Ground Ephemeris Translation Software

SWITCH TO API

Description

The Ground Ephemeris Translation Software is a flight software module that converts a ground station’s state information into a standard ephemeris message format. This translation enables ground-based assets to be represented in the same coordinate framework as orbital objects, facilitating unified analysis of communication links, visibility windows, and relative motion between spacecraft and ground stations. The module extracts position data from the ground state message and formats it as an ephemeris message compatible with other navigation and tracking algorithms in the simulation.

---

Example Use Cases

• Communication Link Analysis: Provide ground station ephemeris for link budget calculations and visibility determination with orbiting spacecraft.
• Tracking System Integration: Enable ground stations to be processed by the same tracking algorithms used for orbital objects.
• Relative Navigation: Compute relative position and velocity between spacecraft and ground stations using a common ephemeris interface.
• Multi-Asset Coordination: Represent both space and ground assets in a unified coordinate framework for constellation operations.

---

Module Implementation

The Ground Ephemeris Translation Software is a Software component that processes a Ground State Message and produces an Ephemeris Message containing the ground station’s position in the inertial reference frame.

Input Message

The module requires a single input message:

Input Message	Type	Description
In_GroundStateMsg	GroundStateMessage	Ground station state including position in the inertial frame

The ground state message provides the position of the ground station location $L$ with respect to the inertial origin $N$, expressed in the inertial frame $\mathcal{N}$.

Position Transformation

The ground station position is extracted directly from the input message and assigned to the ephemeris output:

$${\mathbf{r}}_{B/N}^{\mathcal{N}}={\mathbf{r}}_{L/N}^{\mathcal{N}}$$

where:

• ${\mathbf{r}}_{B/N}^{\mathcal{N}}$ is the body position in the ephemeris message (output)
• ${\mathbf{r}}_{L/N}^{\mathcal{N}}$ is the ground station location position from the ground state message (input)

The position vector is copied to a new Vector3 instance to ensure independence from the source message.

Velocity Handling

Ground stations are assumed to be fixed on the planetary surface for the purposes of this translation. The velocity in the inertial frame is set to zero:

$${\mathbf{v}}_{B/N}^{\mathcal{N}}=0$$

This simplification is appropriate because:

1. The ground state message does not contain velocity information
2. The ground station’s velocity due to planetary rotation is typically handled separately by the ground object’s dynamics
3. For most link analysis purposes, the instantaneous velocity of the ground station is negligible compared to spacecraft velocities

Output Message

The module produces an EphemerisMessage containing:

Field	Symbol	Units	Description
Position_BN_N	${\mathbf{r}}_{B/N}^{\mathcal{N}}$	m	Position vector in inertial frame
Velocity_BN_N	${\mathbf{v}}_{B/N}^{\mathcal{N}}$	m/s	Velocity vector in inertial frame (zero)

Update Sequence

At each simulation time step, the module performs the following operations:

1. Validate Input: Check that the ground state message is connected; skip update if missing.
2. Extract Position: Copy the ground station position from the input message to the output ephemeris.
3. Set Zero Velocity: Assign a zero velocity vector to the output ephemeris.

Coordinate Frame Considerations

The ground state message provides the ground station position in the inertial frame $\mathcal{N}$, which is typically the J2000 Earth-centered inertial reference. This position already accounts for:

• The ground station’s geodetic coordinates (latitude, longitude, altitude)
• The planetary rotation angle at the current simulation time
• The planet’s position in the solar system barycentric frame

The resulting ephemeris message is therefore directly compatible with spacecraft ephemeris data for relative position calculations.

---

Assumptions/Limitations

• The ground station is assumed to be stationary in the inertial frame for velocity purposes; rotational velocity due to planetary spin is not included in the output.
• The ground state message must already contain the position in the inertial frame; no geodetic-to-inertial conversion is performed by this module.
• Velocity data is not available from the ground state message and is set to zero; applications requiring ground station velocity must compute it separately.
• The module performs a direct copy of position data without filtering, smoothing, or validation.
• No acceleration data is computed or output; higher-order derivatives are not available.
• The ephemeris message uses the same timestamp as the input ground state message; no time interpolation is performed.
• The module does not account for tropospheric or ionospheric effects on the apparent position.
• Tidal deformations and tectonic motion of the ground station are not modelled.
• The output is suitable for geometric calculations but does not include