Light Detection and Ranging (MET LIDAR)

See also MET Overview

See also MET File Naming

The main objective of the Lidar is to measure the vertical distribution of scattering particles (both dust and clouds) in the Martian atmosphere. These measurements are expected to provide estimates of:

The lidar on the Phoenix mission will record the height profile of laser backscatter from the dust and cloud layers that drift past the landing site with a view that is continuous in time. The attenuation of the lidar signal can be used to derive the optical extinction coefficient and this is related to atmospheric properties. For example, the extinction of solar radiation due to dust has a first order effect on heating of the surface of Mars. The measured optical extinction can also be related to the amount of scattering material. For example, for a thin cloud the extinction coefficient measured by a lidar can be used to derive the ice water content within the cloud. Such a measurement will be of vital importance to the mission goal to investigate water on Mars. The   combination of the lidar and passive remote sensing with in situ sampling will provide a view of the interacting processes that determine the local weather at the surface and also the role of the atmosphere in the water cycle on Mars.

A basic atmospheric lidar (light detection and ranging) emits pulses of laser light into the atmosphere then detects and records the backscattered light as a function of time. The time resolved signal is converted to distance using the speed of light and a factor of two to account for the round-trip path length.

The lidar transmitter is based on a diode pumped, passively Q-switched, neodymium-doped yttrium garnet (Nd:YAG) laser. This configuration was chosen for its robustness and technological maturity as much as for its suitable lidar performance. The pump diode array emits at a wavelength of 808 nm and is used to provide the energy to the Nd:YAG crystal within the laser cavity. Lasing is inhibited within the cavity through the use of a saturable absorber. The photon density within the oscillator cavity builds to a level where the saturable absorber bleaches and the energy within the cavity is dumped as a laser pulse over a very short time (a passive Q-switch). For this laser the output light pulse has a length of approximately 10 ns and an energy of 0.7 mJ.

After the laser cavity emits its light pulse, part of the optical energy is converted from a wavelength of 1064 nm to 532 nm by second harmonic generation in a Potassium Titanyl Phosphate (KTP) crystal. The laser output is then expanded by a factor of 10 in order to reduce the divergence to 0.5 mrad. A small fraction of the outgoing laser pulse is 'picked off' and separated into the two wavelength components in order to measure the relative amplitude of each wavelength with photodiodes. The signal from the photodiode detecting the 1064 nm pulse is used to trigger the data acquisition electronics, providing a zero-time reference, and also to shut off the pumping diodes.

The atmospheric backscatter signals are collected by an afocal reflective telescope and split into the two relevant wavelengths using dichroic mirrors. The 1064 nm backscatter is detected by an Avalanche Photo-Diode (APD), and recorded using 14-bit analog to digital conversion (ADC). The 532 nm backscatter is detected by a Photomultiplier (PMT) and the signal  is recorded using both ADC and photon counting. Photon counting is required to record the weak signals from heights above 5 km and up to 20 km.

Source: LIDAR Instrument Catalog File