1、 Rec. ITU-R M.1467-1 1 RECOMMENDATION ITU-R M.1467-1*Prediction of sea area A2 and NAVTEX ranges and protection of the A2 global maritime distress and safety system distress watch channel (Question ITU-R 92/8) (2000-2006) Scope Recommendation ITU-R M.1467 provides guidance to administrations for pre
2、dicting sea area A2 and NAVTEX coverage areas by taking into account variations in the propagation conditions. These coverage areas can be confirmed by measurement. This information is provided for administrations that are upgrading, or planning to upgrade, their shore-based facilities for global ma
3、ritime distress and safety system (GMDSS) operation in the A2 sea area. The ITU Radiocommunication Assembly, considering a) that the International Convention for Safety of Life at Sea (SOLAS) 1974, as amended, prescribes that all ships subject to this Convention shall be fitted for the global mariti
4、me distress and safety system (GMDSS) by 1 February 1999; b) that some administrations have yet to establish A2 services for the GMDSS; c) that Question ITU-R 92/8 identifies the need for promulgation of minimum performance criteria for the protection of the service, and guidance to accelerate the u
5、pgrade of shore-based facilities for GMDSS operation in the A2 sea area, recommends 1 that administrations currently upgrading, or planning to upgrade, their shore-based facilities for GMDSS operation in the A2 sea area should base such upgrading on the information contained in Annex 1. Administrati
6、ons are invited to develop appropriate software to perform the calculations described in Annex 1. *This Recommendation should be brought to the attention of the International Maritime Organization (IMO). 2 Rec. ITU-R M.1467-1 Annex 1 Prediction of A2 and NAVTEX ranges 1 Overview In order to establis
7、h a new A2 sea area it is necessary to account for variations in the propagation conditions. A2 coverage is by groundwave, which is largely stable, enabling the extent of the service area to be confirmed by measurement, as is recommended by the IMO, before committing capital expenditure. The design
8、criteria to be used for establishing A2 and NAVTEX sea areas are defined by the IMO in Annex 3 to their Resolution A.801(19). 2 Prediction of A2 and NAVTEX ranges 2.1 IMO performance criteria The criteria developed by the IMO for determination of A2 and NAVTEX ranges are reproduced in Table 1 and sh
9、ould be used in the determination of ranges for A2 and NAVTEX services. TABLE 1 Performance criteria for A2 and NAVTEX transmissions Distress channel Radiotelephony DSC ARQ NBDP NAVTEX Frequency (kHz) 2 182 2 187.5 2 174.50 490 and 518 Bandwidth (Hz) 3 000 300 300 300 Propagation Groundwave Groundwa
10、ve Groundwave Groundwave Ships power (W) 60 60 60 Ships antenna efficiency (%) 25 25 25 25 RF full bandwidth signal/noise ratio (S/N) (dB) 9 12 18 min(1)8 Mean Tx power below peak (dB) 8 0 0 0 Fading margin (dB) 3 Not stated 3 IMO reference for above Res. A.801(19) Res. A.804(19) Rec. ITU-R F.339 Re
11、s. A.801(19)Availability required (%) 95(2)Not stated Not stated 90 DSC: digital selective calling NBDP: narrow-band direct printing (1)Stated as 43 dB(Hz) under stable and 52 dB(Hz) under fading conditions with 90% traffic efficiency. (2)Availability can be relaxed to 90% in cases where the noise d
12、ata used or performance achieved can be proven by measurement. Rec. ITU-R M.1467-1 3 2.2 Achieving the required quality of signal 2.2.1 The effect of received noise On a very quiet site, man-made noise dominates below 4 MHz and galactic noise above. These combine, at the receive antenna with seasona
13、l levels of atmospheric noise, and also transmitter sideband noise, as shown in Fig. 1. Recommendation ITU-R P.372 should be used to account for atmospheric and normal man-made noise levels. Paragraph 3.5 should be used to ensure that the levels of transmitter sideband noise and intermodulation prod
14、ucts reaching the receive antenna by groundwave do not exceed the tolerable limit for protection of the A2 DSC watch frequency. 2.2.2 C/N required for single sideband (SSB) radiotelephony In order to maintain the intelligibility of a received SSB radiotelephony signal it is necessary to provide the
15、operator with a minimum AF signal/noise plus distortion ratio (SINAD), which in turn defines the RF C/N required at the receive antenna. The capture range for an A2 receive system should be calculated assuming an RF C/N density figure of 52 dB(Hz) at the shore-based receive antenna. This will ensure
16、 that a ships transmitter operating with a peak-to-mean ratio of 8 dB provides the shore-based operator with a 9 dB S/N in a 3 000 Hz bandwidth, as stipulated by the IMO. The receive antenna and multicoupler should be designed to offer good linearity to minimize the risk of intermodulation products
17、being generated on the watch frequencies. With good electronic design the noise generated within the receive system itself can be ignored below 3 MHz. 2.2.3 C/N required for NAVTEX broadcasts The transmit range for NAVTEX broadcasts should be calculated assuming an RF C/N density figure of 35 dB(Hz)
18、 at the ships antenna. This will ensure that the NAVTEX receiver is provided with an RF S/N of 8 dB in a 300 Hz bandwidth. 4 Rec. ITU-R M.1467-1 2.3 Accounting for ships topside noise Topside noise refers to the environmental noise generated by ship-borne machinery, and other sources, and a figure i
19、s required for entry into NOISEDAT and other programs. Table 2 shows a number of published figures, and for reference purposes includes galactic and quasi-minimum noise levels, which is accepted as representing the best achievable noise floor. TABLE 2 Naval environmental categories for topside noise
20、 Environmental category dB below 1 W ref. 3 MHz DOD Cat 1 mobile platform 137.0 IPS ship (ASAPS and GWPS) 142.0 AGARD ship 148.0 Quasi-minimum noise 156.7 Noise galactic (Rec. ITU-R P.372) 163.6 ASAPS: advanced stand alone prediction system GWPS: groundwave prediction system The Australian Departmen
21、t of Defence (DOD) and Advisory Group for Aeronautical Research and Development (AGARD) have both published relevant figures. The AGARD figure represents a naval vessel under normal cruise conditions, whilst the DOD figure represents the maximum level under battle conditions with all machinery in op
22、eration. The levels of noise to be expected on commercial vessels can be expected to range between these figures. The IPS Radio and Space Services (IPS) of the Australian Department of Industry have adopted an intermediate figure in their GWPS, which is well accepted as representing the noise level
23、encountered on container vessels, pleasure cruisers, and utility ships. This figure, 142 dBW, should be used in prediction of coverage area of shore-based GMDSS transmitters. 2.4 Determination of external noise factor, Fa, for the required availability An A2 area in the GMDSS is defined as the area
24、within which ship stations can alert shore stations by using DSC on MF and communicate with the shore stations using MF radiotelephony (class of emission J3E). The communications ranges for voice signals are shorter than for DSC and the IMO criteria for determination of A2 areas should therefore be
25、based on the communication of voice signals. The range achieved by a transmitter or a receiver depends upon the radiated power, the propagation loss, and the ability of the receiver to discriminate between the wanted signal and the unwanted noise or interference. The level of each component in the r
26、eceived signal will drift as the propagation conditions change with time, and therefore arrive at the receive antenna in varying proportions. The final system design should therefore ensure that the level of the signal will exceed the level of the noise by an adequate amount for an adequate proporti
27、on of the time. This proportion is called the availability, and is determined by quantifying the behaviour of the signal and the noise with time as shown in Fig. 2. Rec. ITU-R M.1467-1 5 Equation (1) should be used to calculate an upper value Fafor the external noise factor which corresponds to the
28、required availability: 22stamaDDFF += dB above k T0B (1) where: Fam: median external noise factor Ds: variation in signal level expected for the required time percentage, to which is ascribed the figure of 3 dB specified by the IMO as fading margin Dt: variation in noise level expected for the requi
29、red percentage of time. 90% availability is required for NAVTEX broadcasts, and so the upper decile value Dushould be substituted for Dtin equation (1). 95% availability is required for A2 coverage. To achieve this, substitute Dt= Du+ 3 dB in equation (1). First Famand Dushould be determined by runn
30、ing the Noise1 program, which comes with the ITU NOISEDAT package. The program requests seasons required, site location, frequency, level or category of man-made noise, and type of data output required (select Fa), local mean time, and statistical parameters required (select overall median). For pre
31、diction of external noise factor on ship stations, the reference figure of 142 dBW should be used to account for topside noise, if no better data is available. The data is presented in seasonal blocks as shown in Table 3, the data fields being explained in Table 4. 6 Rec. ITU-R M.1467-1 TABLE 3 Samp
32、le NOISEDAT output LAT = 51.45, LONG = 57.56, DUMMY SITE WINTER FMHZ = 2.182, QUIET RURAL NOISE OVERALL NOISE TIME BLOCK ATMO GAL MANMADE OVERALL DL DU SL SM SU 0000-0400 59.3 44.2 43.9 59.6 7.2 9.2 2.3 3.5 2.6 0400-0800 54.0 44.2 43.9 54.5 4.1 1.9 3.2 3.4 2.7 0800-1200 28.2 44.2 43.9 45.9 4.3 9.0 2
33、.2 3.4 1.3 1200-1600 31.0 44.2 43.9 46.0 4.2 8.9 2.2 3.3 1.3 1600-2000 53.5 44.2 43.9 53.9 10.4 12.2 3.6 3.9 2.9 2000-2400 54.3 44.2 43.9 55.2 7.2 9.2 2.3 3.7 2.6 TABLE 4 Fields presented for use in the NOISEDAT output Field Symbol Description TIME BLOCK Time block during which original measurements
34、 were made ATMO Level of atmospheric component GAL Level of galactic component MANMADE Level of man-made component OVERALL FamMedian level of FaDL DlLower decile of deviation from median DU DuUpper decile of deviation from median SL DlStandard deviation of DlSM FamStandard deviation of FamSU DuStand
35、ard deviation of DuThe median and upper values for Fashould be organized as shown in Table 5, and the seasonal spread in the value of Fafor the required availability should be plotted as a bar graph in Fig. 3. This presentation enables the process to be reviewed if any anomalies occur. Rec. ITU-R M.
36、1467-1 7 TABLE 5 External noise factor, FaMedian value, FamFafor required availability stamDDF22+Time block Winter Spring Summer Autumn Winter Spring Summer Autumn 0000-0400 59.6 55.9 52 52.2 71.7 65.2 60.2 60.9 0400-0800 54.5 43.7 45.9 46 66.8 56.2 55.6 59.5 0800-1200 45.9 45.9 45.8 45.9 55.4 55.4
37、55.3 55.4 1200-1600 46 41.9 37.7 45.8 55.4 54.8 52.5 55.7 1600-2000 53.9 43.2 43.6 43.9 66.5 59.7 59.5 58.2 2000-2400 55.2 55 54.4 55.8 64.9 63.2 61.4 64.3 IMO Resolution A.801(19) states “Administrations should determine time-periods and seasons appropriate to their geographic area based on prevail
38、ing noise levels”. 8 Rec. ITU-R M.1467-1 2.5 Accounting for propagation by groundwave 2.5.1 Introduction Horizontally polarized waves will not propagate along the surface of normal ground, as the electric vector runs tangential to the surface causing a current to flow, which results in absorption an
39、d heavy transmission losses. For this reason groundwaves have to be vertically polarized, and can only be generated by a vertical antenna, or to a limited extent by an antenna which is not perfectly horizontal, either because one end is higher than the other, or because the elements droop. The prime
40、 mover for groundwave propagation is the cymomotive force (c.m.f.) exerted by the transmit antenna. In free space, power flux-density (W/m2) decreases inversely with the square of distance, and so the field strength decreases inversely with distance and has a value equal to the product of c.m.f. and
41、 distance. The c.m.f. is synonymous with the effective monopole radiated power (e.m.r.p.), which is the power (kW) which would have to be fed into a short lossless monopole to achieve the same c.m.f., and in dB terms the two have the same value. A short lossless monopole on a perfect ground fed with
42、 1 kW has a c.m.f. of 300 V, which is the reference used in the groundwave curves given in Recommendation ITU-R P.368. Subsequent calculation of the transmitter power required should take account of the following losses associated with the antenna: the transmitter output power may be de-rated by an
43、antenna offering a poor match; power will be absorbed by the ground and the feeder; whereas an ideal monopole will produce maximum radiation along the ground, the radiation from a real antenna will peak a few degrees above the ground and tuck in to a lower value along the ground. 2.5.2 Proof of perf
44、ormance tests IMO Resolution A.801(19) stipulates that the range of the A2 sea area should be verified by field strength measurement. The c.m.f. of any shore-based transmitter and antenna should therefore be determined by operating the transmitter continuously at peak power, and measuring the result
45、ing field strength using a portable field strength meter. This should be done on an arc around the station with an approximate radius of 1 km in the required directions of propagation. The precise location of the antenna and each measurement point should be fixed using a GPS navigator. The c.m.f. on
46、 each bearing is then the product of field strength (mV/m) and range (km) for each measurement point. The antenna drive point current should also be recorded before and after the measurement. The procedures in this Recommendation should be used by administrations to determine the c.m.f. required to
47、establish coverage, which should then be demonstrated by the equipment supplier, effectively eliminating uncertainties in performance due to local ground conditions, and the antenna and station earthing system. 2.5.3 Determination of extent of A2 service area The extent of the A2 service area is det
48、ermined by the range over which SSB communication is effective at 2 182 kHz between ship and shore. The ship is considered to be fitted with a 60 W transmitter, feeding a short monopole antenna with an efficiency of 25%, as given in Table 1. The range is fixed by the maximum distance at which the sh
49、ip can be from the shore station to produce an S/N of 9 dB in a 3 kHz bandwidth out of the receive antenna at the shore station. The shore transmit station must transmit sufficient power to return the same S/N at the output of the ships receive antenna. Rec. ITU-R M.1467-1 9 The range in both directions depends upon the sensitivity of the receive antenna, which depends upon the levels of natural and man-made noise present, and the ability of the antenna to discriminate between the wanted signal and the unwanted radiated noise. Although some improvement can b
