[Claim 1] A radar system for detecting objects in an underwater environment, comprising: a plurality of sentry unit node devices in communication with a plurality of antennas; wherein the plurality of antennas include at least one transmitter (Tx) antenna and at least one receiver (Rx) antenna, and/or at least one antenna capable of both transmitting and receiving electromagnetic waves; wherein each of the plurality of sentry unit node devices includes at least one signal generator and at least one signal processor; wherein at least one of the plurality of sentry unit node devices is operable to emit at least one transmission signal to a target area through at least one of the plurality of antennas; wherein one or more of the plurality of sentry unit node devices is operable to receive at least one return signal from the target area; wherein the at least one signal processor of the one or more of the plurality of sentry unit node devices is operable to analyze the at least one return signal; wherein the at least one signal processor is operable to detect and/or locate at least one target object in the target area based on the at least one return signal; wherein the underwater environment is a saltwater environment; and wherein the at least one transmission signal is an extremely low frequency (ELF) signal.

[Claim 2] The system of claim 1, wherein the plurality of sentry unit node devices is connected to at least one buoy or at least one other flotation device.

[Claim 3] The system of claim 2, wherein the at least one buoy or at least one other floatation device includes an onboard energy storage device and/ or energy generator operable to supply energy to the at least one sentry unit node device.

[Claim 4] The system of claim 1, wherein the plurality of sentry unit node devices are connected in series by at least one power cable and at least one data cable.

[Claim 5] The system of claim 1, wherein the plurality of sentry unit node devices and/or the plurality of antennas are buried beneath a seabed of the underwater environment.

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AMENDED SHEET (ARTICLE 19) [Claim 6] The system of claim 1, wherein the plurality of sentry unit node devices and the plurality of antennas are connected via fiber-optic communication, coaxial cable communication, and/or twisted pair cable communication.

[Claim 7] The system of claim 1, wherein the one or more of the plurality of sentry unit node devices is operable to receive at least one signal emitted by at least one subsurface vessel.

[Claim 8] The system of claim 1, wherein the system further includes at least one transponder operable to receive one or more above water signals and re-emit the one or more above water signals directed at the plurality of antennas.

[Claim 9] The system of claim 1, wherein the at least one signal processor is further operable to determine a velocity of the at least one target object.

[Claim 10] The system of claim 1, wherein each of the plurality of antennas is separated by at least 300 meters.

[Claim 11] The system of claim 1, wherein a multiplicity of the plurality of antennas are directly connected to one of the plurality of sentry unit node devices.

[Claim 12] The system of claim 11, wherein a multiplicity of the plurality of antennas are connected by cables in series with at least one of the multiplicity of the plurality of antennas directly connected to the at least one sentry unit node device.

[Claim 13] The system of claim 1, wherein the plurality of sentry unit node devices are connected to at least one surface energy storage device and/or at least one surface energy generator.

[Claim 14] A radar system for detecting ferrous and non-ferrous metals in an underwater environment, comprising: at least one support vessel; and an antenna system including at least one signal generator, at least one transmitter (Tx) antenna, at least one receiver (Rx) antenna, and at least one signal processor; wherein the at least one Tx antenna and the at least one Rx antenna are fixed in a cross-polarized orientation with each other or electrically isolated by distance and/or topography; wherein the at least one Rx antenna is substantially perpendicular to a direction of travel of the at least one support vessel; wherein the at least one signal generator is operable to emit at least one transmission signal to a target area through the at least one Tx antenna;

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AMENDED SHEET (ARTICLE 19) wherein the at least one transmission signal is an extremely low frequency (ELF) signal; wherein the at least one Rx antenna is operable to receive at least one return signal from the target area; wherein the at least one signal processor is operable to analyze the at least one return signal; wherein the at least one signal processor is operable to detect and/or locate at least one target object in the target area based on the at least one return signal; and wherein the underwater environment is a saltwater environment.

[Claim 15] The system of claim 14, further comprising a graphical user interface

(GUI), wherein the GUI is operable to display a visualization of the at least one target object in the target area.

[Claim 16] The system of claim 14, wherein the antenna system includes a plurality of Rx antennas for each of the at least one Tx antennas.

[Claim 17] The system of claim 14, wherein the at least one transmission signal is a plurality of transmission signals, and wherein the plurality of transmission signals includes at least two different frequencies and/or at least two different power levels.

[Claim 18] The system of claim 14, wherein the at least one signal processor is operable to identify at least one constructive interference zone and at least one destructive interference zone in the target area.

[Claim 19] The system of claim 14, wherein the support vessel is an active tow fish, a remotely operated vehicle (ROV), or an autonomous underwater vehicle (AUV).

[Claim 20] The system of claim 14, wherein information from the at least one signal processor is transmitted to land-based and/or water-based systems via fiber-optic communication, wired communication, and/or low frequency sub-channel radio frequency (RF) communication.

[Claim 21] The system of claim 14, wherein the at least one signal processor is operable to distinguish between different types of metal comprising the at least one target object.

[Claim 22] The system of claim 14, wherein the at least one Tx antenna and the at least one Rx antenna includes a plurality of Tx antennas and a plurality of Rx antennas, wherein the plurality of Tx antennas and the plurality of Rx antennas are interlinked via fiber-optic communication, wired communication, and/or low frequency sub-channel radio frequency (RF) communication.

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AMENDED SHEET (ARTICLE 19) [Claim 23] The system of claim 14, wherein the at least one Tx antenna and the at least one Rx antenna includes a plurality of Tx antennas and a plurality of Rx antennas, wherein the plurality of Tx antennas and the plurality of Rx antennas are arranged in a pattern to form a radio frequency (RF) fence, and wherein the radar system is operable to identify movement across the RF fence.

[Claim 24] The system of claim 14, wherein the at least one Rx antenna includes a plurality of Rx antennas, wherein the plurality of Rx antennas is arranged in at least two parallel lines.

[Claim 25] The system of claim 14, wherein the at least one signal processor is operable to determine a relative geolocation and/or an absolute geolocation of the at least one target object using a geolocation system.

[Claim 26] The system of claim 14, further comprising at least one amplifier board, wherein the at least one Rx antenna is connected to the at least one amplifier board using at least one discrete resistor network arranged in a parallel and/or series configuration, and wherein at least one switch and/or at least one digitally controlled relay is operable to adjust gain on at least one amplifier on the at least one amplifier board.

[Claim 27] The system of claim 14, further comprising at least one amplifier board, wherein the at least one amplifier board includes at least one narrow band filter to allow at least one high power out-of-band signal to be transmitted without interfering with reception of at least one in-band signal.

[Claim 28] A method for detecting ferrous and non-ferrous metals in an underwater environment, comprising: at least one support vessel traversing a target area; at least one signal generator emitting at least one transmission signal to the target area through at least one transmitter (Tx) antenna; at least one receiver (Rx) antenna receiving at least one return signal from the target area; at least one signal processor analyzing the at least one return signal; and the at least one signal processor detecting and/or locating at least one target object in the target area based on the at least one return signal; wherein the at least one transmission signal is an extremely low frequency (ELF) signal; wherein the at least one Tx antenna and the at least one Rx antenna are fixed in a cross-polarized orientation with each other;

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AMENDED SHEET (ARTICLE 19) wherein the at least one Rx antenna is substantially perpendicular to a direction of travel of the at least one support vessel; and wherein the underwater environment is a saltwater environment.

[Claim 29] The method of claim 28, further comprising the support vessel traversing at least one portion of the target area multiple times.

[Claim 30] The method of claim 28, wherein the at least one transmission signal is a plurality of transmission signals, wherein the plurality of transmission signals includes at least two frequencies and/or at least two power levels.

[Claim 31] The method of claim 28, further comprising the at least one signal processor identifying at least one constructive interference zone and at least one destructive interference zone in the target area.

[Claim 32] The method of claim 28, further comprising adjusting gain of at least one amplifier on at least one amplifier board using at least one switch and/or at least one digitally controlled relay, wherein the at least one Rx antenna is connected to the at least one amplifier board using at least one discrete resistor network arranged in a parallel and/or series configuration.

[Claim 33] The method of claim 28, further comprising the at least one signal processor identifying at least one metal comprising the at least one target object.

[Claim 34] The method of claim 28, further comprising the at least one signal processor determining a three-dimensional (3D) distribution of the at least one target object on and/or below a floor of the underwater environment.

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AMENDED SHEET (ARTICLE 19)