The selection of the range of azimuthal overlap is called topology
in the derive program. The cable twist covers a range from -90 degrees up to
+450 degrees, in order to allow the full coverage the indicated and the
finally commanded azimuth are converted into this range and not reduced to
the fundamental range.
	Sources that culminate south of the zenith (apparent declination with
a value smaller than the latitude of the observatory) will cover an azimuth
range from a few degrees through about 360 - the few degrees, and the full
range is always covered. 
	Sources that culminate north of the zenith will have an azimuth
range from a little over 270 degrees through 360 degrees up to 90 degrees.
This range can be covered twice within the travel limits. For historical
reasons the smooth hardware ranges are called as below:
	 270 to 450 	 NORTH
	 -90 to  90	 SOUTH

Topology has an automatic selection of the azimuth range. For 'southern'
sources it unwinds the range and uses 0 to 360, for 'northern' sources
it will select the range which is closer at the instant that LOAD is
pressed.
For technical, calibrating and scheduling reasons one might want to override
the automatic choice. This can be done by an input to OBSINP
	TOPO NOR (TH)
	TOPO SOU (TH)
	TOPO any other character combination > AUTO

For 'northern' sources there is no problem involved with any of the choices
above. 
For 'southern' sources the following difficulties arise:
TOPO SOUTH : If the observation starts at an azimuth < 270 degrees
and crosses 270 degrees, this would require a rotation of the antenna during
the observation: this will not occur.
TOPO NORTH : if an observation starts at an azimuth  < 90 degrees and does
not finish before the source reaches 90, the observation stops by hitting
the travel limit.
If an observation is requested and the starting azimuth is far ahead of 270
degrees, this choice is overruled.

Execution steps:
When load is pressed, the actual azimuth of the starting point is not
known until the drive program is executing once. Therefore the TOPO request
are converted to an integer as follows
Northern source:	TOPO AUTO	> IF_NORTHSOUTH = -1 (decide later)
			TOPO NORTH	> IF_NORTHSOUTH = 1 
			TOPO SOUTH	> IF_NORTHSOUTH = 2 
Southern source:	TOPO AUTO	> IF_NORTHSOUTH = -8 (decide later)
			TOPO NORTH	> IF_NORTHSOUTH = -9 (decide later)
			TOPO SOUTH	> IF_NORTHSOUTH = -10(decide later)

The routine TOPOLOGY interpretes the integer, receives the commanded and the
indicated azimuth. If the integer is negative, it once makes the decicion
of which range to use and modifies the integer accordingly. As a function
of the integer, now positive, it modifies the commanded azimuth by
adding or subtracting 360 degrees in deciarcsecs.