Communications and Data Handling Dr Andrew Ketsdever MAE

Communications and Data Handling Dr Andrew Ketsdever MAE 5595 Lesson 10

Outline • Communication Subsystem – Introduction – Communications Architecture (uplink/downlink) – Data Rates – Budgets and Sizing • Data Handling Subsystem – Introduction – Requirements and design – Sampling Rates – Quantization

Communications Subsystem • Function – Transmits data to ground station(s) – Receives commands and data from ground station(s) • Deals with concerns arising from – Modulation scheme – Antenna characteristics – Propagating medium – Encryption

Simple Communication Architecture

Alternate Communication Architectures

Communication Architectures

Communication Architecture

Military Communications Architecture

Radio Frequency Bands • Microwaves: 1 mm to 1 m wavelength. The microwaves are further divided into different frequency (wavelength) bands: (1 GHz = 109 Hz) – – – – – P band: 0. 3 - 1 GHz (30 - 100 cm) L band: 1 - 2 GHz (15 - 30 cm) S band: 2 - 4 GHz (7. 5 - 15 cm) C band: 4 - 8 GHz (3. 8 - 7. 5 cm) X band: 8 - 12. 5 GHz (2. 4 - 3. 8 cm) Ku band: 12. 5 - 18 GHz (1. 7 - 2. 4 cm) K band: 18 - 26. 5 GHz (1. 1 - 1. 7 cm) Ka band: 26. 5 - 40 GHz (0. 75 - 1. 1 cm) V band: 50 – 75 GHz W band: 75 – 111 GHz • Care required since EU and other countries may use different designations. Do not confuse with RADAR bands.

Modulation Schemes • Modulation – Variation of a periodic waveform to convey information • Modulation Schemes – Pulse Modulation – Amplitude Modulation – Frequency Modulation – Phase Modulation How can you communicate with someone on the other side of the lake?

Modulation Schemes • Carrier signal typically a sinusoid - Easy to recreate Period, P Amplitude, A Phase shift,

Amplitude Modulation

Frequency Modulation

Phase Modulation

Modulation Binary Phase Shift Keying Quadriphased Phase Shift Keying Frequency Shift Keying Multiple (8) Frequency Shift Keying

Link Design • Signal to Noise Pulse shape for illustration purposes only – would use sinusoidal waveform Frii’s Transmission Formula (ratio of received energy-perbit to noise-density):

Signal to Noise SNR = Eb R / (No)

d. B Language • d. B or Decibels are power ratios • Pref = 1 W or 1 m. W (d. BW or d. Bm respectively) • P(d. Bm) = P(d. B) +30 • Examples – 1 W – 1000 W = = 0 d. BW = 30 d. Bm 60 d. Bm • Attenuation – 1 d. B attenuation implies that 0. 79 of the input power is left – 10 d. B attenuation implies that 0. 10 of the input power is left – 1000 d. B attenuation implies that 0. 001 of the input power is left

Frii’s Transmission Formula Given Frii’s Transmission Formula: a) Write equation in terms of transmit power b) Express in logarithmic (d. B) form

Comm Subsystem—Design Transmitter Link Contributions n Effective Isotropic Radiated Power: n Antenna gain Measure of how well antenna concentrates the power density n Ratio of peak power to that of an isotropic antenna n

Comm Subsystem—Design Frii’s Transmission Formula Break formula into pieces… æ 1 Eb Pt Ll Gt Ls La Lr L p Gr = = (Pt Ll Gt )(Ls La Lr L p )(Gr )çç N 0 k. TS R k. TS receive è carrier power at transmitte r losses carrier power density at receive antenna EIRP antenna gain 1 N 0 öæ 1ö ÷÷ ç ÷ øè Rø reciprical of data rate

Comm Subsystem—Design Transmitter Link Contributions n Antenna gain: n for parabolic antenna: • function of imperfections in antenna • typical 0. 55 for S/C, 0. 6 – 0. 7 for GS n may approximate as:

Comm Subsystem—Design Transmitter Link Contributions n EIRP Tradeoff between transmitter power and antenna gain (for same frequency and antenna size) n Typical EIRPs: n 100 d. BW for ground station n 20 -60 d. BW for S/C n Example: n Case 1 Case 2 Pt 25 W 1 W Ll 0. 8 Gt 5 125 • Same EIRP 100 • Much different 75 deg 15 deg

Comm Subsystem—Design Receiver Link Contributions n Receiver figure of merit: n System noise: n Antenna noise sources: n Galactic noise, Solar noise, Earth (typically 290 K), Man-made noise, Clouds and rain in propagation path, Nearby objects (radomes, buildings), Temperature of blockage items (feeds, booms) Receiver noise sources: n Transmission lines and filters, Low noise amplifiers n n Values given in SMAD Table 13 -10

Comm Subsystem—Design Typical System Noise Temperatures

Comm Subsystem—Design Transmission Loss Contributions n n Free space path loss: Pointing loss: • Valid for e /2 (identical antennas) • Contributions from both antennas

Comm Subsystem—Design Transmission Loss Contributions Atmospheric loss, La n Due to molecular absorption and scattering n Oxygen: 60 GHz, 118. 8 GHz n Water vapor: 22 GHz, 183. 3 GHz (seasonal variations as much as 20 -to-1) n SMAD Fig 13 -10 n Rain loss, Lr n Strong function of elevation angle n May want to accept short outages rather than design for continuous service n SMAD Fig 13 -11 n

Comm Subsystem—Design Transmission Loss Contributions (La)

Comm Subsystem—Design Modulation Schemes

Comm Subsystem—Design Modulation Schemes

Data Handling

Data Handling—Intro Driving Requirements • Two main system requirements – Receives, validates, decodes, and distributes commands to other spacecraft systems – Gathers, processes, and formats spacecraft housekeeping and mission data for downlink or use by an onboard computer. • The data handling (DH) subsystem has probably the least defined driving requirements of all subsystems and is usually designed last – Based on the complexity of the spacecraft and two performance parameters: 1) on-board processing power to run bus and payloads and 2) storage capacity for housekeeping and payload data – Meeting requirements is a function of available flight computer configurations

Data Handling—Intro Driving Requirements • System level requirements and constraints – – – – – Satellite power up default mode Power constraints Mass and size constraints Reliability Data bus requirements (architecture and number of digital and analog channels) Analog interface module derived requirement Total-dose radiation hardness requirement Single-event charged particle hardness requirement Other strategic radiation requirements (EMP, dose rate, neutron flux, operate through nuclear event, etc. ) Software flash upgradeable

Data Handling—Intro Functions • Subsystem known by a variety of names – TT&C: Telemetry, Tracking, and Control (or Command) – TTC&C: Telemetry, Tracking, Command, and Communication – TC&R: Telemetry, Command Ranging – C&DH: Command Data Handling – CT&DH: Command, Tracking and Data Handling • Functions – Receives, validates, decodes, and distributes commands to other spacecraft systems – Gathers, processes, and formats spacecraft housekeeping and mission data for downlink or use by an onboard computer.

Data Handling—Intro Functions

Data Handling—Intro Functions • CT&DH Functions: – Aid in orbit determination (tracking) – Command S/C (command) (concerned with the uplink) – Provide S/C status (telemetry) (concerned with the downlink) • Gather and process data • Data handling – Make payload data available (telemetry) (concerned with the downlink) • Sometimes, the payload will have a dedicated system rather than using the bus – CT&DH functions often performed by OBC (On-Board Computer) • Comm Functions: – Deals with data transmission concerns (encryption, modulation scheme, antenna characteristics, medium characteristics) These will be discussed in Comm lessons.

Data Handling—Intro Functions—Command Handling • Commands may be generated by: – The Ground Station – Internally by the CT&DH computer – Another subsystem • Types of commands – Low-level On-Off: reset logic switches in SW (computer controlled actions) – High-level On-Off: reset mechanical devices directly (i. e. latching relays, solenoids, waveguide switches, power to Xmitter) – Proportional Commands: digital words (camera pointing angle, valve opening size)

Data Handling—Intro Functions—Data/Telemetry Handling • Housekeeping: – – – Temps Pressures Voltages and currents Operating status (on/off) Redundancy status (which unit is in use) … • Attitude: might need to update 4 times/sec • Payload: case-by-case payload health and payload data

DH Subsystem—Design Acquiring Analog and Digital Data Point-to-point digital data interface Digital In Digital network interface Sel MUX Digital Out Flight Computer Op Amp ADC DAC Analog In Op Amp Analog Out Shared data bus

DH Subsystem—Design Acquiring Analog Data—Op Amps • All real world data interfaces are analog – Sound – EM Spectrum: light, IR, UV, Gamma rays, X-rays, etc. – Motor speed, position • Usually analog signal levels on the input side are weak (payload sensor, receiver, telemetry level signal) – Need to boost signal level through Operational Amplifier otherwise known as “Op Amp” • On the output side, must match signal levels with equipment (transmitter, actuator, etc. ) – Use Op Amp to match systems

DH Subsystem—Design Acquiring Analog Data—Op Amps VCC i=0 Inverting input Non-Inverting input VP eg =0 VN + Zin = ¥ - Vo + Zout =0 -VCC

DH Subsystem—Design Acquiring Analog Data—Op Amps

DH Subsystem—Design Acquiring Analog Data—ADC • Once analog data is converted to “readable” level, we must convert it for use by the flight computer • Accomplished through Analog-to-Digital Converter (ADC) – Reverse process is Digital-to-Analog Converter (DAC) • Changes continuous signal into 1’s and 0’s representation – Sampling: choosing how often to measure signal – Quantization: choosing how many levels to approximate signal • Must tradeoff reconstructed signal quality versus bandwidth of data – Driven by mission requirements: accuracy, bandwidth, CPU processing speed, data storage, etc.

DH Subsystem—Design Acquiring Analog Data—DAC • Sampling rate considerations – Many samples → good signal representation, but takes lots of bits (bandwidth) – Few samples → low bandwidth, but not so good signal representation • Nyquist Criteria for sampling: fs 2 fm – fs = sampling frequency – fm = maximum frequency of sampled signal • Example: Human ear hears sounds in the frequency range from 20 Hz to 20 k. Hz. Audio compact discs represent music digitally and use a sample rate of 44. 1 k. Hz (2. 2 X human max frequency)

DH Subsystem—Design Acquiring Analog Data—DAC Sampling Rate

DH Subsystem—Design Acquiring Analog Data—ADC Quantization • Quantization level considerations – Many levels → good signal representation, but lots of bits (bandwidth) – Fewer levels→ low bandwidth, but not so good signal representation

DH Subsystem—Design Acquiring Analog Data—Quantization

DH Subsystem—Design Multiplexing • Used when sharing common wire for multiple sets of data – Need method to sequence data into telemetry stream EPS 12 separate data lines (dedicated) … CT&DH OBC 1 shared data line (multiplex data)

DH Subsystem—Design Multiplexing • Frames – Rigid telemetry structure, synchronous (pre-defined) communications. – A schedule for using the data bus, where the most crucial information (like ADACS) is sent more frequently than slowly changing, or non-critical data (for example TCS).

DH Subsystem—Design Multiplexing Example n Simple GEO EM surveillance satellite that receives traffic on one frequency, encrypts and transmits on a different frequency. Consider that each subframe is 250 msec long. Define the following messages/rates: n n n M 1: Send ADACS data to payload – 1 Hz M 2: Get RX’d data from Comm and send to CT&DH OBC – 8 Hz M 3: Send TX data to Comm – 8 Hz M 4: Get thermal data from TCS – 1 Hz M 5: Get battery voltage, supply current from EPS – 1 Hz M 6: Get fuel levels from Propulsion – 1 Hz

DH Subsystem—Design Multiplexing Example n n n M 1: Send ADACS data to payload – 1 Hz M 2: Get RX’d data from Comm and send to CT&DH OBC – 8 Hz M 3: Send TX data to Comm – 8 Hz M 4: Get thermal data from TCS – 1 Hz M 5: Get battery voltage, supply current from EPS – 1 Hz M 6: Get fuel levels from Propulsion – 1 Hz

DH Subsystem—Design Multiplexing Example Solution

DH Subsystem—Design DH Design and Sizing

Software Engineering Do. D Software statistics (The Problem) DOD Software Expenditures (according to one Army Study) 51% of all failures are blamed on bad requirements (by the way, only 2% of the working software is on time, under budget)

Software Engineering Software Growth Trends (The Need) 10000 F-22 PROJECTED MISSION CONTROL: GROUND STATION SHUTTLE/OPERATIONAL SKYLAB 2 B-2 APOLLO 7 GEMINI 12 1000 SHUTTLE/OFT APOLLO 17 F-15 E B-1 B MANNED SYSTEMS SHUTTLE/OFT B-1 A C-17 PROJECTED F-16 C/D Thousands of Code Memory Locations 100 (i. e. size of executable software) GEMINI 3 P-3 A AWACS S-3 A APOLLO 11 E-2 C P-3 A GEMINI 2 F-111 PERSHING 11(ED) SKYLAB 2 F-111 TITAN 34 D (IUS) VIKING MERCURY 3 C-5 A PERSHING 11(AD) A-7 D/E 10 PERSHING 1 Flight Date TITAN 1960 MANNED A/C POSEIDON C 3 PERSHING 1 A 70 UNMANNED INTERPLANETARY VOYAGER MANNED SPACE MARINER VENUS MERCURY SURVEYOR 65 UNMANNED TRIDENT C 4 GEMINI 3 TITAN IIIC 1 UNMANNED SYSTEMS F-15 APOLLO 7 MISSILE GALILEO 75 80 MANNED SPACE CONTROL 85 90 95

Software Engineering Software Increasingly matters

Software Engineering What can go wrong (The Errors) • H. M. S. Sheffield – sunk by a missile its software identified as being “friendly” • Patriot clock drift – Missed Mach 6 scud by 0. 36 sec clock drift that occurred over a continuous 4 -day usage period • NASA Mariner 1 – $80 million missing comma (DO 17 I = 1 10 vs. DO 17 I = 110 vs. DO 17 I = 1, 10 ) • SDI laser and Space shuttle mirror – Shuttle positioned to bounce a laser positioned at 10, 023 miles vs. 10, 023 feet • USS Yorktown – Zero entered as data caused a divide by 0 error, cascading errors caused complete shut down of the ship’s propulsion system for an hour (ship was eventually rebooted) • Ariane 5 – Non-critical component failure shut down system including critical components shoved a 64 bit float number in a 16 bit integer space • Mars Climate Orbiter and Polar Lander failures – English units (pounds-force seconds) used instead of metric units (Newton-seconds) – Flight software vulnerability to transient signals shut down descent engines early • Titan IVB-32/Centaur (Milstar) – Misplaced decimal point in avionics database