Natural Motion Circumnavigation RPO Sequence
The Natural Motion Circumnavigation (NMC) RPO sequence generates the initial conditions for an NMC.
The sequence moves the chase satellite from the current position to the starting point of the NMC. It does this via a set of forced motion waypoints that, by default, moves the chase in a near-straight line along the path connecting the current relative position to the start position of the NMC. If you prefer a simple hop instead of a series of waypoints, then set the Way_MaxDegrees parameter to a large value (>180 deg).
The degrees of freedom in the NMC are:
- size
- orientation
- translation
You can vary these parameters to achieve many different NMC configurations. This sequence returns the state to the beginning of the NMC, and you can either add a Coast (little or no differential force) or Maintain NMC sequence after this sequence to propagate in the NMC.
For the special case of an “Equidistant NMC” — the NMC at a near-constant distance to the target — choose the inputs such that the RBar_CrossTrack_Offset is +/-√(3) times the Semi-Minor Axis. Then the VBar_CrossTrack_Offset, InTrack_Offset, and InTrack_Drift are all 0.0. In such a case, the resulting NMC is approximately a constant distance to the target, with that distance being two times the semi-minor axis. Also, for this case, the NMC crosses the VBar.
The figure below shows a complex NMC.
Sequence parameters
| Option | Description |
|---|---|
| SemiMinor_Axis | Enter the semi-minor axis size of the NMC. |
| RBar_CrossTrack_Offset | Enter the crosstrack offset distance when the NMC crosses the positive RBar. |
| VBar_CrossTrack_Offset | Enter the crosstrack offset distance when the NMC crosses the positive VBar. |
| Init_Phase_Angle | Enter the initial phase angle at the initial state of the NMC. This angle is relative to the nadir direction and is measured counterclockwise. The valid range is +180 to -180 degrees. |
| Transfer_Duration | Enter the transfer time from current position to the start of the NMC. The transfer uses forced-motion waypoints to connect from the current position to the start of the NMC. |
| InTrack_Offset | Enter the intrack offset distance from target center. This offsets the NMC relative to the target. If you want a target-centered NMC, enter a value of 0. |
| InTrack_Drift | Enter, in distance units, the NMC drift per revolution. This causes a drifting of the NMC each revolution; it is sometimes referred to as the “corkscrew” relative orbit. |
| Way_MaxDegrees | Enter the maximum spacecraft degrees between hops for the forced motion part that connects the current position to the start of the NMC. |
| Engine_Name | Enter, as a string, the name of the finite maneuver engine model. |
| Maneuver_Type | Select the type of maneuver, "Impulsive" or "Finite," from the drop-down list. |
| WhoAmI | Enter, as a string, the name of the current satellite. |
| ThisSequence | Enter, as a string, the name of this sequence. This is nominally NMCircumnav, but if you insert the sequence multiple times in the MCS, Astrogator adds a monotonically increasing number to the end of the name (NMCircumnav1, NMCircumnav2, etc.). |