Introduction to Rendezvous and Proximity Operation Sequences in STK

Rendezvous and proximity operations (RPO) mission planning can be a very complex process. STK's Astrogator capability has a robust feature set that enables complex RPO mission planning. The Astrogator RPO mission planner requires significant expertise in various key skills to efficiently and effectively plan an RPO mission. These skills include:

  • A thorough understanding of relevant orbital mechanics
  • A strong background and experience with Astrogator capabilities
  • A thorough understanding of numerical issues using a differential corrector, such as which parameters are numerically safe to target, numerical perturbations and maximum steps, etc.
  • Basic scripting using JScript, VBScript, Python, or MATLAB, both in general as well as within the Astrogator scripting tool

To approach RPO mission planning using Astrogator, you often need years of experience to obtain effective levels of skill in all these areas. To help mitigate this overhead, AGI has developed a set of prebuilt RPO sequences that enables an RPO mission planner with reduced training to plan a mission quickly and effectively. This topic introduces you to the following aspects Astrogator RPO prebuilt sequences:

 

Configuring STK to work with the RPO sequences

The RPO sequences and associated support objects are available directly from the STK installation with an STK Premium Space or STK Enterprise license. To gain access to the RPO objects and configure your STK session to use them, open the Utilities menu within STK and select Astrogator > RPO Setup. The Astrogator RPO Setup dialog box will appear. Select the Use Python check box to download the RPO sequences in Python rather than in the default VBScript format.

This tool functions to configure your STK session for use with RPO work. You should view this as a mechanism to reconfigure your STK experience. Click Load Template Files to initiate the ingestion of the following RPO components:

  • RPO sequences
  • Support sequences and supporting calculation objects for the Component Browser
  • Several reference/template satellites for your scenario

This configures your STK session for RPO work. Since RPO work involves interactions of one satellite with respect to another, all these support objects interact with one another. The RPO support objects reflect interdependencies, and you should keep this behavior in mind when working with the reference/template satellites. If the template satellites have inadvertently been removed from the scenario, leading to a bad scenario state, then click Reload Template Satellites at the bottom of the Astrogator RPO Setup dialog box. If you selected the Use Python check box for the initial RPO loading, the check box remains selected for the reloading of template satellites, which would also have the original accompanying Python scripts.

Interacting with RPO sequences

The basic premise of RPO sequences is to arrange them in a time-ordered fashion as you follow the mission design. The first step is to define the reference vehicle using the Set_Reference_Vehicle sequence. The reference vehicle is generally the target satellite, but it can be a position relative to the target satellite, as is the case in the rendezvous phase of the RPO mission. Before executing the MCS of the Astrogator RPO satellite, you must configure the target satellite. In the template objects that ship with STK, the default target satellite is named “Target”.

You must propagate the target satellite and ensure there is sufficient ephemeris generated to cover the mission timeframe. The target satellite can point to any satellite in the scenario, but you must set the reference satellite in the Follow segment of the target satellite prior to running its MCS.

The next step is to define the initial conditions of the chase satellite. You can accomplish this with either a rendezvous sequence (e.g., GEO_To_GEO_Rendezvous_Drifting) or a Set_Initial_State sequence, which defines the initial state relative to the reference vehicle. After this, you may begin proximity operations.

Limitations and constraints

Autosequences

VBar_Approach and RBar_Approach use autosequences that require the mission planner to populate them prior to using other associated sequences.

Finite engine thrust level

If you want to use a finite maneuver solution, a concern is the choice of the finite engine thrust level. If you choose a thrust level that is too small, maneuvers overlap and the sequences fail. If a thrust level is too large, the maneuvers may be too short. It is difficult to physically realize a repeatable Delta-V when the finite maneuver durations are small (less than a second). For this reason, you should use thrust sizes between 0.1 N and 10 N — assuming 1000 kg total mass, with the effective acceleration being the key factor — for the proximity operations portion. Rendezvous operations generally require larger thrust levels, as the associated Delta-V is generally larger than the proximity operations equivalent.

Orbit type

All the example pictures in the detailed descriptions are for a GEO RPO mission. These sequences work for all orbit regimes (not just GEO), but natural motion sequences work best for circular orbits, and the TearDrop sequence is limited to circular orbits. You can only use GEO near-circular orbits for GEO_to_GEO_Rendezvous_NoLead, GEO_to_GEO_Rendezvous_Drifting, and Exit_GEO sequences. You should generally limit transfer times to be less than a revolution, so check the period of the target satellite. You can modify this limitation if necessary. The RPO Delta-V generally rises as the velocity of the target orbit increases. So for LEO circular orbits, you can expect the required Delta-V to be larger to accomplish the same RPO mission. Additionally, for LEO cases, the differential drag becomes an issue at low altitudes, even if the area, shape, size, and mass of the chase and target vehicles are identical. HEO RPO missions are possible; however, the forced motion required going through perigee is large and you may wish to suspend the forced motion going through perigee.

Propagator

By default, the RPO sequences use a propagator that contains a “full force” model — drag, solar radiation pressure, Sun, Moon, and a 21x21 geopotential. A few sequences require that you limit the maximum step size so that the propagator does not miss crossing conditions. In such cases, Astrogator provides a propagator called “Small_Step” and sets the maximum step size to 60 seconds. Occasionally, a sequence will use Hill’s equations to seed the initial guess for an RPO maneuver, but the propagator performs all calculations using a full force model based on the initial guess. Astrogator's scripting tool determines the initial guesses. The scripting tool also controls execution and enforces certain logic based on your input parameters.

Astrogator uses the scripting tool in three places:

  1. Forward and backward sequences
  2. A profile in a target sequence
  3. Embedded into a differential corrector or optimizer profile, executed prior to each iteration

The first two types represent the primary uses of the scripting tool. The third type applies to some forms of rendezvous.

Sequences

You will not use the following four sequences directly in a mission sequence: InTrack_Finite, InTrack_Impulsive, Radial_Finite, and Radial_Impulsive. However, these do appear as autosequences for approach sequences RBar_Approach and VBar_Approach.

Additionally, you use the following four sequences only with required reference satellites that are added to the scenario when needed: MatchOrbit_Reference, NMC_Reference, Perch_Reference, and VBar_Reference.

Passive safety

When designing RPO sequences using Astrogator, you can run a passive safety analysis to help ensure the target and actor (chase) satellites do not collide. See the Passive Safety Tool topic.

List of sequences and associated functions

There are many sequences available for the proximity operations phase. Below is a list of the sequences by category and alphabetically, with a brief description and a link to the specifics page. Some sequences such as VBar, NMCircumnav, TearDrop, FMCircumnav, FollowSun, and others transition from the current position to move to the starting position of the selected proximity operation. You define the transition duration, and then the sequence uses forced-motion waypoints to move from the current position to the start position of the selected proximity operation. There are other sequences that require the initial position to be correct for that sequence, such as VBar_To_NMCircumnav, RBar_To_NMCircumnav, NMCircumnav_To_VBar, NMCircumnav_To_RBar, VBar_To_RBar, RBar_to_VBar, and others. In those sequences, the input state must already be at the appropriate conditions (e.g., on the VBar). Other sequences such as Coast, Stop_RelRate, Stop_PlaneCross, and others stop propagation at a condition.

After you load the sequences into STK, you can view them in the component browser as subfolders of the MCS Segments library, inserted into an MCS via the Insert New Segment menu. The RPO sequences are in categories for ease of access.

Here is a list of the current RPO sequences:

RPO Sequence Description
Configuration
Coast Propagate to a stopping condition.
Set Initial State Set the relative initial state position and velocity.
Set Reference Vehicle Identify the current and reference satellites.
Set Delta V Add a user-specified Delta-V to the current satellite.
Update Spacecraft Parameters Update spacecraft parameters such as mass, area, drag coefficient, coefficient of reflectivity, etc.
Differential Forces
Maintain NMC Maintain a natural motion circumnavigation with differential forces.
Maintain VBar Maintain a VBar offset in the presence of differential forces.
Forced Motion
FMCircumNav Perform a prescribed circumnavigation.
FMWaypoints Move in a straight line between points given as relative to the target.
Follow Sun Maintain a position along the line between the target and the Sun.
Hop Perform a single maneuver to move between relative positions.
Hop MinDV Perform a single maneuver by using the optimal transfer time to minimize Delta-V.
HopAndStop Perform a single maneuver between relative positions and then stop all relative motion.
Perch EqualSpacing Maintain a perch point using FMWaypoints of equal spacing.
Perch MaxError Maintain a perch point using the maximum error method.
TearDrop Enter a relative orbit resembling a tear drop shape.
VBar_Hop Hop along the VBar using half of a natural motion circumnavigation.
Matched Forces
NMCircumNav Generate the initial conditions for a natural motion circumnavigation.
VBar Move to an orbit offset from the target vehicle along the VBar.
Rendezvous
Exit EccVector Exit proximity while maintaining the eccentricity vector of the target.
Exit GEO Exit a GEO RPO mission.
GEO to GEO Rendezvous Drifting Chase to a target GEO from a drifting GEO condition.
GEO to GEO Rendezvous NoLead Rendezvous a chase GEO satellite to a target GEO satellite.
Lambert Match Orbit Use a Lambert arc as an initial guess to match the orbit of a target satellite while minimizing fuel usage.
Lambert Rendezvous Use a Lambert arc as an initial guess to rendezvous with a target while minimizing fuel usage.
Specialized
MatchPlane SingleBurn Move the chase satellite to match the target satellite's orbit plane.
Phase Change Adjust orbit phasing by drifting the chase satellite to a point relative to a target.
Stop PlaneCross Stop propagation upon reaching a relative plane crossing.
Stop RelRate Stop propagation based on a relative rate condition.
Stop RelMotion Stop all relative rates now.
Specialized RBar and VBar
NMCircumnav to RBar Stop an NMC relative orbit on the RBar, if it crosses the RBar.
NMCircumnav to VBar Stop an NMC relative orbit on the VBar, if it crosses the VBar.
RBar Approach Approach or depart a target satellite along the RBar.
RBar Hop Hop along the RBar using half of a tear drop.
RBar to NMCircumnav Initiate an NMC from the RBar.
RBar To VBar Move from the RBar to the VBar using a partial NMC.
VBar Approach Approach or depart a target satellite along the VBar.
VBar to NMCircumnav Initiate an NMC from the VBar.
VBar To RBar Move from the VBar to the RBar using a partial NMC transfer.

Support sequences

Support sequences are not used directly by RPO sequences. They are included in autosequences and for specific operations as needed. You can find a list of these support sequences in the Support Sequences topic.