Spacecraft Communications Chain

What Information Does a Signal Carry?

• Command uplink: ground → spacecraft
• Telemetry / science return: spacecraft → ground
• The waveform also provides timing, range, and Doppler measurements (see navigation and tracking signals)

The Return-Link Chain

• Spacecraft: avionics and RF hardware inside the spacecraft bus; the antenna and feed launch the radio wave
• Return link, block by block:
  1. C&DH (command and data handling) collects data
  2. Encode / modulate (Digital Modulation)
  3. PA (power amplifier); a low-loss RF feed connects the PA to the antenna
  4. TX antenna
  5. RX antenna (ground station)
  6. LNA — keep loss before the LNA small (LNA placement)
  7. Demodulate / decode → ground receiver and mission-data processing
• Functional locations are schematic: e.g. DSN beam-waveguide optics carry the RF from the dish to the receiver equipment (photos: LRO; Goldstone 34 m beam-waveguide antenna)

The Receiver Must Recover the Bits

• RF/IF conversion → carrier + timing recovery → filter + symbol decisions → channel decoder
  • IF = intermediate frequency
• The receiver needs carrier and timing references before making symbol decisions
• Its error performance depends on signal energy relative to noise (Energy per Bit, Bit Error Rate)
• The receiver and decoder get examined in detail in the next class (Demodulation, BER, and Coding)

Navigation and Tracking Signals

Purpose	Observable or reference	Example
Navigation	Known timing/code/phase references	GPS time-tagged ranging codes
Historical navigation	Carrier Doppler over a pass	Transit navigation
Bearing reference	Carrier/subcarrier phase relationship	VOR
Spacecraft tracking	Range and Doppler over time	Orbit estimation

• Range is a line-of-sight distance measurement
• Doppler constrains line-of-sight velocity
• A trajectory solution uses measurements over time
• Ranging tones and PN ranging on the same carrier: subcarriers
• Reference: JPL DSN 810-005, Module 206 (Telemetry General Information)