| Date: | 2003-08-28 18:49:00 | |
| Sender: | Van S Husson, ILRS CB <Husson, Van <Van.Husson@honeywell-tsi.com>> | |
| Subject: | [SLR-Mail] No. 1123: Meteorological Devices Best Practices | |
| Author: | Van S Husson, ILRS CB | |
| Content: | ******************************************************************************** SLR Electronic Mail 2003-08-28 18:49:00 UTC Message No. 1123 ******************************************************************************** Author: Van S Husson, ILRS CB Subject: Meteorological Devices Best Practices Dear Colleagues: Meteorological errors from our sites have been a common problem in our technique, which adversely influences final SLR data quality. As we push towards 1 millimeter (mm) accuracy, we need to eradicate meteorological errors as an error source. An error of just 1 millibar INDUCES an elevation dependent range bias of 7 and 3 mm at 20 and 90 degrees elevation, respectively. We also can not neglect the humidity measurement. A 10 % humidity error, at a temperature of 20 degrees Celsius, INDUCES an elevation dependent range bias of 1 and 0.4 millimeters at 20 and 90 degrees, respectively. The impact of a humidity error is temperature dependent, the higher the temperature the larger the error. The temperature measurement is not critical for tropospheric refraction, but is a critical measurement for systems which have far calibration targets (i.e. calibration distance >1 Km). Below is a list of recommended procedures and manufacturers: BEST PRACTICES -------------- Each site should have a least one near-by independent meteorological data source (i.e. local airport, weather station, etc.) for comparison purposes. We recommend that met. comparisons been done automatically at least daily and tabulated (i.e. create a database) in order to detect any long term trend in the differences. At least ONCE EVERY 2 years, we strongly encourage that your met. device be calibrated versus a known standard. This is currently being done within the NASA network and a similar campaign was initiated in 2002 within EUROLAS. Through regular calibrations and daily comparisons any errors that might creep into the measurements would be traceable and therefore recoverable. Any met. data correction algorithms should be sent to the ILRS Central Bureau (cb@ilrs.gsfc.nasa.gov) and footnoted in your site log. It is also imperative that your barometric sensor either be at the same height as your optical axes OR that the pressure measurement be corrected for height in on-site data processing. A one meter height difference equates to an approximate 0.1 millibar pressure change. Meteorological devices with ports/gills are preferable to devices with tubing. Tubes can become easily kinked or clogged. Tubing should also be shielded from the wind. MANUFACTURERS ------------- Based on the site logs, the two most popular manufacturers of meteorological sensors are Paroscientific (16 sites) and Vaisala (6 sites). The Paroscientific MET3 is the most widely used model within the ILRS. Below are URLs of these two manufacturers: Vaisala Web Site is: http://www.vaisala.com/ Paroscientific Web Site is: http://www.paroscientific.com/ Paroscientific MET3A description is located at: http://www.paroscientific.com/met3a.htm The key features that should be strongly considered in the purchase of any met. device for our application is: 1. Automated readings, 2. the long term stability and absolute accuracy of the barometric sensor, and 3. rapid humidity recovery time after saturation. The purchase and proper installation/care of a state-of-the-art meteorological device coupled with periodic calibrations should significantly improve our data quality. The cost of a good met. device is a relatively small price to pay factoring in the total price of an SLR system especially and its potential benefits. From: ”Husson, Van” ******************************************************************************** |