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For the complete documentation index, see 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.

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SMA maintenance in Flight Plan

When Flight Plan is available for your account, select a spacecraft and Add phase. Choose SMA maintenance, select its thruster, and enter Reference SMA (km) and Allowed deviation (± km). Save the phase; reopen Phase overview to edit it. Timing belongs to the waypoints. The selected thruster supplies its existing physical limits and automatically selects the chemical or electric maintenance algorithm. Changing thrusters updates that routing; there is no separate algorithm choice. Choose Compute maneuver plan and supply an explicit UTC planning window. The request uses the saved phase revision and the existing operational starting orbit. Review progress or a reported failure, then open the resulting proposal under Maneuver plans. Review its retained inputs, planned burns, timing, consumption and SMA Analysis. Approve or reject using the existing permitted actions. Open approved burns under Burns & analysis to review the stored record and available OD evidence in the same workspace. Later phase edits affect future computations; earlier proposals retain their original revision and inputs. Approval and elapsed dates do not confirm execution. This workflow uses the chemical and electric SMA maintenance algorithms and their parameter semantics. Osculating is retired for new computations; historical results remain available. Standalone Planner also derives the algorithm from its selected thruster. Its configuration is described below.

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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 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. 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. 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.

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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
The Analysis chart carries the same reference-frame indicator badge 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.

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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