Communication. Provides the interface between ground and the spacecraft Functions:

Transcription

1 Telecomm

2 Communication Provides the interface between ground and the spacecraft Functions: Lock onto the ground station signal (carrier tracking) Receive uplink and process it (command reception and detection) Process and transmit spacecraft data (telemetry modulation and transmission) Receive, process and transmit ranging signal [to help determine spacecraft position] (ranging) Antenna pointing

3 Communication Some terminologies Power Flux Density (F) of an omni-directional power source Flux = Power Surface Area of Sphere P = 4πR Antenna s Gain: the ratio of energy it transmits in the primary direction to energy if it was transmitting omni-directionally (isotropic). The larger the gain, the better (or more efficient) the transmission. (example: flash light vs. light bulb) Gain 4πA = η 2 = λ 2 ( πd ) η 2 λ A = area of the antenna d = diameter of the parabolic antenna λ = wavelength of singal η = aperture or antenna efficiency (0.55 to 0.7 for parabolic) 2

4 Communication Some terminologies Effective Isotropic Radiated Power (EIRP): the equivalent amount of power that an omni-directional antenna would need to have to emit the same power level to a target EIRP = P t G t P t = transmitter s power output G t = transmitter s antenna gain Received signal strength S = PG t t 2 λ 4πR G R 4 G R = receiver s antenna gain 2 λ πr = space loss (0 to1) space loss refers to the reduction of power while traveling away from the power source

5 Communication Some terminologies Bandwidth: the range of frequencies the receiver is designed to received Noise power is the power of the electromagnetic radiation because it is not at absolute zero K N = ktb k = Boltzmann s constant (1.381x10-23 joules/k) T = receiver system s temperature (K) B = receiving system s bandwidth (Hz) Signal-to-Noise Ratio (SNR or S/N) S N = PG t kb t 2 λ 4πR G T R should be >1 for effective communication i.e., your signal is louder than the noise Recall: λ = c / f, where c = speed of light and f = frequency (Hz)

6 Command and Data Handling Receives, validates, decodes, and distributes commands to other spacecraft systems Gathers, process, and formats spacecraft housekeeping and mission data for downlink or onboard use Spacecraft time keeping

7 Command and Data Handling Subsystems raw data CDHS Command (uplink) Payloads command Telemetry (downlink)

8 Command and Data Some Terminologies Handling Carrier Signal: the base or known frequency signal; example: a sinusoidal wave Modulation: the change or modification of the carrier signal; a direct input Amplitude modulation (AM): changing the amplitude of the carrier signal Frequency modulation (FM): changing the frequency of the carrier signal Phase modulation: delayed of the carrier signal Example: a internet modem (derived from the term modulator) converts audiofrequency (AF) tones to digital computer signals (1 s and 0 s); 1 s for high tone and 0 s for low tone Multiplexing: having more than one modulation signal sent on the same carrier signal (sharing the bandwidth) Each separate signal becomes a subchannel of the carrier signal (now known as the channel) Type of multiplexing: frequency division multiplexing (FDM) for analog communications and time division multiplexing (TDM) for digital communications.

9 Command and Data Handling S/C and P/L Information modulator Telemetry Signal Command (uplink) Data Handling Amp Command information de-mod Command Signal Telemetry (downlink)

10 Command and Data Handling Data Handling Data I/O CPU Memory I/O S/C and P/L Information Command I/O I/O Command information

11 Thermal

12 Thermal The thermal control subsystem controls the operating temperature environment of the spacecraft Controls the amount of heat going in and out of a spacecraft Most components are less reliable when operated beyond their design operating temperature Keep propellant from freezing Some instruments have to be operating at very cold temperatures (IR sensors require about 70K or -316 deg F)

13 Thermal Heat Sources: Sun, Earth, and internal 1358 W/m 2 ~237 W/m 2 ~407 W/m 2

14 Thermal Heat Transfer Convection: transfer via flowing fluids (e.g., air or water) Conduction: transfer within materials (non-fluid) Radiation: transfer via electromagnetic waves q = σεat where q = heat - power transfer per unit time (W) σ = Stefan - Boltzmann's constant (5.67x10 ε = emissivity (0to1) A = surfacearea of 4 the body (m T = black body temperature(k) 2 ) -8 2 W/m k 4 )

15 Thermal Control Method Radiators Heat pipes Surface coating

16 Structures

17 Structures and Mechanisms Structures Mechanically support all other subsystems Attaches spacecraft to launch vehicle Provide shielding for components Satisfy all strength and stiffness requirement Support of wires, propellant feed line, brackets, etc. Mechanisms High Cycle: Antenna gimbals, Solar array drives, Reaction wheels Low Cycle: Gravity gradient boom, Solar array or antenna deployments, cover removal

18 Structures and Mechanisms Structural Loads Static Loads Support components during integration Pressurized tanks Mechanical pre-loads Thermoelastic loads Dynamic Loads (axial, lateral, torsional loads) Transport (to launch site or test facilities) Launch vehicle Acoustic loads Wind loads Attitude control actuation Thermal cycling Mechanical operations

19 Structures and Mechanisms Mechanics of Materials Three main load type Strength: stress, strain, and shear Vibration Thermal

20 Structures and Mechanisms σ = Mechanics of Materials Strength Stress: Load = Area P A Young s Modulus: E = σ ε P: linear E: elastic limit Y: plastic deformation Strain: ε = ΔL L Thermal Expansion ΔL = α( ΔT)L α = coefficient of thermal expansion

21 Structures and Mechanisms Mechanics of Materials Vibration Natural frequency is the frequency at which a unforced system vibrates if given an impulse (i.e., pendulum or swing) At this natural frequency the system is in resonance The lowest natural frequency is the fundamental frequency where f n = 1 2π k m k = stiffness(spring constant) m = mass

22 Structures and Mechanisms Mechanics of Materials Vibration (continues) Without energy dissipation the harmonic motion will continue forever (theoretically) An object in resonant can continue building up amplitude (i.e., periodic force to increase amplitude while swinging, breaking glass, Earth quakes and bridges) Example: story building fundamental frequency is about 0.5 Hz. The magnitude 8.1 earth quake in Mexico City in 1985 has a forcing vibration of 0.5 Hz Damping can be done through energy dissipation

23 Structures and Mechanisms Structural Design σ DESIGN FS < σ ALLOWABLE Allowable stress depends on type of stress and material 1) Option Ultimate test of a dedicated qualification article Design Factors of Safety Critical for Personnel Safety Not Critical for Personnel Safety Yield Ultimate Yield Ultimate ) Proof test of all flight structures ) Proof test of one flight unit of a fleet ) No structural test (Source: SSAM, Table 12.5 SMAD, Table DOD-HDBK-343, MIL-HDBK-340 and MSFC-HDBK-505A offer similar options.)

24 Structures and Mechanisms Material Advantages Disadvantages Aluminum Steel Heat-resistant alloy High strength vs. weight Ductile; tolerant of concentrated stresses Easy to machine Low density; efficient in compression High strength Wide range of strength, hardness, and ductility obtained by treatment High strength vs. volume Strength retained at high temperatures Ductile Relatively low strength vs. volume Low hardness High coefficient of thermal expansion Not efficient for stability (high density) Most are hard to machine Magnetic Not efficient for stability (high density) Not as hard as some steels Magnesium Low density very efficient for stability Susceptible to corrosions Low strength vs. volume Titanium High strength vs. weight Low coefficient of thermal expansion Hard to machine Poor fracture toughness if solution treated and aged Beryllium High stiffness vs. density Low ductility and fracture toughness Low short transverse properties Toxic Composite Can be tailored for high stiffness, high strength, and extremely low coefficient of thermal expansion Low density Good in tension (e.g., pressurized tanks) Costly for low production volume; requires development program Strength depends on workmanship; usually requires individual proof testing Laminated composites are not as strong in compression Brittle; can be hard to attach

25 Launch Structures and Mechanisms Atlas V Acceleration due to: - Thrust - Vibration - Acoustic (pressure wave)