# Fast Gravity, Gravity Partials, Normalized Gravity ... Fast Gravity, Gravity Partials, Normalized

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NASA Contractor Report 188243

Fast Gravity, Gravity Partials, Normalized Gravity, Gravity Gradient Torque and Magnetic Field: Derivation, Code and Data

Robert G. Gottlieb McDonnell Douglas Space Systems - Houston Division Houston, Texas

Prepared for Lyndon B. Johnson Space Center under contract NAS9-17885

National Aeronautics and Space Administration Lyndon B. Johnson Space Center Houston, Texas . . . .

February 1 993

https://ntrs.nasa.gov/search.jsp?R=19940025085 2018-07-29T19:38:18+00:00Z

CONTENTS 1.0 Summary .......................................................................................................... 1 2.0 Introduction ........................................................................................................ 2 3.0 The Gravitational Potential Function .................................................................... 3 4.0 The First Partial ................................................................................................... 6 5.0 The Second Partial ............................................................................................... 9 6.0 Computational Considerations ........................................................................... 14 7.0 First and Second Partials Using Normalized Coefficients .................................... 15 8.0 Gravity Gradient Torque ........................ ... ...................................................... 19

8.1 Point Mass Gravity Model .......................................................................... 19 8.2 General Gravity Model ............................................................................... 21 8.3 Formulation Validation .............................................................................. 23

........................................................ 9.0 GeomagneticField ..................... ........ 25 10.0 Conclusions ........... .... ................................................................................ 2 8 1 1.0 Acknowledgments .............................................................................................. 29 12 . 0 References ........................... .. ...................................................................... 3 0 Appendix A ................................................................................................................ 3 1

A 1 Spec of Fast-Gravity-Model ...................................................................... 3 2 A.2 Body of Fast-Gravity-Model. ..................................................................... 3 3 A 3 Body of Nonnallzed-Gravity-Model .......................................................... 40 A.4 Body of General-Gravity-Gradient ............................................................ 4 9 A.5 Spec of Fast-Magnetic-Model ................................................................ 50 A6 Body of Fast-Magnetic-Model .................................................................. 51

Appendix B ............................................................................................................ 55 B . 1 4 x 4 Gravity Model Test Case from Ref . (21 .............................................. 56 B.2 5 x 5 Gravlty Model Test Case from Ref . 121 ................................................ 57 B.3 Gravity Gradient Torque Test Case ........................................................ 58 B.4 Magnetic Field Vector ............................................................................. 59

MbfBmxa Figure 1 . Position of a Particle of Mass in the Body Ards System ....................... 19

........ . Figure B- 1 Execution Times for Fast Normalized & Ref (21 Gravity Models 60

........ . Figure B-2 Execution Times for Fast Normalized & Ref 121 Gravity Models 61

L i S u K m h .................................................. Table 1 . Table of Derived Legendre Functions 14

0 NASA CR-188243 January 1993

This report contains the development of a recursive, non-singular method for computing the first and second partials of the gravitation potential, with respect to position, using both unnomalfied and normalized harmonic coefficients. When unnormalized coeffi- cients are used, every attempt was been made to bufld a 'fast" algorithm. When normal- ized coefficients were used. the algorithm developed uses a more stable. albeit more complex, recursive algorithm for the derived Legendre functions. Even so. the normal- ized algorithm is still quite efficient. The normalized algorithm should be quite stable and portable for model sizes exceeding 180x180 in degree and order. Efficiency in com- putation was gained by precomputing everything that was not a function of the state and by using singly dimensioned arrays wherever possible as well as arrays of pointers to arrays.

A complete derivation of the gravity gradient torque resulting from a full (nxn) gravity model is given since it uses the second partial of the potential developed earlier.

A complete derivation of the geomagnetic field vector was included since the computa- tion of the magnetic field is so similar to that of the gravitational field.

Ada code for all of the algorithms is included.

Test cases compare the algorithms to each other and to previously published data.

January 1993 NASA CR- 188243

This report is basically a rewrite of Ref 121. with some useful additions. First of all, by examining the derived Legendre functions that are used to compute the gravitational acceleration it is noted that some of them are not functions of the state and hence may be computed only once. This fact is used to speed up the computation of gravity and its partials.

Secondly, a derivation using normallzed gravity coefficients and a superior recursion for- mula for the derived Legendre functions is presented. As the size of gravity models increases, an algorithm using normalized coefficients becomes more attractive since the unnormallzlng process requires the computation of terms on the order of 2n! Even for models of size 50x50 this would be a number so large ( - 10'~') that some computers might not be able to compute it.

Algorithms are developed. using both normalized and unnormalized gravity coefficients. that compute the first and second derivatives of the potential function. This yields the gravitational acceleration, and the partial derivative of the gravitational acceleration with respect to the position vector. The partial derivative of the gravitational acceleration is needed in the computation of the state transition matrix for both estimation and optimi- zation. In addition. the partial derivative matrk can 6e applied to the problem of com- puting general gravity gradient torque.

Next. a general gravity gradient torque derivation fs presented that uses the second par- tial of the potential developed in the previous section.

Since the geomagnetic field is defined in terms of Legendre functions, a derivation of the geomagnetic field is included which is very similar in form to the gravity derivation.

And finally. Ada code as tmplemented in the Ada Simulation Development System (ASDS). 191, is given for the various algorithms in addition to test cases that verify the validity of the derivations.

2 NASA CR- 188243 January 1993

3.0 The Gravitational Potential Functfog

The gravitational potential function is normally written

where p is the gravitational constant, ae is the equatorial radius. r is the magnitude of the position vector. 8 = (x, , x2, x3) , and

are the associated Legendre functions and Pn are the Legendre polynomials. Also. we have the sine of the latitude

E = x3/r

and the longitude is computed from

For notational convention. we define a potentfal function U to be

Ut -V and write eq(2- 1) as

Given the equation for E above, P, , becomes

where

The are known as derived Legendre functions.

Note that

January 1993 NASA CR- 188243 3

Also define.

The C,, and the S,, are the unnormalized cosine and slne gravity coefficients that result from the mass distribution of the planet. When these coefficients are published, they are published in normalized form. The relationship between the normalized and unnormalized form is given by

where

(n - m)! ( 2 n + 1) (2- ti,,) N ( n m) = ( n + m)!

Where ti,, is 1.0 if m = 0. and is zero othenvise.

The derivation will proceed using unnormalized notation because the derivation is some- what simpler. In a later section, a derivation using normalized coefficients will be devel- oped. The normalized form does not require the computation of tenns on the order of (n+m)! This may be desirable on some computers where very large or small numbers may cause a problem. For now, the potential can be written

This form is especially useful since c. C,. and S, can be calculated recursively and the singularity at the pole (p = 0 ) can be avoided.

The unnormallzed derived Legendre functions may be calculated recursively a number of different ways. In 111 and [2] the were computed from

4 NASA CR- 188243 January 1993

In 131. seven recursion algorithms were compared numerically for stability. Unfortu- nately. eq(3- 13) was not among those studied. Of the seven algorithms studied, two were clearly superior. The simpler of these is

P;: = ( (2n - I ) E ~ - , - (n+m- 1)P;:-,)/(n- m), (m

- The gravitational acceleration vector. g. is calculated as the first partial derivative of U with respect to the planet-fixed vector. z. kom eq(3- 12). we have

We note that

ap ax Now. using the definitions of - and - given earlier. ax ax

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