The instrument has been modified to measure polarization and a separate channel has been added to measure both polarization and signal amplitude variations as function of receiver field of view. These modifications allow discrimination between ice and water clouds and measurements of multiple scattering. Along with these modifications, the high resolution etalon, used in the earlier versions of the HSRL, has been replaced with an iodine absorption filter. Also, the system has been redesigned for use in a semitrailer. Other changes include new data acquisition system, temperature controlled environment for the receiver optics, and improved optical design. The system parameters of the HSRL are summarized in Table 1.
The changes to the HSRL have improved the measurement capabilities of the HSRL. With the new HSRL, the measurements of the optical parameters of the atmosphere below 35 km can be made and therefore, the measurements can cover clouds from water clouds to high altitude cirrus clouds and also measurements of stratospheric aerosol layers can be performed. Clouds with optical depths up to 3 can be measured. This means that most of the cirrus cloud cases can be fully observed and the cloud bases of the thick water clouds can be measured up to 300-500 m inside the cloud. This has been achieved by using a high pulse repetition rate, small pulse energy per laser pulse, and very fast photon counting electronics. Furthermore, the averaging time required for a good signal to noise ratio has been decreased by improving the optical transmission of the system. The use of the iodine filter, the controlling of the pressure of the etalons and the temperature of the optics, and the locking of the laser wavelength to an iodine absorption peak have increased the stability and the reliability of the system. Therefore, the need for frequent calibrations is eliminated and a stable operation can be maintained over a long period of time. The enhanced calibration technique has improved the accuracy of the HSRL calibrations.
Table 1: HSRL specifications (April 26, 1994)