This paper presents our group's most recent passive millimeter-wave (MMW) measurements made using a 94-GHz Stokes-vector radiometer. Included are images and analyses of treeline data. These data were collected to investigate the possible use of passive MMW sensors to perform the helicopter collision avoidance task. The treeline data presented were collected in both the summer and winter. The results of the analysis show that in the winter the detection of the treeline can be straightforward because of an oftenlow horizon sky brightness temperature. The contrast between the tree branches and the horizon are seen in the data to be about 10-15 K. The summer case, however, shows a horizon sky-to-tree brightness temperature ratio of about 1. A simple statistical analysis of the summer image shows that the trees, in our case, can be distinguished from the horizon sky based upon the statistical parameters alone.