Omnisys has designed and produced 58 Water Vapor Radiometers, one for each of the antennas at the European Southern Observatory ALMA telescope in Chile. The expected project lifetime is at least 15 years.  

The Omnisys Water Vapor Radiometer, or WVR, is a powerful support equipment used to compensate for the phase propagation delay, which is due to differences in the amount of water vapor in the atmosphere above antennas. Omnisys was awarded the project in June 2007. By December 2010, in line with the schedule, the last radiometer was accepted by ALMA. Several critical subsystems were included in the delivery, such as Quasi Optics, Front-End, Back-End and Control and Power modules.


  • Water Vapor compensation is done by measuring the energy on the 183.31GHz spectral line of water. The measurement is calibrated by alternate measurements on well defined hot or cold loads and the sky, a so called Dicke-Switch radiometer.
  • The overall accuracy of the WVR is 2K brightness temperature over the range from 50K to 370K. To be able to reach this accuracy, the Front-End and parts of the Back-End is temperature controlled to better than 0.1K.
    • Sensitivity: 80-100mK on the four separate channels.
    • Stability: 100mK during 10 minutes and 10 degree tilt.
    • Quasi optics: One flat mirror, three active mirrors and a chopper wheel with mirrors for hot and cold loads.
    • Front End: One corrugated horn antenna, one low noise (NF=7.5 dB) integrated Schottky mixer and LNA (30 dB), local oscillator system based on a DDS controlled 15 GHz VCO and active multiplier chain.
    • Back End: A four channel filter bank with diode detectors and 50 dB of distributed stable amplification.
    • Control and power: Thermal control, motor control, computing and communication.
    • Weight: 23kg.
    • Power: 230V AC supply, 65W during operation.
    • Interface: CAN bus, also possible to run via Ethernet port.


The Omnisys WVR is used to compensate antennas for the effect of water vapor in the atmosphere, which cause distortion to signals. More information about the instrument is available by clicking the link below:

Correcting for the Effects of the Atmosphere by Richard Hills, ALMA Project Scientist

Alma 3D