Design and Development of an On-Board System in Satellite-Based Automatic Identification Systems (AIS)

Parent Category: 2017 HFE

By Roopa Narayan Naik and Dr. V Sambasiva Rao

Abstract

The Automatic Identification System (AIS) is mainly used to enhance the ability to identify ships and improve sailing safety. In the existing AIS, the allocation of message slots is self-coordinated only within each organized area. When several organized areas are covered during a pass of the satellite, AIS messages sent in the same time slot from different organized areas will reach the satellite simultaneously, colliding with one another. Special algorithms are required to extract the mixed signals.

The main aim of this paper is to configure a system which receives mixed signals from different ships and sends them to the ground station. To process these received mixed signals, first signals are sent to satellite on board, where analog signals are converted to digital signals, stored on the Onboard System, modulated and sent to the ground station for further processing. In ground stations, demodulation is performed using special algorithms to obtain the original signals. In this paper, simulations of the system have been carried out and implemented on a Field Programmable Gate Array (FPGA).

Introduction

As terrorism threats increase, major coastal cities are highly prone to sea/waterborne terrorist attacks. Supervising ships, managing the traffic flow into harbors, and preventing the accidental collision of ships has become a major task.

The Automatic Identification System is used to enhance the ability to identify ships, and improve sailing safety. In AIS, the messages from the existing VHF radio equipment (AIS) onboard the ships can be received from space so as to monitor the maritime traffic on a worldwide basis and also provide long range identification service. This technology enables various ships to exchange their information such as name, position, speed, type, course, destination etc. from ship to ship, as well as ship to ground station, in real time. The AIS concept plays an important role in navigational safety, environmental protection at sea, and on shore.

The satellite-based AIS, which is also called as Space based AIS, is developed as a global surveillance system that uses Low Earth Orbit satellites that carry AIS transponders to receive information from ships, process it, and then relay that info to the ground station for monitoring. Similar to other satellite communication and navigation systems, satellite-based AIS also consists of five components; LEO satellites in space, AIS equipment onboard ships, the ground station, the user, and communications links. Figure 1 shows the general model of AIS.

Figure 1 • General Model of AIS [1].

The satellite receives signals from various ships in the antenna coverage area. The signals transmitted from ships in the antenna coverage zone are received by the satellite with different amplitudes, due to varying ranges and different frequencies due to Doppler shifts and time delays, resulting in the collision of AIS Messages within a single slot. In this condition, at each instant, the received signal is a mixture of AIS messages with a certain ratio depending on received power level and different channel characteristics.

The GMSK modulated signals transmitted from ships in the visible zone are received by the antennas present on a LEO satellite. The received signals are digitized using ADC. This digitized data is stored on a solid state recorder during orbit. The stored data is modulated to the ground station using QPSK modulation. At the ground station QPSK demodulation has already been carried out. Further GMSK demodulation can be performed to obtain the original data.

Figure 2 • Slot collision of AIS Messages [5].

On-Board System Design

Three signals coming from different ships collide in the same time slot due to the varying ranges and frequencies caused by Doppler shifts. For demonstration purposes, three signals are mixed in the ratio 1:4:8 and sent to the ground station. According to statistical analysis, signals can collide in different ratios and only three mixed signals can be separated.

Figure 3 • Schematic of AIS transceiver architecture.

Figure 4 shows the Block Diagram of mixed signals originating from ships to the ground station.

Figure 4 • Block Diagram of Mixed Signals in MATLAB System Generator.

Table 1 • Simulation Parameters for Mixed Signal Transfer.

B. Design of Random Data Source

A counter is used to generate random bit and store in ROM as shown in figure 5. MATLAB code has been used in the counter block.

Figure 5 • Random Data Source in MATLAB System Generator

C. Level Shifter

For the GMSK Modulation the data should be a bipolar signal. The output of the Random Data Source is a unipolar signal which is converted to a bipolar signal using a level shifter as shown in Figure 6. The level shifter shifts the amplitude of the incoming signal to +0.5v to - 0.5v. The signal from the level shifter is amplified to +1v to -1v by using a gain block for convenience.

Figure 6 • Level Shifter in MATLAB System Generator.

D. NCO

A numerically controlled oscillator (NCO) is the digital implementation of a voltage controlled oscillator as in Figure 7. The NCO is designed for the desired frequency sine wave or cosine wave generation with a control word calculated for that desired frequency.

Figure 7 • NCO Implementation in MATLAB system generator.

E. GMSK Modulation

A signal is first shaped using shaping frequency and then multiplied with carrier frequency to get the GMSK modulated wave as shown in Figure 8.

Figure 8 • Implementation of GMSK Modulation in MATLAB System Generator.

F. Analog to Digital Conversion

A GMSK modulated wave is digitized in the On-Board system using an analog to digital (A/D) converter. The block schematic is shown in Figure 9.

Figure 9 • Analog to Digital Conversion Block in System Generator.

G. Storage

Digitized data is stored in single port RAM of the FPGA. Storage block in the system generator is shown in Figure 10. RAM has three inputs i.e. address, write enable and data, and one output port. When write enable is high, data is written into the memory location.

Figure 10: Storage Block in System Generator.

H. QPSK Modulation

Data read from the memory is QPSK modulated for transmitting to the ground station. The block schematic is shown in Figure 11.

Figure 11. Implementation of QPSK Modulation in MATLAB System Generator.

I. QPSK Demodulation

The QPSK Demodulation is done using a Costas Loop and is shown in Figure 12. The I channel low pass filter and Q channel low pass filter is designed to 153.6 KHz which allows the data to pass through and allows the second harmonic components and the loop filter is designed to 15.36KHz.

Figure 12. Implementation of Costas Loop in System Generator.

Finally, I and Q-channel outputs are muxed to get QPSK data.

J. Top module of On-Board System Design

Top module implementation in MATLAB system generator is shown in figure 13.

Figure 13. Top Level Module Implementation in MATLAB System Generator.

K. Simulation Results

Simulation results are shown in Figure 14.

Figure 14 • Simulation Results of the On-board System Design Displayed on a Scope.

L. Hardware Outputs

The total system after simulating in the system generator, transferred to the FPGA and the test results are shown from Figure 15 through Figure 18.

Figure 15 • Output of Mixed GMSK Signal on CRO.

Figure 16 • Output of ADC and Stored Data on CRO.

Figure 17 • Output of QPSK Modulated Signal CRO.

Figure 18 • Output of QPSK Demodulation on CRO.

Conclusion

In this paper, an On-Board System has been designed and implemented on a FPGA. At the transmitter side, the GMSK source signals coming from three different ships are mixed in the ratios 1:4:8 and are sent to the satellite On-Board where these signals are digitized using an ADC, stored on a FPGA single port RAM and sent to the ground station with the help of QPSK modulation. At the receiver side, QPSK demodulation has been accomplished. Further GMSK demodulation can be performed to recover the original signals.

Acknowledgements

My thanks to Principal Dr. K. S. Sridhar and Dr. T. S. Chandar, HOD of EC Department, PES UNIVERSITY, for providing the requisite infrastructure to complete this work in timely fashion. I would like to also thank Swetha G M for her kind support, skillful guidance, and constant supervision in the successful completion of this project.

About the Authors

Roopa Narayan and Dr. V. Sambasiva Rao are with the Dept. of Electronics and Communication Engineering, PES University, Bengaluru, India. roopanaik9193@gmail.com; vsrao@pes.edu.

References

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