performance characteristics and design trades for an iss hybrid doppler wind lidar
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Performance characteristics and design trades for an ISS Hybrid
Doppler Wind LidarG. D. Emmitt and S. Wood
Simpson Weather AssociatesCharlottesville, Va
ISS Winds Mission Science WorkshopMiami, 2011
Outline• Instrument design issues and data products• The DLSM and OSSEs• The global coverage• The sampling pattern• The key atmospheric variable for evaluating the expected
performance of a hybrid DWL– Clouds
• Cloud climatology• Cirrus in the tropics
– Aerosols• Background and enhanced
– Wind variability• Simulated performance profiles• Summary
Attribute Goal
Vertical depth of regard (km) 25Vertical resolution Tropopause to 25 km Top of BL to tropopause Surface to top of BL
42
1 (.25)Horizontal resolution (km) 350 (35)# of tracks 1# of perspectives within target volume
2
Horizontal component error (m/s) Above BL Within BL (includes sampling RMSE)
< 32 (1)
Numbers in () are desired
Data Goals for Wind Lidar on ISS
Instrument Design Issues
• Vertical coverage in cloudy regions (much of the globe)– Hybrid approach• Direct detection molecular for volumes with low
aerosol content (mid/upper troposphere and lower stratosphere)• Coherent detection for volumes with clouds and
sufficient aerosols (dust layers and lower troposphere)– Number of telescopes– Dwell times
ISS Wind Lidar Concept
• Hybrid Doppler Wind Lidar:– Coherent detection (2 um) for aerosol and cloud returns– Direct detection (.355 um) for molecular returns
• Two fixed telescopes provide forward and aft perspectives
• Variable dwell times allow a high spatial resolution (~ 28km) for the coherent system while allowing the direct system longer integration (~ 84 km)
Instrument parameters used in performance simulations for WISSCRS flown on the ISS
*At the fundamental 1.06um for direct detection. The utility wavelength is .355um** Includes: Pre LRE optics (.48), IF (.7) and LRE throughput (.74)
Parameter WISSCRS/ISSCoherent(2um)
WISSCRS/ISSDirect (.355um)
Orbit altitude (km) 350 350Orbit inclination (deg) 52 52Nadir angle (deg) 35 35Energy/pulse (J)* .25 .8PRF (Hz) 10 100Aperture (m) .5 .5EAP .49 15.7# perspectives per cycle 2 2Optical eff** .51 .25Heterodyne mixing eff .36Detector eff .8 .5Integration time (sec) 12(4, .1) 12Misalignment loss .42Beam split . .48 Filter throughput .17Edge sensitivity .007 Recycling factor/margin 1.6Conversion eff .45Wallplug eff * .014 .092Duty cycle (%) 100 100Power required (Watts) 178 869Vertical layer depth (m) 1000/500/250 2000/1000Beta(50) backscatter (/m/sr) 2.8 x 10-9
The DLSM
• Used since 1988 to simulate DWL performance for OSSEs using inputs from Nature Runs.– Clouds– Subgrid scale wind variability– Aerosol distributions
• Simulate both direct and coherent detection• Stress realistic characterization of random and
systematic errors.
Doppler Lidar Simulation Model
DLSM* simulations for use in OSSEs
• Emphasis on the tropics.• Scaling GWOS down to 350km• Uses GWOS instrument performance parameters listed
in a prior slide• Clouds in T511 Nature Run modified to conform to
ISCCP coverage statistics• Has been understating very thin to subvisual cirrus
effects which would be very positive for coherent detection coverage and slightly negative for direct detection accuracies.
* Doppler Lidar Simulation Model
ISS Wind Lidar Coverage for three orbits
The Sampling Pattern
• Illuminated volume– Coherent (250m long cylinder, ~ 2m diameter
footprint)• Spectral domain processing provides information on
turbulence intensity, BL depth, precipitation fall velocities– Direct (2000m long, ~ 50m diameter)
• Accuracy a function of intensity of return, presence of clouds/aerosols could be derived.
• Sampling pattern, a ground perspective• Pattern over a hurricane
4 second dwell pattern for both direct and coherent (fore and aft perspectives)
4 second dwell pattern (fore and aft sampling)
Single shot coherent samples (~ 700meter intervals)
Clouds and Aerosols
532 nm Total Attenuated Backscatter
Seze, Pelon, Flamant, Vaughn, Trepte and Winker
Sub-visual Cirrus
• Until this year, simulations done using the DLSM in support of OSSEs have not included sub-visual cirrus derived from the nature run
• Recent published studies of very thin and subvisual cirrus have documented a climatology (5 years, in one case) of these upper tropospheric clouds.
• We have modified the DLSM to generate sub-visual cirrus from the ECMWF T511 Nature Run and are currently assessing the realism of the derivation.
Simulated thin and sub-visual cirrus
GWOS/ISS
Single shot threshold sensitivity
Natural Variability of 2 mm Backscatter
Backscatter (m-1sr -1)10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3
Land
Mid-Upper Troposphere
Volcanic
Subvisual Cirrus
Maritime PBL
Continental PBL
Ocean
Background
Enhanced
Lower Troposphere
Surface GWOS/ISS
CALIPSO (derived from 532um)
Clouds
Simulated WISSCR’s performance using the DLSM with the T511
ECMWF Nature Run
Aerosol/cloud subsystem of the Wind Lidar on ISS20 – 10 N
Cirrus returnsOpaque clouds
Aerosols
Aerosol/cloud subsystem of the Wind Lidar on ISS10 -0 N
Aerosol/cloud subsystem of the Wind Lidar on ISS0 – 10 S
Aerosol/cloud subsystem of the Wind Lidar on ISS10 -20 S
Molecular subsystem of the Wind Lidar on ISS10 – 0 N
Molecular subsystem of the Wind Lidar on ISS 0 - 10 S
Molecular subsystem of the Wind Lidar on ISS10 -20 S
Summary• Platform attitude drift does not appear to be a major factor in DWL
data quality using pointing knowledge over pointing control.• Clouds will be a major factor in DWL coverage
– Direct detection (molecular) is negatively effected by the high clouds in the tropics.
– However, coherent provides winds within most of the high level clouds and within the lower troposphere below.
• The hybrid technology approach provides the best vertical coverage for science investigations in the tropics– A direct detection subsystem is critical to tropospheric/stratospheric
exchange investigations– The coherent subsystem is critical for accurate, high spatial resolution
measurements in cloudy scenes and in the lower troposphere.
Japanese JEM-EF
–Accommodates 9 experiment payloads–Nominal 500kg payloads–3kW 120VDC per payload–5 Mbits/second download data rates for
single payload– .8 x 1.0 x 1.8 meters –Access to cooling loop for thermal
management
JEM-EF
Key accommodation issues related to instrument performance
• High frequency vibrations (> 1 Hz)• Slow attitude changes (+- 10 degrees)• Power to PL (average and peak)• Thermal management• Orbital debris• Data rates (uplink and downlink)
Accommodations Summary
• The ISS offers an attractive orbit for focusing the Wind Lidar resources on the lower latitudes where ageostrophy is most dominant.
• Assuming that an ISS mission would be regarded as a research science with a focus on the tropics, instrument lifetime, duty cycle and data downloads would be negotiable.
• At this time, no accommodation “show stoppers” have been identified. Just completed a NASA evaluation within the IDL and MDL at GSFC
Molecular subsystem of the Wind Lidar on ISS52N – 52S
Aerosol/cloud subsystem of the Wind Lidar on ISS52N – 52S
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