Ground-Based Scanning Radiometer (GSR)

NOAA/ETL has designed a multi-frequency scanning radiometer operating from 50 to 380 GHz. The radiometers are installed into a scanning drum scanhead which is mechanically and electrically compatible with the NOAA aircraft instrument, the Polarimetric Scanning Radiometer (PSR). The PSR system has operated in more than 15 experiments and has over 600 flight hours of successful operation observing the Earth's surface and atmospheric parameters. The ground-based instrument, GSR, uses the submillimeter scanhead (PSR/S) with 11-channels in the 50-56 GHz region, a dual-polarization measurement at 89 GHz, 7-channels around the 183.31 GHz water vapor absorption line, dual-polarized channels at 340 GHz, and 3-channels at 380 GHz. It also has a 10.6 micrometer infrared radiometer within the same scanhead. All of the radiometers are mounted within a rotating scanhead, use lens antennas, and view two external reference targets during the calibration cycle. New thermally stable calibration targets with high emission coefficients have been designed for the purpose. In addition, each of the radiometers' design includes two internal reference points for more frequent calibration. The beam widths of the GSR channels are 1.8° and can be averaged to given beam-widths consistent with the ARM MWR (4.5 to 5.5°).

Schematic diagram of the GSR calibration and scanner system. The GSR scanhead periodically moves out of the framework for zenith viewing on a trolley system, and shares time observing the atmosphere and the two temperature-controlled blackbody reference targets.	GSR in process of assembly on March 7, 2004 in Barrow, AK. This photo shows the GSR framework with the external calibration targets and the scanhead with the radiometers installed are shown.
GSR radiometers mounted on the scanning drum scanhead.	GSR operating in Barrow, AK, during the WVIOP04, on March-April 2004.

Unique features of the GSR radiometer include:

• Potential for inclusion of several important radiometric bands in a single co-aligned scanning system
  
  
• Use of two common large thermally stabilized calibration targets with up to 100-120 degrees centigrade of hot-cold temperature spread
  
  
• Complete shielding of calibration targets from vertically-falling hydrometeors
  
  
• Absence of reflecting mirrors normally used to implement scanning
  
  
• Absence of radome materials between the radiometer antennas and sky
  
  
• Horizon-to-horizon sky field of view of all radiometers
  
  
• Inclusion of co-aligned 10-um thermal IR sensor for cloud detection
  
  

The GSR was operated at Barrow, Alaska, during the Arctic Winter Water Vapor Intensive Operating Period 2004 (WVIOP04) from March 9 to April 9, 2004. The primary purpose of the instrument was to measure temperature, water vapor, and clouds, at cold (-20 to -55 °C) and dry (PWV < 5 mm) conditions.

• Typical Target, continuous scan, and air-mass dwell for GSR
  
  
• GSR sample data during the WVIOP04
  
  
• Artic Winter Water Vapor IOP 2004 preliminary results
  
  

Typical Target, scan and air-mass dwell for GSR

The GSR has a flexible and software programmable angular-scanning sequence that is repeated every two minutes. The sequence starts with the GSR being inside the calibration house and viewing the hot calibration target for 3 seconds. During the next step, the GSR remains in the calibration house and views the cold target, again for 3-seconds. The scanhead then moves out of the calibration house and moves to the atmospheric-scanning position, where it moves from air mass = 3.5 to a sequence of air mass dwells of 2-seconds each. Between the air mass dwells the radiometer moves continuously to the next scan position. Thus the radiometer acquires both continuous and dwell data for the atmosphere with two-point calibration data in between. For channels in the transparency windows, both 2-point and tip-curve calibration methods can be used. In addition to the external calibration, the radiometer also switches between hot and cold internal calibration loads. The GSR basic record length is 10ms.

GSR in motion samples (mpg) from Barrow, Alaska, on March 10 2004

GSR sample data during the WVIOP04

Short time series of data taken on March 11, 2004, during the WVIOP. From the channels in the 50-60 GHz Oxygen band it is possible to note that the strongest channels from 55.5 to 56.3 GHz clearly show the presence of a thermal inversion. Conversely, the weakest channel at 50.3 GHz will allow tip curve calibration. For all of the channels, the time spent dwelling at the separate air mass dwell points can be seen. From the channels around the 183.31 GHz Water vapor line we note that the strongest channels from 183.31 ±0.5 and ±1 are close to saturation. Conversely, the weakest channels from 183.31 ± 15, ±12, and ±7 all will allow tip curve calibration. From the channels around the 380.2 GHz water vapor line we note that the strongest channel at 380.2 ±4 shows the presence of a thermal inversion.

Time series of Tb between 50 and 60 GHz.	Time Series of Tb for channels around the 183.31 GHz water vapor line.
Time series of Tb near the 380.2 GHz water vapor line.

Publications

• Ed R. Westwater, Susanne Crewell and Christian Mätzler, Surface-based Microwave and Millimeter wave Radiometric Remote Sensing of the Troposphere: a Tutorial, IEEE Geosciences and Remote Sensing Newsletter, March 2005, 134.


• Ed R. Westwater, Marian Klein, and Vladimir Leuski, Al Gasiewsk, Taneil Uttal, and Duane Hazen, Domenico CiminiM Vinia Mattioli, Bob L. Weber, Sally Dowlatshahi,Joe A. Shaw, Jim Liljegren ,B. M. Lesht, Bernie Zak; The 2004 North Slope of Alaska Arctic Winter Radiometric Experiment Proceedings Of The 14th ARM Science Team Meeting, March 22-26, 2004, Albuquerque, New Mexico.


• E. R. Westwater, M. Klein, V. Leuski, A. J. Gasiewski, T. Uttal, D. A. Hazen, D. Cimini V. Mattioli B. L. Weber, S. Dowlatshahi, J. A. Shaw, J. S. Liljegren , B. M. Lesht, and B. D. Zak, Initial Results from the 2004 North Slope of Alaska Arctic Winter Radiometric Experiment Proc. IGARSS'04 (in press).


• Ed R. Westwater, Susanne Crewell,and Christian Mätzler, Frontiers in Surface-based Microwave and Millimeter Wavelength Radiometry Proc. IGARSS'04 (in press).