Optical Receiver

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Description

The Optical Receiver is a specialized Receiver component designed to receive laser-based free-space optical communication from Optical Transmitter components. Unlike radio-frequency receivers that can acquire signals through antenna patterns, optical receivers require direct line-of-sight to the transmitter. The narrow beam width of laser communication provides high data rates and improved security, but demands precise alignment and unobstructed optical paths between communicating terminals.

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Example Use Cases

• Inter-Satellite Links: Receive high-bandwidth data from other spacecraft in a constellation via laser crosslinks.
• Deep Space Communication: Acquire high-rate optical signals from distant spacecraft.
• Secure Communications: Exploit the narrow beam width of laser links to minimize interception risk.
• Ground Station Downlink: Receive high-speed optical data from spacecraft at optical ground terminals.

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Module Implementation

The optical receiver extends the base Receiver class, adding line-of-sight validation and integration with external rendering engines for accurate occlusion detection.

Transmission Type

The optical receiver operates exclusively with optical transmitters. The transmission type is set to Optical, ensuring the Telemetry System only establishes links with compatible Optical Transmitter components. Links from radio-frequency transmitters are not created.

Line-of-Sight Requirement

Unlike RF receivers where signals can propagate through various paths and antenna patterns determine gain, optical communication requires an unobstructed geometric line-of-sight between transmitter and receiver. The link is only valid when:

$$\text{IsConnected}=\text{IsLineOfSight}\wedge \text{FrequencyMatch}\wedge \text{GeometryValid}$$

The IsLineOfSight flag is determined by the Optical Transmitter based on beam geometry, or can be updated by external systems performing ray-casting.

Collector Orientation

The optical receiver includes a configurable collector rotation that defines the orientation of the optical aperture relative to the component’s local coordinate system:

$${\hat{\mathbf{n}}}_{\text{collector}}={\mathbf{R}}_{\text{collector}}\cdot \hat{\mathbf{z}}$$

where ${\mathbf{R}}_{\text{collector}}$ is the collector rotation matrix. This allows the receiver aperture to be oriented independently of the component mounting.

External Line-of-Sight Control

The optical receiver provides an interface for external systems (such as rendering engines like Unreal Engine) to update the line-of-sight state. This enables accurate modelling of:

• Planetary body occultation
• Spacecraft self-shadowing
• Structural occlusion by solar panels, antennas, or other components
• Atmospheric effects and cloud cover for ground stations

The SetLineOfSight method updates the link state for a specific transmitter:

$${\text{Link}}_{tx\to rx}.\text{IsLineOfSight}=\text{los}$$

When line-of-sight is blocked, the link’s IsConnected flag becomes false and no data transfer occurs, regardless of signal strength or frequency alignment.

Device Status Integration

The optical receiver can be controlled by an external device status message. When connected, data reception is only processed if the device is marked as active:

$$\text{ProcessData}=\text{DeviceStatus.IsActive}\wedge \text{BaseConditions}$$

This enables integration with payload controllers, power management systems, or operational mode logic. If the device status indicates inactive, all incoming data processing is skipped for that simulation step.

Data Reception

When line-of-sight is established and the device is active, the optical receiver processes incoming data using the standard Receiver pipeline:

1. Packets are received from connected optical transmitters
2. Propagation delay is applied based on distance
3. Bit errors are applied based on link quality
4. Complete messages are reassembled and made available for consumption

The effective bit rate is determined by the Optical Transmitter’s dynamic bit rate, which accounts for off-axis angle and range degradation.

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Assumptions/Limitations

• Only Optical Transmitter components can transmit to an optical receiver; RF transmitters are incompatible.
• Line-of-sight is binary; partial obscuration or atmospheric attenuation gradients are not modelled.
• The collector rotation defines aperture orientation but does not affect receiver gain or field of view calculations.
• External ray-casting systems must explicitly update line-of-sight state; the receiver does not perform its own occlusion checks.
• When device status is inactive, all data processing halts but the link state is preserved.
• The receiver does not model acquisition and tracking dynamics; alignment is assumed instantaneous once line-of-sight is established.
• Background light interference (solar, Earth albedo) is not included in the link calculation.
• Atmospheric scintillation effects for ground-based receivers are not modelled.