Optical Transmitter

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Description

The Optical Transmitter is a specialized Transmitter component that uses laser-based free-space optical communication to transmit data to Optical Receiver components. Unlike radio-frequency transmitters that rely on antenna patterns and link budget calculations based on signal propagation, optical transmitters require direct line-of-sight between the transmitter and receiver. The narrow beam divergence of laser communication provides high data rates and improved security, but demands precise pointing and unobstructed optical paths.

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Example Use Cases

• Inter-Satellite Links: High-bandwidth data relay between spacecraft in a constellation using laser crosslinks.
• Deep Space Communication: Long-range optical communication for science missions requiring high data throughput.
• Secure Communications: Exploit the narrow beam width of laser links to minimize interception risk.
• Ground Station Downlink: High-speed data transfer from spacecraft to optical ground stations.

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Module Implementation

The optical transmitter extends the base Transmitter class, adding line-of-sight validation, beam geometry calculations, and dynamic bit rate scaling based on pointing accuracy and range.

Transmission Type

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

Line-of-Sight Determination

Unlike RF transmitters where signals can propagate through various paths, optical communication requires an unobstructed geometric line-of-sight between transmitter and receiver. The transmitter validates each potential link by checking:

1. Angular Constraint: The receiver must fall within the transmitter’s beam divergence cone
2. Range Constraint: The receiver must be within the maximum transmission range (if configured)

For a receiver at position ${\mathbf{r}}_{\text{rx}}$ and a transmitter at position ${\mathbf{r}}_{\text{tx}}$ with beam axis $\hat{\mathbf{b}}$, the off-axis angle is:

$$\theta ={\cos }^{-1}\left(\hat{\mathbf{b}}\cdot \hat{\mathbf{d}}\right)$$

where $\hat{\mathbf{d}}$ is the unit vector from transmitter to receiver:

$$\hat{\mathbf{d}}=\frac{{\mathbf{r}}_{\text{rx}}-{\mathbf{r}}_{\text{tx}}}{|{\mathbf{r}}_{\text{rx}}-{\mathbf{r}}_{\text{tx}}|}$$

The link is valid only if:

$$\theta \le \frac{{\theta }_{\text{div}}}{2}$$

where ${\theta }_{\text{div}}$ is the beam divergence angle.

Range Validation

If a maximum range $R_{\text{max}}$ is configured (non-zero), the link is additionally constrained by:

$$|{\mathbf{r}}_{\text{rx}}-{\mathbf{r}}_{\text{tx}}|\le R_{\text{max}}$$

When $R_{\text{max}}=0$, range limiting is disabled and the transmitter can communicate at any distance, subject to other link budget constraints.

Emitter Orientation

The beam axis direction accounts for both the transmitter’s world orientation and any configured emitter rotation. Additionally, alignment errors (from attached error models) are incorporated:

$$\hat{\mathbf{b}}=\left({\mathbf{R}}_{\text{world}}\cdot {\mathbf{R}}_{\text{emitter}}\cdot {\mathbf{R}}_{\text{error}}\right)\cdot \hat{\mathbf{z}}$$

where:

• ${\mathbf{R}}_{\text{world}}$ is the transmitter’s world rotation matrix
• ${\mathbf{R}}_{\text{emitter}}$ is the configured emitter rotation offset
• ${\mathbf{R}}_{\text{error}}$ is the alignment error rotation (about the right axis)

Dynamic Bit Rate Scaling

The effective bit rate degrades as the receiver moves off-axis or approaches the maximum range. The bit rate is scaled based on both angular and range factors:

$$R_b=R_{b,0}-\left(\frac{\theta }{{\theta }_{\text{div}}}\right)\cdot \frac{R_{b,0}}{2}-\left(\frac{d}{R_{\text{max}}}\right)\cdot \frac{R_{b,0}}{2}$$

where:

• $R_{b,0}$ is the initial configured bit rate [bps]
• $\theta$ is the off-axis angle to the receiver [deg]
• ${\theta }_{\text{div}}$ is the beam divergence angle [deg]
• $d$ is the distance to the receiver [m]
• $R_{\text{max}}$ is the maximum configured range [m]

This formulation means:

• At perfect alignment ($\theta =0$) and zero range ($d=0$), the full bit rate is available
• At the edge of the beam ($\theta ={\theta }_{\text{div}}$), bit rate is reduced by 50%
• At maximum range ($d=R_{\text{max}}$), bit rate is reduced by an additional 50%

Complex Line-of-Sight

When ComplexLineOfSight is enabled, the optical transmitter supports integration with external rendering engines (such as Unreal Engine) to perform ray-casting for occlusion detection. This allows accurate modelling of:

• Planetary body occultation
• Spacecraft self-shadowing
• Atmospheric effects

External systems can update the line-of-sight state using the UpdateLineOfSight method, or block all links using BlockLineOfSight.

Device Status Integration

The optical transmitter can be controlled by an external device status message. When connected, transmission is only permitted if the device is marked as active:

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

This enables integration with payload controllers, power management systems, or operational mode logic.

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Assumptions/Limitations

• Only Optical Receiver components can receive from an optical transmitter; RF receivers are incompatible.
• The beam is modelled as a simple cone; complex beam profiles (Gaussian, Airy) are not simulated.
• Atmospheric attenuation and scintillation effects are not explicitly modelled.
• The bit rate scaling model is linear; actual optical link performance may follow different degradation curves.
• Alignment error is applied as a simple rotation about a single axis.
• When complex line-of-sight is disabled, only geometric beam constraints are checked; occlusion is not considered.
• The transmitter does not model acquisition and tracking dynamics; pointing is assumed instantaneous.
• Doppler shift effects on the optical carrier are not modelled.
• Background light interference (solar, Earth albedo) is not included in the link calculation.

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References

[1] M. R. Spiegel, S. Lipschutz, and D. Spellman, Vector Analysis (Schaum’s Outlines), 2nd ed. New York, NY, USA: McGraw-Hill, 2009.