CA2734960A1

CA2734960A1 - Calibration load

Info

Publication number: CA2734960A1
Application number: CA2734960A
Authority: CA
Inventors: Robert Paul Spurrett; Richard John Wylde; Axel Murk
Original assignee: ABSL Power Solutions Ltd
Current assignee: ABSL Power Solutions Ltd
Priority date: 2008-08-22
Filing date: 2009-08-20
Publication date: 2010-02-25
Also published as: GB0815352D0; EP2326928A1; WO2010020816A1

Abstract

A calibration load that may be used in calibrating a radiometer comprises a hollow disk (16, 18) of thermally conductive material defining a cavity (20), with an aperture (22) at the centre of one face of the disk. The aperture communicates with the cavity and is centred about an axis (11). The cavity (20) is defined between opposed surfaces (16, 18), and it tapers in the direction away from the axis, and these opposed surfaces are covered with radiation-absorbing material. A reflector (24) is arranged such that any radiation that enters the aperture (22) parallel to the axis (11) would be reflected on to a radiation-absorbing surface in the cavity. The reflector (24) may be generally conical. This calibration load (10) provides a black body of the dimensions of the aperture, or of the dimensions of the disk if the outer surface around the aperture is radiation-absorbing.

Description

Calibration Load Introduction The present invention relates to a calibration load that may be used for calibrating the sensors of a radiometer and in particular but not exclusively to calibration loads for calibrating radiometers operating at microwave frequencies.
Radiometers may be used to sense properties of remote objects, for example from satellites for observing the atmosphere and land surfaces, and for observing for example, stratospheric gases such as ozone, water vapour or chlorine species, deducing temperature profiles and water in the troposphere, as well as studying land topography. Microwave radiometers may operate at a frequency typically in the range 10 GHz to 800 GHz, depending on what measurements are to be made.
Components of a radiometer will have response characteristics that may be dependent on temperature. In the case of multi-channel radiometers designed to work over multiple frequency bands, different channels may have different temperature response characteristics and this can result in poor quality data.

Since the output signals of the radiometer are to some extent dependent upon the response characteristic of the radiometer, and any differences between different channels of a multi-channel radiometer, it is necessary to calibrate the radiometer at intervals during use.
This typically involves pointing the radiometer at one or more known sources of radiance. These may be calibration loads integrated within the radiometer or known external sources of radiance.

Conventionally calibration loads are either at ambient temperature i.e. the same temperature as the radiometer or they are heated or cooled, for example with a cryogen, such as liquid nitrogen. For example, by configuring the radiometer to alternately view a cold calibration load and then a hot calibration load, the response characteristic of the radiometer can then be calibrated.

In the case of satellites in space, typically a two point calibration scheme is used, whereby a view to deep space provides a known cold reference source of radiance (cosmic background radiation, which corresponds to a perfect emitter at a temperature of 2.73 K), and a calibration load provides a reference source which is preferably towards the upper end of temperatures that are to be sensed. Such calibration devices are sometimes called calibration hot loads, or black bodies.

There is a growing need for higher resolution data at lower microwave frequencies, which requires use of large aperture radiometers, and consequently there is an increasing demand for larger calibration loads. Known calibration loads are black body targets, that is to say objects with very high absorption and therefore very high emissivity at the operating frequency of the radiometer.
The temperature of the calibration load must be accurately known, and ideally should be uniform over the field of view of the radiometer. The temperature can be measured using a thermometer such as a platinum resistance thermometer. However ensuring high emissivity, preferably above 0.999, over a large area is difficult to achieve, particularly if there are limitations on the dimensions of the calibration load.

If the emissivity of the calibration load is not sufficiently high then extraneous signals may be reflected into the radiometer and may degrade the precision of the calibration.

Invention According to a first aspect of the present invention there is provided a calibration load comprising a hollow structure of thermally conductive material defining a cavity, the cavity being defined between opposed walls, and with an aperture in one wall communicating with the cavity such that an incident axis of the load extends through the aperture, and the cavity tapering in a direction away from the axis, wherein the opposed walls defining the cavity are covered with radiation-absorbing material, and wherein there is a reflector arranged such that any radiation that is incident on the aperture parallel to the incident axis would be reflected on to a radiation-absorbing surface of a wall in the cavity.
Preferably the calibration load is symmetrical around the axis. Hence the invention preferably provides a calibration load comprising a hollow disk of thermally conductive material defining a cavity, with an aperture at the centre of one face of the disk communicating with the cavity, the disk and the aperture being centred about an axis, the cavity being defined between opposed surfaces and tapering in the direction away from the axis, wherein the opposed surfaces defining the cavity are covered with radiation-absorbing material, and wherein there is a reflector within the cavity arranged such that any radiation that enters the aperture parallel to the axis would be reflected on to a radiation-absorbing surface of the cavity.
Preferably the radiation reflector is generally conical, with a straight, elliptical or parabolic profile although a pyramidal reflector may alternatively be used.
Preferably the cavity is of circular or annular cross-section (in a plane orthogonal to the incident axis).
Preferably the cavity tapers to zero width at its furthest distance from the axis.

Any radiation incident on the aperture would be reflected on to a radiation absorbing surface of the cavity, being initially incident on the surface around the aperture, and undergoes repeated reflections between the opposed surfaces of the cavity. Since those surfaces are covered with radiation-absorbing material, any such incident radiation is very effectively absorbed. Hence the aperture acts as a perfect black body for the radiation. The radiation-absorbing material must be appropriate for the radiation to which the radiometer is sensitive. In particular the calibration load may be used in conjunction with an infrared radiometer or a microwave radiometer. If it is to be used for calibrating a microwave radiometer, then the radiation-absorbing material must be microwave-absorbing material.
It therefore acts as a black body emitter, at least as regards microwaves.
Although the reflector may be conical (that is to say, the surface being a straight line in axial section), it may alternatively be curved slightly concavely (that is to say the surface in axial section being a concave curve) arranged to focus radiation incident along the axis towards a focal region beyond the surface surrounding the aperture.

The calibration load described above provides a near perfect black body of the dimensions of the aperture. If a larger black body is required, this can be provided by a larger aperture, or by covering the outer surface of the disk around the aperture with a non-reflective structure. This implementation of the invention may be particularly effective if, as is usually the case, the sensitivity of the radiometer is concentrated towards its axis.

According to a second aspect of the present invention there is provided a calibration load comprising a hollow structure of thermally conductive material, the structure comprising an outer wall defining a radiation-absorbing surface generally orthogonal to an incident axis of the calibration load and defining an aperture in the outer wall, and the structure defining a cavity, the cavity being defined between opposed walls, and the chamber tapering in a direction away from the axis, wherein the opposed walls defining the cavity are covered with radiation-absorbing material, and wherein there is a reflector arranged such that any radiation that is incident on the aperture parallel to the incident axis would be reflected on to a radiation-absorbing surface of a wall in the cavity.

In both aspects of the invention the outer surface surrounding the aperture may define an array of tapered grooves, for example concentric V-grooves of progressively greater radius, or an array of pyramids or cones covering the surface, and coated with radiation-absorbing material. In this case the effective area of the black body is the entire projected area of the disk.
Preferably the width of the aperture is at least 10%, more preferably at least 15%, of the width of the disk or outer wall; a preferred embodiment has the aperture with a width preferably between 15% and 25% of the width of the disk, most preferably about 20%. Preferably the depth of the cavity at its centre is less than 30% of its width, more preferably less than 20% of its width.
If the calibration load is for calibration of a microwave radiometer, the reflector is preferably an absorber of infrared radiation, as temperature uniformity between the reflector and the remainder of the structure is desirable, and therefore the reflector is preferably provided with an infrared-absorbing coating, for example a matt black paint or a black anodised surface. Such surface finishes do not change the highly reflective nature of the surface to microwaves.

Preferably the thermally-conducting material of the structure is a metal, for example copper or aluminium.
Aluminium is a preferred material. The microwave-absorbing material may be a polymer loaded with an absorbing particulate material, for example an epoxy resin loaded with iron powder.

Detailed Description The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which:
Figure 1 shows a perspective view of a calibration load;
and Figure 2 shows a cross-sectional view of the calibration load of figure 1.

Referring to the figures, a calibration load 10 consists of an aluminium structure 12 of generally disk shape, of diameter 500 mm and of total thickness 75 mm, mounted on a backing plate 14, so the calibration load 10 has an axis 11 (indicated by a broken line) through its centre. The casting 12 may be in two parts: a lower part 16 and an upper part 18 that are joined around their periphery 17, and which each slope linearly away from the periphery 17 so as to define a cavity 20 with a V-shaped cross-section. The upper part 18 defines a central circular aperture 22 of diameter 105 mm (i.e. just over 20% of the diameter of the calibration load 10), so that the aperture 22 and the periphery 17 are coaxial with the axis 11; while the lower part 16 defines a central cone 24 that projects through the aperture 22, and has a base that is also of diameter 105 mm, so the cone 24 is coaxial with the axis 11.

In this example the cone 24 is shown as having a curved shape (in axial section). For example it may have a parabolic curvature, and this curvature may be such as would focus any radiation incident parallel to the axis 11 towards a focal point beyond the internal surface of the upper part 18; this would tend to distribute any such incident radiation over an annular region of the surface of the upper part 18 concentric with the aperture 22 but spaced radially away from it.

The outer surface of the upper part 18 is entirely covered by an array of pyramids 26, of graded sizes so that the tops of the pyramids 26 define a plane. The pyramids 26 are each of generally square plan, defined between a multiplicity of radial lines and a multiplicity of concentric circles, as shown in figure 1. The aluminium structure 12 may be made by milling disks of aluminium, one to form the lower part 16 and the other to form the upper part 18. The pyramids 26 may also be made by milling, so they are integral with the aluminium structure 12, for example milling of concentric circular V-grooves and then milling of radial V-grooves.

The outer surfaces of the pyramids 26 are coated with microwave-absorbing material. The opposed sloping surfaces of the lower part 16 and the upper part 18 (within the cavity 20) are also completely covered with a uniform layer of microwave-absorbing material (not shown), which may be for example of thickness 2 or 3 mm.
In contrast, the surface of the central cone 24 has a black anodised finish, so that it is an absorber of infrared radiation but a very good reflector of microwaves.

It will be appreciated that if a beam of microwaves were incident on the calibration load 10 parallel to and centred on the axis 11, the central part of the beam would pass through the aperture 22 and be reflected by the cone 24, onto the absorbing coating on the opposed sloping surfaces of the lower part 16 and the upper part 18. Any microwaves not absorbed would undergo successive reflections between those opposed sloping surfaces, further absorption taking place on each successive reflection. Consequently the aperture 22 is a very effective absorber of microwave radiation. Similarly, the outer part of the beam, if it is larger than the aperture 22, would be incident on the pyramids 26 where again it would be absorbed by the absorbing coating, and any microwaves not absorbed would undergo successive reflections between opposed sloping surfaces of adjacent pyramids 26, further absorption taking place on each successive reflection. Consequently the array of pyramids 26 is also a very effective absorber of microwave radiation. It follows that the calibration load 10 will act as a near perfect black body, and can be assumed to emit radiation characteristic of a near perfect black body of its temperature. The calibration load 10 therefore includes several thermometers, preferably platinum resistance thermometers (not shown) embedded in the aluminium, so that the mean temperature of the calibration load 10 can be measured accurately.
Thus the calibration load 10 may be installed on a satellite within the view of a microwave radiometer. At intervals the microwave radiometer would be aimed along the axis 11 of the calibration load 10, and the microwave radiation emitted by the calibration load 10 detected.
Since the calibration load 10 acts a near perfect black body radiator, the microwave intensity emitted by it depends only on its temperature, and may therefore be used in calibrating the radiometer. It would usually be combined with observations with the radiometer aimed at deep space from which the microwave radiation is the cosmic background radiation, and corresponds to black body radiation from a source at about 2.73 K.

For accurate calibration the temperature of the emitting surface must be known accurately. A problem with some previous designs of calibration load is that the emitting surface may be heated by incident solar radiation; although a thermometer embedded in an aluminium structure enables the temperature of the adjacent aluminium to be measured, the thermal conductivity of the microwave-absorbing coating is typically much less than that of aluminium, so there is a risk that there may be a significant temperature difference between the outer emitting surface and the vicinity of the thermometer. This problem is significantly alleviated by the calibration load 10, because the radiometer will be at least predominantly sensitive to radiation from the aperture 22, and the corresponding emitting surfaces are the sloping surfaces on opposite sides of the cavity 20, and these surfaces are well shielded from solar radiation.

It will also be appreciated that a calibration load installed on a satellite is thermally linked to its environment by any mechanical connections to the spacecraft, and also by radiative coupling. Radiative coupling may include views to parts of the spacecraft structure, deep space, and intermittent views of the sun.
Preferably the calibration load 10 is therefore shielded, where possible, from transient views to deep space or to sunlight, and is preferably connected to the spacecraft structure by a coupling that provides for some thermal conduction.

It will be appreciated that the calibration load 10 described above is given by way of example only, and that it may be modified in various ways while remaining within the scope of the present invention. For example the dimensions are given by way of example only. The aperture 22 is preferably as large as possible, so that the radiometer is predominantly or exclusively sensitive to radiation from the aperture; this depends on the field of view of the radiometer, but preferably at least 80%
and more preferably at least 90% of the sensitivity is to radiation from the aperture 22. Typically it can be assumed that the sensitivity of the radiometer will vary in a Gaussian fashion with radial distance from the axis, as a consequence of the associated optics. However, the larger the aperture, for a given diameter of calibration load 10, the shorter is the absorbing cavity 20 in the radial direction. Preferably the aperture 22 occupies not more than 25% of the total width of the calibration load 10, to provide sufficient depth to the absorbing cavity 20 in the radial direction. In some cases, where the field of view of the radiometer is smaller than the aperture 22, the outer surface of the upper part 18 may be substantially flat or uniformly sloped, without the pyramids 26 and without requiring any microwave-absorbing coating (although it may be provided with a microwave-absorbing coating in any event, in case the radiometer is not accurately aligned with the axis 11).

In a further modification, the reflecting cone 24 may have a reflective metallic surface. And the reflecting cone 24 may have a different shape, for example being strictly conical (so as to be a straight line in axial section). Furthermore the reflecting cone 24 may be slightly shorter than shown, so that its point does not project through the aperture 22. Additionally, the reflective cone may be finished in a material which is highly emissive at thermal infrared frequencies to improve the thermal uniformity of the cavity.
In a further modification, a calibration load may be provided by a structure defining an outer wall (corresponding to the upper part 18, with the central aperture 22 and the non-reflective structure provided by the pyramids 26), the structure also defining a single conical absorbing cavity behind the outer wall whose axis extends away from the incident axis of the calibration load, and with a reflector so that any radiation incident on the aperture 22 parallel to the incident axis 11 would be reflected into the conical absorbing cavity. In this modification, to minimise the depth of the structure, the axis of the conical absorbing cavity is preferably orthogonal to the incident axis.

Claims

1. A calibration load comprising a hollow structure of thermally conductive material defining a cavity, the cavity being defined between opposed walls, and with an aperture in one wall communicating with the cavity such that an incident axis of the load extends through the aperture, and the cavity tapering in a direction away from the axis, wherein the opposed walls defining the cavity are covered with radiation-absorbing material, and wherein there is a reflector arranged such that any radiation that is incident on the aperture parallel to the incident axis would be reflected on to a radiation-absorbing surface of a wall in the cavity.

2. A calibration load according to claim 1 wherein the structure is a disk, with an aperture at the centre of one face of the disk communicating with the cavity, the disk and the aperture being centred about an axis.

3. A calibration load as claimed in claim 1 or 2 wherein the radiation reflector is generally conical.

4. A calibration load as claimed in claim 3 wherein the radiation reflector has a surface that in axial section is a concave curve.

5. A calibration load as claimed in any one of claims 1 to 4 wherein the cavity is of circular cross-section and tapers to zero width at its furthest from the axis.

6. A calibration load as claimed in any one of the preceding claims wherein the depth of the cavity at its centre is less than 30% of its width, more preferably less than 20% of its width.

7. A calibration load as claimed in any one of the preceding claims wherein the outer surface of the structure around the aperture is provided with a radiation-absorbing surface.

8. A calibration load comprising a hollow structure of thermally conductive material, the structure comprising an outer wall defining a radiation-absorbing surface generally orthogonal to an incident axis of the calibration load and defining an aperture in the outer wall, and the structure defining a cavity, the cavity being defined between opposed walls, and the chamber tapering in a direction away from the axis, wherein the opposed walls defining the cavity are covered with radiation-absorbing material, and wherein there is a reflector arranged such that any radiation that is incident on the aperture parallel to the incident axis would be reflected on to a radiation-absorbing surface of a wall in the cavity.

9. A calibration load as claimed in claim 7 or claim 8 wherein the non-reflective surface on the outer surface of the structure around the aperture is defined by an array of V-grooves, coated with radiation-absorbing material.

10. A calibration load as claimed in claim 9 wherein V-grooves extend in radial directions and in circumferential directions, so as to define an array of pyramids.

11. A calibration load as claimed in any one of the preceding claims wherein the width of the aperture is at least 10%, more preferably at least 15%, of the width of the structure.

12. A calibration load as claimed in claim 11 wherein the aperture has a width between 15% and 25% of the width of the structure.

13. A calibration load for calibration of a microwave radiometer as claimed in any one of the preceding claims wherein the reflector is provided with a surface that absorbs infrared radiation.

14. A calibration load as claimed in any one of the preceding claims wherein the thermally conductive material comprises aluminium.

15. A calibration load substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

16. A satellite including a radiometer and a calibration load as claimed in any one of the preceding claims.