Use measurements to refine the path

Navigation combines observations with a model to estimate the spacecraft’s state. Tracking measurements may constrain range, line-of-sight velocity or angular direction in different ways. Their errors and geometric limitations produce an uncertainty in the predicted arrival, not one perfectly known dot.

A trajectory correction — open definition is a planned velocity change intended to improve the future path. Its timing and direction follow the estimated state and mission constraints. More corrections are not automatically better: an unnecessary maneuver consumes resources and introduces its own execution uncertainty.

In a simple straight-line sensitivity exercise, a constant transverse velocity difference of 0.01 meter per second acts for 100,000 seconds. Multiplying gives a one-kilometer positional difference. Real trajectories require gravitational propagation, but the example shows why a small early velocity error can matter later.

It also explains why timing matters. An early correction can allow a small velocity change to influence the path over a long interval. A later correction may use better navigation information but have less time to act. The preferred choice depends on both measurement quality and the trajectory’s response.

Small errors can accumulate

A commanded burn duration is not the same as a measured velocity change. Thruster behavior, pointing and timing affect the executed result. After a maneuver, the team updates the state estimate using appropriate tracking and spacecraft information, then compares the resulting arrival prediction with the mission’s acceptable region.

Suppose a fictional landing corridor is narrow east-west but broad north-south. An uncertainty ellipse with a large east-west extent can be troublesome even if its total area seems modest. Direction and orientation matter. Ask which measurements or correction opportunities most reduce the uncertainty relevant to the actual encounter requirement.