ISS Naked-Eye Visibility Data 9-21 Jan 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
and its mass by 17 tons to currently 97 tons. 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 Earthís shadow, if the sky is without overcast and
haze.

The OSF Orbital Visibility schedules at present cover 3,406 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 entry.
 

List of Cities Served

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.

ISS Altitude Update

ISS Altitude History

Two-Line Keplerian Orbital Elements

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 1/19/01 8:00am EST:

ISS

1 25544U 98067A   01019.54189815  .00035177  00000-0  30262-3 0  5234

2 25544  51.5748 120.3903 0009480  11.1418  17.8275 15.68915085123832

Name………………………………ISS

NORAD ID#………………………….25544

Epoch Year…………………………1

Epoch Day………………………….19.5419  1/19/01  8:00am EST

Mean Altitude (km)………………….361.962

Period (min)……………………….91.78

Apogee (km)………………………..368.352

Perigee (km)……………………….355.572

Inclination (degrees)……………….51.5748

Right Ascension of Ascending

  Node (RAAN, degrees)………………120.3903

Eccentricity……………………….0.000948

Argument of Perigee (degrees)………..11.1418

Mean Anomaly (degrees)………………17.8275

Mean Motion (revs. per day)………….15.68915

Decay Rate…………………………0.00035177

Epoch Revolution……………………12383

Element Set#……………………….523

Visible up to Latitude (degrees)……..70.4
                                                   1/19/01 1:13 PM EST

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

Curator:

SAIC Information Services