Understanding the Prediction Tables

To get maximum use of the satellite prediction tables it is necessary to understand the significance of each column. That information is presented first. A few strategies for observing and Frequently Asked Questions round out our explanation.

Prediction table column definitions

Here is a detailed explanation of the prediction tables. Below is a fragment of a prediction for Pembroke, MA. We will use this as an example.

 EST Wed  AZ EL RANG  PH  START  AZ EL  PH    END   AZ  EL PH  Ag  APO  PER  IN REF  MAG  OBJ   Pembroke 29Oct97
17:27:40 265 56  581 115 [22:00 176  0  97] [33:00 344  1  97]  6  489  477  97 4.0  3.4 11252 Meteor 1-29 r
17:27:46 342 79  589  82 [21:26 227  1 147] [32:06  44  9  36]  6  581  573  57 2.0  0.7 21701 UARS
17:28:00  68 52  984  43 [20:20 344  0  94] [35:40 148  0  67]  6  790  767  74 3.5  2.9 11511 Cosmos 1125 r
17:29:12  95 50 1242  43 [21:17 168  2  89] [37:57  13  0  69]  6  991  959  83 3.5  3.4 21088 INFORMATOR 1 R/B

The AZ column is the azimuthal direction of the satellite in degrees at the time of maximum elevation. Zero (or 360) represents north, and 90 represents east. This standard is most commonly used in satellite observing, but some astronomers use zero degrees for south, so keep that in mind to avoid possible confusion.

The EL column is the elevation of the satellite in degrees at the time of maximum elevation. 90 is straight up, and 45 is halfway up. Some observers prefer to use altitude (sometimes abbreviated ALT) for this quantity.

The RANG column represents the range to the satellite in kilometers at the time of maximum elevation. This is also essentially the minimum range of the pass. Yes, it's pretty amazing that we can see a satellite which is maybe 20 or 30 feet long at a range of 500 km with the naked eye!

The PH column represents the phase angle in degrees. This is the angle seen from the satellite between the sun and the observer. The significance of this angle is great. If this angle is 90 deg, say, then half the satellite is illuminated by the sun, similar to a half moon. A phase angle of zero corresponds to a full moon, and higher than 90 degrees is less illumination than half. Satellites aren't nearly as uniform in shape as the moon, so you can't always tell how bright they are going to be, but clearly a small phase angle helps a lot. This angle changes quite a bit over the pass, but only the value at maximum elevation is reported.

The START bracketed columns give the AZ, EL, and PH at a different time, the starting time of the pass. Only the minutes and seconds are reported, to save space. The hour is always clear. For example, the UARS satellite in the example above has a maximum elevation time of 17:27:46. The START time is given as 21:26, which is really 17:21:26. In other words, the satellite breaks the horizon 6:20 prior to attaining maximum elevation.

The END bracketed columns give the AZ, EL, and PH at the ending time of the pass. Again the hours are omitted from the time. The interpretation is the same as the START columns.

The Ag field gives the number of days between the date of the prediction and the elements used in the prediction. If a satellite is in a high enough orbit (see APO and PER, below), and not being controlled with maneuvering jets, then it can be predicted pretty accurately weeks and even months ahead. For example, a rocket body in a 1000 km orbit should be predictable for months. For low orbits, say 300 to 400 km, atmospheric drag causes much more unpredictable effects, and getting a prediction within a week or so is very helpful. For maneuvering space objects like the Mir space station or the space shuttle, it is important to get elements as fresh as possible for best results. Sometimes something which is about to re-enter may be in an orbit of say 200 km apogee by 700 km perigee. This type of thing is very hard to predict more than a few orbits ahead.

The APO field is the apogee, the maximum orbit height above the earth, in km. The PER field is the perigee, the minimum orbit height above the earth, in km. If the orbit is circular, these numbers will be very close. If they are significantly different, then the orbit is not circular, and the satellite moves more quickly near perigee. Sometimes you can see a satellite moving faster than usual across the sky near perigee.

The IN field is the inclination, the angle between the orbit and the equator, in degrees. If this is zero, then the orbit lies directly over the equator. If it is 90, then the orbit goes right over the poles. Most of the satellites that we can see are have orbits in the range of 400-1000 km. At that height, a low-inclination satellite never becomes visible from the more northerly latitudes. For example, many space shuttle missions are launched into a 28.5 deg inclination orbit at a pretty low orbit of maybe 400 to 500 km. In Boston, at 42 deg latitude, such missions never rise higher than about 6 deg above the horizon, so they are hard to see. The same thing is true for the Hubble Space Telescope which is at a 28 deg inclination. Shuttles and HST are among the brightest and easiest to see objects at lower latitudes. Of course sometimes the shuttle docks with the Mir; in that case it has the inclination of the Mir and is easily seen.

The REF field is the reference magnitude of the satellite, in optical magnitudes. Smaller numbers here are brighter, as a legacy from the ancients who invented this system. This reference magnitude is a possible brightness at 1000 km range and 90 deg phase angle. These numbers come from a variety of satellite watchers on the net. Sometimes they are typical values, and other times they are average values. This number gives you an idea of the size of a satellite; smaller REF means larger satellite. In some cases this value is reported as ?.? rather than a number. These are satellites for which there is no predicted magnitude.

In some cases the reference magnitude is followed by a letter as supplied by Mike McCants in the magnitude reference file for his excellent quicksat prediction program. Mike says,

The following code letters are used: "d" means that the object has decayed or returned. "h" means that the object is in a high altitude orbit. The class letter "c" means the object has a classified orbit. The class letters "f" and "t" indicate that the object is of interest because it is tumbling. The class letter "s" indicates that the object has tumbled in the past, but is currently steady. The class letter "p" indicates that accurate positional observations are of interest to the Royal Greenwich Observatory.

The MAG field is the predicted magnitude at maximum elevation for this pass. This is derived from the REF field by correcting for the actual range and phase angle. Since satellite brightness can change quite a bit due to the strange shapes of satellites, and the REF field itself is not unimpeachable, your mileage may vary. A star chart can give you an idea of the magnitudes of stars for comparison. The Big Dipper stars range from 1.7 to 3.4 in brightness. In cases where the REF field is unknown (?.?), a value of 5.4, which represents a fairly big satellite, is used to estimate the MAG field. This is pretty optimistic, but it flags satellites which are very close as candidates for viewing.

The OBJ column is the object number of the satellite. I use a US-style single number assigned by the Air Force, as do many people in the U.S. Unfortunately, there is also an international number for each satellite, and you may see satellites referred to in this way as well. For example, #23705 is is also known as 1995-058B. The single number is easier to handle in software, being conveniently stored as an integer. The international approach has the advantage of telling us that this object was the second cataloged piece of launch #58 of 1995. It is easy to figure out that 1995-058A is a related piece, while it is not as apparent whether 23705 is related in any way to 23704, for example. This two-number problem is not going to go away, so get used to it.

Strategies for observing

There are a couple of strategies for observing with my predictions. If you just want to see what's going over, save the predictions from your browser in text format, and print them in landscape format. With a small font (like a 7 point) I can fit about 50 on a page, so one or two pages does the job. When you see a satellite, you can make a note on the page, and then stick it in a notebook so you'll know what you saw.

I observe usually from my driveway (high elevation, south or west viewing) or my deck (east). From there I know exactly what direction is 50 deg azimuth, for example. Get a compass if you are observing from someplace where you have any doubt. Start out looking for the brightest stuff like Mir, UARS, etc. The dimmer stuff can be seen if there is good visibility.

To decide what to look for, look first in the MAG (magnitude) column for a low number. Visualize the whole orbit, by looking at the START and END bracketed columns to tell where the satellite begins and ends visibility, and at what times. It is most likely the satellite will be brightest near the AZ and EL values given first, since it is closest there, so pay special attention to that AZ/EL location at the appropriate time. It's a good idea to look a little early because sometimes the satellite will be brighter earlier, and it could even show up a little early. (To let your eyes adjust, go out at least several minutes early.)

If a satellite is too dim to see with the naked eye, you can try using binoculars. Don't try this until you have seen a bunch of satellites with the naked eye and are sure you know what they look like, and what a typical trajectory looks like. If you know where to look, binoculars make it possible to see many dim satellites, even those not on the page.

It has been shown that the eyes can take as much as 20 or 30 minutes or even an hour to fully adjust to very low light conditions. I find that 5 minutes seems to work pretty well for the light conditions that I usually see. If you want a better chance to see satellites, take a 10-year-old kid with you. Their eyes are amazingly better than adults at this sort of thing, and my son will usually spot dim satellites first. Taking advantage of averted vision (looking a little away from the spot at which you expect to see the satellite) can also give you a little more sensitivity.

If you see a satellite and are curious which one it was, note the exact time at which it attains maximum elevation, if possible, as well as the azimuth and elevation. That, coupled with the direction of travel (N to S, for example) is generally enough to figure it out in retrospect. The best aid to retroactive identification is an accurate watch.

Frequently Asked Questions

What are all these satellites, anyway?

I'm not in the business of identifying satellites. Check out the many web sites and books written about satellites to find out about them. The short descriptions shown in the predictions are whatever was contained in the identification field of the two-line-elements used to predict them. I should tell you that the many satellites identified as Cosmos are Russian (or old Soviet) satellites, and those names which end in "r" or "R/B" are rocket bodies which launched satellites into orbit. These are often bigger than the satellites they launched and easier to see.

Can't you tell us which stars the satellites are going to go past to make it easier than using this azimuth/elevation scheme?

No, I can't, because I'm not an astronomer. It is possible to convert the az/el/time predictions plus the latitude and longitude of the observer to a right ascension/declination pair which can then be looked up on a star chart to find out where you should be looking on the celestial sphere, and what stars are nearby. As an engineer, I find az/el easier to deal with!

I live in New York. Can I use the Boston predictions to see satellites?

Get out your globe and think about it. Most of these satellites orbit the earth in about 90 minutes, so they are moving about 17,000 MPH, or about 300 miles a minute. If a bright satellite is in an orbit 800 km above the earth, and is visible going north to south over Boston at 10:00 EST, it should come by New York a minute or so later, and a little farther toward the west than it appeared from Boston.

You can always learn more about it and predict them yourself by consulting the ever useful Visual Satellite Observer's Home Page!