Instruments Overview:

All sites in MPLNET currently use the Micro Pulse Lidar (MPL), which was developed at NASA Goddard Space Flight Center (GSFC) in the early 1990s. The MPL was patented and subsequently licensed to industry for commercial sales beginning in the mid 1990s.

The MPL (Spinhirne et al. 1995) is a compact and eye-safe lidar system capable of determining the range of aerosols and clouds by firing a short pulse of laser light (at 523, 527, or 532 nm) and measuring the time-of-flight from pulse transmission to reception of a returned signal. The returned signal is a function of time, converted into range using the speed of light, and is proportional to the amount of light backscattered by atmospheric molecules (Rayleigh scattering), aerosols, and clouds. The evolution of the MPL from the initial Spinhirne et al. (1995) optical design to the standard design now used in MPLNET is described in detail by Campbell et al. (2002) and Welton and Campbell (2002), including on-site maintenance, and calibration techniques.

Post 2002 enhancements include a new data system, telescope, fiber-coupled detectors, and a new laser. These changes do not significantly alter the basic MPL optical design but increase system reliability and allow for more in-field repair options. In addition, the MPL company introduced a new model of the MPL called the miniMPL. The miniMPL was designed to be more compact and portable, and uses a modified optical design. However, the MPL and miniMPL are very similar in performance, except with a higher signal-to-noise ratio for the MPL relative to the smaller miniMPL.

Post 2013 changes: The original MPL design from the early 1990s was unpolarized, providing only total signal power. The MPL was first polarized with an approach described by Flynn et al (2007), and was sold by Sigma Space Corporation for several years. The initial polarized MPL design was not used in MPLNET due to 1) use of nematic liquid crystal polarizer that resulted in slower data rates and potential corruption of data specifically for clouds, and 2) lack of a proper instrument characterization and stability study of the polarized design and its calibration procedures. A new polarized MPL design was developed with NASA and Sigma Space Corporation around 2013, following the same basic design concept from Flynn et al but using a ferroelectric liquid crystal (FLC) for faster data rates and a slightly modified measurement strategy to accommodate the difference in polarizer properties. The FLC polarized MPL was characterized for several years at NASA and design iterations with Sigma Space eventually lead to a new commercial polarized MPL now sold for several years. Welton et al (2018) present the new polarized lidar and discuss calibration procedures. A more detailed paper is in preparation describing the new polarized MPL and calibration/performance studies.

Note, the MPL divison at Sigma Space was realigned to Leica Geosystems Inc in 2018, and then sold to Droplet Measurement Technologies which is the current MPL company.

Commercial Off The Shelf (COTS) MPL Systems:

COTS MPL systems may not meet MPLNET specifications. In order to meet MPLNET specifications, COTS MPLs must be inspected at our GSFC facility. This process is performed for all COTS MPLs in MPLNET. Any required modifications to the COTS MPL are performed prior to operation in MPLNET. The NASA MPLNET project does not inspect COTS MPLs sold to non-MPLNET partners, nor does it support data processing for non-MPLNET partners. It is the responsibility of the commercial manufacturer to ensure high quality of COTS MPLs sold to customers outside MPLNET. It is also the responsibility of the non-MPLNET customer to operate the COTS MPL correctly and apply proper data processing and analysis techniques. As a result, the performance and data quality of COTS MPL systems are not the same as our MPLNET systems and data products.

In recent years, other small aerosol and cloud lidars have become commercially available. Some are designed for possible network use. We are currently evaluating some of those for possible support in MPLNET.

* full citations for references available on our publications page