1. Pointing Determination for a Coherent Wind Lidar Mission
J. Marcos Sirota, Christopher Field
Sigma Space Corp.
Michael Kavaya
NASA LaRC
January 2006

2. Outline Background Information Proposed system for coherent wind lidar
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: t+100 ms, 768 m, 103 mrad
Return light: t+3.1 ms, 24 m, 3.5 mrad
First Aft Shot
t + 46 s
Nadir tilt rate
~ 1mrad/ms
7.7 km/s
90° fore/aft angle
in horiz. plane
30°
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°
233 km
120 shots = 12 s = 86 km
165 km
1/10 s = 722 m
165 km

4. Pointing Geometry - Side View
Earth-Orbiting Doppler Wind Lidar
Pointing Geometry - Side View
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 qL qT ZL qS ZT
qT - qL
SPACE
R E
ATM
ATM
EARTH
Lidar Beam
Direction

5. 2 “Horizontal” Wind Lines, 400 km, 30 deg nadir Azimuths = ±45, ± 135 deg
Target
Sample
Volume
Horiz.
Resol.
350 km
S/C Ground
Track
78 km
330 km

6. Two “Horizontal” Wind Profile “Lines”

7. 4 Horiz Wind Lines 90 deg. 4 2 45 deg 6 1 8 5 7 3 115 deg. 155 deg.
LaRC/Kavaya-

8. Four “Horizontal” Wind Profile “Lines”

9. Space-Based Coherent Doppler Wind Lidar System Schematic
Transmitter Laser
PZT
Injection
Locking
Driver
Loop
Detector
Mirror
Power
Amplifier
Pulsed Laser Oscillator BS
Master Osc.
Laser MM
Optics
Isolator
Transmitter
Beam
Telescope, Scanner, and Pointing
Determination System
10mm
Frequency BS
LO Laser
Locking
Det.
Nadir Angle
Compensator
FB Control
Nadir Angle
Compensator
Signal
Beam
2mm
Local Oscillator
Laser
Control
Signal
l/4 BS
Polarizing BS
Lens
10mm
Alignment
Mirror
Detector &
Pre-Amplifier
90/10 BS
Control
Signal
Scan
Controller
Laser Controller
Scan Controller
Command
&
Data
Data
IF Receiver
A/D
Management
System
Recorder
Data Transmitter
INS/GPS

10. LOS Wind Measurement Sequence
Orbiting Doppler Wind Lidar at 400 km
LOS Wind Measurement Sequence
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

11. 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 = ± 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

12. 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.

13. 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

14. SRS Functional Block Diagram

16. 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.

17. Predicted Bus Stability

18. Spacecraft motion with Solar Panel Articulation
(Case 1)
Star Trajectory in LRS
20 urad
~ 1 sec

19. Spacecraft motion with Solar Panel Articulation
(Case 2)
Star Trajectory in LRS
~ 200 urad
~ 1 sec

20. Normal Flight, No Solar Array Articulation
s = 1.2 urad

21. Stellar Reference System in ICESat
ICESat II Concept
What did you accomplish during the year?
What is the significance of these results?
SRS
Old system of equal function

22. Space-Based Coherent Doppler Wind Lidar System Schematic
Transmitter Laser
PZT
Injection
Locking
Driver
Loop
Detector
Mirror
Power
Amplifier
Pulsed Laser Oscillator BS
Master Osc.
Laser MM
Optics
Isolator
Transmitter
Beam
Telescope, Scanner, and Pointing
Determination System
10mm
Frequency BS
LO Laser
Locking
Det.
Nadir Angle
Compensator
FB Control
Nadir Angle
Compensator
Signal
Beam
2mm
Local Oscillator
Laser
Control
Signal
l/4 BS
Polarizing BS
Lens
10mm
Alignment
Mirror
Detector &
Pre-Amplifier
90/10 BS
Control
Signal
Scan
Controller
Laser Controller
Scan Controller
Command
&
Data
Data
IF Receiver
A/D
Management
System
Recorder
Data Transmitter
INS/GPS

23. Pointing determination system concept for Wind Lidar Mission
Lateral Transfer Retroreflector
Laser Camera /Star Tracker
Transceiver Telescope
Counter-rotating Ring
Frame Motor with Absolute Encoder
Silicon Wedge

24. 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).

25. Pointing knowledge analysis

26. 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

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. 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).