2008

Stable Calibration of Raman Lidar Water-Vapor Measurements

Data from occasional radiosonde campaigns and routine laboratory lamp measurements are utilized.

A method has been devised to ensure stable, long-term calibration of Raman lidar measurements that are used to determine the altitude-dependent mixing ratio of water vapor in the upper troposphere and lower stratosphere. Because the lidar measurements yield a quantity proportional to the mixing ratio, rather than the mixing ratio itself, calibration is necessary to obtain the factor of proportionality. The present method involves the use of calibration data from two sources: (1) absolute calibration data from in situ radiosonde measurements made during occasional campaigns and (2) partial calibration data obtained by use, on a regular schedule, of a lamp that emits in a known spectrum determined in laboratory calibration measurements.

In this method, data from the first radiosonde campaign are used to calculate a campaign-averaged absolute lidar calibration factor (t1) and the corresponding campaign-averaged ratio (L1) between lamp irradiances at the water-vapor and nitrogen channel wavelengths. Depending on the scenario considered, this ratio can be assumed to be either constant over a long time (L = L1) or drifting slowly with time.

The absolutely calibrated water-vapor mixing ratio (q) obtained from the ith routine off-campaign lidar measurement run is given by
qi = Pi/ti = LPi/P′i

where Pi is water-vapor/nitrogen measurement signal ratio, ti is the unknown and unneeded overall efficiency ratio of the lidar receiver during the ith routine off-campaign measurement run, and P′i is the water-vapor/nitrogen signal ratio obtained during the lamp run associated with the ith routine off-campaign measurement run. If L is assumed constant, then the lidar calibration is routinely obtained without the need for new radiosonde data. In this case, one uses L = L1 = P1′/t1, where P1 is the watervapor/ nitrogen signal ratio obtained during the lamp run associated with the first radiosonde campaign.

If L is assumed to drift slowly, then it is necessary to postpone calculation of qi until after a second radiosonde campaign. In this case, one obtains a new value, L2, from the second radiosonde campaign, and for the ith routine off-campaign measurement run, one uses an intermediate value of L obtained by simple linear time interpolation between L1 and L2.

This work was done by Thierry Leblanc and Iain S. McDermid of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it.. NPO-45955

White Papers

Fundamentals of Vector Network Analysis Primer
Sponsored by Rohde and Schwarz
Noncontact Differential Impedance Transducer
Sponsored by Kaman
Removing the Gap Between ECAD and MCAD Design
Sponsored by Mentor Graphics
Meeting Industry Standards for Sanitation in The Food Industry
Sponsored by Able Electropolishing
dB or not dB? Everything you ever wanted to know about decibels but were afraid to ask...
Sponsored by rohde and schwarz a and d
Avionics Reliability – Thermal Design Considerations
Sponsored by Mentor Graphics

White Papers Sponsored By: