1、 Report ITU-R M.2169(12/2009)Improved satellite detection of AISM SeriesMobile, radiodetermination, amateurand related satellites servicesii Rep. ITU-R M.2169 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio-frequency
2、spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regional Radiocommunica
3、tion Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be used for the submission of patent st
4、atements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series of ITU-R Reports (Also availa
5、ble online at http:/www.itu.int/publ/R-REP/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodetermination, amateur and related satellite s
6、ervices P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management Note: This ITU-R Report was approved in English
7、by the Study Group under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2010 ITU 2010 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU. Rep. ITU-R M.2169 1 REPORT ITU-R M.2169 Improved satelli
8、te detection of AIS (2009) 1 Introduction Satellite detection of AIS messages has been requested by administrations. In response, studies were made that were documented in Report ITU-R M.2084 Satellite detection of automatic identification system messages. The report considered the operational and t
9、echnical characteristics of the shipborne AIS and the requirements and limitations of reception of AIS messages from satellites. Further studies were proposed to deal with the technical limitations that hinder detection of AIS-equipped target ships in high-traffic areas (e.g. the Dover Straits, Sing
10、apore, North East United States of America, and the northern Gulf of Mexico) on the designated AIS VDL (VHF Data Link). The technical limitations specifically cited were: 1 the length of the AIS message in the time slot (insufficient time buffer for the satellite detection range); 2 the large number
11、 of messages in the satellite antenna footprint (excess reuse of the time slots in the VDL as detected by the satellite); 3 the difficulty satellite AIS has in distinguishing signals between AIS messages and communications from terrestrial services within the satellite antenna footprint (coverage pa
12、ttern). This report on further studies shows the need for a special AIS message that is shortened in length and can be transmitted on a special reporting schedule on two other frequencies when those frequencies are designated. Accordingly, a new Recommendation ITU-R M.1371 AIS message (Message 27) a
13、nd new Radio Regulations (RR) Appendix 18 (AP18) frequencies for this service are proposed. 2 Solving the problem of overlapping messages (blurred reception) The differences in propagation delay between vessels and the satellite causes the AIS messages to overlap. Thus, the slot composition must be
14、adjusted to increase the time buffer so that the AIS transmissions can be received by the satellite in separate slots without a time overlap. Table 1 serves as the basis for the proposed new Message 27 slot composition shown in Table 5 based on an allowance for the “propagation time delay difference
15、 (bits)”. Tables 2 to 4 show the technical parameters, reporting intervals and data packet structure of the AIS messages that are currently approved. 2 Rep. ITU-R M.2169 TABLE 1 Satellite AIS reception technical parameters AIS satellite propagation calculations Constants: Speed of light (m/s) 299 79
16、2 458 AIS bit-time 9 600 bit/s (ms) 0.1041667 Nautical mile (km/nm) 1.852 Calculations: Orbit No. 1 Orbit No. 2 Orbit No. 3 Satellite orbital altitude (km/nm) 600/324 948/512 1 000/540 Slant range to horizon (km/nm) 2 831/1 529 3 604/1 946 3 709/2 003 Ground range to horizon (km/nm) 2 664/1 438 3 28
17、1/1 772 3 359/1 814 Difference in propagation distance (km/nm) 2 231/1 205 2 656/1 434 2 709/1 463 Propagation time delay difference (ms) 7.44 8.86 9.04 Propagation time delay difference (bits) 71 85 87 Typical satellite visibility (min)(1)10.2 13.6 14.0(1)These satellite visibility periods are for
18、a typical overhead pass of the satellite based on a circular, polar-orbiting satellite for the indicated satellite altitudes and a target ship located at 40 latitude. Typical visibility periods will vary depending on a number of factors including satellite inclination angle and target ship latitude.
19、 TABLE 2 Overview of shipboard AIS technical parameters AIS parameters Values Frequencies 161.975 and 162.025 MHz Channel bandwidth 25 kHz Platforms Class A ships, Class B ships, coast stations, navigation aids Power 12.5 W (Class A); 2 W (Class B) Antenna type(1)1/2 dipole Antenna gain(1)2 dBi with
20、 cosine-squared vertical elevation pattern; Minimum gain = 10 dBi Cable loss(1)3 dB (estimated) Receiver sensitivity 107 dBm for 20% packet error rate (PER) (minimum) 109 dBm for 20% PER (typical) Modulation 9 600 bits/s GMSK Multiple access mode TDMA (self-organizing, random, fixed and incremental)
21、 TDMA frame length 1 min; 2 250 time-slots Rep. ITU-R M.2169 3 TABLE 2 (end) AIS parameters Values TDMA slot length 26.7 ms; 256 bits (see Table 4) Message types 26 types Message length 1 to 5 slots with 1 slot being the dominant type Periodic message interval 2 s to 6 min transmit intervals (see Ta
22、ble 3) Required D/U protection ratio 10 dB at PER = 20%(2)(1) Typical parameters not defined in Recommendation ITU-R M.1371. (2)Parameter specified in IEC 61993-2. TABLE 3 Reporting intervals for AIS messages AIS platform Reporting interval Dynamic information: Coast station 3 1/3 to 10 s interval (
23、10 s nominal) Class A ship 2 s to 3 min interval (approximately 7 s average) (see Table 4) Class B ship 5 s to 3 min interval (30 s nominal) Search and rescue aircraft 10 s interval Aid to navigation 3 min interval Static and voyage information 6 min interval Safety and administrative messages As re
24、quired Data message As required TABLE 4 Default AIS data packet bit structure Power ramp up 8 bits Training sequence 24 bits Necessary for synchronization Start flag 8 bits Data field 168 bits Default length Cyclic redundancy code 16 bits Necessary for error detection End flag 8 bits Buffer 24 bits
25、(typically, the last 20-bit positions are empty) Necessary to accommodate bit stuffing, propagation and repeater delays, and jitter Total 256 bits 4 Rep. ITU-R M.2169 TABLE 5 Modified AIS packet bit structure for satellite reception Slot composition Bits Notes Ramp up 8 Standard Training sequence 24
26、 Standard Start flag 8 Standard Data field 96 Data field is 168 bits for other single-slot messages. This field is shortened by 72 bits to support the satellite AIS system buffer CRC 16 Standard End flag 8 Standard Satellite AIS system buffer 96 Bit stuffing = 4 bits Synch jitter (mobile station) =
27、3 bits Synch jitter (mobile/satellite) = 1 bit Propagation time delay difference = 87 bits Spare = 1 bit Total(1) 256 Standard (1) Only 160 bits are used in the 17 ms transmission. Thus the proposed data field for Message 27 is shown in Table 6. TABLE 6 Proposed new data field for AIS satellite dete
28、ction Message 27 Parameter Number of bits Description Message ID 6 Identifier for this message (similar to all other messages) Repeat indicator 2 Repeat indicator value should be 3 User ID 30 MMSI number Position accuracy 1 As defined for Message 1 RAIM flag 1 As defined for Message 1 Navigational s
29、tatus 4 As defined for Message 1 Longitude 18 Longitude in 1/10 min (180, East = positive, West = negative Latitude 17 Latitude in 1/10 min (90, North = positive, South = negative SOG 6 Knots (0-62); 63 = not available = default COG 9 Degrees (0-359); 511 = not available = default Status of current
30、GNSS position 1 0 = Position is the current GNSS position; 1 = Reported position is not the current GNSS position = default Spare 1 Set to zero, to preserve byte boundaries Total number of bits 96 NOTE 1 There is no time stamp in this message. The receiving system is expected to provide the time sta
31、mp when this message is received. Rep. ITU-R M.2169 5 3 Solving the problem of the large number of messages in the satellite antenna footprint The large number of messages in the satellite antenna footprint (excess reuse of the time slots in the VDL as detected by the satellite) is attributable to b
32、oth the large number of ships and the reporting rate. Studies show that 100% of AIS Class A ships can be detected if: an appropriate reporting rate for the AIS Class A ships is selected; coastal ships within range of an AIS base station are eliminated; and the AIS Class B is eliminated from satellit
33、e reception. 3.1 Selecting a reporting interval for satellite reception of the AIS Class A The proposed reporting interval for the proposed new Message 27 is 3 min in order to give the best detection probability at a typical observation time of 13.6 min as shown in Fig. 1. The reports from administr
34、ations on simulation results (for AIS Class A) also support this proposed reporting interval. Note that a 2-channel system doubles the loading capacity and provides interference protection. FIGURE 1 Optimizing the reporting rate for AIS Class A ships Report 2169-01Detection statistics with 3rd AIS s
35、atellite channel(assuming uniform ship distribution)Number of ships in satellite footprint 0 20 000 40 000 60 000 80 000100806040200Probabilityofdetection(%) 3-minutes interval6-minutes interval3-minutes; 2-channelsAssumptions: 1 Uniform ship distribution 2 Target ship located at 40 latitude 3 160 b
36、it packet length (256 bits normal packet 96 bits buffer for Message 27) 4 Polar orbiting satellite with 13.6 min average satellite visibility period. Conclusions: 1 A 3-minutes reporting interval can achieve 100% detection of AIS Class A ships 2 AIS Class A already has an available 3-minutes schedul
37、e (refer to Table 3) 3 A 2-channel system increases loading capacity and provides interference protection. 6 Rep. ITU-R M.2169 3.2 Eliminating coastal ships within range of an AIS base station An AIS base station transmits a special base station Message 4 that is received by all ships within radio r
38、ange of that station. Since that AIS base station presumably receives AIS position reports from all ships within that range, it is not necessary for those ships to transmit the proposed AIS satellite Message 27. Therefore, it is proposed that when a ship receives an AIS base station Message 4, the s
39、hip should reset the 3-minutes message timer for the proposed Message 27. This provision will greatly improve the probability of detection by reducing the number of reports. 3.3 Eliminating the AIS Class B from the satellite reception Reports from administrations on simulation results indicate that
40、both AIS Class A transmissions at 12.5 W and Class B “CS” transmissions at 2 W have sufficient signal margins to support satellite AIS reception. However, the satellite footprint is too wide to accommodate the expected combined population of both classes of AIS. Furthermore, some administrations hav
41、e stated the need, based on simulation results, to request that special frequencies be designated for transmitting the proposed new Message 27. Therefore, this proposal is to add the new proposed Message 27 to the AIS Class A only with a message priority level 4 and on the other frequencies. In that
42、 case, the AIS Class A equipment would not be required to receive on those frequencies and the reporting interval could be implemented with RATDMA based solely on the activity of AIS 1 and AIS 2 without the need to monitor the satellite-designated channel to further qualify candidate time slots for
43、transmission. 4 Operating frequencies for satellite detection of AIS Separate frequencies for satellite detection of AIS should be considered from within RR AP18 because the tuning range of the shipborne AIS Class A is limited to these frequencies. At the time of development of this report, besides
44、the secondary mobile satellite service (MSS) allocations referred to in RR No 5.227A, there is no other MSS allocations within the frequency range covered by RR AP18. With respect to possible additional AIS frequencies for satellite detection, Report ITU-R M.2084 indicated that the interference envi
45、ronment resulting from the existing services in those bands must be taken into account in determining the feasibility of accommodating satellite AIS in any given band or channel, due to the satellite antenna footprint that overlaps both land and sea. Examination of RR AP18 indicates that only 4 freq
46、uencies (channels 16, 70, 75 and 76) are exclusively dedicated to maritime use and restricted from terrestrial use on a global basis. Channel 70 is exclusively dedicated for DSC for distress and calling and channel 16 is exclusively dedicated for distress and calling, therefore making them both unav
47、ailable for AIS purposes. Footnote n), referring to channels 75 and 76, currently limits power to 1 W for radio transmissions because they are adjacent channels to channel 16 (the distress and calling channel). It should be noted that at least one administration uses channels 75 and 76 for low-power
48、 (1 W) maritime communications in-port. Transmission of Message 27 at 12.5 W only once every 3 min for 17 ms, alternating between channels 75 and 76 together with the restriction to not transmit when the ship is within range of an AIS base station, would not interfere with voice communications on an
49、y of these channels 16, 75 and 76, and it would be detectable over a 1 W radio transmission. Rep. ITU-R M.2169 7 Indeed, studies1(the JSC Report) have shown that AIS transmissions do not significantly degrade voice communications on the adjacent channels due to the shortness of the duration of one AIS time slot compared to the time intervals used by human speech. The JSC Report used the metric of “voice articulation score” (VAS) to make this assessment, and it concluded that the VAS on the channel adjacent to the AIS was 90-95% for the normal reporting inte
