Comparison of PWV measurements determined from co-located water vapour monitors used in the Thirty Meter Telescope site testing campaign

The 20 mum (15 THz) Infrared Radiometer for Millimetre Astronomy (IRMA) monitors a narrow spectral band containing only water vapour molecular transitions. When used in conjunction with an accurate atmospheric model (BTRAM), it is possible to determine absolute precipitable water vapour (PWV) in a column of atmosphere to high accuracy. Flux calibration of IRMA is accomplished by using a calibrated blackbody source. The resulting PWV measurements can be used to determine atmospheric opacity and thus the potential to conduct infrared astronomical observations at the site. Since January 2007, three calibrated IRMA units have been deployed in the Americas as part of a site selection effort for the Thirty Meter Telescope (TMT) project. The three units were operated in parallel while co-located and viewing the same atmosphere. We present the parallel observation data, model sensitivity studies, and error analysis.