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This package contains a partial port to C of Dr TS. Kelso's PASCAL library
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sgp4-plb26a. This library contains files with functions for calculating the
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position and velocity of a satellite using the NORAD SGP4 and SDP4 routines
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and from these the azimuth, elevation, range and range rate of same. Routines
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are also included for calculating the position of the sun and the illumination
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status and eclipse depth of the satellite.
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The port to C includes most of the sources needed for making the above
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calculations and for reading and checking two-line Keplerian element sets
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(TLE's) from a file, as available from many sources on the Internet. Some of
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the files in the PASCAL library are PC-specific and have not been ported to C
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or they have been substituted or modified with some functions I developed
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myself. A simple main() function is included to demonstrate the use of the C
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sgp4/sdp4 library and functions for reading and testing a TLE set from a file.
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This package should contain the following files:
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2. The COPYING file with copyright details.
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3. sgp4sdp4.c: The source-code of the ported SGP4 and SDP4 routines and some
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extra functions for testing and setting flow control flags.
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4. sgp_in.c: Functions for reading a TLE set from a file and verifying,
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converting and pre-processing the Keplerian element set.
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5. sgp_math.c: Various mathematical functions used by other routines.
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6. sgp_obs.c: Functions for calculating the geodetic position of the observer
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and converting the ECI position and velocity of a satellite to
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observer-centered azimuth, elevation, range and range rate. Also
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calculating the observer-centered position of the sun.
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7. sgp_time.c: Functions needed to calculate and convert time in various
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formats, e.g. Julian, calender etc.
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8. solar.c: Functions for calculating the position of the sun.
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9. main.c: A basic main() function to demonstrate the use of the sgp4/sdp4
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library to read a TLE set and calculate and print satellite
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10. sgp4sdp4.h: A header file that should be #include'd in all source files
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11. Makefile: A simple makefile for building the 'ephem' binary.
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Run make in the sgp4sdp4 source directory to compile the binary executable.
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Run ephem to start the test program, but please note you need to include a
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file in the same directory including TLE sets in NASA format as available
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on the Internet. There are fresh sets uploaded regularly at Dr TS Kelso's
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site at www.celestrak.com . Please note that you should edit main.c to
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enter the directory path for this file, it is currently set at amateur.txt
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and to also enter the observer's geodetic coordinates (lat and lon in rads
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and altitude in km). This is only a skeleton test application!
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It appears that the original NORAD routines have been modified to
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correct some discrepancies in the calculated satellite position data,
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at least for some TLE sets of deep-space orbits. I have incorporated
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a fix for one bug at the end of the Lyddane modification in the
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'dpper' section of the Deep routine, as suggested by Rom Matson.
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There are also suggested fixes for the discrepancy in satellite
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position data at 0 epoch time, between SDP4 and SGP routines. I have
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not made any changes to the NORAD routines, as the fixes appear to be
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for newer, modified SGP4/SDP4 routines. I will leave this to Celestial
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Mechanics programming gurus to be safe!
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There may well be (even should be) some hidden bugs in the ported C
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source as I made a big effort to streamline the flow and structure of
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the FORTRAN routines, especially getting rid of all goto's. Although
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I have been careful with the changes, the TLE sets in NORAD's report
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do not result in program flow in all the branches of control statements
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so that its not possible to check the integrity of the C source in all
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circumstances. However I have made sure that the results produced by my
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C routines agree to the 5th or 6th digit of NORAD's examples.
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The most complicated part of the NORAD routines is the 'dpsec' part of
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the Deep routine where most of the changes to flow control and
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structure were made. This is were bugs affecting the deep-space
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routines may be hiding. I have conducted tests using KD2BD's PREDICT
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tracker using modified tle sets to force program flow through all the
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branches of if() statements and after fixing a number of bugs, I have
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close agreement with predict with a wide variety of orbital element sets.
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Still, there is no guarantee that some circumstances will not trigger a
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Using the sgp4sdp4 package:
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===========================
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In order to use the ephemeris functions in sgp4sdp4 you will need to
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#include the sgp4sdp4.h header in your own source files. There are
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comments in all files describing the various functions and there use.
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There are also comments in the main() function describing the steps
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needed to get results from the package.
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The routines that calculate satellite position (SGP4 and SDP4) all
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have the same interface as follows:
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(double tsince, tle_t *tle, vector_t *pos, vector_t *vel ).
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tsince is the time interval in minutes since epoch time.
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tle_t *tle is a pointer to a structure that holds the TLE set of
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the satellite. Please see norad.h for details.
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vector_t *pos, vector_t *vel are structures holding the calculated
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Cartesian position x, y, z and velocity xdot, ydot, zdot data
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for the satellite. Please see norad.h for details.
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Please note that all three arguments are safer declared static if
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they are declared outside the main() function.
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All the flags controlling program flow must be cleared before a new
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TLE set is used or when a different ephemeris function is called.
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This is done by using ClearFlags(ALL_FLAGS); See main.c for details.
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I have ported the sgp-plb Pascal routines to C so that I can use them
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in my satellite tracking projects. I am releasing this source-code in
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the hope that other developers can benefit, as I have benefitted from
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available open source software.
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I can make no guarantee explicit or implicit as to the integrity and
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usability of these software modules and I cannot be held responsible
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for any damage or loss incurred by using them. Use the material in
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this package at your own risk.
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Dr. TS. Kelso maintains an excellent web-site with a lot of information
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and software for tracking celestial objects. A pdf version of the
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NORAD Spacetrack report #3 is also included: http://celestrack.com/
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Version 0.1: Was the first relased after local bug-fixing.
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Version 0.2: Function Date_Time was re-written to include calculation
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of tm_wday and tm_isdst members in the struct tm type value it returns.
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Version 0.3: A suggested modification to the 4-quadrant AcTan() function
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was incorporated in order to make it agree with the original NORAD code.
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Thanks to Mr. Ed Savage for this.
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Neoklis Kyriazis <neoklis@my-deja.com> August 31 2001.
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src/sgpsdp/2_README
