Underwater communications

1. An environmental monitoring system comprising at least one underwater environmental measurement device and a transmitter for transmitting data from the measurement device in water to a receiver in air at an above water station using low frequency electromagnetic and/or magnetic signals, and/or a receiver for receiving data and/or commands from a transmitter in air at the above water station using low frequency electromagnetic and/or magnetic signals, wherein the transmitter and receiver include a magnetically coupled antenna.

2. A system as claimed in claim 1 wherein measured data is modulated on the electromagnetic or magnetic signal.

3. A system as claimed in claim 1 , further including a processor, the processor executing one or more command signals received by the underwater receiver from the above water station.

4. A system as claimed in claim 1 , wherein the submerged transmitter is operable intermittently according to a predetermined pattern of transmission periods or when instructed to be so operable by one or more command signals received by the underwater receiver from the above water station.

5. A system as claimed in claim 4 , wherein the pattern of transmission periods is capable of being changed by one or more command signals received at the submerged receiver from a remote location.

6. A system as claimed in claim 1 , wherein the submerged receiver is operable substantially only during periods when a command signal might be expected periodically or according to a predetermined schedule.

7. A system as claimed in claim 1 , wherein the transmitter is caused to transmit data as and when it becomes available.

8. A system as claimed in claim 1 , wherein the underwater device is operative to report the condition of its battery supply periodically, or on a received command, or when the battery reaches a certain state of discharge.

9. A system as claimed in claim 1 , wherein the underwater device is capable of performing a self-testing or assessment process on receipt of a command signal and transmitting the result of the process to the above water station.

10. A system as claimed in claim 1 , wherein one or more of the antennas is omnidirectional.

11. A system as claimed in claim 10 wherein the omni-directional antenna comprises three orthogonal antennas.

12. A system as claimed in claim 1 further comprising a waterproof housing, wherein the measurement device and transmitter/receiver circuitry are included in the housing.

13. A system as claimed in claim 1 wherein the transmitter is operable to transmit a signal to allow its underwater location to be determined.

14. A system as claimed in claim 13 wherein the position location signal is a regular operational signal or a signal sent as a result of a command for the purpose or a signal sent in response to one or more predetermined conditions detected by the measurement device.

15. A system as claimed in claim 1 wherein the measurement device is operable to measure one or more of the following: flow rate; water depth by pressure sensing; water depth by ultrasonic techniques; temperature; acidity/alkalinity (pH); conductivity; dissolved oxygen content; chlorophyll density; salinity and presence of chemicals such as ammonium salts and nitrates; presence of other specific pollutants determined by chemical analysis sensors; presence of blue-green and other potentially toxic algae; particulate content; and turbidity.

16. A system as claimed in claim 1 , including an intermediate relaying station.

17. A waterproof sonde comprising at least one sensor for sensing at least one environmental parameter, and a transmitter for transmitting low frequency electromagnetic and/or magnetic signals carrying data from the sensor to an above-water location using a magnetically coupled antenna.

18. A sonde as claimed in claim 17 comprising a receiver for receiving low frequency electromagnetic or magnetic signals from the above-water location using a magnetically coupled antenna.

19. A sonde as claimed in claim 17 wherein the sensor is operable to measure one or more of the following: flow rate; water depth by pressure sensing; water depth by ultrasonic techniques; temperature; acidity/alkalinity (pH); conductivity; dissolved oxygen content; chlorophyll density; salinity and presence of chemicals such as ammonium salts and nitrates; presence of other specific pollutants determined by chemical analysis sensors; presence of blue-green and other potentially toxic algae; particulate content; and turbidity.