Synchronized SETI - The Case for "Opposition"
  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. 
Robin H.D. Corbet (USRA)