## Dc

Routine Function

RECUR Given a user-supplied function. RECUR performs a standard recursive estimation to process a single observation and update the estimate of a state vector to provide a least-squares fit to a set of observations.

Integration of Differential Equations

RUNGE General Runge-Kutta integrator, to be-used with a user-supplied routine for computing derivatives.

203.2 Time-Conversion Routines Routine Function

JD Converts year, month, and day to Julian date, using the algorithm of Fliegel and Van Flandern [1968],

DATE Converts Julian date to year, month, and day. JD and DATE

together provide the basis for all calendar time conversions.

TCON40 Converts time in the format YYMMDD.HHMMSS to seconds from 0 hours UT, September 1, 1957.

TCON20 Converts time in seconds from 0 hours UT. September I. 1957, to the form: YYMMDD.HHMMSS. TCON20 and TCON40 together provide examples of how any general time conversion can be performed easily using JD and DATE.

2033 Ephemeris Routines

Routine

Function

### Analytic Ephemeris Utilities

ELEM Converts position, velocity, and gravitational constant of the central body into classical Keplerian elements. Handles hyperbolic, parabolic, circular, or elliptical orbits.

d)RBGEN Two-body orbit generator that computes position and velocity given Keplerian elements, time from epoch, and gravitational constant of the central body. Useful for Earth or Moon orbits.

RAGREN Computes the right ascension of the Greenwich meridian ( = sidereal time at Greenwich, see Appendix J) in degrees. Uses a first-order method accurate to 0.01 deg for times from 1900 to 2100 A.D.

EQU1N Rotates coordinates from mean equator and equinox of time 1 to mean equator and equinox of time 2, using a first-order method accurate to 0.01 deg for time periods of 50 years or less.

Routine

### Function

MAGFLD Set of routines that compute the Earth's magnetic field vector at any desired time and position according to the International Geomagnetic Reference Field described in Appendix H.

SUNIX Computes the position of the Sun using a rapid analytical technique accurate to 0.012-deg arc length over the period 1971 to 1981. (The epoch date of the parameters is 1900; thus we anticipate that the accuracy should remain close to this limit for times beyond 1981.)

SMPOS Computes positions of the Sun and the Moon using an analytic technique which includes 21 perturbation terms for the Moon and 2 perturbation terms for the Sun. It is accurate to within 0.25-deg arc length for the Moon and 0.012-deg arc length for the Sun over the period 1971 to 1981. (The epoch date of the parameters is 1900; thus we anticipate that the accuracy should remain close to these limits for times beyond 1981.)

PLANET Computes positions of all nine planets using a two-body heliocentric orbit generator. Accurate to 0.02-deg arc length for times within ±2 years of the epoch. Elements and epoch time may be updated periodically using values from the American Ephemeris and Nautical Almanac.

Ephemeris Utilities Which Read Data Sets