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Generalized Mechanism of SOTDMA and Probability of Reception for Satellite-based AIS

Anindya Harchowdhury Department of E&ECE

Indian Institute of Technology Kharagpur Kharagpur-721302, India

[email protected]

Kalyan Bandyopadhyay Department of E&ECE

Indian Institute of Technology Kharagpur Kharagpur-721302, India

[email protected]

Binay Kumar Sarkar Amitabha Bhattacharya Department of E&ECE Department of E&ECE

Indian Institute of Technology Kharagpur Indian Institute of Technology Kharagpur Kharagpur-721302, India

[email protected]

Kharagpur-721302, India [email protected]

Abstract— Satellite based ‘Automatic Identification System’ (AIS) for global maritime applications is of current interest. The challenge of the system mainly lies in the detection capability of the receiver due to multiple transmissions from different ships received at the satellite simultaneously. To estimate the performance of the receiver proper modeling of the channel access scheme is important. In this paper we propose a generalized methodology of the Self Organized Time Division Multiple Access (SOTDMA) in an extended observation area for satellite based AIS. The performance of the proposed scheme is compared with an existing model and more realistic reception performance by a satellite based receiver is shown.

Keywords-AIS class-B; SOTDMA; Organized area

I. INTRODUCTION Today global maritime surveillance is an urgent need all

around the world. This is because of increasing number of cargo transport, illegal exchanges, and growth in global terrorism. The countries, especially those which are surrounded by water face big challenge in controlling the vessel movement and transaction through water bodies. Many countries are trying to build their own monitoring system to track and detect the vessels moving on their waters.

One such monitoring system is ship based Automatic Identification System (AIS). It is a navigation system equipped on the vessels to broadcast messages in maritime VHF band. One way it keeps vessels aware to avoid collisions while on the other way, it contributes as an important vessel monitoring services to coast guards as well as search and rescue organizations [1]. AIS is intended to be applicable for short range (20-30NM) ship-to-ship or ship-to-shore communication.

Recently interests in detecting and tracking ships beyond VHF coverage range has grown for better handling hazardous cargo, strengthening national security and to counter illegal operations at sea. Satellite based AIS can be a favorable solution to monitor beyond VHF coverage limitation to cover any given area on earth. Satellite based AIS faces some technical challenges which were not considered for class-B vessels in the ITU recommendations [2]. These are as follows:

1) Collision of transmissions from different self organized areas simultaneously in time, to be received at satellite.

2) Variable doppler offset.

3) Low carrier to noise ratio.

4) Relatively high and variable propagation loss.

These challenges play significant role in determining the capacity of a satellite based AIS. Capacity of the receiver can be estimated through modeling of channel access mechanism. SOTDMA is a new multiple access protocol being used in AIS. Proper modeling of SOTDMA will help to identify the real scenario of channel utilization. According to the concept of SOTDMA, ships within a self organized area maintain timing synchronization to transmit messages amongst themselves. Each self organized area can be thought of as a circle of 20NM radius. In [3], the self organized areas have been kept static to model the observation by a satellite in a big field of view where multiple self organized areas come in sight.

In this paper we propose an algorithm to model satellite based observations for SOTDMA systems. Here organized areas are individual ship centric and dynamic at the same time, which resembles the reality. This paper is organized as follows: II gives a brief idea about AIS system. III talks about the ideas of space based AIS. IV briefly describes the problem statement. Section V shows the proposed method in view to a realistic modeling of the problem. VI and VII show the results and summarize the work respectively.

II. AIS, THE FRONIER OF VESSEL MONITORING

A. Introduction to AIS AIS was included under the United Nation's SOLAS

convention [4]. One of the major reasons of marine collisions is lack of sufficient communication among vessels over VHF band. Vessels equipped with AIS are intended to avoid collisions at sea. It can produce more information for assessment of dangerous situations. According to the regulation of IMO and SOLAS, AIS must be implemented on the vessels


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that are larger than 300 GT. Functionally AIS should satisfy ship-to-ship or ship-to-shore communication operated at two VHF channels: 161.975MHz and 162.025MHz. Though radar has increased accuracy in vessel monitoring and tracking services, its performance for detecting small vessels and identifying a particular vessel from a group of vessels is not good. Mountainous islands could also be a bar for radar detection. So radar is not being used beyond shore based VHF coverage limitation. To improve navigational safety and reduce the scope of environmental pollution, AIS could be a better choice providing with more vessel information and detecting vessels with wider coverage area [5, 6].

B. AIS messages and communication protocol AIS broadcasts information in packets, where each packet

containing 256 bits are transmitted at 9600 bps, and use GMSK modulation over two VHF carriers. AIS messages can be categorized as following: a) static messages, b) dynamic messages, c) voyage related messages, and d) safety related messages.

To execute AIS operation there are four different access schemes for controlling access to the data transfer medium. The access schemes are: SOTDMA, ITDMA, RATDMA, and FATDMA [1]. The TDMA technology provided for collision avoidance in data transmission makes the link robust. At power on, an AIS unit forms a dynamic directory of all the ships in its radio visibility (20NM) for one minute. Then it starts transmission by allocating a new slot using ITDMA. RATDMA is used to allocate a slot which is not preannounced. SOTDMA is used for autonomous transmission. Among the aforementioned channel access techniques SOTDMA is of major importance because it is being used for continuous transmission. For dual channel AIS the parameters used to implement SOTDMA technique are as follows [2]:

Nominal start slot (NSS) is the first slot to be used by a station to announce itself on a data link, which is selected within NI from current slot ahead.. For dual channel,

.B ANSS NSS NI= + (1)

where subscript A and B identify two separate channels. Nominal increment (NI) is a given number of slots and is derived from,

2250 / .NI Rr= (2)

Selection interval (SI) is the number of slots (0.2*NI) within which nominal transmission slot (NTS) has to be selected. NTS should be selected about NS within SI from it. NTS is the slot for transmission over the data link and NSS is the first nominal slot (NS) after network entry.

0.1*ASI NS NI= − to 0.1*ASI NS NI= + (3)

( * 2* )B BNS NSS n NI= + , (0 0.5* )n Rr≤ < (4)

BNS is the nominal slot used for channel B. The reporting interval TΔ is

60 / .T RrΔ = (5)

Each message transmission period is 26.67 msec. Thus each minute 2250 slots can be used for transmission through a single channel. So for dual channel the capacity is 4500 slots.

III. SPACE BASED AIS

Though AIS was designed to be used for maritime terrestrial communication, there has been a growing need for satellite reception of AIS signals for monitoring. After carrying out a feasibility study we found that AIS signals can be received through LEO satellites maintaining enough link margin ( ≈ 10dB at 600 km. altitude).

According to SOTDMA algorithm, ships within a single SOTDMA are synchronized with each other; therefore avoid collision of transmissions amongst each other. As the communication protocol is a TDMA algorithm, and there are multiple of such dynamic SOTDMA regions get formed, AIS antenna from space may receive a plurality of signals at same slot and therefore leads to loss of information. The problem of slot collision occurs mainly due to

a) Transmission from different SOTDMA regions at same slot, to be received by the satellite simultaneously [3].

b) Transmission from different SOTDMA regions in consecutive slots, to be received simultaneously because of different propagation delay [3].

IV. PROBLEM DEFINITION

Collision free reception from space serves major role in determining the performance of a space-based AIS system. So it's very important to model the slot allocation issue in a realistic manner. In [3], a concept of 'organized area' has been used to model the reception scenario from space. The key constraints considered over there and our observations on those are as follows:

i. Each organized area is of 40*40 NM2. Within an organized area all ships maintain same SOTDMA [3].

But standard VHF coverage is up to 20NM from transmitter or receiver. So there could be handful of ships in an 'organized area' which are not in line of sight of some other ones in that organized area in reality. In Fig.1, C, a self organized area includes ship1, 4, and 5. Ship5 is in the center and ship4 is at one edge keeping distance of 20 NM. So ship1 cannot have communication with ship 4 in reality. In a big field of view this could happen quite often.

ii. In a satellite field of view (FOV) there would be thousands of such organized area. But one ship from an organized area cannot talk to another one from neighboring organized area in spite of being within VHF coverage.

In reality this should not happen. According to Fig.1 ship 1 from C should have communication with 2 from ship A and 3 from B.

iii. Number of ships in an organized area is predefined.

Random distribution of ships would be more realistic.

As this paper focuses on AIS class-B ships which are large in


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5

2

1

3

A B

20 NM

4

C

Figure 1. Multiple SOTDMA regions

number, we must consider the observations mentioned above.

V. PROPOSAL OF NEW METHODOLOGY

A new scheme has been developed to realize the performance of satellite based AIS in terms of probability of reception of ships. In a big field of view where thousands of vessels are in action, the dynamics of SOTDMA is

implemented keeping the above mentioned problems in mind. For satellite reception, ship movement is recorded for a complete pass over time of a LEO satellite with given orbital parameters. The process steps followed during simulation are as follows:

i. Ship distribution in a field of view to the horizon is taken randomly.

ii. Before starting the transmissions, each ship keeps track of all the other ships in its coverage and selects own NSS within NI randomly.

iii. Thereafter first ship selects the NTS within SI using (2). The other ships in its coverage select their SI leaving the booked ones apart.

iv. The processes (ii, iii) go on for all the ships. It may though happen that the ships which already booked NTS are being checked again. The NTS should be kept same without any change.

v. In this simulation the ships are assumed to be operated in autonomous mode [1]. The process followed at first minute is shown in Fig.2.

vi. After first minute of transmission the positions of each ship will change. Each ship will search for the other

Time < 1? Ship_no=1?

Selec t NSS withinNI

Check no. ofships in coverage Select NSS

within NI

Select NTS amongthe remaining slots

within SI

Transmit

NTSselected?

Check no. ofships in

coverage

Store NTS forthe NTS

selection of otherships in coverage

Selec t NTSw ithin SI

At each minute checkwhether any of the ships

in SOTDMA synchronizedarea shares same NTS

w ith any other

ships w ith same NTS w illrandomly select NTS amongthe free slots w ithin individual

SI

Start

Ship_no=1?

Yes

No

Yes

No

No Yes

Yes

No

Figure 2. Generalized network model


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ships in its coverage and their NTSs. If one ship is found to have same NTS with some other one which was previously not in first one's radio coverage, any of the ships will be randomly picked up and will change its NTS. The same will happen when more than two ships are found to have same NTS. Fig.2 represents the process flow for this step.

For the simulation of ship reception algorithm, two previously mentioned collision mechanisms have been employed.

VI. RESULTS & DISCUSSION

Simulation results show a comparative study between ‘organized area’ based approach and the proposed method. In first one network model is kept same as in [3], but instead of using square grids circular cells have been considered here and moreover the FOV of the satellite is also kept circular to realize omnidirectional propagation of RF signal. Ships are assumed to be equal in number in each ‘organized area’. In the proposed system ships are gaussian distributed. Report rate (Rr) is kept equal for all the ships. Fig. 3 shows the comparison of performances between the two systems for satellite antenna

Figure 3. Ship reception probability as a function of number of ships in

FOV (1600NM swath width, Rr=5)


FOV(1931NM swath width, Rr=5)


FOV (2468NM swath width, Rr=5)

gain of 3.25dB. The proposed system provides better clash free reception probability than the ‘Organized area’ based system when number of ships in FOV is small, and worse when number of ships increase beyond 1000. Again when the number of ships is larger than 2500, the proposed system shows very small but steady difference from the other one. Fig. 4 and Fig. 5 represent the same phenomenon where receiving antenna gains are 3 dB and 2.8 dB respectively. While probability of reception is high, proposed system shows worse performance than the ‘organized area’ based system, and when the reception probability is very less both the system exhibit nearly equal performance.

VII. CONCLUSION

This research was emphasized to model network algorithm for observing from satellite to obtain realistic AIS receiver performance in terms of probability of ship reception. Significant differences between the performances of two methods are observed though some similarities are also found. The performance also differs in great amount from the square grid approach as in [3].

REFERENCES [1] IMO, "International convention for the Safety of Life at

Sea(SOLAS)",Chapter V" Safety of Navigation", Regulation 19, 1974/1980.

[2] ITU, Recommendation, ITU-R M.1371-4 (04/2010): "Technical characteristicsfor an automatic identification system using time-division multiple access in VHF maritime mobile band".

[3] HØye G (2004): Observational Modelling and Detection Probability for Space-based AIS Reception-Extended Observation Area, FFT/RAPPORT-2004/04390.

[4] HØye G (2004): "Ship Detection Probability Analysis for a Possible Long-range AIS System" , FFT/RAPPORT-2004/04383.

[5] Mr. Jorge Arroyo, "The Automatic Identification System- Then,now,and in future", Proceedings, Spring 2011.

[6] Bin Lin and Chih-Hao Huang, "Comparison Between ARPA Radar and AIS Characteristics for Vessel Traffic Services", Journal of Marine Science and Technology, Vol.14, No.3, pp. 182-189(2006).

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