Airborne Raman Ozone and Temperature Lidar (AROTEL)

Measurement Description: Goddard's AROTEL lidar is configured around three key components:

• the light source,
• detector assembly, and
• data processing instrumentation.

The detector assembly consist of a telescope to collect the signal, discriminators to minimize noise counts, photomultiplier tubes to convert the signal photons into electrical pulses and both bandpass filters and beamsplitters to isolate the wavelengths of interest. Splitters transmit/reflect the various spectral components with high efficiency thereby insuring that most of a desired wavelength's signal goes to a particular PMT. Bandpass filters have high transmission only at the wavelength of interest, out of band rejection is typically 105 with a passband of ~1nm. Blocking extends from ~305nm to beyond 700 nm at which point the quantum efficiency of the PMT's bialkali photocathode is negligible. Because of their high quantum efficiency, high gain and low noise, photomultipliers are used to detect the return signals. Hamamatsu's R7400 PMT is employed because of its low noise (<100 cps) and extremely compact design (total length <5 cm). Linear PMT operation over a range spanning five orders of magnitude requires the photocathode be protected from the strong initial returns found just above the aircraft; otherwise the background count rate is no longer flat thus making it difficult to correctly separate out weak signals from the background. This protection occurs by using four different techniques. First, attenuated signals are sent to separate PMT's; this allows for collection of signals closest to the aircraft. In the highest sensitivity channel, used to measure signals farthest from the aircraft, complete blockage of the optical path between the telescope and PMT is produced by a rotating mechanical shutter (resembling an aircraft propeller) thereby preventing any light from reaching the photocathode. A membrane mirror provides up to three orders of magnitude blocking by rapidly change its radius of curvature (~1 microsecond) while the third technique involves reducing the PMT's gain from 10+6 to 0 ( in approximately 1 microsecond) by reversing the voltage on two of the dynode's (typically 1 & 2). This method provides satisfactory protection for all but the most intense pulses. Since the detectors have limited dynamic range, multiple channels are used for each wavelength. Much of the pre-integration effort is aimed at finding the appropriate solution for this problem.

Data will be collected using photon counting and analog detection techniques. Acquired data will be stored with a two minute time resolution. Between acquisitions, previous data will be analyzed and profiles of ozone, temperature, and aerosols will be available for viewing on the aircraft CCTV system.

Theory

Ozone: Ozone is measured using the DIAL (differential absorption lidar) technique that employs two distinct wavelengths, one strongly absorbed by ozone (308 nm) and the other having minimal absorption (355 nm). Returns are generated by Rayleigh and Raman scattering, therefore two wavelengths are transmitted and four wavelengths are collected. The Raman signals will be used to extract ozone in the presence of aerosols or clouds. Taking the ratio of the absorbed and not-absorbed signals removes instrumental parameters (such as detection efficiency and telescope size) and provides an expression relating ozone number density to the difference in the slopes of the two signal returns. Data will be acquired in 2 minute intervals (a 24 km horizontal footprint) with a vertical resolution ranging between 0.5 to 1.5 km depending upon altitude and integration time. Profiles of ozone are expected from near the aircraft to approximately 35-40 km. Measurement precision will be better than 10% depending upon altitude and acquisition time.

Temperature: Temperature profiles from just above the aircraft to beyond 40 km are measured using both Rayleigh and Raman scattering. Signal returns are directly proportional to atmospheric number density, which for a system in hydrostatic equilibrium can be related to temperature through the ideal gas law. Rayleigh scattering is employed down to an altitude of ~25 km (depending upon the aerosol loading of the stratosphere) while Raman scattering is used from there to the aircraft. Because of the strong returns, Rayleigh scattering is the preferred mechanism to generate signals but serious contamination by aerosols below ~25 km necessitates employing Raman scattering (which is, to first order, not affected by aerosol scattering). The algorithm is initialized at a maximum altitude (currently a pressure altitude of 1 mbar - ~48.4 km ) using Goddards Data Assimilation Model and rapidly converges to the correct value within three scale heights. Profiles are acquired in 2 minutes intervals (24 km horizontal footprint) with a vertical resolution of between 0.5 - 2.0 km depending upon altitude. Precision is 1-2 K for a two minute measurement.

Additional information on both the instrument and the theory can be found at the following web site: http://hyperion.gsfcw.nasa.gov/Ground_based/