Final Version Frank Stocklin Ron Vento Bob Summers May Data Systems Micro-Arcsecond Imaging Mission, Pathfinder (MAXIM-PF)

Final Version Data Systems Page 2 LAI-Maxim-PF May Goddard Space Flight Center Data Systems Topics  Ops Concept  Driving Requirements and Assumptions  Selected Configuration and Rationale  Signal Margin Summary  Component Power/Mass/Cost Summary  Risk Assessment  LASER option  Backup

Final Version Data Systems Page 3 LAI-Maxim-PF May Goddard Space Flight Center OPS CONCEPT  HUB to Free Flyers(FF)  UHF  Coherent for ranging/ 60 Kbps duplex data transfer  CDMA  simultaneous receive of 6 FF’s  Time share transmits to 6 FF’s  may also be able to simultaneous transmit to FF’s if necessary-needs some NRE  LASER reflector FF to HUB to determine relative position  HUB to Detector  S-Band  34 kbps/5.5 Kbps using HGA’s w/omni backup  Simultaneous receive/transmit with HUB to FF  LASER reflector to determine relative position  Detector to Ground  X Band to DSN  5 Mbps/5 Kbps  15 minute dump/day

Final Version Data Systems Page 4 LAI-Maxim-PF May Goddard Space Flight Center  Launch Date: August 2015  Mission Life: 4 years required/5 year goal  Nominal Orbit: L2 Location  Stellar pointing  One HUB S/C & 6 identical Free Flyers located in a spherical arc forming a radius of m  50 Kbps to/from  One Detector S/C located at 20 KKM from HUB  34/5.5 Kbps to/from  Distance from HUB to FF’s must be determined  RF ranging will be course & LASER will be fine  Distance from HUB to Detector must be determined  RF ranging will be course & LASER will be fine  Formation flying  Maintained by continuous RF & LASER Data Systems Driving Requirements & Assumptions

Final Version Data Systems Page 5 LAI-Maxim-PF May Goddard Space Flight Center  No FF inter-communications  Data Latency: None  Telemetry BER =10 -5  Selective redundancy appropriate Data Systems Driving Requirements & Assumptions

Final Version Data Systems Page 6 LAI-Maxim-PF May Goddard Space Flight Center Selected Configuration & Rationale Free Flyers  UHF selected because of ease of antenna design to minimize nulls  Transponder design from current transceiver design*  CDMA used to enable simultaneous communication with 6 FF’s  Ranging enabled by use of PN code  FF’s will compute range to HUB  60 Kbps duplex link between HUB & FF’s  Baseline approach is to time share transmissions from HUB to FF’s  Possible to design for simultaneous transmissions-needs some NRE  Laser  Used for range and position of the HUB to FF’s * Prototype will fly on STS this summer

Final Version Data Systems Page 7 LAI-Maxim-PF May Goddard Space Flight Center Selected Configuration & Rationale HUB  UHF/S-Band Transponders (2)  S-Band  2 omnis  Fixed HGA (0.3 M)  2 HPAs (10 watts)  Transmit/receive 34 kbps/5.5 kbps to/from detector (operational mode)  Transmit/receive 50 bps with detector (coarse ranging and emergency)  UHF  2 omnis (or patches)  Transmit 60 kbps to each of 6 FFs (time shared - effective rate received at each FF is 10 kbps)

Final Version Data Systems Page 8 LAI-Maxim-PF May Goddard Space Flight Center Selected Configuration & Rationale Detector  S-Band  2 transponders  2 omnis  Fixed HGA (0.3 M)  2 HPA (10 watts)  Transmit/receive 5.5kbps/34 kbps to/from HUB (operational mode)  Transmit/receive 50 bps with HUB (coarse ranging and emergency)  X-Band  2 Transponders  2 omnis  2 gimbaled HGAs (0.5 M)  Transmit/Receive 50 Kbps/5 kbps with DSN 34 M (using S/C HGA)  Transmit/Receive 50 bps/5 kbps with DSN 34 M (using S/C omni)  Ranging available

Final Version Data Systems Page 9 LAI-Maxim-PF May Goddard Space Flight Center Data Systems Selected Configuration & Rationale MOC DSN 34 M Science & Hskpg Command HUB 34M Free Flyers(6) DETECTOR LASER FF’s to HUB RF LASER HUB to DETECTOR X Band

Final Version Data Systems Page 10 LAI-Maxim-PF May Goddard Space Flight Center Selected Configuration and Rational Functional Free Flyer Block Diagram C&DH LASER CMD/TLM Multi Channel UHF transponder Hybrid Diplexer Omnis/ patches To HUB

Final Version Data Systems Page 11 LAI-Maxim-PF May Goddard Space Flight Center Selected Configuration and Rational Functional HUB Block Diagram C&DH UHF Omnis/ patches 0.3M S- Band Reflector LASER Multi CH UHF/S Band Transponder(2) CMD/TLM Diplexer Hybrid S-Band Omnis Hybrid HUB to FF communications HUB to Detector communications 6 Channels from FF’s CDMA RF Switch Diplexer To Detector 6 LASER Reflectors HPA (2)

Final Version Data Systems Page 12 LAI-Maxim-PF May Goddard Space Flight Center Selected Configuration and Rational Functional Detector Block Diagram C&DH S Band Omnis 0.5M X- Band Reflector S Band Transponder(2) CMD/TLM Diplexer Hybrid X-Band Omnis Hybrid Detector to HUB communications Detector to Ground communications RF Switch Diplexer 1 LASER Reflector RF Switch HPA (2) X Band Transponder(2) 0.3 M HGA

Final Version Data Systems Page 13 LAI-Maxim-PF May Goddard Space Flight Center Maxim_PF Signal Margins

Final Version Data Systems Page 14 LAI-Maxim-PF May Goddard Space Flight Center Power/Mass/Cost Summary Free Flyer

Final Version Data Systems Page 15 LAI-Maxim-PF May Goddard Space Flight Center Mass/Cost/Power Summary HUB

Final Version Data Systems Page 16 LAI-Maxim-PF May Goddard Space Flight Center Mass/Cost/Power Summary Detector *Includes gimbals, booms, deployment hardware ComponentPower (peak/Average)MassCost S/X-band Omni Antennas (2 each)4 kg$200K X-Band xpndr (2)42/25 watts8 kg$1.0 M S-Band xpndr (2)22/22 watts7 kg$1.0 M S-Band HPA (2)40/40 watts8 kg$1.0 M S-Band HGA (fixed)4 kg$2.0 M X-Band HGA (2) ( gimbaled) 16/1 watts24 kg *$6.0 M Hybrids, diplexers, switches, misc10 kg$500K Laser reflector Included in instruments Total120/88 watts65 kg$11.7 M

Final Version Data Systems Page 17 LAI-Maxim-PF May Goddard Space Flight Center Data Systems Cost Summary FreeFlyer (6) $0.8 M HUB $4.5 M Detector $11.7 M Ground station $2.4 M (4 years)** TOTAL$19.4 M* *Laser cost included in instruments ** Includes 1 hr pre/post pass time

Final Version Data Systems Page 18 LAI-Maxim-PF May Goddard Space Flight Center LASER OPTION  Laser data link between the HUB and Detector  Eliminates two 0.3 M antennas  4 kg and $2 M savings on both HUB and Detector.  RF transponders and HPAs still required for coarse ranging and emergency modes  Requires 1.9 kg,and 1.9 watts on both HUB and Detector  $1 M NRE and $0.5 M per flight unit  Net difference from RF  -2.1 kg, + 2 watts, -$0.5 M (Detector-Includes all NRE)  -2.1 kg, + 2 watts, -$1.5 M (HUB) ** Exact details are given in the backup charts

Final Version Data Systems Page 19 LAI-Maxim-PF May Goddard Space Flight Center Data Systems Risk Assessment  Some NRE to make current transceiver design to transponder  Multi channel receive for HUB is an evolving capability but is not a concern for the time frame of this mission  Simultaneous transmission of 2 independent signals (HUB to FF & Detector) is also doable but should be encouraged(funded) to make it happen  Simultaneous transmission of 6 signals (HUB to FF’s) is probably doable but needs to be funded & demonstrated  Basic design is low-medium risk

Final Version Data Systems Page 20 LAI-Maxim-PF May Goddard Space Flight Center Back-Up Charts

Final Version Data Systems Page 21 LAI-Maxim-PF May Goddard Space Flight Center UHF HUB/Freeflyer - Freeflyer/Hub 50 kbps *** DOWNLINK MARGIN CALCULATION*** GSFC C.L.A.S.S. ANALYSIS #1 DATE & TIME: 5/14/ 2 14: 9:47 PERFORMED BY: R. VENTO LINKID: MAXIM FREQUENCY: MHz RANGE: 0.5 km MODULATION: BPSK DATA RATE: kbps CODING: RATE 1/2 CODED BER: 1.00E-05 OMNIS AT 300 DEG 1 MILLIWATT PARAMETER VALUE REMARKS USER SPACECRAFT TRANSMITTER POWER - dBW WATTS 02. USER SPACECRAFT PASSIVE LOSS - dB 5.00 NOTE A 03. USER SPACECRAFT ANTENNA GAIN - dBi 0.00 NOTE A 04. USER SPACECRAFT POINTING LOSS - dB 0.00 NOTE A 05. USER SPACECRAFT EIRP - dBWi POLARIZATION LOSS - dB 0.30 NOTE A 07. FREE SPACE LOSS - dB NOTE B 08. ATMOSPHERIC LOSS - dB 0.00 NOTE A 09. RAIN ATTENUATION - dB 0.00 NOTE A 10. MULTIPATH LOSS - dB 0.00 NOTE A 11. GROUND STATION ANTENNA GAIN - dB 0.00 NOTE A 12. GROUND STATION PASSIVE LOSS - dB 5.00 NOTE A 13. GROUND STATION POINTING LOSS - dB 0.00 NOTE A 14. SYSTEM NOISE TEMPERATURE - dB-DEGREES-K NOTE A 15. GROUND STATION G/T - dB/DEGREES-K BOLTZMANN'S CONSTANT - dBW/(Hz*K) CONSTANT 17. RECEIVED CARRIER TO NOISE DENSITY - dB/Hz MODULATION LOSS - dB 0.00 NOTE A 19. DATA RATE - dB-bps NOTE A 20. DIFFERENTIAL ENCODING/DECODING LOSS - dB 0.00 NOTE A 21. USER CONSTRAINT LOSS - dB 0.00 NOTE A 22. RECEIVED Eb/No - dB IMPLEMENTATION LOSS - dB 3.00 NOTE A 24. REQUIRED Eb/No - dB 4.25 NOTE B 25. REQUIRED PERFORMANCE MARGIN - dB 0.00 NOTE A 26. MARGIN - dB * MAXI04 * Minus 7.8 dB when supporting 6 Freeflyers simultaneously

Final Version Data Systems Page 22 LAI-Maxim-PF May Goddard Space Flight Center X-band Downlink Detector to 34M BWG 5 Mbps HGA *** DOWNLINK MARGIN CALCULATION*** GSFC C.L.A.S.S. ANALYSIS #1 DATE & TIME: 5/14/ 2 14:36:44 PERFORMED BY: R. VENTO LINKID: 11 FREQUENCY: MHz RANGE: km MODULATION: BPSK DATA RATE: kbps CODING: TURBO BER: 1.00E-05 S/C 0.5 METER ANTENNA 99% AVAILABILITY PARAMETER VALUE REMARKS USER SPACECRAFT TRANSMITTER POWER - dBW WATTS 02. USER SPACECRAFT PASSIVE LOSS - dB 3.00 NOTE A 03. USER SPACECRAFT ANTENNA GAIN - dBi NOTE A 04. USER SPACECRAFT POINTING LOSS - dB 0.50 NOTE A 05. USER SPACECRAFT EIRP - dBWi POLARIZATION LOSS - dB 0.50 NOTE A 07. FREE SPACE LOSS - dB NOTE B 08. ATMOSPHERIC LOSS - dB 0.50 NOTE A 09. RAIN ATTENUATION - dB 1.00 NOTE A 10. MULTIPATH LOSS - dB 0.00 NOTE A 11. GROUND STATION ANTENNA GAIN - dB NOTE A 12. GROUND STATION PASSIVE LOSS - dB 0.00 NOTE A 13. GROUND STATION POINTING LOSS - dB 0.00 NOTE A 14. SYSTEM NOISE TEMPERATURE - dB-DEGREES-K NOTE A 15. GROUND STATION G/T - dB/DEGREES-K BOLTZMANN'S CONSTANT - dBW/(Hz*K) CONSTANT 17. RECEIVED CARRIER TO NOISE DENSITY - dB/Hz MODULATION LOSS - dB 0.00 NOTE A 19. DATA RATE - dB-bps NOTE A 20. DIFFERENTIAL ENCODING/DECODING LOSS - dB 0.00 NOTE A 21. USER CONSTRAINT LOSS - dB 0.00 NOTE A 22. RECEIVED Eb/No - dB IMPLEMENTATION LOSS - dB 3.00 NOTE A 24. REQUIRED Eb/No - dB 1.00 NOTE A 25. REQUIRED PERFORMANCE MARGIN - dB 0.00 NOTE A 26. MARGIN - dB 0.75 MAXI06 NOTE A: PARAMETER VALUE FROM USER PROJECT - SUBJECT TO CHANGE NOTE B: FROM CLASS ANALYSIS IF COMPUTED

Final Version Data Systems Page 23 LAI-Maxim-PF May Goddard Space Flight Center S-band Detector/Hub - Hub/Detector 5.5 Kbps - 34 Kbps HGAs *** DOWNLINK MARGIN CALCULATION*** GSFC C.L.A.S.S. ANALYSIS #1 DATE & TIME: 5/14/ 2 12:17:48 PERFORMED BY: R. VENTO LINKID: MAXIM FREQUENCY: MHz RANGE: km MODULATION: BPSK DATA RATE: kbps CODING:TURBO BER: 1.00E-05 S/C ANTENNAS ARE 0.3 METERS AT 300 DEG TURBO CODES PARAMETER VALUE REMARKS USER SPACECRAFT TRANSMITTER POWER - dBW WATTS 02. USER SPACECRAFT PASSIVE LOSS - dB USER SPACECRAFT ANTENNA GAIN - dBi USER SPACECRAFT POINTING LOSS - dB USER SPACECRAFT EIRP - dBWi POLARIZATION LOSS - dB FREE SPACE LOSS - dB ATMOSPHERIC LOSS - dB RAIN ATTENUATION - dB MULTIPATH LOSS - dB GROUND STATION ANTENNA GAIN - dBi M, EFF: 55.0% 12. GROUND STATION PASSIVE LOSS - dB GROUND STATION POINTING LOSS - dB SYSTEM NOISE TEMPERATURE - dB-DEGREES-K GROUND STATION G/T - dB/DEGREES-K BOLTZMANN'S CONSTANT - dBW/(Hz*K) CONSTANT 17. RECEIVED CARRIER TO NOISE DENSITY - dB/Hz MODULATION LOSS - dB DATA RATE - dB-bps DIFFERENTIAL ENCODING/DECODING LOSS - dB USER CONSTRAINT LOSS - dB RECEIVED Eb/No - dB IMPLEMENTATION LOSS - dB REQUIRED Eb/No - dB REQUIRED PERFORMANCE MARGIN - dB MARGIN - dB 1.72 MAXI02

Final Version Data Systems Page 24 LAI-Maxim-PF May Goddard Space Flight Center - X-Band DSN 34 M BWG to Detector 5 Kbps HGA TABLE 0.5 S/C ANTENNA UPLINK DATE & TIME: 05/14/02 15: 1:25 MAXIM PF FREQUENCY MHZ GROUND ANTENNA BWG POWER K WATTS PARAMETERS UNITS VALUES ESTIMATED TOLERANCES (MAX RNG: (MIN RNG: DB KM KM 10.0 EL) 90.0 EL) FAV ADV EFFECTIVE RADIATED POWER DBM FREE SPACE DISPERSION LOSS DB ATMOSPHERIC LOSS DB POLARIZATION LOSS DB SPACECRAFT ANTENNA GAIN DBI SPACECRAFT PASSIVE LOSS DB MAXIMUM TOTAL RECEIVED POWER DBM SPACECRAFT ANTENNA NULL DEPTH DB MINIMUM TOTAL RECEIVED POWER DBM SYSTEM NOISE DENSITY DBM/HZ IF NOISE BANDWIDTH( KHZ) DB-HZ IF NOISE POWER DBM IF SNR (MIN) DB CARRIER CHANNEL CARRIER/TOTAL POWER DB RECEIVED CARRIER POWER DBM CARRIER LOOP NOISE BW( 800. HZ) DB-HZ NOISE POWER DBM CARRIER/NOISE DB REQUIRED CARRIER/NOISE DB AVAILABLE CARRIER MARGIN DB REQUIRED PERFORMANCE MARGIN DB NET MARGIN DB COMMAND CHANNEL (PCM/PSK/PM) COMMAND/TOTAL POWER(MI=1.10 RAD) DB RECEIVED COMMAND POWER DBM PREDETECTION (PSK) NOISE BW( KHZ) DB-HZ PREDETECTION (PSK) NOISE POWER DB PREDETECTION (PSK) SNR DB COMMAND DATA RATE ( 5.000KBPS) DB-BPS AVAILABLE ENERGY PER BIT/NOISE DENSITY DB DECODER DEGRADATION DB REQUIRED ENERGY PER BIT/NOISE DENSITY (BER=E-5) DB AVAILABLE COMMAND MARGIN DB REQUIRED PERFORMANCE MARGIN DB NET MARGIN DB

Final Version Data Systems Page 25 LAI-Maxim-PF May Goddard Space Flight Center *** DOWNLINK MARGIN CALCULATION*** GSFC C.L.A.S.S. ANALYSIS #1 DATE & TIME: 5/15/ 2 10:39:22 PERFORMED BY: R. VENTO LINKID: 11 FREQUENCY: MHz RANGE: km MODULATION: BPSK DATA RATE: kbps CODING: TURBO BER: 1.00E-05 S/C 0.5 METER ANTENNA 99% AVAILABILITY PARAMETER VALUE REMARKS USER SPACECRAFT TRANSMITTER POWER - dBW WATTS 02. USER SPACECRAFT PASSIVE LOSS - dB 3.00 NOTE A 03. USER SPACECRAFT ANTENNA GAIN - dBi 0.00 NOTE A 04. USER SPACECRAFT POINTING LOSS - dB 0.00 NOTE A 05. USER SPACECRAFT EIRP - dBWi POLARIZATION LOSS - dB 0.50 NOTE A 07. FREE SPACE LOSS - dB NOTE B 08. ATMOSPHERIC LOSS - dB 0.50 NOTE A 09. RAIN ATTENUATION - dB 1.00 NOTE A 10. MULTIPATH LOSS - dB 0.00 NOTE A 11. GROUND STATION ANTENNA GAIN - dB NOTE A 12. GROUND STATION PASSIVE LOSS - dB 0.00 NOTE A 13. GROUND STATION POINTING LOSS - dB 0.00 NOTE A 14. SYSTEM NOISE TEMPERATURE - dB-DEGREES-K NOTE A 15. GROUND STATION G/T - dB/DEGREES-K BOLTZMANN'S CONSTANT - dBW/(Hz*K) CONSTANT 17. RECEIVED CARRIER TO NOISE DENSITY - dB/Hz MODULATION LOSS - dB 0.00 NOTE A 19. DATA RATE - dB-bps NOTE A 20. DIFFERENTIAL ENCODING/DECODING LOSS - dB 0.00 NOTE A 21. USER CONSTRAINT LOSS - dB 0.00 NOTE A 22. RECEIVED Eb/No - dB IMPLEMENTATION LOSS - dB 3.00 NOTE A 24. REQUIRED Eb/No - dB 1.00 NOTE A 25. REQUIRED PERFORMANCE MARGIN - dB 0.00 NOTE A 26. MARGIN - dB 0.90 MAXI12 NOTE A: PARAMETER VALUE FROM USER PROJECT - SUBJECT TO CHANGE NOTE B: FROM CLASS ANALYSIS IF COMPUTED X-band Downlink Detector to 34M BWG 5 Kbps OMNI Mode

Final Version Data Systems Page 26 LAI-Maxim-PF May Goddard Space Flight Center S-band Detector/Hub - Hub/Detector 50 bits OMNIs *** DOWNLINK MARGIN CALCULATION*** GSFC C.L.A.S.S. ANALYSIS #1 DATE & TIME: 5/15/ 2 10:29:54 PERFORMED BY: R. VENTO LINKID: MAXIM PF FREQUENCY: MHz RANGE: km MODULATION: BPSK DATA RATE: kbps CODING: TURBO BER: 1.00E-05 PARAMETER VALUE REMARKS USER SPACECRAFT TRANSMITTER POWER - dBW WATTS 02. USER SPACECRAFT PASSIVE LOSS - dB 5.00 NOTE A 03. USER SPACECRAFT ANTENNA GAIN - dBi 0.00 NOTE A 04. USER SPACECRAFT POINTING LOSS - dB 0.00 NOTE A 05. USER SPACECRAFT EIRP - dBWi POLARIZATION LOSS - dB 0.30 NOTE A 07. FREE SPACE LOSS - dB NOTE B 08. ATMOSPHERIC LOSS - dB 0.00 NOTE A 09. RAIN ATTENUATION - dB 0.00 NOTE A 10. MULTIPATH LOSS - dB 0.00 NOTE A 11. GROUND STATION ANTENNA GAIN - dB 0.00 NOTE A 12. GROUND STATION PASSIVE LOSS - dB 2.00 NOTE A 13. GROUND STATION POINTING LOSS - dB 0.00 NOTE A 14. SYSTEM NOISE TEMPERATURE - dB-DEGREES-K NOTE A 15. GROUND STATION G/T - dB/DEGREES-K BOLTZMANN'S CONSTANT - dBW/(Hz*K) CONSTANT 17. RECEIVED CARRIER TO NOISE DENSITY - dB/Hz MODULATION LOSS - dB 0.00 NOTE A 19. DATA RATE - dB-bps NOTE A 20. DIFFERENTIAL ENCODING/DECODING LOSS - dB 0.00 NOTE A 21. USER CONSTRAINT LOSS - dB 0.00 NOTE A 22. RECEIVED Eb/No - dB IMPLEMENTATION LOSS - dB 3.00 NOTE A 24. REQUIRED Eb/No - dB 1.00 NOTE A 25. REQUIRED PERFORMANCE MARGIN - dB 0.00 NOTE A 26. MARGIN - dB 0.03 MAXI10 NOTE A: PARAMETER VALUE FROM USER PROJECT - SUBJECT TO CHANGE NOTE B: FROM CLASS ANALYSIS IF COMPUTED

Final Version Data Systems Page 27 LAI-Maxim-PF May Goddard Space Flight Center HUB - DETECTOR LASER COMMUNICATIONS Concept: A low power laser communications link can exploit the precision alignment of the spacecraft to provide low rate data links with simple, low power, lightweight equipment.  Assumptions:  Operates only when both spacecraft are in operational attitude.  A low bandwidth RF link is used to control Hub and Detector spacecraft positioning into the operational attitude.  Approach:  Use low power “laser pointer” technology for the transmitters.  Use a different frequency from the beacon to avoid interference.  Simplify layout by using separate optics from beacon and star tracker.  Use simple modulation without forward error correction.  Requirements:  Operate at a range of 20,000 kilometers between spacecraft.  Communicate Forward data continuously from the Detector to the Hub at 5500 bps.  Communicate Return data from continuously from the Hub to the Detector at 34,000 bps.

Final Version Data Systems Page 28 LAI-Maxim-PF May Goddard Space Flight Center Laser communications links  Transmitters:  671 nm, 10 & 50 mW GaAs diode lasers.  500 microradian beam divergence (simple lens).  Higher power version of 5 mW “laser pointer.”  Receivers:  10 cm (4”) spacecraft telescope.  3.5 dB Implementation Loss; 2.0 dB Pointing Loss.  Limited motion gimbal.

Final Version Data Systems Page 29 LAI-Maxim-PF May Goddard Space Flight Center WEIGHT AND POWER ESTIMATE Using parametric model and engineering estimates: Note: 10 mW transmitter will require less power (< 100 mW).

Final Version Data Systems Page 30 LAI-Maxim-PF May Goddard Space Flight Center COST & SCHEDULE ESTIMATE COST  Based on COTS laser technology; still requires a receiver.  Assumes that fundamental R&D is completed; designs exist.  NRE to adapt existing designs to specific spacecraft: ~$1M.  Recurring engineering for flight units: $0.2M to $0.5M. SCHEDULE ESTIMATE (FLIGHT EQUIPMENT)  NRE: ~ 6-12 months  Recurring Build & Test: ~ 6-12 months

Final Version Data Systems Page 31 LAI-Maxim-PF May Goddard Space Flight Center SUMMARY  Simple, low power “laser pointer” transmitter still requires a receiver with a telescope.  Eliminating gimbals requires precise co-alignment, though  Gimbals, if needed, can be very limited motion.  Fixed geometry of spacecraft eliminates need for “look ahead.” Therefore  Sharing the telescope for both transmit and receive could be better:  Increased transmitter gain allows smaller telescope, or  Can use even lower power lasers, and  Would allow much higher data rates.  Little impact on mass and power.  Scalability very good (either alternative) through:  Changing transmitter power (first choice up to about 100 mW).  Use coding and/or better modulation (second choice).  Increasing receiver telescope aperture (last choice).

Final Version Data Systems Page 32 LAI-Maxim-PF May Goddard Space Flight Center SHARED TELESCOPE ALTERNATIVE EXAMPLE  Reduced shared aperture to 2.5 cm.  Decreased laser power to 2 mW and 10 mW. SCALABILITY