ISS Transit Prediction Help

ISS Transit Prediction

OVERVIEW

This application generates International Space Station (ISS) Transit Predictions for the Sun, Moon and Planets.

The user specifies a location, which includes latitude, longitude and elevation. The application downloads the latest orbital information (Two Line Elements or TLEs). The application generates a transit prediction map which contains prediction paths for each transit within a specified alert radius.

Specify and save multiple locations
Save multiple prediction maps for later viewing
User specified Two Line Elements
View maps with Google Earth

USER INTERFACE

The main screen provides 4 buttons:

Location - Press this button to add or select the prediction generation location
Two Line Elements (TLE) - Press this button to download TLEs
Generate Prediction - Press this button to start the prediction generation
View Prediction - Press this button to view the prediction map or text file

The options menu provides the following:

Locations - Press to add, select, edit or delete saved locations
Predictions - Press to view, share or delete saved prediction maps
Settings - Press to set user preferences
DEM Files - Press to list or delete the downloaded DEM Data
Help - Press to display this help page
About - Press to display application version, credits and links

LOCATIONS

Add a named observing location by clicking the "Add Location" button accessible from the locations screen.

The location coordinates can be entered in any of three methods:

Manually - Enter latitude, longitude and elevation in the text boxes. Positive values represent North and East, negative values South and West. Elevation can be entered in meters or feet above sea level depending on the current prediction units setting. As the coordinates are typed, the map pans to the location. Press the search button to search for a location.
Map Input - Use the map to zoom and pan to a location. Pressing the Set button sets the location name, coordinates and elevation in the text boxes. The elevation of the current coordinates is retrieved using the specified elevation data source setting. Switch between map and satellite modes by toggling the Map/Sat button.
GPS - By pressing the GPS button, the application uses GPS to obtain location coordinates and elevation.

Use the locations page accessible from the options menu to edit and delete saved locations.

TWO LINE ELEMENT (TLE) SETS

The application requires satellite orbital Two Line Elements to accurately generate a prediction. The elements are downloaded from the NASA website, by default. If the NASA website is unavailable, the elements are downloaded from the CelesTrak website. The TLE source can be changed in the Two Line Elements download setting. Each element contains an "epoch" time. The closer the epoch time is to the prediction time, the more accurate the prediction. NASA generates new elements about twice a week, which normally span about 12 days. CelesTrak generates new elements every few hours. Unlike the NASA elements, the CelesTrak elements do not span into the future. The TLE button gives an indication if a new set of elements are available.

Pressing the TLE button downloads the latest elements from the default TLE Source. 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 prediction text file includes the number of days that the TLE set spans (TLE Span). The specified prediction span setting should be no longer than a day or so past the TLE Span.

The prediction text file includes 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 a location has been entered and TLE have been downloaded, press the "Generate Prediction" button to begin prediction generation. The progress bar gives an indication of how long it will take to complete the process. Depending on your CPU speed, it can take a few minutes to generate the predictions. Pressing the cancel button will cancel the prediction.

VIEWING PREDICTIONS

Once the prediction generation is complete, the prediction map or text file can be viewed. Pressing the View Prediction button brings up the previously generated prediction map. The text button within the map view displays the prediction text. The Map/Sat button switches between Map mode and Satellite mode.

Within the map view press the Google Earth button to view the map in Google Earth. Press the save button to save the prediction for later viewing.
Use the predictions page accessible from the options menu to view, share and delete previously saved prediction maps.

SETTINGS

The following settings can be specified via the settings page:

Transit Bodies - Brings up a page which specifies the transit bodies for which predictions are generated from the following:
- Moon (ISS in shadow)
- Moon (ISS in sunlight)
- Mercury
- Venus
- Mars
- Jupiter
- Saturn
- Sun
Transit Body Update Times - Can be specified via the Transit Bodies page. A setting is provided for the Sun, Moon and Planets. These settings are used to update the position in the sky of the transit body at the interval specified. Shorter update times give more accurate predictions. Larger update times give faster predictions.
Minimum Transit Elevation - 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).
Prediction Units - Specifies if distances should be measured in metric (meters, kilometers) or imperial (feet, miles) units
Alert Radius - Specifies the maximum distance from the selected location to look for transits (up to 160.9 kilometers or 100 miles)
Prediction Span - Specifies the time span to look for transits (starting now, up to 11 days)
Timezone - Specifies timezone offset from Coordinated Universal Time (UTC), negative numbers are west, positive numbers are east. Check the box to have the timezone automatically selected.
TLE Download - Specifies automatic or manual TLE download. In automatic mode, elements are automatically downloaded from the NASA website, if available; otherwise they are downloaded from the CelesTrak website. In manual mode, the user specifies the TLE source and must manually press the TLE button to initiate the download.
Specify an elements file to use for prediction. This setting is provided to allow use of separately downloaded elements (e.g., from SpackTrack.org). The element file must be present in external memory in the folder displayed within the TLE file section screen. Only ISS elements can be specified. Each TLE must be preceded by an identification line which starts with "ISS". For example:

ISS
1 25544U 98067A 13293.55472222 .00014076 00000-0 25605-3 0 8456
2 25544 051.6517 207.1515 0002900 046.2774 302.5149 15.49789271854148
Elevation Data Source - Specifies how the application obtains elevation data:
- Download Digital Elevation Model (DEM) data from the U.S. Geological Survey (USGS) - Predictions are more accurate
- - Specify whether DEM data is automatically downloaded or not
- Use the elevation of the selected location for all elevations - Predictions are less accurate
Default GPS - Specify if GPS should be on automatically when entering a location
File Storage - Specify whether stored files should be saved using External or Internal memory. Note: Google Earth viewing feature is only available for predictions saved to external memory.
Language - Change the language. An application restart is required.

DEM DATA

The DEM Data is downloaded as needed from the USGS website via the Elevation Data Source setting page. The USGS has organized world elevations into "tiles" of 40 degrees of longitude by 50 degrees of latitude. Only the DEM Data near the selected location is needed. Typically this is only one tile, but could be as many as 4 files, if the selected location is near (within 5 degrees of) a DEM tile boundary. For example, a location in Los Angeles, CA (118W, 34N) would need the DEM data in tile W140N40. However, a location in San Francisco, CA (122W, 38N) would need two tiles (W140N90 and W140N40). The DEM data is provided in a compressed format. The application downloads and uncompress the needed DEM data.

View ISS Transits Predictions on the web