Phil Liebrecht Assistant Deputy Associate Administrator and Deputy Program Manager Space Communications and Navigation NASA HQ Meeting the Communications and Navigation Needs of Space missions since 1957 “Keeping the Universe Connected” Public University of Navarra, 2010 [email protected] 1 Operations and Communications Customers 2 Space Operations 101 Relation Between Space Segment, Ground System, and Data Users* Ground System Command Commands Requests Spacecraft and Payload Support • Mission Planning Telemetry • Flight Operations • Flight Dynamics • Perf. Assess., Trending & Archiving Space Data Users • Anomaly Support Segment Data Relay and Level (S/C) Mission Zero Data Processing Mission Data Data • Space-to-ground Comm. • Data Capture • Data Processing • Network Management • Data Distribution • Quick Look * Based on Wertz and Wiley; Space Mission Analysis and Design 3 Communications Theory- Basic Concepts Transmitter and Receiver must use the same language Noise causes interference a) Figures it – no errors (Identify and quantify) b) Figures it – corrects Must be loud enough ENERGY! c) Figures it – incorrectly (message in error) d) Cannot figure- recognizes E L an error T M E XP AIN L E e) Cannot figure- ? TRANSMITTER Distance weakens the sound RECEIVER (calculate the loss) CHANNEL Has to correctly interpret the message - the most difficult job The fundamental problem of communications is that of reproducing at one point either exactly or approximately a message selected at another point. 4 Functional - End to End Process Polarizer To reflector (dish)- Captures Polarizer To reflector (dish) - Directs focuses the signal and focus signal Separate incoming signal Separate incoming signal Diplexer Diplexer from outgoing signal from outgoing signal Amplify the modulated Amplifier Amplify the received signal signal (Low Noise) Amplifier Analog signal Analog signal Ranging Reconstitute the Modulate symbol signal symbol stream Ranging Modulator stream on to the carrier The Demodulator signal Medium Reference Carrier Symbol stream (bits) signal signal Soft Convert to a coded Reconstitute the Symbol Encoder symbol stream bit stream Decoder stream (bits) Bit stream Bit stream Constitute the data Organize data for formats and forward Processor transmission- Frames, Processor the data (Frames, Files) Files as addressed Digital data System Data Mission Data ….. Capture desired Camera Sensor Status information convert to digital format Desired measurements 5 Antenna Gain and Propagation Loss 1 λ 4πA = = 2 Pr Pt 2 Ae Pt 2 e 4πR 4πR λ Propagation Loss (1/LP) Receive Antenna Gain (Gant) In dB: Pr = Pt – LP + Gant – (Lother) 6 Antenna – Operation – Directional Antenna – Size Matters 200 Visible 180 160 140 Infrared 120 ) ic 100 80dB 80 Microwave Gain (dB Gain 60 40 20 Arecibo HST Keck DSN 70m 0 0.1 0.3 1 3 10 30 100 300 1,000 JWST Collecting Dish Diameter (m) Assumes antenna efficiency of 100% 7 What is Noise? • Noise is additional “signal” that does not correspond to the information you are trying to convey. • Noise in a signal takes several forms: – Signal noise • Amplitude noise – error in the magnitude of a signal • Phase noise – error in the frequency / phase modulation – System Noise • Component passive noise • Component active noise (amplifiers, mixers, etc…) – Environmental Noise • Atmospheric noise • Galactic noise • Precipitation • Noise introduces error in the “ideal” modulation and signal in free space which results in errors in the received signal at the destination 8 Direct Sequence Spread Spectrum A DS-CDMA signal is generated by multiplication of a user data signal by a code sequence. Source: Jack Glas, T.U. Delft. http://www.wirelesscommunication.nl/reference/chaptr05/cdma/dscdma.htm) 9 Receiving Direct Sequence Spread Spectrum Signals Signal x(t) Recovered Symbol (Data) x x Filter data rate R spreading code spreading code signal g(t) signal g(t) chip rate Rch chip rate Rch Rch ≈ ≥ 10 symbol (data) rate • Multiplication by the spreading signal once spreads the signal bandwidth. • Multiplication by the spreading signal twice recovers the original signal. • The desired signal gets multiplied twice, but the jamming signal gets multiplied only once. • g(t) must be deterministic, since it must be generated at both the transmitter and receiver, yet it must appear random to authorized listeners. – Generally g(t) is generated as a pre-defined pseudo-random sequence of 1s and –1s through the use of prescribed shift registers. 10 Spreading: Effect of Spread Spectrum G(f) Jammer with total power J JO = J/W Before Spreading w Gss(f) J'o = Jo (W/Wss) After Spreading wss 11 Digital Modulation Bi-Phase Shift Keying (BPSK) Quadrature Phase Shift Keying (QPSK) 12 CC/RS Concatenated Coding Performance Capacity Gap = 2.8 dB Coding Gain = 8 dB 13 Metric Tracking Ranging • Range is the distance from ground station to spacecraft. – Usually measured as “Round-trip” ranging: station, to spacecraft (R1), to station (R2). AKA, RTLT, Round-trip light time. – Distance = (RTLT/2) * light speed • Methods – Tone (signal itself) – PN (pseudo Noise) R1 R2 14 Metric Tracking Doppler = Range Rate An observed/perceived change in the frequency of a radio wave due to the rate of change in distance (relative velocity) between transmitter and receiver. Doppler compensation 15 Spacecraft-Quasar Differential Angular Techniques • Common-modes out most error sources • Leaves accurate measure of S/C-to- Quasar separation angle 16 GPS Space Applications Critical to a Range of Space Operations, Science, and Exploration Enterprises GPS services already enable: GPS / QZSS monitoring station at Kokee Park, Hawaii (NASA –JAXA 2009 Agreement) • Real-time On-Board Autonomous Navigation: –Reduces the burden and costs –Enables new methods of spaceflight such as precision formation flying and station-keeping • Attitude Determination: –Used on the ISS • Earth Sciences: –Remote sensing tool supports Jules Verne ATV during rendezvous with ISS in 2008 atmospheric and ionospheric sciences • GPS relative navigation used –Geodesy, and geodynamics -- from • Future navigation on ATV to be performed via combined GPS/Galileo receiver monitoring sea level heights and climate change to understanding the gravity field • International space agencies planning to use similar robust capabilities from Galileo and other GNSS constellations 17 Space Communications and Navigation Systems Engineering Fundamentals • To achieve successful space communication with microwaves you need: – Line of sight between the space vehicle and the receiver – Ability to point the antennas at each other (PAT) – Need sufficient received signal power compared to background noise • Large antennas • Ultra low noise cooled preamplifiers • Large transmitters (ground stations) – Use of error correcting coding and disruption tolerant protocols to improve performance in noise – Typically integrated metric tracking measurements – With increased distances these become even more important (and difficult) 18 SCaN Current Networks The current NASA space communications architecture embraces three operational networks that collectively provide communications services to supported missions using space-based and ground-based assets Near Earth Network - NASA, commercial, and partner ground stations and integration systems providing space communications and tracking services to orbital and suborbital missions Space Network - constellation of geosynchronous relays (TDRSS) and associated ground systems Deep Space Network - ground stations spaced around the world providing continuous coverage of satellites from Earth Orbit (GEO) to the edge of our solar system NASA Integrated Services Network (NISN) - not part of SCaN; provides terrestrial connectivity 19 Orbits and View Deep Space Network TDRSS Provides continual tracking to spacecraft above 30,000 km altitude .3 complexes around the Earth .34 to 70 meter antennas on the Earth Goldstone View TDRSS Space Network Provides continuous tracking of Earth Madrid Canberra spacecraft below GEO View View .Constellation of relay spacecraft at three GEO nodes around the Earth .5 meter tracking antennas in space 30,000 Km Near Earth Network Provides tracking of near earth LEO (600 orbiting spacecraft. Km) .10 to 18 meter antennas on the Earth View looking down on the Earth’s North Pole Together they provide nearly continuous coverage across the Solar System! 20 Near Earth Orbit Ground Network 180 165 150 135 120 105 90 75 60 45 30 15 0 15 30 45 60 75 90 105 120 135 150 165 180 90 Svalbard, Norway 90 Poker Flat, Alaska NSC DATALYNX 75 75 60 Sweden 60 USN 45 45 University of Alaska, Wallops Island Fairbanks USN 30 30 Merritt Island, Hawaii Ponce De Leon 15 USN 15 0 0 15 15 South Africa 30 Santiago USN Perth 30 University of Chile USN 45 45 60 60 75 McMurdo Station, Antarctica 75 90 90 180 165 150 135 120 105 90 75 60 45 30 15 0 15 30 45 60 75 90 105 120 135 150 165 180 21 Near Earth Network Support Limits H OF USER GROUND BASED ANTENNA PAT SP L AC FIELD OF VIEW A EC IT R B A R F O T USER SPACECRAFT Wallops Island provides Near Earth Network Support CAN ONLY SUPPORT UP TO 20% OF USER ORBIT, UNDER IDEAL CONDITIONS. MANY ORBITS GO UNSUPPORTED OR HAVE VERY LIMITED SUPPORT. RESPONSE TO USER SPACECRAFT EMERGENCIES DEPENDENT ON ORBIT POSITION. DATA RATES LIMITED. 22 Near Earth Network Resources SG1 SG2 SG4 SGS, Longyearbyen, Svalbard Antennas WGS, Wallops Flight Facility Spitzbergen, Norway Wallops Island, Virginia McMurdo Ground Station 23 Commercial Network Support 13M 13M USN South