customizing computations with stk plug-ins using compiled code

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Customizing computations with STK Plug-ins using compiled code

Dr. Vince Coppola – Analytical Graphics, Inc.

October 2005


Outline

• What is an STK plug-in?

• Motivation: TDRS SRP Model

• COM based plug-ins

• Access constraint example

• Questions and answers


What is an STK plug-in?

• Custom software that STK uses during computations

• Purpose– Customize models used in STK computations to specific

needs


What is an STK plug-in? (cont.)

• Differs from STK/Connect– Not used to control task flow

• Differs from STK/X and 4DX– Not used for custom GUIs– Not used for creating a custom application


Example: TDRS SRP model

• TDRS– GEO Communication satellite– Solar panels track Sun– Solar sail, AW C-band antenna, and SGL antenna are nominally

Earth-pointing

• Need– Generate ephemeris using a high-fidelity force model

• Force Model Contributors– Gravity field (solid tides?) (ocean tides?)– Third body gravity (Sun, Moon, others?)– General relativity effect– Solar radiation pressure (SRP)


TDRS SRP model (cont.)

• Solar radiation pressure force– Depends on particle absorption and reflection– Depends on surface orientation– Surface orientation varies along the trajectory

• Generic model: spherical shape– Accel = Cr * (Area/mass) * Illumination * Irradiance / c– Directed along apparent sun line– Supported by STK



• Better model for TDRS– Based on Pechenick, K. and Hujsak, R. (1987)– Account for surface orientation of main parts– Account for diffuse and specular reflection– Accel = Cr *(1.0/mass)* Illumination * Irradiance / c *

{A1 k + A2 k x ( k x N ) + A3 k x ( k x M ) }– k – along sun line, N normal to solar panels, M radial– A1, A2, A3 depends on area, reflection values



• Use a HPOP Plug-in – Code the model in a computer language of choice

• C++, C#, VB.NET, Perl, VBScript, Jscript, etc.

– Adhere to the published plug-in interface• Sample code provided

– Choose to use the plug-in from the HPOP GUI• Just like the other force model settings

– Set any plug-in specific parameters from the GUI• Similar to the other force model settings

– Configure the remaining force model parameters– HPOP generates the ephemeris using the force model



• What you don’t code (nor test, doc, maintain)– All the other force model contributions– The numerical integration algorithm– The VOP formulation– The time regularization algorithm– The covariance propagation algorithm

• Save time and avoid risk– Code only the custom part


Purpose

Plug-ins provide the means for custom modeling while leveraging STK’s tested

generic computational framework

Only code the custom part of the model


HPOP plug-in uses

• Attitude dependent SRP

• Attitude dependent Drag

• Density model

• Satellite thermal emission

• Micrometeorite particle collisions

• Re-entry drag and lift

• Outgasssing (thrust without mass loss)– Models non-physical accelerations


Outgassing


Performance

143168

329

0

50

100

150

200

250

300

350

C++ C# Jscript

90 days propagation

seco

nds


Plug-in evolution

• STK 4.3 introduced plug-in scripts (2002)– Perl, VBScript, Matlab scripts– AGI-developed interface

• Installation• Calling conventions• Debugging ideas• Troubleshooting


Available plug-in points

• 15 plug-in points– Astrogator– Vector Tool– Access constraint– Attitude simulator– Communications modeling


Plug-in scripts

• Use– Only needed when generic models are insufficient– Several customers rely on this capability

• Limitations– 3 languages – Perl, VBScript, Matlab– Not as fast as compiled code– Non-standard function calling conventions– Requested input data must be known a priori – Unable to get/set plug-in parameter data


COM-based plug-ins

• Computer programmer-friendly plug-ins– Based on Microsoft’s COM technology

• STK 6.1 introduced COM Plug-ins (2004)– HPOP

• STK 6.2 (2005)– Astrogator engine model, Astrogator attitude controller

• STK 7.0 (2005)– Alpha prototype for Access constraints, dynamic phase

array antenna


COM

• Component Object Model – Standard Microsoft protocol– Many languages (Windows operating system)

• C++, C#, VB.NET, Perl, VBScript, Jscript

– Sun Java• No samples yet customer interest?

– Microsoft J++ / J#• No samples yet


Plug-in concepts

• Focused on a computational task– HPOP: evaluating the force model– Attitude controller: steering an engine– Engine model: modeling thrust and flow rate– Access constraint: providing a visibility measure

• Plug-in point– Own triggering events– Provides interface for setting/getting input/output data


Plug-In Points

• Must modify STK to make a plug-in point– Identify a computation– Identify a need for user customization– Identify the triggering events– Identify inputs and outputs

• Made available as need arises


Matlab plug-ins

• Previous capability supported Matlab– Compatible with Matlab version 6.x and 7.x

• Matlab is COM-aware– Can use COM objects– Can be used as a COM object

• Recommendation– Use a scripting language plug-in to STK– Call Matlab from scripting language


HPOP plug-in

• Custom force model

• Supported by STK and ODTK

• Plug-in scripts– Astrogator propagator plug-in– Custom force model– Migration to COM plug-ins soon!


HPOP plug-in events

• Init()

• PrePropagate()

• PreNextStep()

• Evaluate()

• PostEvaluate()

• PostPropagate()

• Free()


Hpop plug-in events (cont.)

PostPropagate()

PrePropagate()

while needing to take another step

{ PreNextStep()

Attempt the step

while haven’t taken a good step

{

Assess whether step is good

}}


Hpop plug-in events (cont.)

Attempt the step:

Evaluate force model :

Evaluate()

Perform preliminary computations

Compute all force model contributions

PostEvaluate()

Add all force model contributions


Calling convention

• Plug-in scripts passed input / output data

• COM based plug-ins pass interfaces– Each event has an interface appropriate for that event– The interface has properties and methods for getting

inputs and outputs

bool

Evaluate( IAgAsHpopPluginResultEval interface)


Calling Convention (cont.)

bool


illum = interface.SolarIntensity

if(illum > 0)

{

interface.SunPosition(eSRP, eInertial,

sunX, sunY, sunZ)

… <compute SRP acceleration> …

interface.AddAccleration(eInertial, aX, aY, aZ)

}


Calling Convention (cont.)

Init( IAgUtPluginSite siteInterface)

PrePropagate( IAgAsHpopPluginResult interface)

PreNextStep( IAgAsHpopPluginResult interface)


PostEvaluate( IAgAsHpopPluginResultPostEval

interface)

PostPropagate( IAgAsHpopPluginResult interface)

Free()


Exception handling

• Event calls return a Boolean value– True: everything is okay– False: an unrecoverable error occurred

• Turns this event off – no longer called

• STK code captures exceptions raised by a plug-in– Exception text sent to message viewer

• Certain interface function may fail– Sets a defined error code


Inputs and outputs

• Specific to each interface

• Ephemeris

• Variables pertaining to the plug-in point– HPOP: Cd, Drag Area, Cr, SRP Area, Mass, etc.– Engine Model: thrust, mass flow rate, Isp– Attitude Controller: Euler angles, quaternion

• Methods and properties


Documentation

• Compiled help file (stkPlugins.chm)– Plug-in points– Triggering events– Interface methods and properties– 7 language syntaxes shown

• VB.NET, VB6, C#• Managed C++, Unmanaged C++• VBScript, Perl


Evaluate method


Evaluate method (cont.)


Eval interface


SunPosition method


SunPosition_Array method


IAgUtPluginSite interface

• bool Init( IAgUtPluginSite site)

• Message Viewer– Write messages to message viewer

• Vector Tool– Vectors, axes, points, angles, etc– Ephemeris and attitude of any object expressed in any

components

• Astrogator Calc Objects


Building plug-in components

• Scripting languages– Windows script host– Text editor

• Compiled languages– Microsoft Visual Studio (C++, C#, VB.NET)


Installation

• Standard COM registration

• Registration in the Windows registry– GUID– Regsvr32

• C++ components• Perl, VBScript, Jscript components

– Regasm • .NET components (C#, VB.NET)

• Must register on every machine you want to run on


Debugging

• Available for compiled plug-ins– Able to debug plug-in code when running STK– Must exit STK before re-building the plug-in

• Not available for scripting languages– Create a log file instead– Use message viewer (but not too much)– Advantage: Can edit text file with STK running

• Helps to reduce cycle time


TDRS SRP model

• Init()– Return true to turn plug-in On

• PrePropagate()– Check whether SRP is On– Pre-compute some variables

• PreNextStep, PostPropagate(), Free()– If debug mode, output msgs else do nothing



• Evaluate()– Compute SRP based on formula– Use interface->AddAcceleration()

• PostEvaluate()– If debug mode, output diagnostic messages to viewer



• Plug-in parameter data– Diffuse reflectivity, specular reflectivity– ‘Debug’ settings (messages sent to MessageViewer)

• IAgUtPluginConfig– Implement this interface to set / get parameter data– GetPluginConfig()

• Register parameter data with STK

– VerifyPluginConfig()• Determine whether user settings are acceptable



AddBoolDispatchProperty (

scope,

"DebugMode",

"Turn debug messages on or off",

"DebugMode",

eFlagNone )



AddDoubleDispatchProperty (

scope,

"Reflectivity_Specular",

"Specular reflectivity coefficient",

"SpecularReflectivity",

eFlagNone )


Choosing a plug-in language

Language SpeedCode

complexityMemory Debug

Scripting Slower Simple Automatic No

C#, VB.NET

Fast Medium Automatic Yes

C++ Fastest DifficultManage

yourselfYes


Using COM in plug-in scripts

• Post STK 7.0– Migration to use COM based plug-ins

• Able to use COM plug-ins from VBScript– Restricted to inputs / outputs available– Must use AGI-defined interface– Get handle to the COM component on initialization– Route inputs to the COM component– Have COM component compute values– Return using plug-in script mechanism


Access constraints

• Constraints are often measures– Elevation angle– Range– Ground elevation angle– Sunlight

• Used for visibility and for Figures Of Merit (FOM)– Coverage: visibility to a grid of points– FOM: quality of coverage during visibility times


Figure of merit


Example: image quality

• Sensor taking a picture of ground – Oblique geometry of sensor with respect to ground– Sensor properties (focal length, pixel resolution)– Ground properties (sun elevation)– Time effects

• Create an access constraint plug-in– Use as an image quality metric


Ground sampled distance

• Built in GSD constraint

• pp is the pixel pitch

• range is the slant range

• θ is the ground elevation angle

• f is the focal length

sinf

rangeppGSD


Ground spot size and optical ratio

• Ground spot size due to the optics

• D is the optical diameter

• λ is the wavelength

• The optical ratio

sinD

rangeGSS

Dpp

f

GSD

GSSQ


Alternate GSD

• Can perform trade studies on GSD as we vary Q

• Create a plug-in constraint– Implement GSD as function of Q

sin

DQ

rangeGSD


Q Variation Study

Q = 0.583Q = 0.750Q = 1.000Q = 1.250Q = 1.500Q = 1.750Q = 2.000


NIIRS

• National Image Interpretability Rating Scale– “Image quality for remote sensing systems,” Robert

Fiete, Eastman Kodak, Opt. Eng. 38(7), July 1999.– For electro-optical systems

• Create a plug-in access constraint

SNR

GHRERb

GSDaNIIRS

344.0656.0log

)(log251.10

10

10

inches in


NIIRS Results

Q = 2.000


Questions & answers

customizing computations with stk plug-ins using compiled code

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