Transmitter Power Model

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Description

The Transmitter Power Model creates a coupling between a Transmitter component and the Power Bus electrical network. This model enables realistic power consumption simulation during data transmission, where the transmitter draws additional power from the spacecraft power system when actively transmitting. The model also enforces power availability constraints, preventing transmission when insufficient voltage is available.

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Example Use Cases

• Power Budget Analysis: Evaluate spacecraft power consumption during communication passes.
• Battery Sizing: Determine energy storage requirements for mission communication profiles.
• Operational Constraints: Model transmission blackouts during eclipse or low-power conditions.
• System Integration: Couple telemetry operations with electrical power system performance.

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Module Implementation

The transmitter power model extends the base PowerNodeModel class, integrating the transmitter into the spacecraft electrical network as a variable-resistance load. The Power Bus computes the voltage at the node based on the circuit topology and component characteristics.

Equivalent Circuit

The transmitter power model is represented in the electrical network as a resistive element. When idle, the transmitter presents a base resistance to the circuit. During transmission, the resistance decreases to draw additional current corresponding to the RF output power.

Resistance Calculation

The power model dynamically adjusts its resistance based on the transmission state. When the transmitter is idle, the resistance equals the configured base resistance:

$$R=R_{\text{base}}$$

When the transmitter is actively transmitting and input voltage is available ($V_{\text{in}}>0$), the resistance is reduced to draw the required power. Using the relationship $P=V^2/R$, the resistance during transmission is calculated as:

$$R=R_{\text{base}}-\frac{V_{\text{in}}^2}{P_{\text{tx}}\cdot N_{\text{bands}}}$$

where:

• $R_{\text{base}}$ is the base resistance when idle [Ω]
• $V_{\text{in}}$ is the input voltage from the Power Bus [V]
• $P_{\text{tx}}$ is the transmitter RF output power [W]
• $N_{\text{bands}}$ is the number of frequency bands being transmitted

To maintain compatibility with circuit simulation and prevent numerical instabilities, the resistance is clamped to a minimum value:

$$R\ge 10^{-12}\Omega$$

Power Consumption

The actual power consumed by the transmitter during transmission can be derived from the circuit parameters:

$$P_{\text{consumed}}=\frac{V_{\text{in}}^2}{R}$$

When transmitting, this approximates:

$$P_{\text{consumed}}\approx P_{\text{tx}}\cdot N_{\text{bands}}$$

assuming the base resistance contribution is small relative to the transmission power draw.

Transmission Enable Control

The power model controls the transmitter’s ability to transmit based on power availability. The CanTransmit flag on the parent transmitter is set according to:

$$\text{CanTransmit}=\{\begin{array}{ll}\text{true}& \text{if }{V}_{\text{in}}>0\\ \text{false}& \text{otherwise}\end{array}$$

This ensures that the transmitter cannot operate when no power is available, such as during a power system fault or when the spacecraft is in a low-power safe mode.

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Assumptions/Limitations

• The model assumes a purely resistive load; reactive power components are not modelled.
• RF amplifier efficiency is not explicitly modelled; the power draw represents total electrical consumption.
• The base resistance represents quiescent power consumption but does not scale with operating mode.
• Multi-band transmission power scales linearly with the number of active bands.
• The minimum resistance clamp may introduce small numerical errors in extreme cases.
• Thermal effects from power dissipation are not coupled to the transmitter thermal model.
• Transient power draw during transmission startup is not modelled; power consumption is instantaneous.