Brenner, Martin (25 Νοεμβρίου 2009). «Pilot Weather Balloon (Pibal) Optical Theodolites». Martin Brenner's Pilot Balloon Resources. California State University, Long Beach. Ανακτήθηκε στις 25 Ιουλίου 2014.
doi.org
dx.doi.org
Thyer, Norman (Μάρτιος 1962), «Double Theodolite Pibal Evaluation by Computer», Journal of Applied Meteorology and Climatology (American Meteorological Society) 1 (1): 66–68, doi:10.1175/1520-0450(1962)001<0066:DTPEBC>2.0.CO;2