Ground Station Link

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Description

The Ground Station Link is a model that bridges the Ground Station access calculations with the Telemetry System link budget computations. When a Transmitter or Receiver is added to a ground station, this component is automatically created to manage the relationship between ground station visibility (azimuth, elevation, range) and communication link availability. The model ensures that telemetry links are only active when the ground station has geometric access to the target spacecraft.

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Example Use Cases

• Pass Scheduling: Coordinate communication windows with ground station visibility periods.
• Link Availability Analysis: Determine when spacecraft telemetry links are viable based on ground station access constraints.
• Network Operations: Model realistic ground station contact schedules for mission planning.
• Handover Analysis: Evaluate communication continuity when transitioning between ground stations.

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Module Implementation

The ground station link model is automatically attached to a Ground Station when an antenna component is added. It maintains a mapping between each DataLinkMessage and its corresponding AccessMessage, synchronizing the access state with the link availability.

Link-Access Mapping

For each antenna on the ground station, the Telemetry System creates links to compatible antennas on spacecraft. The ground station link model associates each of these data links with an access message computed by the parent ground station:

$$\text{LinkAccess}:\text{DataLinkMessage}\to \text{AccessMessage}$$

This mapping enables the model to query ground station visibility for each individual communication link.

Access Override

The ground station link updates the access override flag on each data link based on two conditions:

$$\text{IsAccessOverride}=\text{OverrideAccess}\vee \text{IsAccessible}$$

where:

• $\text{OverrideAccess}$ is a flag on the parent Ground Station that forces all links to be accessible regardless of geometry
• $\text{IsAccessible}$ is the geometric access state from the corresponding AccessMessage

When the access override is true, the Telemetry System treats the link as having valid line-of-sight, enabling data transfer. When false, the link is blocked regardless of signal strength.

Access Calculation Bypass

When a new link is added, the model sets the IsAccessCalculated flag to false on the data link message. This indicates to the Telemetry System that celestial body access calculations should be bypassed in favor of the ground station’s own access determination:

$$\text{IsAccessCalculated}=\text{false}$$

The ground station access calculation already accounts for:

• Minimum elevation angle constraints
• Maximum range limitations
• Line-of-sight occlusion by the planetary body

Link Lifecycle

Links are dynamically added and removed as the simulation evolves:

Event	Behaviour
Antenna added to ground station	New link-access pairs are registered
Antenna removed from ground station	Associated link-access pairs are unregistered
Each update step	Access override flags are synchronised

This ensures the ground station link model accurately reflects the current antenna configuration.

Integration with Ground Station Access

The access state used by this model is computed by the parent Ground Station based on:

$$\text{IsAccessible}=(\alpha \ge {\alpha }_{\text{min}})\wedge (R\le R_{\text{max}})\wedge \text{LineOfSight}$$

where:

• $\alpha$ is the current elevation angle to the spacecraft [deg]
• ${\alpha }_{\text{min}}$ is the minimum elevation constraint [deg]
• $R$ is the slant range to the spacecraft [m]
• $R_{\text{max}}$ is the maximum range constraint [m]
• $\text{LineOfSight}$ indicates no occlusion by the planetary body

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Assumptions/Limitations

• The model assumes one-to-one correspondence between data links and access messages.
• Access calculations are performed by the parent Ground Station; this model only propagates the results.
• The access override affects the entire link; partial access degradation is not modelled.
• When OverrideAccess is enabled on the ground station, all geometric constraints are bypassed.
• The model does not account for antenna-specific pointing constraints beyond the ground station’s global access calculation.
• Atmospheric effects on link availability (weather, rain fade) are not included in the access determination.
• Multiple antennas on the same ground station share the same access calculation for a