The ASOS is jointly administered and used by the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense. The network is designed to support weather forecasting activities, aviation operations, and the needs of the meteorological, hydrological, and climatological research communities and private sector communities.
According to Program Manager Rick Ahlberg, the main objectives of the new ASOS wind measurement sensor modernization were: increased reliability, reduced need for maintenance (particularly in winter), ice-free operation and the sensors’ ability to incorporate the WMO Gust Standard.
The wind sensor modernization is a part of an overall ASOS Product Improvement Program. Mr. Ahlberg says that the improvements include better observing capability, reporting accuracy, and consistency.
The overall program objective was to acquire a commercial off-the-shelf (COTS) sensor that could be modified to meet the requirements of the NWS and FAA. The wind instrument needed to be ice-free, and contain no moving parts which would wear down or be contaminated by sand, salt or dust.
After a rigorous evaluation, Handar Inc’s COTS sonic sensor was chosen. The sensor was modified to meet the NWS specifications. In 1999, Vaisala Oyj acquired Handar Inc. The sensor is now known as the special ice-free model of the Vaisala Ultrasonic Wind Sensor WS425. The sensor incorporates Vaisala’s proprietary equilateral triangle design, which always has one redundant wind measurement path and therefore solves the known turbulence problem.
The Vaisala Ultrasonic Wind Sensor WS425 is an array of three equally spaced transducers which radiate and receive ultrasonic pulses in a horizontal plane. The sensor measures transit times in both directions for each of the three transducer pairs. The wind speed and direction are then derived from these six transit time measurements. The Ice Free Wind Sensor for the NWS is capable of providing the WMO standard 3-second average gust, whereas the existing cup anemometer provided a five-second average gust. This capability improves responsiveness and reflects the gusts more accurately.
As mentioned before, the fundamental design of the ASOS Ice Free Wind (IFW) relies on the standard Vaisala Ultrasonic Wind Sensor WS425. However, a few improvements and ASOS specific modifications have been made.
In order to guarantee operation even in the most difficult conditions, the sensor has an enhanced heating feature. There are heater foils inside the ceramic transducers, the transducer arms, and the birdspike. The power consumption is divided as follows: three transducers total 24 W, three transducer arms total 76 W, and 1 birdspike 15W. The total heating power is 150W.
To ensure the sensor’s ASOS compatibility, it has a 10m long sensor cable, power supply with
heating control circuitry, and power relays to control the heating current. Additionally, in order to guarantee trouble-free and safe communications, there is an optical modem that isolates the sensor from the rest of the ASOS infrastructure. An installation kit is included in the delivery to make the sensor mechanically compatible with the ASOS system.
During the course of the project, the sensor was subjected to extensive electrical, environmental, field, and functional testing.
Test Manager Michael Sturgeon has been responsible for the tests on the NWS’ side. Although the sensors’ functional accuracy requirement for wind speed is +/-2 knots, or 3% up to 125 knots, Michael has also tested the sensor performance beyond its limits. The sensor has shown good performance with wind speeds up to 157 knots. The following graphs show some of the test results.
(Reprinted with permission by Vaisala)
— Fondriest Environmental offers a large selection of research-grade wind measurement technology, including field-rugged sensors from Vaisala. Furthermore, Fondriest offers many weather sensor options for accommodating a wide range of project needs. For more information about the Vaisala Ultrasonic Wind Sensor WS425 and additional weather sensors, call a Fondriest Environmental product specialist at 888.426.2151 or email email@example.com.