Spacecraft Command and Telemetry Systems Spacecraft Telemetry System. ACQUISITION SENSORS...

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    Mark Zimmerman Typewritten Text Satellite RF Communications and Onboard Processing

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  • Special Characteristics of Space Links (2)

    The satellite is constantly moving -

    1

    • Antennas must be constantly pointed

    • Doppler shift complicates receiver design Example:

    ∆f/f can be ± 25 ppm for LEO satellites (± 50 kHz at S-band).

    • Poor station coverage, short pass times – continuous coverage would require hundreds of ground stations – may need data storage – special communications orbits (geostationary, Molniya) – data relay satellite

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  • Tracking and Data Relay Satellite System (TDRSS)

    2

    NASA Standard TDRSS Transponder

    5.9 x .5 x 4.5 inches, 5 kg

    5 W output, 40 W DC input Source: “Space Network Users’ Guide,” NASA GSFC, 1988

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  • Optimizing the Beamwidth to Cover a Given Cone

    ● ● ●

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  • Antenna Considerations for Spacecraft

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    Standard PCM Formats

  • M-ary Phase Shift Keying (m = 8)

    For Pε small,

    7

    Power Spectra

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  • Properties of Gaussian Noise

    • Probability density of amplitude

    • Average value, mean square value

    • Time behavior, autocorrelation function

    • Power spectral density, “white” noise

    • Noise temperature

    where k is Boltzmann’s constant =

    2

    221( ) 2

    x

    p x e σ σ π

    − =

    ( ) ( ) ( )vR v t v t dtτ τ ∞

    −∞ = +∫

    2( ) ( ) j fv vG f R e d π ττ τ

    ∞ −

    −∞ = ∫

    231.38 10 W/Hz/K−×

    o (W/Hz) (W/Hz K)

    NT k

    =

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  • Receiving System Figure of Merit - G/T

    • The receiving system antenna gain divided by the system noise temperature provides a convenient “figure of merit” to compare receiving stations.

    9

    • Two systems having the same G/T will (to first order) have the same link performance.

    Example: Gr = + 50.0 dBi = 100,000

    Ts = 200 K

    G/T = = 500 = + 27.0 dB/K 100,000

    200

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    State of the art!

  • Shannon’s Channel Capacity

    • Consider a channel with bandwidth W and signal-to-noise ratio S/N.

    10

    • Do not use this upper bound for design!

    • By letting N = No W and W ∞, can show that error-free digital communication cannot take place below E/No = -1.6 dB (ln 2)

    • High performance exacts a price: bandwidth spreading, abrupt thresholds, complex coding/decoding equipment, computational delays

    C = W log2 ( 1 + ) S

    N

    • In 1948 Claude Shannon proved “there exist” codes and modulations which permit error-free communication, provided the bit rate does not exceed

    Claude Shannon 1916-2001

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  • Supraluminal (faster-than-c) Communications

    • Wormholes. Spacewarps through higher dimensions.

    • Would supraluminal communications violate the Causality Principle?

    • Can a particle be accelerated to c? • Can a particle have a velocity > c?

    - Tachyons: how generate, modulate, detect?

    References: “Particles That Go Faster Than Light,” Gerald Feinberg, Sci. Amer., 222, 2, Feb. 1970

    Timescape, Gregory Benford, Simon & Schuster, 1980 A Brief History of Time,