In Plane Phasing Adjustment Scenario and Script [Last saved 4/19/2006.  Scenario originally created by Mike Loucks, SEE]

4-17.  Original scenario:

This scenario was created in response to a customer’s scenario and request.  A satellite begins in a circular orbit and we wish to place the satellite at a different True Anomaly in the same orbit.  We will accomplish this by placing the satellite into a higher circular orbit (with a chosen height difference, or DH), coasting or phasing in this higher orbit for some period of time, and then lowering back to the original orbit.  We have created a reference Satellite, ‘XSS-Ref’ which is the original state propagated for 4 days.  The Mission Control Sequence, or MCS in question is in the ‘XSS-11’ Satellite.

Each 2-maneuver transfer is a ‘Hohmann’ transfer.  There are 4 maneuvers:

1.       DV1, to raise the apogee of the initial orbit to be a DH above the original circular orbit altitude. (‘Raise Orbit’ targeter)

2.      DV2, to circularize the orbit at the higher altitude. (‘Circularize’ targeter)

3.      DV3, to lower the perigee back to the altitude of the original circular orbit (‘Lower Orbit’ targeter)

4.      DV4, to circularize at the original orbit. (‘Circularize2’ targeter)

A coast, or ‘phasing’ segment takes place between DV2 and DV3.  The duration of this coast depends on the original DH chosen, and the desired final phase angle, or separation. 

The 5 targeters in the STK/Astrogator scenario correspond to the 4 maneuvers listed above (the inside targeters) and the one phasing targeter, which is the ‘Target Phase’ targeter.

 The scenario itself is set up to target a user-entered DH and then calculate the maneuvers and phasing necessary to achieve the user-entered final phase separation (which we measure in the scenario as a relative argument of latitude, with respect to a reference satellite that stays in the original orbit).

The enclosed MS Excel spreadsheet contains a VBA macro that cycles through values of DH for a given desired final separation.  In this case, since the scenario starts off using a positive DH, we assume the desired phase is negative (i.e. we drift back away from the reference as we raise up).

The user enters the Starting DH, End DH and Step in DH for the script, along the desired final phase angle.  The script then sets DH value into the ‘Raise Orbit’ targeter, and sets the Phase Angle into the ‘Target Phase’ targeter.  The MCS is run, and once it converges, the various DV values and the total duration of the transfers are extracted to the spreadsheet.  The DH is incremented by the DH step, and the process repeated until the loop is complete.  The ‘Total DV’ column is calculated in Excel, the rest are fed into the spreadsheet from the VBA script.

The script is easily user-modified, and can be accessed through the MS Excel Spreadsheet.  Note that the spreadsheet can be operated from the embedded HTML window within STK, or externally.  If you wish to run it internally, you may want to run it when the window is not maximized within the STK Integrated space.  Otherwise, the Astrogator targeter progress windows will obscure the excel window during the execution of the script.

4-19 modifications

I have made some slight modifications to the excel spreadsheet, the VBA script and MCS.  While before we had the propagate segments stopping on Apoapsis and Periapsis, now I’m using Delta mean anomaly.  We may even want to change this to delta-arg of latitude at some point.  The setup of the MCS assumes that the circularization burns will take place at an apse.  If we aren’t careful with such circular orbits, we can do these in the wrong place.  Since our targeters assume the burns are taking place at an apse and are setup to use impulsive burns,  they will not converge if we are somewhat off-apse.  This is why the final targeter, ‘Circularize2’, has 2 components of the burn.  This will account for any off-apse offset we might have, and still allows us to get our FPA to zero.

I’ve also set some of the targeters to clear corrections before each run, and am clearing some from the script.  The new version of the scenario should be more robust. 

I’ve found that initial starting DH offsets of less than 15 km are problematic.  There probably are ways to reconfigure the scenario to deal with this, but I’ve not taken the time to do so yet.  For now, just stay above 15 km as your starting point and it should work fine.

Finally, I’ve decided that we should run the excel script outside of STK.  It works better this way.