Lenses of Hyperspectral Video Recording Instruments of Satellite Surveillance and Record Systems
Nowadays, using the Russian spacecrafts "Resurs-P" No. 1 and No. 2 the detailed broad-band hyperspectral electrooptical observation of earth surface is performed for the first time .
For example, deciphering and determination of type and state of vegetation, soil identification, structure of contamination film on the surface of water refer to the short list of activities, which can be performed by hyperspectral space photographs. The peculiarity of hyperspectral video recording equipment of the spacecrafts "Resurs-P" consists in the capability of simultaneous surveying of the same section of earth surface within several narrow spectral ranges from 0.4 to 1.1 µm. It allows obtaining the high-informative images of earth surface for the solution of the tasks of customers: Ministry of Natural Resources and Environment of the Russian Federation, Ministry of Emergency Situations of the Russian Federation, Ministry of Agriculture of the Russian Federation, Federal Agency for Fishery, Federal Service for Hydrometeorology and Environmental Monitoring of Russia etc. (Fig. 1).
Spacecrafts solve the following tasks in very effective manner:
• compilation and update of general geographical, thematic and topographic maps;
• control of environmental contamination, ecological control in the regions of geological exploration works and mineral resources mining, control of water conservation and protected areas;
• inventory of natural resources (agricultural and forest lands, pastures, offshore operation areas), compilation of land cadastre and control of economical processes in order to ensure the rational activities in various sectors of economy;
• information provision of the search activities with respect to oil, natural gas resources, ore and other deposits of mineral resources;
• control of building development, obtainment of data for engineering evaluation of the area for the benefit of economic activities;
• information support for laying the mainlines and large-size buildings, motor roads, oil and gas pipelines, communication systems;
• evaluation of ice situation;
• surveillance of emergency areas for the purpose of monitoring of natural disasters, accidents, catastrophes and evaluation of their consequences and planning of remediation activities.
It is obvious that the number of tasks for future spacecrafts will be expanded. The technical solutions of optical schemes of the lenses intended for use in the systems of space surveillance have the crucial importance in ensuring the quality of information received from satellite systems . In particular, they will allow maintaining the competitiveness of electrooptical instruments of the Russian spacecrafts in future, which are planned for launching in the years to come. Let us consider four types of optical schemes, application of which significantly expands the number of tasks made with respect to the satellites of remote Earth sensing.
One of these tasks consists in the study of lightning, specifically one of its new types, which was recorded for the first time entirely by accident in 1989 and received the name "sprite". Sprite refers to the rare type of lightning discharges, a certain likeness of lightning striking in mesosphere and thermosphere. It is difficult to distinguish between different sprites but it is well known that they occur during the period of heavy thunderstorm at the height of 50–130 kilometers (at the same time, the height of formation of "standard" lightning is not more than 16 kilometers) and reach up to 60 km in length and 100 km in diameter. Sprites occur in tenth fractions of second after the strike of very severe lightning and last for the period of less than 100 milliseconds; most often they propagate simultaneously upwards and downwards, and the propagation downwards is noticeably larger and faster.
Sprites are polychromatic. The difference between colors can be explained by the various pressure and composition of atmosphere at different heights. At the height of 70 km the nitrogen gives red luminescence, and the closer to earth the greater pressure and amount of oxygen are, and the color of sprite changes to blue, light blue and white. Up until now, we have very little information about the physical nature of sprites. The photograph given in Fig. 3 shows how high the sprites can rise, and it appears that together with aurora, meteors and noctilucent clouds they seem to touch the space edge.
Taking into account the presence of chromaticity in sprites, the optical system, which allows recording them, must have the narrow operating spectral range. Due to dimensions, it is reasonable to provide its width with built-in interference filter.
In order to solve this task, the high-aperture wide-angle lens (Fig. 4) with built-in interference filter, which provides the operating spectral range from 0.7769 to 0.7779 µm, was estimated. The lens had the focal distance F = 9 mm, relative aperture 1:1 and angular field of view 98°. High light intensity and required quality of image in the suggested seven-lens objective are reached using the method of alternation of high and low refractive indices of glasses. Constructive data of the objective is given in table. Obtained scheme with specified technical characteristics is implemented with weight indices within the limits of 1070 g. Calculated values of sizes of confusions circles and graphs of frequency-contrast characteristics of the objective are given in Fig. 5 and 6.
Since this objective refers to the category of wide-angle systems, the distortion values are very interesting from the point of view of reliability of obtained final image. Graph of field distortion and curvature is shown in Fig. 7. According to the data obtained during calculations, it was managed to achieve the distortion, value of which does not exceed 27.9%. And in general this value is the classical distortion value for hundred-degree objective. Influence of such distortion can be demonstrated in quite graphic manner by the distortions, which occur during the transmission of square image (Fig. 8).
During the surveillance from spacecrafts, the medium and IR range of wavelengths is also actively used; it allows performing the recording through the haze, mist, smog, dust, smoke and clouds. Taking into account the fact that orbital spacecraft can perform the repeated recording of the same area of earth surface only with the interval of three days, the topicality of this task becomes obvious.
In order to obtain the operative information under adverse conditions, the spacecrafts are equipped with the optical systems operating within different spectral ranges. Equipment of surveillance systems not with multiple objectives for the operation within individual spectral ranges (visible, near, medium or far IR range) but with the objectives, which are able to operate within several ranges, became the obvious aspiration when solving such tasks.
Specifically for this purpose, two-range 300 mm objective (Fig. 9) with the relative aperture 1:3.5 and operating spectrums Δλ1 = (0.4–0.9) µm and Δλ2 = (1.5–1.7) µm was estimated. Obtained scheme with specified technical characteristics is implemented with mass parameters within the limits of 1680 g. Qualitative characteristics of two-range objective within every operating range are given in Fig. 10–11. It should be noted that during the calculation of frequency-contrast characteristic of the objective the following weight coefficients for different wavelengths of operating ranges are applied. Within the range Δλ1: 0.4 µm – 0.4; 0.5 µm – 0.6; 0.6 µm – 1; 0.7 µm – 1; 0.8 µm – 0.6; 0.9 µm – 0.3. Within the range Δλ2: 1.5 µm – 0.5; 1.55 µm – 0.7; 1.6 µm – 1; 1.65 µm – 0.7; 1.7 µm – 0.5.
The main peculiarity of such objective operating within the spectral range of 400–1700 nm consists in the capability of determination of atmosphere state and identification and recording of the special state of earth surface, individual components of which can significantly influence on image quality. For example, when analyzing the photographs obtained due to the use of such objective, white clouds can be distinguished from snow and ice, and different types of dust and smoke in the air can be distinguished from water steam.
However, for more accurate and efficient correction of photographs, the optical systems of other category are used – long-focus objectives with small field of view, which are intended for the operation within one spectral range, as a rule.
The long-focus objectives with the focal distance F=440 mm, relative aperture 1:3.28 and the same linear field 64 mm, which are given below, can be applied in order to solve such tasks, within the spectral ranges, which have been already considered, Δλ1 = (0.4–0.9) µm and Δλ2 = (1.5–1.7) µm. The optical scheme of the variant of such long-focus objective for the spectrum Δλ= (0.4–0.9) µm is shown in Fig. 12. The calculation is made for the combination of glass grades CaF2, STK19, LK5 and TF10. The total weight of optical components in obtained scheme does not exceed 2960 g. The qualitative characteristics of long-focus objective when operating within the spectral range from 0.4 to 0.9 µm are given in Fig. 13. When calculating the frequency-contrast characteristics of the objective for different wavelengths the following weight coefficients were applied: 0.4 µm – 0.4; 0.5 µm – 0.6; 0.6 µm – 1; 0.7 µm – 1; 0.8 µm – 0.6; 0.9 µm – 0.3.
The optical scheme of long-focus narrow-field objective with analogous technical parameters (focus, linear field of view and relative aperture) for operating spectral range from 1.5 to 1.7 µm has different form and is distinguished by the lower number of optical components (Fig. 14). The calculation is made for the combination of glass grades STK3, CaF2, BK8, LK5 and K8. The total weight of optical components in obtained scheme is 2360 g. It should be noted that the weight decrease of this variant of long-focus objective in comparison with the previous one is stipulated by the decrease of number of components, although the objective length increased by more than 70 mm. Qualitative characteristics of long-focus objective during the operation within the spectral range from 1.5 to 1.7 µm are given in Fig. 15. When calculating the frequency-contrast characteristics of objective for different wavelengths the following weight coefficients were applied: 1.5 µm – 0.7; 1.55 µm – 0.9; 1.6 µm – 1; 1.65 µm – 0.9; 1.7 µm – 0.7.
Taking into account the character of the tasks solved by satellite and other space surveillance systems, very important parameter of objectives installed in them consists in the consistency of image plane position in case of ambient temperature variation or thermal stability of objective. All considered objectives were evaluated by the value of image plane shift within the range of positive temperatures 5–35 °C.
When making the mechanical components of objectives from aluminum alloy D16, which has the coefficient of temperature expansion αD16T = 0.0000238 (1/mm·°C), the calculated shift of the plane of the best device by ±15 °C from the specified temperature 20 °C in this category of objectives does not exceed 0.3 mm.
Optical calculations demonstrate that such values can be compensated by axial displacement of one of the lenses, in other words, by variation of the relevant air gaps. It gives grounds for the use of common body with fixed mounting of the main components and photodetector and application of one compensating ring of movable lens, and the lens clamping to compensating ring can be performed with the help of commonly used spring steal ring, which compensates the temperature deformations (in our case, less than ±0.4 mm). Constructively such "floating" lens position is suggested to be arranged on the basis of us of the ring made of material with high value of temperature expansion coefficient, for example, material PA TU6–05–988–87. In order to compensate the inaccuracies occurring due to quite large length of compensating ring, if needed, the construction of "double" compensating ring can be used. In such case, it is important to provide the opportunity of compensation of coefficient of linear expansion of teflon F4 with the relevant construction of ring mounting .
Application of considered objectives allows analyzing the state of vegetation, coastline and offshore strip in details on the basis of obtained photographs, finding and identifying the set objects in more accurate manner. They will give opportunity to the satellite optical systems to take distinct photographs of surface despite the dust, soot and other interferences, which are present in the atmosphere of Earth.
Besides, in short-wavelength infrared region of spectrum the differences between materials become well noticeable, and it is of absolute interest during the prospecting of mineral resources, in control of water resources (Fig. 16) and in many other scopes of application.
When completing the instruments intended for the purposes of monitoring of forest areas, the preference should be given to multispectral objectives, which generate the multi-zone photographic images of forests and allow obtaining the photographs with high deciphering quality, in case of surveying during the whole surveying period, in summer and early automn.
In conclusion it should be noted that the results of space survey are in demand by military industry as well for the cases of detection and classification of objects. Thus, use of satellites of remote sensing of Earth is equally stipulated by the needs of military and civil users.