CHALLENGES IN OBSERVING SATELLITES

by Paul D. Maley

Satellite observation is not always easy. While I document a number of common problems below, you should not be deterred. I have been able to overcome every one of these problems and you should be able to also. Probably the worst difficulty is not having a dark-sky site. My backyard is full of 100-foot tall trees, so the only place for me to observe is from the driveway in front of my house. I can see about 21 streetlights including several very bright ones located within 150 feet of my driveway.

There are two streets very close by where cars often direct their headlights onto my driveway. The worst problem is an unsym- pathetic next-door neighbor that, without fail, will keep her brilliant driveway lamps on just when I start to observe.

I am able to observe from the northeast to east to south half of the sky, but the rest is relatively blocked by trees and house structure. Here are views to the north, south, east and west.

 

 

 

To counter the light, I have erected a folding cardboard shield, which protects me from some lights on most occasions. During the 5 years I have lived here I was nearly run over in my own driveway by a drunk driver who left tire marks on the driveway and lawn as he sped down the street at 1am on a Saturday morning. In spite of this I am often able to make very useful observations of objects even as faint as 10th magnitude.

If my cats stare out the window, they assume that if they look long enough a bird will come close enough to the window that they will be able to eat for lunch. Like the cats, most of the time I think I can get information on any object if I stare at it long enough or watch it enough times. For some satellites, even this is not enough. One confounding problem is that the orbital elements being used are in error. It is wise sometimes to examine the drag term (first derivative of mean motion) to see how it varies if, after several tries, you do not successfully spot it.

Another common problem is that the satellite is so faint you just cannot pick it up in your telescope/binoculars. This may be due to low reflective coverings/paint or small radar cross-sectional area.

Beyond operator error in using a satellite look-angle prediction program, one source of problems can be the location of the observing site. The coordinates should be accurate to within at least 0.1 degrees of longitude and latitude. Altitude variation should not be a factor.

Phase angle is a problem that may be often overlooked. A satellite may appear, for example at a rather substantial promising elevation of 45 degrees above the horizon, but is located geometrically between you and the sun. This makes it hard for the satellite to reflect enough light back down to the observer. You can understand this when you think about the moon. The moon is brightest when it is more opposite the sun and more of its disk is sunlit (gibbous to full phase) than when closest to it (crescent phase).

Like bad cheese, another confounder is the age of the orbital elements. Some satellite visibility programs do not give out information on how ‘fresh’ the orbital information is that is being used to generate the look angles. It is best to utilize the latest information available from web resources so that the age is usually not more than a few days.

Then what if you see two satellites moving along through the field at the same time moving in the same direction? In the images below this actually happens. Contrary to the arrow directions, both are moving right to left in the first photo below.

After more than a minute has passed both objects have separated further, but still travel in the same direction at almost the same velocity and they remain in the same field of view. Both had the same optical characteristics and moved in very similar trajectories. Space is getting fairly crowded in certain orbital inclinations so that such encounters do occur on occasion.

A further problem is related to how close the satellite may be in relation to the shadow of the earth. The satellite normally is visible by reflected sunlight, and you should allow for a significant portion of the pass to be available to you. Many satellites require at least one or two minutes of continual observation to enable a flash period to be determined.

Proximity to twilight or to the moon or other light sources impact ability to track them. It is often tempting to observe a satellite only to find out that the sun is only a few degrees below the horizon rendering success out of your reach.

Other more common confounders are interference due to light pollution, terrain masking, and clouds. The distance from the observer to the satellite can also inhibit observation depending on how large or small the target is. If the satellite is simply too far away to be seen, you must wait for the slant range to be reduced to a more practical level. If the satellite is subject to orbital maneuvers such as a Soyuz, Progress, Space Shuttle or similar type of craft, you will need the latest orbital element set to achieve success.

As a satellite nears reentry, the rate of orbital decay influences the drag term causing the elements to be come somewhat unreliable. A similar effect may also be present on a small object that is light and can be influenced by solar radiation pressure. Lunar and solar perturbations impact still other satellite orbits causing them to be early/late.