μephem

Overview

μephem is a tiny ephemeris utility that generates orbital state vectors for the major bodies in our solar system. The μephem source is a single C-file with a few hundred lines of code, making it easily ported to other languages or made to run on embedded systems.

Usage

uephem <file> <item> <t0> [<t1> <resolution>]

• file must be a JPL Planetary and Lunar Ephemerides binary ephemeris file. For an overview of the available ephemerides, see https://ssd.jpl.nasa.gov/planets/eph_export.html. Little-endian ephemeris files can be downloaded from ftp://ssd.jpl.nasa.gov/pub/eph/planets/Linux. Big-endian ephemeris files can be downloaded from ftp://ssd.jpl.nasa.gov/pub/eph/planets/SunOS. μephem is capable of reading both little and big-endian files, regardless of the host endianness. .bsp ephemeris files are not supported.
• item is the ID number of an ephemeris item to evaluate. See the table below for more details.
• t0 and t1 are the start and end Julian dates. e.g. 2451545.0 corresponds to the J2000.0 epoch.
• resolution is the number of time points to evaluate between t0 and t1.

Items

ID	Name	Output	Units	Center
0	Mercury	x,y,z,vx,vy,vz	km,km/d	SSB
1	Venus	x,y,z,vx,vy,vz	km,km/d	SSB
2	Earth-Moon barycenter	x,y,z,vx,vy,vz	km,km/d	SSB
3	Mars	x,y,z,vx,vy,vz	km,km/d	SSB
4	Jupiter	x,y,z,vx,vy,vz	km,km/d	SSB
5	Saturn	x,y,z,vx,vy,vz	km,km/d	SSB
6	Uranus	x,y,z,vx,vy,vz	km,km/d	SSB
7	Neptune	x,y,z,vx,vy,vz	km,km/d	SSB
8	Pluto	x,y,z,vx,vy,vz	km,km/d	SSB
9	Moon	x,y,z,vx,vy,vz	km,km/d	Earth
10	Sun	x,y,z,vx,vy,vz	km,km/d	SSB
11	Earth nutation	d(psi),d(epsilon),vd(psi),vd(epislon)	rad,rad/d	Earth
12	Lunar mantle libration	phi,theta,psi,vphi,vtheta,vpsi	rad,rad/d	Moon
13	Lunar mantle angular velocity	omega_x,omega_y,omega_z	rad/d	Moon
14	TT-TDB	t	s	Earth

Note: Items 11-14 may not be present in some ephemeris files.

Configuration & Building

CMake is required to configure and build the application.

Run the following commands to configure and build a release executable:

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build

Ephemeris File Format

JPL ephemeris binary files consist of a series fixed-size records. Record size can vary between ephemeris files. A file's record size can be inferred through its file size along with the time information given in its header.

The first record in a file contains the header, the structure of which is described below:

typedef struct
{
	// Comment text.
	char comments[3][84];
	
	// First set of constant name labels.
	char cnames1[400][6];
	
	// JD start, JD end, record duration.
	double time[3];
	
	// Number of constants in the second record.
	int32_t nconst;
	
	// Constant values AU and EMRAT.
	double constants[2];
	
	// Record offset, coeffs per component, and coeff sets per record, for items 0-11.
	int32_t table1[12][3];
	
	// DE version number.
	int32_t denum;
	
	// Continuation of table1 for item 12.
	int32_t table2[3];
	
	// Second set of constant name labels.
	char cnames2[nconst - 400][6];
	
	// Continuation of table2 for items 13 and 14.
	int32_t table3[2][3];
	
} de_header;

Note: Some ephemeris files store additional constant names in the header between table2 and table3 if the limit of 400 constant names has been reached.

The second record is simply an array of double values for the constants named in the header. Each of the remaining records contain two double values: the Julian date start and end times of the record, followed by an array of double Chebyshev polynomial coefficients for each item in the ephemeris.

License

μephem is licensed under the MIT license. See LICENSE.md for details.