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Satellite Information (State Vectors)

NASA SkyWatch provides you with some flexibility when you input state vectors. State vectors describe the position and velocity of a satellite at a given time and are the heart of NASA SkyWatch. The more accurate the input state vector is, the more accurate the results will be. For this reason, it is suggested that any state vector you use have a time tag as close to present time as possible.

There are two ways to input state vectors into NASA SkyWatch -- manually and automatically from a Mission Control Center ephemeris. An ephemeris is simply a collection of state vectors in chronological order.

The Manual Method

From the "Select Spacecraft" pull down menu, select "User Input" from the list of available vector sources. This tells NASA SkyWatch that you intend to type a state vector into the fields provided.

Skipping ahead a little in the menu, next, enter the coordinate system of the data you wish to enter. NASA SkyWatch will allow you to enter the vector in the Cartesian systems of Mean of 1950 (M50), Rotating True of Date (TDR) or Mean of 2000 (J2K). You can also use the commonly used two-line element sets, or TLE. Note that you do not need all the parameters on the TLE to proceed with processing. NASA SkyWatch will accept the six indicated parameters and then convert them into a M50 Cartesian state vector for internal processing. Note that the six parameter labels for the state vector will change when you select a TLE set so you know exactly which field is for which parameter. The six required parameters obtained from a two-line element set are as follows:

Inclination (degrees).
Right Ascension of the Ascending Node (degrees).
Eccentricity (nondimensional).
Argument of Perigee (degrees).
Mean Anomaly (degrees).
Mean Motion (revolutions per day).









If you are entering data for any of the Cartesian coordinate systems, be sure to select the proper units for the state vectors parameters. You can choose either feet or meters by simply indicating your choice from the units pull down menu.

Next, type in the vector time. This time must be in the form of DDD:HH:MM:SS.SSS in Greenwich Mean Time, or GMT, for Cartesian vectors or the TLE format -- decimal Julian days -- when entering TLE data. These labels will change accordingly. Shorthand notation is allowed when entering times in GMT so you won't have to type all the numerals. For example, for a vector dated March 27, 1998, at 6:00 a.m. GMT, you can simply type in "1998,86,6,," and NASA SkyWatch will reformat the time display to the standard display. Note that a comma (,), a colon (:) or a semicolon (;) can be used to separate numbers.

Finally, type in the six parameters in the fields provided. Pay attention to the labels to the left of the fields so you don't type the right information into the wrong field. Take care when typing all the numbers as mistakes often occur and your results can be dramatically incorrect.

The Automatic Method

You also have access to a Mission Control Center, or MCC, ephemeris during space shuttle missions and for the International Space Station. An ephemeris is simply a collection of state vectors in chronological order generated in the MCC. The state vectors that are generated in the MCC are far and above the most accurate available, and their use is highly recommended.

To use the ephemeris data, simply select the vehicle you want from the vector source pull down menu. This will give you the most up to date state vectors available. The time of the vector that is presented will be the first vector in the ephemeris. You may want to select a vector closer to current time to take advantage of any maneuvers the Flight Dynamics Officers have planned to support mission objectives. Once a time has been selected, NASA SkyWatch will retrieve the vector from the ephemeris and fill out the fields associated with the vector. There is no need for you to type anything other than a time. NASA SkyWatch will take care of all the rest! Note that the vector cannot be changed once it has been retrieved. To use another vector, you must type in a new vector time.

NASA SkyWatch also automatically retrieves two-line element set data. This provides you access to several other NASA satellites without having to manually input any data. You can tell if the satellite information is in the form of a two-line element set if "(TLE)" is appended after the satellite name. Also, like an ephemeris, a representative weight and area are computed and displayed on the Input Tab.

Threshold Time

For either the automatic method or the manual method, the threshold time field is provided for you to decide when to begin searching for acquisitions. NASA SkyWatch was developed to search for one acquisition at a time starting at the vector time. However, if the vector you are using is several days old or you are interested in acquisitions several days in the future, you would need to look at each acquisition starting from the vector time until the time you are interested in. The threshold time gives you the flexibility to specify when to begin looking and to bypass unwanted passes. It functions exactly like the vector time field and will be updated each time the vector time field is updated. Note that the default threshold time is the current time as specified by your system clock. Also, the threshold time is not available for entry ephemeris processing since the entire ephemeris is generally less than one hour in duration.

Entry Ephemeris Processing

You can also specify an entry ephemeris for processing. An entry ephemeris is also a collection of state vectors in chronological order but start after the de-orbit burn and end at approximately 10,000 feet. Normal state vector propagation does not apply for an entry ephemeris since the shuttle is in true atmospheric flight. Typically, an entry ephemeris will have a state vector each guidance cycle (~10 sec). Entry sightings are most impressive and can be seen at just about any time of day or night. The traditional constraints of satellite sighting simply do not apply for entry sightings because of the bright plasma trail left by the re-entering orbiter. In fact, entry sightings are best when the orbiter and the observer are both in complete darkness. The STS-82 entry (913K MPEG) over Houston, Texas, was one of the most dramatic ever photographed. Daytime entry sightings are possible, but the plasma trail will not be as visible. The shuttle will be a bright point of light moving across the sky. Be sure to stick around a minute or two after the sighting to hear the sonic boom!

Be careful when selecting an entry ephemeris for processing. Each entry ephemeris is to a specific landing sight at a certain time and has "(ENTRY)" appended to the menu selection. They are usually described by what landing site and what orbit the de-orbit burn occurs on. For example, "KSC168 (ENTRY)" would be for a de-orbit to the Kennedy Space Center, or KSC, in Florida on shuttle orbit 168. "EDW169 (ENTRY)" would be for a landing at Edwards Air Force Base in California one orbit later. Don't be disappointed if you don't get an opportunity to view an entry sighting. Entry sightings are usually not seen by as many people since the shuttle is lower to the ground and many times approaches KSC and Edwards from the ocean.

Curator: Kim Dismukes | Responsible NASA Official: Amiko Kauderer | Updated: 09/23/2011
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