David Cappellucci

Acappella Publishing

© Copyright 1995-9

All Rights Reserved

Satellite viewing is a fascinating and rewarding hobby. The information presented here will help you understand what makes it possible to see a satellite hundreds of miles in space and how to actually see a satellite.

A satellite is visible from the ground when the sun has set and it is dark where you are, but the satellite, flying high overhead, is still in bright sunlight. What you are seeing is the sunlight reflecting off of the satellite. The brightness of a satellite depends on several factors:

You can think of a satellite as having phases like the moon. A satellite is brightest when the sun is to your back as you are looking toward the satellite. This is the same geometry that makes the moon "full" once a month. When the sun has just set and you are looking for a satellite in the western portion of the sky, you are looking toward its "dark side". This is similar to the moon when it is "new". You should generally try to find passes to the East in the evening and to the West in the morning.

The two easiest satellites to see, Mir and the Space Shuttle, are the largest ones currently in operation. The new International Space Station (ISS) will be even easier to see when completed. Large satellites are generally easier to spot than are small satellites. Size is not the only factor, however. The material used to manufacture the satellite can make a big difference. Satellites built from a reflective material should be easier to see than those that are made from dark materials. A good example is the EGP, a Japanese geophysical satellite. EGP is less than a meter in diameter and is built like a "disco ball", covered with highly reflective mirrors. This is one of the most interesting objects to observe since it "sparkles" as it rotates in its orbit. Another example are the Iridium satellites. These are ordinarily just barely visible to the nake eye, but occassionally they will "flare" into extremely bright objects when sunlight reflects off of the main mission antenna (MMA).

Even if a satellite is large, it may not be visible if it is not well oriented to the observer as it passes over-head. Most rocket bodies are large and easy to see. They tend to be cylindrical in shape and several meters in length. If a rocket body is oriented so that it is viewed end-on, it will probably not be visible. If the rocket body is oriented so that it is viewed from the side, it will probably be very bright. Many satellites, especially spent rocket bodies, tumble or rotate as they move. As their orientation changes, they appear to flash at regular intervals.

All you really need to view satellites are your eyes. To make your viewing experience more enjoyable and productive, the following items can be useful:

Without a pair of binoculars, you can only see the brightest of satellites, especially if you live in or near a city. When purchasing binoculars, you should look for a pair with a relatively wide field of view and good light gathering capabilities. A pair of 7x35 is generally good enough. Most astronomy magazines advertise binoculars and give tips on what to buy.

Accurate time keeping is critical in viewing satellites. If your watch is off by more than a few seconds, you can miss a pass. A good digital watch, preferably with a lighted dial, is generally sufficient, but a watch by itself is only part of the story. You must also be able to correctly set the time. Most radio stations broadcast a time signal at the top of the hour. These are generally accurate, but to get the best time signal, you can call the National Institute of Standards and Technology (NIST) in Boulder, Colorado at 1-303-499-7111 or the US Naval Observatory Master Clock at 1-719-567-6742.. These numbers send out a continuous recorded message with regular time signals.

You can also get the same time signal over your short-wave radio. NIST broadcasts continuously updated signals on stations WWV and WWVH. The signals are broadcast at the following frequencies: 2.5MHz, 5MHz, 10MHz, 15MHz, and 20 MHz (WWV only).

If you have suitable software, you can also get accurate time updates over the internet using the network time protocol (NTP). An example of this type of software is the Atomic Clock program from Parson's Technology. It allows you to update your computers clock at the click of a button.

When watching for satellites, you will find yourself looking up a lot. You can help your neck and back by investing in a good lounge chair. In addition, when you are standing up and trying to look straight overhead, it is very difficult to hold your binoculars steady and to keep that position for very long. I use a rocking chair mounted on a swivel. This allows me to rotate the chair and lean back while following a satellite across the sky. A pair of armrests on the chair allow me to support my arms and hold the binoculars steady while looking.

The best way to use SatSpy is to make printouts of the passes you want to see and then take them outside with you. In order to see the paper, you need to use a light source. You should use a small flashlight with a red filter for the bulb. This will help your "night eyes" while still being able to see the paper.

If you are not familiar with the stars and constellations, a planisphere or star-chart is helpful in orienting yourself to the sky.

I find a clipboard and a pen useful for holding my papers and writing notes. If you are doing your viewing from a cold climate, warm clothing is essential. A warm drink makes it more pleasant.

Also depending on your climate, you may need bug repellent to keep from feeding the mosquitoes and other biting insects.

Since the motion of a satellite is independent of any action you may take (unless you happen to work for a space agency), you can only find it along its predicted path of motion. If your objective is to observe many different satellites or don't care on which pass you observe a particular satellite, you can plan your viewing to take advantage of preferred pass orientations.

The best way to find a satellite is to predict when it passes close to a familiar object. This can be a star or a constellation. If possible, always choose a pass that goes through a familiar constellation. You can use the "find" feature of the Skytrace window to identify constellations and stars. You can also use the drag-n-drop time labels to mark the times of closest approach to your marker objects.

Your best viewing comes when the sun is to your back relative to the sun. In the evening, this means that the best passes will be those to the East. In the morning, the best passes are to the West.

You can categorize passes by their culmination or zenith. If a pass culminates (reaches the highest point in its pass) too low to the horizon, you will probably not be able to see the satellite due to atmospheric attenuation and the greater distance to the object. A pass that culminates straight over-head has the least atmospheric distortion, but has percent illumination of at most 50%. (You will also notice a sore neck after following a few passes straight over-head.) I find that the best passes are those directed away from the sun and between 30 and 75 degrees in elevation.

At night, of course! But seriously, this does not tell the whole story. A satellite is visible because it is dark where you are but the satellite is still flying in daylight. This means your viewing location must be close to the edge of the envelope of the earth's shadow which suggests viewing in evening or morning.

After the sun goes down in the evening or before it comes up in the morning, we go through a period of transition from day to night known as twilight. Astronomers use three classifications for twilight based on the sun's elevation angle relative to your local horizon: civil twilight, nautical twilight and astronomical twilight. SatSpy computes the start and end times for all three forms of twilight for your location on any date.

There is no absolute best time to start your viewing. The sky is usually too bright during civil twilight to see any satellites. Towards the end of civil twilight or the beginning of nautical twilight, you may be able to see some bright satellites. I have been able to observe the Shuttle in the evening when no stars were visible. The best time to start your viewing is near the beginning nautical twilight. You can use SatSpy to judge the end of the viewing period by noting the number of passes that are still in sunlight as indicated by the color green.

Depending on your latitude, you may notice large variations in your window of opportunity for satellite viewing. If you live at a high latitude, either in the northern or southern hemisphere, you will find that you can observe satellites long into the night during the summer. This is because your location does not pass as deeply into the earth's shadow as it does during the winter. If you live beyond the Arctic or Antarctic circles where it is daylight 24 hours a day in the summer, you will not be able to see any satellites in that season. In the winter, the situation is reversed. The viewing period tends to be much shorter. At 40 degrees north latitude, you can typically view satellites for only an hour or so in the winter. At lower latitudes there is less seasonal variation in viewing times.

The orbit planes of all satellites, to a greater or lesser degree, slowly precess, or rotate, about the earth's rotational axis. This orbital precession causes the position of the orbit plane to constantly change relative to a viewer's position on the earth. Given the fact that the visibility conditions (viewer in darkness, satellite in sunlight) impose strict constraints on whether or not a satellite is viewable, this means that a particular satellite will not always be viewable every evening. You may observe a satellite on several successive evenings and then not see it again for several months.

There are several different types of satellites that you can view. Here are some of the categories and selected satellites for viewing in each category.

This category (as of late 1998) consists of two systems, the Russian space station MIR (and its support vehicles) and the US Space Shuttle. MIR has been in orbit since 1986 while the Space Shuttle typically flies five to eight missions a year, usually lasting less than 10 days. These two systems are the brightest satellites currently visible. Starting in the spring of 1995, several missions resulted in rendezvous and docking of the space shuttle and the MIR complex. These missions resulted in spectacular satellite viewing opportunities. Many more dockings are planned over the next few years.

Starting in the late 1998, assembly began on the new international space station. It is being placed in an orbit similar to that of MIR. This means that you can expect many good opportunities to view both space shuttles and other vehicles during assembly as well as the new space station itself when it is completed.

Earth observation satellites carry a wide variety of sensors designed to provide multispectral views of the earth. These satellites usually fly in sun-synchronous orbits and are therefore visible from any place on earth. Typical satellites in this category include the US Landsat series and the French Spot satellites. One of the brightest is SeaSat 1 which is an ocean science satellite launched in 1978.

These satellites observe and help predict the earth's weather. There are two groups of satellites in this category, those in low-earth orbits and those in geosynchronous orbits. For the purposes of viewing satellites, only those in low-earth orbits are viewable. Satellites in geosynchronous orbits are simply too far away to be observed without a relatively large (and expensive) telescope.

There are several low-earth orbiting weather satellite constellations that are interesting for viewing. These include the US NOAA and DMSP satellites, the Russian Meteor satellites and the Chinese FenYung satellites. All of these satellites are in highly inclined orbits and are viewable from all latitudes. The Meteor and FenYung satellites are the easiest (brightest) to spot while the DMSP satellites are a little more difficult.

This class of satellites relay communications signals between different points on the earth. There are three types of orbits used by communications satellites: low-earth, molnyia and geosynchronous. For the purposes of satellite observation, only those in low-earth orbit are of interest. Among the visible satellites in this group are the Canadian Alouette satellites and Echo, which is no longer in orbit.

A whole new class of communication satellites is now going into operation. Premier, from an observing standpoint, is the Iridium constellation. This consists of 66 satellites placed low earth orbit in 6 orbital planes to ensure global coverage to achieve its primary goal of worldwide cellular telephone communications. The satellites are relatively small, but each one posesses three highly polished main mission antenna. Under certain (predictable) conditions, sunlight will reflect off of these mirrors causing the satellite to "flare". Magnitudes of -7 or brighter have been observed. By contrast, Venus is magnitude -4 at its brightest.

Navigation satellites send out radio signals that are picked up by special receivers and used to precisely locate the position of the receiver anywhere on or above the surface of the earth. These satellites normally orbit in constellations consisting of 10 or more satellites. Two such constellations are the US GPS series and the Russian Glonass series. The satellites in both constellations are in orbits that are much too high to be seen without a telescope. Several spent boosters from GPS launches are still in orbit and make good viewing targets.

Much of what is in orbit today was placed there by the US and USSR during the years of the cold war. There are several missions of interest to the militaries of both nations. These missions include reconnaissance (spy satellites), communications, early warning of missile launches and weather predictions. Many of the best satellites for viewing are the spy satellites in low-earth orbit. Because of the nature of their missions, the satellites tend to be quite large and in very low orbits. Unfortunately, the element sets for this class of satellites are not published by any official agency. You can occasionally find element sets from amateur groups who compile these elsets from their own observations.

Of the 7000 or so objects in orbit, more than two-thirds fall into the category of "debris". This includes spent rocket bodies that accompanied a payload into orbit, payloads that are no longer active, pieces of space-junk resulting from the explosion of a satellite in orbit and such exotic items as wrenches, cameras and gloves accidentally dropped by astronauts and cosmonauts. For the purposes of satellite viewing, the best objects in this group are the spent payloads, especially those from the Russian and Soviet COSMOS launches. Many of these objects are quite large, the size of a passenger bus or more, and are quite easy to see.

On many occasions I have gone out into the night air, a stack of papers in hand, and binoculars around my neck. Upon sitting down, I found my papers not in order; I was not looking in the right direction; and I had not allowed enough time to get oriented before the pass. Consequently, I missed an interesting pass. You can avoid these problems by doing a little planning before heading outside. You should:

Different people approach this hobby with different objectives. Depending on what you are trying to do, you will be constrained to a greater or lesser degree in picking your passes for a particular viewing session. Before you begin your planning you should consider the following questions:

Do I want to spot a particular satellite?

If you are trying to spot a specific satellite, such as MIR or the Space Shuttle, you are the most constrained in that you can only see the satellite on one or two passes a day. Depending on the orientation of the orbit, you may not be able to see the satellite for several months and will have to wait until the orbit plane precesses to where you get good morning or evening passes. To see the satellite, you must take what you get in terms of passes.

Do I want to see members of a certain class of satellites?

If you are trying to see members of a certain class of satellites such as the DMSP or Meteor weather satellites, you are less constrained. You may be able to see one or more of the individual birds on a given viewing session. To see the entire class, however, may require a viewing strategy spread over several months.

Do I want to see as many different satellites as possible?

This is the least constrained, and to me, the most rewarding, type of viewing. When it does not matter which satellites you are trying to see in a given session, you are free to choose the best passes from the numerous ones available.

The best method for choosing your passes is to use SatSpy's Viewing Opportunities window. This function brings up a window showing the passes as scheduling bars sorted in time order from top to bottom.

When planning a session, you should first compute your passes using this window, then open the corresponding Multiple Skytraces window from the pop-up menu and finally use the Tile command (under the Windows menu) to expand both windows to maximum size. This allows you to see both the time ordering of the passes along with the corresponding skytraces. You can use the find "Sat Name" option on the Multiple Skytraces window to map a skytrace back to a particular scheduling bar (by comparing names).

You should try to group your passes so that you have clusters that are reasonably close together in terms of time and background. That is, over the viewing interval, you may want to find one or more groups of passes that fall within a few minutes of each other and that all pass near a certain constellation. By grouping the passes by time, you may be able to set up your session to minimize your out-doors time (a significant advantage during winter viewing). By grouping the passes by region, you can minimize the amount of movement necessary to readjust your position to view a different region of the sky.

Be careful not to pick passes that have key features, such as passing near a landmark star or constellation, at very nearly the same time. If your landmark objects are in different regions of the sky, you may not have enough time to re-orient yourself between look-times and may end up missing both passes.

On occasion, you may be fortunate to find passes where you may see more than one satellite simultaneously. This happens more often than one might think. I have observed numerous cases of satellites "flying in formation", that is, flying close together along a similar skytrace. Even more interesting, I have observed occasions when to satellites appear to collide head-on in space (even though they are in fact many kilometers apart).

Once you have identified the passes you want to look for, the next step is to print them out and organize them for your viewing session.

After finding a good-looking pass under either the Multiple Skytraces or Viewing Opportunities window, use the Single Skytrace window to print out the pass. Before printing however, use the "Find Times" option to attach drag-n-drop labels to significant time points. These are points on the skytrace where the satellite passes near a landmark star or constellation. You want to mark as many key time points as you can find. Once you are satisfied with the labeling, use the print command from the pop-up menu or toolbar to print out the pass.

Once you have printed a skytrace, you should examine it and decide when you first expect to see the satellite. Each pass should have one point at which to begin searching. Use a pencil or high-lighter pen to mark this point. You may also want to have some system for marking secondary acquisition points.

Once you have printed and marked all of your passes for a single viewing session, you should sort the papers. I generally sort the passes in time order using the time of the primary acquisition point and put the sorted papers in a clip-board for easy handling.

You can make your viewing sessions more productive and enjoyable by preparing your viewing spot before you go outside to view satellites.

You should find a location for your chair that is free from viewing obstructions for the passes you plan on viewing. You may need to identify different locations for different sky regions so as to avoid obstacles such as trees and buildings. If you need to switch positions during an evening's session, be sure to allow yourself enough time to relocate between passes.

A small table or platform next to your favorite chair can be very useful for holding your binoculars, flashlight, clip-board, drinks and any other items you may need. If you use a table, make sure that you can easily reach it from your chair as you move from pass to pass during your viewing session.

Be sure to place all of your viewing paraphernalia where they are easy to reach. Nothing is more frustrating that fumbling around in the dark looking for your flashlight or binoculars when you know you are missing a pass.

Once you have prepared your viewing materials and location, be sure to allow enough time to position yourself before the first pass.

When you head out to your viewing spot (which hopefully you have already prepared), you should allow yourself a few moments to get into your chair and locate all of your materials. You will probably need to use your flashlight to do this.

If you go outside from a lighted enclosure just before your first pass or if you use a non-filtered flashlight, you need to allow enough time to allow your night vision to become established. Depending on your age, this may take from a few seconds to a minute or two.

Once you have your night vision, you need to get your bearings relative to the night sky. You should take a moment and look up and try to find some familiar constellations and the north star. You should then look for the landmarks you established for your first pass.

This question probably has a different answer for each and every observer. For myself, I usually try to give myself at least two to three minutes before going outside. Since I am able to view satellites from my back deck, this is usually plenty of time. If I am in an unfamiliar location, I usually try to give myself at least five minutes to get ready.

Once you are in position and you know a satellite should appear soon, how do you find it?

If you know that the satellite you are looking for is bright, you should look toward the landmark star or constellation using only your naked eye. Focus your eyes on the background stars and wait for the satellite to appear. If you do not see the satellite at the exact time, do not give up; satellites sometimes appear a little late (or early).

Looking for a dim satellite is the same as looking for a bright satellite except that you will need to use binoculars to see it. In addition, since the field of view of a pair of binoculars is less than that of your eyes, you may need to scan around the landmark location in order to find the satellite.

You will not always find all of your targeted satellites. I consider it a successful session if I see more than half of the satellites I am looking for. If you have not found your satellite within a minute or so of its predicted appearance time, you will probably not see it on this pass. At that point, it is best to give up on this pass and move on to the next.

The Single Skytrace printouts provide a notes section for recording your observations. You should make a habit of filling out the information. This will enhance your viewing experience by allowing you to keep track of which satellites you have "bagged" and to record anything unusual you may have witnessed.

You can estimate a satellite's visual magnitude by comparing its brightness with that of a known star. If you find a star that is about the same brightness as the satellite, you can use SatSpy's "Find Star" feature to get the magnitude of the star. As you accumulate your own visual magnitudes, you can add them to SatSpy's internal set by editing the EMP-MAG.DAT file.