Return to Human Space Flight home page

NASA SkyWatchIntroduction | What's New? | FAQ | Help > Variables Tab Help > Sighting Constraints

Sighting Constraints

In order for a sighting of a satellite to be made, there are 4 conditions that must be satisfied. First, the satellite must be above the horizon at the observer location. Second, the satellite must be in daylight. Third, the side of the satellite in daylight must be roughly facing the observer. And finally, the observer must be in darkness. When all constraints have been achieved, a sighting is possible.

NASA SkyWatch allows the user to alter these sighting constraints. It is important to remember that not all satellites have the same type of viewing possibilities. The default parameters are typical but somewhat optimistic based on recommendations of experienced watchers.

Minimum Solar Separation
This parameter determines if the lit side of the satellite is facing in the rough direction of the observer. Generally, the observer needs to be between the sun and the satellite in order for a sighting to take place. It is assumed that the satellite can only reflect light back toward the sun and not ahead of itself. This is usually true for most satellites. However, for larger targets with solar arrays, like the International Space Station, light can actually be reflected ahead which makes more viewing potential. For this reason, it is reasonable to lower the maximum solar separation to as low as 60 degrees.

Maximum Solar Elevation
This parameter defines what "darkness" is at the observer location. Since one of the constraints for sighting is for the observer to be in darkness, it is required to define how dark darkness really is. From past experience, the sun needs to be approximately 6 degrees or more below the horizon in order not to wash out the sky.

Minimum Elevation
It is possible to acquire a satellite right when it crosses the horizon. This is likely for electronic acquisition. However, for actually seeing a satellite, haze and ambient light make viewing almost impossible below about 15 degrees above the horizon. For exceptionally clear nights, picking up the sighting at below 15 degrees can be challenging but possible.

Maximum Search
When using the "Next Sighting", "SkyWeek", or "Super SkyWeek" buttons on the "Table" tab, NASA SkyWatch will look for a sighting until it finds one or it reaches this maximum number of days.

Curator: Kim Dismukes | Responsible NASA Official: Amiko Kauderer | Updated: 09/23/2011
Privacy Policy and Important Notices