LORAN (Long Range Navigation)

LORAN (Long Range Navigation)

1. Accuracy of a hyperbolic system is a function of the baseline distance

   • Stations could be spread out
   • System would become more accurate
   • Fewer stations would be needed
   • Best to be within baseline
     • LOP are closer

2. LORAN A

   • Skywave Synchronised Loran ( SS LORAN)
     • Allow for greater distance
     • Average accuracy of 1 mile
     • Disadvantage
       • Indicator located close to either or both stations
         • Erratic reception occurred when angle of reflection reaches critical angle
         • Radio waves exhibited increasing penetration power
           • Go through partially or entirely into 'E' layer
   • At least 3 transmitting station in each chain
     • 1 master
     • 2 slave
   • Differences in arrival time of pulses from pair of stations
     • Transposed to Loran lattice chart
       • Position could be plotted
   • Range
     • Max 1,400 miles at night
     • 500 - 700 miles by day
       • Groundwave
       • 350 - 500 miles at night
   • each pulse 40 µs interval
     • Named Pulse Repetition Interval (PRI) Signal
     • Varies with each station
       • Lasted b/w 29,000 µs & 40,000 µs
         • H-rate pairs: 30,000 µs
         • L-rate pairs: 40,000 µs
         • S-rate pairs: 50,000 µs
   • Fixed time interval b/w master & slave transmission
     • time for signal to travel from master to slave, plus one-half P.R.I., plus 'coding delay'
   • No absolute sync of receiver time base with transmitter required
     • Difference between time of arrival of two pulses, and not actual time taken for each pulse to reach receiver
       • No need for an absolute synchronization of the receiver time base with the transmitter
   • Sync of receiver's time base
     • Length of trace on CRT matches PRI
       • Failing to match, causes the pulses to appear to be drifting left or right
   • Identification of station pairs
     • Radio frequency channel
       • Channel 1: 1.95 MHz
       • Channel 2: 1.85 MHz
       • Channel 3: 1.9 MHz
       • Channel 4: 1.75 MHz
     • Pulse repetition rate
       • To economize on frequency channels
         • Stations operated on same frequency channels
       • Make sweep repetition rate of indicator same as pulse repetition rate of pair
         • Desired pair will be stationary, the rest will be drifted across the scan and ignored
       • 2 switches
         • Basic pulse repetition rate
           • Low
             • 25 pulse per second
           • High
             • 33 1/3 pps
           • Slow
             • 20 pulse per second
         • Specific pulse repetition rate
           • Differ from basic by small amount
           • 8 specific rates: 0-7
       • 96 separate station pairs using 4 frequency pairs
     • 3 character identification symbol
       • 1st character: channel
       • 2nd character: basic pulse repetition rate
       • 3rd character: specific pulse repetition rate
   • Shortcoming
     • Pulse length of 40 µs

3. LORAN B

   • Phase comparison of LORAN A
   • Attempt to redesign LORAN properly & add new features
   • No attempt made to achieve phase coherence in transmitted pulse in original design
     • Impossible to have phase comparison
   • Phase comparison increased accuracy to about half cycle (about 0.25 µs)
   • Operation in chains instead of pairs
   • Timing control & timing measurements were made on pulse carrier
     • After cycle identification from pulse envelope
       • Very precise timing measurement
   • Considerable phase distortion on land & sea
     • Resulted in major differences in phase to envelope timing depending on azimuth from transmitter

4. LORAN C

   • Chains of 3,4,5 stations
     • 1 Master station
       • Master burst of 9 pulses
         • Each pulse duration of 200 microseconds
           • Each interval b.w pulses is 1000 microseconds
           • Last pulse has 2000 microseonds
     • Other secondary stations
       • Secondary burst of 8 pulses
   • Transmitting at 100 kHz carrier wave
     • GRI is the key by which a receiver can identify and isolate signal groups from a specific chain
   • Each chain have unique Group Repetition Interval (GRI)
     • Time interval for master station to repeat
     • E.G. Great Lakes Chain GRI 8970, 89700 µs, in multiples of 10 µs
     • GRI consideration
       • Baseline lengths between master and secondaries
         • Cannot be lesser than time to get to slaves
       • Number of slaves
       • Geography
       • Other nearby chains for interference
       • Sky wave cross rate interference
       • Duty cycle of transmitter
         • Faster GRI, higher average transmitted signal
           • Final stage transmitter requires more cooling
   • All transmitter work on same frequency
   • Emission Delay (Unique to each station)
     • Duration between secondary transmits after master transmit
   • Coding Delay (Unique to each station)
     • Removes local (near-field) discrepancy between envelope and carrier
   • Baseline
     • 1200 - 1900 km (600 - 1000 nm)
   • Sky Wave rejection
     • Presence of delayed skywave
       • Cause distortion in pulse shape & change carrier phase within pulses of received signal
       • Delay in skywaves to arrive at receiver than groundwave
     • To avoid skywave contamination
       • Receiver selects zero-crossing of specified carrier cycle at front end of pulses of master & slave
         • Usually 3rd cycle
         • Ensures time interval measurement is made using uncontaminated part of pulse
         • Receiver compares envelop (rough shape) of received pulse with stored envelope
           • Rough measurement
           • To locate 3rd cycle
   • Phase Coding
     • Carrier phase of selected pulses is reversed in a pattern
       • Reduce effects of interference & noise on time difference measurement
       • Distinguish master & slave
     • Repeated every 2 GRI
     • Determines 1st peak is upwards of downwards
   • Time difference of master & slave
     • Precision of 0.1 µs
       • Correspond to 30 metres
   • Precision clock
     • Must not exceed a few tens of nanosecond
     • System Area Monitor (SAM) control
       • Measurement equipment
       • Adjustment to clock of secondary station
         • Timing is kept at predetermined value
       • Advantage
         • Time difference close to monitor area will be very stable
     • Time Of Emission (TOE) control
       • Used by Northwest European Loran-C System (NELS)
       • Arrival times of signal from adjacent transmitter are measured relative to local clock at each transmitter
         • Measurements from all stations are sent by permanent data link to control station where time deviation of each transmitter's clock can be calculated
           • Computed adjustments are returned to the individual transmitter for clock synchronization
       • Disadvantage
         • Time difference measurement over coverage area vary slightly with seasonal changes
           • Ground wave propagation speed
       • Advantage
         • Better overall performance monitoring throughout the coverage area
         • Modeling & prediction of timing variations are made easier
         • More accurate timing from signals
         • Better accuracy for cross-chain and master independent use
         • Better accuracy for Rho Rho navigation (circular navigation method)
         • No monitor sites needed
   • Additional Secondary Factor (ASF)
     • Small variations in velocity of propagation between sea water & different land masses
     • Corrections applied for compensation
       • Improve accuracy
     • Depends on conductivity of earth's surface
       • Sea water - high conductivity ASF 0
       • Dry soil, mountains, ice - low conductivity
         • Travel more slowly
         • Substantial ASF delays
         • Degradation of absolute accuracy
     • Distributed as electronic database
   • Service Integrity
     • Monitored to detect signal abnormalities, render the system unusable for navigation
       • "Blink"
   • Accuracy
     • Varies from 185 m to 463 m (0.1 to 0.25 nautical miles)
       • Depends on position of coverage area
     • Repeatable accuracy
       • Measure of observer's ability
       • Return to same position visited previously using same navigation system
       • 18 m t0 100 m
   • Propagation Anolomies
     • Encountered in urban areas
     • Improvement can be done using differential Loran C
       • DLoran-C
       • Similar to differential GPS
   • Future
     • GPS provided better performing replacement
       • Basic GPS fails
         • Accuracy
           • Difference between measured position at any given time to actual or true position
         • Availability
           • Ability to provide service throughout a flight operation whenever needed
         • Integrity
           • Ability to provide timely warnings to users or to shut itself down when it should not be used for navigation
       • FAA develop Wide Area Augmentation System (WAAS) to meet requirement
     • Enhanced Loran C Data Channel Project
       • Unidirectional communications information for GPS integrity and potentially for GPS correction data
     • Enhance Loran (e-Loran)
       • Accuracy of 8m
       • Includes additional pulses which can transmit auxiliary data such as DGPS corrections
   • Developed for long range high accuracy

5. Accuracy

   • Short distance
     • Lines cross at angles close to 90 degrees
   • Angles reduce with range
     • Inaccuracy increase with range
   • Accuracy of fix depends on crossing angle
   • Long ranges, hyperbolic lines approximate straight lines radiating from the center of the baseline.
     • Resulting pattern of lines becomes increasingly parallel as the baseline distance becomes smaller in comparison to the range
       • Increase area

6. Loran D

   • Short range, high accuracy
   • 16 pulse transmission instead of 8
     • Interleaving an additional 8 pulses after each pulse of the current Loran-C pulse pattern
       • Known as Supernumary Interpulse Modulation (SIM)

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