From: NASA HQ
Posted: Thursday, August 16, 2001
A new star is rising. The International Space Station (ISS) assembly consists of the energy and control block FGB "Zarya" , the U.S. connecting module "Unity", the Russian Service Module "Zvezda" and the first element, Z1, of the future solar-array-carrying truss, on top of the Node Unity. During Shuttle mission 3A (STS-92), the third docking adapter PMA-3 was also added, followed in the next mission, 4A (STS-97), by the second truss element, the photovoltaic module P6 which was mounted on top of the Z1 where it deployed two gigantic solar array wings measuring 240 feet tip-to-tip. It will later be moved outboard to become part of the port-side outrigger of the solar array truss. P6 has increased ISS power by up to 62 kilowatts. On 10 February '01, Space Shuttle flight STS-98 on ISS mission 5A further added the 28 ft (8.5 m) long U.S. Laboratory module Destiny, increasing the station's mass to currently 112 tons and its dimensions to 171 ft (52 m) long, 90 ft (27.4 m) high and 240 ft (73 m) wide; it now surpasses Mir and the U.S. Skylab in terms of habitable volume. It is visible to the naked eye as a bright star in the morning or evening star, appearing or disappearing at the horizon or in EarthÝs shadow, if the sky is without overcast and haze.
The OSF Orbital Visibility schedules at present cover 3,410 locations
worldwide. To determine if your data for your city is available click
on the "List of Cities Served" link below and scroll through the list
(alphabetized by city name). If you do not find your city/location on
the list, for the time being, we ask that you to select the nearest listed
Please note that the times reported in the U.S. Cities tables are in the a.m./p.m. format familiar to most people in the United States. The times reported in the Non-U.S. Cities tables are in 24-hour format most commonly in use elsewhere.
NOTES: Included are only major cities currently in the range of visibility with maximum spacecraft elevations over the horizon larger than 20 degrees. The data are also valid for suburban regions around these cities with slight changes in Direction of Movement and Max. Elevation.
Jesco von Puttkamer
The "Two-Line Elements" (or TLE) format generally used by PC-based satellite tracking programs contain all necessary numerical data describing the orbit (position, flightpath and motion) of a satellite such as Mir or the coming ISS, as well as its exact position along that orbit at a specific reference time (the "epoch"). This format dates back to the days when NORAD (North American Aerospace Defense Command, today US Space Command) still used IBM punched cards on its computers. Thus, because each card could only carry one line, today's Two-Line Elements were "Two-Card Elements" back then. TLE files are always in ASCII format, and when they are copied or moved around with "Clip and Paste" commands, non-proportional fonts (like Courier) must be used to preserve the exact positions of the digits and their spacings.
To completely describe not only the size and shape of an orbit but also its orientation around its central body (for Mir, that would be the Earth, of course), only five independent quantities called "orbital elements" are required. The object in question can be anywhere on that closed path as long as its position at a specified time is not given. Thus, a sixth element is required to pinpoint the satellite's position. From this position, the satellite tracking program then calculates "forward", in effect predicting the object's locations at any desired future time. The real world is not ideal, however, and therefore all orbits are influenced by various disturbances called "orbital perturbations"; in the case of Mir and the Space Shuttle, such "perturbations" might include applications of thrust from the craftsí maneuvering jets as well as naturally-occurring conditions.
To fully include these perturbations in the predictions would be impractical for PC-based calculation routines. Thus, with time, their influences pile up, causing increasingly noticeable deviations of the real orbital path from the predicted one. To take care of that, predictions need to be "refreshed" periodically with up-to-date TLEs based on the most recent radar tracking measurements of the responsible organizations such as US Space Command.
The element data used by our TLE's to describe the orbit size and shape are: the Mean Motion (2nd line position 53-63) and the Eccentricity (2nd line pos. 27-33). Mean Motion is used because, according to Kepler, an object in an elliptical orbit moves at periodically varying speed, depending on its distance from the mass center at its focal point. From the Mean Motion (in degrees per second) we can calculate the orbital period and, with the Earth's gravitational constant, the semi-major axis of the elliptic orbit (which could, in rare cases, reduce to a perfect circular orbit). With the Eccentricity, the apogee (farthest point) and perigee (closest point) of the ellipse can be determined and, with the known Earth's radius, their altitudes above Earth and also the mean altitude. (When not referring specifically to Earth, we are using "apoapsis" and "periapsis", or "apofocus" and "perifocus" for these characteristic points of an elliptic orbit).
For determining the orientation of the orbit about the Earth, the TLE also contains the Inclination (2nd line pos. 09-16) of the orbit plane in degrees measured from the Earth's equatorial plane, the Right Ascension of the Ascending Node (RAAN, 2nd line pos.18-25), and the Argument of Perigee (2nd line pos. 35-42). The ascending node is the point where Mir crosses the Earth's equatorial plane in the northerly direction. (The opposite point is the descending node, and the line connecting both points is called the Line of Nodes). RAAN, measured in degrees, is the angular distance of the ascending node from the line pointing to the Vernal Equinox on the ecliptic (the point where the Sun crosses the celestial equator in spring around March 21). Argument of Perigee defines the orientation of the elliptical orbit's semi-major axis: measured in Mir's orbit plane in the direction of motion, it is the angle between its ascending node and its perigee.
The sixth element is the Mean Anomaly (2nd line pos. 44-51), which is used for calculating the satellite's exact position at a particular time ("epoch") from perigee.
The first line of the TLE file, under the name, contains the US Space Command-assigned Catalog Number of the object (often called the "NORAD Number"), the Epoch Year and Epoch Date (pos. 19-32) and other identifiers of interest. In line 2, pos. 64-86 are reserved for the number of revolutions accumulated at epoch.
The two-line elements are not the only factors necessary to predict the orbit of the Mir Space Station for the purposes of these visibility tables. Many additional factors must be taken into account to ensure the reasonable precision of these predictions over the dates covered by the tables.
Following are the Two-Line Elements and Translated Orbital Data for the International Space Station as of 8/15/01 7:55am EDT:
ISS 1 25544U 98067A 01227.49670139 .00221527 00000-0 28627-2 0 3485 2 25544 51.6374 149.6428 0010342 10.8985 332.3916 15.57609104156336 Name....................................ISS NORAD ID#...............................25544 Epoch Year..............................1 Epoch Day...............................227.4967 8/15/01 7:55am EDT Mean Altitude (km)......................394.534 Period (min)............................92.45 Apogee (km).............................401.539 Perigee (km)............................387.530 Inclination (degrees)...................51.6374 Right Ascension of Ascending Node (RAAN, degrees)..................149.6428 Eccentricity............................0.0010342 Argument of Perigee (degrees)...........10.8985 Mean Anomaly (degrees)..................332.3916 Mean Motion (revs. per day).............15.57609 Decay Rate..............................0.00221527 Epoch Revolution........................15633 Element Set#............................348 Visible up to Latitude (degrees)........71.3 8/15/01 12:54 PM EDT
NOTES: Included are only major cities currently in the range of visibility, with maximum spacecraft elevations over the horizon larger than 10 degrees. The data are also valid for suburban regions around these cities, with slight changes in Direction of Movement and Max. Elevation.
Pickup Time: The local time of day that the spacecraft becomes visible on the horizon.
Direction of Movement: The spacecraft will appear in the first direction
and travel across the sky, rising to the "Maximum Elevation" and
disappearing at the horizon in the second direction shown. These compass directions
are understood to embrace an angular field of 22.5 degrees each, with their
symbols defined as follows:
N: North NW: Northwest NNW: North-Northwest SSE: South-Southeast E: East SW: Southwest WNW: West-Northwest ESE: East-Southeast S: South NE: Northeast WSW: West-Southwest ENE: East-Northeast W: West SE: Southeast SSW: South-Southwest NNE: North-Northeast
Responsible NASA Official:
Jesco von Puttkamer
SAIC Information Services
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