Electronic System of Position Fixing
Loran C System
The term “Loran” is an acronym for LOng RAnge Navigation (LORAN).
Loran-C provides better than 0.25 nautical mile (460 meters), absolute accuracy for suitably equipped users within the published areas.
Users can return to previously determined positions with an accuracy of 18 to 90 meters using Loran-C in the time difference repeatable mode.
Loran-C transmitters are organized into chains of 3, 4 or 5 stations. Within a chain, one station is designated “Master” (M) while the other “Secondary” stations identified by the letters W, X, Y and Z. Different secondary designations are used depending on the number of station in a chain. This is summarized in the table below.
Master with 5 secondaries
M V, W, X, Y, Z
Master with 4 secondaries
M W, X, Y, Z
Master with 3 secondaries
M X, Y, Z
Canadian West Coast 5990
Master with 2 secondaries
M X, Y
Power levels can range from as low as 11 KW to as high as 1.2 MW.
The Loran-C navigation signal is a structured sequence of short radio frequency pulses on a carrier wave centered at 100 kHz. All secondary stations send pulses in bursts of eight, whereas the Master signal, for identification purposes, has an additional ninth pulse burst.
The sequence of signal transmissions consists of a pulse group from the Master (M) station followed at exact time intervals by pulse groups from the secondary stations.
The time interval between the reoccurrence of the Master pulse is called the Group
Repetition Interval (GRI).
Each Loran-C chain has a unique GRI.
Loran-C pulse structure and sequencing.
Since all Loran-C transmitters operate on the same frequency, the GRI is the key by which a receiver can identify and isolate signal groups from a specific chain.
In naming the chains, the GRI is included. As an
This means the time interval is 89700 microseconds.
The rightmost zero is always implied and the GRI is always in multiples of 10 microseconds.
In old Loran-C receivers, the operator had to actually set this number to receive the chain.
GRI’s are chosen on the basis of:
Baseline lengths between master and secondaries. If the distance between the master and first secondary is say 1000 kms, the radio signal will take 33,000 microseconds to get to the slave so the GRI cannot possibly be less than that.
Number of slaves that have to be accommodated - they all have to have delays so that there is no possibility of them crossing over anywhere in coverage area.
Other nearby chains with consideration given to interference.
Skywave cross-rate interference.
Duty cycle of the transmitters - a faster GRI means the average power of the transmitted signal is higher so the final stage in the transmitter requires more cooling. With average baseline lengths and three slaves, the minimum GRI cannot be much less than 50,000 microseconds.
Each Loran-C pulse has an approximate duration of 200 microseconds (μs).
The interval between pulses within a pulse group is 1000 μs, except for the last two pulses at the Master, which have a 2000 μs interval.
1000 μs 1000 μs 1000 μs 1000 μs 1000 μs 1000 μs 1000 μs 2000 μs
This above illustrates the points on the Loran-C pulse envelope that define the start time, the time of maximum envelope power and the stop time of the pulse.
Two other important characteristics are associated with Loran-C signals, namely emission and coding delay.
If the master station is taken as a reference, the emission delay refers to how long it takes before the secondary transmits after the Master has done so.
The coding delay is a very small correction that removes the local (near-field) discrepancy between the envelope and carrier.
Both parameters are measured in microseconds and are uniquely associated with each secondary station.
BASELINES AND COVERAGE
An imaginary line drawn between the Master and each secondary station is called the baseline.
The continuation of the baseline in either direction is called a baseline extension.
Typical baselines are from 1200 to 1900 km (say 600 to 1000 nautical miles).
Chain coverage is determined by:
The power transmitted from each transmitter in the chain,
The distance between them and
How the different transmitters are oriented in relation to each other (the geometry of the chain).
SKY WAVE REJECTION
A frequency of 100 kHz was chosen for the Loran-C carrier wave to take advantage of propagation of the stable ground wave to long distances.
However, the presence of delayed sky waves, reflected from the ionosphere, cause distortions of the pulse shape and change the carrier phase within the pulses of the received signal.
Not only those, the skywaves take longer to arrive at the receiver than the ground wave, so their presence complicates the computation.
To avoid sky wave contamination, the Loran-C receiver selects a zero crossing of a specified carrier cycle at the front end of the pulses transmitted by master and secondary stations.
Making the cycle selection early in the ground wave pulse - usually the third cycle is employed - ensures that the time interval measurement is made using the uncontaminated part of the pulse.
But how is the third peak selected when the start time of the pulse is not known?
To solve the problem, the receiver compares the envelope (the rough shape) of the received pulse with a stored envelope.
This process is called the “rough measurement”. When the third peak is finally located, the phase of the signal can be determined. The phase of the signal can be zero or pi radians.
Precise control over the pulse shape at the transmitter also ensures that the selected zero crossing can be identified reliably by the receiver.
Zero Crossing: This diagram illustrates the third cycle in the Loran pulse.
To reduce the effects of interference and noise on time difference measurements, and to assist in distinguishing between master and secondary stations, the carrier phase of selected transmitted pulses is reversed in a predetermined pattern.
The pattern is shown below, where a minus sign indicates an inverted pulse (180° phase shift), and a plus sign means no phase shift. This pattern is repeated every two GRI’s.
Simply stated, phase coding determines whether the first peak in the pulse is upwards or downwards.
TIME DIFFERENCE MEASUREMENTS
The basic measurements made by Loran-C receivers are to determine the difference in the time-of-arrival (Time Difference, TD) between the master signal and the signals from each of the secondary stations of a chain. Each TD value is measured to a precision of about 0.1 microseconds (100 nanoseconds) or better. As a rule of thumb, 100 nanoseconds correspond to about 30 metres. The principle of time difference measurements in hyperbolic mode is as illustrated.
Time Difference Measurements
Today’s state-of-the-art, solid-state Loran-C transmitters are adapted for automatic operation.
The functions are monitored at the Control Centre, which has the capability of initiating corrective action using data communications.
Loran-C Receiver Latitude/Longitude Corrections
Today’s Loran-C receivers are equipped with microprocessors which are designed to internally compute the latitude and longitude co-ordinates of the receiver, based on the Time Difference (TD) readings, and directly display these values.
This may reduce the need to possess Loran-C charts, though it is still required.
The latitude/longitude computation may be based upon a pure sea water path.
This leads to errors if the Loran-C signals from the various stations involve appreciable overland paths since the speed of the signal will decrease by varying amounts, depending on the nature of the earth’s surface over which it is passing.
Loran-C operates by measuring the difference in arrival times of the signals from the different stations in the Loran-C chain, and thus any unforeseen variation in the speed of a signal will result in an error in the latitude/longitude reading.
Note that when the receiver is being used in the time difference mode (time difference readings being used to manually plot lines of position on a Loran-C chart), these errors are minimal and the system should be accurate to within ¼ nautical mile.
This is because the Loran-C lattice on a nautical chart has already been adjusted to allow for the signal variation as it travels over land.
It is therefore necessary that before using the latitude/longitude feature of the receiver, to check the manufacturer’s operating manual to determine if corrections are necessary and how they may be applied to compensate for overland paths in order to obtain a greater fix accuracy.
The correction can be applied in either of two forms:
insertion of a correction when the vessel is at a known location, or
the insertion of a correction factor that is determined from a table or chartlet.
The latter is called an Additional Secondary Phase Factor (ASF) correction, and can be used to ascertain the numeric value to apply. These corrections will normally be valid only within 50 to 100 miles of the location at which the correction was inserted because of the changing effects of landmass on the Loran signals in the different areas.
To achieve high positioning accuracy within the service area, Loran-C transmitter stations are equipped with a bank of atomic clocks, which provide the timing for the transmitted Loran-C signal.
Precise navigation with Loran-C demands that the error in the timing system must not exceed a few tens of nanoseconds. For Northwest European Loran-C System (NELS), it is specified that a station’s clock shall not deviate by more than 30 nanoseconds from the clocks of the neighbouring stations. Achieving this precision in timing it is necessary to continuously measure the time deviation between the clocks in the system.
ADDITIONAL SECONDARY FACTOR (ASF)
A Loran-C receiver computes distances from Loran-C transmitting stations using the time of arrival measurements and the propagation velocity of the radio ground wave to determine position.
Small variations in the velocity of propagation between that over seawater and over different landmasses are known as the Additional Secondary Factor, or ASF.
Corrections may be applied to compensate for this variation. Such corrections may improve the absolute accuracy of the Loran-C service in positions where the received Loran-C signal passes over anything but seawater on its way from transmitter to receiver. The values of ASF depend mainly on the conductivity of the earth’s surface along the signal paths. Seawater has high conductivity, and the ASFs of seawater are, by definition, zero. Dry soil, mountains or ice generally have low conductivity and radio signals travel over them more slowly, giving rise to substantial ASF delays and hence degradation of absolute accuracy.
Fortunately, ASFs vary little with time, and it is possible to calibrate the Loran-C service by measuring ASF values throughout the coverage area.
Loran-C stations are constantly monitored to detect signal abnormalities, which would render the system unusable for navigation.
“Blink” is the prime means by which the user is notified that the transmitted Loran-C signal does not comply with the system specifications. Blink also indicates that the Control Centre cannot ensure that the signal complies with these specifications, for instance, as a result of discontinuation of data communications linking the Control Centre to the stations. Blink is a distinctive change in the group of eight Loran-C pulses that can be recognized automatically by a receiver so the user is notified instantly that the Loran-C chain blinking should not be used for navigation.
Blink starts at a maximum of 60 seconds after detection of an abnormality. Automatic blink initiated within 10 seconds of a timing abnormality may be added where Loran-C is extensively used for aviation purposes.
The Loran-C service will support an absolute accuracy varying from 185 meters to 463 meters (0.1 to 0.25 nautical miles), depending on where the observer is within the coverage area. Absolute accuracy defines a user’s true geographic position (latitude and longitude). Repeatable accuracy is a measure of an observer’s ability - by using a navigation system such as Loran-C - to return to a position visited previously using the same navigation system. Loran-C repeatable accuracy is sometimes as good as 18 meters and is usually better than 100 meters within the coverage area.
FUTURE OF LORAN-C
In 1997, an independent study was conducted in the
At the ICAO CNS/ATM implementation conference held in
The reason given was that the possibilities of
jamming, solar events, etc., were now better understood. Excellent though GPS
may be, its problem is that it is so low powered that the signal can easily be
blanked out or disrupted - as demonstrated at an 1997
The notion of GPS as sole means of navigation is dead. Suitable backup systems cited are triple inertial, VOR/DME and LORAN-C.
As of September 1998, the American DoT confirmed that the existing LORAN-C chains will be maintained and upgraded, at least to 2008, “in the transition period to satellite based navigation”.
Loran’s wavelength and signal strength enable it to penetrate into areas where GPS has difficulty because of line-of-sight blockage as in urban or forested situations. Loran can even penetrate some buildings.
The most recent draft European Radio Navigation Plan
(ERNP) and European Commission report (