target.cpp

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// SPDX-License-Identifier: GPL-2.0-only
//
// Copyright (c) 2007-2008 Marc Brooker and Michael Inggs
// Copyright (c) 2008-present FERS Contributors (see AUTHORS.md).
//
// See the GNU GPLv2 LICENSE file in the FERS project root for more information.
 
/**
 * @file target.cpp
 * @brief Defines classes for radar targets and their Radar Cross-Section (RCS) models.
 */
 
#include "target.h"
 
#include <cmath>
#include <optional>
#include <stdexcept>
 
#include "core/logging.h"
#include "math/geometry_ops.h"
#include "serial/libxml_wrapper.h"
 
using math::SVec3;
 
namespace
{
    /**
     * @brief Load the target gain axis from an XML element.
     *
     * @param set The interpolation set to insert the samples into.
     * @param axisXml The XML element containing the gain samples.
     */
    void loadTargetGainAxis(const interp::InterpSet* set, const XmlElement& axisXml) noexcept
    {
        XmlElement tmp = axisXml.childElement("rcssample");
        while (tmp.isValid())
        {
            XmlElement const angle_element = tmp.childElement("angle", 0);
 
            if (XmlElement const gain_element = tmp.childElement("rcs", 0);
                angle_element.isValid() && gain_element.isValid())
            {
                const RealType angle = std::stof(angle_element.getText());
                const RealType gain = std::stof(gain_element.getText());
                set->insertSample(angle, gain);
            }
 
            tmp = XmlElement(tmp.getNode()->next);
        }
    }
}
 
namespace radar
{
    RealType IsoTarget::getRcs(SVec3& /*inAngle*/, SVec3& /*outAngle*/, RealType /*time*/) const noexcept
    {
        return _model ? _rcs * _model->sampleModel() : _rcs;
    }
 
    FileTarget::FileTarget(Platform* platform, std::string name, const std::string& filename, const unsigned seed,
                           const SimId id) :
        Target(platform, std::move(name), seed, id), _azi_samples(std::make_unique_for_overwrite<interp::InterpSet>()),
        _elev_samples(std::make_unique_for_overwrite<interp::InterpSet>()), _filename(filename)
    {
        XmlDocument doc;
        if (!doc.loadFile(filename))
        {
            throw std::runtime_error("Could not load target description from " + filename);
        }
 
        const XmlElement root(doc.getRootElement());
 
        loadTargetGainAxis(_elev_samples.get(), root.childElement("elevation", 0));
        loadTargetGainAxis(_azi_samples.get(), root.childElement("azimuth", 0));
    }
 
    RealType FileTarget::getRcs(SVec3& inAngle, SVec3& outAngle, const RealType time) const
    {
        // TODO: the handling of arbitrary rcs models needs to be validated and expanded to cover edge cases.
        // 1. Calculate the bistatic angle bisector in the GLOBAL frame.
        const SVec3 global_bisector_angle = inAngle + outAngle;
 
        // 2. Get the target's own rotation at the current time.
        const SVec3 target_rotation = getRotation(time);
 
        // 3. Transform the global angle into the target's LOCAL frame for lookup.
        const SVec3 local_aspect_angle = global_bisector_angle - target_rotation;
 
        // 4. Use the local aspect angle (bisector is halved) to look up RCS.
        const auto azi_value = _azi_samples->getValueAt(local_aspect_angle.azimuth / 2.0);
 
        if (const auto elev_value = _elev_samples->getValueAt(local_aspect_angle.elevation / 2.0);
            azi_value && elev_value)
        {
            // Return the raw RCS value (proportional to power), not its square root.
            const RealType rcs = *azi_value * *elev_value;
            return _model ? rcs * _model->sampleModel() : rcs;
        }
 
        LOG(logging::Level::FATAL, "Could not get RCS value for target");
        throw std::runtime_error("Could not get RCS value for target");
    }
}