SATELLITE OBSERVING ----------------- John Pazmino NYSkies Astronomy Inc email@example.com www.nyskies.org 2002 May 1 initial 2012 March 7 current
Introduction ---------- 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 binoculars.
History ----- 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.
Telcomms ------ 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 UNIX. 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.
Metrics ----- 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.
Instruments --------- 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.
Accessories --------- 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 orbit.
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. 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 magnitude. 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 satellites. 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.
Excentricity ---------- 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 --------- 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.
Precession -------- 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.
Launches ------- 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.
Reentries ------- 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 record.
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.
Features ------ 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 informative. 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.
Conclusion -------- 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.