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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.

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Step 4: SMA Parameters

Configure the target semi-major axis and tolerance band for the maintenance algorithm. Planner Step4 Sma Parameters
FieldDescriptionUnitConstraints
Nominal SMATarget semi-major axis valuekmMust be > 6,371 km (Earth’s radius). Auto-populated from the spacecraft’s current orbit. Up to 3 decimal places.
Threshold SMATolerance band around the nominal valuekmMust 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.

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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.

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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.

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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). 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
This visualization confirms that the optimizer successfully keeps the SMA within the defined tolerance band throughout the plan duration.

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Key Concepts

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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.

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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.
  • Planner — Planner overview, shared wizard steps, and plan details
  • Burn Plan — View and manage individual spacecraft maneuvers
  • Core Concepts — Foundational orbital mechanics concepts