  If the signals that are being sought in SETI programs do exist but are very
brief, for example because they are produced intermittently to conserve energy,
then it is essential to know when these signals will arrive at the Earth.
Different types of synchronization schemes are possible which vary in the
relative amount of effort which is required by the transmitter and the
receiver. Here the case is made for a scheme which is extremely simple for the
receiver: make observations of a target when it is at maximum angular distance
from the Sun (i.e. "opposition"). This strategy requires the transmitter to
have accurate knowledge of the distance and proper motion of the Sun and the
orbit of the Earth. It is anticipated that within about the next 10 to 20 years
it will be possible to directly detect nearby extra-solar planets of
approximately terrestrial mass. As any extraterrestrial transmitters are
expected to have significantly more advanced technology it is therefore not
unreasonable to expect that these transmitters would be able to detect the
presence of the Earth and measure its orbit at even greater distances. In
addition to the simplicity to the receiver of implementing this strategy it has
the advantage that opposition is typically the time when observations are
easiest to make anyway. A number of all-sky surveys that have already been
performed naturally contain tiny "opposition surveys" within them. A full
all-sky opposition survey would require extensive time to complete with a
single moderate field of view telescope but different types of arrays might be
employed instead including some systems already under construction.
