1、 Rep. ITU-R M.2123 1 REPORT ITU-R M.2123*Long range detection of automatic identification system (AIS) messages under various tropospheric propagation conditions (2007) 1 Introduction In the 1990s, the International Maritime Organization (IMO), the International Telecommunications Union (ITU) and th
2、e International Electrotechnical Commission (IEC) adopted a new navigation aid known as the Automatic Identification System (AIS) to help improve safety-of-navigation, maritime traffic control and efficiency of maritime commerce. The primary purpose of the AIS is to facilitate the efficient exchange
3、 of navigation and voyage data between ships, and between ships and shore stations. The technical characteristics of the AIS using time division multiple access (TDMA) techniques in the VHF maritime mobile band are described in Recommendation ITU-R M.1371. Like most VHF terrestrial systems, the maxi
4、mum range of AIS communications is normally governed by line-of-sight and diffraction mode propagation mechanisms. Assuming typical technical parameters of AIS equipment, maximum reliable ship-to-ship radiocommunications over sea water is in the range of 20-25 NM. Shore stations, with high antennas,
5、 can reliably receive AIS messages from ships at distances of up to 20 to 35 NM, depending on antenna heights above sea level. The safety-of-navigation and traffic control functions provided by the AIS dictate a requirement for high communications reliability in which a high percentage of the AIS me
6、ssages are detected and corrected decoded. Because of the continued growing importance of the AIS traffic, a need has arisen to monitor shipping at distances from shore greater than can be achieved via these conventional propagation mechanisms. Recommendation ITU-R M.1371 introduces the concept of l
7、ong range detection of AIS data but does not define a specific communications mechanism to accomplish long range AIS detection. As contrasted with normal AIS safety-of-navigation functions, this long range AIS capability does not necessarily need the same high degree of communications reliability. T
8、his lower reliability requirement follows from the fact that it is necessary to only detect a fraction of the AIS messages sent from a given ship to accomplish the goal of updating ship locations on a regular basis. This Report addresses long range monitoring of ship locations at sea through detecti
9、on of AIS messages. Section 2 focuses on concepts that may enhance long range detection of ships at sea by AIS coast stations. Section 3 assesses whether long range detection has similar spectrum sharing characteristics with other co-channel mobile systems as that of normal AIS safety-of-navigation
10、functions. Section 4 provides an overall summary. *This Report should be brought to the attention of Radiocommunication Study Group 3, the International Maritime Organization (IMO), the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) and the Comit Internation
11、al Radio Maritime (CIRM). 2 Rep. ITU-R M.2123 2 Concepts to enhance the long range detection capabilities of AIS coast stations 2.1 Off-shore platforms One mechanism to effectively supplement the shore-based reception range of AIS ship messages is through the installation of AIS receivers on off-sho
12、re platforms. Two cases are considered herein off-shore oil platforms and off-shore weather buoys. 2.2 Oil platforms In some parts of the world, off-shore oil platforms are extensively deployed along coastal areas. These platforms are typically very large, as illustrated in Fig. 1, and have extensiv
13、e radiocommunications capability with the nearby mainland. Consequently, in such areas, oil platforms can provide a very useful base on which to locate AIS receiving equipment to enhance shore-based coverage. FIGURE 1 Typical offshore oil platform The first step in evaluating the range enhancements
14、possible with the use of offshore oil platforms is the development of a baseline capability for conventional shore-based reception. Table 1 gives representative technical parameters applicable for the AIS ship to shore link. Using these parameters and a radio propagation model as described in Append
15、ix 3, Fig. 2 was developed which shows the median predicted received power for AIS ship to shore radiocommunications as a function of shore station antenna height. The results from Fig. 2 indicate a median radiocommunication range of about 38 to 60 km (20 to 32 NM) for the antenna heights shown. Off
16、shore oil platforms have tower heights considerable higher than the coast station heights used above and consequently have larger communications range with ships at sea. Rep. ITU-R M.2123 3 TABLE 1 Representative technical parameters for AIS ship-to-shore communications Technical parameter Typical v
17、alue Ship Transmit power 41 dBm Antenna line losses 3 dB Antenna gain 2 dB Antenna height 10 M Shore station Antenna gain 5 dBi (omnidirectional) Antenna line losses 3 dB Receiver sensitivity 107 dBm for 20% per (minimum) 109 dBm for 20% per (typical) Median received signal level from edge of servic
18、e range 104 dBm Antenna height 10 to 50 m FIGURE 2 Predicted AIS median received power at shore station 4 Rep. ITU-R M.2123 In those cases where there are sufficient numbers of such oil platforms, a line of AIS-equipped oil platforms appropriately located surrounding a key port can provide reliable
19、AIS reception coverage at up to triple the distance from coast stations alone. Because of the extensive communications equipment already installed on most oil platforms, backhaul of the received AIS data to shore, either via satellite or undersea cable, generally presents no problems. One such locat
20、ion is the Gulf of Mexico on the Southern Border of the United States of America. Figure 3 gives an overview of the oil platforms deployed in this area. Also shown as dark circles on the figure is one possible arrangement of AIS-equipped platforms. This arrangement of platforms extends reliable AIS
21、reception coverage for ships to at least 120 NM from two major port areas, Houston, Texas and New Orleans, Louisiana in the United States of America. FIGURE 3 Example of off shore oil platform deployment near New Orleans LA, United States of America The communication range enhancement as a result of
22、 anomalous propagation modes, primarily atmospheric ducting, discussed later in this section would also be applicable for AIS receivers installed on off-shore platforms. Consequently, on an intermittent basis, AIS reception coverage would extend further out to sea. For the Gulf of Mexico example des
23、cribed here, climatic conditions are often conducive to atmospheric ducting. It is concluded that in areas where off-shore oil drilling occurs, installation of AIS receivers on these platforms can increase the coverage range of shore-based AIS detection by a factor of up to three. A line of well-pla
24、ced platforms can provide complete extended-range coverage around key port facilities. 2.3 Weather buoys As in the case of oil platforms, off-shore weather buoys are also used along coastal areas. Figure 4 illustrates a typical 10 m buoy. While the density of deployment is typically much lower than
25、the example described above for oil platforms, their use is much more widespread in many coastal areas. Rep. ITU-R M.2123 5 FIGURE 4 Typical offshore weather buoy These platforms offer an additional opportunity for the installation of AIS receivers to supplement shore-based AIS detection. However, u
26、se of off-shore weather buoys presents several additional challenges. The much smaller size of the weather buoys significantly limits the possible height for placement of the antennas; consequently the detection range is smaller than normal shore based reception. Table 2 presents representative tech
27、nical parameters for a ship-to-weather buoy link. TABLE 2 Representative link parameters for AIS weather buoy communications Technical parameter Typical value Ship parameters Transmit power 41 dBm Antenna line losses 3 dB Antenna gain 2 dB Antenna height 10 m Weather buoy Antenna gain 2 dBi Antenna
28、line losses 1 dB Antenna height 3 m Median signal level from edge of service area 104 dBm Receiver sensitivity 107 dBm for 20% per (minimum) 109 dBm for 20% per (typical) Using analysis methods as described earlier, the reliable communication range of weather buoy AIS detection is estimated to be ap
29、proximately 33 km (18 NM). 6 Rep. ITU-R M.2123 FIGURE 5 AIS communication range from typical weather buoys Figure 6 shows one example of the distribution of off-shore weather buoys along the East Coast of the United States of America. As contrasted with the use of oil platforms, the lower density an
30、d shorter communication range of AIS-equipped weather buoys typically could not provide enhanced AIS coverage in all directions from a key port area. For this example, installation of AIS receivers on several weather buoys could provide limited enhanced AIS coverage along major shipping lanes approa
31、ching the port of New York but not full umbrella coverage. FIGURE 6 Example of off shore weather buoy deployment Rep. ITU-R M.2123 7 One significant limitation on the use of weather buoys to enhance AIS range is the mechanism to backhaul the data to shore. While the use of undersea cables is impract
32、ical, communication to shore via satellite communications is possible. However, the reliance on solar energy as the source of power limits satellite communications to intermittent, low data rate communications. This limitation, in turn, would require the use of on-board signal processing with the AI
33、S receiver so that only significant new or updated ship information would be sent and all unnecessary repetitive AIS data eliminated. One option to add the backhaul AIS data to the existing weather data channel via LEO and geostationary earth orbiting (GEO) weather satellite links was investigated.
34、The weather data transmitters installed on the weather buoys typically operate on an intermittent, low-duty-cycle, basis at a data rate of 300 to 1 200 bit/s on a channel shared by many other weather buoys. The sharing of a single uplink channel among many buoys generally limits the transmissions fr
35、om a given buoy to about once per hour. In order to not significantly impact the primary weather functions, any added AIS data must be limited to some small fraction of the existing weather traffic. With these combined limitations, the use of the existing satellite weather communication channel for
36、AIS derived data on an operational basis appears impractical. The alternative is to include a separate transmitter on the weather buoy capable of communicating with an existing LEO satellite communications network. The communications requirements of such a link can be estimated as follows: a) Assume
37、 that the on-board AIS signal processing limits the data forwarded via the satellite link to only a single message when a ship enters its AIS communications zone and one message when a ship leaves the zone. b) Assume that the average ship traffic along the associated shipping lane is X inbound ships
38、 and Y outbound ships per hour. Under these assumptions, the net number of AIS messages forwarded via the satellite link would then be 2X + 2Y per hour. Under any reasonable estimate of ship traffic, the transmitted AIS message rate via the satellite link would be quite low. In summary, the installa
39、tion of AIS receivers on off-shore weather buoys can provide useful extended range coverage along key shipping routes, although full umbrella coverage will typically not be possible. 2.4 Anomalous propagation Another concept to take advantage of the lower reliability requirement for long range AIS d
40、etection is to supplement normal coverage via line-of-sight and diffraction propagation modes by placing additional reliance on anomalous propagation mechanisms. Several ITU-R Recommendations address the characteristics of a number of these mechanisms including: Tropospheric scatter Atmospheric duct
41、ing Meteor burst Rain scatter Sporadic-E Two of these mechanisms, tropospheric scatter and atmospheric ducting, are investigated in the following paragraphs. 2.4.1 Tropospheric scatter Tropospheric scatter (hereinafter called troposcatter) is a mode of transhorizon radio wave propagation that result
42、s from the random reflections and scattering from irregularities in the dielectric gradient density of the troposphere. This propagation mode is applicable from below 100 MHz to above 8 000 MHz and may extend for distances of several hundred kilometres. 8 Rep. ITU-R M.2123 Although it is included he
43、rein under the general category of anomalous propagation, the propagation loss resulting from this effect, although quite large, can be sufficiently steady and predictable to support reliable long range communications. Because of the typically large propagation losses involved, it is clear at the ou
44、tset of this study that reliable reception of AIS messages from ships at sea via troposcatter propagation will not be possible using current AIS coast station designs. The discussion herein focuses on design factors that may allow reliance on tropospheric scatter for long range AIS detection. Since
45、the characteristics of ship-borne AIS equipment is fixed and cannot be modified in the near term, the key factors are the propagation loss, coast station receiver sensitivity, receiver antenna gain, receiver signal processing, and radio noise. a) Propagation loss Appendix 1 describes in detail two I
46、TU-R Recommendations that address the characteristics of troposcatter propagation. Recommendation ITU-R P.617 addresses propagation loss from the standpoint of the design of transhorizon communications systems in the frequency range 200 MHz to 5 GHz. Recommendation ITU-R P.452 addresses the evaluati
47、on of interference via various propagation mechanisms, including tropospheric scatter, for the frequency range 100 MHz to 50 GHz.1Although the latter recommendation is not directly applicable to the present subject, consideration of the recommendation is useful here in that it demonstrates that prop
48、agation trends continue smoothly, free from unexpected results, as low as 100 MHz. A third propagation model was examined which similarly showed consistent trends as low as 20 MHz. Consequently, for purposes of this report, the propagation methodologies described in Recommendation ITU-R P.617 are as
49、sumed applicable when extrapolated to 162 MHz. Figure 7, drawn from Appendix 1, describes troposcatter loss as a function of distance and reliability statistics. The curves were developed based on a ship height of 10 m and a receiving antenna height above average terrain of 50 m. However, since troposcatter propagation losses over water are relatively insensitive to antenna heights, the curves will be generally applicable at most practical shore station heights. FIGURE 7 Tropospheric propagation loss at 162 MHz 1The subject recommendation is focused on interference consideration
