Satgen 538  Satellite Height Pt2  by GM4IHJ  (BID SGEN538) 1999-07-17

Satgen 537 discussed some of the advantages which accrue if a Low
Earth Orbit (LEO) satellite can be orbited between 1200 and 1800 kms
altitude. Rather than, much lower around 750 kms altitude , where most
of the digital satellites go. The digitals have to stay at low
altitude because of their susceptibility to radiation damage. But
analog satellites have survived in much higher orbits, for many years.

There are other advantages in addition to those discussed in
Satgen537. Not least is one which should appeal to anyone
contemplating the launch of a LEO sat equiped with an analog S or L
band transponder. Doppler shift at these frequency bands can be very
difficult.  The total doppler shift of a LEO satellite as it crosses
your sky , is about the same irrespective of its altitude ie circa 80
kHz total for a 2400 to 435 MHz subtractive mixing transponder. This
amounts to an average doppler rate of about 8kHz per minute for a 750
km altitude pass of 10 minutes duration, but only 4 kHz per minute
average for a 1500 km altitude pass of 20 minutes duration. A feature
which makes a tremendous difference to communications continuity.

Another important effect of satellite altitude and increased time a
particular satellite pass is in range of your station, is the effect
this time extension has in slowing down the rate at which the
satellite crosses your sky. Thereby bringing the rate at which you
have to follow it in both azimuth and elevation , down to much slower
more managable proportions. Such that the usual LEO scramble requiring
an 8 armed octupus like operator, trying to follow doppler, azimuth
and elevation changes, all at the same time is somewhat ameliorated.  

Extra time in range also allows for more QSOs per pass or longer QSOs.
This was most noticable when Oscar 8 in a lower orbit followed, Oscar
7. QSOs with distant stations ie Scotland to Kentucky on Oscar 8 ( old
friends N4AR and W4AUZ), were dreadfully short , almost antisocial in
their haste, before contact was lost. Whereas Oscar 7 QSOs got into
states well beyond Kentucky and the QSOs were easy and unhurried.

These advantages due to extra height become very obvious if one tables
the size of the satellite footprint ( the area it sees from a
particular height) with altitude.
Height -          750km       1000km       1250km       1500kms
Footprint angle   26.6        30.1         33.8         36
Diameter in kms   5826        6704         7520         8000

A situation which means that most of USA is in range from Scotland
with the high flier, and any station above 54 latitude can catch part
of almost every orbit of a satellite in a polar orbit . Which allows a
Scottish station access to all of Canada and Alaska, over the pole. An
area which is not densely populated but it does have quite a few
excellent operators, able to take part in, the often unusual,
propagation conditions, which exist in that part of the world .