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Communications and Data Handling

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.

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Communications and Data Handling

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  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 = 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.

  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 • Carrier signal typically a sinusoid • - Easy to recreate Period, P   Amplitude, A Phase shift, 

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

  17. Signal to Noise SNR = Eb R / (No)

  18. dB Language • dB or Decibels are power ratios • Pref = 1 W or 1 mW (dBW or dBm respectively) • P(dBm) = P(dB) +30 • Examples • 1W = 0 dBW = 30 dBm • 1000W = 30 dBW = 60 dBm • 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—DesignTransmitter 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. Comm Subsystem—Design Frii’s Transmission Formula Break formula into pieces… æ ö P L G L L L L G ( ) æ ö E 1 1 ( ) ( ) ç ÷ t l t s a r p r = = b ç ÷ P L G L L L L G ç ÷ t l t s a r p r N kT R kT R è ø è ø 0 S S receive carrier losses antenna power at reciprical 1 gain transmitte r of data N rate 0 carrier power density at receive antenna EIRP

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

  23. Comm Subsystem—DesignTransmitter Link Contributions • 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: • Same EIRP • Much different 

  24. Comm Subsystem—DesignReceiver Link Contributions • Receiver figure of merit: • 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 • Values given in SMAD Table 13-10

  25. Comm Subsystem—DesignTypical System Noise Temperatures

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

  27. Comm Subsystem—Design Transmission Loss Contributions • 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

  28. Comm Subsystem—Design Transmission Loss Contributions (La)

  29. Comm Subsystem—DesignModulation Schemes

  30. Comm Subsystem—DesignModulation Schemes

  31. Data Handling

  32. Data Handling—IntroDriving 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—IntroDriving 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. Data Handling—IntroFunctions • 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.

  35. Data Handling—IntroFunctions

  36. Data Handling—IntroFunctions • 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. Data Handling—IntroFunctions—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)

  38. Data Handling—IntroFunctions—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

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

  40. DH Subsystem—DesignAcquiring 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

  41. DH Subsystem—DesignAcquiring Analog Data—Op Amps V CC i=0 Inverting - VP - input e =0 V g o Non- Inverting VN + + input = 0 Z out ¥ = Z - V in CC

  42. DH Subsystem—DesignAcquiring Analog Data—Op Amps

  43. DH Subsystem—DesignAcquiring 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.

  44. DH Subsystem—DesignAcquiring 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 2fm • fs = sampling frequency • fm = 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)

  45. DH Subsystem—DesignAcquiring Analog Data—DAC Sampling Rate

  46. DH Subsystem—DesignAcquiring 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

  47. DH Subsystem—DesignAcquiring Analog Data—Quantization

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

  49. DH Subsystem—DesignMultiplexing • 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).

  50. DH Subsystem—DesignMultiplexing Example • 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: • 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

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