SMU and TMU alignment is part of the configuration and maintenance of the antenna and instruments, respectively, and ought to be fixed thereafter. However, the SMU is repeatedly exercised during operations, the TMU occasionally so, providing scope for errors in their positions and hence in pointing. The TMU particularly seems susceptible to small, unrequested changes in its position. Again, not normally a major concern, and assumed absent during routine analysis of pointing data, their effects are as given by MTUN021.