Vessel

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Description

The Vessel is a Vehicle that represents a maritime object on the Earth’s surface. It can be used to simulate ships or connect to maritime data streams to represent real vessel positions. The vessel is constrained to the surface of a WGS-84 (World Geodetic System) globe, which models the true non-spherical shape of the Earth. The vessel outputs standard AIS (Automatic Identification System) position reports for integration with maritime systems.

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Example Use Cases

• Ship Tracking: Represent vessel positions from AIS maritime data streams.
• Ground Station Visibility: Simulate ship-based tracking stations for spacecraft communication analysis.
• Maritime Operations: Model surface vessels for multi-domain simulation scenarios.
• AIS Integration: Generate position reports compatible with maritime communication standards.

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Module Implementation

The vessel extends the base Vehicle with maritime-specific properties and surface-constrained motion.

Heading and Velocity

The vessel orientation is defined by a heading angle $H$ measured clockwise from true north. When the vessel speed $v$ is non-zero, the velocity vector is computed from the heading:

$$\theta =90°-H$$ $$\vec{v}=\left[\begin{array}{c}v\cos \theta \\ v\sin \theta \\ 0\end{array}\right]$$

where $\theta$ converts the heading (clockwise from north) to a standard mathematical angle measured counter-clockwise from east.

Course Over Ground

The course over ground (COG) represents the actual direction of travel based on the vessel’s surface velocity. It is calculated from the velocity components:

$$\mathrm{COG}=\arctan \left(\frac{v_x}{v_y}\right)$$

where $v_x$ is the eastward velocity component and $v_y$ is the northward velocity component. When the vessel is stationary, the COG defaults to the current heading. The result is wrapped to the range $[0°,360°)$.

AIS Position Report

The vessel automatically generates an AIS position report containing:

• True Heading: The bow angle in degrees
• Course Over Ground: Direction of surface velocity in degrees
• Speed Over Ground: Vessel speed converted to knots ($1m/s=1.94384\mathrm{kn}$)
• Position: Latitude and longitude in WGS-84 coordinates

Maritime Identification

The vessel supports identification via Maritime Mobile Service Identity (MMSI), the standard unique nine-digit identifier used in maritime communications.

Surface Constraint

The vessel is always positioned on the Earth’s surface using the WGS-84 ellipsoid model. Location changes are restricted to Earth regardless of any planet parameter specified.

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Assumptions/Limitations

• The vessel is constrained to the Earth’s surface; it cannot represent objects on other celestial bodies.
• The model does not simulate hydrodynamic effects, wave motion, or ocean currents.
• Altitude is measured relative to the WGS-84 reference ellipsoid, not mean sea level.
• Course over ground assumes flat-Earth geometry for velocity decomposition over short distances.
• The AIS position report does not include all fields defined in the full AIS standard.