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.

It is often the case that to ensure high emissivity (and therefore low reflectivity) the calibration loads are made of corrugated or pyramidal structures. These corrugated/pyramidal structures are provided as regular arrays and are often made from, or coated with, microwave absorbing material. Calibration with such known devices still results in errors because they do not behave as a perfect black body with an emissivity of 1.0 and zero reflectivity.

This is particularly the case where the frequency of the calibration has a wavelength that corresponds to multiples of the periodicity of the regular arrays. This is a well know phenomenon termed 'Bragg reflection' and at least for those wavelengths, the calibration load is not a black body.

The present invention seeks to provide a calibration load that will ameliorate the difficulties associated with known devices.

According to a first aspect of the invention there is provided a calibration load for use in calibrating an electromagnetic sensor, said calibration load including at least one surface for absorbing electromagnetic radiation, said surface being provided with a plurality of protruding elements, at least a proportion of said protruding elements each having an axis that extends to form a portion that protrudes from the surface, with at least part of the axis of each of said protruding elements being offset by an angle from a perpendicular to the surface, such that said protruding elements can controllably absorb radiation directed at the surface thereby minimising the amount of radiation reflected from the surf ace .

The protruding elements are each provided , at a first end, with a base which can be fastened to a baseplate; and at a second end, remote from the first end, with a tip, said tip being at an angle offset to an axis that is perpendicular to the base.

Preferably, the protruding elements are mainly of a conical shape, as this minimises the presence of corners or edges at which diffraction can occur. However, pyramids may be used. However, pyramids of various shapes may be used, for example hexagonal, triangular or square .

The shape, e.g. the hexagonal or square shape may extend along the length of the protruding element or only partially along its length. More typically, the base of a protruding element will be hexagonal and the end of the element leading to the tip will be conical. Having a hexagonal base means that elements can be placed on a base plate such that the perimeter of each base is in contact with perimeters of adjacent elements so there is no risk of radiation reaching the base plate. This arrangement in effect tiles or tesselates the base plate with the protruding elements, ensuring that no area of the base plate is visible to incident radiation.

The length of the axis of each of the protruding elements may vary with some being longer that others and there may be areas of the elements that have the same axial length but other areas may have different axial lengths in order to increase the randomness of the array.

It is envisaged that the protruding elements may extend at a range of different offset angles from the perpendicular. Alternatively all the protruding elements may have an offset angle that is the same. In any event the protruding elements are arranged with the offset angles forming a random or pseudo-random array. The angle of offset may depend on the frequency of the radiation directed at it, but typically the angle of offset ranges from between 2° and 30°, more preferably between 2° and 20° and even more preferably 1.5° and 10°.

It is preferred that the protruding elements have a radiation-absorbing coating on at least part of their surface .

The coating may be uniform but preferably the coating is non uniform, with the coating being thicker in the area where the protruding element extends from the surface .

The calibration load may be provided by individual elements that are mounted on a base plate, but it is envisaged that an alternative structure may be provided, for example by casting where the elements are formed as an integral structure.

The calibration load may be circular with protruding elements being arranged as a random array that forms a radiation reflective surface for the calibration load. It is envisaged that there may be a varying density across the array with there being an increased density of elements towards the perimeter of the circle than in the centre of the circle.

According to a second aspect of the invention there is provided an electromagnetic sensor incorporating a calibration load, the calibration load including at least one surface for absorbing electromagnetic radiation, said surface being provided with a plurality of protruding elements, at least a proportion of said protruding elements each having an axis that extends to form a raised portion projecting from the surface, with at least part of the axis of each protruding element being offset by an angle from a perpendicular to the surface, such that said protruding elements can controllably absorb radiation directed at the surface thereby minimising the amount of radiation reflected from the surface.

Preferably the electromagnetic sensor is a radiometer, more particularly a microwave radiometer.

According to a further aspect of the invention there is provided a satellite including a calibration load or an electromagnetic sensor according to a first or second aspect of the invention.

It is to be understood that although individual embodiments are described, the descriptions in relation to one embodiment may apply to another embodiment.

Figure 1 shows a calibration load according to the prior art;

Figure 2 shows a protruding element of a calibration load according to the invention;

Figure 3 shows a sectional view of the protruding element of figure 2;

Figure 4 shows a sectional view of an alternative to the protruding element of figure 2;

Figure 5 shows a plan view of an array of protruding elements according to an embodiment of the invention; and Figure 6 shows a plan view of a calibration load incorporating an array of protruding elements as shown in figures 2 and 3.

As shown in Figure 1, black body arrangements that are used in calibration loads are known. However, such arrangements include cones A that are in regular arrays. The regular arrays will result in Bragg reflections of some wavelengths in certain directions, so that at least for those wavelengths the calibration load is not a black body .

Figure 2 shows a side view of a protruding element according to the present invention. The element is shown generally as 7 and it has a base 8, which is preferably hexagonal, and extending from it is a conical element 9. In this example the axis of the cone is offset by about 6° from the axis that extends orthogonal to the hexagonal base 8.

Figure 3 shows individual elements that have a nonlinear taper towards their end 10. The elements also have a coating 11, which is thinner towards the end of the element than at its base. Figure 4 shows an alternative arrangement where the individual elements have a uniform taper 12, and the coating 11 is also thicker towards the base of the element than at its tip. In both the elements of figure 3 and of figure 4 the protruding element is primarily of metal, to ensure good thermal conduction, and the coating provides microwave absorption. The coating 11 may be of a plastic loaded with a particulate material, for example epoxy resin loaded with iron powder. The elements may be of aluminium. They are mounted on a baseplate 15. Figure 5 shows a view from above of part of an array 18 of protruding elements 20 with offset tips. In this view the protruding elements 20 are shown as pyramids on square bases. A large number of these protruding elements 20 are assembled (for example onto a baseplate 15), but they are arranged in different orientations, randomly, so that there is no repeated pattern that would risk Bragg reflections. The entire array 18 constitutes the calibration load. It would incorporate thermometers, for example platinum resistance thermometers, embedded within the metal components of the protruding elements 20.

Figure 6 shows a plan view of part of an array 24 of conical protruding elements 7 (as in figure 2), each of which is provided with a coating 11 of microwave- absorbing material as described in relation to figure 3. The elements 7 have hexagonal bases 8 so they form a hexagonal array, but the inclined cone axes are arranged in random orientations, so there is no repeated pattern that would risk Bragg reflections. The entire array 24 constitutes the calibration load. It would incorporate thermometers, for example platinum resistance thermometers, embedded within the metal components of the protruding elements 7.

It will be appreciated that the examples given above are by way of example only, and that various modifications can be made while remaining within the scope of the present invention. In particular the randomness of the array may be increased by providing protruding elements 7 of different heights, so that both the heights and the orientations of the offset angles vary randomly in the array.

By way of example the random arrangement of the protruding elements (which may be pyramidal or conical) may be established by an iterative analysis process. If both the offset angles and the lengths are to be random, the process would start by generating three sets of random numbers: the first two sets being between the limits of the variability of the offset angles from the perpendicular, in two orthogonal directions; and the third set of random numbers being between the limits of variability of the length of the axis. One random value from each set would be generated for each of the protruding elements. It will be appreciated that such set of random numbers can be generated using a standard spreadsheet function.

Once the angular offset and lengths have been provided with random values in this manner, an iterative process is preferably carried out to ensure that, where any flat sides continue up the protruding element, the offset angles of adjacent protruding elements have not been made so large that the absorbing quality of the surface would be compromised. In this case, the angle between each pair of adjacent facing surfaces would be calculated and compared to a standard value. For example if the angle between two such facing surfaces were to be greater than say 40°, more preferably if it is greater than 30°, then the randomisation process may be repeated (at least for the protruding elements where this angle is greater than the specified value) .

Alternatively the above approach may be used to provide a random arrangement of conical or pyramidal protruding elements whose lengths do not vary, each projecting from a hexagonal base; in this case only two sets of random numbers would be needed, corresponding to the offset angles of the cone axes from the perpendicular to the base relative to two orthogonal directions, say the directions of X and Y axes in the plane of the base plate .