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 the Space Shuttle and 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
NASA website. 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 (*.dat): 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 NASA website and the user-selectable prediction options.
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 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.
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)
- Space
Shuttle: Generate predictions for the Space Shuttle. This
selection is only valid during a Space Shuttle mission.
- 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 or Space Shuttle 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 are downloaded from the NASA
website.
Each TLE contains an "epoch" time. The closer the epoch time
is
to the prediction time, the more accurate the prediction.
NASA
normally generates new TLEs about twice a week, which normally span
about 12 days. The Transit Satellite / TLE Source button
gives the user an
indication
if a new set of NASA TLEs are available. The user can check
for
the latest TLEs from NASA from 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 NASA. The program will
automatically download new TLEs as part of the transit prediction
generation, so this step is optional. The ISS NASA TLE file is stored
in a file called NASA_ISS.tle in the "files.iss" sub-directory of the
prediction directory. NASA Space Shuttle TLEs are stored in
NASA_SpaceShuttle.tle. 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 NASA.
The Space
Track website generates
TLEs on the order of every few hours; however, the Space Track TLE
epoch is always in the past, thus the predictions generated with them
are only good for one day or so. Registered Space-Track.org users can
obtain TLE data directly by selecting the Space-Track.org TLE Source
button. A login window appears prompting the user for their
Space-Track.org username and password. Space-Track.org users
are subject to the terms of the Space-Track.org User Agreement,
available here.
To use TLEs from sources other than NASA or Space-Track.org, the user
must 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:
- -d <Prediction Directory>
- -f <User Parameters File>
- -o <Output Prediction File>
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"
Java Runtime Environment (JRE) Version 6.0 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:
- 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)
Screen Resolution: 800x600 or larger
Connection to the
Internet (The program by default downloads the latest ISS Two Line Element set from the
NASA
website).
Optionally the user can download Two Line Elements from the
Space-Track.org website. This requires a Space-Track.org user account.
Back
to the ISS Transits Predictions Page