1. Trajectory Design and Maneuver Planning with STK / Astrogator
John Carrico
Analytical Graphics, Inc.

2. Tech Support: support@stk.com;
STK
Tech Support:
(8 am to 8 pm Eastern, or leave message)

3. Agenda What is Astrogator? History Examples & Exercises Installation
File Configuration
Sharing Your Scenarios

4. Astrogator: What is it?
“Astrogator” appears in Science Fiction; Follow-on to Navigator module.
Interactive orbit maneuver and space mission planning tool
For use by spacecraft operations and mission analysis staff
Fully integrated within STK
For Earth-orbiting, formation-flying, constellation, Geostationary, Lunar, Lagrange-point, and interplanetary missions

5. History: Swingby->Navigator->Astrogator
Swingby developed at Goddard Space Flight Center/Flight Dynamics Division by Computer Sciences Corp. (CSC) in 1989
Swingby commercialized to become Navigator (CSC; 1994); purchased by AGI
Astrogator developed to meet GSFC/Flight Dynamics Analysis Branch request for a COTS solution (Jan 1997)
Requirements from Swingby, and other institutional software
Swingby/Navigator algorithms, and some code, in Astrogator

6. Example: GTO to Mars From a Geostationary transfer orbit to Mars
Arrive on 17 Dec, 2003
Lunar Swingby changes plane (look in VO)
Mission Control Sequence (MCS)
Colored Segments
Trajectory draws while calculating
Segment summary report
Quick view of segment times

7. Exercise 1: The Mission Control Sequence
Initial Modified Keplerian: Rp = 7000 km; inc = 28.5º; e = .01; Epoch = 4 July ‘00
Propagate orbit to 3rd perigee
Generate Segment Summary Report
Create new 2-D window
ECI Orthographic
Graphics
Animation start time
Show ‘All’ orbit pass
Set scenario “as default”

8. Exercise 1 (Cont’d) Name and save scenario! AutoSave
Good rules of thumb:
Create new folder for each scenario
Name folder the same as the scenario
AutoSave
Will save work every 5 minutes (by default)
User can change save interval: Application Basic Properties

9. Example: Double Lunar Swingby (DLS)
Two Gravity Assists (used for Wind and Geotail missions)
Note MCS; Relationship of colors to trajectory
Uses launch model (Simple Ascent)
Use burnout information from launch vehicle
Note use of maneuver segments
Target on B-Plane for gravity assist
Copy (with Ctrl-C) time of maneuver or of gravity assist off of MCS panel; Paste (Ctrl-V) into 2-D window and hit enter for visualization

10. Background: “Delta-V”
Unique velocity at every point in orbit
If you change the velocity at a given point on an orbit, you are on a new orbit
A simple change in velocity is an impulsive “Delta-V”
Firing an engine for a finite duration changes the velocity

11. Exercise 2: Maneuvers 1 km/s impulsive maneuver at perigee
Local axes: VNC
Propagate to 2nd apogee
Change segment color
Draw trajectory as calculated
Turn off propagate on apply
Adjust 2-D view with mouse

12. Exercise 2 (Cont’d)
Graph Altitude and True Anomaly

13. Example: To The Moon Transfer from low-Earth orbit to the Lunar orbit
Earth-centered trajectory
Propagator Selection
Definition in Astrogator Component Browser
Moon-centered trajectory
Segments may have different force models and even different central bodies
Segment summary coordinate system choice
Compare Blue Moon scenario
Air Force Academy project
Low Delta-V transfer to the Moon via “Weak Stability Boundary” (WSB)

14. Exercise 3: Propagators and Force Models
Add 2nd maneuver at apogee
Velocity = 1.0 km/s
Normal = -2.0 km/s
Propagate 1 day
Type “1 day”; converted to units
Use CisLunar Propagator
Astrogator Component Browser
Create & use new 12x12 gravity force model for last segment

15. Exercise 3 (cont’d) Graph semi-major axis and inclination
Look at Initial State Segment Summary in Earth-Fixed coordinates

16. Example: 2-Maneuver Transfer
Orbit transfer
Propagate segments
Type in Delta-Vs
Repeat count for stopping conditions
Let the targeter search for answer (Shows on 2-D window)
Station contact: “Access”

17. Example: 2-Maneuver Transfer (cont’d)
Help System:
Uses HTML Browser
“Explore STK”
Main Page
Module Help
Context-Sensitive Help
Math
Exercises

18. Exercise 4: Targeting maneuvers
Use ‘V’ component as control
Select Radius of Apogee (Ra) as a result; observe value in Segment Summary
Select Ra as a constraint ( km)
Action = Run Targeter

19. Exercise 4 (Cont’d) Look at radius value at apogee
These are not two- body orbits
Lunar and Solar perturbations
Move propagate into target sequence
Target on Radius Magnitude (R Mag) at the real apogee
Apply corrections

20. Exercise 5 Target second maneuver (2 x 2) Reasons for non- convergence
Use Velocity and Normal components as controls
Ecc = 0; Inc = 5º
Reasons for non- convergence
Max num iterations
Max step on control
No correlation
Most problems are “square”; n controls and n constraints (n x n)

21. Example: Ground Track Control
LandSat-7
Repeating Ground Track (233)
World Reference System (WRS)
Reference Longitude (295.4°)

22. Example: Ground Track Control (cont’d)
Drift to ground track error near km
Perform maneuver so turn-around within –1.0 km error
Ground Track Error at Descending Node
1st Guess and Actual maneuvers plotted

23. Example: Formation Flying
Model and Control Multiple Satellites
Reference Satellite:
Real
Virtual
Target on values relative to reference satellite(s)

24. Example: Ascent to Formation
Plan maneuvers to obtain formation
Plan maneuvers to maintain formation

25. Example: Constellation
Multiple Satellites spread around a planet
Control to reference satellite:
In-plane reference
Other-orbit-plane reference
Phased ascent from single launch vehicle

26. Example: AutoBurn Capability to model repetitive sequences
“Event Loop” philosophy
Auto-burn capability using multiple stopping conditions
Repeat count for stopping conditions

27. Example: Mars Climate Observer
Uses auto-burn to simulate aero- braking

28. Exercise 6: AutoBurn Initial State: Modified Keplerian Elements:
Radius of Periapsis = 7000
Eccentricity = 0.01
Create auxiliary sequence in Control Sequence Browser
New Sequence: “Small Burn”
Insert 1 segment: Impulsive Delta-V; 0.1 km/s along velocity
In Main Sequence, add 2nd stopping condition
Stop on periapsis.
Run new sequence “Small Burn”
Run and look at propagate summary for list

29. Example: Scripting
Useful for parametric studies, Monte-Carlo analysis, automation, etc.
Support for Perl, MATLAB®, and user programs (e.g., C, C++)
Uses STK/Connect with TCP/IP socket
Script can be run on same or separate computer
User can write custom user interface

30. Example: Monte-Carlo Error Analysis
Perl script to repeat same sequence with random perturbations of initial launch burnout values
Script set control parameters
Script retrieves STK calculated results
Script calculates statistics
Maneuvers can be automatically planned
Results include all output data: Elements; Fuel-used; Delta-V; Access times and durations…

31. Example: Engine Modeling
Integrate accelerations due to engine thrust model; Integrate mass loss based on Specific Impulse (Isp)
Finite maneuver modeling setup
Engine model setup: thrust and Isp
Polynomial test utility
Out of fuel message
Blow-Down or Pressure-Regulated
Effect of changing thrust
Thrust efficiency
Multiple thrusters
Attitude input/output

32. Exercise 7: Engine Modeling
MCS:
Initial state
Propagate to perigee
Finite Maneuver: second duration along velocity
Create new polynomial engine model
Thrust: 400 (N) (N/Pa) x Pressure
Isp: 150 (s) (s/Pa) x Pressure
Polynomial test utility: P = 1000 kPa; T = 293º K
Run in Blow-Down mode
Multiple thrusters: Use 2 Engines, half duration.
Attitude input

33. Example: Low Thrust Super-synchronous transfer to geo-synchronous
Multiple stopping conditions: stops on first satisfied
Multiple stopping conditions is the “Logical OR”
Stop propagation if ‘A = x’ OR ‘B = y’
Constraint on stopping conditions
Finds stopping conditions and then checks constraints
Multiple constraints allowed
Constraints is the “Logical AND”
Stop propagation if ‘A = x’ AND ‘B = y’ (within a tolerance)
Stop propagation if ‘A = x’ AND ‘B < y’
Stop propagation if ‘A = x’ AND ‘B > y’

34. Example: Maneuver Calibration
Calculate thruster efficiency based on orbit determination (OD)
Use calibration factor to improve subsequent maneuvers
Baseline trajectory
Simulate OD
Calibrate
User must decide on mass flow rate

35. Example: Lagrange Point - MAP
Microwave Anisotropy Probe
Launch in spring 2001
Small-amplitude Lagrange point orbit
Display in rotating coordinates
Phasing loops before Lunar gravity assist
Create Sun-Earth-SC angle graph
Astrogator Browser

36. Example: NGST Next Generation Space Telescope
Large Amplitude Libration Point
Self-targeting for launch window calculations (+ 10 min)
Can override trajectory segment colors (Turn off “Use Trajectory Segment Colors)

37. Installation Astrogator is part of regular STK Activated with license
Full STK installation on each PC
C:\Program Files\AGI\stk\…
User Install
Each user has own data area on each machine
D:\stk\jcarrico

38. File Configuration Scenarios in a subdirectory share objects
Rule of Thumb: One scenario per subdirectory
(or very similar scenarios)
Helps when zipping files for someone else
Astrogator subdirectory holds user-defined objects
Flag as Read-Only for configuration control
User “config” subdirectory
Styles
Read-Only Astrogator components

39. Sharing Your Scenarios
Using Winzip
Recurse subfolders
Make sure to include the “Astrogator” subfolder