Pointing Determination for a Coherent Wind Lidar Mission

1
Pointing Determination for a Coherent Wind Lidar
Mission

• J. Marcos Sirota, Christopher Field
• Sigma Space Corp.
• Michael Kavaya
• NASA LaRC
• January 2006

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Outline

• Background Information
• Wind Lidar Mission Concept
• Pointing Determination in GLAS/ICESat
• ICESat attitude data
• Proposed system for coherent wind lidar
• Analysis of pointing control and determination
  requirements and solutions.
• Summary

3
Orbiting Doppler Wind Lidar at 400 km
Second shot t100 ms, 768 m, 103 mrad
First Aft Shot t 46 s
Return light t3.1 ms, 24 m, 3.5 mrad
Nadir tilt rate 1mrad/ms
7.7 km/s
90 fore/aft angle in horiz. plane
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FORE
AFT
400 km
467 km
6.1 m (86)
180 ns (27 m) FWHM (76)
2 lines LOS wind profiles 1 line horiz wind
profiles
45
7.2 km/s
32.1
120 shots 12 s 86 km
233 km
165 km
1/10 s 722 m
165 km
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Pointing Geometry - Side View
Earth-Orbiting Doppler Wind Lidar
qT arcsin(RE ZL) sinqL/(RE ZT)
VL
RT
ZL 400 km qL 30 deg. ZT 4 km (example) qT
32.1 deg. VL 7676 m/s RT 462 km R E 6371
10.7 km
qT
qL
ZL
qS
ZT
qT - qL
SPACE
R E
ATM
ATM
Lidar Beam Direction
EARTH
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2 Horizontal Wind Lines, 400 km, 30 deg
nadirAzimuths 45, 135 deg
100 km Target Sample Volume
Horiz. Resol. 350 km
S/C Ground Track
78 km
330 km
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Two Horizontal Wind Profile Lines
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4 Horiz Wind Lines
90 deg.
4
115 deg.
2
45 deg
6
1
155 deg.
8
- 25 deg.
- 155 deg.
5
7
- 120 deg.
- 60 deg.
3
7
LaRC/Kavaya-
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Four Horizontal Wind Profile Lines
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Space-Based Coherent Doppler Wind Lidar System
Schematic
Transmitter Laser
Injection
PZT
Locking
Driver
Loop
Detector
Mirror
Power Amplifier
Pulsed Laser Oscillator
BS
Transmitter Beam
MM Optics
Master Osc. Laser
Isolator
Telescope, Scanner, and Pointing Determination
System
10mm
BS
Frequency
LO Laser
Det.
Locking
Nadir Angle Compensator
Nadir Angle Compensator

FB Control
Signal Beam
2mm
Control
Local Oscillator Laser
Signal
BS
Polarizing BS
Lens
10mm
Alignment Mirror
90/10 BS
Detector Pre-Amplifier
Control Signal
Scan Controller
Scan Controller
Laser Controller
Command

Data
Data
A/D
IF Receiver
Management System
Recorder
Data Transmitter
INS/GPS
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LOS Wind Measurement Sequence
Orbiting Doppler Wind Lidar at 400 km

• Aim scanner to next desired direction pre-shot
  pointing control, 2 deg.
• Tune LO laser to remove predicted gross motion
  and earth rotation pre-shot pointing
  knowledge, 0.2 deg.
• Measure LO laser frequency error and tune
  electronic mixer to compensate
• Fire laser pulse
• Keep receiver axis well aligned for 3 ms
  Stability A 6.6 mrad/3 ms
• Optically mix, electronically mix, and digitize
  backscattered signal
• Divide data into time/range/altitude bins NALT
  22
• Combine shots aimed in same direction, if desired
  NACC 60 Stability B 0.2 deg./12 sec.
• Estimate frequency
• Remove residual spacecraft and earth rotation
  caused frequencies Final pointing knowledge, 60
  mrad
• Assign time, location, altitude, and direction to
  each LOS velocity
• Repeat above sequence for other desired
  cross-track distances NCT 4
• Repeat above sequence for aft perspectives
  collocated with fore perspectives NPER 2

On Orbit
On Orbit Or Ground Processing
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Pointing Knowledge, Control, and Stability
Requirements

• Pre-shot control
• to ensure that Doppler shift is within LO laser
  tuning range
• a) 2 deg. from -ZLV, scanner fore or aft, if
  4000 MHz LO tuning range
• b) 6.7 deg. from -ZLV , scanner fore or aft, if
  4500 MHz LO tuning range
• Pre-shot knowledge
• to allow LO to be tuned for sufficiently small
  heterodyne beat frequency
• 0.2 to 0.5 deg.
• affects receiver bandwidth and data quality
• Stability, t 0 to 3 ms, for each shot
• 7.1 mrad, 1 s, for budgeted 3 dB 1 s SNR loss
• Stability, while staring for shot accumulation
  (for 0.3 m/s LOS error)
• nadir 0.2 deg., azimuth 0.3 deg. (beam azimuth
  at 45 deg. to wind)
• up to 30 sec.
• Final post-mission knowledge (for 0.3 m/s LOS
  error)
• 60 mrad 0.0034 deg. 12 arcsec (scanner
  azimuth angle at 45 deg. to fore or aft)
• Will require use of lidar surface return data for
  this Shuttle Hitchhiker mission

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Background InformationICESat

• The Geoscience Laser Altimeter System on ICESat
  carried the first laser pointing determination
  system in a Lidar space mission.
• It determines the laser pointing direction w.r.t.
  the stars with an accuracy of 7.5 microradians
  per axis for every laser shot (40 Hz).
• The system includes star and laser imagers, a
  high precision gyroscope, and cross-reference
  optical sources.

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Geoscience Laser Altimeter System Measurements

• Surface Altimetry
• Range to ice, land, water, clouds
• Uses time of flight of 1064 nm laser pulse
• Digitizes transmitted received 1064-nm pulse
  waveforms
• Laser-beam pointing from star-trackers, laser
  camera gyro
• 3 cm single shot range resolution
• 7 urad angular resolution

• Atmospheric Lidar
• Laser back-scatter profiles from clouds
  aerosols
• Uses 1064 nm 532 nm pulses
• 75 m vertical resolution
• Analog photon counting detection
• Simultaneous, co-located measurements with
  altimeter

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SRS Functional Block Diagram
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ICESat Bus

• The ICESat bus was selected based on its pointing
  accuracy and stability. The Ball Global Imaging
  System 2000 is an imaging-based platform where
  the attitude control and determination system
  were designed for accurate pointing control and
  stability during image acquisition of high
  resolution Earth scenes from orbit.

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Predicted Bus Stability
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Spacecraft motion with Solar Panel Articulation
(Case 1)
Star Trajectory in LRS
20 urad
1 sec
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Spacecraft motion with Solar Panel Articulation
(Case 2)
Star Trajectory in LRS
200 urad
1 sec
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Normal Flight, No Solar Array Articulation
s 1.2 urad
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Stellar Reference System in ICESat
ICESat II Concept
SRS
Old system of equal function
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Space-Based Coherent Doppler Wind Lidar System
Schematic
Transmitter Laser
Injection
PZT
Locking
Driver
Loop
Detector
Mirror
Power Amplifier
Pulsed Laser Oscillator
BS
Transmitter Beam
MM Optics
Master Osc. Laser
Isolator
Telescope, Scanner, and Pointing Determination
System
10mm
BS
Frequency
LO Laser
Det.
Locking
Nadir Angle Compensator
Nadir Angle Compensator

FB Control
Signal Beam
2mm
Control
Local Oscillator Laser
Signal
BS
Polarizing BS
Lens
10mm
Alignment Mirror
90/10 BS
Detector Pre-Amplifier
Control Signal
Scan Controller
Scan Controller
Laser Controller
Command

Data
Data
A/D
IF Receiver
Management System
Recorder
Data Transmitter
INS/GPS
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Pointing determination system concept forWind
Lidar Mission
Lateral Transfer Retroreflector
Laser Camera /Star Tracker
Transceiver Telescope
Counter-rotating Ring
Frame Motor with Absolute Encoder
Silicon Wedge
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Star Tracker Errors Per Axis

• Single frame errors for HD-1003 (example)
• - 2 arcsec (1s) pitch and yaw (ST coordinates)
• - 40 arcsec (1s) roll
• If at 45 degree to Nadir it translates to
• 30 arcsec per axis per frame
• Filtered solution (Star tracker plus Inertial
  Reference Unit) shall yield about 3 arcsec per
  axis (1s).

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Pointing knowledge analysis
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Stability analysis
Requirement

• Stability of ICESat-class spacecraft is adequate
  for round-trip per shot requirement
• Orbital motion compensation with aft-optics
  mirror is necessary for multi-shot integration

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Requirement Compliance Analysis

• 1. How will we have pre-shot pointing control? To
  ensure that the gross Doppler (spacecraft and
  earth motions) is within the tuning range of the
  tunable LO laser. (/- 2-7 degrees)
• a. Spacecraft slew rates for ICESat-class bus
  have demonstrated this level of pointing control.
• b. Fine pointing can be achieved with the
  aft-optics beam steering mechanism.

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Requirement Compliance Analysis

• 2. How will we have pre-shot pointing knowledge?
  To allow the setting of the tunable LO frequency
  so that the return signal is within the bandwidth
  of the detector and electronics. (/- 0.2
  degrees, or 3.5 mrad)
• Pointing knowledge will be obtained from the
  Laser Sensor, Attitude Determination System, and
  Scanner Encoder to within 20 urad per axis.

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Requirement Compliance Analysis

• 3. How will we hold the line of sight of the
  receiver stable while waiting for the laser light
  to return from the earth? To avoid more SNR
  lossthan is budgeted. (8 microradians 1 sigma
  over 7 ms).
• The stability of the spacecraft is sufficient to
  comply with this requirement. If we wish to
  compensate for the 3.1 urad from orbital motion
  then a fixed-angle wedge or tilt mirror can be
  introduced on the path between fire and return.

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Requirement Compliance Analysis

• 4. How will we hold the line of sight stable
  while we are accumulating several shots to make
  one wind measurement? To avoid smearing the angle
  at which we probe the atmosphere which will add
  error to the wind estimate. (/- 0.2 degrees over
  12 seconds).
• The Nadir Compensation Mechanism will provide
  compensation form shot to shot, holding the line
  of sight stable until the end of integration.

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Requirement Compliance Analysis

• 5. How will we achieve the final pointing
  knowledge for each shot? To allow minimum error
  in reporting the measured wind's direction to the
  user. (/- 60 microradians assuming earth surface
  is not available to use for reference).
• The Laser Reference Sensor plus the Scanner
  Encoder shall provide knowledge for every shot
  fired w.r.t the stars to better than 20 urad per
  axis.

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Summary

• Pointing requirements for a space based coherent
  wind lidar mission can be met with space proven
  technology, and some current miniaturization
  efforts.
• Same design could be used to adapt the system to
  various platforms, i.e dedicated craft or
  multi-instrument (NPOESS).