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A measurement device, such as a laser radar, is described that comprises a transmitter portion for transmitting radiation to a remote probe volume, a receiver portion for detecting radiation returned from the remote probe volume and a processor for analysing the detected return radiation. The device

A measurement device, such as a laser radar, is described that comprises a transmitter portion for transmitting radiation to a remote probe volume, a receiver portion for detecting radiation returned from the remote probe volume and a processor for analysing the detected return radiation. The device is suitable for transmitting and receiving radiation through a window portion having associated window cleaning apparatus (e.g. wiper 62) for cleaning said window portion in response to a cleaning activation signal. The processor is arranged to generate a cleaning activation signal for activating window cleaning apparatus when the properties of said detected return radiation are indicative of a reduction in transmission through at least part of said window portion. In one embodiment, a conically scanned lidar is described that generates a cleaning activation signal when the power of detected return radiation varies as a function of the angle of transmission of radiation from the device.

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The invention claimed is: 1. A measurement device comprising a transmitter portion for transmitting radiation to a plurality of points within a remote probe volume through a window portion, a receiver portion for detecting radiation returned through said window portion from the remote probe volume

The invention claimed is: 1. A measurement device comprising a transmitter portion for transmitting radiation to a plurality of points within a remote probe volume through a window portion, a receiver portion for detecting radiation returned through said window portion from the remote probe volume and a processor for analysing the detected return radiation, said window portion having associated window cleaning apparatus for cleaning said window portion in response to a cleaning activation signal, wherein the processor is arranged to determine the average return power of detected return radiation collected from each of a plurality of mutually exclusive angular segments, each angular segment corresponding to a range of angles of transmission of radiation from the device and to generate a cleaning activation signal for activating window cleaning apparatus when the power of detected return radiation varies as a function of the angle of transmission of radiation from the device. 2. A device according to claim 1 wherein the transmitter portion comprises a scanning mechanism. 3. A device according to claim 1 wherein said plurality of angular segments comprises at least 4 angular segments. 4. A device according to claim 1 in which the processor is arranged to determine a first angular segment having the highest average return power and a second angular segment having the lowest average return power, wherein the processor generates a cleaning activation signal if the average return power of the first angular segment substantially exceeds the average return power of the second angular segment. 5. A device according to claim 4 wherein a cleaning activation signal is produced if the average return power of the first angular segment exceeds the average return power of the second angular segment by a factor greater than two. 6. A device according to claim 1 in which the processor is arranged to calculate the standard deviation of the average return power for each segment from the mean average return intensity of all segments, wherein a cleaning activation signal is produced if the standard deviation exceeds a predetermined value. 7. A device according to claim 1 wherein the processor is arranged to determine the normalised return power for each measurement, said normalised return power being calculated from the power of the detected return and an associated return power reference value. 8. A device according to claim 1 wherein the processor is arranged to output an error signal if a cleaning activation signal is persistently generated. 9. A device according to claim 1 comprising communication means for contacting a remote centre. 10. A device according to claim 1 wherein the transmit beam formed by the transmitter portion and the receive beam defined by the receiver portion are arranged to continually intersect in the remote probe volume during use. 11. A device according to claim 1 wherein said transmitter portion and said receiver portion comprise a monostatic transceiver. 12. A device according to claim 1 wherein the processor is arranged to determine any Doppler shift of return radiation detected by the receiver portion. 13. A device according to claim 12 wherein the processor is arranged to calculate wind speed from the Doppler shift of backscattered radiation collected by the receiver portion. 14. A device according to claim 12 wherein the processor is arranged to analyse the Doppler power spectrum of each measurement for the presence of rain. 15. A device according to claim 14 wherein analysis of the Doppler spectrum comprises analysing the “spikiness” of each Doppler spectrum. 16. A device according to claim 1 wherein the transmitter portion, in use, transmits continuous wave radiation to the remote probe volume. 17. A device according to claim 1 wherein the transmitter portion, in use, transmits pulsed radiation to the remote probe volume. 18. A weatherproof device comprising a device according to claim 1 and a weatherproof housing, said weatherproof housing comprising a substantially transparent window portion and a window cleaning mechanism wherein the device is arranged to transmit and receive radiation through the window portion of the weatherproof housing. 19. A weatherproof device according to claim 18 wherein the window cleaning mechanism comprises a wash and wipe mechanism. 20. A method of acquiring a measurement comprising the steps of; (i) transmitting and scanning radiation to a plurality of points within a remote probe volume, (ii) detecting radiation returned from the remote probe volume, (iii) analysing the detected return radiation using a processor to determine the average return power of detected return radiation collected from each of a plurality of mutually exclusive angular segments, each angular segment corresponding to a range of angles of transmission of radiation from the device; and (iv) generating a cleaning activation signal for activating window cleaning apparatus when the power of the detected return radiation varies as a function of the angle of transmission of radiation from the device wherein steps (i) and (ii) further comprise transmitting and receiving radiation through a window portion having associated window cleaning apparatus for cleaning said window portion in response to a cleaning activation signal.