L16 Pre-flight

L16 Pre-flight#

Block 2: RF Electronic Warfare — Lesson 16: EP for Communications Work this before L16. Pre-flights are never collected — in class, anyone may be cold-called to present any question at the board, and that recitation is the participation grade.

Assumed pre-class reading#

  • L15 wrap-up

  • The Jupyter Book Reading page for L16 — the narrowband-link vulnerability, spread spectrum (DSSS and FHSS), processing gain, and low probability of detection.

Quiz questions (5 items, ~5 minutes)#

Q1. (Multiple choice) Why is a plain narrowband datalink easy to jam?

  • [ ] (a) It uses too much power

  • [ ] (b) All of its energy sits in one small band, so a jammer can concentrate all of its power there

  • [ ] (c) It hops too quickly to track

  • [ ] (d) It is always encrypted

Q2. (Multiple choice) Spread spectrum works by:

  • [ ] (a) Increasing transmit power until the jammer is overpowered

  • [ ] (b) Spreading the signal over a much wider band with a shared code, so it looks like noise to anyone without the code

  • [ ] (c) Encrypting the message with a one-time pad

  • [ ] (d) Narrowing the bandwidth to avoid the jammer

Q3. (Multiple choice) The processing gain of a spread-spectrum link is:

  • [ ] (a) \(G_p = 10\log_{10}(B_{\text{spread}} / B_{\text{data}})\), i.e., \(10\log_{10}\) of the spreading factor

  • [ ] (b) The transmit power divided by the jammer power

  • [ ] (c) The number of bits per second

  • [ ] (d) The carrier frequency divided by the bandwidth

Q4. (Multiple choice) In DSSS, multiplying the received signal by the matching PN code (despreading) does what?

  • [ ] (a) Spreads the signal and concentrates the jammer

  • [ ] (b) Concentrates the signal back to narrowband while spreading the jammer’s power across the band

  • [ ] (c) Removes all thermal noise

  • [ ] (d) Increases the data rate

Q5. (Short answer) A protected link has 30 dB of processing gain and faces a jammer 20 dB above the signal (J/S = +20 dB). Explain whether the link can still close, and name the property that lets GPS be received below the noise floor.

Contents