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Explanatory Notes

ISS as seen from the Shuttle Observing Satellites
When can satellites be seen?
Observing the ISS
The Daily Predictions Page for All Visible Satellites
The Pass Details Page
The ISS 10-day Prediction Page
Accuracy
Contacts

Introduction
Many people have probably noticed satellites passing overhead on a clear night and wondered what they were. We maintain this set of dynamic Web pages which generate predictions of visible satellite passes for any point on the Earth's surface. Our aim is to promote public interest in spaceflight, and it can be a very satisfying experience to make the predictions for an evening and then to observe and identify some of the satellites. To the best of our knowledge, this site is unique on the Internet because it is the only one to offer dynamically generated predictions for ANY location using the latest orbit data.

Observing Satellites
There are over a hundred objects orbiting the Earth which can be seen with the naked eye, and this means that there will be several passing over your location every evening, wherever you are. Due to its large size and relatively low orbit, the International Space Station ISS is a very bright object and can be easily seen, even under poor viewing conditions. Satellites can be seen as star-like objects moving quite rapidly across the sky, and most newcomers are surprised at how fast they move. They can be distinguished from a high-flying aircraft because they normally don't blink, make no sound, and occasionally can be seen entering or leaving the Earth's shadow. The apparent speed is similar to a high-flying aircraft, because the ISS, for example, travels about 30 times faster but is 40 times higher up. Satellites in higher orbits will have a slower apparent motion which is roughly inversely proportional to their height. Sometimes, a satellite will suddenly become much brighter for a few seconds, when the angle between the observer, the satellite and the sun is just right and the sunlight glints off the solar panels or some other part of the structure. Many of the visible objects are spent rocket upper stages which are tumbling and this causes a periodic increase and decrease in brightness with a period of a few seconds which can easily be observed.

The best way to observe satellites is to generate the predictions page for the particular evening you are interested in as late as possible, and then print it out if you can. That way, you will have the latest, most accurate predictions available to refer to. It is always a good idea to give your eyes a minute or so to adjust to the darkness before you start looking, and if possible, find a spot away from street lights which can spoil the viewing conditions.

For a good introduction to observing visible satellites, see the Visual Satellite Observer's Home Page.

When can satellites be seen?
Satellites are visible only when the sun is already well below the observer's horizon but the satellite is still in sunlight because of its altitude of several hundred kilometers above the Earth's surface. This means that visible passes only occur within about 2 hours after sunset, or 2 hours before sunrise. All the predictions assume that the sun has to be at least 6° below the horizon, otherwise the sky will still be too bright. Satellites in low orbit complete approximately 15 orbits around the Earth each day, and can rise above the horizon several times at a particular location. Only occasionally, though, will the lighting conditions be right for it to be visible. Usually, it will either pass over during the daytime, or deep in the night when the satellite is also in the Earth's shadow.

Observing the ISS
The ISS is much brighter than the other visible spacecraft and is of particular interest to many people because it is almost permanently manned and is often in the news. For this reason we have developed a special page with predictions for the next 10 days just for the ISS. ISS visible passes usually occur in batches lasting for several days, then there will be a gap of several days before another set starts. These batches also alternate between early morning and evening passes, and for most people, the evening passes happen at more socially acceptable hours! What this can mean is that if you are unlucky enough to generate the prediction summary at the start of a gap, the prediction software might not find any passes within the search period. The search period, or "look ahead" time is limited to 10 days because longer term predictions become increasingly more inaccurate. However, to spare the user frequent visits to our site to check if the ISS can be seen, we give a rough estimate of when the next batch of passes will start.

The ISS's orbit is inclined at about 51.6° to the equator. This means that it can NEVER be seen from the polar regions. If you live at latitudes further north or south than 60°, the ISS will never rise high enough to make observing worthwhile.

The Daily Predictions Page for All Visible Satellites
This page gives a table of the visible satellite passes for all the satellites in our database. The predictions are generated for a particular evening or a morning, and we make this distinction to reduce the load on our server since most people are only interested in evening passes. Only passes which reach an elevation of at least 20° and a magnitude of +5.0 are shown. For an explanation of visual magnitudes, visit the Visual Satellite Observer's Home Page. First the information about the observers location and time zone are given, and then comes a table with the actual predictions which looks like this;

SatelliteStartsMax. ElevationEnds
NameMag.TimeEl.Az.TimeEl.Az.TimeEl.Az.
Mir 0.9 17:07:42 20° S 17:08:55 26° SE 17:10:09 20° E
Cosmos 1043 4.1 17:14:38 20° NE 17:15:24 23° E 17:16:14 20° E
Cosmos 1005 4.2 17:21:43 20° N 17:24:01 64° W 17:26:27 20° S

The first column gives the name of the satellite, and those marked as rockets are the spent upper stages of rocket launch vehicles. A few are marked as debris, and these are the results of explosions or collisions between satellites. By clicking on the name, another page can be displayed giving more information about that particular spacecraft and its orbit. The second column gives the calculated visual magnitude at the point of maximum elevation and can be used as a rough indication of how bright the satellite will appear. The third column gives the starting time of the pass, and by clicking on it, the Pass Details page for that particular pass is displayed. For this summary page, the pass start time is defined as the time when the satellite reaches an elevation angle of 20° or, if it is then still in shadow, when it leaves the Earth's shadow. The next two columns give the elevation (angle above the local horizon) and azimuth (compass direction) at the start. The next three columns give the time, elevation and azimuth at the highest point of the pass. Finally, the last three columns give the time and location at the end of the pass. This is defined similarly to the start, i.e. when the satellite drops below 20° or enters into shadow if that happens first.

The ISS 10-day Prediction Page
This page gives a table of the upcoming opportunities to see the ISS over the next 10 days. Predictions much further than this into the future are pointless because the uncertainties in the orbital predictions grow rapidly with time. Only passes which reach an elevation of at least 20° are shown, because lower ones are difficult to observe. First, the information about the observers location and time zone are given, and also the age of the orbit data used to generate the predictions, which give an indication of how accurate the predictions are likely to be (see Accuracy). The table listing the visible passes looks like this;

DateStartsMax. ElevationEnds
TimeEl.Az.TimeEl.Az.TimeEl.Az.
28 Feb 19:32:00 20° NW 19:33:58 70° SW 19:34:04 69° SW
01 Mar 18:34:22 20° N 18:35:39 27° NE 18:36:58 20° E

The first column gives the date when the pass occurs, and by clicking on it, the Pass Details page for that particular pass is displayed. Next comes the start time, which for the summary is defined as the time when the ISS reaches an elevation angle of 20° or, if it is then still in shadow, when it leaves the Earth's shadow. The next two columns give the elevation (angle above the local horizon) and azimuth (compass direction) at the start. The next three columns give the time, elevation and azimuth at the highest point of the pass. Finally, the last three columns give the time and location at the end of the pass. This is defined similarly to the start, i.e. when the ISS drops below 20° or enters into shadow if that happens first.

If there were no evening passes found within the look ahead time, an estimate of the date when the next evening pass season starts is given after the table. If no passes at all were found, the date of both the next morning and evening pass season is given.

The Pass Details Page
This page shows all the details for a particular pass. After the date and observer's location and time zone, the sun rise/set time is given, followed by the elevation angle of the sun at the highest point of the pass. If the moon is above the horizon, it elevation is also given. Then comes a table giving the main events of the pass in their correct time sequence as follows;

Event Time Elevation Azimuth Distance (km)
Rises above horizon 19:28:51 NW 2227
Reaches 20° elevation 19:31:59 20° NW 941
Highest elevation 19:33:57 70° SW 405
Enters shadow 19:34:04 69° SW 407

The entries in the table should be fairly obvious. The distance given is from the observer to the satellite.

Accuracy
The accuracy of the predictions is dependent upon several factors. Firstly, a satellite's orbit is affected by the tenuous upper atmosphere which creates a small, but continuous drag force. This drag is proportional to the density of the ionosphere, which is notoriously difficult to predict. Solar activity can increase the density by over a hundred times more than its average value. Secondly, because of this residual air drag, some active satellites such as the ISS regularly re-boost themselves to a higher orbit. These boost manoeuvres are also unpredictable.

The orbit predictions are essentially calculated by taking the actual measured position and velocity of a satellite at a given point in time (the epoch), and propagating forwards in time taking account of the major forces, which are the Earth's gravitational attraction and the air-drag. The propagation is done using the standard SGP8 model developed by NASA. Given the uncertain nature of the air drag force acting on the satellite, the further the orbital position is predicted into the future, the more innaccurate it will become. This means that the latest orbital parameters should always be used, and the predictions should be regenerated just before observations are planned. We download the latest orbital parameters every day from the Space Track web site run by the USAF Space Command for use in the predictions. The parameters themselves are regularly updated by processing tracking data. This means that if you re-load the prediction page after a few days delay, you will notice that the predictions change slightly, in the light of the updated orbital parameters. Due to the nature of the drag force and the manoeuvres, which (usually) act only along the direction of flight, the times of the pass events are affected much more than the angles.

So what kind of accuracy can one expect? For the ISS, if the time between the orbit epoch, which is given in the satellite summary information page and the time of the predicted pass is less than a week, the times should be accurate to within a few seconds. If you generate a prediction 20 days in advance, however, and don't regenerate it nearer the time of the actual pass, the errors could grow to a few minutes. Other satellites are in higher orbits and are much less affected by the uncertain air drag forces because the air is much thinner at higher altitudes. Furthermore, many of the observable objects are old rocket boosters which are no longer making manoeuvres and so for these objects, the accuracy of the predictions will be even better.

Contacts and Feedback
Heavens-Above welcomes feedback and suggestions for improvements to these pages, but please read the important notes in the FAQ before sending mail to Chris Peat (Chris.Peat@heavens-above.com).

 

DLR