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 other satellites as specified by the user.
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.
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:
- User Parameter data
files (*.cfg): Stores user specific information including latitude and
longitude so that the user need not enter the information manually each
time the program runs.
- Transit Prediction
text files (*.txt): Once a transit prediction is generated, the user
saves the prediction file. The user chooses the name of the text file.
- Google Earth Plot
files (*.kml): These files share the same name as the prediction text
file, but with the .kml extension.
- Google Maps Plot
files (*.goo): These files share the same name as the prediction text
file, but with the .goo extension.
- Optional user
downloaded Two Line Element files (*.tle):
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, the user selected satellite list, 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).
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 used by this program is the USGS EROS Archive -
Digital Elevation - Global 30 Arc-Second Elevation (GTOPO30).
The data can be downloaded via the USGS EarthExplorer web page.
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 gt30w140n40.dem.
However, a user in San Francisco, CA (122W, 38N) would need two files
(gt30w140n90.dem and gt30w140n40.dem).
The USGS provides DEM data 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:
- Latitude of user
location
- Longitude of
user location
- Elevation Above
Sea Level (ASL) of user location
- Timezone offset
from Greenwich Mean Time (GMT), negative numbers are west, positive
numbers are east. Don't forget to adjust for Daylight Saving Time as
appropriate
- Metric flag
specifies if distances should be measured in kilometers or miles
- Alert Radius
specifies the maximum distance from the user location to look for
transits (up to 100 miles)
- Span specifies
the time span to look for transits starting today (up to 11 days)
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:
- Sun
- Moon (Satellite in shadow)
- Moon (Satellite in sunlight)
- Mercury
- Venus
- Mars
- Jupiter
- Saturn
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:
- ISS (default): Generate predictions for the ISS (Satellite ID 25544).
Users can select other satellites (e.g., CSS or HST) within the satellite selection
drop-down box. Satellites can be added to the drop-down box by clicking
the plus (+) button.
- NORAD ID: Generate predictions for the satellite NORAD ID entered in the text box.
- Satellite from user TLE file: Generate predictions for a
satellite found within a user specified TLE file. Predictions are
generated for a single satellite. The program selects the
satellite from the first Two Line Element encountered within the file.
ISS transits can also be generated from a user
TLE files with this option.
The program requires satellite orbital Two Line Elements (TLE)
to accurately generate a prediction. By default the TLE files for the ISS are
downloaded from the CelesTrak supplemental TLEs web page.
There is also an option to generate ISS 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.
An option is also provided to download TLEs from the CelesTrak current NORAD data web page.
This option is required for non-ISS satellites.
The CelesTrak supplemental TLEs are derived from NASA, while the CelesTrak
NORAD 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 NORAD 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 NORAD 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 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:
- -d <Prediction Directory>
- -f <User Parameters File>
- -o <Output Prediction File>
- -t <User TLE File> (optional)
- -i <Satellite NORAD ID> (optional, defaults to ID of ISS, 25544)
- -s <TLE Source> (optional, NORAD, SUPPLEMENTAL or NASA, defaults to NORAD for non-ISS, SUPPLEMENTAL for ISS)
The first three switches must be supplied at the command line.
The User TLE File switch is optional, and allows a user to
specify their own Two Line Element file. The Satellite NORAD ID
switch is optional, and allows for predictions for other satellites.
The TLE Source is optional, and allows for TLE Source selection. Supplemental
and NASA sources are only valid for the ISS. 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"
Java Runtime Environment (JRE) Version 8.0 or better, get it here.
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:
- User
Data Files: 1 KB each
-
Prediction Text Files: Up to 100 KB each
- Google Earth Plot Files: Up to 200 KB each
- Google Maps Plot Files: Up to 200 KB each
-
Generation Files (files.iss): Up to 100 KB
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).
Back
to the ISS Transits Predictions Page