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Communications Architecture in Space Systems: Designing Efficient Satellite Links

This lecture outlines the fundamental aspects of communications architecture in space systems, detailing the arrangement of satellites and ground stations involved in data transmission. It covers design stages from defining mission objectives to sizing the communication system. Key considerations include determining communication duration and distances, data rates, modulation and coding methods, and antenna constraints. The lecture highlights the importance of a well-defined architecture to ensure effective command, control, data relay, and telemetry in space exploration.

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Communications Architecture in Space Systems: Designing Efficient Satellite Links

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  1. Lecture 6: Telecommunications From you host … Dr. H

  2. Introduction “A communications architecture is the arrangement, or configuration, of satellites and ground stations in a space system, and the network of communication links that transfer information between them.” L & W, SMAD • Communications design requires us to think about the following issues: • Over what time durations and over what distances does the spacecraft need to communicate with Earth? • What does the spacecraft need to communicate & how often? • How much command, control and decision making is local and howmuch is centered on Earth?

  3. Steps in Defining a Communications Architecture • Identify communication links • Define mission objectives • Define mission requirements • Determine the architecture • Determine data rates for each link • Specify accuracy required • Determine sampling rates, quantization levels. • Design each link • Select frequency band • Select modulation, coding • Determine antenna size, beamwidth constraints • Determine transmitter power constraints • Estimate received noise, interference powers • Calculate antenna gains & transmitter power • Size the comm system • Select antenna configuration • Calculate antenna size • Estimate antenna masses • Estimate transmitter masses

  4. Comm. Architecture Defined by Function Spacecraft Relay Spacecraft

  5. Step 2: Determine Data Rates for Each Link • What information must be communicated and how fast? • Analog-to-digital conversion: • Sample frequency >2.2 x (max input frequency) • Divide the range of the analog signal into M =2n levels – n = no. of bits per sample • Mean-square noise power = (V)2/12, where V = Vfull-scale/M • Signal-to-quantization noise power ratio = (M2-1). So you need smaller steps for weaker signals.

  6. Step 2: Determine Data Rates • No. of bits per sample determined by mission requirements • Data rate = (No. samples/sec.) X (No. of bits/sample)Abbreviated: bps

  7. Step 2: Data Rates for TT&C • Monitoring: • Several hundred functions might be sampled, but at a low rate • Typical data rate ~ 50bps • Transmitting commands: • Usually ~ 1/sec. • Command message is typically ~ 48 to 64 bits • Tracking: • Ground station measures range or range rate for computing orbit ephemeris • For typical parameters of existing TT&C systems see table on next slide

  8. Step 2: Existing TT&C Systems

  9. Step 2: Data Rates for Data Collection • Example: A geostationary satellite with a radiometer which scans the entire Earth in 20 min. with 1 km resolution

  10. Step 2: Data Rates for Data Relay • Data relay systems typically re-transmit data with a receiver/transmitter combination called a transponder • Transponder bandwidths of commercial communication satellites are usually 36 MHz or 72 MHz. • Maximum data rate can be several times the bandwidth, depending on the modulation and the receiving station size

  11. r Step 3: Link Design~Electromagnetic Signal Propagation~ • Isotropic antenna: • Point source with total power, P • Total power flowing through any spherical surface of radius r remains = P • Therefore, power per unit area flowing to a receiver distance r away is proportional to 1/r2

  12. Step 3: Link Design~Electromagnetic Signal Propagation~ • But real transmit antennas have directionality. • They radiate preferentially toward the axial direction • As a result, the power per unit area transmitted along the axis is: • Gt x (power radiated by an isotropic antenna) • And this defines the transmitting antenna gain, Gt

  13. Noise r P P =transmitter power Ll = transm.-to-anten. line loss x Gt LsLa Gt = transm, anten. Gain Ls = space loss =(/4r)2 La = transm. path loss x Gr Ar Gr = receive anten. Gain Ar= effective receive antenna area Step 3: Link Design

  14. Step 3: Link Design – Link Equation Derivation

  15. Step 3: Link Equation Derivation - Continued

  16. Step 3: Link Design Equations in dB

  17. Step 3: Link Design Equations in dBTypical Noise Temperatures In s/c Comm Links

  18. Step 3: Detailed Procedure for Link Design

  19. Step 3: Detailed Procedure for Link Design- Continued

  20. Step 4: Sizing the Communications SystemEstimated transmit power + aperture size  System mass

  21. FINI

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