Passive vs. active — two sensing modes
Line up passive and active sonar by the quantities that show up in their equations. The goal is to keep target-radiated sound and echo conceptually separate.
Passive listens to target-radiated sound
A passive sonar uses the sound the target is emitting on its own: machinery noise, propeller cavitation, marine-mammal vocalizations, flow noise, and so on. Because nothing is transmitted from our side, passive is a natural fit for monitoring where exposing your own position through transmission is undesirable, and for continuous observation of ambient sound.
However, when the target is quiet, passive detection becomes difficult. A single hydrophone also has trouble recovering range directly, which is another limitation.
Active looks at the echo
An active sonar transmits its own ping and listens to the reflection. Here TS (target strength) comes into play. TS is a measure of how effectively a target returns sound toward the sonar. Targets with larger TS produce stronger, easier-to-pick-up echoes.
The reason the basic active equation contains 2TL is that the sound undergoes transmission loss twice — once outbound and once on return. Passive is one way from target to receiver, so we only need one TL for a first pass.
What each mode makes easy or hard
Simplifying, active makes range easy; passive makes non-transmitting surveillance easy. That is the introductory summary. In practice, both modes can extract a lot more information via array processing, tracking, frequency analysis, and multiple receiver locations, but this single distinction is enough to start.
Passive
listen only / no transmission / a single sensor has trouble yielding range directly
Active
ping + echo / TS matters / range falls out naturally from round-trip time
From Chapter 2 to Chapter 3
Chapter 2 laid out the acoustic-physics fundamentals — speed of sound, wavelength, and round-trip time. The basic equation range = c × t / 2 applies when sound follows the path transmitter → target → receiver. In this chapter we focus on how that path differs between passive and active. Passive uses a one-way path target → receiver, while active uses a round-trip transmitter → target → receiver, so transmission loss (TL) is counted once for passive and twice for active. This is the basis for why TL and 2TL appear in the SNR equations of Chapter 4.
Monostatic vs. bistatic — definitions
Active sonars are classified into two configurations by how the transmitter and receiver are placed.
- Monostatic: transmitter and receiver are co-located. Hull-mounted sonars or echo sounders, where the same platform emits the ping and listens for the echo, are typical examples.
- Bistatic: transmitter and receiver are at different locations. The separated geometry can yield additional information about target position, but synchronization and calibration become necessary.
Unless otherwise noted, the SNR equations and the way TL is counted in this course assume a monostatic configuration.
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Chapter 3 / Practice 1
Unanswered
Q11. Which mode uses sound the target emits on its own?
Which mode uses the sound radiated from the target itself for detection?
Show hint
Listening to radiated sound is the passive mode.
Show reasoning
A passive sonar detects and classifies targets by listening to the radiated noise or vocalizations they emit on their own.
Chapter 3 / Practice 2
Unanswered
Q12. Which equation does TS appear in directly?
In the basic sonar equations, target strength (TS) appears directly mainly in which mode?
Show hint
Think about which mode reasons about echo strength.
Show reasoning
TS describes how much sound a target reflects back, so it shows up directly in the active sonar equation, which uses the echo.
Chapter 3 / Practice 3
Unanswered
Q13. Why 2TL appears in the active equation
Pick the best reason why the simplified active sonar equation contains
2TL.Show hint
Sketch the path transmitter → target → receiver.
Show reasoning
The transmitted sound experiences TL on the outbound leg to the target, and the reflected sound experiences TL again on the return leg.
Chapter 3 / Practice 4
Unanswered
Q14. Best match for a quiet but reflective target
Suppose the target emits almost no sound of its own, but returns a reasonable echo when insonified. On a first-principles basis, which mode is more directly useful?
Show hint
Passive assumes there is target-radiated sound to listen to.
Show reasoning
When the target is not emitting sound on its own, passive detection is difficult, so active — which can rely on the reflection — is more direct.
Chapter 3 / Practice 5
Unanswered
Q15. What a single-hydrophone passive observation struggles to deliver directly
If you are only listening passively with a single hydrophone, which quantity is the hardest to determine directly?
Show hint
Active gives you round-trip time and arrays give you bearing, but a single passive hydrophone sees relatively little.
Show reasoning
A single passive channel cannot recover range directly; you need additional geometric information or multiple observations.
Key takeaways from this chapter
- Passive listens to sound emitted by the target; active uses the echo of its own ping.
- TS appears directly in the active sonar equation, because it is about reflected sound.
- Active carries a
2TLfactor; passive uses a single TL on a first pass.