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Pointing Determination for a Coherent Wind Lidar Mission
J. Marcos Sirota, Christopher Field Sigma Space Corp.
Michael KavayaNASA LaRC
January 2006
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
7.7 km/s
400 km 467 km
233 km
165 km
165 km
30
32.145
180 ns (27 m) FWHM (76%)
6.1 m (86%)
1/10 s = 722 m
Return light: t+3.1 ms, 24 m, 3.5 rad
First Aft Shott + 46 s
120 shots = 12 s = 86 km
90° fore/aft anglein horiz. plane
FOREAFT
Second shot: t+100 ms, 768 m, 103 rad
2 lines LOS wind profiles1 line “horiz” wind profiles
Orbiting Doppler Wind Lidar at 400 km
7.2 km/s
Nadir tilt rate~ 1rad/ms
Pointing Geometry - Side View
VL
Lidar BeamDirection
L T
S
R E
ZT
ZL
EARTHATM
SPACE
ATM
RTZL = 400 kmL = 30 deg.ZT = 4 km (example)T = 32.1 deg.VL = 7676 m/sRT = 462 kmR E = 6371 10.7 km
T = arcsin[(RE + ZL) sinL/(RE + ZT)]
T - L
Earth-Orbiting Doppler Wind Lidar
2 “Horizontal” Wind Lines, 400 km, 30 deg nadirAzimuths = ±45, ± 135 deg
78 km
330 km
Horiz.Resol.350 km S/C Ground
Track
100 kmTargetSampleVolume
Two “Horizontal” Wind Profile “Lines”
7LaRC/Kavaya-
400 km orbit, 45 degree nadir angle25, 65, -120, 115, -155, 155, -60, -25 azimuth
pattern. 1.06 sec. to scan
-500
-400
-300
-200
-100
0
100
200
300
400
500
0 1000 2000 3000 4000
Along-Track (km)
Cross-Track (km)
25 deg.
65 deg.115 deg.
155 deg.
- 155 deg.
- 120 deg.
- 25 deg.
- 60 deg.
1
2
3
4
5
6
7
8
90 deg.
45 deg
4 HorizWindLines
Four “Horizontal” Wind Profile “Lines”
Space-Based Coherent Doppler Wind Lidar System Schematic
Detector
Injection LockingLoop
PZTDriver
Pulsed Laser Oscillator
Recorder
BS
Mirror
MMOptics
Scan Controller
IF Receiver
Detector &Pre-Amplifier
Local OscillatorLaser
A/DData
Command& DataManagementSystem
Laser Controller
BS
BS
FrequencyLocking
FB Control
Transmitter Laser
LO Laser
Data Transmitter
Lens
Polarizing BS
Master Osc.Laser
Isolator
INS/GPS
SignalBeam
TransmitterBeam
AlignmentMirror
Det.
ControlSignal
Nadir AngleCompensator
PowerAmplifier
90/10 BS
2mm
/4
ControlSignal
ScanController
10mm
10mm
Telescope, Scanner, and PointingDetermination System
Nadir AngleCompensator
1. Aim scanner to next desired direction [pre-shot pointing control, 2 deg.]2. Tune LO laser to remove predicted gross motion and earth rotation [pre-shot pointing
knowledge, 0.2 deg.]3. Measure LO laser frequency error and tune electronic mixer to compensate4. Fire laser pulse5. Keep receiver axis well aligned for 3 ms [Stability A: 6.6 rad/3 ms]6. Optically mix, electronically mix, and digitize backscattered signal
7. Divide data into time/range/altitude bins [NALT = 22]
8. Combine shots aimed in same direction, if desired [NACC = 60] [Stability B: 0.2 deg./12 sec.]9. Estimate frequency10. Remove residual spacecraft and earth rotation caused frequencies [Final pointing knowledge,
60 rad]11. Assign time, location, altitude, and direction to each LOS velocity
12. Repeat above sequence for other desired cross-track distances [NCT = 4]
13. Repeat above sequence for aft perspectives collocated with fore perspectives [NPER = 2]
On Orbit
LOS Wind Measurement Sequence
On Orbit Or
Ground Processing
Orbiting Doppler Wind Lidar at 400 km
• 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 rad, 1 , for budgeted 3 dB 1 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 rad = 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
Pointing Knowledge, Control, and Stability Requirements
Background Information:ICESat
• 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.
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
Geoscience Laser Altimeter System Measurements
SRS Functional Block Diagram
•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.
ICESat Bus
Predicted Bus Stability
1207.5
1208
1208.5
1209
1209.5
1210
0 200 400 600 800 1000 1200 1400 1600
LRS x axis (arcsec)
LRS Y axis (arcsec)
20 urad
Spacecraft motion with Solar Panel Articulation (Case 1)
~ 1 sec
Star Trajectory in LRS
140
145
150
155
160
165
170
0 200 400 600 800 1000 1200
LRS X axis (arcsec)
LRS Y axis (arcsec)
~ 200 urad
~ 1 sec
Spacecraft motion with Solar Panel Articulation (Case 2)
Star Trajectory in LRS
1200
1210
1220
1230
1240
1250
1260
0 200 400 600 800 1000 1200 1400 1600
LRS X axis (arcsec)
LRS
Yax
is (a
rcse
c)Normal Flight, No Solar Array Articulation
= 1.2 urad
SRS
ICESat II Concept
Old system of equal function
Stellar Reference System in ICESat
Space-Based Coherent Doppler Wind Lidar System Schematic
Detector
Injection LockingLoop
PZTDriver
Pulsed Laser Oscillator
Recorder
BS
Mirror
MMOptics
Scan Controller
IF Receiver
Detector &Pre-Amplifier
Local OscillatorLaser
A/DData
Command& DataManagementSystem
Laser Controller
BS
BS
FrequencyLocking
FB Control
Transmitter Laser
LO Laser
Data Transmitter
Lens
Polarizing BS
Master Osc.Laser
Isolator
INS/GPS
SignalBeam
TransmitterBeam
AlignmentMirror
Det.
ControlSignal
Nadir AngleCompensator
PowerAmplifier
90/10 BS
2mm
/4
ControlSignal
ScanController
10mm
10mm
Telescope, Scanner, and PointingDetermination System
Nadir AngleCompensator
Pointing determination system concept forWind Lidar Mission
Transceiver Telescope
Silicon Wedge
Frame Motor with Absolute Encoder
Laser Camera /Star Tracker
Lateral Transfer Retroreflector
Counter-rotating Ring
Star Tracker Errors Per Axis
• Single frame errors for HD-1003 (example)
- 2 arcsec (1) pitch and yaw (ST coordinates)
- 40 arcsec (1) 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 (1).
Pointing knowledge analysis
Telescope/backplane magnification 5LRS laser pointing determination error 1 uradLRS to Star Tracker stability 2 uradAttitude solution error 15 uradTransfer optics (LTR) 5 uradScanner Encoder accuracy 10 urad
Pointing determination error per axis 18.19 urad 34.68 urad
Stability analysis
Spacecraft jitter (max) 0.2 urad/msecSpacecraft motion amplitude 200 urad
Shot return time 3.1 msecMisspointing due to jittter for return pulse 0.62 urad 3.5 uradMisspointing due to jittter for 30 sec integration 200 urad
Orbital rate 1.03 urad/msecMisspointing due to orbital rate for 30 sec 30.9 mrad
1.77 degrees 0.2 degrees
• 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
Requirement
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.
• 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.
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.
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.
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.
Requirement Compliance Analysis
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).