Observing Satellites
When can satellites be seen?
Observing Mir
The Daily Predictions Page for All Visible Satellites
The Pass Details Page
The Mir 10-day Prediction Page
Accuracy
Realtime Orbit Display
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 russian space station Mir 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 Mir, 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 Mir
The Mir space station 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 (not always for reasons
its controllers would wish!). For this reason we have developed a special page with predictions for the next 10
days just for Mir.
Mir's 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 Mir can be seen, we give a rough estimate of when the
next batch of passes will start.
Mir'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°, Mir 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;
Satellite | Starts | Max. Elevation | Ends |
Name | Mag. | Time | El. | Az. | Time | El. | Az. | Time | El. | 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 Mir 10-day Prediction Page
This page gives a table of the upcoming opportunities to see Mir 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;
Date | Starts | Max. Elevation | Ends |
Time | El. | Az. | Time | El. | Az. | Time | El. | 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 Mir 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 Mir 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 | 0° | 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
Mir 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
Orbital Information Group (OIG) of
NASA 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 Mir, 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.
Realtime Orbit Display
Those users running Windows 95 or Windows NT
and an ActiveX capable Web browser
can view a realtime display of Mir's orbital
position. The display looks similar to the above picture, and makes use of an
ActiveX control.
The first time you try to load
this page, your browser will have to download the ActiveX control and several large auxilliary
files from the server. This can take several minutes,
depending upon your Web connection speed, so please be patient. Subsequent visits to the page
will be very fast, because the components are already available on your hard disk.
The display shows the current position of Mir in its orbit, and is continuously updated. The user's
location is also indicated, together with several orbital parameters. The features of the display include;
- Shading of the day/night areas of the world.
- The portions of its orbit where Mir is in the earth's shadow are shown as a dotted line.
- The region of the Earth where Mir is above the horizon is shown as a "circle" around the current position.
- The observer's location is indicated by the label "You" and the region where Mir would be above
the observer's horizon is also shown as a circle.
- The user can click on the display mode button to flip between the globe type view shown above, and
a flat map view (shown below).
- The globe view shows the actual height of Mir's orbit correctly scaled relative to the earth.
- Both display modes are completely redrawn every 10 minutes, to keep Mir close to the centre, and to ensure
accuracy of the day/night shading.
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).
Developed and maintained by
Chris Peat, Heavens-Above GmbH
Please read the updated
FAQ before sending e-mail.
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