Innovative Solutions in Nano and Pico ‑ s atellite Communications April 19 th , 2013

1. Innovative Solutions in Nano and Pico‑satellite CommunicationsApril 19th, 2013 Danilo Roascio

2. Outline • Introduction to Nano and Pico-Satellites • Ground CommunicationSegment • Current Architecture Limitations • Improvements with Analog RF • From analog to digital… • Software DefinedRadios

3. Nano and Pico-satellites

4. Nano and Pico-satellites • Conceptbornaround 2001 • Low-costvectorsallowfor easy access to space • Interested parties include research/educational entities, industries and localgovernments • Great educational return, manyuniversitiesaround the world are developing/operating small satellites Need for cost reduction during development and production

5. Costreduction Whichare the consequencesof "cheapness"? • Commercial Electronics: • re-use of existingtechnology • proper design to tolerateradiation, vacuum, vibrations, etc. • Standardizationneeded: • today, design considered a “trade secret” • design for multiple missions, focus on the payload • protocolsinteroperability • sharedknowledge, betterresults

6. GENSO – Ground Stations

7. GENSO Overview GENSO consists of three main types of entity: • Ground Station Servers, GSS Software • Running on every GENSO ground station • Controls the existing local hardware setup • Communicates with many GENSO spacecraft • Used by radio amateurs/students • Mission Control Clients, MCC Software • Running at each GENSO mission control centre • Interfaces to existing local control software • Communicates with the local spacecraft via GSSs • Used by spacecraft controllers • Authentication Servers, ASL Hardware & Software • Monitor and control the network status • Provide authentication and network security • A set of mirrored servers, one on each continent • Controlled by GENSO Administration

8. GSS – Ground Station Server • User can be any radio amateur station • Most educational stations are ham stations • User simply configures the GSS with the details of their station hardware. Then, all processes are automatic (if desired). • The Controller module reports to the AUS and receives lists of GENSO spacecraft details • The Scheduler automatically plans and executes downlink passes for GENSO spacecraft, and also allows MCCs to book bidirectional sessions • All mission data is stored locally for the operator, and also sent automatically to the spacecraft controllers • The Hardware Abstraction Layer ensures that the GSS can work with almost any hardware • Specific drivers are required for each piece of local hardware, a large library of which will be packaged with the GSS. (Users can write more and contribute!)

9. PoliTo Ground Station ROTOR 2 ROTOR 1 TNC RELAYS RADIO

10. GSS – Software Schema Rotators Radios TNCsModems Relays

11. GUI

12. Channel security

13. Microsatellites – Current Security Approach • RF uplink/downlink channels allocated in free frequency bands • no licensing • wider choice of RF components • ground station uses amateur equipment • Downlink frequency/data format public • improves mission visibility • scientific data can be collected by third parties • Uplink frequency/command format kept secret • security through obscurity • prevents protocol standardization • exposes missions to security risks

14. Microsatellites – Current Security Weakness (I) • Attack to this system requires: • replay/discovery of the command format  used to require proximity to uplink GS now may prevent full GS network exploitation

15. Microsatellites – Current Security Weakness (II) • Attack to this system requires: • discovery of the uplink frequency easy with SDRs/modern hardware f t

16. Security goals • Confidentiality • keeping the message secret to unauthorized observersless important in downlink • Dataintegrity • ensuring information has not been altered either intentionally or not • Authentication • verifying that the message is actually coming from its intended sender and is not, e.g., beingre‑played less important in downlink

17. Possible Security Scheme • Encryption of the uplink / private downlink channel • security becomes independent of frequency/command format protocol/software re-use becomes feasible • Nonce mechanism • prevents replayof recorded commands • Different choices for authentication: • identity guaranteed by encryption may be subject to tampering • secure authentication slower

18. Security Scheme – Encryption • Symmetric-key cipher • faster encryption/decryption • public-key schemes heavy/not needed • synchronous stream cipher • transmission errorsdo not propagateto blocks (as in self-synch stream ciphers, ECB block ciphers) or to whole parts of message (as in CBC block ciphers) • frame synchronization provided by underlying layer 1 • missing/added bits unlikely with PLL transceivers • periodic key update may be desirable

19. Security Scheme – Encryption Standard • Salsa20/121 (or heavier Salsa20/20 variant) • recommended scheme fromeSTREAMproject (Profile 1 – Software)2 • 256-bit key,~3 times faster than AES-256(Salsa20/12)3 • only 8 rounds broken as of today4, 2251 time complexity • reduced instruction set, no lookup tables,scales wellon processors with less than 32-bit parallelism • optimized AES-128 achieves >280kbps @ 8 MIPS onMSP430 architecture5; extrapolating, AES-256 should achieve ~200kbps6, Salsa20/12 should achieve >500kbps

20. Conclusions • A secure small satellite communication system is shown to be feasible with existing standardson low-power embedded processors. • Previous work shows that the required bit-rate is achievable even at reduced clock speeds, with a subsequent power consumption reduction. • Unfortunately, usually, the first rule always is “don’t fix it, if it ain’t broke”.To move away from an insecure and limiting system, the push may finally come from emerging GSNs. • However, public key algorithms are slow on microcontrollers and may require dedicated programmable logic.

21. RF Architectures

22. Attitude Determination Systems • Common attitude determination systems applied to nano-satellites: • solar cells (low accuracy if used for power generation) • sun sensors (good accuracy but not available in eclipse) • star trackers (big optics needed) • magnetic field sensors (low accuracy,subject to disturbances from satellite subsystems) • GPS sensors (complex hardware)

23. RF tracking • Techniques for object tracking with RF signals are well known in radar systems: • simultaneous lobing monopulse radars: • the reflected pulse is received by two antennas and is combined with an interference network at RF level to obtain ΔAz and ΔEl • the interference network is narrowband and increases complexity in the RF stage • phase comparison monopulse radars: • the reflected pulse is received by two antenna and an actual measurement of the phase difference is used to obtain ΔAz and ΔEl

24. Phase Measurement • This approach allows to use the existing antenna and COTS components • Phase shift (R ≫ d): • Δφ = ±180° (multi-cycle delays not considered): • d = λ/2 → Δθ = ±90° • d ≈ 7 cm → Δθ≈±63°

25. RF splitting • Signals from antennas are split with hybrid power dividers • Phasing section applies proper delay to antenna feeds to obtain the desired beam • Coupling factor < -3 dB to avoid waste of power in transmission • Measurement system works in parallel with and independently of existing transceivers

26. Phase Measuring Receiver • Common superheterodyne structure (good interfering signals resilience, easily tunable.) • How to measure phase shifts? → PLL loop locked on first signal (reference), phase detectors (PFD) provide phase evaluation directly at IF level. • With proper PLL loop filter, the system keeps working also with modulated data. • COTS components (-90 dBm sensitivity, 3 dB NF, incl. losses.)

27. Conclusions • The RF tracking methods used historically in radars can be applied with COTS components to the nanosatellite architecture and to small satellites in general. • The proposed system remains totally independent from existing transceivers and will also work with modulated signals. • The RF carrier phase tracking system should be able to fill the gap between coarse solar cell systems and accurate sun/star trackers. Without additional structural requirements other than the ones already present for antennas. • The analog section is quite complex. Is there a way to make things more flexible?

28. A software approach…

29. Common Small Satellite Antennas • At lower frequencies • Dipoles • Almost omnidirectional • Easy to deploy • Helixes • Flexible design choices • Shorter than a dipole but harder to deploy • At higher frequencies • Quarter wavelength patches/PIFA • Almost omnidirectional (3 dB) • Compact • Half wavelength patches • Slightly more directional • Bigger

30. Design choices • The usual choice is “omnidirectional” • communication is possible if attitude is unknown • simpler design • compact • Higher gain may provide better bitrates, but: • may require ground station tracking • more complex both in design and operation • Range compensation is possible, but requires a complex feeding network Traditional antennas design is a trade-off

31. Phased arrays • Signals delayed andscaled to obtain thedesired beam: • static feeding networksquickly become complex and expensive • electronically steered arrays add the need for custom RF designs and speciality components

32. Digital Beamforming • Several elementsare sampledindependently • Signal weighting isdone in the digital domain • Digital, programmable beamsteering becomes reality • Smart antenna concept can be implemented on-board • adaptive algorithms iteratively optimize the beam to changing electromagnetic conditions

33. Why digital transceivers in Cubesats? (I) • Traditional PLL transceiver • Simplest, lowest power, cheapest • Complete system-on-chip • Proprietary architecture • Protocol constraints • Application specific capabilities • Vendor lock-in • No flexibility (FSK demod. shown)

34. Why digital transceivers in Cubesats? (II) • Software defined transceiver • Complex, higher power consumption, expensive • IF/Baseband independent of front-end • Band of transceiver changed by swapping front-end • Re-use of IF/Baseband code on future hardware • Total control of modulation choices • Bitrate, error detection/correction, encryption, low level protocols • Complex modulation schemes: spread spectrum, CDMA • Pre-distortion, equalization (Receiver) (Transmitter)

35. Why digital transceivers in Cubesats? (III) • Software defined transceiver (cont.) • Dynamic control of modulation choices • Dynamic bitrate based on link conditions (explicit bitrate control or window flood control) • FSK telemetry transmission for most of the orbit, higher performance modulation when above main ground station • Smart antenna platform • Array of low-power receivers able to digitally steer antenna beam • Smart antenna dynamic/iterative algorithms • In-orbit reconfigurable radio • Research test bed like NASA's CoNNeCT/SCaN project • Orbit-to-orbit, orbit-to-ground, groud-to-ground repeater

36. SDR Hardware

37. From analog to digital • Digital after IF2: easy, fIF2 ≈ 0, done in commercial devices since the ’90s, higher noise, complex circuit • Digital before IF1: hard, high power consumption (conversion and processing) • Digital after IF1: trade-off, power consumption becoming acceptable, lower data rate

38. How to optimize the hardware? • Challenge: • Low Power Consumption (RX) • Ground mobile designs favor linearity and sensitivity • In-orbit requirements are different: • Uplink: low data rate, high reliability, low power consumption • Downlink: high data rate, high transmission power • In-orbit design goals are different: • No interferers No need for linearity (up and downlink) • Powerful ground equipment No need for sensitivity (uplink)

39. Lowering Analog Power Consumption • Reduction in mixer linearity • Lower IP3 requirement • Lower LO drive • Lower LO amplification • More efficient mixers • Higher IP3-LO power differential • Lower sensitivity • Lower amplification at RF and IF frequencies • Stable RF levels • No need to operate VGAs at high attenuation and low efficiency point

40. Lowering Digital Power Consumption • LowPower/High Integration requirements • Limited number of commercial devices • Alternatives: • Discrete components lowlevel of integration, hard to design with, hard to assemble, high degree of customization,optimizationpossible • Integratedcircuits high level of integration, easier design process, easierassemblyprocess, applicationspecific • Selectionexample, A/D/A interfaces: • AnalogDevices AD9862/AD9963 • Texas Instruments AFE7222/AFE7225

41. Isfurtheroptimizationpossible? • Hardware choices are limited • Software choices are (almost)unlimited! • Goals: • Low distortion high precision in computation • High integration as little logic as possible • Low operation power as little logic as possible and as static as possible P = PS + α C V2 f

42. Precision in computation TX • Simulinkevaluation of how SNR isaffected by: • Sample bit-widthchanges • Sample ratechanges RX

43. Lowering Digital power consumption • Processing partially on low power A-to-D/D-to-A front end (AFE) • Proof-of-concept DBPSK transmitter on FPGA: • Multirate implementation, multistage polyphase decomposition of FIR filters • Lowering clock speeds reduces dynamic power consumption with negligible effect on computational accuracy • Compact implementation lowers gate count

44. Multirate decomposition • Matlabevaluation of howfilteraccuracyisaffected by: • Bit-widths of filterinputs/outputs/internalproducts • Coefficientquantization

45. Precision in computation (III) • Matlabevaluation of how SNR isaffected by: • Bit-widthsof filterinputs/outputs/internalproducts • Filterstructure

46. Reducing board area • High level of integration • all the transceiver functions implemented by 5 ICs (PA/LNA excluded) • signal processing and supervision soft core on FPGA • Board occupation evaluation • Top components red, bottom components blue • Realistic support circuitry provision • Optimized placement 9 cm × 9 cm

47. Currentsystem (I)

48. Currentsystem (II) • Solid zones conservative, dashedzones to be verified. • Bottom (blue) analog, top (red) digital. • Supply close to PA. • PA and LNA as far aspossible. • 6 layers minimum. • Estimated power consumption (PA/LNA excluded): • during TX: 1700 mW • during RX: 1500 mW 9 cm 9 cm

49. Conclusions • SDR design is complex • Peculiar satellite reqsallow for relevantoptimizations • Proper digital design can further optimize the system • ICs integration level is able to fit the SDR in aCubeSat • There's still a "power price" to pay • SDRs can't (and won't) match PLL transceiver power rating • Is it worth to “pay the price” of an SDR for • in-orbit re-configurability, • dynamic communication adaptation, • complex modulations, • easier reuse across missions, • and to further extend mission goals?

50. Conclusions and Future Works