1. Communications and Data Handling Dr Andrew Ketsdever MAE 5595 Lesson 10

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

3. 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

4. Simple Communication Architecture

5. Alternate Communication Architectures

6. Communication Architectures

7. Communication Architecture

8. Military Communications Architecture

9. Radio Frequency Bands Microwaves: 1 mm to 1 m wavelength. The microwaves are further divided into different frequency (wavelength) bands: (1 GHz = 10 9 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.

10. 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?

11. Modulation Schemes Amplitude, A Phase shift,  Period, P   Carrier signal typically a sinusoid - Easy to recreate

12. Amplitude Modulation

13. Frequency Modulation

14. Phase Modulation

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

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

17. Signal to Noise SNR = E b R / (N o )

18. dB Language dB or Decibels are power ratios P ref = 1 W or 1 mW (dB W or dB m respectively) P(dB m ) = P(dB) +30 Examples –1W=0 dB W =30 dB m –1000W=30 dB W =60 dB m Attenuation –1 dB attenuation implies that 0.79 of the input power is left –10 dB attenuation implies that 0.10 of the input power is left –1000 dB attenuation implies that 0.001 of the input power is left

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

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

21. Break formula into pieces…    rate data of reciprical 1 gain antenna receive antenna receiveat densitypowercarrier losses rtransmitte atpower carrier 0 11 0                RkT GLLLLGLP R GLLLLGLP N E N S rprastlt S rprastlt b Comm Subsystem—Design Frii’s Transmission Formula EIRP

22. Antenna gain: for parabolic antenna: may approximate as: function of imperfections in antenna typical   0.55 for S/C,   0.6 – 0.7 for GS Comm Subsystem—Design Transmitter Link Contributions

23. EIRP Tradeoff between transmitter power and antenna gain (for same frequency and antenna size) Typical EIRPs: 100 dBW for ground station 20-60 dBW for S/C Example: Case 1Case 2 PtPt 25 W1 W LlLl 0.8 GtGt 5125 EIRP100  75 deg15 deg Same EIRP Much different  Comm Subsystem—Design Transmitter Link Contributions

24. Comm Subsystem—Design Receiver Link Contributions Receiver figure of merit: Values given in SMAD Table 13-10 System noise: Antenna noise sources: 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: Transmission lines and filters, Low noise amplifiers

25. Comm Subsystem—Design Typical System Noise Temperatures

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

27. Atmospheric loss, La Due to molecular absorption and scattering Oxygen: 60 GHz, 118.8 GHz Water vapor: 22 GHz, 183.3 GHz (seasonal variations as much as 20-to-1) SMAD Fig 13-10 Rain loss, Lr Strong function of elevation angle May want to accept short outages rather than design for continuous service SMAD Fig 13-11 Comm Subsystem—Design Transmission Loss Contributions

28. Comm Subsystem—Design Transmission Loss Contributions (L a )

29. Comm Subsystem—Design Modulation Schemes

31. Data Handling

32. 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

33. 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

34. 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 and Ranging –C&DH: Command and 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

36. 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.

37. 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—Command Handling

38. 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 Data Handling—Intro Functions—Data/Telemetry Handling

39. DH Subsystem—Design Acquiring Analog and Digital Data Flight Computer Digital In Digital Out ADCDAC Point-to-point digital data interfaceDigital network interface Op Amp Analog InAnalog Out MUX Sel Shared data bus

40. 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

41. V CC + V o - i=0 + - e g =0 VNVN -V CC Z in =  Z out =0 Non-Inverting input Inverting input VPVP

42. DH Subsystem—Design Acquiring Analog Data—Op Amps

43. 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—ADC

44. 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: f s  2f m –f s = sampling frequency –f m = maximum frequency of sampled signal Example: Human ear hears sounds in the frequency range from 20 Hz to 20 kHz. Audio compact discs represent music digitally and use a sample rate of 44.1 kHz (2.2 X human max frequency) DH Subsystem—Design Acquiring Analog Data—DAC

45. DH Subsystem—Design Acquiring Analog Data—DAC Sampling Rate

46. 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—ADC Quantization

47. DH Subsystem—Design Acquiring Analog Data—Quantization

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

49. 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

50. M1: Send ADACS data to payload – 1 Hz M2: Get RX’d data from Comm and send to CT&DH OBC – 8 Hz M3: Send TX data to Comm – 8 Hz M4: Get thermal data from TCS – 1 Hz M5: Get battery voltage, supply current from EPS – 1 Hz M6: Get fuel levels from Propulsion – 1 Hz 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: DH Subsystem—Design Multiplexing Example

51. M1: Send ADACS data to payload – 1 Hz M2: Get RX’d data from Comm and send to CT&DH OBC – 8 Hz M3: Send TX data to Comm – 8 Hz M4: Get thermal data from TCS – 1 Hz M5: Get battery voltage, supply current from EPS – 1 Hz M6: Get fuel levels from Propulsion – 1 Hz DH Subsystem—Design Multiplexing Example

52. DH Subsystem—Design Multiplexing Example Solution

53. DH Subsystem—Design DH Design and Sizing

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

55. Software Engineering Software Growth Trends (The Need) Thousands of Code Memory Locations (i.e. size of executable software) Flight Date

56. Software Engineering Software Increasingly matters

57. 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. DO17I = 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