Sonar can be used at any depth. All other means of observation and communication are more dangerous because they require bringing the submarine up to periscope depth. Visual observation, radio communication, and navigational updates all require running near the surface, where submarines are most vulnerable. Two-way communication requires submarines to come to periscope depth and break the surface with an antenna, risking detection. Communications satellites have helped minimize the risk by greatly reducing the time spent at periscope depth to exchange data.
Radar is an acronym for radio detection and ranging.
A radar system transmits a radio beam, then detects and measures the echo when the beam bounces off an object. This provides information about both the target's direction and range. Like periscopes and radio antennas, however, radar masts protruding above the water provide vital information at the expense of increased risk of detection. Submarines generally use radar only on the surface when leaving or entering port. A sound signature is the unique, repetitive noise created by every ship's rotating machinery, such as engines, propellers, and pumps.