Hybrid Doppler Wind Lidar

Global Tropospheric Wind Sounder (GTWS): A Strategy for Obtaining Operational Wind Profiles from Space

3. Hybrid Doppler Wind Lidar (Emmitt)

 Given that the GTWS reference designs for both direct and coherent detection DWLs were large, expensive, and would require significant technology advances, interest has increased in a hybrid DWL instrument that uses both direct and coherent detection. This may be the optimum solution, reducing cost, time, and risk. The hybrid technology roadmap is consistent with the combined roadmaps for direct and coherent, with the addition of developing shared technology, shared scanner, a shared spacecraft, and launch.

Hybrid DWL addresses the key technology challenges of lasers, detectors, telescopes and pointing by using coherent and direct lidars for different parts of the atmospheric depth of regard, for regions where they individually perform best. Coherent technologies perform better in those parts of the atmosphere where the direct detection lidar is weakest and vice versa. For either direct or coherent lidar alone to meet GTWS requirements, laser power, optics, and pointing are driven by conditions in the most challenging parts of the altitude range. Hybrid DWL promises to reduce technology demands on both. While use of two lidars instead of one adds some complexity, it also changes and lowers the most serious technology challenges. The hope is that this will reduce mission risk, cost, and time to launch date.

3.1 Notional Hybrid DWL Concept

Hybrid DWL was first proposed in 1995 as Wind Observing Satellite/Hybrid (WOS/H). It capitalizes on the strengths of both direct and coherent technologies. It uses coherent DWL for probing the lower troposphere with highly accurate wind velocity measurement at and below clouds and in regions of enhanced aerosols throughout the troposphere. In and through partly cloudy scenes, the coherent DWL is most likely to make a useful observation since the backscatter tends to increase towards the earth’s surface and a single shot may be sufficient to make an accurate observation. Direct DWL (molecular reflection) is used for broad coverage of the mid/upper cloud free troposphere and lower stratosphere with modest accuracy. For the hybrid DWL, lower cost is expected from reducing the investment in “very large” individual DWLs (direct or coherent detection alone); sharing a launch; sharing a platform; sharing pointing control, data collection, mission management, science team, and other resources.

 3.2 Early IPO Feasibility Study

The NPOESS IPO funded a hybrid DWL feasibility study (Emmitt, 2000). Initially, reference individual coherent and direct detection systems were developed in 1999-2001 for use in trade studies. A common target data product was defined and each detection system was scaled to obtain the same data product. For each individual technology, this yielded very large systems. For comparisons, a hybrid reference system was designed to yield the same or better data products.

3.3 Current Hybrid DWL Concepts

Recently, the reference (or baseline) DWL concepts have been revised from earlier versions to take advantage of (1) the GTWS community data requirements resulting from a collaboration of many individuals and distributed as official draft requirements and (2) findings of the GSFC ISAL and IMDC reference designs. The results of the hybrid feasibility study are summarized here in a series of tables.

Table 3.3-1 is a summary of general mission parameters common to both the stand-alone DWLs as well as the hybrid missions. These common parameters are used in simulating the expected performance of the various DWLs being evaluated. Details of these data product evaluations are provided in Section 3.4.

 Table 3.3-2 contains parameters for the IPO Baseline (based upon ISAL Coherent Reference Design) and the hybrid coherent detection subsystem. Many system parameters are the same, but important differences in pulse energy, energy per LOS data product, average laser power, telescope area, and instrument mass all are significant and address the key technology concerns found in the coherent reference design. The average laser power, a major concern in the baseline, is reduced by a factor of 16. Telescope area is a major factor in instrument weight, scanning, and pointing, and is reduced by a factor of 2.5.

Table 3.3-1. General mission parameters for IPO Hybrid feasibility study  

Table 3.3-2. IPO Coherent Detection Subsystem Parameters

Table 3.3-3 contains the system parameters for the IPO Baseline and Hybrid Direct Detection Subsystem alternatives compared to the baseline from the GSFC Direct Detection Reference Design. The important differences are found in greatly reduced pulse energy, pulse repetition frequency (prf), telescope area and mass, and total average power for the instrument. All of these improvements address key technology concerns in the Direct DWL Reference Design. Two alternative hybrid configurations are considered for the direct detection hybrid sub-system, IPO1 and IPO2. IPO2 used a lower prf and 4 azimuths per LOS Data Product instead of 8 (e.g., 2 cross track bi-perspective observation lines instead of 4, which equates to some reduction in horizontal resolution across track relative to the stated requirements). IPO1 represents a system that meets the GTWS requirements with no negotiation. It must be noted that the system efficiencies used for the direct detection trade study were those that have been demonstrated in field lidars times a factor of two. For direct detection, in particular, improvements in system photon efficiencies of at least a factor of two are expected using techniques such as photon recycling.

Table 3.3-3.  IPO Baseline and Hybrid Direct Detection Subsystem Parameters

Table 3.3-4 summarizes critical instrument parameters for the combined IPO Hybrid DWL. These instrument alternatives provide significant relief in the key technology areas relative to either direct or coherent lidar alone. The total average power of the instrument includes power for lidar, thermal control, data system, and scanner motors. Power is not included in these figures for shared subsystems such as power distribution unit or data system. The inclusion of the IPO2 option is in recognition of the difficulty of downsizing the direct detection concepts from the performance demanded in the mid-upper cloud free troposphere and lower stratosphere.

 Table 3.3-4. Parameters for combined IPO Hybrid DWL

* DD is Direct Detection, CD is Coherent Detection

3.4 Simulated Data Products for IPO Study Concepts

As part of the GTWS effort, an existing Doppler Lidar Simulation Model (DLSM) was updated to enable improved simulations of the performance of various DWL concepts, including the hybrid DWL. In particular, the model was made more current with inputs from both the direct and coherent detection engineering communities. We do not describe the DLSM here in any detail but provide examples of a summary output called a data product performance profile.

 Figure 3.4-1 Performance profile for IPO coherent detection baseline

Figure 3.4-1 is a performance (or data) profile for the IPO coherent detection baseline system described in Column 2 of Table 3.3-2. In this case, and in all cases shown here, the T213 Nature Run provided by the European Center for Medium Range Weather Forecasts (ECMWF) for OSSEs is used for simulating DWL performance. The T213 Nature Run, along with Digital Elevation Models (DEMs), provides values on a three dimensional grid for winds, clouds, water vapor, and topography. The backscatter (molecular and aerosol) used is based upon the GTWS reference atmospheres. For aerosol backscatter, two modes of vertical distribution are recognized. One is the background mode (cleanest mode) and the other the enhanced mode (convectively pumped aerosols into the troposphere).  Based upon the GLOBE (Global Backscatter Experiment) missions, a log-normal variability is added to the mean profile values.  This variability is assumed correlated over regions on the order of 200-400 km.

In Figure 3.4-1, the background mode of aerosols is used for the design atmosphere for a stand-alone coherent DWL. The gray areas represent no data, due primarily to cloud obscuration and the very cleanest tail on the background mode distribution. The hatched areas denote useful returns from the tops of dense clouds and within thin cirrus clouds. The error shown here is the LOS measurement error projected on to the horizontal plane (HLOS). The color of the error is directly related to the SNR and modeled atmospheric variance as used in an algorithm (provided in part by Rod Frehlich, CIRES, University of Colorado ). One statement this figure makes is that a coherent 2 micron DWL with 96 optical watts (8.0 Joules/pulse and 12 pulses/second) output and a .75 meter aperture could meet the GTWS requirements better than 95% of the time (clouds permitting) during the background aerosol mode. However, those instrument design points are driven by the need to meet the science data requirements in the upper troposphere where the aerosol concentrations can be several orders of magnitude lower than those in the lower troposphere and clouds.

Figure 3.4-2 Performance profile for IPO direct detection (double edge) baseline, molecular backscatter only

Figure 3.4-2 is the performance profile for the IPO direct detection baseline system (molecular backscatter only). A double-edge detector is used in this case. Adding an aerosol detector that would meet data requirements in the lower troposphere would increase the power demands. Note that 125 watts at .355 microns would require 300-400 watts at the fundamental of 1.06 microns. In this case, the direct detection system is being sized to get as much of the power as is “reasonable” for the technology. Alternative direct detection approaches remain to be studied.

Figure 3.4-3 is the performance profile for a hybrid system and the background mode of aerosols.  In this case we have a 6 watt (optical) / .5 meter aerosol coherent subsystem combined with a 36 watt (at .1.06 micron) / 1.0 meter molecular direct detection subsystem. It is clear that the hybrid not only meets the threshold requirements but exceeds them in accuracy where the coherent system has the sensitivity.

Figure 3.4-3 Performance profile for hybrid system using background aerosol backscatter

Figure 3.4-4 is for the same instrument configuration as that in Figure 3.4-3 except the enhanced backscatter mode is used. In this case, the objective data accuracies are met with the coherent DWL over nearly 70% of the troposphere below 12km. The direct detection fills in most of the remaining volume with threshold accuracies. Together, the two systems meet the science data requirements (with margin) while reducing the power, weight and size (all cost determinants) from that of either stand-alone concept.

Figure 3.4-4 Performance profile for hybrid system using enhanced aerosol backscatter