistles for the given application(s).

– Will demand significant human factors insights/evaluations.

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What Loran of the 21st Century means to the user

All modes & users benefit from having a redundant or back-up navigation system. * Sea -- maritime domain awareness vulnerability mitigation -- harbor/harbor approach (with improved ASF corrections) * Air -- GBNA .3 RNAV redundant system -- NPA * Land -- higher accuracy applications e.g., vehicle monitoring w/ surveyed or real-time ASF… no specific requirements defined/identified -- terrain or locations where satellites are blocked. -- WX user… lower cost sondes *Timing-- tighter tolerances for applications (comms, utilities, labs) -- time tags on signal -- Precision time recovery at < 100 ns level

All modes and user benefit from having an alternative or back-up timing system. Stratum 1 sources of time and frequency synchronization.

Surveillance and control systems will have an alternative navigation source.

GPS RAIM applications will have more options to fill potential RAIM holes.

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What Loran of the 21st Century means to the user (continued)

Proven means of LF communications.

Eliminates real-time dependency on one system…GPS.

Provides an independent system and complimentary system to GPS.– Lat/long-based area navigation that is transparent to the user.– Transparent timing and frequency capability.

Less expensive operations – GPS and other terrestrial based systems are costly to operate and maintain; GBNA face significant recapitalization challenges.

An international market and interest in Loran as a navigation communication system.

Congressional interest and support.

Likely be market interest in Loran … Boeing, Rockwell, Goodrich, Free Flight are interested and are working on Loran options even w/o a firm policy statement.

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Questions and Answers

Now… ask away

Later: gunther_thomas@ bah.com or 401-885-0079

On your own: http://KSN-team.faa.gov/AND/and700/loranc

For your reading enjoyment:– “The Analysis of Loran-C Performance, its Suitability for Aviation

Use and Potential System Enhancements” FAA AND-700, June 2002

– “Navigation and Landing Transition Strategy” FAA ASD-1, June 2002