DTIC ADA277805: Atmospheric Effects on Airborne Lasers for Tactical Missile Defense: Clouds and Turbulence

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AD-A277 805
IDA DOCUMENT D- 1082
ATMOSPHERIC EFFECTS ON AIRBORNE LASERS
FOR TACTICAL MISSILE DEFENSE:
CLOUDS AND TURBULENCE
DTIC
SELEC'T
pMARR311994
SF1
F
0 E. Bauer
R. R. Beland
(USAF-PUGP/OPA)
January 1993
*mM Prepawdf or
S0
Ballistic Missile Defense Organization
* Appmved fr public release; dlalrbullen unlimited.
94 3 31 002
0
INSTITUTE FOR DEFENSE ANALYSES
1801 N. Beauregard Street, Alexandria, Virginia 22311-1772
IDA Log No. HO 91-40634
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
January 1993 Final--June-December 1991
4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
Atmospheric Effects on Airborne Lasers for Tactical Missile Defense: G - MDA 903 89 C 0003
Clouds and Turbulence
6AUTHORS) T -T-R2-597.12
Ernest Bauer, Robert R. Beland
PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) a. PERFORMING ORGANIZATION
REPORT NUMBER
Institute for Defense Analyses
1801 N. Beauregard St. IDA Document D-1082
Alexandria, VA 22311-1772
SPONSORINGIMONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORINGIMONITORING
AGENCY REPORT NUMBER
Ballistic Missile Defense Organization
The Pentagon
Washington, DC 20301-7100
11. SUPPLEMENTARY NOTES
12a. DISTRIBUTIOWAVAILABIUTY STATEMENT 12b. DISTRIBUTION CODE
Approved for public release; distribution unlimited.
13. ABSTRACT (Maximum 200 words)
In the context of Tactical Missile Defense (TMD), an Airborne (High-Energy) Laser (ABL) is investigated for its possible
utility for boost-phase kill of a tactical ballistic missile, at a range of 100-1,000 km. Here we examine the effects of
clouds and atmospheric turbulence, including possible compensation by Adaptive Optics (AO) techniques, which can
interfere with the functioning of this concept. To avoid the effect of clouds, the aircraft carrying the laser should fly as
high as possible, preferably above the tropopause. At 15 km (49 kft) there will be no problem at mid- and high latitudes,
but in the tropics conventional aircraft cannot fly high enough to avoid possible interference from towering
cumulonimbus, which have been found upon occasion as high as 20 km. Regarding the effects of atmospheric
turbulence on beam propagation, if the airplane flies high enough that the optical viewing path avoids cloud, there
would probably be no major problems with turbulence at ranges of not more than 100 km. However, for longer ranges,
there may be significant effects of phase coherence, beam wander, and beam broadening that could require
compensation by AO. There are critical uncertainties in models of atmospheric turbulence, suggesting that more data
are needed. Work at USAF/Phillips Laboratory and at MIT/Lincoln Laboratory is attacking the problems of appropriate
compensation for atmospheric turbulence by AO techniques, but the current level of effort is not adequate to support
deployment of an ABL system for TMD on a near-term basis.
14. SUBJECT TERMS 15. NUMBER OF PAGES
Atmospheric propagation, laser, clouds, turbulence 54
16. PRICE CODE
17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. UMITATION OF ABSTRACT
OF REPORT OF THIS PAGE OF ABSTRACT
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NSN 7540-20-55(0 Standard Form 298 (Rev. 2-89)
Peoibed byANSJ SW(cid:127).Z W31
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IDA DOCUMENT D-1082
ATMOSPHERIC EFFECTS ON AIRBORNE LASERS
FOR TACTICAL MISSILE DEFENSE:
CLOUDS AND TURBULENCE
* E. Bauer Accesio,.Fo"
,
AN ý c RA L'd
R .R .B el a n d
(USAF-PL/GP/OPA) I :\.
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January 1993 Dist (cid:127),,,'o-oraiI
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INSTITUTE FOR DEFENSE ANALYSES
Contract MDA 903 89 C 0003
Task T-R2-597.12
ACKNOWLEDGMENTS
We wish to thank Chief Augustine, USAF; Keith Gilbert, BDM Corp.,
Albuquerque, NM; Jan Herrmann, MIT Lincoln Laboratory; and Bill Rose, Rose
Engineering & Research, Inc., for helpful discussions. We also thank the reviewers,
Brett Ellerbroek, USAF/PL; Rex Finke, IDA/STD; Darryl Greenwood, MIT Lincoln
Laboratory; Michael Tavis, The Aerospace Corporation, and Edwin Townsley, IDA/STD,
for their careful and thorough review of this document.
This work was done as part of the POET (Phase One Engineering Team) TMDAT
(Tactical Missile Defense Advanced Technology) Study of summer 1991, supported by
SDIO.
iii
0
ABSTRACT
In the context of Tactical Missile Defense (TMD), an Airborne (High-Energy) Laser
(ABL) is being investigated for its possible utility for boost-phase kill of a tactical ballistic
missile, at a range of 100-1,000 km. The propagation of the laser beam through the
atmosphere can be stopped by clouds and atmospheric turbulence. To avoid the effect of
clouds, the aircraft carrying the laser should fly as high as possible, preferably above the
tropopause where there are very few clouds. Normal aircraft flight altitudes should be
adequate at mid- and high latitudes, but not in the tropics. Atmospheric turbulence is
highly variable, but generally falls off with increasing altitude. If the airplane flies high
enough to avoid clouds, it is probable that for ranges of 100 km or less the effects of
turbulence are not serious. However, there exist no experimental data on laser beam
propagation through long-range near-horizontal paths, and available models of atmospheric
turbulence are inadequate. Theoretical analysis summarized here suggests that for ranges in
excess of 100 km there may be significant effects that could require compensation by
Adaptive Optics, explicitly deformable transmitter mirrors. There are critical uncertainties
in models of atmospheric turbulence, suggesting that experimental flight data are needed.
Current work is attacking the problems of appropriate compensation for atmospheric
turbulence by Adaptive Optics techniques, but the level of effort is not adequate to support
deployment of an ABL system for TMD on a near-term basis.
v
CONTENTS
Acknowledgments ................................................................................... i
Abstat ................................................................................................ v
Figures ............................................................................................ ix
Tables .............................................................................................. x
Summary ........................................................................................ S-1
1. THE PROBLEM ............................................................................... 1
2. CLOUDS .................................................................................. 9
3. ATMOSPHERIC TURBULENCE ..................................................... 13
3.1 Introduction and Overview ....................................................... 13
3.2 The Parameter C,2 ........................... ................................... . . 16
3.3 Geometrical Considerations ....................................................... 20
3.4 Near-Field Vs. Far-Field Conditions ............................................ 21
3.5 Weak Fluctuations ................................................................ 21
3.6 Beam Wander ...................................................................... 22
3.7 Beam Broadening .................................................................. 23
3.8 Some Results .......................................................................... 24
3.9 Summary ............................................................................... 26
4. ADAPTIVE CORRECTION FOR TURBULENCE ................................ 29
5. OTHER CONCERNS ................................................................... 31
5.1 Thermal Blooming ................................................................ 31
5.2 Ducting ............................................................................. 33
6. SUGGESTED MEASUREMENT AND STUDY PROGRAM .................... 35
6.1 Clouds ............................................................................... 35
6.2 Optical Turbulence ................................................................ 35
7. DISCUSSION ........................................................................... 39
References ........................................................................................ 41
vii
FIGURES
S-i. The Problem Addressed .................................................................... S-1
S-2 Optical Turbulence in the Atmosphere: Structure of the Parameter Cn2 ............. S-4
1. The Problem Addressed .................................................................. 1
2. Schematic Location of Tropopause and Jet Streams (Js = Subtropical Jet,
Jp = Polar Jet) in Winter .................................................................. 3
3. Optical Turbulence in the Atmosphere: Structure of the Parameter Cn2 ................ 4
4. Models of the Jet Stream: Wind Speed and Turbulence .................................. 5
5. Adaptive Compensation and Isoplanatism ................................................ 14
6. Comparison of SLC Day and Night and HV 5/7 Models ............................ 18
7. Comparison of the SLC Night and AFGL AMOS Models, Both Displayed
Relative to the Mountaintop Surface ................................................... 19
8. Atmospheric Turbulence Strehl Ratio for Aircraft at 13 km and
Target at 20 kmi, for SLC and HV/57 Models ........................................ 25
9. Aerosol Extinction Profile in the Atmosphere ........................................ 32
10. Sample Temperature Profile in a Subtropical Atmosphere Showing
Ifigh Thermal Gradients ................................................................ 33
ix
TABLES
S- 1. Environmental Issues for Airborne Lasers ............................................... S-2
1. Some Representative Trajectory Data .................................................... 2
2. Mean Seasonal Heights of the Tropopause ................................................. 2
3. High and Total Cloudiaess at Representative Locations in the
Northern Hemisphere .................................................................... 10
4. Global Frequency of Occurrence of Cirrus Clouds ................................... 11
5. Effective Path Lengths Through Turbulence as a Function of
Elevation Angle a (d = 0. 1 Ian) ......................................................... 20
6. Variance of Log Amplitude .............................................................. 21
7. Values for the Lateral Coherence Length Po (m) ......................................... 24
8. Values for RMS Beam Radius ps (m) .................................................... 24
x
SUMMARY
The effectiveness of an optical laser weapon depends, in large part, on the
characteristics of the optical path the beam must traverse. Among the factors influencing
that path for airborne lasers intended to engage tactical ballistic missiles are clouds and
turbulence.
In the context of Tactical Missile Defense (TMD), an Airborne (High-Energy) Laser
(ABL) is examined for its possible utility for boost-phase kill of a tactical ballistic missile,
at a range of 100-1000 km. Figure S-I sketches the problem addressed, and Table S-i
outlines environmental issues. Here we examine the effects of clouds and of atmospheric
turbulence--including its possible compensation by Adaptive Optical (AO) techniques with
a deformable transmitter mirror-that can interfere with the functioning of this concepL
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II
9 1091 IM
Figure S-1. The Problem Addressed
S-1