Re: Hello & Question
jbarker@arinc.com
Wed, 22 Nov 1995 08:46:02 -0500
Kenny:
Your question is a good one. You are, however, stating two conditions
that are only partially related.
First: the fact that the satellite passes directly over you only means
that the ground trace of the satellite's orbit (the plot of the nadir)
happened to pass directly through your location. Any satellite in
orbit around the Earth that has an inclination equal to or greater
than the latitude of your location, could pass directly over you at
some time, but unlikely that it would do so on every pass.
Second: You noted that the satellite came into view on an azimuth of
about 175 deg, passed directly overhead, and left your view at an
azimuth of about 350 degrees. This indicates that the satellite is on
it's ascending node. The inclination of the orbit is roughly 90 to 95
degrees. An orbit with an inclination of greater than 90 degree is
called a retrograde orbit because the ascending node is moving against
the direction of the Earth's rotation. Any satellite that has an
inclination of about 85 degrees to about 100 degrees are often
referred to as "polar" orbiting satellites, meaning that they pass
over the polar regions on every pass. There are a lot of low Earth
polar orbiting satellites. The principle ones that also have a
retrograde orbit (greater than 90 deg incl) are imaging satellites
such as Landsat, SPOT, and weather satellites. At the right altitude,
a retrograde polar orbit has the advantage that it can compensate for
precession of the orbit resulting from the rotation of the Earth on
it's axis. A retrograde orbit can be designed so that the satellite
will pass over the same places periodically (some number of orbits
later). This makes comparison of images easier because they are taken
at the same angle. For example, Landsat has a revisit time of about
14 days. Or the orbit can be designed so that the satellite crosses
the equator going north (the beginning of the ascending node) each day
at the same time. This is a technique often used by polar orbiting
weather satellites.
There are also some polar orbiting communications satellites.
On a particular pass, a polar orbiting satellite will not necessarily
pass directly over you.
Designing an orbit for a satellite is always a tradeoff between a
number of factors. It's even harder for a constellation of
satellites.
Also, please include you return e-mail address in all correspondence.
Without it it is necessary to respond to all of SeeSat. Some subjects
are not of interest to all memebers.
Jeff Barker jbarker@arinc.com
______________________________ Reply Separator _________________________________
Subject: Hello & Question
Author: seesat-l@iris01.plasma.mpe-garching.mpg.de at SMTPGATE
Date: 11/21/95 9:25 PM
>Message was resent -- Original recipients were:
To:
seesat-l@iris01.plasma.mpe-garching.mpg.de--------------------------------------
-----------------------------------------
Hello all,
I am Kenny and I am a new subscriber. I am very new to viewing
satellites and have a few questions.
I live in Kenner Louisiana and have a great view NORTH over Lake
Pontchatrain from about 270 degrees to 90 degrees. My southern view is
somewhat light polluted though :-(.
I have lots of luck seeing Mir, shuttle and HST by using my
computer software. Now here's the question.....
Sometimes, when I am stargazing with binoculars at 90 degrees
(overhead), I see a fast moving object moving from about 175 degrees to
350 degrees (almost directly north). Is this a strange occurence? Am I
seeing a "polar" sat? I see these objects up to one and a half hours
after sunset.
Does anyone know what I am looking at?
Hope I made sense....
Kenny