Electronic Surveillance and Intelligence Measures
The main method of detection for naval forces in the modern era is by by electronic measures, either by active radar, or by electronic surveillance, that is to say the passive reception of electronic signals. We will briefly discuss the pros and cons of both the former, Active radar emissions, and ESM. Both are extremely useful assets to a naval commander and both should be utilised, but it's important to know when and how it's appropriate to utilise radar and ESM.
Active radar
The principal of radio detection and ranging is fairly simple. A transmitter emits radio waves of the electromagnetic spectrum, on a given frequency. If something returns the wave, the receiver picks it up and measures where it is and how far away it is based on the speed at which the wave returns. Radar is specifically effective at returning signals from objects which are electrically conductive. This makes steel very visible on radar, and, say, paper, not. We call a radar that is sending out transmissions an 'active' radar since it is actively transmitting. Active radars are very good at finding out where things are. Unfortunately, radiating one's active radar sort of gives away ones own position, which is where the next point comes in.
The principal of radio detection and ranging is fairly simple. A transmitter emits radio waves of the electromagnetic spectrum, on a given frequency. If something returns the wave, the receiver picks it up and measures where it is and how far away it is based on the speed at which the wave returns. Radar is specifically effective at returning signals from objects which are electrically conductive. This makes steel very visible on radar, and, say, paper, not. We call a radar that is sending out transmissions an 'active' radar since it is actively transmitting. Active radars are very good at finding out where things are. Unfortunately, radiating one's active radar sort of gives away ones own position, which is where the next point comes in.
ESM systems
Although they're not the same, ESM systems are comparable to a 'passive' radar, that is to say a radar with only a receiver switched on. ESM operates on the principle that the energy emitted by a radar travels further than the actual useful range of the radar set. This means that a sensitive receiver can pick up transmissions of a radar (and also other forms of sensors), translate those electronic signals into something more comprehensive, and from there analyse the situation. For instance, if an ESM system detects the operation of a SPY-1 radar, there is a good suggestion that a ship using the SPY-1 system is in the area. Passive sensors are limited in that they require the enemy to emit and to stay emitting. Time and more contacts are a requirement to make a reasonable assessment, but the early-warning of ESM systems is indispensable.
Although they're not the same, ESM systems are comparable to a 'passive' radar, that is to say a radar with only a receiver switched on. ESM operates on the principle that the energy emitted by a radar travels further than the actual useful range of the radar set. This means that a sensitive receiver can pick up transmissions of a radar (and also other forms of sensors), translate those electronic signals into something more comprehensive, and from there analyse the situation. For instance, if an ESM system detects the operation of a SPY-1 radar, there is a good suggestion that a ship using the SPY-1 system is in the area. Passive sensors are limited in that they require the enemy to emit and to stay emitting. Time and more contacts are a requirement to make a reasonable assessment, but the early-warning of ESM systems is indispensable.
Switched off
There's a third state we'll spend a moment discussing: that is when a radar is turned completely off. Not even passive receivers are operating; the whole system is depowered. This means that the radar is completely undetectable by ESM and also reduces the chance of it being detected by active radar, seeing as it will now come up only as the aggregate of its components and not as a bunch of electrics using up a lot of power. On the other hand, depowering a system means it has to be repowered later on.
There's a third state we'll spend a moment discussing: that is when a radar is turned completely off. Not even passive receivers are operating; the whole system is depowered. This means that the radar is completely undetectable by ESM and also reduces the chance of it being detected by active radar, seeing as it will now come up only as the aggregate of its components and not as a bunch of electrics using up a lot of power. On the other hand, depowering a system means it has to be repowered later on.
Radar horizon
Note that radar travels to the horizon, like the Mk 1 Eyeball, or electro-optical equipment (although radar has a slightly longer horizon.) For a 23-metre mast, the radar horizon is 16-kilometres. See here for a radar horizon calculator. This means that aircraft are specifically very important for reconnaissance because their altitude means they can see much further.
Note that radar travels to the horizon, like the Mk 1 Eyeball, or electro-optical equipment (although radar has a slightly longer horizon.) For a 23-metre mast, the radar horizon is 16-kilometres. See here for a radar horizon calculator. This means that aircraft are specifically very important for reconnaissance because their altitude means they can see much further.
So?
The intelligent reader will already have come to the conclusion that we find ourselves now discussing. That is to say, the two principal modes of detection, active radar and ESM, have both their uses as important pieces of technology with which to gain the required intelligence to commit to an attack. Both have their uses. The intelligent reader will also have come to another conclusion: the first found is the first destroyed. The use of emissions control -- that is, the tactical control of when and how you broadcast sensors, is extremely important. Give your position away, and you die.
The intelligent reader will already have come to the conclusion that we find ourselves now discussing. That is to say, the two principal modes of detection, active radar and ESM, have both their uses as important pieces of technology with which to gain the required intelligence to commit to an attack. Both have their uses. The intelligent reader will also have come to another conclusion: the first found is the first destroyed. The use of emissions control -- that is, the tactical control of when and how you broadcast sensors, is extremely important. Give your position away, and you die.