ISS Transit Prediction Details


OVERVIEW

This program generates user specific International Space Station (ISS) Transit Predictions for the Sun, Moon and Planets.  The program can also generate predictions for a satellite specified within a user orbital TLE (Two Line Element) file.

The user enters location data, including latitude and longitude.  By default, the program downloads the latest orbital information (Two Line Elements) from the web. The program then generates a transit prediction text file which contains prediction details for each transit within a user specified alert radius.  This text file roughly follows the same format used by Thomas Fly and his ISS Transit Alert Service.

The program can also generate Google Earth and Google Map plot files.  The Google Earth plot file can be opened with the Google Earth program.

The Google Maps plot file utilizes the Google Maps API and can be displayed via the ISS Transit Prediction Google Maps web page.

PREDICTION DIRECTORY

The program requires the user to create and specify a directory on the user hard drive to contain files which are created and used by the program during the generation of the transit predictions. This Prediction Directory can be located anywhere on the user hard drive and contains the following types of files:

The program creates a sub-directory inside the Prediction Directory called "files.iss". The program saves various files here during the generation of the prediction. Included are the Two Line Element sets obtained by default from the web and the user-selectable prediction options.

An optional "TLE.iss" sub-directory holds TLE Support data used to convert Orbital Ephemeris State Vector data downloaded from NASA into Two Line Elements. This data is needed when NASA is selected as the TLE source. (see next section).

An optional "GeoidHeight.iss" sub-directory holds Geoid Height data (see next section).

An optional "DEM.iss" sub-directory holds Digital Elevation Model (DEM) data (see next section).

OPTIONAL DATA

The user can optionally download Geoid Height Data and Digital Elevation Model data which is used by the program to more accurately compute the transit prediction.  In the absence of this data, the program will use the user specified elevation to roughly compute the transit path.

The Geoid Height Data can be downloaded here.  The data is provided in a compressed (zip) format and should be unzipped into the Prediction Directory.  The zip file provides the "GeoidHeight.iss" directory and contents.  The amount of hard disk space required for this file is listed in the System Requirements section.

The DEM Data can be downloaded from this USGS web page.  The page displays a map of the world, where users can click on specific tiles to download the DEM data.  The world is organized into "tiles" of 40 degrees of longitude by 50 degrees of latitude.  Only the DEM Data which contains the user location is needed.  Typically this is only one file, but could be as many as 4 files, if the user is near (within 5 degrees of) a DEM file boundary.  Refer to the world map to determine which tiles are needed for your specific location.  For example, a user in Los Angeles, CA (118W, 34N) would need the DEM data in file W140N40.DEM.  However, a user in San Francisco, CA (122W, 38N) would need two tiles (W140N90 and W140N40).  The DEM data is provided in a compressed format.  The user needs to create a "DEM.iss" sub-directory in the Prediction Directory.  The user then downloads the compressed DEM data and extracts the *.dem file to the DEM.iss sub-directory, using a file extraction utility such as WinZip.  Make sure any "smart" file extraction options, such as "automatic CR/LF insert" are NOT checked.  Verify the file size of each DEM file; they should be exactly 57,000,000 bytes (56,250 KB).  Once the DEM file has been extracted, the original compressed data file can be deleted to save disk space.

The user can also optionally download TLE Support data which is used by the program to convert Orbital Ephemeris State Vectors from the NASA website to Two Line Elements used in generating the transit prediction. In the absence of this data, the user must select a TLE source other than NASA.

The TLE Support data can be downloaded here. The data is provided in a compressed (zip) format and should be unzipped into the Prediction Directory. The zip file provides the "TLE.iss" directory and contents. The amount of hard disk space required for this file is listed in the System Requirements section.


USER SPECIFIC DATA

The first time a user runs the program, they must manually enter in their specific user parameters.  Once entered, the parameters can be saved to a User Parameter File.  On subsequent invocations, the user can load a previously saved User Parameter File to recall their location specific data.

The following User Parameters are required before a prediction can be generated:


PREDICTION OPTIONS

The Prediction Options button brings up a window which allows the user to select the transit bodies for which predictions are generated from the following:
The default is to generate predictions for all transit bodies.

The windows also allows the user to select the minimum elevation angle for which transits predictions are generated.  The default value of 0 degrees generates predictions for the entire sky -- Horizon (0) to the local Zenith (90).

These options are automatically retained each time the program is run.

TRANSIT SATELLITE SELECTION

By default the program generates transit predictions for the International Space Station (ISS).  By clicking the Transit Satellite / TLE Source button, the user can specify predictions for other satellites.  The Transit Satellite / TLE Source window provides the following satellite selection options:


TWO LINE ELEMENT SETS

The program requires satellite orbital Two Line Elements (TLE) to accurately generate a prediction. By default the TLE files are downloaded from the CelesTrak supplemental TLEs web page. An option is also provided to download TLEs from the CelesTrak current data web page. There is also an option to generate TLEs from the Ephemeris State Vector data downloaded from the NASA website.  Note: This option requires the optional TLE.iss support data to be downloaded as discussed here. The CelesTrak supplemental TLEs are derived from NASA, while the CelesTrak current TLE is derived from space-track.org. Each TLE contains an "epoch" time. The closer the epoch time is to the prediction time, the more accurate the prediction. The CelesTrak Supplemental and NASA TLEs are normally generated about twice a week and span about 12 days. The CelesTrak Current TLE is generated every few hours, but do not span into the future. The Transit Satellite / TLE Source button gives the user an indication if a new set of TLEs are available. The user can check for the latest TLEs within the program by clicking the Check TLE button in the Transit Satellite / TLE Source dialog window.

The Download TLE button allows the user to download the latest TLEs from the web. The program will automatically download new TLEs as part of the transit prediction generation, so this step is optional. The ISS TLE file is stored in files within the "files.iss" sub-directory of the prediction directory.   If transits are found during the generation of a prediction, the TLEs used for the prediction are displayed within the Transit Prediction text file. The TLEs can be imported into various planetarium programs which can display the path of the satellite across the sky.

As the prediction time grows near, the user may desire to download a more recent TLE file than the one provided by CelesTrak Supplemental or NASA.  The CelesTrak Current TLE are generated on the order of every few hours; however, the TLE epoch is always in the past, thus the predictions generated with them are only good for one day or so.

To use other TLEs, the user can download the TLE file from the source (via an external browser or program) and place it in the prediction directory. The files should have the extension ".tle". The user then clicks the user TLE file option within the Transit Satellite / TLE Source dialog window to specify the downloaded TLE file.

The program displays the number of days that the TLE set spans (TLE Span) from within the TLE Source dialog window.  In general the user specified span should be no longer than a day or so past the TLE Span.

After a completed prediction run, the program displays the longest difference between TLE epoch and prediction epoch (TLE Age) in days.  The closer the TLE Age is to zero days, the more accurate the prediction.


GENERATING PREDICTIONS

Once the user parameters have been entered (and optionally saved to a User Parameter File), the user clicks the "Generate Prediction" button to begin prediction generation.  The progress bar indicates how long it will take to complete the process.  Be patient, depending on your CPU speed, it can take a few minutes to generate the predictions.  The user has an option to cancel, at any time.  Once the predictions are complete, the user can view the predictions, save them to a text file and optionally create the prediction plot files.



COMMAND LINE MODE

Note: Command line mode is intended for advanced users, comfortable with running programs via a terminal window or shell tool.

Command line mode allows ISS Transit Predictions to be generated with specific command line switches.  This mode disables the program's Graphical User Interface (GUI).  This allows for unattended generation via a scheduled task or cron job.  The following switches are accepted in this mode:

All three switches must be supplied at the command line.  The Prediction Directory must already exist (i.e., have been created by running the program's GUI).  The User Parameter file must also exist and be present in the Prediction Directory.  The specified output prediction file must have the ".txt" extension.  If the output prediction file exists, it will get overwritten automatically in this mode.  Google Maps and Google Earth prediction plot files (extensions .goo and .kml) are also saved automatically. 

The main jar file can be found here: http://pictures.ed-morana.com/ISSTransits/predictions/ISSTransitPrediction.jar.  In addition to the main jar file, the library jar files are found here: http://pictures.ed-morana.com/ISSTransits/predictions/lib.  All library files need to be present in a "lib" subdirectory.

As an example, the main jar file could be downloaded to the directory "C:\ISS", while the library files are downloaded to "C:\ISS\lib".  The main jar file could be run once without command line switches, to create a prediction directory in "C:\ISS\PredictionDir".  The user parameter file and prediction options file could also be created.  Optional DEM and Geoid Height data could also be downloaded.

Once all the files are in place, the program could be run via the following commands:

cd C:\ISS\

java -jar ISSTransitPrediction.jar -d "PredictionDir" -f "myParametersFile.dat" -o "predict.txt"



SYSTEM REQUIREMENTS

Java Runtime Environment (JRE) Version 8.0 or better, get it here.     Not sure which version you have? Check this page or this page.

CPU Speed:  No requirement, but the faster the CPU, the faster the generation

Hard disk space required for ISS Transit Prediction Program: 2 MB

Hard disk space required for User Prediction Directory:
Hard disk space required for optional Geoid Height Data: 4 MB

Hard disk space required for optional DEM Elevation Data: 55 MB per tile (Up to 4 tiles maximum)

Hard disk space required for optional TLE Support Data: 50 MB

Screen Resolution: 800x600 or larger

Connection to the Internet (The program downloads the latest Orbital Elements from the web).




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