Remote Sensing And Satellite Basics

INTRODUCTION

Survey like in any other field is a very important and crucial part of Data Collection in Engineering too. It’s the backbone of any Engineering project. A good Survey data helps the designer in visualizing and thus designing the projects with more applicability to the reality. As the Science and Technology is developing at unprecedented speed, the field of surveying too has evolved matching its footsteps with it parallelly. This field of chains, Compass, abney levels has on modern era (or say Digital Era) has been more reliant on Total stations and Prisms. Coming to this short revolutionary time of Artificial Intelligence (Let’s say “Era of AI”) surveying techniques for engineering has adopted various new methods too. One of the huge topic in modern surveying is Remote Sensing these days which has completely revolutionized the way we perceive and conduct Surveying.

Coming to the meaning itself, Remote sensing is the method of collecting data without any physical access to the source of data made possible by various sensors synchronized to get desired result. Camera is one of the most commonly used example of remote sensing Device which uses an optical sensor to get the desired imagery data. Usually, sensors used for remote sensing are strategically placed and are often moving. Satellite Images are among the one of the mostly used remotely sensed data which can be used for various purpose like: Estimating the Vegetation Health, Mapping possible Landslide Zones, Calculating Volume of water in a Lake etc. Weather forecasting, Climate change study, National Defense Planning and strategy all heavily rely on remotely sensed data. 

Characteristics of Remotely Sensed Data

                                    Remotely Sensed data depends on the nature of sensor used as per necessity of the work. This sensing ranges along the Various range of data that human beings cannot detect. Let’s shift our focus to Electromagnetic Spectrum for this Purpose.

                                             As only narrow spectrum of wavelength is detectable to natural human sensors, Remote Sensing devices can sense larger and wider bands of electromagnetic Spectrum. Hence different bands(wavelength) of data are captured for various kinds of purpose using remote sensing devices. So let’s Dive into these different bands of remotely Sensed Data:

  1. Visible Range

           This Range of waves are visible to human light and  have wavelength in between 400 nanometers to 700 nanometers with blue light along shorter wavelength(400nm) and red along longer(700nm) encompassing all colors in between them.

  1.       Waves Shorter than Visible Spectrum

              This Range Includes shorter Ultra Violet rays hence highly energetic waves as it is less than 400nm. X-Ray and Gamma ray are other bands of shorter and very energetic waves beyond ultraviolet rays.

  1.      Waves Longer than Visible Spectrum

             This waves includes waves Larger than 700nm which on shorter side includes Near Infrared followed by Thermal (mid- IR) and Far Infrared rays. Near Infrared is used for vegetation assessment while Infrared rays are used to sense the temperature as heat waves travel as infrared rays.

     Modes of Acquisition:

                     These data can be extracted from sensors mounted on various devices like:

  • Drone
  • Orbiting Satellites
  • High Flying Airplanes
  • Here, We’ll be focusing on Satellites and the modes of data acquisition through them. First Let’s be familiar with how imagery is captured through satellites. Though Satellites are geo-stationary too, most of the satellites acquire data by revolving around the location to be remotely sensed. Resolution of the image depends on the nature of work and quality of data required. These satellites revolve round the Erath and pass the data as soon as they get into proximity of receiving stations. Satellites can only capture the image of certain part of Earth at a time(called as single swath). Hence the images captured are clipped together and a complete required surface is constructed.
  • Though satellites move very fast (about 27,000kmph), It takes some time to capture the Data for entire Earth. Also, the resolution of data is a crucial factor that affects the cycling period of a satellite. Higher the resolution of data, longer it takes for a complete satellite cycle. Since the resolution of pixel is dependent on the nature of work, the satellite image to be used varies.  So here are some of the Satellites with their resolution and return period :

·         Satellite

Resolution

Return Period

Modis

250 m (bands 1–2) 500 m (bands 3–7) 1000 m (bands 8–36)

1-2 Days

Landsat

80 m

16 days (+/- 15 mins)

Sentinel

10m (bands 1-2) and 20m (bands 3-11)

2-3 days

       There are many other satellites but these three are most reliable and widely used satellites for imaging. These data can be aquired from sites like :

Google Earth pro

USGS Earth explorer

Sentinel ESA

Sentinel-2 Playground

Future of Remote Sensing

                      Remote sensing has a huge implications and future ranging from deep neural network based image analysis and computer based analysis which to some extent is currently applicable and turning out to be very useful for various fields. Coming to hardware part NASA jet Propulsion Laboratory recently launched a satellite AVIRIS (Airborne Visible InfraRed Imaging Spectrometer) which wan sense the data from visible up to 2500nm from 400nm which includes 224 bands of data which shows how wide the field of remote sensing is going to be.

To Conclude, remote sensing is a very wide applicable field and has much more sources and applications. Hope this article was helpful to you in establishing stepping stone on this subject. 

 

 

Credit: Rashik Raj Koirala
Rashik Raj Koirala is a civil engineer by profession and is well known for is works on media and so on. Follow the link below to know more about him.