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IDL JPL Ephemeris and Solar System Timing

JPLEPHEM - Read and Interpolate JPL Planetary Ephemeris

This package provides routines to open, read, and interpolate the JPL Planetary Ephemeris in FITS format. The JPL ephemerides provide the positions and motions of the major planetary bodies in the solar system, including the earth, moon and sun, to very high precision. The JPL ephemerides are provided as blocks of Chebyshev coefficients, which, when interpolated, reproduce the original JPL numerical integrations within 1.5 cm.

This package has been tested and verified using the JPL planetary ephemerides DE200 and DE405. The raw files provided by JPL must be converted to FITS format using the BINEPH2FITS utility found in the AXBARY package. For convenience, FITS format ephemerides for the years 1950-2050 are provided by FTP. Users must also have the IDL Astronomy Library installed in order to access the FITS files.

Using this package is rather straightforward. There are essentially two steps involved. First, the ephemeris file is opened and read with JPLEPHREAD; second, the table is interpolated to the desired date and time. Here is an example of finding the position of the earth on New Year's Day, 2000:

IDL>   JPLEPHREAD, 'JPLEPH.200', pinfo, pdata, [2451544D, 2451545D]
IDL>   JPLEPHINTERP, pinfo, pdata, 2451544.5D, xearth, yearth, zearth, $
                 /EARTH, posunits='AU'

Here, the requested date was JD 2451544.5, the julian date of the new year. The cartesian position of the planet earth is returned in xearth, yearth and zearth, in astronomical units. Users can also request other units, and body velocities are also available. By default the origin of coordinates is the solar system barycenter, but any body can be chosen for the origin.

The procedure JPLEPHTEST is provided to test the ephemeris interpolation routines based on test data sets provided by JPL. The new procedure JPLEPHMAKE can be used to generate new ephemeris tables, based on tabulated positions and velocities, for use with JPLEPHINTERP.

Dec 21 201114 kb  
Oct 02 201226 kb  
Jan 30 20055 kb jplephtest.prooptional  
Jun 03 20028 kb jplephmake.proCreate new ephemerides  

Full package -- I am also making a full package available, which contains programs, FITS-format DE200 and DE405 ephemerides from 1950-2050, and test data provided by JPL. Beware, these files are more than 16 MB each!

May 19 201616504 kb  
May 19 201616491 kb cmephem.tar.gz  

Other resources

   Markwardt HTTP Site for JPL Ephemerides in FITS format
      JPLEPH.200 - JPL-DE200 (older but very well known)
      JPLEPH.405 - JPL-DE405 (more recent and precise)
      testpo.200 - Test data for DE200 (use with JPLEPHTEST)
      testpo.405 - Test data for DE405 (use with JPLEPHTEST)

   AXBARY, barycentering and timing for X-ray Astronomy, by Arnold Rots.

   HORIZONS, JPL Web-based ephermis calculator (Ephemeris DE406)

GEOGRAV - Estimate gravitational (geo) potentials

The routines GEOGREAD and GEOGRAV read gravitational potential model files and interpolate them to the desired position.

GEOGREAD is used to read a geopotential file into memory. The user must have a description file which contains a description of the model in IDL syntax. The format of this file is described in the documentation of GEOGREAD. The actual model coefficients must be in a file of the same root name. This routine is designed to read coefficients from standard model files available on the internet; the description file contains entries which describe which character columns from the input file contain the appropriate data.

Once the data have been read into memory, the geopotential can be expanded to any position outside of the body using GEOGRAD. Both the gravitational potential and the acceleration are computed at the desired positions. Users can limit the calculation to lower degree or order than is available. Several choices of units are also available.

Below are the routines and description files for several popular gravitational models. Within the description file is the URL where the original data can be downloaded (I can also supply the models upon request).

May 24 201210 kb  
Sep 26 20048 kb  
Jan 05 20041 kb egm96.descModel descriptions  
Jan 05 20041 kb gemt2.descModel descriptions  
Jan 05 20041 kb jgm3.descModel descriptions  

NUTATE - Compute high precision earth precession, nutation and orientations

The procedure HPRNUTANG computes values of the earth orientation-related angles, including precession and nutation, which are used for high precision earth-based astronomy applications.

It is the goal of this procedure to provide all angles relevant in determining the position of an earth station, as measured in an earth-fixed coordinate system, and converting to space-fixed coordinates. This is useful in applications where observations by a station in the earth-fixed frame are taken of an astrophysical object which is in the non-rotating space-fixed frame.

The procedure EOPDATA reads, interpolates and returns Earth orientation parameters used for precision earth-base astronomy applications. The values returned are PMX and PMY (series for polar motion); UT1-UTC (series for earth rotation); and DEPS and DPSI (corrections to the standard IAU 1980 Earth nutation model). These quantities are available from the International Earth Rotation Service web page.

The procedure HPRSTATN computes the coordinates and velocities of an earth station, whose position is known in the ITRF, in J2000 equatorial earth-centered inertial coordinates (ECI). This may be useful in any application where an earthbound observatory is used to collect data on a non-terrestrial phenomenon.

NOTE: The user is responsible for downloading and maintaining an up-to-date file of earth orientation parameters from the International Earth Rotation Service. These can be found at one of the IERS web pages:

This interfaces are somewhat provisional. See OPEN QUESTIONS in the documentation. Comments are invited.

May 19 201632 kb  
Nov 18 20108 kb  
May 19 201613 kb  

SRVADD - Add vector velocities according to special relativity

The function SRVADD performs addition of velocity 3-vectors according to special relativity. SRVADD operates in two inertial frames, a "lab" or stationary frame, and a "rocket" or moving frame (moving at vector velocity V with respect to the lab). Another body is moving with vector velocity U1 as measured from the rocket frame. The function SRVADD computes the vector velocity of the body as seen from the lab frame.

This routine handles arbitrary 3-vector velocities. None of the velocities are constrained to lie along the X-axis.

Jul 29 20025 kb  

SRVDOPP - Compute relativistic doppler effect

The function SRVDOPP computes the relativistic doppler shift between two inertial reference frames. SRVDOPP computes the doppler shift of a photon as observed in the "rocket" frame, whose direction and frequency are known in the "lab" frame. This routine handles arbitrary 3-vector velocities and photon propagation directions, and thus by definition the transverse doppler effect.

Jul 29 20024 kb  

TDB2TDT - Relativistic Clock Corrections in Solar System

The function TDB2TDT computes relativistic corrections that must be applied when performing high precision absolute timing in the solar system.

According to general relativity, moving clocks, and clocks at different gravitational potentials, will run at different rates with respect to each other. Thus, for the most demanding astrophysical timing applications, times in the accelerating earth observer's frame must be corrected to an inertial frame, such as the solar system barycenter. This function provides the most important correction term: from clocks at the geocenter (TT) to the solar system barycenter (TDB), hence the name TDB2TT. The user is responsible for the observatory-dependent components, described in the documentation.

The method of computation of TDB2TDT in this function is based on the analytical formulation by Fairhead, Bretagnon & Lestrade, 1988 (so-called FBL model) and Fairhead & Bretagnon 1990, in terms of sinusoids of various amplitudes. TDB2TDT has a dominant periodic component of period 1 year and amplitude 1.7 ms. The set of 791 coefficients used here were drawn from the Princeton pulsar timing program TEMPO version 11.005 (Taylor & Weisberg 1989).

May 19 201648 kb  

TAI_UTC - Compute (TAI - UTC) time difference (Leap Seconds)

The function TAI_UTC computes the difference between International Atomic Time (TAI) and Universal Coordinated Time (UTC), in seconds.

After 01 Jan 1972, the two time systems are synchronized, except for a number of leap seconds added to account for the varying rate of rotation of the earth. While TAI is a continuous atomic time system, UTC is a civil time system which may have discontinuities where leap seconds are introduced. This function computes the differences between the two time systems.

NOTE - the leap second file must be kept up to date as new leap seconds are introduced. Download it from the US Naval Observatory at

May 19 201613 kb  

TZOFFSET - Compute timezone offset from GMT for any date

The function TZOFFSET computes the time zone offset between the local time zone and GMT for any date.

The time zone offset is defined here as the number of seconds of time West of the Greenwich Meridian. Equivalently, it is the number of seconds that must be *added* to local time in order to transform it to GMT.

The user may input the date, T, as either seconds elapsed since 1970-01-01T00:00:00, or in Julian days (if /JULIAN is set). The input time may be either expressed in the user's local time zone (if /LOCAL is set) or in UTC.

The results of TZOFFSET are only as good as your operating system's timezone information. If your system's timezone tables are incomplete or erroneous, then so will be TZOFFSET's output.

TZOFFSET computes the timezone offsets for your system's current time-zone. To compute the offset for another different time zone, you will need to reset your system's notion of the timezone. On Unix and Mac OS X systems, this can be done by setting the "TZ" environment variable with SETENV.
May 19 201614 kb  

JBEPOCH - Compute Julian Day to/from Julian or Besselian Epoch

The function JBEPOCH computes the Julian or Besselian Epoch year number from a given Julian day number. Epochs of this form are often given in the astronomical literature as B1950.0 or J2000.0, but they can be different.

Besselian year numbers are measured in tropical years of about 365.2422 days. Julian year numbers are measured in years whose lengths are exactly 365.25 days of 86400 second lengths. The "/J" or "/B" keywords identify which year numbering system is being used.

JBEPOCH also computes the inverse transformation, from Julian or Besselian epoch to Julian Day, by specifying the /TO_DAY keyword.

Mar 22 20023 kb  

LITMSOL - Solve light time equation for solar system bodies

The procedures LITMSOL and LITMSOL2 solve the light time equation between two moving bodies in the solar system. Given the time and position of reception or transmission of a photon, this equation determines the time of transmission or reception at the other solar system body. Since both bodies may be moving, the equation must be solved iteratively.

LITMSOL (OBSOLETE) requires the trajectories of solar system bodies to be described by either a JPL ephemeris, or by a JPL-like ephemeris generated by JPLEPHMAKE. This routine calls JPLEPHINTERP.

LITMSOL2 is more flexible and allows the trajectory of the body to be described by any IDL function. The user is required to define this function, which could, for example, perform a lookup of a JPL-like trajectory ephemeris. LITMSOL2 also allows user-defined "extra" delays, which might include Shapiro delay or spacecraft transponder delays.

Nov 18 201012 kb litmsol2.proRecommended  
Nov 08 200812 kb  

Copyright © 1997-2010 Craig B. Markwardt
Last Modified on 2016-05-19 15:49:49 by Craig Markwardt