2953

GROUNDYAVE, OVER-THE HORIZON, RADAR DEVELOPMENT AT NORDCO

A,M. Ponsford', S.K. Srivastava' and T.N.R. Coyne"

'NORDCO Limited, P.O. Box 8833 St. John's, Newfoundland, AlB 3T2

"Defence Research Establishment Ottawa Ottawa, Ontario. KIA OZ4

ABSTRACT In recent years there has been a considerable interest in the development of groundwave, over­the-horizon, (GYOTH) radars for remote sensing in the ocean environment. An integral part of the development is the progressive refinement of software models to predict the performance of these radars, under differing tasks and differing environmental conditions. The model must provide estimates of transmission loss, sea clutter, external noise, and target cross-section as well as take account of the radar system parameters.

A series of experiments are planned to evaluate the performance of different radar configurations including multifrequency and sampled aperture systems. The experimental results will also be used for evaluating and improving software models. An experimental radar has been developed at Cape Bonavista in Newfoundland. The radar comprises an existing LORAN-A transmitter (1.95 MHz, 1 MY peak power) and a co-located receiving system consisting of a linear array of eleven doublets over an aperture of 850 meters.

Preliminary trials were conducted during the of 1988. Using a modified LORAN-A receiver, detection in excess of 300 km was achieved. system has now been upgraded by changing to dynamic range digital receiver and adding a signal processing and control unit.

fall

The a high VME-bus

A full experimental program involving ground truthing and provision of controlled targets is planned for the spring and summer of 1989. The radar was also in operation during the Labrador Ice Margin Experiment (LIMEX '89), when radar

propagation was partly over pack ice. This data is presently being analyzed.

Keywords: Groundwave radar, Remote sensing, Iceberg detection, Signal processing

INTRODUCTION This paper describes the research work being undertaken at NORDCO to further the development of groundwave radar as a remote sensor. An integral part of the development is the progressive refinement of software models to predict the performance of these radars. One such model is presently at an advanced stage of development at NORDCO. Our interest, at the present time, is primarily in the long range detection and tracking of point targets such as icebergs, ships and aircrafts. The E!xtraction of environmental data concerning; sea state, ocean surface currents and surface wind, is also of interest.

The three categories of point targets quoted offer differing detection problems. The iceberg has a small forward velocity of usually less than a knot, long coherent integration periods, in the order of 10's of minutes can therefore be used to discriminate its radar return from that of the ocean. Ships generally travel at speeds of less than 30 knots, hence their Doppler returns will again usually lie within the clutter spectrum. In this case, however, the velocity of the ship will limit practical coherent integration periods to the order of 100's of seconds (Ponsford et al., 1987). Low flying aircraft travel at much higher velocities and their Doppler returns will lie well away from the Bragg returns of the ocean. Detection will be limited, in a well designed system, by either external noise or additional clutter as discussed later. Against this is the fact that the coherent integration time will now be restricted to the order of 10's of seconds. The signal processing architecture must therefore be structured to cater for these differing processing scenarios.

Our experimental program has been structured to evaluate the pred:ictions of software models and to obtain environmental data concerning noise levels and spectral occupancy around the Newfoundland coast line. An el{perimental groundwave radar facility has been established on the Cape Bonavista peninsula in Newfoundland. This radar presently operates at 1.95 11Hz and is being used to confirm the long range detection capability of this frequency as predicted by the modelling.

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SOFTWARE MODEL The necessity to predict the performance of GWOTH radars under differing tasks and environmental conditions has led to the development of a generalized computer-simulation package. The main components of the software model and some important features are briefly described. A detailed description of the software structure and required input parameters is given in Bryant et al. (1988). The model consists of modules that provide estimates of sea clutter, transmission loss and external noise level. The information is then used for predicting detection of any given target of known radar cross section.

The clutter estimate is based on the analytical model proposed by Walsh and Srivastava (1987, 1988). Their model for first and second-order cross sections for a shore based radar system is similar to the well known model given by Barrick (1972). However, their first order result gives a continuum in addition to two Bragg peaks as shown in Figure 1a. The Doppler shift in the continuum increases with increasing frequency of sinusoidal components approximating ocean waves and, except for the Bragg peaks, it does not follow the principle of conventional Doppler shift based on the velocity. This theoretical behaviour of first­order Doppler shift has been observed by others (Wait, 1969; Crombie, 1971). If the interest is in the detection of low speed targets (ships and icebergs), the first-order continuum is unimportant as the dominating clutter is the second-order (Figure 1b). On the other hand, the detection of low flying targets may be limited by this continuum, in some cases, instead of the external noise presently accepted as the limiting factor. This needs further investigation for quantifying its effect. Moreover, for a ship or platform based radar system the model predicts an additional second-order clutter. This clutter may be viewed as the effect of interaction of the source with the surrounding ocean waves. This additional sea scatter results in a very slow cutoff of the clutter spectrum outside the Bragg peaks. This. may be a strong limiting factor in detection of high speed ships and low flying targets. The above additional clutter features require experimental verification.

The software for the radio propagation loss is based on the standard smooth spherical earth propagation model. The effect of ocean waves on the propagation is included as an added loss via modified surface impedance (Walsh and Srivastava 1988). The estimates for man-made, atmospheric �nd galactic noises are based on the standard CCIR radio noise data. Figure 1c illustrates the total (target signal plus clutter) power spectrum received from a fixed range and the noise spectrum for the; system, environmental, and target, parameters given in Figure 1. The detection �erformance of the target as a function of range is Illustrated by the target visibility profile presented in Figure 1d. Thus for a given signal to clutter, or signal to noise, threshold level maximum detectable range can be readily determined.

In any target detection model an estimation of the target cross section is essential. A software model has been developed for estimating the average cross section of icebergs. The software is based on the analytical model proposed by Walsh (Walsh, 1983 and Walsh and Srivastava, 1984). The cross

section depends upon the contour of a given iceberg at the water line. Using an averaging scheme similar to that proposed by Walsh et al. (1986), the software provides an average cross section for a given iceberg waterline contour area regardless of the contour shape. Figure 2 illustrates the models prediction for various iceberg sizes at different operating frequencies. Based on those plots, the average cross section in the HF band may be estimated independent of frequency as follows:

i) Small, medium, large and very large icebergs (2-30 MHz) Cross Section = 0.06A

ii) Bergy Bit (5-30 MHz) Cross Section = 0.07A

iii) Growler (20-30 MHz) Cross Section = 0.08A

where A is the water line contour area. The cross section for bergy bits and growlers in the lower HF band are significantly smaller than given above. Also, their normalized cross section varies significantly with size and frequency as shown in Figure 2.

EXPERIMENTAL RADAR SYSTEM An experimental radar has been developed at Cape Bonavista in Newfoundland. The radar comprises an existing LORAN-A transmitter and a co-located receiving system consisting of a linear array of

eleven doublets. Specifications for the radar system are summarized in Table 1. A critical part of the radar system is the receiver and signal processing unit (Ponsford and Bagwell, 1988). The present receiver, designed to operate over the entire HF band, consists of three stages of high level mixing (to a 25 kHz, +1- 20 kHz baseband) followed by 125 kHz, 16 bit analogue-to-digital conversion. A spurious free dynamic range of greater than 120 dB is achieved in the analogue stages. The 16 bit digital output is transferred via a data Buffer to a VME based signal processor. At this stage all raw data is stored to digital tape for off-line analysis. In addition selected range samples can be processed in real time and their Doppler spectra displayed. Specifications for the receiver system are listed in Table 2.

A schematic diagram of the signal processing structure is presented in Figure 3. The range samples at the output of the RF-to-Digital converter are bandpassed filtered using a finite impulse response (FIR) digital filter. To enable the extraction of both positive and negative Doppler frequencies, without ambiguity, the range samples are initially frequency translated to base band using a complex sinusoid. The data is low pass filtered to remove the unwanted product and then stored. This process is repeated for each transmitted pulse that occurs during the coherent integration period. The task of extracting the Doppler frequencies is completed by applying an FFT to the time accumulated, range gated, data set. The resulting power spectrum, for each range sample, is then available for further analysis.

A consequence of undersampling the received signal at the pulse repetition rate is the aliasing of noise from the receiver bandwidth into the unambiguous Doppler Bandwidth. This results in an increase in the noise level. This becomes a significant factor when detection is noise limited.

CONCLUSIONS The experimental radar facility described in this paper will be operational during the summer of 1989. Its high pulse power and low frequency are suited to the long range detection and tracking of large targets such as ships and icebergs. The eXperimental programme is designed to evaluate this capability as well as to gather performance data for the testing and refinement of software radar modelling. Development of such models is a necessary step if the full potential of HF groundwave radar is to be realized.

REFERENCES

Barrick, D.E., "Remote Sensing of sea state by radar", in Remote Sensing of Troposphere, V.E. Derr, Ed., GPO, Yash., DC, Ch.12, 1972.

Bryant, D.S., A.M. Ponsford and S.K. Srivastava, "A computer package for the parameter optimization of groundwave radar", Proc. Oceans '88, Vol. 2, pp. 485-490, 1988.

Crombie, D.O., "Backscatter of HF radiowaves from the sea", in Electromagnetic Probing in Geophysics, J.R. Yait, Ed., The Golem Press, Boulder, Colorado. pp. 131-162, 1971.

Ponsford, A.M., D.J. Bagwell, D.G. Money and M.H. Gledhill. "Progress in ship tracking by HF groundwave radar". lEE Conf. Radar 87, Conf. Pub 281, 1987.

Ponsford A.M. and D.J. Bagwell, "Receiver design for HF groundwave radar". IEEE Conf. HF Radio Systems and Techniques, Conf. Pub 284, 1988.

Wait, J.R. "Concerning the theory of scatter of HF radio ground waves from periodic sea waves", ESSA Tech. Rep. ERL 145-00-3, GPO, Yash., DC, 14 pp., 1969.

Yalsh, J., "Propagation and scatter for mixed paths with discontinuities and applications to the remote sensing of sea ice with HF radar", C-CORE Tech. Rep. 83-16, Memorial University of Newfoundland, St. John's, Canada, 105 pp., 1983.

lIalsh, J. and S.K. Srivastava, "Model development for feasibility studies of HF radars as ice hazard remote sensors", OEIC Tech. Rep. N00397, Memorial University of Newfoundland, Canada, 212 pp., 1984.

Walsh, J. and S.K. Srivastava, "Rough surface propagation and scatter 1. General formulation and solution for periodic surfaces", Radio Sci., Vol. 22, No. 2, pp. 193-208, 1987.

lIalsh, J. and S.K. Srivastava, "Rough surface propagation and scatter with applications to ground wave remote sensing in an ocean environment", Proc. AGARD (NATO) EPP Specialists' Meeting on Scattering and Propagation in Random Media, No. 419, pp. 23.1-23.15, 1988.

Yalsh, J., B.J. Dawe and S.K. Srivastava, "Remote Sensing of icebergs by ground wave Doppler radar", IEEE J. Oceanic Eng., Vol. DE-II, No. 2, pp. 276-284, 1986.

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ACKNOWLEDGEMENTS

The work has been supported by the Canadian Department of National Defence and the Canada-Newfoundland Offshore Development Fund Agreement.

.

TABLE 1

Bonavista GWOTH Radar Specifications

TRANSMITTER I'.ak power: 1 M W ISOdBWI

flulse width: 50 JJ seconds between 3 dB points

of a raised cosine pul ••

Fluls. repetition freque"cy : 25Hz

Frequency : 1.95 MHz

Meln power: lapprox.llkW 130dBWI

Antenna 1/4 wavelength monop�e with earth rNlt.

RECEIVER ARRAY Unear array of 11 doublets

E:lement type: VALCOM 54 foot whip antenna

Effective aperature 846 metras

Theoretical beamwidth : 10 degrees

!Ioresight : 110.75 degre ..

TABLE 2

Receiver and Signal Processor

FEATURES

-Receiver

- Triple frequency conversion

.,... Frequency range 1.9 MHz - 30 MHz

_ Spurious free dynamic range: RF sections> 120 dB

IF Idiglt.U > 90 dB

- Noise figure approximately 12 dB

- Data rata 12:5 kHz 16 bit

-VME Based prclcessor

- 32 Bit 68020 Microprocessor operating at 25 MHz

-Data Storage

- 320 MBy," ham disc

- 150 MByto. 9-tracl< tape

- 2 GByto. cartridge tape

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lal First Order Sea Clutter

20

10

.� 0

CI -10 <II � -20

e -30 u � -40

� 0.

<II

-50

-70

-80

.1 hY fV[

W\IV . , in �, I

-0,50 -0.25

In n. 11--:::Ji

0,00

\ lI�ru, , VII \AI

'lJl 1 0.25 0.50

Doppler Frequency (Hz)

(bl Second Or.der Sea Clutter

I 20

iii :E 10

c

� " -10 e <II

: -20

e -30 u

� -40

� -50 <II

/' "0 -60

.� �

-70

-80 o z -0.50

" -'-

f t\ IV'

/ ."" -�

-0.25 0,00 0,25 0.50

Doppler Frequency (Hz)

(cl Total Doppler Spectrum

20 N

J: 0 � -20

:E -40

� -60 � -80 0. <II

, I lAIL I\AI

<A� ------.\L ---�I--.!--� � -100

� -120 � -140

� -160 'w ¥ -180 tr

-200

-0.50

, ,

-0.25 0.00 0.25 0.50

Doppler Frequency (Hz)

(dl Target Visiblity Profile TARGET: Medium Size Iceberg

� ., �

� 0 ... 'll >

.� �

tt:

10'

System Parameters 20

Operating Frequencv (MHz) 1.95 Tsrget

Clutter Pulse Length (microsecond.) 50

-20 Peek Transmitter Power tkWI 1000 Noise Pul •• Repetition Frequency IHI' 25 -40

-60 TX Antenna Geln (dB) Number of Element. In RX Antenna 11 -80 Element Spacing Un wavelength) 0.5

-100

r---..---- _ Raised Cosine Weight 1

-120 RX Antenne Gain ldB) 16 r--.:.:..: -140 ' .

�� RX Antenna Beam (degrees) - -- -------- - -.------�- - ----- ------ System Gain IdBI -160

-180 "'- Coherent Integration Time (seconds) 1800

10.0 31.6 100,0 316 .2 1000.0

Range (kilometres)

FIGURE 1 Software Model Output

GROWLER

ICEBERG SIZE CLASSIFICATION

BERGY BIT SMAll I MEDIUM I LARGE

.... _.'ItH. -'-'-'-" " .• lIHI

-----�-- IS.,lIH.

----- 'II.IlIH.

- - - - U .• ItH,

- - - - ",II.,I/H.

1�' L-----------L-----______ L-__________ L-__________ L-________ �

10' 10' 10' 10' 10' la'

Area (sQ. m)

FIGURE 2 Aver age normalized iceberg cross section vs. water l ine contour area.

Environmental Parameters Wind Speed (m/a) 15 Wind OlrectiOf'l (degree,' 90

Noise Area (1.2,3,4,5,6) 5

t10ur Block 11-00-04. 2"04-08. etc.)

Sel.on (1-Dec-Jan-Feb. 2"Mer-Apr- May. etc.)

External Nolle Level (dBW/Hz) -144

Target Parameters Speed Imlsl 0.25 Olrection (degreesl 0

Range to Target (kilometres) 100 Target Cron Section (dB sq.m) 2S

Suaband C

I".nuo�. 5 •• n.,c.n" ..... 25 kHz

!l � I I • I I I I I � --125 -100 -75 -50 -25 0 25 50 75 100 125

-so -25

Sampling Fraquency of 125 k

FreQ,uency Shifting by

e)(pl-]50'7fti

FreQuancy Shifted Signal

25 50

Low PIIU Filter

B .. eband Output

FIGURE 3

Recovery of Baseband Doppler Signals