1
Introduction
1.1 The Loran-C and Chayka systems are long-range radionavigation
systems, operating at an assigned frequency of 100 kHz, utilizing pulses with known
group repetition intervals from spaced transmitting stations. Lines of position are
determined by the measurement of the differences in the time of arrival of these
pulses.
1.2 Receivers for the Loran-C or Chayka system, or both, intended for
navigational purposes on ships with maximum speeds not exceeding 35 knots should, in
addition to the general requirements contained in resolution A.694(17), comply with the
following minimum performance requirements.
1.3 The equipment should comply with these standards within 7.5 min of being
switched on.
1.4 Definitions of terms used in these performance standards are given in section
8.
2
Performance Standards for the Reception of Loran-C and Chayka Signals
2.1 The receiver should be capable of taking measurements of time differences,
and of calculating and displaying the current position estimate. The processing should
be such that the combined timing accuracy of each time difference used in the navigation
solution is better than 0.3 μs.
2.2 The receiver, together with its antenna, should have the capability of fully
automatic acquisition of the signals, cycle selection and tracking. The receiver should
allow, but not require, operator assistance to acquire the secondaries.
2.3 The receiver, together with its antenna, should be capable of providing the
combined timing accuracy specified in 2.1 under the following conditions:
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.1 in signals of field strength between 17.8 μV/m and
316 mV/m (25 dB/μV/m to 110 dB/μV/m);
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.2 with a range of 0 dB to 60 dB between the signal levels of the stations
being used for the navigation solution;
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.3 with an envelope-to-cycle difference (ECD) within the range of +2.4
μs to –2.4 μs; and
-
.4 with a minimum signal-to-noise ratio of –10 dB for
acquisition in a noise level range of 4 μV/m to 5.6 mV/m (12 dB/μV/m
to 75 dB/μV/m).
3
Interference Protection
3.1 The receiver should meet the requirements of these standards:
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.1 when two near synchronous near band sources of interference of 0 dB
signal to interference ratio (SIR) are present; and
-
.2 in the presence of an interference source of –60 dB SIR relative to the
weakest Loran-C or Chayka signal specified in 2.3.1. The interfering signal should
be amplitude modulated at 30% at 1,000 Hz, and its frequency should lie outside
the band 50 kHz to 200 kHz.
3.2 The receiver should meet the timing accuracy and lock-on requirements of
these standards in the presence of cross-rate interference at a level as high as the
strongest Loran-C or Chayka signal being used.
3.3 The receiver should distinguish between signals received by ground or sky
waves in the service area and should lock on in the presence of skywave interference
having delays from 37.5 μs to 60 μs, with strengths of 12 dB to 26 dB
respectively, both delays and strengths being measured relative to the groundwave
signal.
4
Processing
4.1 The operator should be able to override any automatic selection
of chains or stations.
4.2 Maximum time to lock-on should not exceed 7.5 min under any of the conditions
specified in sections 2 and 3. It should be possible to select those secondaries which
are to be locked and tracked.
4.3 The receiver, together with its antenna, should conform to these standards
when subject to ship motion of roll, pitch and yaw under the following conditions:
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.1 at speeds up to 16 knots (3.3 μs/min time difference rate of
change on the baseline) in any horizontal direction and at accelerations up to 3
knots/min (0.6 μs/min/min time difference acceleration); and
-
.2 at speeds between 16 and 20 knots (4 μs/min time difference rate
of change) the receiver should provide a combined accuracy of 0.45 μs or
better.
5
Display of Positional Information
5.1 The receiver should be capable of displaying time difference
measurements and may allow the display of geographical positions calculated from time
difference measurements.
5.2 A receiver should be capable of displaying at least two time differences
selected by an operator, either sequentially or simultaneously, with the following
facilities:
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.1 a display of at least six digits providing a read-out to 0.1 μs
for each preselected pair of stations;
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.2 identification of the pairs of stations between which the time
differences have been measured;
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.3 when time difference information is displayed sequentially, provision
should be made for holding it on the display for as long as it is required,
without interrupting the continuous updating of time differences by the
receiver;
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.4 where provision is made for manually entering corrections in order to
display corrected positions, a clear warning indication that the position has been
corrected should be provided. It should be possible to display the applied
correction with a polarity sign;
-
.5 where provision is made for entering precomputed (additional secondary
factor) corrections for a given area, in order to display co-ordinates
automatically corrected, a clear indication should be provided that the
co-ordinates are corrected. Details should be given in the equipment handbook of
the propagation model on which these corrections are based;
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.6 where geographical co-ordinates are displayed, any additional error due
to the co-ordination calculation should not be greater than the equivalent of 0.1
μs. The receiver should be capable of presenting co-ordinates in the
form of degrees, minutes and hundredths of minutes. The display should indicate
whether latitudes are North or South and longitudes are East or West. Latitude
degrees should be displayed by two digits and longitude degrees by three digits;
and
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.7 means may be provided to transform the computed position based upon
(World Geodetic System) WGS 84 into data compatible with the datum of the
navigational chart in use. Where this facility exists, the receiver should
indicate that co-ordinate conversion is being performed and should identify the
co-ordinate system in which the position is expressed.
6
Warning Devices
Warnings should be provided to indicate that:
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.1 any station being used is blinking;
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.2 the signal has been lost; and
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.3 a cycle identification error has been detected.
7
Ancillary Equipment
Loran-C and Chayka receivers may be fitted with outputs to allow the
connection of peripheral equipment. Data from these outputs should be in digital form
and should comply with Publication IEC 1162.
8
Definitions
8.1 Group repetition interval (GRI)
The GRI designates the particular chain to which the receiver is adjusted. The
four-digit numerical designation from 4,000 to 9,999 is the time in tens of microseconds
between successive Master group transmissions; e.g. Loran-C chain 9,930 repeats its
transmissions at intervals of 99,300 μs.
8.2 Time difference (TD)
Time difference is the difference between the arrival times of the signal from two
specified stations.
8.3 Envelope-to-cycle difference (ECD)
Envelope-to-cycle difference is the time relationship between the phase of a Loran-C or
Chayka carrier and the time origin of the envelope waveform. Zero envelope-to-cycle
difference is defined as the signal condition occurring when the 30 μs point of
the Loran-C or Chayka pulse envelope is in time coincidence with the third positive zero
crossing of the 100 kHz carrier.
8.4 Loran-C or Chayka signal level
For the purpose of this standard, the level of a Loran-C or Chayka signal is the
root-mean-square (RMS) level of a carrier wave (CW) signal having the same peak-to-peak
amplitude as the Loran-C or Chayka pulse envelope 25 μs after the beginning of
the pulse. The 25 μs point is referred to as the standard sampling point
(SSP).
8.5 Lock-on
A receiver has completed lock-on when it has acquired, and is tracking, the signals of
the selected stations. Lock-on time is the interval between the time that the receivers
are switched on, or the selected chain or stations are changed, and the time that
lock-on is achieved. The lock-on time does not include any time required to tune notch
filters.
8.6 Combined timing accuracy
Root sum of squares of mean and standard deviation of the time difference error.
8.7 Noise level
For the purpose of performance specification and testing, the noise is considered to
have a uniform spectral density prior to filtering. It should be filtered by a single
resonator LC filter having a centre frequency of 100 kHz and a 3 dB bandwidth of 30 kHz
feeding a load of 50 ohms. The defined noise level is the true RMS level measured at the
filter output.
8.8 Near synchronous interference
Interference by a carrier with a frequency difference from the nearest spectral line
within the bandwidth of any post-sampling averaging or filtering process.