M.A. Holdaway , Satoki Matsushita , M. Saito 
July 11, 1997
Keywords: phase stability, turbulence, Chajnantor, Pampa la Bola
We present a preliminary comparison of seven weeks of phase stability site testing data from the Pampa la Bola and Chajnantor sites, which are separated by only about 8~km. The comparison indicates that the overall distribution of rms phase errors is similar for the two sites (the median rms phase error is about 17\% higher at Pampa la Bola). However, at any given time the rms phase on the site testing interferometers' 300~m baselines may be very different at the two sites, sometimes by as much as a factor of 10 different. Presumably, the frequent significant difference in the two site's rms phase is due to the effects of the local topography on turbulence. Being 5~km downwind from a mountain which is 600-800~m higher than the site results in an increase of about 20-30\% in the rms phase, and increases the thickness and scale of the turbulence. Both these trends are very noisy, so chaotic local effects play a large role as well.
The wide range of phase stability seen at the two sites at the same time has implications for observing and phase monitoring for the MMA's 3~km array, the LMSA's 2~km array, and the 10~km Atacama Array, as there will frequently be times when the phase stability varies greatly over the array.
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