Kavaya1

1
Thoughts on the Lidar Operating Point for the
Upcoming NexGen NPOESS Wind Mission Instrument
Design at GSFCMichael J. KavayaNASA Langley
Research Centermichael.j.kavaya_at_nasa.govWork
ing Group on Space-Based Lidar WindsMonterey,
California USA5 8 February 2008
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History and Motivation

• NASAs instrument and mission hybrid Doppler
  wind lidar (HDWL) Global Wind Observing Sounder
  (GWOS) study took place at GSFC in September and
  October 2006
• The study assumed a 400 km orbit height, a 45
  degree lidar nadir angle, and the Global
  Tropospheric Wind Sounder (GTWS) Demonstration
  wind measurement requirements (added in 2006 with
  NASA HQ and GTWS SDT approval now hopefully
  renamed the 2008 NASA/NOAA Science
  Demonstration requirements)
• The GWOS study team cleverly solved the large
  moving lidar scanner problem with four fixed
  50-cm diameter telescopes to obtain 2 lines of
  vector wind profiles
• The GWOS study lidar operating points were CDWL
  at 0.25 J pulse energy and 5 Hz DDWL at 0.8/0.36
  J and 100 Hz
• The NRC Earth Science Decadal Survey endorsed
  the HDWL mission in January 2007
• The NPOESS/IPO planned spacecraft orbit height
  is 824 km, and winds is their 1 unaccommodated
  measurement
• NPOESS/IPO/Dr. Stephen A. Mango has funded an
  instrument study at GSFC for an HDWL mission at
  824 km. The study is scheduled for 25 29 Feb.
  2008.
• The best use of 1 week of the GSFC IDC team is
  to narrow down the lidar operating points before
  the study begins

3
Geometry400 vs. 824 km45 degree nadir
angleSpherical earth
400 48.5 s/350 km 824 53.1 s/350 km
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Geometry400 vs. 824 km45 degree nadir angle
5
Connecting Lidar Parameters to Measurement
Performance
Coherent Detection Doppler Wind Lidar

• Equal Performance Parameter Linkages
• Note better vertical resolution is a smaller
  value of Vert Res
• If decrease (improve) Vert Res by a factor F,
  then increase E by a factor , or increase
  PRF by F, or increase D by
• If increase number of azimuth angles NAZ by a
  factor F, then increase PRF by F , or increase
  Vert Res by F, or increase E by , or increase
  D by
• If increase R by a factor F, then increase D by
  F, or increase E by F2, or increase PRF or Vert
  Res by F4

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Connecting Lidar Parameters to Measurement
Performance
Direct Detection Doppler Wind Lidar

• Equal Performance Parameter Linkages
• If decrease (improve) Vert Res by a factor F,
  then increase E or PRF by F, or increase D by
• If increase number of azimuth angles NAZ by a
  factor F, then increase PRF or Vert Res or E by
  F, or increase D by
• If increase R by a factor F, then increase D by
  F, or increase E or PRF or Vert Res by F2

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NWOS Mission StudyThoughts on Lidar Parameter
Strategy

• From geometry only, going from 400 km to 824 km,
  the increase in range is effectively a factor of
  2.16 (assuming 45 deg. nadir and spherical
  earth). We consider restoring the original lidar
  velocity measurement performance with either
  changes in only pulse energy, or only PRF, or
  only optical diameter
• The vertical resolution requirement for
  operational is x 2 smaller (harder) than for
  science demonstration
• The number of azimuth angles for operational is x
  2 greater than for science demonstration

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NWOS Mission StudyThoughts on Lidar Parameter
Strategy

• Combining Tables 2 and 3 to see the total effect
  of operational vs. science demonstration
  requirements, neglecting time to advance by
  horizontal resolution (10 effect)
• Combining Tables 1 and 4 to see the total effect
  of higher orbit height and operational
  requirements
• Comparing Table 1 with Table 5, it seems prudent
  to perform the NWOS Mission Study using the same
  Science Demonstration requirements as were used
  in GWOS. We assume this choice below (Table 1)

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NWOS Mission StudyThoughts on Lidar Parameter
Strategy

• Using the factors in Table 1 and the original
  GWOS mission parameters, we get the actual values
  of the parameters

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NWOS Mission StudyThoughts on Lidar Parameter
Strategy

• The lasers required input power and heat removal
  are proportional to the product of the energy and
  PRF. This was dominated by the direct lidar
  in GWOS.
• The lasers mission lifetime is inversely
  proportional to PRF (not counting PRF effect on
  energy)
• The lasers mission lifetime decreases as pulse
  energy increases (not counting E effect on PRF)
• The data rate is proportional to PRF. This was
  dominated by the coherent lidar in GWOS.
• For the coherent lidar, it is very attractive to
  stay at 50 cm and let the pulse energy and PRF
  rise. A pulse energy of 1.2 J at 2 Hz has been
  demonstrated at LaRC. A possible NWOS operating
  point for coherent would be 1.2 J at 5 Hz.
• The direct lidar, with 50 cm, must increase the
  average laser power by a factor of 4.7 somewhere
  between the extremes of 3.76 J at 100 Hz, and 0.8
  J at 470 Hz. Perhaps this eliminates 50 cm for
  the direct lidar.
• A possible NWOS operating point for direct would
  be 1000/450 mJ, 190 Hz, and 0.7 m. Coherent may
  not use the entire 0.7 m, but only the center of
  the mirror.

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NWOSScience Demonstration Requirements
EXAMPLE COHERENT OPTIONS 1.2 J 5 Hz 6 W 0.5 m
EXAMPLE DIRECT OPTIONS 3.76 J (1.69) 100
Hz 376 W 0.5 m 0.8 J (0.36) 470 Hz 376
W 0.5 m 1.0 J (0.45) 190 Hz 190 W 0.7 m 1.2
J (0.54) 158 Hz 190 W 0.7 m 1.4 J (0.63) 135
Hz 190 W 0.7 m