Akademisk Radioklubb


Author: LA9SSA

Unified software rotor control over the local network

ARK has recently collected all rotor controllers on a single Raspberry Pi-device and made these available for software control from all Linux machines on the local network. We’ve also enabled rotor control from N1MM on Windows. This post outlines how we did it.

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Introduction to the libpredict API

libpredict is an ANSI C library for predicting satellite orbits based on TLEs, developed by ARK. This was primarily developed for use in flyby, but can also be useful on its own. If you just want to track a satellite, flyby is usually a better choice, but if you want to go down to a deeper level and be able to apply satellite prediction to more advanced and complex usecases in a more flexible way, libpredict might be suitable.

The goal of libpredict was mainly to separate the satellite calculations from predict for use in its fork, flyby, and enable reuse of the API in other satellite applications. C implementation became a requirement due to the well-defined binary compatibility for C libraries and the use of C in both predict and flyby. While the core routines are in C, we will also at some point be providing high-level bindings for other languages like python. See also: Development of flyby and libpredict.

This post outlines in detail how libpredict can be used to track satellites in a programming language, and is long and technical and probably mostly for those with special interest in the topic. If life gets too frustrating and boring, you can scroll down to the plots and rest your eyes on colorful satellite tracks:-). An earlier post, Satellite tracking using flyby, gives a top-down motivation for why we are doing this at all and a more user-friendly approach to satellite tracking.

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Satellite tracking using flyby

A lot of satellites typically have beacons, interesting data transmitters or transponders that can be received or used by radio amateurs. Knowing when and where the satellites are passing overhead is essential. Flyby, through its companion library libpredict, is a software package for estimating such satellite positions. See also: Development of flyby and libpredict.

Exact calculation of the position of the satellites in complicated gravitational fields is very difficult, so simplified perturbation models are generally used. The exact position and other parameters of the satellite is measured (typically by NORAD) at a given time (epoch time). These properties can then be estimated at other times by using a simple gravity model for earth, and including the more complicated gravitational effects from earth and deeper space using perturbation theory. Given updated parameters and “small” time differences from the epoch time (less than a couple of weeks is probably good enough), we can generally predict the position of the satellite to a high enough accuracy that we can point our antennas towards the predicted position and expect the satellite to be there. The set of measured parameters (after being crunched through a related model) is called a TLE, while the prediction model is called SGP4/SDP4. The details on this are not really that important, you can generally assume that a TLE corresponds to a satellite, and that it can be used to calculate a time-dependent position using the appropriate prediction software.

The satellite is speeding overhead while we are approximately standing at rest. The electromagnetic waves transmitted by the satellite will therefore be doppler shifted when we receive them. The satellite transmits at a specific frequency known a priori, but the received frequency is shifted significantly and will change continuously as the satellite passes overhead. The satellite will experience the same thing with respect to our transmitted frequency if we are transmitting anything towards it. Without proper correction of our transmitted signal, the satellite might not even receive anything within its fixed frequency range. In addition to pointing the antenna in the right direction, it is therefore important to also adjust the frequencies correctly. Flyby does both, using the satellite prediction library libpredict, and can automatically communicate the frequency and position changes to a rig and antenna controller using hamlib.

This post will outline the steps involved from compiling Flyby to tracking satellites. Continue reading

Development of flyby and libpredict

ARK has been developing satellite tracking software based on a fork of predict-g1yyh, which is in turn a patched version of predict. We have long been using predict for TLE-based satellite tracking, but in order to control our radios and antenna rotors, we have also been forced to patch predict with hamlib support. In the end, fueled by annoyances over minor or major bugs often due to a slightly insane codebase, our need for changes outweighted our patience for trying to push patches upstream. We therefore decided to fork into a new software package called flyby.

The goal of flyby has been to improve the original code base of predict, while retaining most of the features and feel of the original application. On the way, we decided that the best way to do this was to split all functionality for satellite calculations into a separate C library, libpredict. This has also enabled us to reuse the pure satellite tracking in other applications in a sensible and easy way.

While some remnants of code dating back to a FORTRAN implementation of SGP4 and SDP4 from the seventies and some associated weirdness are still present under the hood, libpredict defines what we believe is a sensible public API with well-defined data structures for doing satellite calculations and extracting information from TLEs. Among the included features are prediction of both absolute (WGS84) and relative (azimuth, elevation) coordinates, along with other relevant properties for a given TLE at a given time. It can also predict the arrival time (AOS) for a satellite, and the corresponding departure time (LOS), and calculate the doppler shifts throughout a pass. The use of ANSI C makes binary compatibility well-defined. Bindings to other programming languages can also easily be made. Version 1.0 has been released, though flyby currently uses the latest development version planned for release 2.0. This version will be released once we freeze some fluctuating parts of the API. We are also planning to possibly upgrade the internal calculation algorithms to the most recent version of the reference implementation of SGP4, but this will be done in a backwards-compatible way. We have, however, been doing our own cleanups of the C version of the original SGP4/SDP4 code to make it more robust, and the expected improvements in numerical accuracy are probably minor at best.

With libpredict in place, along with numerical tests to check the correctness of its prediction results, we were able to remove a large bulk of code from flyby and replace it with more readable calls to libpredict. Redundant functionality was removed, like the network socket for providing satellite calculations (as this now is available for any application through an inclusion of libpredict), and the TLE-editor, so that you don’t have to destroy your TLEs when accidentally entering the wrong submenu, like shown for the unfortunate user below.

Removing unnecessary code made cleaning up the rest of the application more manageable. We believe we have been able to make the code more robust against accidental errors, race conditions and general weirdness, and made it more readable and easier to add new, advanced features or fix new and old bugs. Some unavoidable insanity still remains, but the larger bulk should hopefully have less insane characteristics.

Most of this, except for the removal of certain features, is generally unnoticable by the end user, however. But we have not only made the code more robust. We have also implemented new features!:-) At first glance on some of the screens, flyby looks very much like predict-g1yyh with a different colour scheme, but it goes deeper than that.

  • Main menu: We have replaced the static main menu with a straight jump to the multitrack view over the current state of all displayed satellites, from where further options can be selected (see screenshot at the start of this post).
  • Multitrack view: We have made it possible to show more satellites than the number of lines available in the terminal :-). Search features and submenus for tracking options are also implemented.
  • The fields in the QTH editor now behave as actual fields, and you don’t have to be afraid of destroying your QTH coordinates when you accidentally enter the editor.
  • Satellites in the TLE database can be disabled/enabled for display, when you have too many TLEs in your TLE database or you are interested in only a few specific satellites.
  • Flyby now follows the XDG base directory standard for its configuration files, TLE files and the transponder database.
  • As a consequence of the above, TLE files are not limited to the single TLE file defined in ~/.predict/predict.tle, but merges all TLE files defined in ~/.local/share/flyby/tles/, /usr/local/share/flyby/tles/ and /usr/share/flyby/tles/. Multiply defined TLEs have a precedence ordering where the user-defined TLEs take precedence over any system-wide TLEs (and/or the most recent TLE). This can be useful for TLE updating and providing defaults, and also makes it easier to compile TLEs from several sources. It is no longer necessary to merge TLE-files manually into a single file. (The ease of adding a lot of TLEs also necessitates the earlier mentioned whitelisting feature :-).)
  • As previously mentioned, flyby supports rotor and rig control through hamlib using rotctld and rigctld in single track mode, which provides an easy way to support a wide range of rigs and rotor controllers.
  • A transponder database editor has been implemented, so that you don’t have to edit its weird format manually anymore.
  • Option parsing is now done using GNU longopts, and the arguments have been revised and (hopefully) better documented in --help.

Formats of all options files and database files are backwards-compatible with the file formats used in predict.

More, minor features will also be added, like improving the whitelisting options for more intuitive ease of use, apply layout improvements and remove the size limit on the number of satellites. Layout-wise, there does not have to be limits on the number of TLEs anymore, whereas original predict was limited to the number of letters in the alphabet due to the shortcuts for selecting TLEs, and also had limitations on the multitrack view.

Finally: Why still use ncurses? Well, why not?:-P While it seems old-fashioned, and sometimes/often is far more horrendous to program than just using a normal GUI toolkit built for the purpose, it is lightweight and doesn’t need the availability of Xorg. This can be nice when running remotely. Some potential for nice visualization of the satellite tracks is lost, but the display can in some ways be made to be as elegant as or more elegant than a normal GUI application. Maneuvering can be more convenient when using only a keyboard and sensible shortcuts, and it’s always handy to just apply CTRL + C to immediately jump out of the program, like for other CLI applications.

We will be publishing a series of blog posts detailing the usage of flyby and libpredict.