> ## Documentation Index > Fetch the complete documentation index at: https://docs.valar.space/llms.txt > Use this file to discover all available pages before exploring further. # SMA Maintenance > Maintain semi-major axis within a tolerance band using automated maneuver planning > For the complete documentation index, see [llms.txt](/llms.txt). SMA (Semi-Major Axis) Maintenance is a station-keeping algorithm that computes a sequence of maneuvers to keep a spacecraft's semi-major axis within a defined tolerance band. It corrects for natural orbital decay caused by atmospheric drag, gravitational perturbations, and other forces. This page documents the SMA-specific configuration. For the shared wizard steps (Overview, Initial State, Time Range, Review) and general Planner functionality, see the [Planner overview](/features/planner). ## Step 4: SMA Parameters Configure the target semi-major axis and tolerance band for the maintenance algorithm. Planner Step4 Sma Parameters | Field | Description | Unit | Constraints | | ----------------- | --------------------------------------- | ---- | ---------------------------------------------------------------------------------------------------------------- | | **Nominal SMA** | Target semi-major axis value | km | Must be > 6,371 km (Earth's radius). Auto-populated from the spacecraft's current orbit. Up to 3 decimal places. | | **Threshold SMA** | Tolerance band around the nominal value | km | Must be > 0. Default: 10 km. Up to 3 decimal places. | The optimizer will compute maneuvers to keep the semi-major axis within **Nominal ± Threshold** km. ### SMA Plot Below the input fields, a real-time plot shows the spacecraft's semi-major axis over the configured time range. The plot displays: * **Spacecraft SMA** — the osculating semi-major axis line * **Nominal line** — solid gray reference at the target value * **Threshold lines** — dashed red lines at upper and lower bounds * **Tolerance zone** — shaded area between the threshold lines The plot updates as you change the nominal and threshold values. A reference-frame indicator badge appears next to the plot title showing the inertial frame the semi-major axis is computed in (e.g., `GCRF`). Hover the badge for the full frame name; click **Learn more** to open the [reference frame catalog](/platform-overview/reference-frames). The badge also appears on the Inclination and Keplerian-element charts elsewhere in the SMA Maintenance planner views — they all share the same propagation frame. ### Auto-Population When a spacecraft is selected, the **Nominal SMA** field is automatically populated with the spacecraft's current semi-major axis value (extracted from the latest Keplerian elements). You can override this value manually if needed. ## Analysis Tab When viewing an SMA Maintenance plan in the details panel, the **Analysis** tab displays a chart showing how the computed maneuvers maintain the semi-major axis within the tolerance band (visible in the [Planner page screenshot](/features/planner#page-layout)). The chart shows: * **Osculating SMA** — the spacecraft's semi-major axis over the plan time range, including the effects of each computed maneuver * **Nominal reference** — solid line at the target SMA value * **Threshold bounds** — upper and lower bounds with shaded tolerance zone The Analysis chart carries the same [reference-frame indicator badge](#sma-plot) as the wizard-step preview — the propagated SMA is expressed against the same inertial frame. This visualization confirms that the optimizer successfully keeps the SMA within the defined tolerance band throughout the plan duration. ## Key Concepts ### Tolerance Band The range **\[Nominal SMA - Threshold, Nominal SMA + Threshold]** defines the acceptable operating window. When the spacecraft's SMA drifts toward a threshold boundary, the optimizer schedules a corrective maneuver to push it back toward the nominal value. ### Semi-Major Axis The semi-major axis defines the size of an orbit — it is half the longest diameter of the orbital ellipse. For a circular orbit, it equals the orbital radius. For more details, see [Core Concepts](/platform-overview/core-concepts). ## Related Pages * [Planner](/features/planner) — Planner overview, shared wizard steps, and plan details * [Burn Plan](/features/burn-plan) — View and manage individual spacecraft maneuvers * [Core Concepts](/platform-overview/core-concepts) — Foundational orbital mechanics concepts