1、 Rep. ITU-R M.2084 1 REPORT ITU-R M.2084*Satellite detection of automatic identification system messages (2006) 1 Introduction In the early 1990s, the International Association of Maritime Aids to Navigation and Lighthouse Authorities (IALA) first proposed the development of a universal shipborne sy
2、stem to improve the maritime safety and efficiency of navigation, and to help protect the marine environment. Subsequent to that proposal, the International Maritime Organization (IMO), the ITU, and the International Electrotechnical Commission (IEC) adopted a new navigation system now known as the
3、Automatic Identification System (AIS) to help achieve these goals. The primary purpose of the AIS is to facilitate the efficient exchange of navigational data between ships and between ships and shore stations to significantly improve safety of navigation and promote improved control and monitoring
4、of maritime events. The technical characteristics of the current AIS system using time division multiple access (TDMA) techniques in the VHF maritime mobile band are described in detail in Recommendation ITU-R M.1371. As described in that Recommendation, the AIS is designed to operate autonomously a
5、nd automatically to exchange short messages among ships, coast stations, and navigational aids within a 20 to 30 nautical miles (NM) (27 to 56 km) range primarily using a self-organizing form of TDMA. Messages include data such as ship identification, location, course and speed. Under requirements o
6、f the International Convention for the Safety of Life at Sea (SOLAS), the installation and use of AIS is mandatory on all ships of 300 gross tons or more engaged in international voyages. In 2008, all ships of 500 gross tons or more engaged in national voyages should also be equipped with AIS. AIS e
7、quipments designed for this mandatory carriage requirement are designated as Class A units. A lower power version intended for voluntary carriage, called Class B, is under development. Since its introduction, the AIS has proven very successful in meeting the original goals set by the IALA. Recently,
8、 a need has evolved for the capability to detect and track ships at distances from coastlines that are larger than can be accomplished by normal terrestrial VHF communications. Requirements of these long-range applications such as better handling of hazardous cargo, improved security, and countering
9、 illegal operations suggest a need to detect approaching ships at distances of 200 NM (370 km) from shore and beyond. This Report introduces satellite detection of AIS as one means of accomplishing long range ship detection. The report addresses its technical feasibility, examines satellite capacity
10、 under various conditions and examines possible methods for improving satellite capacity. The remaining portions of this document are organized into eight subsections as follows: operational and technical characteristics of AIS, overview of satellite detection of AIS, link budget analysis, intra-sys
11、tem interference analysis (Class A only, Mixed class A and Class B, and non-uniform ship distribution), compatibility with incumbent mobile systems, techniques for improving performance and sharing, and summary. *This Report should be brought to the attention of the International Maritime Organizati
12、on (IMO), the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA), and the International Maritime Radio Association (CIRM). 2 Rep. ITU-R M.2084 2 Operational and technical characteristics of shipborne AIS To assist in functionally describing and understanding the
13、 nature of satellite AIS detection, the basic characteristics of conventional terrestrial AIS as described in Recommendation. ITU-R M.1371 are summarized in the following paragraphs. AIS functions as a ship-to-ship and ship-to-shore communication system in which AIS-equipped ships periodically trans
14、mit short fixed-length TDMA messages including data such as identification, location, course, speed, and other status information. The associated AIS receivers aboard ships and shore stations detect this information from all nearby ships, thus providing a comprehensive picture of the local environme
15、nt to supplement radar and other navigation aids. The TDMA signal structure is based on a one minute frame divided into 2 250 time-slots with each message nominally occupying one time-slot. In the normal mode, these identification messages are periodically transmitted alternately on two VHF maritime
16、 channels that have been designated for this purpose. Ship location is obtained from an on-board electronic position-fixing device. TDMA timing is obtained from the GNSS receiver built in the AIS station. With the two channels, the total capacity of AIS is 4 500 one-slot messages/min. AIS is designe
17、d around an access scheme called self-organizing TDMA. Through this technique, the system functions without a central TDMA controller, as is typical in fixed-assignment TDMA schemes. By continuously sensing the AIS signals in the local environment and “announcing” its next intended transmission slot
18、, coordination is achieved by all participating ships in the local environment and conflicts in use of a given time-slot are minimized. Other TDMA access schemes are also used for certain message types. The RF and data technical parameters of AIS are summarized in Table 1. As described in the Table,
19、 the basic message length is 256 bits with the last 24 bits serving as a buffer to accommodate propagation and repeater delays, timing jitter and extra bits due to bit stuffing. Typically, the last 20-bit positions are empty. The characteristics of antenna and associated transmission line parameters
20、 to be installed on AIS equipped ships are not defined in the basic ITU Recommendation but are added herein to more fully define the AIS characteristics. In practice, two types of antennas are in common use, a 1/2 dipole and a 5/8 end-fed monopole with gains ranging from 2 to 4.5 dBi. In order to be
21、 conservative for this study, the dipole is assumed having a maximum gain of approximately 2 dBi with a simple cosine-squared elevation gain pattern. The transmission line type and length varies with the installation. For purposes of this paper, a 3 dB loss is assumed to account for cable and other
22、miscellaneous losses associated with the AIS ship transmitter. The default data packet bit structure is shown in Table 2. TABLE 1 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,
23、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 cosine-squared vertical elevation pattern; Minimum gain = 10 dBi Cable loss(1)3 dB (estimated) Rep. ITU-R M.2084 3 TABLE 1 (end) AIS parameters Values Receiver sensitivity 107 dB
24、m for 20% packet error rate (PER) (minimum) 109 dBm for 20% PER (typical) Modulation 9 600 bits GMSK Multiple access mode TDMA (self-organizing, random, fixed and incremental) TDMA frame length 1 min; 2 250 time-slots TDMA slot length 26.7 ms; 256 bits (see Table 2) Message types 22 types Message le
25、ngth 1 to 5 slots with 1 slot being the dominate type Periodic message interval 2 s to 6 min transmit intervals (see Table 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 2 Default da
26、ta packet bit structure Power ramp up 8 bits Training sequence 24 bits Necessary for synchronization Start flag 8 bits Data 168 bits Default length Cyclic redundancy code 16 bits Necessary for error detection End flag 8 bits Buffer 24 bits (typically, the last 20-bit positions are empty) Necessary t
27、o accommodate bit stuffing, propagation and repeater delays, and jitter Total 256 bits To accommodate the various functions performed by AIS, 22 message types are defined in the standard, which can be grouped into four categories: dynamic, static and voyage, safety and administrative, and data. The
28、dynamic messages, transmitted periodically, comprise the largest volume of traffic in the AIS environment. One key variable is the rate at which the different platforms transmit these periodic messages. For several platform types a range of reporting intervals are defined in the standard depending o
29、n the ship dynamics such as speed and course. Table 3 summarizes the message reporting intervals for the various platforms. As will be shown later, the message reporting interval plays an important role in the performance of satellite detection of AIS. As shown in Table 3, the reporting interval for
30、 Class A ships varies over a wide range from every 2 s to every 3 min depending on the dynamic status of the ship. In order to determine a long term average transmission interval for Class A ships, it is necessary to have an estimate of the distribution of the ships among the various dynamic status
31、situations. Table 4 lists the status categories, their respective reporting interval and an estimate of the percentage of ships in each category at any given time. From this data, an overall estimate for the reporting interval was determined. 4 Rep. ITU-R M.2084 TABLE 3 AIS message reporting interva
32、ls AIS platform Reporting interval Dynamic information: Coast station 3 1/3 to 10 s interval (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
33、interval Static and voyage information 6 min interval Safety consequently satellite coverage of a given ship location will not be continuous. Full global coverage and the use of a modest number of earth stations necessitate the Rep. ITU-R M.2084 5 need to use store and forward techniques for the rec
34、eived AIS data. However, for detection and monitoring of ships up to several thousand nautical miles from a coast, the large satellite footprint on the Earth allows real time download of data during the visibility period of the satellite. Several key technical factors distinguish satellite AIS detec
35、tion from conventional ship-to-ship and ship-to-shore AIS detection, specifically receiver sensitivity, antenna gain pattern, and reliability requirements. Measured data reported for AIS shipborne receivers show that off-the-shelf receivers are typically more sensitive than the receiver sensitivity
36、required in the AIS specifications. Using low noise amplifiers (LNAs) and optimum detection schemes, further improvement in AIS satellite receiver sensitivity is possible. Countering these improvements, however, is the need for larger than optimum receiver bandwidths to accommodate Doppler shifts of
37、 up to about 3.5 kHz. Taking these factors into account, a baseline sensitivity of 118 dBm for a 1% packet error rate (PER) and 120 dBm for a 20% PER are used herein for the AIS satellite receiver. The initial satellite system will use a wide beam satellite antenna. Broadbeam antennas used on LEO sa
38、tellites can generally be categorized into two groups. One commonly used type is one in which the peak gain is directed omni-directionally towards the horizon with lower gain towards the sub-satellite point. With this type of antenna, the change in antenna gain with off axis angle partially compensa
39、tes for the changes in propagation loss resulting in a lower variation in signal level as off-axis angles vary. The other antenna category is of a more conventional type with peak gain directed towards the sub-satellite point. For purposes of this study the latter type is assumed having a peak gain
40、of 6 dBi and a 3 dB beamwidth of 100. For the gain pattern of the main lobe, a model often used in ITU-R studies is used herein as follows: G() = GMB 12(/3dB)2where: G(): satellite antenna gain (dBi) at off axis angle (degrees) GMB: satellite antenna main beam gain (dBi) 3dB: satellite antenna 3 dB
41、beamwidth (degrees) The performance requirements of AIS satellite detection are also significantly different than the terrestrial counterpart. Conventional AIS, like most communications systems, aims to successfully receive and decode most of the associated transmitted messages with moderate to high
42、 reliability. For purposes of monitoring ships using satellite detection of AIS, high communications reliability is not required. For ships within a few hundred nautical miles of a coast, updates of the ship locations every hour may be sufficient and for ships further at sea, location updates every
43、four hours or even every twelve hours may be sufficient. As will be shown later, intra-system interference results in the loss of a very large percentage of received AIS ship messages. For example, for a single satellite overpass, up to 99% or more of the AIS ship messages can be lost and the goal o
44、f updating ship locations on a regular basis can still be achieved. To achieve ship location updates every 12 h, it is necessary to successful decode only one of the more than 360 messages received (0.3%) from a given ship during this period. This is explained in more detail later. The two frequenci
45、es that have been designated as channels within the maritime mobile service for the terrestrial AIS function are not allocated on an exclusive basis. Rather, these channels and adjacent channels are allocated and used throughout various regions of the world for other mobile service applications incl
46、uding VHF public correspondence stations (VPCS) in the maritime mobile service and land mobile radio systems. Unlike conventional terrestrial AIS systems that can co-exist with other co-frequency transmitters through geographical separation, the satellite antenna beam covers a large geographical are
47、a, thereby receiving transmissions by multiple AIS ship transmitters simultaneously, as well as mobile systems operating inland. Satellite AIS must be able to 6 Rep. ITU-R M.2084 successfully operate in the interference environment resulting from existing services. The performance of satellite AIS o
48、perating with existing services is examined in 9. Table 5 takes into account the above discussion to summarize the characteristics of the AIS satellite used for this study. TABLE 5 Assumed characteristic of AIS satellite link AIS satellite parameters Values Satellite Constellation 1 to 6 satellites
49、Altitude (km) 950 Inclination (degrees) 82.5 Period (minutes) 104 Earth footprint 3 281 km radius (at the horizon) Antenna Gain (GMB) (dBi) 6 Beamwidth (3dB) (degrees) 100 Pattern GMB 12 (/3dB)2Polarization Near circular Circular to linear polarization conversion loss (dB) 3 Receiver Noise figure at LNA input (dB) 3 Required Eb/N0for BER = 105(dB)13 including implementation loss Line/filter losses prior to LNA (dB) 2.5 Sensitivity at LNA (dBm) 118 for 1% packet error rate (PER) 120 for 20% PER Protection ratio (for co-channel, coincident-in-time signals) (
