Design of a High Resolution Multispectral Scanner for Developing Vegetation Indexes

Design of a High Resolution Multispectral Scanner for Developing Vegetation Indexes Rishitosh kumar sinha*, Roushan kumar mishra, Sam jeba kumar, Gunasekar. S Dept. of Instrumentation & Control Engg. S.R.M University, Tamil Nadu, India *Corresponding author, Email Id: rishisrmice@gmail.com Abstract: The collection of accurate, timely information of the world s food and fiber crops will always be important. The collection of such information s using in situ techniques is expensive, time consuming, and often impossible. An alternative method can be done by analyzing the multispectral image. Most of the research in this area has involved the analysis of the Landsat multispectral sensor, thematic mapper, SPOT HRV data using digital image processing techniques. The goal has often been to reduce the multiple bands of data down to a single number per pixel that predicts or assesses such canopy characteristics as biomass, productivity, leaf area index, amount of photosynthetically active radiation consumed, or vegetation ground cover. This work develops a MSS (multi spectral scanner) which can capture images of various vegetation cover & of other land characteristics in the wavelength region varying from (400 1100 nm), with a resolution of 10 m, covering a swath (subject width) of 151.7 km at a time. The choice of wavelength ranges enables the MSS to delineate between features such as waste grass, dry bare soil, green grasses etc. The choice of materials to develop this miniaturized MSS enables it to sustain the adverse conditions at an altitude of 650 900 km. The volume occupied by the MSS is 150x10x10 mm respectively which enables it to get fit into NANO & PICO satellites. The design of optics of this MSS with an angle of view equal to 12.50 deg makes it flexible so as to act as other massive MSS S capable of producing multispectral images under its respective wavelength region. The detector material has been selected so that it can absorb almost all the radiations falling on it. The revisit time or period being 1.64 hrs & inclination angle of the MSS has also been taken under considerations so as to maintain the preciseness of the image captured by the MSS. Incorporating this optical design we can easily & accurately develop various vegetation indexes so as to perform the analysis of timely information s of the world s food & fiber crops. I. INTRODUCTION In this present trend of miniaturization, technology is taking various shapes so as to generate solutions without much labour & material concern. It hardly bothers about the objective, but still considerations have been given to how we achieve it. In this work also we are giving more concern to how we are achieving our objective, & exactly not giving any sort of concern to those comparable technologies which have been generated earlier to do the same purpose. There are n numbers of ways to achieve something, but according to present trend it s like how easily you achieve it, & how miniaturized the technology is. Also this miniaturization should not affect the quality of the outcome when the same is asked for comparison with the existing technologies. A new approach has been developed in this, where we are developing vegetation indexes for the type of vegetations being done, incorporating the optical design of multi spectral scanner (mss). Suitable band selections have been done to delineate the 1

various types of vegetation covers present on the earth surface. By analyzing the image obtained from the scanner we are developing these vegetation indexes which can clearly describe the type of vegetation being done over a particular area on the earth surface. II. OBJECTIVES To be a little brief in the initial steps, the objective of this miniaturized technology is mainly aiming towards designing a multi spectral scanner which can delineate the various features of type of vegetation being done on the earth surface. Since, it has been a tough task always to determine the vegetation index by analysis of the Landsat multispectral sensor, thematic mapper, SPOT HRV data using digital image processing techniques. So, images captured by this scanner can be analysed, & then a set of data providing details about the type of vegetation, or in simple words an index can be prepared for the type of vegetation being done. The design of this multi spectral scanner enables us to achieve our objective. The specifications of the MSS can be summarized as below: Table 1. Specifications of MSS System parameter Altitude Velocity Swath Wavelength Range Volume View Angle Resolution Specifications 650 900 km 7.512 x 10 3 m/s 151.7 km 400 1100 nm 150x10x10 mm 12.5 degrees 10 meters Time Period 1.64 hrs Payload Develop vegetation index Mass 100 gms Power < 4 watts Consumption This is the detailed specification of the MSS, designed as per the objective so as to achieve it. This MSS can be widely used in PICO/NANO satellites, as the specifications of the MSS matches with the specifications given for sensor of these satellites. III. Design of MSS This MSS is designed mainly for capturing high resolution images, i.e. a resolution upto 10 meters. We can incorporate this design with both PICO & NANO satellites. The area covered on earth at one shot is between the range 140 160 kms. The exposure time of the lens is yet not decided, but will be decided as it s confirmed. The tilt angle of the scanner is yet not obtained. The use of proper materials has been done to make it suitable so as to sustain the advanced conditions of the low earth orbits. A. List of tangible elements in the (i) (ii) design Two convex lens, made of silicon with focal length 2o mm, CCD detector, 2

(iii) Scanner with casing of silicon mirrors & also with an appropriate coating. B. Mathematic realization of image from the optics The optical radiation reflected back from the field of view falls on the first convex lens, & it happens during the exposure of the scanner towards the earth. C. Case 1. Refraction at the first curved surface Focal length of lens = 20 mm, Radius of lens = 10 mm (R), Object distance (here object is the field of view of the scanner) = 7x 10 8 mm, Refractive index of air = 1 (n 1 ), Refractive index of lens = 1.5 (n 2 ). So, from the formula for refraction at curved surfaces, we have n 2 / V n 1 / U = n 2 n 1 / R Where, V is the image distance obtained after first refraction at the first lens. 1.5 / V 1 / (7 X 10 8) = (1.5 1) / 10 Here, 1 / (7 X 10 8 ) = 0 (assumption) 1.5 / V = 0.5 / 10 or V = 30 mm (Positive sign of V indicates that the image is formed at a distance of 30 mm from right hand side of the lens). Nature of image formed after first refraction will be as: (i) Real, & between F & 2F, (ii) Inverted, (iii) Diminished (m< 1) Here M is magnification. Magnification=size of image/size of object = distance of image/distance of object In this magnification, ( ) sign is there since the object distance for a convex lens is always negative, when ratio is taken, it comes as negative in nature. Verification by the relation M=V/U or I/O. So, after the first refraction, the image is diminished. So, we need to obtain an enlarged form of the object, i.e. we need to view the subject in an enlarged format, but not in diminished format. So, a second refraction has to be performed, so as to make it fit into the requirements. D. Refraction at the second curved surface The second refraction is done so as to fulfill the requirements. Keeping in mind that the image should be enlarged after the refraction, & the distance of the image, i.e. the position of the image formed should fall under the dimensions provided, so as to make it fit into NANO/PICO satellites. So, in the second refraction, the object for the second lens will be the image formed by the first lens. After refraction from the first lens, the image 3

is formed between F & 2F of first lens. Since, the focal length of both the lenses are same, the first image formed between F & 2F will be acting as an object placed between F / & 2F / for the second lens. So, it s quite clear that position of object for the second lens is between F / & 2F / of the second lens on the left hand side of the lens. Focal length of second lens= 20 mm, Object distance = 30 mm, Radius = 10 mm, Refractive index of air = 1(n 1/ ) Refractive index of lens= 1.5(n 2/ ) So, from the formula of refraction at the curved surfaces, we have as n 2/ / V n 1/ / U = n 2 / n 1 / / R 1.5/V 1/ (30) = (1.5 1) / R From the above equations, we get V= 90 mm. Nature of image formed after the second refraction will be as (i) Real, (ii) Inverted, so after second refraction, the first image which was inverted in nature, after it s second refraction became non inverted in nature, (iii) Enlarged (m> 1), the diminished image gets enlarged by the second refraction, & it happens by the same factor, & then it forms an image with actual dimensions. E. Swath of the MSS In the working of the MSS, the scanner has an opening slit which opens when it gets proper exposure of light. Every surface has its own optical radiation, these radiations are continuously radiated from their surfaces, when an exposure is performed, the optical radiations emitted by the surfaces falls on the exposed part, & then this radiation falls on the detector after it s proper refraction, & then an image is formed. This swath is also called as the footprint of the MSS, & it depends on the focal length, narrower is the swath more is the focal length of lenses of MSS. If we know few things about the sensor, then we can easily determine the footprint or swath of the MSS. Before this, few things have to be understood very much clearly, i.e. the size of the film or detector where the final image is being formed can t be greater than 20 mm, or it s restricted to 20 mm, it can be less than but should not be more than 20 mm, so that these values matches up with the exact cross section of the MSS. Also this assumption is done by keeping in mind that there should be exact & proper magnification relations. F. Clarifications of assumptions We are assuming that the size of the detector or film or negative should not exceed 20 mm, or we are taking the size of it as 20 mm. but this assumption will be valid when it satisfies this magnification. Magnification=size of image/size of object = distance of image/ distance of object From the design of optics, it is clear that the image is formed at a 4

distance of 90 mm, & the object is kept at a distance of 30 mm from the second lens respectively. Magnification= 90/30 = 3 Let s see, the assumption what we have made is correct or not, on basis of this magnification. Hence, footprint is the object size depicted from the image being obtained, & we have to determine it's size. 1 α= 2 Tan (d/2b) = 2 Tan 1 (20/2x90) α= 12.68 degrees(angle of view) Now, object size of second lens can be calcu;ated from here using the formula, Object size= 30 x sine(12.68) = 6.58 mm Fig 1. Angle of view Here, α is the angle of view for the lens. α can be calculated from here as: Tan α/2= (d/2)/b, where d is the size of image, & b is the distance of image from the pole of the lens. or α = Tan 1 (d/2b). Now, in the design of optics, we can show like after the second refraction of first image. Fig 2. Refraction at curved surfaces On, comparing this design with the above magnification explanations; we have Already assumed 'd'= 20 mm, 'b'= 90 mm. G. Checking this value We had assumed that the final image or the size of the detector is 20 mm, & also we have shown that the magnification value for that is 3. So, as we know that Magnification= size of image/ size of object 3= 20/size of image Therefore, size of image= 6.666 mm. So, the size of object= 6.666 mm According to our assumptions towards the size of detector, we got the size of object as 6.666 mm. Previously, we have found that the size of object is 6.58 mm, therefore in both the case we are getting approximately same value, therefore our assumptions is very much correct & absolutely accurate. So, in this way we found the object size as 6.66 mm, we have to find the size of the swath being captured by 5

the first lens, which is also the footprint of the sensor. Therefore, for our MSS the resolution of our scanner is 151.7 kms/15000 pixels = 10 meters. I. Time period of MSS Fig 3. Refraction at curved surfaces Here, from the above image we got the value of 1 α= 2 Tan (d/2b) = 2 Tan 1 (6.58/2x30) = 12.51 degrees. Therefore, the swath, footprint of the MSS= subject distance x sine(angle of view) = 7 x 10 8 mm x sine(12.51 deg) = 151.7 kms This is the swath of the MSS, or we can say that the MSS captures 151.7 kms in one shot from an altitude of 700 kms. H. Resolution of the sensor The resolution of a sensor is referred to as the precision it provides towards the image. The resolution of the image is simply the number of pixels in the image divided by the area covered by the image. When we say 10 meters imagery or resolution, then we can say that one pixel is covering 10 meters of image data. If we say we havev10 meters imagery, then also it means that we have one pixel of image data for every ten meters of subject. This MSS rotates around the earth. The total distance at which the sensor is placed from earth is R. Where, R = R earth + altitude of the MSS = 6.37 X 10 6 m + 0.7 X 10 6 m = 7.07 x 10 6 meters Also it revolves around the massive body, i.e earth, which has a mass of 5.98 x 10 24 kg. Also, the gravitational force constant acting here will be G = 6.673 x 10 11 Nm 2 /kg 2. So, the velocity of MSS with which it orbits earth will be = SQRT [(G.M)/R) = SQRT [ (6.67 X 10 11 X5.98X10 24 )/7.07 X 10 6 ] = 7.512 X 10 3 m/s Therefore, the time period of satellite or MSS, or we can say the revisit time of our MSS will be found by the relation: T 2 /R 3 = 4 X pi 2 /G.M T= SQRT[( 4 x pi 2 x R 3 )/G.M] = SQRT [(4 x 3.14 2 x (7.07 x 10 6 ) 3 / 6.673 x 10 11 x 5.98 x 10 24 = 5912.86 seconds T= 1.62 Hours From the above calculations, we can conclude that a same point can be revisited within 1.62 hrs by the MSS. Resolution= total area of swath/total number of pixels 6

Fig 4. Producing the final image This is the final design of our optics towards the development of MSS. A three dimensional design has also been prepared to somewhat enhance the specifications of MSS. IV. Development of Vegetation Indexes The Normalized Difference Vegetation Index (NDVI) is a simple numerical indicator that can be used to analyze remote sensing measurements, typically but not necessarily from a space platform, and assess whether the target being observed contains live green vegetation or not. Very low values of NDVI (0.1 and below) correspond to barren areas of rock, sand, or snow. Moderate values represent shrub and grassland (0.2 to 0.3), while high values indicate temperate and tropical rainforests (0.6 to 0.8). Thus, NDVI was one of the most successful of many attempts to simply and quickly identify vegetated areas and their "condition," and it remains the most wellknown and used index to detect live green plant canopies in multispectral remote sensing data. Once the feasibility to detect vegetation had been demonstrated, users tended to also use the NDVI to quantify the photosynthetic capacity of plant canopies. Live green plants absorb solar radiation in the photosynthetically active radiation (PAR) spectral region, which they use as a source of energy in the process of photosynthesis. Leaf cells have also evolved to scatter (i.e., reflect and transmit) solar radiation in the near infrared spectral region (which carries approximately half of the total incoming solar energy), because the energy level per photon in that domain (wavelengths longer than about 700 nanometers) is not sufficient to be useful to synthesize organic molecules. A strong absorption at these wavelengths would only result in over heating the plant and possibly damaging the tissues. Hence, live green plants appear relatively dark in the PAR and relatively bright in the near infrared. By contrast, clouds and snow tend to be rather bright in the red (as well as other visible wavelengths) and quite dark in the nearinfrared. The calculations of NDVI will be performed using this formula: NDVI = (NIR RED) / (NIR + RED) or, put another way, NDVI = (Ch2 Ch1) / (Ch2 + Ch1) Where, RED is the reflectance for Channel 1 and NIR is the reflectance for Channel 2. The addition and subtraction is done to normalize the values to restrict them to a range of 1 to +1. From MSS, high resolution images will be obtained. These images can be used to generate the idea of the vegetation being done on the earth surface. From this study, an idea can be developed towards the status of vegetation reached. 7

V. Summary & Conclusions The need for collecting informations on world food & fiber s crops lead to the design of this MSS. With the aid of high resolution images, using the NDVI formula we can easily develop the vegetation indexes. The MSS has been developed for PICO/NANO satellites. This MSS being kept at an altitude of 650 900 kms, with a time period of 1.64 hrs, captures images with a resolution of 10 meters, in the spectral range of 400 1100 nm. The detector used here is a CCD detector because of it s capability to produce images with more clarity when compared with CMOS detector. Silicon mirrors have been used for covering the entire casing of MSS, & even lenses are made up of silicon to ensure light weight of the whole optics. The mass & volume occupied by our MSS is very much adaptable for PICO/NANO satellites. This miniaturized design enables these satellites to be used in this field of remote sensing. The 3D design of the MSS is achieved through the CATIA software. With this we can realize the specifications of the MSS. The tilt angle of the MSS is still under considerations, & it will be upgraded further after study. The angle of view for each lens is given in the specifications of the MSS. towards the development of vegetation indexes. o o o Books VII. References Ajoy Ghatak, OPTICS, IIT Delhi. G.C. Agarwal, PHYSICS FOR COMPETITIONS. An article in a journal Nurit Agam, A vegetation index based technique for spatial sharpening of thermal imagery, SCIENCE DIRECT. A web page http://en.wikipedia.org/wiki/f ile:globalndvi_tmo_200711_l rg. VI. Acknowledgements This work is supported by SRM University, & it s going to be implemented in the NANO satellite being launched by this university. A logistical support was given by remote sensing department of this university 8