ALERT field stations consist of one or more sensors, a transmitter/data logger, and antenna, usually enclosed in a standpipe structure or gage house. The station, which is designed for simplicity and vandal resistance, can operate effectively under adverse weather conditions and is powered by a rechargeable battery lasting for up to a year without service. Five types of sensors are currently used:

• Rain Gage: Precipitation is measured by a tipping bucket located under a funnel assembly on top of a standpipe structure. For every 1 millimeter (0.04 inch) increment of precipitation, the tipping bucket generates an input which activates the transmitter and sends a data transmission. The data transmitted includes the station identification and the accumulated rainfall value. Each sensor is assigned a unique identification number which is used for data filing and processing. By transmitting an accumulated rainfall value rainfall amounts can be determined by the base station computer even if some radio transmissions are not received.
  
  
• Snow Gage:  The snow gage consists of an electrically heated tipping bucket rain gage. Snow falling into the gage funnel is melted and measured by the tipping bucket. The resulting "water equivalent" of snow is transmitted as rainfall.
  
  
• Water-Level Sensors: Stilling well water level floats, pressure transducers, or nitrogen bubblier-type water level sensors are used to monitor the water level in streams, flood control channels, reservoirs, retarding basins, and dams. Whenever the incremental change in water level exceeds a programmed quantity the transmitter sends out the current water level. The selection of the programmed increment value is based on the characteristics of each station, usually the increment is between 0.05 and 0.25 feet.
  
  
• Wind Speed - Direction Sensor: Wind speed and direction are measured using two different devices. The mechanical Wind Sensor consist of a weather vain assembly connected to a potentiometer which responds to changes in wind direction and produces a linear voltage output and a propeller connected to an electrical pulse counter. Wind speed is determined by the time it requires to generate a given number of pulses which are equal to a wind run of 2 kilometers. The Ultrasonic wind sensor measures the time of travel of ultrasonic sound pulses between three probes located at the vertices of an equilateral triangle. The shift in speed of sound between the three probes determines the wind speed and direction.
  
  
• Temperature Sensor: Temperature sensors consist of a two-terminal integrated circuit transducer that provides a linear output current proportional to the absolute temperature. Temperature sensors are protected from the effects of overheating due to solar radiation by a Solar Radiation Shield.

The same type of electronic package containing the transmitter and data logger is used for all the field sensors described. The electronic package is available in two configurations: 1) a two input -  rain gage - water level configuration, and 2) a weather station - configuration which allows for up to 5 sensor inputs. Thus, more than one sensor can be installed at a station utilizing a single transmitter. The transmitters are the critical component of the ALERT System. Manufacturers supplying ALERT capable equipment include HydroLynx, High Sierra Electronics, and Handar. Orange County is using Sierra-Misco/HydroLynx equipment. Sierra-Misco which supplied the majority of Orange County's equipment is no longer in business, however the Sierra-Misco's equipment is supported by HydroLynx for replacement and repair. The most common problem encountered with the transmitters is battery failure. In remote areas or when it is practicable solar cells are used to maintain the battery charge.

System Components

ALERT System
Repeater Stations
Field Stations
Return Frequencies

Weather Forecasts
EMWIN System

Weather Imaging