Satgen334   Satellite Software Pt1   by GM4IHJ                19 Aug 95

The series of Satgen bulletins which will carry this title , are not
concerned with any specific software products. This will not be a review
of GM4IHJs favourite software. The fact is that some software is good for
one type of operation, some for a different type of operation. So what
these bulletins will try to do, is to explain and discuss , the advantages
and disadvantages of certain software features.

AZIMUTH and ELEVATION . In the begining, you enter up to date Keplerian
elements into your chosen software and run your computer program. Almost
all software works out and prints Time at say 2 minute intervals with the
relevant Azimuth and elevation bearings of the satellite from your station
at that time. Azimuth is given using a standard format read in degrees
clockwise from 0 degs = North through 90 degs = East, 180 degs = South and
270 degs = West. Indicating where you should point your antenna to pick up
the satellite. You also get print out of Elevation, the number of degrees
the line of sight from the satellite to your station is above your station
0 degs elevation horizon. 

Here some care is necessary. Very few stations have unobscured horizons
down to 0 degs elevation. At IHJ horizons are Az 0 El 2 ; Az 45 EL 4 ; Az
90 El 7 ; Az 135 El 5 ; Az 180 El 3 ; Az 202 El 2 ; Az 235 to 338 El 0.
These hills effectively block UHF and Microwave sat signals until the sat
rises above the hills. However at HF and to a lesser extent VHF, received
satellite signals are diffracted around these obstacles such that at HF
satellites arriving or leaving at elevations down to -3 degrees are often
audible.

So please be careful. Do not assume computer predictions are the truth,the
whole truth and,nothing but the truth. You will find that the more
operational experience you get , the more you can instinctively include
these local variations into the computed results and get meaningful
practical data.

SUB SATELLITE LAT LONG  It is common practise to include calculation and
print out of the LATITUDE of the point on the earths surface which is
directly below the satellite and the LONGITUDE of this point. Latitude is
measured in degrees 0 at the equator , +90 at the north pole, - 90 at the
south pole. Longitude is measured in degrees west of the Greenwich
meridian ( that line joining the north and south poles of the earth ,
which passes through Greenwich near London England). For most operators
this data is of little use unless you use it to plot the location of the
sat sub point on a map of the earth ( more about maps later ). But do be
careful . Almost all satellite software quotes longitude measured west
from Greenwich. But, by contrast, quite a lot of astronomical software
quotes longitude measured east of Greenwich. A situation which can be very
confusing at times when software authors mix satellite location prediction
and say Solar or planetary location data in the same software Eg when one
wants to show whether the satellite is or is not in sunlight.