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Do you know which technology can have better range with and without battery assitance than RFID?

This is for a school project, we are looking for ways to detect crab traps lost in the sea.

We thought on passive RFID but then found it has a very small range.

Thanks

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  • \$\begingroup\$ You are not saying (among other things) how much range you would like. \$\endgroup\$
    – Nick Alexeev
    Oct 10, 2015 at 0:30
  • \$\begingroup\$ @NickAlexeev the longest possible, at least 500m \$\endgroup\$
    – JorgeeFG
    Oct 10, 2015 at 2:51
  • \$\begingroup\$ Longest possible, you say. In that case, how about Iridium? \$\endgroup\$
    – Nick Alexeev
    Oct 10, 2015 at 3:01
  • \$\begingroup\$ +1 for a good question. I can't imagine why anyone would want to downvote it other than not having a clue. \$\endgroup\$
    – EM Fields
    Oct 10, 2015 at 5:30
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    \$\begingroup\$ @EMFields I can't imagine why anyone would want to upvote this. Reasons to downvote. (1) It's an academic assignment. The O.P. haven' demonstrated his own effort. (2) He did not characterize the problem. Underwater, or on the surface? What's the required distance? How long does it have to work? How will it be used? How much who cares why? As a result we are sent on a wild goose chase anywhere between RFID, Iridium, VLF and SONAR. \$\endgroup\$
    – Nick Alexeev
    Oct 10, 2015 at 9:23

2 Answers 2

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By and large, RF (particularly RFID) simply won't work underwater, particularly at the depths associated with crab pots. RF attenuation of seawater is just too great. Note figure 7 of the paper. At 1 MHz, attenuation is ~40dB/m, and pots at 100 meters down simply aren't going to hear a transmitter of any reasonable power, much less retransmit their IDs. Going to longer wavelengths helps, but not a whole lot.

Nuclear subs managed to listen for radio signals while submerged, but they used extremely low frequencies, long (100 meter) antennas, and bit rates in the vicinity of 1 bit/second. Not exactly useful for your application.

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  • \$\begingroup\$ Is there any other technology beacon-like or something that someone can use to find lost pots? \$\endgroup\$
    – JorgeeFG
    Oct 10, 2015 at 2:52
  • \$\begingroup\$ @WhatRoughBeast Near field RF communication (say, 13 MHz) can work through water. For example, it's used for communicating with implants. However, the useful range of near field RF is comparatively short, because it's near field. \$\endgroup\$
    – Nick Alexeev
    Oct 10, 2015 at 2:59
  • \$\begingroup\$ @JorgeeFG - Sonar would work, but not without active electronics. You could put a microphone, power source (battery) and pinger, so the pot would make noise when it heard a beacon. And that's exactly how it's done in airliner black boxes, so it can be made to work well. But it's expensive. \$\endgroup\$
    – WhatRoughBeast
    Oct 10, 2015 at 14:03
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A technique that's been used for many years is to attach a SONAR transponder to the target, sink it, and then interrogate it from a surface vessel.

In use, once the surface vessel acquires the synthetic echo from the transponder, it traverses a grid until the time between the surface ping and the transponder's response is minimized. When that happens - assuming the surface vessel's transducer is pointing straight downward - the target will be directly below the surface vessel.

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