Tracking Satellites
Philip Chien
© 2006 by Philip Chien, Earth News
Since the earliest days of the space
program, amateur astronomers and sky watchers have viewed
artificial satellites. A satellite is visible when it's
illuminated by the sun while the observer on the ground
is in relative darkness (dusk or dawn). It's the same
solar geometry that permits you to see the glow of dawn
up a mountain top when it's still dark in a valley.
But what's special about satellites is they're moving
rapidly across the sky.
Low altitude satellites travel at 28,000
km (17,500 miles) per hour and take up to nine minutes
to go across the sky. Under the best circumstances,
large, highly reflective satellites can be as bright
as Venus, and under extraordinary circumstances certain
satellites can be even brighter. The best tools for
observing satellites are the naked eye, clear skies,
and a relatively non-light polluted area.
Articles in Scientific American's
"The Amateur Scientist" column at the beginning
of the space age described how to view satellites. In
one article instructions were given for using a sheet
of glass held at a 45 degree angle to observe the satellite's
reflection. Another article described how a short wave
radio could be used to monitor the satellite's transmissions
and use its Doppler shift to determine when the satellite
passed closest to the observer. Later improvements included
graphic charts with grease pencils that could plot a
satellite's orbit on a map. By the mid 1980s, primitive
satellite tracking programs were written for the early
Apple, Commodore, and IBM computers, and they've improved
over time.
Satellite
Tracking Software and Activities
Today satellite tracking is almost
trivial thanks to online satellite tracking programs,
Java real-time satellite tracking maps, and even satellite
tracking programs for PDAs. But what you can do with
satellite tracking still makes it a fascinating hobby
- from novice through expert.
At the novice level, the web site
http://www.heavens-above.com/ will automatically
predict which satellites are visible each day from any
given location on Earth. More advanced activities include:
● Using a shareware tracking
program to make your own predictions or to track a satellite's
location in real-time.
● Using a radio receiver and
frequency counter to track a satellite's Doppler shift
as it passes overhead.
● Listening to radar signals
as they're bounced off of satellites.
● Decoding data from weather
and scientific satellites.
● Creating your own satellite
tracking data based on your visual observations.
● High level analysis of tracking
information.
Orbital Parameters
Satellites obey Newton's laws of physics,
except in lousy Hollywood movies, novels, and inaccurate
stories about the space program. A satellite in an orbit
383,000 km (238,000 miles) above the Earth, like the
moon, takes one month to make an orbit. A satellite
in an orbit about 241 km (150 miles) above the Earth,
like the International Space Station, takes about 90
minutes to make an orbit.
There are several formats for specifying
the orbit of a satellite. One format specifies a satellite's
X, Y, and Z coordinates and velocity at a given point
in time. A more common format specifies the shape of
the orbit and its orientation relative to the Earth.
Satellite tracking programs take this raw data and use
it to predict where the satellite will be at any given
moment. In a "perfect" universe with only
two point mass objects, a satellite tracking program
would accurately predict the satellite's orbit indefinitely.
But the Earth isn't a point mass, and
it isn't homogeneous. The Earth is oblate (its equatorial
diameter is slightly greater than its polar diameter).
Satellites in low altitude orbits can experience drag
from wisps of atmosphere. In certain orbits, perturbations
caused by the sun and moon are important factors. Satellite
tracking programs take these factors into account to
some degree.
The most common format for satellite
tracking information is the Two Line Element
(TLE) format. Actually there are three lines: a title
line with the common name for the satellite and two
lines of numerical data. TLEs for all unclassified satellites
are available on the web, and TLEs generated by amateur
satellite observers for many classified satellites are
also available. There's an archive of all of the TLEs
which were released during the last flight of the shuttle
Columbia on my web site http://www.sts107.info/mission/STS-107%20tles.txt.
First TLE Set Issued During
the STS-107 Mission (Columbia)
Here is the first set of TLEs issued
during the STS-107 mission:
STS 107
1 27647U 03003A 03016.79298380 .00022003 20216-8 46617-4
0 16
2 27647 39.0169 228.2757 0012366 350.5306 131.3443 15.97537444
30
The first line of data begins with
the object's serial number, STS-107.
The second line indicates that STS-107 was the 27,647th
object tracked in space. 03003A indicates
that the launch was the third launch of 2003, and it
was the primary object (A) of that
mission. 03016.7929380 is the Epoch
time as a decimal date (16.798380 days into the year
2003). The next four sets of values aren't important
for most hobbyists. "16" is actually
two values. "1" indicates it's the
first set of tracking data released for the mission,
and "6" is a mod 10 checksum.
The second line of data includes the
following information: The orbit had an inclination
of 39.0169, a Right Ascension of Ascending
Node (RAAN) of 228.2757, an eccentricity
of .0012366, and Argument of Perigee
of 350.5306, a Mean Anomaly of 131.3443,
and it made 15.97537444 orbits
each day. The final digit is a checksum of zero (0).
Don't worry if you don't understand all the values or
what they mean.
From this data it's easy to determine
that the orbital period was 90.137 minutes (1440 minutes
in a day divided by 15.97537444 orbits per day), and
use Kepler's laws to determine that the orbit had a
perigee (low point) of 280.09 km, and a apogee (high
point) of 296.560 km.
The
Columbia Disaster
One of the most fascinating puzzles
that I've participated in solving was a satellite tracking
mystery after the Columbia accident (Fig. 1). After
the accident there was an intense examination of any
data that could help determine what had happened. USSTRATCOM
went through all of its raw radar data to see if there
was something which could have been missed during the
mission. Orbital mechanics experts were able to determine
that an object came off Columbia about a day after launch.
By connecting the dots between the various radar observations,
an orbit could be constructed. It was given the serial
number 27713. Since the object was far smaller and more
dense, it gradually separated from Columbia, its altitude
decreased, and it reentered the Earth's atmosphere on
20 January and burned up.
The analysis of the mystery object's
orbit is described here.
At first only small amounts of information were made
available, including the rough time the object was first
noticed, the day it reentered, a rough size for the
object based on the strength of the radar returns, and
the fact that it reentered over the Pacific Ocean.
How quickly the object's orbit decayed
until it reentered was one of the most important things
to determine. If the object was a light object with
a large surface area, the drag from the Earth's atmosphere
would cause the orbit to decay more quickly than if
it was a small, dense, aerodynamically shaped object.
Determining what the object was made
of was one of the key puzzles which could solve why
the Columbia accident occurred.With the sketchy data
available, amateur satellite tracker Ted
Molczan was able to derive an approximate orbit
that gave an indication of the object's density. Ted
and I were able to determine that the most likely candidate
was a piece of Reinforced Carbon-Carbon (RCC) the gray
thermal insulating material that projects the shuttle
from reentry heat in the areas that receive the most
heating, such as the leading edge of the wing. That
conclusion matched the analysis by the Columbia Accident
Investigation Board that was released several weeks
after we announced our conclusions.
The most likely candidate for the mystery
object is a piece of the T-seal that connects adjacent
RCCs. This was one of the key clues that helped determine
what happened to Columbia. The 0.76 kg (1.67 lb) piece
of foam that fell off during the launch struck the wing
and damaged the RCC. After a day of heating and cooling
cycles, a piece of RCC came loose and gradually drifted
away from Columbia unnoticed. During Columbia's reentry
the intense heat entered the breach in the wing and
led to Columbia's destruction.
Philip Chien is the author of the
book "Columbia-Final Voyage" about STS-107, the story
of the mission and the people on Columbia's last space
shuttle mission. Now available from
Springer Books. More information is available at his
web site.
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