The GAIA System Simulator June 2003RRFWG Meeting at Dresden GAIA Simulator Development Example A...

June 2003 RRFWG Meeting at Dresden

The GAIA System Simulator

GAIA Simulator Development Example

A Java code for Fundamental Algorithms



The GAIA Simulator structure

•Simulator is not GDAAS

•GASS

•GIBIS

•Common Libraries

Java language

•Portability & modular structure as priorities

•Speed ,Performance and Read-at-Once

•Wrapping from C/C++ & Fortranto Java

Conclusions

An example : Fundamental Algorithms

•What are the Fundamental Algorithms?

•Parameters review and Interactions with other Simulator Parts



The GAIA Simulator structure



GDAAS is the ESA contract which has to solve the problem of the GAIA reduction process, giving as a final result a closed, tested package capable of reduce the 5 years mission data in a reasonable time and in a robust self-consistent way (Global Iterative solution).

GAIA Simulator is an “independent” effort to provide the input Raw data to GDAAS contract and other development efforts.

People working with the simulator spent their “scientific” time in the development of this tool.

•Simulator is not GDAAS



GASS GAIA System Simulator

GASS is designed to simulate the telemetry stream of the GAIA mission according to the GDAAS (Gaia Data Access and Analysis Study) specifications, using models of the astronomical objects and instruments.

GASS has a Grafic User Interface and must be downloaded with a compilable version of the whole simulator to run properly

GIBIS GAIA Image and Basic Instrument Simulator

Generates images of all sky features (stars, galaxies, nebulas, background,etc...) as seen by the intruments (at the pixel level) with the aim of develop Quick-look routines, classification, and on-board detection of all kind of objects and events.

GIBIS has a web interface which allows you to send queries via internet.

http://www.ast.cam.ac.uk:8080/gibis



It is planed to release a full compiled and documented version of the whole

GASS simulator v2.0

It will be distributed in a few weeks (under request)

Coordinator : Eduard Masana, [email protected]



http://www.ast.cam.ac.uk:8080/gibisGIBIS v1.4 has a fully functional version throught the internet (password needed)

Coordinator : Carine Babusiaux, [email protected]

pixel level simulator == images



Recent meeting information and other documents (Simulator documentation, User guides,etc.) are going to be available through SWG web page :

http://gaia.am.ub.es/SWG/

To access SWG Work Area, please send an e-mail to :

Xavier Luri : [email protected]



•Common Libraries (gaiasimu package)

Both simulator efforts use the same libraries in order to preserve consistency and to save time duplicating job.

gibisgass

gaiasimu

gaia(from gaiasimu)

universe(from gaiasimu)

utils

g l o b a l

(from gaiasimu)



This libraries are in a package called gaiasimu, which contains the univers models,satellite and intruments models, all required routines, as well as the the used numerical libraries.

It is expected this libraries being “flexible” and portable enough to allow the community use them for their particular simulation purposes (i.e; supernova and other event detection, stocastic microlensing noise, variability detection, etc.)



Java ; the language of the Simulator



•Portable and Modular as a development priority

Algorithm and routines implementation must be as flexible as possible to allow its generic implementation in any of the present/future simulator efforts.

The code must be portable. It could run in any machine with identical results.

The code must be modular. Diferent parts are developed by diferent groups and are being integrated in the same structure.

In the same philosophy, the code must be efficiently documented. Documentation can be found at SWG web page



•Performance, speed and Read-at-Once

The portability of Java (not machine dependent) have some essential counterparts. For example, the maximum precision you can use (using basic algebraic manipulation) is 64 bits :

long : integer value

double : floating point value

Java is not so fast as other languages (as Fortran or C/C++) in some specific tasks (I/O from files for example).

1 bit sign + 63 bits mantise

1 bit sign + 63 bits mantise +1 bit sign + 7 bits exponent

The use of temporary files is HIGHLY not recomended. If a routine must read from a file a set of parameters it is recommended to Read-at-Once the file at the first time the routine is initialised.



If an algorithm is a bottle neck or its translation is very complicated, Java allows wrapping from other languages.

Wrapping must be avoided if it is not considered essential, because it makes the Simulator lose its portability...

•Wrapping from C/C++ and Fortran to Java

... and because the people working in the Simulator structure should have to wrap the routines themselves.



An example :

Fundamental Algorithms



•What are the “Fundamental Algorithms”?

The so-called Fundamental Algorithms (LL-44) are those equations/processes which relate observations with catalog astrometric data.

Due to the nominal high precission astrometry stated for GAIA, a full relativistic treatment of the observation process must be carefully tested and developed.



In this process there are some independent tasks involved :

Main Task

Observables parametrization (parallax, and proper motions, solar system objects orbits), Gravitational Light deflection and relativistic aberration modelization

Related Tasks :

- Reference Frames definition

- Satellite ephemeris model (position , velocity)

- Solar System model (postion and velocity of relevant massive bodies). Eventually, other object properties may be required from the model.

- Time transformations (i.e; simulated telemetry data MUST be given in satellite proper time)

- Attitude parametrization



•Parameters review and interactions



source positionsource velocitytobs

Baricentric position at Observation Time



source positionsource velocitytobs

observer’s position

Parallax Correction



source positionsource velocitytobsobserver’s positionsolar system ephemerisGamma ppn parameter

Gravitational light deflection



source position(*)source velocity(*)tobs(*)observer’s position observer’s velocitysolar system ephemerisGamma ppn parameter

tobs(*) Satellite Proper Time

S vector Field Of view Angles

Observed direction (aberration correction)



source position(*)source velocity(*)tobs(*)

observer’s position observer’s velocity

solar system ephemeris

Gamma ppn parameter

Solar System Model

Satellite ephemeris

Input parameters

Global Parameters

tobs(*) Satellite Proper Time

S vector Field Of View Angles

Time ephemeris

Attitude



Input parameters

Satellite ephemeris

Solar System Model*

Global Parameters

Time ephemeris

Attitude

Generated by the Universe Models (Galaxy model ; Torra et Al)

Chebychev representation (F.Mignard GAIA-FM-13)

Chebychev representation ? (F.Mignard To be done)

General Relativity = 1.0 (A. Einstein)

Chevychev representation ? To be defined

Quaternion formalism (L.Lindegren)



Input parameters

Satellite ephemeris

Solar System Model*

Global Parameters

Time ephemeris Attitude

, ,

Fundamental Algorithms Main Task

Perpendicular Models




Pure PPN perturbative formalism (S.A.Klioner et al)

General Relativistic “semi-analítical” approach (F.de Felice, A.Vechiatto et al.)




Routines will not have direct acces to “perpendicular models”, if they are not coded in Java

In this case, access has to be provided by a GAIA Simulator interface coded in Java. This impose to such routines to be as much “parametric” as possible

What the Simulator people has to say about....



The Simulator interface is partially done waiting to be decide the final structure I/O methods.

For test purposes it has been implemented a Java version of S.A.Klioner routines following the presented structure for the F.A.Core Tasks.

Fundamental.java

Observer.java (satellite ephemeris)

SolarSystemModel.java (solar system massive objects ephemeris)

If it is all intilialised correctly, a call of :

getDirectionOnSatellite(double tobs, double[] x, double[] v)

return the desired S vector.



Some reference performance results

Some simple test permormed with diferent configurations:

Only Sun, Only Jupiter , Sun & Jupiter

This tests have shown that the numerical stability is guaranted at as precision working at 64 bits precision (theoretical defelction has been obtained using :

21

2

ctgrc

GM

o

i



Some initial rounding problems at the time to estimate were solved using an aproximate rule to calculate the angle between to very similar unit vectors :

43

24

1' Onn

Because of initial rounding problems we tryed to perform all calculations at “arbitrary precission”. When the rounding problems were solved in final angle estimation, arbitrary precision showed to give almost the same results as working in standard 64bits.

- Using arbitrary precision operations

12000 sec / 106 stars = 12.1 miliseconds per star

- Using double precision operations

105 sec / 106 stars= 0.105 miliseconds per star

Intel P4 [2.4 GHz]



- I/O flow must be decided being this fact independent of the implementation chosen.

-From the GAIA Simulator point of view, the “Perpendicular” model + a Java Interface; is the best option to keep the modular structure.

-It allows to develop such models independently from F.A.

•CONCLUSIONS



Time ephemerisChevychev representation ? To be defined

-As a crucial part of the telemetry production, a decision must be taken.

-To proceed with the time integration it is needed a Solar System model for ephemeris compatible with the satellite ephemeris.

The GAIA System Simulator June 2003RRFWG Meeting at Dresden GAIA Simulator Development Example A...

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