The lunar module carried two important radar systems that were tested prior to landing. The first checkout was for the rendezvous radar while the CSM was still nearby. This radar worked in conjunction with a transponder on the CSM to give the crew and the LM computer information about how far away the CSM was, how fast it was approaching and in what direction it was located. Although there were backup methods for the spacecraft to rendezvous, this radar was an important component in getting the two spacecraft together.
Its dish antenna was mounted on two axes so that when it started operating, it swept the view in front of the LM, looking for the
CSM until a return signal from the transponder was found. The receiving horn was split into four so that if the dish was not exactly boresighted on the CSM, the received signal would be stronger in one of those four compared to the others. The electronics could then operate to aim the antenna until all four horns received an equal strength signal. The angle of the dish then represented the direction to the CSM.
The rendezvous radar antenna on Apollo 9's LM Spider.
Was this article helpful?