SEA STATE MONITORING HF RADAR CONTROLLER using RECONFIGURABLE LabVIEW FPGA

Shubhada Gadgil #1 , Dharmesh Verma#2,Dr. Meena S. Panse #3, Dr. Kushal Tuckley #4 #1 MTech Electronics student, Electronics Engg. Dept., VJTI, Mumbai, India,ph:9892978397

#2 Scientist, SPNE Divison, SAMEER, IIT Bombay Campus, Mumbai, India #3 Professor, Elect. and Electronics Engg. Dept., VJTI, Mumbai, India

#4 Head, SPNE Divison, SAMEER, IIT Bombay, Mumbai, India

Abstract— High Frequency (HF) Radars are used extensively for ocean observation and to provide synoptic current maps covering thousands of kilometres. HF Radar for Ocean observation is under development at SAMEER, IIT Bombay Campus. The project is funded by the Government of India, Ministry of Earth Sciences.

The controller for this Radar is designed using NI LabVIEW 8.5 Professional System Development platform, National Instruments high precision R series NI PXI 7833 intelligent DAQ with reconfigurable FPGA and onboard memory, NI LabVIEW FPGA compiler and NI PXI 1033 chassis for PC interface.

This design is implemented as two modules. 1. FPGA Module:-generates Carrier modulating chirp, switching pulses, synchronising and control signals for operating the radar, and receives Receiver signal samples in FPGA buffers. This design is compiled and synthesised on Xilinx Virtex-ll reconfigurable FPGA with 3M gates. 2. Host Module: - This module runs on the Host PC. It is used for reconstruction of time domain Signal (TDS), Power Spectrum (PS) and FFT computation of intra and inter sweep radar echo’s, signal processing and wave spectral analysis and display, waveform storage as .jpeg and .bmp files for offline analysis, velocity and significant wave height limit detection and storm/disaster prediction. The FPGA and LabVIEW based design has resulted in a very deterministic system performance, reduced development time and cost and is reconfigurable to adapt new features. Keywords—Pulsed FMCW, LabVIEW FPGA, Radar Sea echo spectrum, TDS, PS, Matrix FFT, Real Time System.

I. SYSTEM OVERVIEW Long range Omni directional coastal HF radar is used to

map surface currents in real time. A transmitter sends out a frequency modulated continuous wave (FMCW) that scatters off the ocean surface and is received by a three channel receiving antennae. Through the received signal spectrum and principles of Doppler shift, the system is able to determine the range, speed and direction of the ocean surface currents.

A. Role Of HF RADAR [1] HF radio formally spans the 3-30 MHz band (with

wavelengths between 10m -100m).A vertically polarized HF signal is propagated at the electrically conductive ocean water surface; when the radar signal is incident on ocean waves that are 3-50 meters long; the signal scatters in many directions.

1) BRAGG’s Scattering:[1][2] The Radar Signal returns back directly to its source with a

Doppler shift of less than a Hertz with respect to the transmitted signal and with maximum strength only when the signal scatters off a wave that has exactly half the transmitted

signal wavelength, and is traveling in a radial path either directly away from or towards the radar. This is known as the Bragg’s principle, and the phenomenon 'Bragg scattering'.

Fig. 1 Bragg scattering phenomena; ocean wave is half the wavelength of transmitted wave. [1]

HF RADAR is used for sea current mapping because the dimensions of the ocean waves associated with HF wavelengths are always present. In the present design, the signal sent from the transmitting antenna has a known frequency of 5 MHz with 60m EM wave and will be reflected with maximum strength with ocean waves of 30m normally present in mid -ocean.

The feature determination of the sea waves is done by identifying Bragg peaks from the received Doppler Spectrum by placing them symmetrically around zero Doppler. (Additional shift Дf in the peaks is due to ocean currents).

Fig. 2 Radar Sea echo spectrum [1] 1) Determining the velocity of ocean waves:

Velocity of ocean waves is determined from the frequency shift in the Bragg peaks as:

Дf = 2 VR /λT (1) Дf = Frequency Shift in Bragg Peaks

VR = Radial Component of Velocity of ocean waves B. Use of FMCW Radar Topology

FMCW transmission is used to determine the range and Doppler from the backscatter spectrum.

1) Determining the Range of Ocean waves: The Radar transmits a FMCW (chirp) signal over the ocean surface. The frequency of a chirp increases linearly with time. A 3-channel receiver (fig. 3) receives the returned signal. The time difference between the transmitted and returned signal gives a beat frequency, proportional to time delay; this time delay is used to determine the range of the desired ocean waves.

2009 International Conference on Advances in Computing, Control, and Telecommunication Technologies

978-0-7695-3915-7/09 $26.00 © 2009 IEEE

DOI 10.1109/ACT.2009.103

395

2) Angular Direction of the Ocean Waves: A three channel receiver antennae consisting of one Omni-directional monopole and two loop antennae oriented at 900 to each other is used for direction determination. The monopole receives the same signal independent of the incoming direction of ocean waves. The amplitudes (A1 and A2) of the signal received by the dipole antennae are proportional to cosθ and sinθ respectively. The ratio of their magnitudes is used to determine the direction of the signal. Angular direction of ocean waves θ =tan-1(A2/A1). (2)

II. THE SYSTEM UNITS

1) The Transmitter Section: The monopole Antenna with vertical polarization transmits a RF carrier of 5MHz modulated with 0-30KHz Chirp. The Radar operates in the HF spectrum. The modulating chirp and TX Switching signal are generated through LabVIEW based design. The Chirp is mixed with a 5 MHz carrier generated by OCXO followed by image rejection filters and Power Amplifiers. The Peak transmit power is 100W and the Average power is 50W. The expected detectable range is 300kms.

2) The Receiver Section: The signal sent out from the transmitting antenna is incident on ocean waves, reflected and received via a three channel receiver (fig. 3). The signal is continuously compared using a simple beat frequency modulator producing an audio frequency tone from the reflected signal and a portion of the transmitted signal. The signal is sampled, analysed to determine wave characteristics.

3) PC Interface in the form of DAQ & automaton S/W[6]: Data transmission from PC to the Transmitter section and reception through three channel Receivers is achieved by LabVIEW Program control using National instruments R Series Intelligent Data Acquisition card with an integrated programmable FPGA and onboard Memory and NI PXI 1033 chassis for PC interface. The Circuit functionality is achieved by creating NI LabVIEW block diagrams using LabVIEW FPGA Module and synthesizing them on the onboard FPGA. The block diagram executes in hardware, giving the user a direct, immediate and tight control of all I/O signals on the PXI device. Data acquisition is achieved with 200KS/s sampling rate. Analog outputs update rate is 1MS/s. Hardware decision making rate is 40 MHz. The DAQ card is interfaced with a PC via PXI 1033 chassis with 5 slots and integrated

Fig. 1 Block Diagram for Ocean Monitoring HF Radar System

controller enabling remote operational. 4) Software and FPGA based Design: The HF Radar

Controller System Software design consists of two Modules: a) FPGA Modules:-The carrier modulating Chirp

generator module, Transmitter and Receiver switching and control signals generator module, Data Acquisition and sampling modules are implemented using LabVIEW Block diagrams and compiled and synthesised on to the Xilinx target FPGA using LabVIEW FPGA compiler.

b) Host Program Modules on PC:-The Host Modules run on the PC. The FPGA modules are embedded in the Host modules using FPGA Reference controls. These modules communicate with each other by parameter passing and using global variables. Host Modules have been developed for providing operator’s console, achieving synchronised transmission of Chirp, Switching Pulses and Control signals, receiving samples of Receiver signal from the FPGA buffers, Reconstruction of TDS, FFT Power Spectrum computation, Frequency analysis, Waveform Characteristics Display, Velocity, Wave Height Limit exceed detection and indication and storage of samples. Time stamped TDS Waveforms and PS are stored as .bmp files (fig. 8) for offline analysis.

c) Transmitter Section Design: The Operator’s console developed on Host PC prompts the user for the Chirp Start and Stop frequencies. The default Chirp Sweep is 0-30KHz.The chirp and other TX/RX switching waveforms are generated by Direct Digital Synthesis method. The DDS is achieved by storing the waveforms in a Look UP Table for phase values ranging from 0 to 3600. The Analog Out (AO) update rate decides the frequency of the generated signal. The amplitude values are passed to embedded FPGA program and subsequently through D/A convertors to Analog Output port lines of the PXI 7833 DAQ card. In order to achieve a 300km range, with the formula,

Range = (velocity of light*propagation time delay)/2, the pulse transmission durations works out to 2ms.The TX is Switched ON for a period of 2ms and Switched OFF for a period of 2ms.TX switching frequency is 250 Hz.

Fig. 4 LabVIEW based design modules indicative Block diagram

396

Fig. 5 Results: - Chirp and Switching pulses using DDS as observed on DSO

Fig . 6 Results Host synchronised Radar Control Signals using DDS at AO2, AO4 and AO5 ports of DAQ card as observed on DSO. The transmitter transmits the carrier of 5 MHz modulated

with a 0-30kHz chirp as seen in fig. 5.The TX switch On and OFF duration is 2ms to achieve a 300kms range. During this 2ms TX switch OFF period, the receiver is switched ON for 2ms to receive the reflected signal. This is repeated for the Chirp Sweep duration of 256ms. After 10ms duration (fig. 6), the Sweep and the TX/RX switching is repeated over a Coastal Observation Cell for 128 times. A rate of change in frequency is used for velocity determination. The sweep repetition rate is 4Hz. The modulating Chirp, switching and control signals are generated by executing radar_control_signals.vi - FPGA target program and a Host Vi. VI’s are LabVIEW Block Diagrams which model the functions of a Virtual instrument. The HOST Program using cluster controls, parameter passing, local and global variables and timed looping structures Communicates with the embedded target FPGA program to achieve the desired synchronised transmission and reception.

d) Receiver Section Design and Data Processing:[10]

Fig. 7 Results: Reconstructed signal of 250 Hz with 1000 samples at 2 µs rate, averaged mode with 128 sweeps over a range cell in Receiver Section Design

Fig. 8 Results:-Date &Time stamped .bmp real time files for offline analysis

In the transmission OFF period of 2ms, nearly 1000 samples of received signal are collected and averaged and stored as a row of an array. 64 such averaged samples are collected in 256ms duration. The Carrier sweep is repeated over a observation cell 128 times and 64 averaged samples collected through FPGA buffers are stored in 128 rows fig(7). The PS of this TDS is found by computing a 64 point Row FFT. These FFT values are stored in a 64x128 Array and provide range information. Applying column FFT to this array gives the rate of change of frequency/velocity information of the ocean waves. Thus data, extracted from sea echo spectrum through the receivers is used for range, velocity and direction determination, velocity and amplitude limit exceed detection and storm/disaster prediction.

III. FEATURES /CONCLUSIONS 1. Deterministic performance with tight control over timings

due to FPGA based design and signal processing capability, lesser development time and reduced cost

2. Real Time system; User friendly operator’s console 3. Reconfigurable/flexible design to adapt new features.

IV. APPLICATIONS AND SCOPE 1. Assisting Navigation, providing data for oceanographic

Current vector plots to Earth science Dept. 2. Disaster warning in case of situations as storm or Tsunami

ACKNOWLEDGMENT I would like to acknowledge VJTI faculty and Rajesh Harsh,

SAMEER, SPNE division for suggestions during the research. REFERENCES

[1] Codar Ocean Sensors - The leaders in HF Radar Technology CODAR Currents Fall Newsletter New SeaSonde Mapping Network –

http:// www.codar.com [2] Coastal Ocean Observation Lab - Rutgers University

http://www.marine.rutgers.edu/mrs/codar.html [3] Dennis B. Trizna, JOHN MOORE, James Headrick, and Robert Bogle,

Directional HF Sea spectrum determination using Doppler Radar Techniques, IEEE journal of oceanic Engineering, Vol. OE -2,JAN -77.

[4] L.R. Wyatt, progress in the interpretation of HF sea echo:,IEEE Proceedings, Vol. 137, Pt. F, No. 2, APRIL 1990. Page(s):139 - 147

[4] “Linear FMCW radar techniques”, Radar and Signal Processing, IEEE Proceedings F,: Oct 1992, Volume: 139, Issue 5, On page(s): 343-350.

[5] NI PXI 7833 Products and Services - National Instruments sine.ni.com/nips/cds/view/p/lang/en/nid/202009-175k

[6] NI PXI-1033 - Products and Services - National Instruments http://www.sine.ni.com/nips/cds/view/p/lang/en/nid/202984 - 45k

[7] NI LabVIEW - The Software That Powers Virtual Instrumentation ... http:// www.ni.com

[9] Rick Bitter, Taqi Mohiuddin,”LabVIEW Advanced Programming Techniques”, CRC press publication, 2007, London, 2nd edition.

[10] Richards Mark A. Fundamentals of Radar Signal Processing McGraw-hill publication, 2005, New York, London, Madrid.

DDS for RX Switching signal 2 ms ON/OFF inverted

DDS for TX Switching signal 2ms ON/OFF Pulses

DDS for Modulating Chirp 0 -30KHz

397