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1/20/2007 10:34:22 AM

Process for Orbit Information Data Exchange

1.Overview

1.1 Objective

To prescribe the content and format to be used when parties exchange satellite orbit information.

1.2 Rationale

Satellite orbit descriptions include many different elements of information. Satellite operators must exchange orbit descriptions for many reasons, such as rendezvous and collaboratively avoiding close encounters. Satellite operators employ orbit estimation techniques convenient and efficient for their individual applications. There are many different techniques. They vary in the set or orbit elements employed, coordinate systems, coordinate reference frames, and the manner in which modelling and measurement uncertainties are accommodated. Cooperation and collaboration require that those affected by a satellite operator’s actions be provided a complete and well defined data set.

This International Standard is part of a series of ISO standards on mitigation of orbital debris. Debris mitigation requirements are established as part of the ISO standard (WD24113). Standards for describing the manner in which orbital information is derived are in normative references below.

2. Approach

. This standard prescribes the content and format for a standard orbit information data set. This standard does not require the exchange of orbit data. If stakeholders decide to exchange such data, this standard prescribes information that must accompany the data so that collaborating satellite owner/operators understand the similarities and differences between their independent orbit determination processes.

3. Discussion

Normative References

American Institute of Aeronautics and Astronautics Standard R-xxx, AIAA Astrodynamic Standards: Specifications, Recommended Practice, and Certification

WD 24113, Orbital Debris Management

WD XXX, Orbit Estimation

Other ESA, ECSS, or national standards that apply to describing orbit estimation processes.

Informative References

Fundamentals of Astrodynamics and Applications, 2nd ed, David A. Vallado, Space Technology Library, 2004

4. General requirements.

Orbit data transmitted to other parties for collaborative space operations must include the following elements of information.

Descriptive Information: Satellite Number, Common Name, International Designator, and Country of Origin, Gravitational Force Information, Non-conservative Force Information.

Satellite State: Orbit Elements and Epoch

Temporal and Spatial Information: Coordinate System, Coordinate Reference Frame, International Earth Reference System (IERS) Parameters and Epoch

State Vector and Covariances: Satellite State of Motion (position, velocity, acceleration, their variances, and coordinate reference); Covariances (estimation state vector solve-for parameters, dimensions, and coordinate reference).

5. Descriptive Information

5.1 Satellite Number: The satellite owner’s numerical designation

5.2 Common Name: Internationally accepted narrative, such as GPS.

5.3 International Designator: Composed of launch year, sequential launch number for that year, and an alphanumeric designator for each separate object that enters orbit from the same launch.

5.4 Earth Gravitation Information: Commonly accepted name of the Geopotential employed. If a unique Geopotential approximation is used, this information must be accompanied by a brief description of the technique.

5.5 Other Forces: Designation of additional massive body gravitation, such as the Moon. Radiation pressure indicators (solar constant, shadow model). Atmospheric Resistance (drag coefficient, ballistic coefficient, representative drag area cross section). Atmospheric density model (common term for approximations based on observable proxies).

6. Satellite Orbit Elements and Epoch

6.1 Satellite Orbit Elements: Regardless of the element set produced by a data provider’s individual technique, Keplerian elements must be provided (right ascension of the ascending node, inclination, semi-major axis, eccentricity, argument of perigee) in an Earth Centered, Inertial reference frame. Expressed in SI units. This unburdens users of complex transformations.

6.2 Epoch: The time at which orbit elements were derived, taken as the time of the last observation employed in orbit determination. Expressed in UTC.

7.0 Temporal and Spatial Information:

7.1 Coordinate System: The triad of axes within which orbit descriptions are founded.

7.2 Coordinate Reference Frame: The realization of a coordinate set, for example, inertial or celestial.

7.3 International Earth Reference System (IERS) Parameters and Epoch: IERS is an international service that provides precise information about the Earth’s orientation, motion of the Earth’s axis, and other important elements required to express physical quantities relative to the Earth.

8. State Vector and Covariances:

8.1 Satellite State of Motion: Mean position, vector velocity, and acceleration in SI units and in an Earth Centered, Inertial reference frame.

8.2 State Vector Variances and Covariances: Estimation state vector solve-for parameters and dimensionality in SI units and an Earth Centered, Inertial reference frame.

STATE VECTOR FORMAT

The importance of having a standard method to transfer data is clearly indicated, and at present, this is incomplete within the astrodynamics community – hence this format for transferring state vector information. This constitutes an initial set of information necessary to align numerical integration programs. Depending on the level of agreement desired, more or less information will be required. Experience from the various comparisons in this paper suggests that these parameters are sufficient to gain a rough comparison between programs. These formats are not intended to be forced upon the community. Rather, they are intended to stimulate discussion, change, addition, etc., so they can become a standard vehicle through which we operate. If additional tests, integrators, force models, etc., are desired, please contact me for additional assistance. Samples are included on the web – www.CenterForSpace.com/downloads

STATE VECTOR DATA

SATELLITE NUM : xxxxxxxxx COMMON NAME: ______INT DES: ______

ORIGIN : ______

EPOCH (UTC) : yyyy mmm ddd hh:mm:ss.sssssss

COORD SYS : ______

POS KM : ±xxxxxxx.xxxxxxxx ±yyyyyyy.yyyyyyyy ±zzzzzzz.zzzzzzzz

VEL KM/S : ±xxx.xxxxxxxxxxxx ±yyy.yyyyyyyyyyyy ±zzz.zzzzzzzzzzzz

ACCEL KM/S2 : ±xx.xxxxxxxxxxxxx ±yy.yyyyyyyyyyyyy ±zz.zzzzzzzzzzzzz

GEOPOTENTIAL : ______DEGREE/ORDER: ____ x ____

ATMOS DRAG : ___ MODEL : ______

THIRD BODY : ___ SOURCE: ______BODIES: ______

SOLAR PRESS : ___ SU MN ME VE MR JP ST UR NP PL

SOLID TIDES : ___ MODEL : ______TERMS : ______

OCEAN TIDES : ___ MODEL : ______TERMS : ______

EARTH ALBEDO : ___ GRID SIZE: ____

ACCELERATIONS : ___ DIRECTION: ______MAGNITUDE : ______

MANEUVERS : ___

1/BC (M2/KG) : ±______CD: ____ AREA(M2): ______MASS (KG): _____

1/SRPC (M2/KG) : ±______CR: ____ AREA(M2): ______

THR ACC (M/S2) : ±______CM OFFSET(M) : ±______

ATTITUDE : ______

EOP DATA : ______

SOLAR WEATHER : ______

INTERPOLATION : ______

SOLAR F10.7 : ______AVG F10.7: ____ AVG AP: ____

SHADOW MODEL : ______

PREC/NUT UP (S): ______

INTEGRATOR : ______STEP MODE : ______

INIT STEP (S) : ______ERROR CONTROL : ______

REG TIME EXP : ______REG TIME STEPS : ______

COV COORD SYS : ___

POS SIGMA (KM) : ±nnnn.nnnnnn ±tttt.tttttt ±wwww.wwwwww

VEL SIGMA(KM/S): ±nn.nnnnnn ±tt.tttttt ±ww.wwwwww

COV COORDINATES: ______DIMENSION: ____

COV SOLVE-FORS : ______

±x.xxxxxE±xx ±x.xxxxxE±xx ±x.xxxxxE±xx ±x.xxxxxE±xx ±x.xxxxxE±xx ±x.xxxxxE±xx

: : : : : :

Many of the parameters would be optional. The format is shown as a static form, but the best implementation would be via XML so that a common form would be exchanged between organizations, but each organization could tailor the specific inputs for their programs from the XML data. In the above format, additional blank lines have been inserted to aid seeing each grouping within the data. The groupings include satellite location data, force models, physical satellite characteristics, input data, integrator controls, and covariance information. The file contains enough information to recreate the ephemeris generation in multiple programs. Also notice that the fields are separated by spaces to assist free-form reading. There may be concern about the size. Using a catalog of 20,000 satellites, each of which have an 8x8 covariance, the total file (static) size is about 50 Mb using the format above. This could easily be compressed. Even though an XML file format would be larger, comparing the uncompressed size to the file size of a recent Microsoft update for XP (95Mb), this seems pretty reasonable for accurate positional information on the entire satellite catalog. Notes for some of the fields are as follows:

Basic satellite information

ORIGIN Text field for the location of processing

GEODYN, GTDS, NAVSPACOM, RayTRACE, STK, etc.

COORD SYS Coordinate system and designator (both are needed)

B1950, J2000, IAU2000

ECI, MOD, TOD, PEF, ECEF, etc.

Force model information

GEOPOTENTIAL Gravitational model – EGM-96, WGS-84/EGM-96, WGS-84, GGM-01, TEG-4, etc.

Although not listed, there should be a one-time transfer of the gravitational parameter, radius of the Earth, angular rotation, and possibly the gravitational coefficients themselves to ensure the same gravity model is in use

ATMOS DRAG Atmospheric models – MSISE90, NRLMSIS00, J70, J71, JRob, DTM, etc.

TIDES Models – IERS 2003, IERS 1996, UT, Other

Terms – nutation dependent, other

Notes about what Solar/Lunar ephemeris used – DE/LE, analytical, other

ACCELERATIONS Duration, orientation, method, etc. for empirical accelerations

MANEUVERS Number, duration, orientation, method, etc. for maneuvers and thrusting profiles. This particular field may need to be expanded to include mass flow rates, engine models, Isp, etc.

Satellite detailed information

BALLISTIC COEFF / SOLAR RAD COEFF

Reciprocal values of the coefficients entered as a combined value, or as component values. Note that the attitude and macro models may be used, and that the area for drag and solar radiation pressure are likely different.

ATTITUDE The attitude may not be known, or may have a file of quaternions, or something else

External Data

EOP / SOLAR WEATHER DATA

ACTUAL, CONSTANT, etc.

INTERPOLATION Used for the EOP and solar weather data

HERMITE, LAGRANGE, etc.

SHADOW MODEL Shadow modeling for SRP. Dual cone uses both umbra and penumbra regions

NONE, CYLINDRICAL, DUAL CONE

PREC/NUT UP Update interval for precession nutation values

Integrator information

INTEGRATOR Integration scheme – RKF78, GAUSSJACK, ADAMSB, other

STEPMODE Type of integration – FIXED, RELATIVE ERROR, REGTIME

INIT STEP Step sizes, not used if relative error is selected

ERROR CONTROL Error control if needed by the integrator, e.g. 1.0 e-15, other

Covariance information

COV COORD SYS Coordinates for sigma values – RSW, NTW, ECI, other

COV COORDINATES Format of the covariance matrix – J2000 ECI, CARTESIAN, EQUINOCTIAL

DIMENSION Size of covariance matrix

COV SOLVE-FORS Parameters included as solve-fors in the covariance