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Groundwave, over-the-horizon, radar development at NORDCO
Abstract
A series of experiments are planned to evaluate the performances of different radar configurations, including multifrequency and sample 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-MW peak power) and a colocated receiving system consisting of a linear array of eleven doublets over an aperture of 850 m. Preliminary trials were conducted during the fall of 1988. Using a modified loran-A receiver, detection in excess of 300 km was achieved. The system has been upgraded by changing a high-dynamic-range digital receiver and adding a VME-bus signal processing and control unit. The radar was in operation during the Labrador Ice Margin Experiment (LIMEX '89), when radar propagation was partly over pack ice.
Development of the Surface Wave Radar test bed at Cape Bonavista,
Newfoundland was initiated in 1985. The facility has been used to
investigate all aspects of surface wave radar and to provide data for
evaluating and developing models of radar performance. Trials have been
conducted at the site that demonstrate beyond-the-horizon detection of
aircraft to 300 km, surface targets to 500 km, and icebergs to 300 km.
The need to predict surface wave radar performance under differing
tasks, system parameters and environmental conditions has led to the
development of a generalized computer simulation package. An integral
part of the development has been the testing and progressive refinement
of different components of the simulation package based on experimental
results and other advances. The simulation software includes modules
that provide estimates of transmission loss, sea clutter, and external
noise levels. Of interest in this paper are the sea clutter models and
in particular two additional scattering features that are predicted and
have been observed to occur outside of the Bragg resonant region. The
first of these additional scattering features is referred to as `off
resonance first order clutter' whilst the second is referred to as `the
second-part of second order clutter'. This latter clutter has been
predicted to occur when scattering from the ocean occurs from both
behind and in front of the transmit point. The scattering mechanism can
be visualized as being similar to a repeated first order scatter.
Results are presented that illustrate the effect of this scattering
mechanism and how it influences the ocean spectrum for surface wave
radar
Ground-wave radars, operating in the HF band, are becoming
increasingly popular for both the remote sensing of the sea surface and
for the detection of ship targets well beyond the normal radar horizon.
This paper details the requirements of the radar receiver and describes
the gating system and the advantages of using digital signal processing
techniques for the narrowband receiver functions. A coastal based pulse
Doppler radar system as shown is considered. An omnidirectional antenna
is used, on transmission, to `floodlight' the sea surface. Directional
information of the reflected target signal is then obtained using a wide
aperture phased array antenna on reception. Effective target detection
in the presence of sea clutter returns involves coherent integration of
the echo, from a target, over a period of time which is in the order of
100's of seconds. A single narrow beam scanned in azimuth, as in
microwave radars, would result in the introduction of an unacceptable
time delay before returning to a given bearing. Thus the radar must use
multiple beam synthesis from the phased array for target tracking and
acquisition
A generalized computer simulation for groundwave radars operating
in the HF band is described. It is shown how it can be used at the
design stage to optimize the radar parameters for a particular
application, which can vary from remote sensing of the sea state to an
early warning system for detection of low-flying aircraft. A review of
groundwave radar characteristics is included
A simple general formulation for propagation and scattering from rough surfaces is proposed based on the concept of using generalized functions in the electromagnetic field problems. The analysis results consist of two coupled integral equations for the fields above and below the surface separating the media, and a boundary condition. An approximate solution of one integral equation may be obtained with certain assumptions made on the refractive index and spatial spectra of the surface profile and fields. A series solution for the surface electric field in the spatial and temporal Fourier transform domain is derived for a good conducting two-dimensional periodic surface, and is demonstrated with the example of an elementary vertical pulsed electric dipole as the source. The solutions are in the form of ground waves with modified surface impedances.
Some properties of the vector integral equation representing a general
formulation for propagation and scattering from rough surfaces are
discussed. It is shown that under certain conditions the equation may be
reduced to a Volterra equation of the second kind in the normal
component of the surface field for an initially unexcited surface, and
hence some confidence may be had in a Neumann series or successive
approximation. Using the formulation, the backscattered field solutions
at ground wave frequencies and resulting radar cross section solutions
derived previously for a model of the ocean surface are presented and
discussed. The source assumed is a vertical pulsed dipole. The choice of
receiving antenna is left arbitrary. The second-order cross section
contains terms in addition to those provided by existing theories. The
additional terms may become very significant in certain Doppler
frequency regions and hence required for ocean clutter estimation in the
detection of certain types of surface targets. Some of the predictions
were verified experimentally.
In recent years several radar techniques have evolved which allow the remote measurement of certain parameters important in the description of sea state. At MF and HF, monostatic and bistatic configurations employing satellites, ships, islands, and/or land based stations can measure the ocean waveheight spectrum with several frequencies via first-order Bragg scatter. At high HF and VHF, the ocean waveheight spectrum can be estimated at a single carrier frequency via secord-order mechanisms; this technique is especially suited to remote sensing via long distance ionospheric propagation. At UHF, it is possible to measure the slope spectrum of the longer ocean waves via cross-correlation of simultaneous Bragg-effect returns at two frequencies. The short-pulse microwave satellite altimeter permits a direct measurement of the significant waveheight of the sea at the suborbital point via the specular point mechanism. Such techniques will be important both for detailed oceanographic study of ocean wave characteristics and for routine monitoring of sea state for maritime/meteorological purposes.
A software detection model has been developed to predict the returned Doppler spectrum for an iceberg target for ground-wave Doppler radars. This software model is based on proposed new estimates for the backscattered Doppler-dependent iceberg cross section for assumed iceberg models, as well as the backscattered Doppler spectrum from the ocean surface. The model includes estimates for forward and reverse transmission losses, based on classical spherical earth derivations. In addition, the transmission losses account for the effects of surface roughness through a modified surface impedance. Standard estimates for man-made and atmospheric noise have been considered in the detection model. A comparison between the results predicted by the detection model and data acquired during an experiment conducted at Byron Bay, Labrador, Canada has been effected. The hardware used for the experiment was an HF Doppler radar operating at 25.40 MHz. The transmitting antenna was a three-element Yagi array and the receiving antenna a 24-element narrow-beam linear array. By using iceberg ground truthing information the Doppler spectrum for individual icebergs was predicted using the software model. The predicted spectra were compared with the received spectra on a target signal-to-noise power ratio basis. The results of this comparison give a degree of confidence to the detection model and show that ground-wave radars are effective ice hazard remote sensors.
Progress in ship tracking by HF groundwave radar
- A M Ponsford
- D J Bagwell
- D G Money
Propagation and scatter for mixed paths with discontinuities and applications to the remote sensing of sea ice with HF radar
- J Walsh