THE ASSEMBLY,
RECALLING
Article 15(j) of the Convention on the International Maritime Organization concerning
the functions of the Assembly in relation to regulations and guidelines concerning
maritime safety,
HAVING ADOPTED by resolution
A.815(19) the IMO Policy for the recognition and acceptance of suitable radionavigation
systems intended for international use to provide ships with navigational
position-fixing throughout their voyages,
RECOGNIZING that the Maritime Safety Committee has identified that the Loran-C and
Chayka systems may be regional components of the world-wide radionavigation system,
NOTING that shipborne receiving equipment for the
world-wide radionavigation system Should be designed to satisfy the detailed
requirements of the particular system concerned,
HAVING CONSIDERED the recommendation made by the Maritime Safety Committee at its
sixty-fourth session,
1. ADOPTS
the Recommendation on Performance Standards for Shipborne Loran-C and Chayka Receivers
set out in the annex to the present resolution;
2.
INVITES Governments to ensure that Loran-C or Chayka receivers carried on ships of their
flag conform to the performance standards set out in the annex to the present
resolution;
3. REQUESTS the Maritime Safety
Committee to keep these Performance Standards under review and to adopt amendments
thereto, as necessary.
Annex Recommendation on performance standards for shipborne LORAN-C and Chayka receivers
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:
.1 in signals of field strength between 17.8
μV/m and 316 mV/m (25 dB/μV/m to 110 dB/μV/m);
.2 with a
range of 0 dB to 60 dB between the signal levels of the stations being used for the
navigation solution;
.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:
.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 microseconds to 60 microseconds, 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:
.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 3knots/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:
.1 a display of at least six digits
providing a read-out to 0.1 μs for each preselected pair of stations;
.2 identification of the pairs of stations between which the
time differences have been measured;
.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;
.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 of the propagation model on which these
corrections are based should be given in the equipment handbook;
.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 digits; and
.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.
5 Warning devices
Warnings should be provided to indicate that:
.1 any station being used is blinking;
.2 the
signal has been lost; and
.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.