Satgen 481  Sedsat Orbit   by GM4IHJ    (BIS SGEN481)     1998-06-13

Talking to radio amateur satellite users about the proposed orbit for
Mode A satellite SEDSAT, which is due to launch this year. It is clear
that its orbit may disappoint users at high latitude sites. The
problem is a simple one. Sedsat is tentatively scheduled to go into a
relatively low inclination orbit inclined at about 28.5 degrees , with
perigee height around 500kms and apogee around 700 kms. All these
figures are provisional. Working out what they mean in practical 
"When can I see it terms", produces :-

At perigee Sedsat footprint is cos(-1) 6371/6871 = 22 degrees
At apogee  Sedsat footprint is cos(-1) 6371/7071 = 25.7 degrees

So Sedsat will briefly see up to Latitude 28.5 + 22 = 50.5 North  or
South at perigee,
Whilst at apogee it will see up to Latitude 28.5 + 25.7 = 54.2 North
or South. 
Please note apogee will not necessarily occur at furthest north or
south, so practical results will rarely see Sedsat above the mean
value of around latitude 52.5 degrees.

For stations, above middle England ,Scotland, Denmark, and all
Scandinavia this means no Sedsat ever. Whilst stations in Canada, all
UK, Holland Belgium , North Germany Poland and much of Russian will
see Sedsat only rarely for meaningful periods of more than a minute or
two at low elevation. Still closer to the equator stations as far
south as New York will have some coverage but it will never match the
more extended cover of say the Radiosport satellites or the Fuji mode
J birds.

Fortunately a lot of other places nearer the equator will get
excellent coverage from Sedsat, which is particularly important for
Third World countries where access to a simple mode A satellite of
this type will be very useful for encouraging new users, who can
access on simple equipment. 

It is equally important to realise this is the first amateur satellite
launch from North America in 20 years. American launches from Florida
are generally confined with few exceptions to low inclination orbits.
Thus taking maximum advantage of the assistance of earth spin from
this relatively low latitude site ( 28.5 N ) . Launching to higher
inclinations as some Florida launched shuttles do, is very expensive
in fuel because of the much reduced earth spin assist, and these
shuttles never go to inclinations much above 55 degrees because they
must avoid the higher inclination route which overflies New York and
much of the US east coast . A launch accident over a densely populated
area such as that being out of the question, with,  Eg the possibility
that Challengers debris could have come down in New Jersey, Virginia
or the Carolinas. Some American military launches do go to high
inclination orbits but they all start from Vandenberg AFB in
California, which has a clear launch field down to the Antarctic.