S and L Microwave Radiometers(SLMR)
 |
||
|
| SLMR Data |  Ground remote sensing page |
Introduction
One of the primary objectives of SGP97 was to establish that the microwave retrieval algorithms for surface soil moisture developed at higher spatial resolution using truck- and aircraft-based sensors could be extended to the coarser resolutions expected from satellite platforms. In support of this objective, a multifrequency single polarization truck-mounted microwave radiometer system was deployed to the ARM/CART Central Facility site during SGP97. The radiometer system consisted primarily of the S and L Microwave Radiometer (SLMR) managed by the USDA/ARS Hydrology Laboratory and maintained in cooperation with the University of Massachusetts. This system was supplemented by a 6-channel stepped frequency C band radiometer also supplied by the University of Massachusetts. A hydraulic boom truck belonging to NASA Goddard Space Flight Center was used as the staging platform for the microwave radiometers.
Objectives
Inclusion of a truck-mounted microwave radiometer system in the SGP97 experiment was based on a number of specific objectives, including to:
- Provide continuous 24-hr TB measurements to complement the once-a-day aircraft microwave data;
- Deploy over representative watershed site (grass pasture and winter wheat);
- Fill in data gaps due to aircraft down time;
- Aid interpretation of potential temporal variations due to duration of aircraft mapping or ground sampling;
- Permit diurnal effects in microwave/soil moisture relationship to be calibrated;
- Acquire energy balance/micrometeorology measurements in parallel at the same site; and
- Allow models or coupling of models to be validated at the point scale before being extrapolated to the entire SGP97 test area.
System Description
| ||||||||||||||||||||||||||||
During SGP97 the SLMR/C truck radiometer system was staged on a 1990 Navstar hydraulic boom truck belonging to the Hydrological Sciences Branch at NASA/GSFC. This vehicle is equipped with a hydraulic boom which permits deployment of sensor packages up to a height of approximately 19 m above the ground. The instrument platform at the end of the boom can be moved to vary incidence angle from 0o (nadir) to 180o (sky), while the boom itself can be rotated 360o in azimuth. In SGP97 the antennas of all of the radiometers were mounted to observe horizontal polarization. At the nominal boom operating height of 12 m and an incidence angle of 15o off nadir, the footprint or spot on the ground observed by the radiometers was on the order of 3.6 to 4.6 m depending on the specified field of view of the individual radiometers. Incidence angle information can be provided by either internal instrument inclinometers or by an independent inclinometer on the truck's instrument grid framework. During SGP97 the internal SLMR inclinometers read 4-4.5o lower than the truck inclinometer. Since the truck inclinometer on the instrument grid was easily calibrated, it was used in the experiment as the true reference angle--the divergence of the internal inclinometers from this reference point was taken into account in positioning the radiometers over the test fields during data taking operations.
In addition to the microwave radiometers, several other supporting instruments were also mounted on the truck platform. A small portable thermal infrared radiometer by Everest Interscience (Model 110) was used to estimate the surface temperature by measuring thermal emission in the 8-14 micron wavelength range. Target location for the microwave radiometers in the test plots was initially achieved with a color video camera installed on the platform. Thereafter, repeat positioning of the radiometers over the same footprint in each field was accomplished using boom geometry (height, azimuthal angle, incidence angle).
System operation and control of the radiometers was performed by a small personal computer in an instrument rack at ground level. Software monitored the thermal status of the radiometers and attempted to maintain thermal equilibrium of the defined goal temperature through the distributed heater network. Data collection was undertaken in one of two modes: manual operation of the microwave system was normally done during the day when the boom was moved between two test fields and two calibration targets on an hourly basis, while automated data measurement or autocollect mode was used for unattended operation of the system at night over a single test field.
Operations Schedule
The nominal operating schedule for SLMR measurements was manual operations daily between 8 am and 4 pm local Oklahoma time, and autocollect operations overnight between 4:30 pm and 7:30 am. During the day measurements were made each hour of the brightness temperature response of the two test fields at the ARM Central Facility site, CF01 grazed pasture and CF02 winter wheat, as well as sky and absorber calibration targets. Most data were acquired at a 15o incidence angle; beginning on 6-28-97, data were also taken at a 40o incidence angle at 8 am and 4 pm local time. During the night data were collected over either CF01 or CF02 (not both) every 30 minutes. Deviations from the nominal operating schedule were generally due to the presence of severe weather or to temporary equipment problems. In addition, SLMR truck operations were suspended whenever a Code Red weather alert was declared by the ARM Facility Manager according to established ARM safety protocols.
|
SGP97 CENTRAL FACILITY SLMR TEST SITES
|
|
Data calibration and quality
Calibration of the SLMR data was accomplished based on assessments of the internal thermal stability of the instruments and their responses to external targets of known brightness temperature (cold sky and ambient microwave absorber ). Data quality issues include observed RFI at S band (observed only when moving boom between sky cal and absorber cal positions - did not appear to affect any data takes), slight movement in C band waveguide causing "jitter" in measurements when boom was moved (solved by waiting 30-60 seconds for instrument to stabilize before data takes in new boom position), and saturation of absorber response at C band due to high ambient temperature of absorber material (open issue). At the present time, only L and S band SLMR data have been included in the SGP97 data base.
Data file formats
Data have been placed in two files, slmrcf01 and slmrcf02, to correspond to each of the two ARM Central Facility fields: CF01 is the pasture/grass field and CF02 is the winter wheat field. All data in both files were acquired at an incidence angle of 15 degrees off nadir. TB values are in Kelvin, TIR values are in degrees C (use the TIR values with some caution - they have not been quality controlled), and dates are in DOY (day of year) format. All times are local Oklahoma Central Daylight Savings Time (CDT). Each line contains the following fields separated by blank spaces:
- DOY
- Hour
- Min
- Sec
- Time (DOY plus fraction of day)
- L voltage
- Calibrated LH TB
- S voltage
- Calibrated SH TB
- TIR (thermal infrared temperature in 8-14 micron band)
- Hour
The number of data points per file will be different between the grass and wheat files for two reasons:
- Since the truck boom was moved manually between the two CF fields, if the ARM Facility Manager called Condition Red in the middle of a data run, data taking ceased immediately and it is possible that a particular data series was not completed. However, normal procedure was to stop after the data run was complete if possible.
- The main reason there is a different number of data points between the files is because SLMR was put into Autocollect mode at night over either the grass or wheat on different days during the month. The experiment started by looking at wheat at night, then switched to grass, then back to wheat stubble, etc. In addition, after the severe rain resulted in water in the antennas on 6-26-97, the SLMR instruments were sometimes shut down at night if a risk of thunderstorms was forecast.
Data Access and Contacts
FTP SiteThe S and L band radiometer data is in the following GES DISC ftp site:
Points of Contact
- For science questions or information about the SLMR contact
- Peggy E. O'Neill
- Code 974
- NASA/GSFC
- Greenbelt, MD 20771
Phone: 301-286-8273- Fax: 301-286-1758
- E-mail: peggy@hydro4.gsfc.nasa.gov
- For more information using SLMR GES DISC data, contact:
- Hydrology Data Support Team
- Goddard Earth Sciences
Data and Information Services Center (GES DISC)- Code 610.2
- NASA Goddard Space Flight Center
- Greenbelt, Maryland 20771
E-mail: hydrology-disc@listserv.gsfc.nasa.gov- Voice: 301-614-5165
- Fax: 301-614-5268
 |
Last update:Thu Jan 28 09:34:12 EST 1999 Page Info: Hydrology Data Support Team -- hydrology-disc@listserv.gsfc.nasa.gov Web Curator: Stephen W Berrick NASA official: Steve Kempler, DAAC Manager -- Steven.J.Kempler@nasa.gov |