SGP4PropWrappers v1.0 C++ wrapper classes to encapsulate the Astrodynamics Standards (AstroStds) libraries

As a extension to the author's AA+ class framework for Computational Astronomy, I was looking for open source code which would calculate the position of Artificial Satellites. What I found is that these algorithms originated in 1970 and was called Simplified General Perturbation 4 (SGP4). For the background to this algorithm, please see https://en.wikipedia.org/wiki/Simplified_perturbations_models. In 1980 this algorithm was made publically available with the publication of Spacetrack Report No. 3. This original code was implemented in Fortran. Since then there has been a number of implementations and re-implementations of SGP4 made available. Open source and commercial implementations include https://github.com/dnwrnr/sgp4, http://www.zeptomoby.com/satellites/trackLibrary.htm, https://github.com/csete/gpredict, https://www.chiandh.eu/comp/soft.html, https://ktrack.sourceforge.net, https://www.qsl.net/kd2bd/predict.html, https://github.com/KJ7LNW/sgp4sdp4, http://celestrak.org/software/tskelso-sw.php and https://github.com/Bill-Gray/sat_code. The original SGP4 implementation is now maintained by US Space Command as the Astrodynamics Standards (AstroStds) libraries. These libraries are export controlled requiring a license but a subset is freely available (in binary form) and redistributable. Included in AstroStds is C wrapper code to use it, along with support for other computer languages. SGP4PropWrappers provide a better (IMHO) C++ wrapping for the AstroStds and includes a fully reworked application to calculate a complete topocentric ephemeris for satellite passes such as the ISS.

The complete list of classes which SGP4PropWrappers provides are:

• SGP4::CDLLLoadError which represents an exception which is thrown when loading an AstroStds library fails.
• SGP4::CGetFunctionError which represents an exception which is thrown when locating functions in an AstroStds library fails.
• SGP4::RETVAL is an enum class which represents the return value from AstroStds methods / functions.
• SGP4::CDllMain encapsulates the functions in the DllMain library.
• SGP4::CEnvConst encapsulates the functions in the EnvConst library.
• SGP4::CTimeFunc encapsulates the functions in the TimeFunc library.
• SGP4::CXAKep provides a C++ class encapsulation of the "XA_KEP_*" Named Constants array type as used by the CAstroFunc and CSGP4Prop classes.
• SGP4::CXAEqnx provides a C++ class encapsulation of the "XA_EQNX_*" Named Constants array type as used by the CAstroFunc class.
• SGP4::CXAClassical provides a C++ class encapsulation of the "XA_CLS_*" Named Constants array type as used by the CAstroFunc class.
• SGP4::CXATopo provides a C++ class encapsulation of the "XA_TOPO_*" Named Constants array type as used by the CAstroFunc::ECIToTopoComps method.
• SGP4::CAstroFunc encapsulates the functions in the AstroFunc library.
• SGP4::CXATle provides a C++ class encapsulation of the "XA_TLE_*" Named Constants array type as used by the CTLE and CSGP4Prop classes.
• SGP4::CXSTle provides a C++ class encapsulation of the "XS_TLE_*" Named Constants array type as used by the CTLE class.
• SGP4::CTle encapsulates the functions in the TLE library.
• SGP4::CXASGP4Out provides a C++ class encapsulation of the "XA_SGP4OUT_*" Named Constants array type as used by the CSGP4Prop::Sgp4PropAll method.
• SGP4::CSGP4Prop encapsulates the functions in the SGP4Prop library.

Features

• Provides a C++ wrapper for v9.3 of the most important non export controlled Astrodynamics Standards (AstroStds) libraries.
• All the various wrapper functions / methods are C++ "const correct" unlike the C wrappers provided with AstroStds.
• All the classes and functions are in a C++ namepace called "SGP4".
• Provides a reimplementation of the Sgp4Prop sample application provided with AstroStds using modern C++.
• The sample application has been extended to generate topocentric satellite ephemeris information. This includes Right Ascension, Declination, Azimuth, Altitude, Range, Right Ascension rate, Declination rate, Azimuth rate, Altitude rate, Range rate, Is the satellite sunlit and is the observer sunlit.
• The sample application allows only passes to be logged when the elevation is positive.
• The sample application allows only passes to be logged where the observer is not in daylight. These additions allow a more useful ephemeris to be generated to help spot satellites such as the International Space Station by yourself.
• A full time constants file generated from the author's AA+ library (tme_constants.txt) is included in the download covering the date range 1956 to 2030. This includes data to allow accurate conversion to and from the following time scales: UTC, UT1, TDT/TT/ET & TAI. A time constants file is required for the SGP4Prop sample application to calculate accurate UT1 values for calculating Greenwich Sidereal Time and ET values for calculating sunlit conditions for the satellite being observed and the observer's location. If you do not use a time constants file during ephemeris generation using the sample application then AstroStds assumes that all time scales are the same which can results in errors upwards of a minute using modern date values.
• The start date, stop date and step size for the ephemeris generated by the sample app can be provided via command line parameters in addition to "6P" cards.
• Will compile on Windows, Linux and Mac as per the AstroStds libraries themselves.
• Classes have no external dependencies on other libraries such as ATL or MFC.
• Hides the details of the AstroStds libraries not working well with C style null terminated strings.
• Makes it easier to deal with the various array types used by the AstroStds libraries.
• Provides methods to control loading the AstroStd libraries from user supplied directories. These methods use exceptions to report loading errors.
• Includes code in the sample application to fully unit test the various wrapper classes and code.

Here is the snippet of the topocentric ephemeris generated by SGP4PropWrappers for the International Space Station for a pass over South East Ireland in late April 2024:

HQ SpOC Sgp4Prop - Version: v9.3 - Build: Jan 19 2024 - Platform: Windows 64-bit - Compiler: OneAPI ifort

EPHEMERIS GENERATED BY SGP4 USING THE WGS-72 EARTH MODEL
COORDINATE FRAME=TRUE EQUATOR AND MEAN EQUINOX OF EPOCH
USING THE FK5 MEAN OF J2000 TIME AND REFERENCE FRAME

Input File = D:\Dev\My Code\SGP4PropWrappers\input\ISS.inp
Observer Geodetic Latitude = 52.500000 Degrees
Observer Geodetic Longitude = -6.560000 Degrees
Observer Height above WGS-72 ellipsoid = 0.000000 KM
Start Time = 0.000000 minute(s) from epoch
Stop Time = 14400.000000 minute(s) from epoch
Step size = 0.500000 minute(s)
Positive satellite elevation passes only
Observer is in darkness satellite passes only

YYYYMMMDDHHMMSS.SSS TSINCE (MIN) RA (DEG) DEC (DEG) AZ (DEG) EL(DEG) RANGE (KM) RADOT (DEG/S) DECDOT (DEG/S) AZDOT (DEG/S) ELDOT (DEG/S) RANGEDOT (KM/S) SUNLIT(S) SUNLIT(O)

1 25544U 98067A 24110.51708638 +.00026201 00000 0 45835-3 0 0999
2 25544 51.6390 246.3171 0004667 76.3880 41.5560 15.5043548044944
Epoch of TLE = 2024Apr19122436.263 UTC

2024Apr20050636.263 1002.0000000 263.8471053 -35.8157907 192.1381634 0.7167164 2257.6378494 0.1508781 0.0332968 -0.1122207 0.0516575 -5.5288317 1 0
2024Apr20050706.263 1002.5000000 268.6443757 -34.7418358 188.5011614 2.2908451 2095.7244639 0.1693867 0.0385978 -0.1308174 0.0532333 -5.2541820 1 0
2024Apr20050736.263 1003.0000000 274.0365520 -33.4794562 184.2526499 3.9056006 1943.1466507 0.1905052 0.0459481 -0.1530533 0.0542920 -4.9034810 1 0
2024Apr20050806.263 1003.5000000 280.0963057 -31.9592510 179.2791772 5.5375108 1802.4691882 0.2137608 0.0558644 -0.1791483 0.0542570 -4.4573891 1 0
2024Apr20050836.263 1004.0000000 286.8718223 -30.0986532 173.4686832 7.1413797 1676.8678080 0.2379451 0.0686574 -0.2087120 0.0522501 -3.8949478 1 0
2024Apr20050906.263 1004.5000000 294.3608279 -27.8136410 166.7367134 8.6413165 1570.1236978 0.2608787 0.0840313 -0.2401913 0.0471247 -3.1977312 1 0
2024Apr20050936.263 1005.0000000 302.4813237 -25.0446766 159.0701720 9.9262181 1486.4398517 0.2794799 0.1005680 -0.2703116 0.0377595 -2.3576535 1 0
2024Apr20051006.263 1005.5000000 311.0527203 -21.7956492 150.5813912 10.8596468 1429.9762104 0.2904242 0.1154679 -0.2940758 0.0237246 -1.3874275 1 0
2024Apr20051036.263 1006.0000000 319.8059362 -18.1690458 141.5439621 11.3128137 1404.0927311 0.2913590 0.1251535 -0.3060810 0.0060628 -0.3282679 1 0
2024Apr20051106.263 1006.5000000 328.4306251 -14.3670603 132.3682707 11.2132120 1410.5105548 0.2820102 0.1268653 -0.3030632 -0.0125983 0.7536606 1 0
2024Apr20051136.263 1007.0000000 336.6427157 -10.6418920 123.5032815 10.5785167 1448.8018997 0.2643229 0.1201912 -0.2858646 -0.0291362 1.7850979 1 0
2024Apr20051206.263 1007.5000000 344.2388014 -7.2197381 115.3144437 9.5077615 1516.5230301 0.2415148 0.1071410 -0.2588978 -0.0414611 2.7082017 1 0
2024Apr20051236.263 1008.0000000 351.1148888 -4.2461228 108.0116600 8.1375446 1609.9041615 0.2168067 0.0908085 -0.2276652 -0.0491563 3.4932445 1 0
2024Apr20051306.263 1008.5000000 357.2526508 -1.7763232 101.6525535 6.5969326 1724.6976524 0.1926099 0.0739323 -0.1965672 -0.0530070 4.1368554 1 0
2024Apr20051336.263 1009.0000000 2.6910283 0.2020290 96.1901894 4.9836755 1856.8313164 0.1703418 0.0582472 -0.1681636 -0.0541959 4.6522917 1 0
2024Apr20051406.263 1009.5000000 7.4987622 1.7385001 91.5252184 3.3611127 2002.7540344 0.1506184 0.0445426 -0.1434837 -0.0537798 5.0596567 1 0
2024Apr20051436.263 1010.0000000 11.7546872 2.8961481 87.5428329 1.7652615 2159.5367042 0.1335429 0.0329858 -0.1226252 -0.0525160 5.3795738 1 0
2024Apr20051506.263 1010.5000000 15.5361488 3.7376333 84.1329366 0.2137822 2324.8363654 0.1189460 0.0234250 -0.1052407 -0.0508820 5.6302159 1 0

From the above, we can see that there will be a pass of the ISS on April 20th 2024 from 05:06 UTC to 05:15 UTC. At 5:10 UTC or 6:10 lrish Summer Time, this pass will reach a max elevation of just over 11 degrees, at a bearing of 141 Degrees (or roughly South East) and a range of 1404 KM. Please note that all date times reported in the ephemeris files are UTC.

The enclosed zip file contains the wrapper class source code and a simple console application which exercises the classes functionality.

Copyright

• You are allowed to include the source code in any product (commercial, shareware, freeware or otherwise) when your product is released in binary form.
• You are allowed to modify the source code in any way you want except you cannot modify the copyright details at the top of each module.
• If you want to distribute source code with your application, then you are only allowed to distribute versions released by the author. This is to maintain a single distribution point for the source code.

Updates

1.0 (20 April 2024)

• Initial public release