Ground Network Transmitter

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Description

The Ground Network Transmitter is a communication component that aggregates multiple Ground Stations into a single logical transmitter endpoint. It extends the base Transmitter class and provides unified access calculations across a distributed network of ground stations, eliminating the need to configure individual transmitters at each station. The transmitter automatically selects the closest ground station with visibility to the receiving spacecraft, enabling seamless handover between stations for uplink operations such as command transmission.

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Example Use Cases

• Command Uplink Network: Model a coordinated network of globally distributed ground stations for commanding spacecraft.
• LEO Constellation Command: Aggregate multiple regional ground stations to maximise command upload windows for low Earth orbit satellites.
• Mission Operations: Simplify command transmission by treating an entire ground network as a single communication endpoint.
• Coverage Analysis: Evaluate network-wide uplink visibility and handover performance without configuring individual station links.
• Ground Station Redundancy: Automatically route command transmissions through the nearest available station.

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Module Implementation

The ground network transmitter attaches to a Ground Station and maintains a list of associated stations. Unlike standard transmitters where each ground station requires its own antenna, this component provides a single transmitter that spans multiple geographic locations.

Station Registration

When the transmitter is attached to a ground station, that station is automatically added to the network:

$$\text{Stations}\leftarrow \text{Stations}\cup \{\text{Parent}\}$$

Additional stations can be registered dynamically using AddGroundStation(). The transmitter prevents duplicate registrations:

$$\text{AddGroundStation}(S):S\notin \text{Stations}\Rightarrow \text{Stations}\leftarrow \text{Stations}\cup \{S\}$$

Stations can also be removed from the network:

$$\text{RemoveGroundStation}(S):S\in \text{Stations}\Rightarrow \text{Stations}\leftarrow \text{Stations}\setminus \{S\}$$

Link Registration

When the Telemetry System creates links from the ground network transmitter, the transmitter disables standard celestial access calculations:

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

This flag indicates that the transmitter will compute access internally using ground station tracking data rather than relying on the telemetry system’s line-of-sight calculations.

Closest Station Selection

For each link, the transmitter determines which ground station is closest to the receiving spacecraft:

$$S^{\ast }=\underset{S\in \text{Stations}}{\mathrm{argmin}}\Vert {\mathbf{r}}_S-{\mathbf{r}}_{\text{rx}}{\Vert }^2$$

where ${\mathbf{r}}_S$ is the position of station $S$ and ${\mathbf{r}}_{\text{rx}}$ is the position of the receiver in the local inertial frame.

The squared distance is used for computational efficiency:

$$d^2=\Vert {\mathbf{r}}_S-{\mathbf{r}}_{\text{rx}}{\Vert }^2=({\mathbf{r}}_S-{\mathbf{r}}_{\text{rx}})\cdot ({\mathbf{r}}_S-{\mathbf{r}}_{\text{rx}})$$

Access Override Calculation

The transmitter overrides the standard access calculation by querying each link’s closest ground station for visibility:

$$\text{IsAccessOverride}=\text{TrackObject}(\text{Root}).\text{IsAccessible}$$

where TrackObject invokes the ground station’s access tracking system, which accounts for:

• Minimum elevation angle constraints
• Maximum range limitations
• Planetary body occlusion

The root object of the receiver’s parent hierarchy is used for tracking, ensuring that spacecraft with multiple receivers are tracked correctly.

Antenna Position for Link Budget

When computing link budget parameters such as distance and pointing losses, the transmitter reports the position of the closest ground station rather than its own position:

$${\mathbf{r}}_{\text{antenna}}=\{\begin{array}{ll}{\mathbf{r}}_{S^{\ast }}& \text{if }{S}^{\ast }\ne \text{null}\\ {\mathbf{r}}_{\text{self}}& \text{otherwise}\end{array}$$

This ensures that link distance calculations reflect the actual geometric relationship between the transmitting station and the spacecraft.

Ground Station Link Bypass

Unlike standard Transmitter components attached to ground stations, the ground network transmitter does not create a Ground Station Link model on its parent station. This prevents duplicate access tracking since the transmitter manages access calculations internally across all registered stations.

Update Sequence

Each simulation step, the transmitter performs the following operations:

Step	Operation
1	Loop through all connected links
2	Determine closest station to each receiver’s root object
3	Query station access for receiver’s root object
4	Set access override flag based on station visibility

This process occurs before the Telemetry System performs link budget calculations, ensuring that access states are current when computing signal and noise power.

Integration with Telemetry System

The ground network transmitter integrates with the Telemetry System through the standard Transmitter interface. Links are created automatically when compatible Receivers exist on other root objects. The key difference is that access calculations bypass the telemetry system’s celestial occlusion checks in favor of the ground station tracking system.

The transmitter inherits all standard transmitter functionality including:

• Packet encoding and transmission
• Propagation delay modelling
• Data rate configuration
• Frequency and bandwidth settings
• Power and gain specifications

Comparison with Ground Network Receiver

The ground network transmitter mirrors the Ground Network Receiver functionality for the uplink direction:

Aspect	Ground Network Receiver	Ground Network Transmitter
Direction	Downlink (space to ground)	Uplink (ground to space)
Data Flow	Receives from spacecraft	Transmits to spacecraft
Station Selection	Closest to transmitter	Closest to receiver
Use Case	Telemetry reception	Command transmission

Both components share the same station registration, closest station selection, and access override mechanisms.

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Assumptions/Limitations

• The transmitter assumes all registered ground stations are compatible with outgoing transmissions; frequency and bandwidth matching is handled by the standard link budget calculations.
• Station selection is based purely on geometric distance; signal quality or station loading are not considered.
• Handover between stations is instantaneous; no handover delay or data loss is modelled during transitions.
• The transmitter does not model antenna slew time at individual ground stations.
• All registered stations are assumed to be operational; station outages or maintenance windows are not tracked.
• Ground station access constraints (elevation, range) are inherited from each station’s configuration; the transmitter does not impose additional constraints.
• The parent ground station’s position is only used if no stations are registered; otherwise, it serves as the default entry in the station list.
• Station removal during active communication may cause temporary link disruption until the next access calculation cycle.
• The transmitter does not model ground station transmit power variations; all stations are assumed to have equivalent transmission capabilities.
• Simultaneous transmission from multiple stations to the same receiver is not supported; only the closest station is