NASA SkyWatch FAQ

• Why does NASA SkyWatch take so long to start?
• NASA SkyWatch won't load and I can't see the user interface, what's wrong?
• Does NASA SkyWatch replace the sighting tables on the NASA site?
• How accurate is NASA SkyWatch?
• Does NASA SkyWatch determine sightings for other satellites or just for the the shuttle?
• Are the ephemerides used by NASA SkyWatch created in Mission Control?
• How do I acquire a vector from an ephemeris or a two line element set?
  
• How accurate does my location have to be?
• Does satellite weight and area really matter?
• Can NASA SkyWatch predict sightings for the re-entering shuttle?

• How can I print SkyTrack?

• Why doesn't my "Enter" key work?

• Can NASA SkyWatch be used to determine if a satellite was visible in the past?

Why does NASA SkyWatch take so long to start?

There is a substantial amount of data to download the first time you access NASA SkyWatch. The application itself is approximately 500K with other text files (cities, solar constants, constellations, etc) totalling another 85K that need to be downloaded as well. A 33 kbps modem will take on the order of 3 minutes to download all files and start the application. Subsequent accesses will not need to download the files again and the start up time will be reduced to about 1 minute or less for initialization.

NASA SkyWatch won't load and I can't see the user interface, what's wrong?

Due to the complexity of NASA SkyWatch, you need to be using a compatible Internet browser. Microsoft Internet Explorer version 4.0 or later or Netscape Navigator version 4.0.6 or later are recommended. Macintosh systems require Internet Explorer version 4.5 or later, plus the Macintosh OS Runtime Environment for Java version 2.1 or later. Additionally, the Java Runtime Environment v.1.4.2 or later is required.

Click here to download:

Does NASA SkyWatch replace the sighting tables on the NASA site?

No, but NASA SkyWatch provides more information than the existing tables. Not only will NASA SkyWatch give digital data, but the Sky Track display makes sightings easier to spot. The tabular data is still available though.

How accurate is NASA SkyWatch?

NASA SkyWatch has a high fidelity numerical integrator along with very accurate acquisition classes that take into account orbital drag and gravity effects of the sun and moon on the satellite. However, the results are only as accurate as the inputs you provide. The most critical input is the state vector. NASA maintains vectors for ISS and the space shuttle and using vectors that are provided by NASA SkyWatch is highly recommended. If you decide to use your own vector or two-line element set, the most recent vector available should be used. Since vector accuracy will degrade over time, vectors more than a week old should be discarded in favor of more up to date data. Translational maneuvers (orbit shaping) should also be accounted for. If you retrieve a vector for ISS or the space shuttle, maneuvers are already accounted for. If you manually input a vector, no maneuvers will be implemented and results may be significantly in error if the vehicle either raises or lowers its orbit. This is generally not a problem for ISS or Mir, but the shuttle will have many such maneuvers during rendezvous missions making sightings difficult.

Does NASA SkyWatch determine sightings for other satellites or just for the the shuttle?

NASA SkyWatch will work on any Earth orbiting satellite provided that you have a valid state vector to start with. NASA routinely provides state vectors for ISS and the space shuttle as a service to the general public. NASA SkyWatch accesses an ephemeris (a collection of state vectors) so you don't have to hunt around for vectors yourself (or type them in). If you are interested in a satellite that is not maintained by NASA directly, you can manually enter the state vector yourself in a variety of coordinate systems. A link at the top of the user interface connects you to a large variety of visible satellites provided by the Goddard Space Flight Center. Successful NASA satellites like the Hubble Space Telescope (HST), the Gamma Ray Observatory (GRO), the Upper Atmosphere Reasearch Satellite (UARS), are also selectable.

Are the ephemerides used by NASA SkyWatch created in Mission Control?

Yes. For Space Shuttle ephemerides, the ephemeris is generated from the control center for the space shuttle. The Flight Dynamics Officers generate this data during the course of their console duties. The same can be said of the ISS ephemeris except the ephemeris is generated by the Trajectory Operations Officer (TOPO) for ISS.

How do I acquire a vector from an ephemeris or a two line element set?

Easy. From the "Select Spacecraft" pull down menu, simply select the ephemeris you wish to use (usually ISS or SHUTTLE). NASA SkyWatch will return the first vector in the ephemeris. If you select a satellite other than ISS or the space shuttle, a GSFC two line element set will be retrieved. In either case, the threshold time to begin searching for sightings will default to your system clock time.

How accurate does my location have to be?

If you can't find a city or town near your location on the pull down menu, you can manually enter your own latitude and longitude. These values don't have to be terribly accurate but the more accurate your inputs are, the more precise the results will be. In general, try to get within one half degree of latitude and longitude (about 30 miles) of your location you intend to view or acquire the satellite.

Does satellite weight and area really matter?

Again, the more accurate the inputs, the more precise the results will be. Weight and area determine how much orbital drag will be experienced by the satellite. So over time, the predictions will degrade. Default values of 100 square feet for area and 5000 lbs for weight are good enough for acquisitions for up to several days in the future.

Can NASA SkyWatch predict sightings for the re-entering shuttle?

Yes. Although in order to accomplish this, you will need to use ephemeris data that NASA SkyWatch provides. Each flight, the flight dynamics officers will publish entry trajectories for various landing opportunities. You will need to select one of this opportunities and process NASA SkyWatch as usual.

How can I print SkyTrack?

You cannot print a SkyTrack image directly. We suggest that you simultanously press the ALT and PRINT SCREEN keys to copy the image to your clipboard. From there, you can paste it into the image editor of your choice, one such as Microsoft Paint. You will be able to print there. We recommend printing in landscape mode and using the B/W button on the Variables tab to reduce ink or toner usage.

Why doesn't my "Enter" key work?

Your "Enter" key performs no useful purpose in NASA SkyWatch. To tab between input fields, use your "Tab" key instead.

Can NASA SkyWatch be used to determine if a satellite was visible in the past?

Yes. All you need to do is enter the time to begin searching for a sighting in the past is to enter the time you want to begin searching in the "Threshold (GMT)" field on the Sat Info Tab. This is true for past as well as future sightings. Remember, the format for the time is year/day:hour:min:sec in Greenwich Mean Time.