target.h

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// SPDX-License-Identifier: GPL-2.0-only
//
// Copyright (c) 2006-2008 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.h
 * @brief Defines classes for radar targets and their Radar Cross-Section (RCS) models.
 */
 
#pragma once
 
#include <memory>
#include <random>
#include <string>
#include <utility>
 
#include "core/config.h"
#include "core/sim_id.h"
#include "interpolation/interpolation_set.h"
#include "noise/noise_generators.h"
#include "object.h"
 
namespace math
{
    class SVec3;
}
 
namespace radar
{
    class Platform;
 
    /**
     * @class RcsModel
     * @brief Base class for RCS fluctuation models.
     */
    class RcsModel
    {
    public:
        virtual ~RcsModel() = default;
 
        RcsModel() = default;
 
        RcsModel(const RcsModel&) = delete;
 
        RcsModel& operator=(const RcsModel&) = delete;
 
        RcsModel(RcsModel&&) = delete;
 
        RcsModel& operator=(RcsModel&&) = delete;
 
        /**
         * @brief Samples the RCS model to produce a value.
         *
         * @return The sampled RCS value.
         */
        virtual RealType sampleModel() = 0;
    };
 
    /**
     * @class RcsConst
     * @brief Constant RCS model.
     */
    class RcsConst final : public RcsModel
    {
    public:
        /**
         * @brief Samples the constant RCS model.
         *
         * @return The RCS value (always 1.0).
         */
        RealType sampleModel() override { return 1.0; }
    };
 
    /**
     * @class RcsChiSquare
     * @brief Chi-square distributed RCS model.
     */
    class RcsChiSquare final : public RcsModel
    {
    public:
        /**
         * @brief Constructs an RcsChiSquare model.
         *
         * @param rngEngine The random number engine to use.
         * @param k The degrees of freedom for the chi-square distribution.
         */
        explicit RcsChiSquare(std::mt19937& rngEngine, RealType k) :
            _gen(std::make_unique<noise::GammaGenerator>(rngEngine, k)), _k(k)
        {
        }
 
        /**
         * @brief Gets the 'k' parameter (degrees of freedom) of the distribution.
         * @return The k value.
         */
        [[nodiscard]] RealType getK() const noexcept { return _k; }
 
        /**
         * @brief Samples the chi-square RCS model.
         *
         * @return The sampled RCS value.
         */
        RealType sampleModel() override { return _gen->getSample(); }
 
    private:
        std::unique_ptr<noise::GammaGenerator> _gen; ///< The gamma generator for sampling the chi-square distribution.
        RealType _k; ///< The 'k' parameter (degrees of freedom).
    };
 
    /**
     * @class Target
     * @brief Base class for radar targets.
     */
    class Target : public Object
    {
    public:
        /**
         * @brief Constructs a radar target.
         *
         * @param platform Pointer to the platform associated with the target.
         * @param name The name of the target.
         * @param seed The seed for the target's internal random number generator.
         */
        Target(Platform* platform, std::string name, const unsigned seed, const SimId id = 0) :
            Object(platform, std::move(name), ObjectType::Target, id), _rng(seed), _seed(seed)
        {
        }
 
        /**
         * @brief Gets the RCS value for the target.
         *
         * @param inAngle The incoming angle of the radar wave.
         * @param outAngle The outgoing angle of the reflected radar wave.
         * @param time The current simulation time (default is 0.0).
         * @return The RCS value.
         */
        virtual RealType getRcs(math::SVec3& inAngle, math::SVec3& outAngle, RealType time) const = 0;
 
        /**
         * @brief Gets the target's internal random number generator engine.
         * @return A mutable reference to the RNG engine.
         */
        [[nodiscard]] std::mt19937& getRngEngine() noexcept { return _rng; }
 
        /**
         * @brief Gets the unique ID of the target.
         *
         * @return The target SimId.
         */
        [[nodiscard]] SimId getId() const noexcept { return Object::getId(); }
 
        /**
         * @brief Sets the RCS fluctuation model.
         *
         * Assigns a new RCS fluctuation model to the target.
         * @param in Unique pointer to the new RCS fluctuation model.
         */
        void setFluctuationModel(std::unique_ptr<RcsModel> in) { _model = std::move(in); }
 
        /**
         * @brief Gets the RCS fluctuation model.
         * @return A const pointer to the RcsModel.
         */
        [[nodiscard]] const RcsModel* getFluctuationModel() const { return _model.get(); }
 
        /**
         * @brief Gets the initial seed used for the target's RNG.
         *
         * @return The initial seed value.
         */
        [[nodiscard]] unsigned getSeed() const noexcept { return _seed; }
 
    protected:
        std::unique_ptr<RcsModel> _model{nullptr}; ///< The RCS fluctuation model for the target.
        std::mt19937 _rng; ///< Per-object random number generator for statistical independence.
        unsigned _seed; ///< The initial seed for the RNG.
    };
 
    /**
     * @class IsoTarget
     * @brief Isotropic radar target.
     *
     */
    class IsoTarget final : public Target
    {
    public:
        /**
         * @brief Constructs an isotropic radar target.
         *
         * @param platform Pointer to the platform associated with the target.
         * @param name The name of the target.
         * @param rcs The constant RCS value for the target.
         * @param seed The seed for the target's internal random number generator.
         */
        IsoTarget(Platform* platform, std::string name, const RealType rcs, const unsigned seed, const SimId id = 0) :
            Target(platform, std::move(name), seed, id), _rcs(rcs)
        {
        }
 
        /**
         * @brief Gets the constant RCS value.
         *
         * @return The constant RCS value, possibly modified by the fluctuation model.
         */
        RealType getRcs(math::SVec3& /*inAngle*/, math::SVec3& /*outAngle*/, RealType /*time*/) const noexcept override;
 
        /**
         * @brief Gets the constant RCS value (without fluctuation model applied).
         *
         * @return The constant RCS value.
         */
        [[nodiscard]] RealType getConstRcs() const noexcept { return _rcs; }
 
    private:
        RealType _rcs; ///< The constant RCS value for the target.
    };
 
    /**
     * @class FileTarget
     * @brief File-based radar target.
     */
    class FileTarget final : public Target
    {
    public:
        /**
         * @brief Constructs a file-based radar target.
         *
         * @param platform Pointer to the platform associated with the target.
         * @param name The name of the target.
         * @param filename The name of the file containing RCS data.
         * @param seed The seed for the target's internal random number generator.
         * @throws std::runtime_error If the file cannot be loaded or parsed.
         */
        FileTarget(Platform* platform, std::string name, const std::string& filename, unsigned seed,
                   const SimId id = 0);
 
        /**
         * @brief Gets the RCS value from file-based data for a specific bistatic geometry and time.
         * @param inAngle The incoming angle of the radar wave in the global frame.
         * @param outAngle The outgoing angle of the reflected radar wave in the global frame.
         * @param time The simulation time at which the interaction occurs.
         * @return The Radar Cross Section (RCS) value in meters squared (m²).
         * @throws std::runtime_error If RCS data cannot be retrieved.
         *
         * This function calculates the target's aspect-dependent RCS. The key steps are:
         * 1.  Calculate the bistatic angle bisector in the global simulation coordinate system.
         * 2.  Retrieve the target's own orientation (rotation) at the specified 'time'.
         * 3.  Transform the global bistatic angle into the target's local, body-fixed frame by subtracting
         *     the target's rotation. This is critical, as RCS patterns are defined relative to the target itself.
         * 4.  Use this local aspect angle to look up the azimuthal and elevation RCS values from the loaded data.
         *
         * NOTE: This function returns the raw RCS value (σ), which is linearly proportional to scattered power.
         * The calling physics engine is responsible for converting this to a signal amplitude by taking the
         * square root.
         */
        RealType getRcs(math::SVec3& inAngle, math::SVec3& outAngle, RealType time) const override;
 
        /**
         * @brief Gets the filename associated with this target's RCS data.
         * @return The source filename.
         */
        [[nodiscard]] const std::string& getFilename() const noexcept { return _filename; }
 
    private:
        std::unique_ptr<interp::InterpSet> _azi_samples; ///< The azimuthal RCS samples.
        std::unique_ptr<interp::InterpSet> _elev_samples; ///< The elevation RCS samples.
        std::string _filename; ///< The original filename for the RCS data.
    };
 
    /**
     * @brief Creates an isotropic target.
     *
     * @param platform Pointer to the platform associated with the target.
     * @param name The name of the target.
     * @param rcs The constant RCS value for the target.
     * @param seed The seed for the target's internal random number generator.
     * @return A unique pointer to the newly created IsoTarget.
     */
    inline std::unique_ptr<Target> createIsoTarget(Platform* platform, std::string name, RealType rcs, unsigned seed,
                                                   const SimId id = 0)
    {
        return std::make_unique<IsoTarget>(platform, std::move(name), rcs, seed, id);
    }
 
    /**
     * @brief Creates a file-based target.
     *
     * @param platform Pointer to the platform associated with the target.
     * @param name The name of the target.
     * @param filename The name of the file containing RCS data.
     * @param seed The seed for the target's internal random number generator.
     * @return A unique pointer to the newly created FileTarget.
     */
    inline std::unique_ptr<Target> createFileTarget(Platform* platform, std::string name, const std::string& filename,
                                                    unsigned seed, const SimId id = 0)
    {
        return std::make_unique<FileTarget>(platform, std::move(name), filename, seed, id);
    }
}