ISS Naked-Eye Visibility Data From Selected Cities 16-21 Aug 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 Earthís 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

entry.

Please note that the times reported in the U.S. Cities

format most commonly in use elsewhere.

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 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