1. Modulating Retro-reflectors for Space Tracking, Acquisition and Ranging using Multiple Quantum Well Technology G. Charmaine Gilbreath, N. Glenn Creamer, W. S. Rabinovich, Timothy J. Meehan, Michael J. Vilcheck, John Vasquez, Rita Mahon, Peter Goetz, and Eun Oh U. S. Naval Research Laboratory, Washington DC 20375 http:mrr.nrl.navy.mil STAR II

2. Objectives: Provide a compact, lightweight, low power method for inter-satellite acquisition, communications and navigation. Approach: Use frequency-tagged corner-cube Multiple Quantum Well (MQW) retro- modulators to provide line-of-sight relative position and orientation between two spacecraft and demonstrate technique in the NRL Dynamic Motion Simulator Facility. Impact: Method is potentially applicable for long (multiple km) to short (10’s of meters) inter-platform ranges and can significantly reduce parasitic payload requirements of the onboard communications, acquisition, and navigation subsystems. STAR II

3. MQW Modulator with Corner-Cube Retro MQW Retro On/Off Signal Driver Gimballed Laser Source Photo- Detector CW Beam Modulated Return Signal Pursuer Spacecraft Target Spacecraft MRR CONCEPT

4. Relative Position Vector: Relative Attitude (Pointing) Matrix:  = yaw  = pitch Az, El from laser gimbal angles Inter-Spacecraft Geometry CONCEPT Az Target S/C Pursuer S/C

5. MQW Retromodulator Picture of Video Payload 2 1 3 5 4 1. Interrogation beam; 2. Modulated beam; 3. Electronic driver; 4. Transmissive MQW modulator; 5. Solid retroreflector- DEVICE : 2 1 3 5 4 1. Interrogation beam; 2. Modulated beam; 3. Electronic driver; 4. Transmissive MQW Multiple Quantum Well “shutter” requires mW; is radiation-hard; and cm-class supports 10+ Mbps with standard corner cube.

6. 7 1 6 8 4 35 2 30 cm Central umbrella array 20 o cant 7 MRR (8) +/-30 o FOV MRR (8) +/-30 o FOV 1 6 8 4 35 2 30 cm Central umbrella array 20 o cant MRR Array For Target Pose Laser beam divergence illuminates entire array Laser gimbal motion equalizes signal returns of MRRs 6-8 to provide relative position vector Signal returns of MRRs 1-5 provide two-axis relative target attitude (pitch, yaw) Combined with pulse TOF measurement, sensor provides relative navigation of ~1 cm in position and ~0.3 deg in attitude

7. Determination of Target Attitude S i = signal return from ith MRR  i = laser angle to ith MRR boresight  = fixed cant angle of MRRs 2-5 Relative Yaw & Pitch Estimation Fundamental Signals If S 3 > S 5 : else : If S 2 > S 4 : else : Diverged laser interrogator beam Center of beam 7 1 6 8 4 35 2

8. Eight element retromodulator array is shown. Each unit is driven with a different code for device discrimination. STAR II: MQW Retromodulator Array

9. Pursuer Control Block Diagram Pursuer Dynamics Target Motion Gyro Accel Divert Control Logic Attitude Control Logic Relative Position Estimator Relative Attitude Estimator TOF Laser Tracker MRR Array 1/s x, y, z Az, El R Torques Forces Attitude Rate Acceleration 1 8 7 6 5 4 3 2 Divert Control Logic: Attitude Control Logic:

10. Upper Target Platform Lower Servicer Platform Control Room National testbed for testing and verification of autonomous rendezvous and capture technologies. This dual platform facility offers 6 degrees of freedom per platform. Space environmental conditions can be programmed into the simulations. Facility workspace dimensions: 30 m x 13 m x 5 m NRL Robotics Laboratory

11. STAR II Target Pursuer In play

12. 1 2 3 4 5 6 7 8 Center of Laser Beam Simulation Actual from GUI STAR II

13. Each MQW modulator is driven by a unique code which is detected, demodulated and translated into a level which is sent to the tracking and acquisition algorithm. 1 8 7 6 5 4 3 2 Code # MRR Code Sequences

14. STAR II Sampled Data detected by APD Sampled Data out of All Signals Matched Filter

15. Signal level Data point Acquisition and centering Chaser aligns with Target Dynamic Target motion STAR II

16. Time Signal Levels equalized

17. STAR II Test Demonstrations T P P P P Commanded Alignment Offset T P T P T P Commanded Slew Angle Alignment to a Stationary TargetTracking a Moving Target

18. STAR II Test Results Alignment to a Stationary Target AZ EL 1  = 1 cm 1  = 0.3 deg

19. STAR II Test Results Tracking a Moving Target AZ EL 1  = 1 cm 1  = 0.3 deg

20. STAR II Test Results Attitude Estimation Error from a Static Test 1  = 0.015 deg

21. Results: MQW retromodulators can provide simultaneous spacecraft-to-spacecraft optical communication and navigation. The navigation solution potentially provides about 1 cm in positioning and 0.3 degrees in orientation. TRL 5 for device and concept Payload: Weight: Retromodulators:.35 oz (10 g) per mounted device so 2.8 oz for 8 Electronics (FPGA + drivers): 8 oz Mechanical Structure: 6 oz (will vary with function) Size: ~2.5 cm x 1.5 cm for a.5 cm mounted device Power: At 1 MHz data rate: 80 mW each device; 740 mW total (incl. Driver) Summary