QUESTION:
What is the reason for switching back and forth between the high and 
low gain antennas? Is this a matter of power consumption? How does 
one antenna differ from the other and in their functions? 


ANSWER from Brian Rush on October 8, 1999:
The High-Gain Antenna (HGA) sends a much stronger signal than either 
Low-Gain Antenna (LGA), allowing the spacecraft to send data to Earth 
at much higher data rates. Therefore, it is always desirable to use 
the HGA when possible.

However, a higher priority is to use the HGA as a sun shield. The HGA 
is large enough to shield the entire spacecraft from the Sun, and 
this is necessary when Cassini is closer than about 5 AU to the Sun.

Starting when Cassini is about 2.7 AU from the Sun, around 1 Feb 2000, the Sun
and the Earth will be close enough from Cassini's point of view that the HGA
can be pointed to the Earth and still work as a sun shield. Thus, 
from that date
onward the HGA will be used. (The HGA was also used a couple times 
within 2.7 AU, when the Earth was almost directly between Cassini and 
the Sun for a short period of time.)

>From the beginning of the mission until next February, one of two 
>LGAs is normally being used. The two LGAs are on different sides of 
>the spacecraft. Depending on where the spacecraft has to be pointed 
>to use the HGA as a Sun shield, one LGA is pointing closer to the 
>Earth and that is generally the one that is used.

As far as I know, power consumption isn't an issue. If it differs between the
HGA and LGAs, it is not as important a factor as getting the higher data rates
with the HGA.