Radiometers

Abstract

Radiometers detect radio wavelength electromagnetic radiation and typically have an antenna ( 16 ), an amplifier ( 18 ) and a detector ( 20 ). All three of these components have response characteristics that may be dependent on temperature, and in the case of systems using radiometer arrays dependent upon temperatures throughout the system. Different temperatures across a multi-channel antenna and differential channel temperature response can result in poor image quality from imaging radiometers. Resolution of a linear array of detector horns is limited by the size of the horns. An imaging radiometer ( 10 ) uses a focussing polariser ( 36 ), a quarter wave plate ( 32 ), a rotating inclined disk ( 28 ), and a detector feed array ( 16 ) to perform a conical scan, and compensates fro variation in operating temperature of a radiometer using one or more of a variety of techniques including calibrating channels relative to each other, calibrating channels using reference temperatures in-situo, and calibrating channels using temperature response predictions stored in the radiometer signal processor ( 22 ). Aspects of the invention also optimise image resolution, image quality and allow calibration.

1 . A scanning radiometer imager having a field of view, focussing optics, a scanner, a detector array, and a control processor adapted to receive signals from the detector array, in which the detector array has an elongate length and has feed elements each feeding respective channels, the said feed elements being spaced along an elongated length or curve in the focal plane (or surface) of the focussing optics, and in which the scanner is in use controlled by the control processor to scan regions of an observed scene over the detector array, and in which the detector array comprises a first line of spaced apart feed elements and a second line of spaced apart feed elements, the feed elements of the first and second lines being offset from each other in the elongated direction of the length or curve.  
     
     
         2 . A radiometer according to  claim 1  in which the second line or curve extends generally adjacent to the first line or curve.  
     
     
         3 . A radiometer according to  claim 1  or  claim 2  which is adapted to conically scan the detector array in the imager field of view.  
     
     
         4 . A radiometer according to any preceding claim in which the feeds on each of the curves or lengths are adjacent to each other, disposed substantially as close to each other as the geometry of the feed elements permits.  
     
     
         5 . A radiometer according to any preceding claim in which there are more than two lines of feed elements.  
     
     
         6 . A radiometer according to any preceding claim adapted to detect radiation in a predetermined band of wavelengths, and in which for the lower end of the band the offset between centres of feed elements of different lines of feed elements is such as to reduce the effective inter-feed element centre spacing in the elongate direction to at worst Nyquist spacing, or near Nyquist spacing.  
     
     
         7 . A method of improving the resolution of an imaging radiometer of the kind having a curved or linear array of detector feed elements and a scanner which scans an observed scene, the method comprising providing an array having a plurality of curves or lines of detector elements, with the curves or lines being adjacent so that the effective feed spacing is smaller than can be achieved with a single array.  
     
     
         8 . A scanning imaging radiometer comprising scanner components, a detector, and a control processor adapted to control the operation of the scanner components; wherein the scanner components comprise a focussing element adapted in use to focus radiation onto the detector and having an optical axis, a detector field of view director comprising a rotatable inclined reflector plate whose normal is adapted to rotate at an angle inclined to the optical axis and a quarter wave plate provided disposed between the reflector plate and the focussing element; and 
 wherein the detector has a detector feed provided disposed between the focussing element and the reflector plate.    
     
     
         9 . A radiometer according to  claim 8  in which detector feeds are provided between the focussing element and the quarter wave plate.  
     
     
         10 . A radiometer according to  claim 8  or  claim 9  in which the focussing element also comprises a polariser adapted in use to transmit radiation of one polarisation and to reflect an orthogonal polarisation.  
     
     
         11 . A radiometer according to any one of  claims 8  to  10  in which the detector comprises an array having a number of feed elements, each feed element of which is adapted in use to receive radiation from a particular direction in the scene at any instant in time, and in which the control processor is adapted to scan each feed element over parts of the imager field of view.  
     
     
         12 . A radiometer according to any one of  claims 8  to  11  in which the detector array is disposed closer to the inclined reflector than the quarter wave plate.  
     
     
         13 . A radiometer according to any one of  claims 8  to  12  which comprises a conical scanner which selects circles or annuli in space in a scene being observed and focuses points or sections of the annuli disposed on different annuli onto respective individual detector elements of the detector array, different points or sections on any one annulus being focussed onto the same detector element, but at different moments in time, the conical scanner directing points or sections from a single annulus onto an individual detector element using the field of view director and focussing optics.  
     
     
         14 . An imaging radiometer having a focuser adapted in use to focus incident radiation onto a detector feed or a detector feed array of detector feed elements for normal imaging operation of the radiometer, and a defocuser adapted to defocus radiation from the scene that falls onto the detector feed or detector feed array so that each detector feed element experiences substantially the same radiation incident upon it, in use.  
     
     
         15 . A radiometer according to  claim 14  in which the defocusing element is the same element as the focuser, the focuser having a focussing configuration and a defocusing configuration.  
     
     
         16 . A radiometer according to  claim 15  in which the focussing element is adapted to be moved angularly to change from the focussing to defocusing configurations.  
     
     
         17 . A radiometer according to  claim 16  in which the focussing element is adapted to be moved angularly about 90° between the configurations.  
     
     
         18 . A radiometer according to any one of  claims 14  to  17  in which the focussing element comprises a polariser.  
     
     
         19 . A radiometer according to  claim 18  in which the focussing element is adapted to focus radiation of one polarisation and to transmit an orthogonal polarisation.  
     
     
         20 . A radiometer according to any one of  claims 14  to  19  in which a reflector and a polarisation-changing element are provided.  
     
     
         21 . A radiometer according to anyone of  claims 14  to  20  having an inclined reflector and a focussing polariser and wherein either the feed array, the inclined reflector, or the focussing polariser, or any combination thereof, can be moved along the imager axis for the purpose of focussing.  
     
     
         22 . A radiometer comprising a detector; a detector feed coupled to the detector; an amplifier receiving signals from the detector and providing amplified signals to a control processor; one or more temperature sensors sensing the temperature of the radiometer or of one or more temperature-sensitive components of the radiometer, and temperature control means; the temperature sensor in use providing temperature signals of the radiometer to the control processor indicative of the temperature of the radiometer or of the one or more temperature sensitive components, and the controller being adapted to control the temperature control means so as to maintain the temperature of the radiometer, or of the one or more temperature sensitive components, substantially constant in use.  
     
     
         23 . A radiometer according to  claim 22  in which the temperature control means comprises at least one thermoelectric device, the control processor controlling the electric current to the or each thermoelectric device in response to the temperature signals it receives.  
     
     
         24 . A radiometer according to  claim 22  or  claim 23  in which individual temperature sensitive elements have their own temperature control means and/or temperature sensors.  
     
     
         25 . A conical scanning radiometer imager having a reflector inclined at a tilt angle to an axis about which the reflector is adapted to rotate in use, in which tilt angle adjustment means is provided adapted to alter the tilt angle of the reflector relative to the axis about which it is adapted to rotate.  
     
     
         26 . A radiometer according to  claim 25  in which the tilt angle adjustment means is adapted to be operable whilst the reflector is rotating.  
     
     
         27 . A radiometer imager having a conical scan pattern in which radiation from a pixel in an observed scene space is directed onto a detector feed for a period of time when the optics of the scanning mechanism of the imager is appropriately aligned, electrical signals from a detector associated with the detector feed being correlated with a point in scene space to which they correspond by a control processor, and in which the integration time over which signals resulting from a particular point in space are allowed to integrate at the detector or control processor is controllable by the control processor, so as to provide an imager which has a variable pixel integration time.  
     
     
         28 . A radiometer according to  claim 27  in which the integration time for each pixel in the image can be varied independently of the integration times in other pixels.  
     
     
         29 . A radiometer according to  claim 27  or  claim 28  which has a rotable scanning disc adapted to scan radiation over said detector, and in which the pixel integration time is associated with the rotation speed of the reflector disc of the scanning optics.  
     
     
         30 . A radiometer according to any one of  claims 27  to  29  in which a control processor is adapted to determine whether the observed scene, or part of a scene, or an object in the scene, is moving faster than a predetermined speed, and is adapted to alter the pixel integration time to vary it if movement above a threshold speed was not established.  
     
     
         31 . A radiometer according to any one of  claims 27  to  30  in which pixel selection means is provided to select which pixels in the observed scene means have a modified integration time.  
     
     
         32 . A radiometer according to  claim 31  in which the pixel selection means is manually operable to select manually particular pixels or groups of pixels to have their integration time modified.  
     
     
         33 . A radiometer according to any one of  claims 27  to  32  in which the imager has an automatic pixel selection routine adapted to identify pixels of a certain category or categories and alter their integration times.  
     
     
         34 . A radiometer having at least one detection channel and a detector adapted to receive radiation acquired in use by the channel, a control processor, and an absolute reference temperature provider, the arrangement being such that the radiometer can be put, in use, in a calibration mode in which it resets the gain and/or offset associated with the, at least one, or each, channel by using signals derived from the reference temperature provider.  
     
     
         35 . A radiometer according to  claim 34  in which the absolute reference temperature provider comprises a holder for holding a substance of known temperature.  
     
     
         36 . A radiometer according to  claim 34  or  claim 35  in which there are two (or more) different absolute reference temperature providers, adapted to present different known temperatures to the radiometer.  
     
     
         37 . A radiometer according to any one of  claims 34  to  36  in which the or each reference temperature provider includes a temperature sensor adapted to provide signals indicative of the temperature of the reference temperature provider to the control processor.  
     
     
         38 . A radiometer according to any one of  claims 34  to  37  in which the or each reference temperature provider(s) comprises a thermo-electric device.  39 , A radiometer according to any preceding claim in which the radiometer has an array of detector feed elements and the or each reference temperature provider is provided adjacent to the detector feed array.  
     
     
         40 . A radiometer according to claim  39  in which the detector feed array and associated read-out connections extend away from the detector elements and are disposed in part of the optical path of radiation that enters the radiometer's image-acquiring aperture, and in which the reference temperature provider(s) are disposed in substantially the same projected position as the read out connections.  
     
     
         41 . A radiometer according to any one of  claims 34  to  40  in which the reference temperature provider comprises defocus means adapted to defocus an observed scene and thereby present substantially the same temperature to all detector channels.  
     
     
         42 . A radiometer according to any one of  claims 34  to  41  which has a switch adapted to cause the detector array to detect radiation from the reference temperature provider instead of focussed information from different scene pixels.  
     
     
         43 . A radiometer according to  claim 42  in which the switch comprises a polarisation altering component adapted to have a scene-observing position and a reference-temperature observing position, the switch being angularly movable between the reference temperature and scene observing positions.  
     
     
         44 . A radiometer according to  claim 42  or  claim 43  in which the switch comprises a quarter wave plate.  
     
     
         45 . A radiometer according to  claim 44  in which the quarter wave plate comprises a meanderline device having its fast or slow axis aligned with the plane of linearly polarised radiation during one of the modes of use and in another of its modes of use aligned at 45° to the plane of radiation incident upon the plate from the image capturing aperture of the device.  
     
     
         46 . A radiometer according to any one of  claims 34  to  45  which has a meanderline plate which for normal imaging of a scene is downstream of a linear polariser and has its fast or slow axis inclined to the plane of polarisation of radiation from the scene that has been linearly polarised prior to interacting with the meanderline plate, and which when in reference temperature observing mode has its fast or slow axis generally parallel to the plane of polarisation of the radiation incident upon it.  
     
     
         47 . A radiometer according to  claim 46  in which a linear polariser is provided upstream of the meanderline device.  
     
     
         48 . A radiometer according to  claim 47  in which the linear polariser is adapted to focus radiation onto the detector array.  
     
     
         49 . A signal processor, or software, which when loaded into a radiometer provide a radiometer in accordance with any preceding radiometer claim, or which when controlling the operation of a radiometer causes the method of any method claim of the invention to be performed.