John Pazmino
 NYSkies Astronomy Inc
 2002 May 1 initial 
 2012 March 7 current
    Ken Brown, NYSkies, gave a presentation about observing Earth 
satellites at the Observing Group on 2000 May 20. This talk rekindled 
my own long-ago affinity for these spacecraft. At starviewings in Carl 
Schurz Park and Central Park. Ken brought satellite passage schedules. 
The sight of the satellites further fueled my interest. 
    Over the rest of 2000 I assembled some notes to relate some 
history of home astronomy involvement with Earth satellites. There 
being a substantial and deep litterature in satellite watching, I 
confined myself to the material pertinent for viewing with only eye or 
    Observing artificial Earth satellites was a favorite pursuit of 
home astronomers since the days of the first Sputniks and Vanguards in 
1957 and 1958. Several members of the Amateur Astronomers Association 
were recruited into the MoonWatch project, operated by Smithsonian 
Astrophysical Observatory, to follow the early satellites. The station 
in New York was atop the [former] RCA tower in Rockefeller Center. 
    I had just one night of work there, on a scramble to catch a 
suspected Soviet satellite. It was so bitterly cold and fiercely windy 
on that winter night, long forgotten from the calendar, that I had to 
pass up on future work. I was a timekeeper, to read a large clock as 
note the times when the telescopists first and last saw the satellite. 
    After a couple years, like around 1961, the station was 
decommissioned by SAO. Stations elsewhere in the country were closed 
by 1962 but a few were kept until, I think, 1964. 
    Since then, there were no formalized home astronomy watches for 
satellites. Individually or in clubs, astronomers continued to follow 
the 'birds' on their own. 
Tools and methods
    For the MoonWatch teams SAO provided predictions of satellite 
passes across the local meridian of their stations. The station's 
telescopes were aimed at intervals of altitude along the meridian to 
form a scrimmage line thru which a satellite eventually had to cross. 
    For home satellite watching, things were far more difficult. The 
1960s were the BC era, before computers/calculettes. Because of the 
Cold War, much of the information needed for tracking satellites was 
not readily available to the public. In many cases the very existence 
of a suspected satellite was denied or on purpose wrongly specified. 
    In the absence of computational tools, the maths were all worked 
out by hand, slideruler, charts, and tables. Most home astronomers, 
lacking grounding in maths, resorted to cunning geometry nad graphical 
methods. Despite the crudelity of these methods, the path of a 
satellite could be laid on a horizon or star chart closely enough to 
recognize the bird in the sky. 
    The theory and physics of satellites were gleaned from space 
science journals and occasional issuances by NASA or an aerospace 
company. In New York, members of the Amateur Astronomers Association 
endowed with college or corporation libraries passed on these articles 
to other satellite enthusiasts. 
    In essence, the orbit was traced around a scale drawing of the 
Earth and coordinates measured from it at one or two minute intervals 
of time. These coordinates were marked on a horizon chart for the City 
or on a planisphere set to each moment during the satellite pass. 
    This was a miserable and tedious task for each satellite pass, but 
we learned more spherical geometry, orbit dynamics, and draftsmanship 
then other wise we cared to. 
    For the brighter satellites the larger newspapers reported, 
usually on the weather and shipping page, the passes. The data were 
the rise time and direction, altitude and direction of midpath, set 
time and direction. No account was taken of the Earth's shadow, depth 
of twilight, or age of the Moon. These reports were extremely useful 
for astronomers to advise nonastronomy friends for an upcoming pass. 
    NASA offered a mail service from which we received a computer 
printout of predictions for the next couple weeks. It took several 
days to get this and sometimes the satellite was already in a new 
orbit away from that in the tables. This happened because the 
influence of the upper atmosphere was yet to be understood. 
Home computers
    The first breakthru in satellite tracking came with the first 
electronic calculettes in the early 1970s, followed by home computers 
in the late 1970s. The number-crunching speed and power of the first 
models exceded any home astronomer's imagination. The maths, once 
skipped over in the technical litterature, could be coded in BASIC 
with output displayed on the screen or sent to the printer. 
    There was one hitch. The actual orbit description for the 
satellite was required. The Department of Defense invented a scheme of 
formatting the parameters of an orbit in a standard layout for 
computer programs to digest. The file had two rows, lines, for the 
orbit data, often called a 'two line element' or 'TLE' file. With the 
Cold War still raging, these were often held back as military secrets. 
    In the early 1980s American Telephone and Telegraph broke up and 
nonAT&T equipment could be attached to the phone mains. A spinoff was 
the new era of interchange of information by telephone and computer. 
Home computer companies marketted modems and bulletin board services 
(BBS) quickly sprang up. 
    Many BBSs catered to astronomy and space science. These boards 
posted in their files section the orbit parameters for assorted 
satellites. At first, we had to copy off the numbers and later key 
them into our tracking program. After a while, program authors 
exploited the standard file layout and wrote their program to directly 
read the file without humans keying in the data. To simplify the use 
of the file as an input for tracking programs, the more or less 
standard filetype is 'TLE'. In some instance, we had to rename the 
file to confirm to the filetype required by some peculiar program. 
    The BBSs also carried the tracking programs. They were written by 
enthusiasts and hobbyists. Depending on the computer system they were 
written for, the output was a text file or a graphic display. With the 
crude displays of the late 1970s to early 1980s, the display often was 
a ASCII-art rendering. Ratty, yes, but still vitally useful for 
spotting the satellite. 
    The slow speeds of telcomms was not a hinderance. Modem were of 
only 300 to 2400 bit/sec! The TLE file and the programs were of small 
bytesize, requiring only a couple minutes to download. 
    BBSs were for the most part one-man shops running off of a single 
system box in the operator's bedroom. They broke down frequently or 
were erratic in service. Despite this deficiencies, bulletin board 
services made life orders easier for observing satellites by providing 
a forum to ask questions, give news, and troubleshoot problems. 
    BBSs communicated thru the regular domestic phonelines. To get 
into a BBS we had to directly dial its phone number. If the BBS was 
outside our local dialing area, we suffered nasty phone bills for the 
long distance call. 
DOS still rules
    By the early 1990s, virtually all home computers were IBM models, 
or compatibles, operating DOS or Windows. Virtually all satellite 
programs were written for these systems, altho there remained pockets 
of loyalty to nonIBM computers. A workaround for nonIBMs was to run an 
IBM emulator and then run the satellite program thru the emulator. 
Today, mid 2002, just about all nonIBM computers have IBM emulators 
for them. 
    A newer operating system for IBMs is LINUX, a translation of UNIX 
for home computers. It is still a minority system but is gaining favor 
among technical folk. Satellite tracking programs are emerging for 
LINUX, some being translations from DOS or Windows, others come from 
    Despite the pervasion of Windows in home computing, DOS remains a 
strong operating system for astronomy applications. DOS satellite 
programs are just as valid today, mid 2002, as they were in the 1980s. 
That's because Windows adds nothing to the computational power of a 
computer not already tapped by DOS. Of course, the methods and theory 
of orbits advanced since then, and the newer programs can well 
incorporate them. Hence, if you got DOS satellite programs, keep them! 
    For nonIBM computers, the TLE files are valid; they are pure ASCII 
textfiles. The results from nonIBM computers are just as credible and 
accurate as from an IBM, given equal competency of program code. 
    Editions of WIndows from Vista and later do not run DOS programs 
consistently. They have no full-screen feature and many DOS programs 
freexe or crash. Install a DOS enulator! This, like an emulator for a 
nonIBM system, allows DOS applications to run normally. 
Russia and the Internet 
    The Cold War ended with the implosion of the Soviet Union in the 
early 1990s. In a frantic effort to restructure its society, Russia 
unlocked its secret satellite information. The US relaxed its secrecy, 
also. Mail and phone requests at American spacefaring agencies were 
freely answered and data was openly supplied. BBSs could update their 
TLE files more frequently; agencies were willing to send the files to 
the BBSs directly as a courtesy. 
    The next quantum step came with the Internet. Altho in use since 
about 1970 among military and university computers, it busted into the 
public realm in mid 1993. The former BBSs reconstituted themselfs into 
Internet services and allnew services blossomed. 
    Internet immediately linked everyone with information sources all 
over the world. We could litterally cruise the satellite databases of 
the former Soviet Union! 
    Internet allows for online processing and distribution of 
satellite predictions. We can now do the work thru a website and print 
out the results, all tailored for our specific location. Some websites 
keep watch for selected satellites and email us a schedule of passes 
and other newsy items. 
    Internet allows us to explore satellites from other countries, 
previously essentially closed to Americans. The space organizations of 
Japan, Europe, China, others, post on their websites information about 
their satellites. 
    Internet opened the libraries of universities and corporations to 
the home astronomer. We can now harvest journals, magazines, articles, 
reports previously unavailable and unknown to us. We are freed from 
multigeneration photocopies or hand copied scribblings. 
    By the end of the 20th century the entire astrodynamics discipline 
was converted to metrics and all proper satellite litterature 
nowayears is written in metrics. Legacy material, before about 1990, 
may contain oldstyle measures. It's wise to keep handy a cribsheet for 
oldstyle units if you refer to such older works. 
    For casual use, the vernacular metric units may be used. In 
technical work, specially for formulae and equations, the Systeme 
Internationale is pretty much required. 
Time standard
    Satellite operations are clocked in Coordinated Universal Time 
(UTC) or, somewhat erroneously, Greenwich Mean Time (GMT). I say that 
because Greenwich Mean Time no longer is uniquely defined. It is 
loosely a synonym for UTC. UTC in turn is commonly called just 
Universal Time (UT), altho there is no such a thing as plain Universal 
Tine. For practical purposes, when you see 'Universal Time' or 
'Greenwich Mean Time', it is really 'Coordinated Universal Time'. 
    The 24-hour clock is used thruout. For casual applications, time 
in AM/PM style may be used. It is amazingly common to mix up, even 
among astronomers, a pass near sunrise for one near sunset or vice 
versa, because of inattention to the AM/PM phase of the hour. 
    Note that since December of 1999 the addition of leapseconds has 
been rare, only a couple thru 2012. For many years there was a dialog 
in the world's timekeeping services to allow Earth rotation i UTC to 
diverege from atomic time with no more use of leapseconds. Leapsecond 
additions are announced a few months before their insertion, usually 
in summer for the next instance on December 31. 
    Time clocked by GPS satellites is not corrected for leapseconds, 
altho it runs at the rate of UTC. The GPS receiver, on which we read 
UTC time, itself adds in the required leapseconds. The GPS signal 
contains this correction, which is sent to the satellites by the 
ground station from time to time. 
    For casual use, like telling your nonastronomy friend about a 
coming pass, the local zonetime may be stated. Be quite careful about 
the shift for daylight savings time and for the crossing of a midnight 
between your location and Greenwich. 
World Trade Center
    There was a sudden tightening of access to certain satellite 
resources following the attacks on the World Trade Center and 
Pentagon, plus the failed attempt in Pennsylvania, on 2001 September 
11. Certain types of bird are no longer carried in the public parts of 
websites. Even the Space Shuttle, starting in 2002 March, withholded 
launch and orbit details until the last minute. 
    Some anonymously accessible websites for satellites now require 
registration, where you provide personal information. As at mid 2002 
this barrier hasn't been too high or deep. Yet we may feel icky about 
divulging personalia to download the mission profile of a spacecraft. 
It comes down to thinking with your belly. 
    You can watch Earth satellites entirely by eye but binoculars are 
a great help. They sharpen the image, darken the background sky, make 
out the dimmer field stars, and help keep your vision concentrated in 
the right direction. Any set is adequate, so long as it is useful for 
regular starviewing. The handheld models of modest magnification are 
best for spontaneous viewing and following a fast moving satellite. 
    An ordinary astronomy telescope is the wrong device for satellite 
watching. Its mount will not allow easy quick aiming and tracking. An 
equatorial mount is a disaster for satellite observing due to its 
totally wrong axis motions relative to the path of the satellite. An 
alt-azimuth mount is a little better but still extremely clumsy. 
    The newer 'goto' instruments can aim at and follow a satellite, 
given current orbit specs. These are downloaded from the scope maker's 
website. It turns out that just about no one uses this feature. It is 
too risky to keep the eye against a slewing eyepiece. 
    Clothing for the weather is essential for health and comfort. Be 
mindful of the drop in temperature and increase of wind from day into 
night. Use the same precautions as for regular stargazing. 
    Starcharts with RA-dec grid are good for plotting the satellite's 
path when expressed in celestial coordinates. Many astronomers are 
more comfortable following the satellite against the stars rather than 
the horizon. Tracking and prediction methods differ on which epoch to 
use, but that for 2000 is adequate for the next decade or so. 
    Lawnchairs are handy for general comfort, altho you may have to 
twist around in them to follow a satellite across a long path. Charts, 
prediction printouts, clock, pencils and notepad, some refreshments, 
are all handy. In sum, bring with you what you would for ordinary 
stargazing, plus the information pertinent to the satellites. 
Satellite designations
    There are two numerical codes to uniquely designate a satellite. 
One is a pure serial number starting from '00001' for the first 
artificial satellite Sputnik 1 launched in 1957. The other is a 
sequence within each year. Sputnik 1 would be '57001' for the first 
satellite in 1957. 
    Both methods are constrained. The sequence code fails for Y2K. 
Codes in the 21st century begin with '00', '01', '02', ... . The 
serial code may fail when the number of satellites excedes 99,999. In 
any case, the systems are locked in and can not be easily redefined. 
    When a launch puts several bodies in orbit, the sequence code is 
suffixed by a capital Latin letter. The main active payload gets 'A' 
and the other pieces from the same launch get other letters in no 
special order. In the serial system, eaxh body gets its peculiar 
serial number. 
    Failed launches, where the payload never achieved a stable orbit, 
are not given any satellite designation. Satellites departing orbit 
for an interplanetry trip and those that permanently merge, like 
segments of ISS, keep their numbers but no longer get own TLE files. 
    Pieces found in orbit after launch -- collision debris, released 
parts, undetected satellites -- get the next open serial number, but 
the sequence number is that for the year and order of the launch. It 
is suffixed with the next open Latin letter. 
    The designation systems are still commonly known by their original 
managing agencies. The agencies changed names over the years but the 
systems never latched onto the new names. We say 'NORAD' or 'satellite 
catalog number' for the serial number; 'COSPAR" or 'international ID' 
for the sequence number. 
    Most satellites have common names from the series of mission they 
are part of or some peculiar name for just that probe. In the TLE the 
first, zeroth, line has the common name for the satellite. This may 
not be the name you would ordinarily refer to the satellite, an other 
designation or technical name, or an earlier name changed later. Yes, 
the name of a satellite can and does alter before launch and while in 
Appearance of satellites
    A satellite appears in the sky as a mere point like a star 
drifting slowly against the stars. It could be taken for a high 
airplane at first. A satellite may be of any brightness from the 
brightest planet to the dimmest star. 
    Clues are the steady motion, straight path in the sky, single 
point of light, and refusal to resolve in binoculars. A satellite may 
wink or blink but still as a single, not multiple. point. 
    None of these alone is sufficient to confirm a satellite but in 
combination and with general sky wiseliness you can generally know 
that a satellite is passing by. 
Number of satellites
    Sticking to those visible by eye, there are scores of satellites. 
The number varies over time with the launch activity of spacefaring 
nations and the removal of satellites from orbit. The latter may be 
deliberate like the Mir or Space Shuttle, or natural like a decay and 
burnup in the upper air. 
    All in all I would hazard that at any given moment there are a 
hundred spacecraft within reach of binoculars all over the Earth. A 
dozen may be seen in the course of a single night from a dark sky. 
    In skies with luminous graffiti, the number of satellites falls 
off precipitously, like for meteors and aurorae. In some American 
towns hope remains only to see the very brightest of satellites. 
    By the 2-thous new satellites tend to be dark in color or texture 
with little intent to be easily seen from the ground. Only ISS remains 
visible to the casual eye for being huge, about 100 x 50 meter, with 
lots of refelctive solar panels. 
Satellite orbits
    Almost all satellites visible to eye and binoculars are in low 
Earth orbit (LEO) with periods around 90 minutes. These are seen 
mainly because they are closer to us and move faster angularly. The 
technical definition of 'low Earth orbit' is one within the height for 
period less than 225 minutes. 
    Higher than LEO there are few satellites within eye and binocular 
range. With a regular starviewing telescope we can spot geostationary 
satellites for they stand still in the sky. Their altitude and azimuth 
remain fixed or drift very slowly. Geostationary orbits are about 
42,150 kilometers from the Earth's center 
Elevation and altitude
    'Elevation' and 'altitude' in astronautics are exactly swopped 
relative to their meaning in astronomy! A satellite's 'elevation' is 
its angular displacement above or below the horizon. This is the 
astronomer's 'altitude'. 'Altitude' for the astronauticist is the 
linear displacement of a point above the Earth's surface. This is the 
astronomer's 'elevation'. 
Viewing hours
    Satellites in LEO are seen best in twilight. They are in sunlight 
but we are in shade of the coming night. The sky around the satellite 
is darkening to improve contrast. A similar reasoning applies to a 
morning twilight sighting. I assume an evening watch because far and 
away the bulk of stargazing is done in this part of the day. 
    In full night the Earth's shadow occupies the whole sky, hiding 
satellites from view. In the early to mid twilight the Earth's shadow 
is in the east opposite the sunset point. A satellite can enter or 
leave the shadow in midsky. It vanishes or pops out in open sky, as 
distinct from being covered or revealed around landscape or cloud. 
    Daytime sightings are practicly impossible, except if the bird 
suffers a mirror reflection of sunlight that strikes us on the ground. 
The Iridium satellites are of this type. Some satellite observers keep 
eye out for their 'flashes' or 'flares'. 
Time drift
    The orbital period of a satellite is surely not a rational 
fraction of a day. A pass occurring at a certain hour will repeat a 
little earlier or a little later on the next day. This drift on the 
clock eventually pushes the pass out of the evening hours, either into 
twilight and daylight or into the late night or predawn hours. This 
effect is superimposed on real displacement of the orbit due to 
    Precession of the orbit and atmospheric drag upset this neat 
pattern. The orbit itself migrates around the equator (precession) and 
decreases its period (drag) on timescales of days or weeks. Successive 
passes, day after day, will not repeat at fixed intervals. 
Satellite brightness 
    The 'magnitude' of a satellite, as an index of apparent 
brightness, is on the same scale as that for astronomy. A satellite 
predicted to be of magnitude +1 should be as bright as a first 
magnitude star. 
    Satellites range in brilliance from threshold dimness to 
dazzlingly bright. The latter are the 'flares' from the radio panels 
(not solar panels) of the Iridium series of bird. 
    Understand well that in most cases the predicted magnitude can be 
badly off by a full magnitude or more. Predicting a satellite's 
magnitude is tough. The best to expect is a crude assessment of 
'bright' versus 'dim'. The magnitude is cited only to the whole 
    Some satellite services cut thru the complexity by asserting all 
satellites are spheres of a nominal reflectivity. Others model a 
satellite with a couple stock shapes and texture.
    A satellite in the west after sunset is dimmer than when it moves 
to the east. In the west it's in a brighter background sky and we see 
more of its night side. When in the east, but away from Earth's 
shadow, the sky is darker and a greater part of the day side faces us. 
The behavior is analogous with that of the Moon. 
    In the end there is no sure way to tell in advance how bright the 
the satellite will be. On the other hand, long experience with certain 
satellites can guide us to know what to look for. 
    The 'absolute magnitude' of an Earth satellite is the apparent 
magnitude is would have if it were 100 kilometers up in the zenith and 
half illuminated by the Sun. Satellites in general appear dimmer than 
their absolute magnitudes because they are higher than 100Km, are seen 
along a slant range far more than 100Km, and be less than half phase. 
Flashing and blinking
    A satellite may have an irregular shape, panels, varied texture 
and color. As the craft tumbles or rotates, it sends to us a varying 
amount of reflected sunlight. The point in the sky oscillates in 
magnitude rythmicly during the passage. 
    The satellite may be visible between its bright and dim phase. The 
amplitude remains within detection of the eye or binoculars. Or the 
satellite may fall out of sight in its dimmer phase. 
    If there is a mirrored part of the craft, it may catch the Sun and 
send down a specially brilliant glint, causing a flash or flare. This 
is usually a one-shot event during a given pass. The Iridium series of 
satellite is famed for its intense flares. These are included in the 
satellite predictions by many websites. 
Earth's shadow
    As the Sun sinks below the horizon the edge of the Earth's shadow 
rises in the sky opposite to the sunset point. From sunset to the end 
of civil twilight you can trace this edge by pointing with both arms 
outstretched at the antisolar point, diametricly opposite from the 
Sun. Then swing the arms outward and downward. After civil twilight 
the shadow border diffuses rapidly and no longer is so simply traced. 
    Within the shadow the satellite is totally invisible. Nonehave 
beacons or other lampss. With no sunlight on it, there is nothing to 
reflect back to us. The paenumbra is deep enough to allow a slow 
winking off or on of the bird within about one second. 
    Computer programs commonly offer an option for the effect of the 
Earth's shadow. When engaged the satellite path  ends or starts at the 
shadow boundary, not at the horizon beyond it. 
Satellie elements files 
    The element files are distributed by NASA, NORAD, and several 
other agencies. They are commonly called NORAD or NASA elements. The 
format is three lines of data placed in specific columns, so a 
satellite tracking program knows where every thing is. The most common 
filetype for the file is 'TLE' for 'two-line elements'. 
    I note the file has THREE lines. The first is line #0 containing 
just the name of the satellite. The other two, lines #1 and #2, 
contain the actual numbers for computing the orbit. 
    The fields in the element set are laid out in specific columns. 
The layout was originally devised to fit the 90-colymn width of an IBM 
punch card, the way in the 1950s to give input to the computers. 
    The file may have elements for many satellites, each with its own 
three-line record, all chained together one after the other. The order 
in the file is typicly that of launch, but it really doesn't matter. 
    The file is a pure ASCII file and can be edited. It is thoroly 
reckless to alter the numerical fields! You can edit the name if you 
want. The International Space Station is not, as you would expect, 
'ISS', but named for its first module,Karya. The Space Shuttle is 
named for the launch sequence, like 'STS-87', not the name of the 
vehicle or mission. 
    You may delete entire sets of element for satellites you are not 
interested in. Be very careful to delete all three lines precisely and 
to close up the resulting gap in the text. This may be handy if you 
download a large file for getting the elements of a certain few 
    Be VERY SURE that you save the edited file as a plain text file 
with no conversion to a wordproc format! It's best NOT to edit the 
file thru a wordproc. Use Notepad or other text editor. 
    The standard element file has no factors for calculating the 
apparent magnitude of the satellite. Hence very few tracking programs 
offer a brightness option. Some individual followers of satellites 
added a couple fields to the element file to allow for brightness 
computation and then write programs to do so. These expanded files are 
useful only for their peculiar programs. 
Orbit dynamics
    For LEO birds the astronomy methods of calculating an orbit are 
sufficient as a start. The orbit is described with the usual orbital 
elements, with altered reference planes and points. The Earth equator 
is the ground plane, not the ecliptic. Longitude along the equator is 
in degrees, not hours like right ascension. 
    Such orbits are not at all rigid in space. Atmospheric drag slows 
the satellite and makes it descend into denser air. Ultimately, if not 
counteracted by rocket firings, the satellite is incinerated by 
friction like a meteor, ending its life. 
    Satellite elements are not like those of asteroids or comets, good 
for some weeks or years. There is, unlike for comets and asteroids, no 
'elements of record' that can be tabulated in some roster of 
satellites. The elements are valid for only a short time, days at 
most, and can be altered by rocket firing, solar pressure, deliberate 
deorbit, air drag, and other forces. Such circumstances terminate the 
validity of the orbital elements. 
    Some tracking program will check the the TLE file's epoch field 
and warn you if the set is too old. 
    Because the elements resemble those of a Kepler orbit for a 
planet they are commonly called 'Keplerians'. This term applies to 
both the collection of elements and to the file that contains it. 
    Earth satellites are in elliptical orbits. Their excentricity 
varies from zero (circular) to over 0.9 (Molniya). The shape is 
governed by the peculiar purpose of the probe and the success or 
mishaps of the launch. 
    While a strongly excentric satellite passes over us, it moves 
faster across the sky when near its perigee; slower, apogee. For very 
excentric orbits the motion at apogee is very slow, almost stationary 
for many hours. In fact, the Soviet Union and then Russia use this 
orbit behavior for their 'stationary' comms satellites. The country is 
so far north that it can not directly place geostationary orbits from 
its launch bases. 
    By putting the satellite in a strong ellipse orbit, with the 
apogee in a far north latitude, the satellite hovers over Russia for 
most of the entire orbital period. It then rapidly swooshes thru 
perigee in a far south latitude. This is the Molniya orbit, after the 
series of satellites that first exploited it. 
    Inclination is the angle between the eastward direction along the 
equator and the downrange direction in the orbit. When this is between 
0 (equatorial orbit) and 90 (polar orbit), the orbit is prograde or 
direct. Inclinations more than 90 (satellite travelling westward) the 
orbit is retrograde. 
    The inclination allows a greater or lesser part of the world to 
see the satellite. Or to allow the satellite to see a greater or 
lesser part of Earth. For LEOs the inclination is closely the extremes 
of north and south latitude over which the satellite can pass thru the 
local zenith. A LEO satellite with inclination 34 degrees can touch 
the zenith at places within 34N and 34S latitude. 
    A band beyond these limits may see the satellite lower in the 
local sky, to the north for the south limit and to the south for the 
north limit. For LEO a satellite can reach at least 20 deg altitude 
from a zone six degrees beyond the limits. 
    For New York, the Space Shuttle when in a standard flight of 
inclination 28 degrees can not be seen at all (we're at 41N latitude). 
If the Shuttle is heading to ISS it must get into an orbit of 52 
degree Inclination and then we do see it. 
    The inclination governs the general path of the satellite across 
our sky. We see a prograde satellite pass southwest to northeast when 
it is heading north and northwest to southeast when running south. A 
retrograde orbit takes the bird southeast to northwest on the north 
arm; northeast to southwest, south arm. 
    In order to maximize the vector headstart from Earth's diurnal 
rotation a launch is typicly made due eastward from the launch site. 
This places the satellite in orbit near its extreme north (for the US 
and Russia) limit of its orbit. This is the latitude of the launch 
site. Once in orbit, the satellite sweeps between this limit as north 
and south latitude. 
    Most Russian launches are of high inclination while most Americans 
ones are of lower inclinations. This disparity results from the 
latitudes of the respective launch sites. In the Cold War the orbit 
inclination for an unknown satellite was a clue for the latitude of 
its launch site. 
Semimajor axis
    This is not used as such in satellite work! In the stead the 
perigee and apogee distances are given. There are two ways to do so. 
One is the layman's method to give the elevation (linear height) of 
the perigee and apogee above the Earth's surface. The other is to give 
the distances from the Earth's center.
    LEO satellites tend to be specified by elevation while those higher 
up tend to be dimensioned by radius from center. Elevation above the 
Earth must be corrected by adding to each parameter the radius of the 
Earth before attempting calculations with it. Earth's radius is, 
closely enough, 6,371 kilometers. 
    There is no official breakpoint for using ground elevation and 
central distance. For birds with perigee-apogee in myriads of 
kilometers, there can be an ambiquity. Before doing any computations 
with the specified figures inquire after their meaning first. 
    The perigee-apogee are commonly written as 'x by y'. This is NOT a 
statement of the semimajor and semiminor axes! Both x and y are on the 
SAME straight line, the line of apsides. Their SUM, plus the diameter 
of Earth (12,742Km) is the major axis. Half is the semimajor axis. 
    Strictly, precession is the swaging of the orbit such that its 
ascending (and descending) node drifts along the equator relative to 
the vernal equinox. In casual use the term refers to the drift of any 
orbit element over time. The evolution of elenets is due to 
geophysical forces like the irregular gravity field of Earth, air 
drag, solar wind pressure. 
    The effect of either sense of precession is to create 'seasons' of 
visibility for a given bird. As the elements evolve day by day,they 
alter the path in the sky over agiven station. When the elements are 
within certainbounds the path is over the stationand we can see the 
satellite. With other values the satellite can miss us. 
Heliosynchronous satellites
    When the inclination and period is rigged to match the Earth's 
rotation, the satellite passes along each longitude meridian at the 
same local hour. The satellite is placed in a polar orbit, inclination 
near 90 degrees, so it tracks along a longitude meridian. One example 
of this heliosynchronous satellite is the Landsat series. 
    The satellite's view of each place enjoys the same solar 
illumination. This situation allows for consistency in resource 
mapping and monitoring, surveillance, and certain telcomms. Such a 
satellite may be seen if and only if it is synched to pass over in the 
evening twilight. 
Geostationary satellites
    A geostationary satellite is placed in an equatorial circular 
orbit, excentricity and inclination zero, and with radius 42,150 
kilometers. At this distance the period is 1 sidereal day, pacing that 
of the Earth, so it stays over the same longitude meridian on the 
equator. This puts the bird in a fixed spot in the sky, keeping the 
same altitude and azimuth all the time. 
    This is the classical case of a telcomms satellite. Its fixed 
location in the local sky allows for fixed ground antennae to converse 
with it and a fixed area of Earth to broadcast to. 
    Sites along the equator to place a geostationary satellite are a 
couple degrees apart, making perhaps 150 in all. The spacing helps 
reduce corss-signal between adjacent sites. They are assigned by 
global rules telcomms companies. They are speicified by the longitude 
they stand over. 
    By parallax from New York, the geostationary satellites sit in a 
belt at -6 degrees declination. The satellite is not perfectly fixed, 
but wanders within about 1/2 degree. This comes from displacement by 
geophysical forces and adjustment by occasional rocket firings. 
    Geostationary birds, from their great height, are quite dim, no 
brighter than 8th magnitude. Because they maintain a fixed alt-azm, a 
starviewing telescope can aim at them without tracking. The satellite 
is distinguished from stars by its stable location in the eyepiece 
while the stars drift behind it in diurnal motion. It returns to a 
given spot among the stars 3.94 minutes earlier each night. 
    We can look for a geostationary satellite more or less at our 
convenience. We usually wait until it stands before prominent 
asterisms or nebulae, one favorite being the Orion Nebula at -5-1/2 
degree delcination. 
    Geostatinary satellites are occasionally eclipse by the Earth's 
shadow. This happens near the equinozes with the Sun, and Earth 
shadow, cross the equator. These sunless periods must be factored into 
the operation of the bird, like witching to battery power or going off-
duty for maintenance for the duration 
    A geostationary satellite is a special case of the general 
geosynchronous satellite. The latter is any bird with period of one 
sidereal day, regardless of orbit inclination or excentricity. The 
geostatinary satellite is a geosynchronous satellite with excentricity 
and inclinaton both zero. 
    For the most part in New York we see no launches. We are too far 
from any spaceport, the nearest being Kennedy some 1,500 kilometers to 
the south. All satellites we see are already established in orbit. Or 
are by chance in reentry over our heads. 
    An exception was the Space Shuttle. When it headed for the 
International Space Station, it traveled from Kennedy north along the 
East Coast. It reacheed New York to the east over the Atlantic Ocean 
about ten minutes after launch. We can see the main engine flare 
rising out of the southeast. At about due east the main engines cut 
off and the Shuttle disappears in mid flight. All this is seen only 
against a dark sky. We miss daytime launches. 
    A rarer exception is a launch from Wallops Island. On occasion the 
rocket is sent to orbital height for placement of a small satellite. 
In such a case we may see the rocket exhaust in the south. The flame 
veers to the east and rapidly fades into the distance. 
    It is not possible to accurately predict when and where a satellite 
will reenter the atmosphere and incinerate. The best we can do is post 
an alert within a couple hours of the anticipated descent. 
    The newer satellites are fitted with means to force a reentry under 
control and command over unpopulated areas of the world. These 
reentries are not visible from New York. 
    The best controlled reentry is that of the Space Shuttle where the 
craft glows from air friction but lands safely. This is out of range 
from the City. The Shuttle reentry path is across the southern United 
States from the Pacific Ocean to landing at Kennedy Space Center. 
    Older satellites with no descent facilities or losing command and 
control are for the most part a lucky shot for us in the City. 
    The Soviet Union was infamous for letting its unwanted or failed, 
satellites decay anywhere without regard to potential harm on the 
ground. The Cosmos 954 was particularly notorious for landing near 
Great Slave Lake, Canada and dumping into it (or nearby, no one really 
knows) its lethally radioactive power unit. 
    A decaying satellite looks like the mother of bolides or 
fireballs. The path in the sky tends to be long, crossing a major 
section of the sky, from the shallow angle of descent and the large 
mass to consume. With satellites being fragile and easily broken up 
under heat and stress, pieces may separate from the main mass to form 
secondary fireballs. Sparks, bursts, flames, smoke may issue from the 
'meteor' and a smoky train may persist for many minutes. 
    In many cases there is no alert issued and we must rely on a 
delayed report that such-&-such satellite reentered over a place at a 
time closely matching that for the stupendous meteor. Even veteran 
skywatchers can get a good scare from a unexpected reentry. 
    A most tragic reeentry was the disintegration of Shuttle Columbia 
on 2003 Febraruy 1 over Texas. Its crew of seven were incinerated 
within seconds. The cause was likely related to loss of many heat-
protective tiles on the vehicle after loose ice from the external fuel 
tank fell on them at launch.. 
Ground track 
    The path on the ground over which the satellite passes is the 
ground track. At all points on this track the bird passes thru the 
local zenith. Due to Earth rotation and precession the track slides 
around the Earth so that it seems on a map or globe of the world to be 
a spiral line. This effect is the 'ball of string' effect, after the 
way string is winded on its armature in a spiral pattern. 
    Satellite tracking programs offer at least the ground track for 
displaying the motion of the satellite. Letting the program run for 
several computer-simulated days will bring out this spiral. It is even 
more dramatic if the program offers a space view of the Earth. 
Range circle
    A satellitecan see only a portion of the Earth's surface. out to 
its own horizon. Within this compass of the Earth, the range circle, 
the satellite sees everything on the ground, neglecting topography and 
weather. The term comes from military artillery to show the territory 
over which a cannon can shoot. 
    The range circle is a geometric line-of-sight circle. Sometimes 
this idea is taken to include the interior volume of the cone whose 
apex os at the satellite and sides are tangent to the Earth. This is 
called the range cone. 
    The radius of the range circle is a function of the elevation of 
the satellite. For all practical purposes it is based on a spherical 
Earth, smooth topography, and no air refraction. 
    Only if we be within the range circle can we see the satellite, at 
least geometricly. When the circle first touches our location the bird 
is just rising. When the center of the circle passes closest to us the 
bird is at its maximum altitude over our horizon. When the range 
circle last touches us the satellite is setting. 
    It is the range circle that allows us to see a satellite beyond 
the north and south limits imposed by the orbit inclination. The 
circle reaches across the adjacent latitude limit. 
Ground and slant range
    The slant range of a satellite is the line-of-sight distance from 
us to the satellite, neglecting air refractions. By reversing the line 
of sight, it's the distance from the satellite to us. When the bird 
rises at our horizon it stands at the greatest slant range from us. 
    The range during the pass diminishes to its least value, then 
increases to the same maximum when the bird sets. The actual minimum 
slant range is a function of the maximum altitude of the satellite. 
The absolute minimum is achieved if the bird passes thru our zenith; 
it is the very elevation of the satellite at that point in its orbit. 
    The ground range is the distance across the Earth's surface from 
us to the satellite's place on the ground track. At the satellite's 
rising or setting, we are on the perimeter of the satellite's range 
circle. The ground range at rising and setting is the radius of this 
range circle. 
    The nearer we are to the center of the range circle, the place of 
the satellite on the ground track, the higher altitude the bird is in 
our local sky. We could plot concentric circles of altitude from 90 
degrees at the center of the range circle to 0 degrees on the 
perimeter. Some satellite tracking programs can do this. 
    The table here gives for steps of 40 kilometers of elevation, from 
100 to 1500, the orbit period, maximum ground and slant range, and 
duration of path. Values are calculated to three places, but you 
should mentally round to the kilometer and tenth minute. A circular 
orbit is assumed. 
    elevation  period        groud         slant        duration 
    kilometer  minute        kilometer     kilometer    minute 
    -------    --------      --------      --------     ---------
     100        86.6210      1121.488      1133.225      4.853564 
     140        87.4254      1323.553      1342.937      5.781251 
     180        88.2323      1496.932      1525.11       6.598915 
     220        89.0416      1650.713      1688.68       7.343575 
     260        89.8534      1789.975      1838.619      8.035717 
     300        90.6677      1917.905      1978.029      8.688053 
     340        91.4844      2036.655      2109          9.309093 
     380        92.3035      2147.767      2233.016      9.90486 
     420        93.1251      2252.387      2351.178      10.47979 
     460        93.9491      2351.396      2464.33       11.03726 
     500        94.7755      2445.489      2573.13       11.57989 
     540        95.6043      2535.227      2678.111      12.10981 
     580        96.4355      2621.071      2779.705      12.6287 
     620        97.2691      2703.402      2878.27       13.13798 
     660        98.1051      2782.544      2974.108      13.63881 
     700        98.9435      2858.773      3067.475      14.13221 
     740        99.7843      2932.325      3158.588      14.61898 
     780       100.6274      3003.408      3247.639      15.09988
     820       101.4729      3072.202      3334.792      15.57552
     860       102.3207      3138.867      3420.193      16.04647
     900       103.1709      3203.544      3503.969      16.51319
     940       104.0235      3266.36       3586.235      16.97611
     980       104.8783      3327.428      3667.092      17.43562
    1020       105.7355      3386.851      3746.63       17.89204
    1060       106.5950      3444.72       3824.934      18.34568 
    1100       107.4569      3501.122      3902.076      18.79682
    1140       108.3210      3556.131      3978.125      19.24569 
    1180       109.1875      3609.819      4053.142      19.69251
    1220       110.0562      3662.25       4127.183      20.1375
    1260       110.9273      3713.484      4200.3        20.58082
    1300       111.8006      3763.575      4272.54       21.02266
    1340       112.6762      3812.576      4343.948      21.46316
    1380       113.5541      3860.532      4414.563      21.90246
    1420       114.4342      3907.489      4484.422      22.34069
    1460       115.3166      3953.486      4553.561      22.77797
    1500       116.2013      3998.563      4622.013      23.21442
Tracking programs 
    No one any more tracks satellites by manual methods, except to 
demonstrate the technique or relive an older era. Today all 
predictions are generated by computer tracking programs or websites. 
While there are several commercial programs there are many perfectly 
competent freewares or sharewares downloadable from the Internet. Many 
Websites with science or astronomy software have satellite programs. 
    Be sure to fetch ones appropriate for your computer. Recall that 
programs for DOS are thoroly adequate for predictions. At worst you 
need a DOS emulator, probably obtainable from the same website. 
    You then need an addiurnate elements file, often from the same 
software website. If not, go to a satellite observing website, which 
will have the files or links to them. For the bright more important 
satellites, you can download just about any current file because it'' 
have these satellites in it. If you want some obscure bird, you may 
end up pulling down several files to locate your peculiar satellite. 
In a few cases, the file opens within the Internet browser for you to 
peruse before downloading it, but usually you get the download panel. 
If so, do 'open' to look at the file. 
    Programs typicly come with element files to practice with. These 
are useless for current predictions but are just fine to muck around 
with orbits in general. One gotcha is that some programs compare the 
epoch of the element set with your computer clock and find the set 
is out-of-date. 
Observing site 
    After installing the program and running it for the first instance 
we must select the observing site, usually called 'station'. Programs 
differ on doing this. Some present a list of major towns from which 
you pick yours. Others ask for keyed-in lat-lon and station name. 
    Once set, save the station for later runs. Some programs also you 
to maintain several sites. On startup the program asks which one to 
work with by picking it from alist. 
    The lat-lon has to be accurate, within a minute of arc, due to the 
parallax on low Earth satellites. A few kilometers one way or an other 
can cause the path in the sky to pass a waypoint star to one side or 
the other. If a satellite at 400km elevation is in the zenith of your 
place, it'll be about 5 degrees off zenith from a site only 35km away. 
This is about the diameter of a large town. Get your own lat-lon from 
a topomap, street atlas, mapping website. 
Load the elements
    The program needs the elements file. It may let you browse your 
computer to find it or insist that it live in a certain directory. It 
may allow files of any name and type or insist on only a certain 
filetype. This is almost always 'TLE'. Read the instructions! 
    Within the element file the program will ask for a satellite. You 
have to either key in carefully and correctly the name or pick the 
name from a list generated from the file. 
    The latter is obviously the better method. Otherwise you have to 
be sure to key in the name exactly as it appears in the file. You may 
have to inspect the file in a text editor first to see just how the 
name is spelled or abbreved. The name is in line #0 of the element 
Time start and step
    We have to specify the start, end, and step for the run. Usually 
this is an hour or so in the evening of a given day. Be sure to enter 
correctly the local time or UT, as the program demands. 
    For New York, UT is five hours AHEAD of EST, with a rollover of 
date if midnight occurs between New York and Greenwich. Stay with 
standard time, just as you do for astronomy. 
    The step increment has to be short to get enough points along the 
path to plot on a map. The duration of pass is only a few minutes; You 
need several points, at one minute intervals, for a good plot. 
    Modern tracking programs come with a broad set of features to 
customize the predictions. These include filters to leave out daylight 
or horizon skimming satellites, passes in large Moon periods, 
accounting for Earth shadow, date and time formats, position of Sun 
and Moon, printout of assorted data like slant range and ground speed. 
Each program offers its own suite of goodies; try them. 
Output display
    The simpler programs output a table of altitude-azimuth for the 
steps of time. This is the bare minimum needed to find the bird in the 
sky. As computer graphics improved, the display became more varied and 
    The display gives assorted data about the satellite. For graphical 
displays there is generally a feature to slow down the action to read 
the screen, pause and resume, run in realtime, capture the graphic. 
    A ground track display shows a flat map of the world with the 
satellite path. This view is so common that many beginner satellite 
watchers think the orbit is a sinusoid around the Earth! The effect is 
glatt distortion caused by the map projection. The orbit ground track 
is a great circle, modified by Earth rotation and precession. 
    A horizon map shows the path in your local sky against the 
horizon. A few brighter stars, planets, Moon may be plotted, too. A 
problem arises because during the passage the sky exercises diurnal 
rotation. It is impossible to plot the path correctly for both horizon 
and stars. For this reason, only the major stars are shown for 
guidance. A few programs ask which fame of reference you want to 
emphasize, horizonor stars. Others draw a new chart at each increment 
of step to minimize the divergence of horizon and star position. 
    The times are marked along the path, with other data you may have 
selected from the program's features. For close intervals of time 
these labels can overlap into illegibility. 
    The horizon chart may be an allsky map like a planisphere or a 
skyscape view in a one direction. Such a chart has the horizon along 
the bottom and the top at around 70 degree altitude. The skyscape map 
is tricky because you may have to set it up before generating the 
plot. If you have no idea where the satellite may be, you could pick a 
direction the satellite doesn't move thru! 
    The space view hovers over the world, which sits in screen center. 
The satellite wraps around and around the Earth like string on an 
armature, the 'ball of string' figure. The scale, distance from Earth, 
surface details, times in various towns, day-night terminator, range 
circle, and other features can be set. 
    The ride-along view is perhaps the sexiest one of all, altho it 
doesn't directly help in finding the satellite in the sky. In this 
display you are on the satellite looking down at Earth. Countries roll 
under you, stars rise and set, you see the changing angular size of 
Earth between perigee and apogee. The viewing direction m/ay be 
selected to see what's behind you, overhead, straight down. Fancier 
displays have a styled console whose gages are readouts of assorted 
data for the satellite. 
File output
    The text parts of output can generally be written to a file for 
later use. This file can be printed to take with you in anticipation 
of the satellite. The program can generate a graphic file of the 
display. also to have with pou as printouts. 
    When the program soen't have its own means of capturing the 
display, you can copy it to clipboard. Paste the image into an image 
viewing program. Clipboard can hold only one file at a time; the next 
one saved into clipboard overwrites the first one. 
    For collecting several graphicss you should use a multiple-capture 
program. Else you must capture andpste the clipboard image for each 
display in turn before copying the next display. 
Photographing satellites
    Taking a picture of the satellite path is just an other skyscape 
picture. The exposure has to be long enough to show substantial trail, 
a minute or two. In places with severe luminous graffiti such long 
exposure may wash out the trail and stars. Take test pictures to 
determine the maximum exposure before excessive washout sets n. 
    Use a wide angle lens to capture sufficient surrounding stars for 
ontext. Trying to aim a tele lens at the satellite and then following 
it thru the sky is very tough. Perhaps a cmaera piggybacked on a goto 
scope that tracks the satellite could work. 
Information sources 
    A websearch on 'observing' 'satellites' 'tracking' 'tle' and 
related terms will turn up scores of hits. Satellite observing is a 
small, but aggressive, area of home astronomy, whose adhaerents 
exploit the Internet for information, fellowship, dialog. 
    The websites are operated by military and spacefaring agencies, 
other government bodies, private individuals, astronomy and space 
clubs, aerospace companies, and aviation/space magazines. The utility 
of the sites varies widely with some having stale news and others 
having up-to-the-minute bulletins. 
    Many websites have current element TLE files or have links to them 
elsewhere. If you can read a file online before downloading it, look 
at the fourth field on line #1. The first three fields are '1' for 
line number, satellite serial number, its sequence number. The fourth 
field is a long decimal number. The first two chars are the 2-digit 
year of latest crossing of the ascending node. The next three chars 
are the day number within the year. The decimal chars are the 
fraction of the day. The newer this date is, the more current are the 
elements. A calendar with day numbers is handy. As a reference, 1/1000 
day is about 90 seconds. 
    In additional to material for finding and following satellites, 
explore the websites dedicated to the very satellite or associated 
space project. They describe the physical structure of the craft, to 
help explain its brightness behavior. The sections on the mission and 
purpose are useful to understand its orbit. 
    Because websites may vanish or change without notice, I can't give 
an enduring list of them. Even a websearch service that updates 
frequently will turn up several dead links. 
    SInce the first days of the Space Age home astronomers watched 
Earth-orbiting stellites. At first there was the Cold War excitement 
totrack a Societ satellite. Later the parctice became a pleasant 
pasttime in itself.
    The coming of calcylettes, computers, telephone modems, Internet 
gave home astronmers thetoolsto accurately and quickly find and follow 
satellites. Some use the available information to play with 
astronautical concepts. 
    While this article as at March 2012 is quite ten years old, 
revisions in the intervening years were surprsingly modest, mostly to 
clean up wording. That's because the fundmental principles of 
satellite watching were fuly matured within a couple years after the 
first Sputniks  The only major change is that the number of bright 
obvious satellites dwindled to just ISS. Perhaps in futureyears some 
new bright stellites will be fielded. 
    Special thanks to Charlie Ridgway, Ken Brown, and Frank Schmidt, 
all of NYSkies, for reviewing and amplifying this article.