2 Concept of space and time; Types of Satellites

Dr. Puneeta Pandey

 

CONTENTS

 

1.  Learning Objectives

 

2.  Concept of space and time

 

3.  Properties of space

 

4.  Models

 

5.  Types of Satellites

 

6.  Conclusions

 

7.  References

 

 

1. Learning Objectives

 

The present module will help us understand the concept of space and time and the role that GIS plays in various domains of space and time. Further, this module would also provide an overview of the types of satellites based on their applications.

 

On completion of this chapter, we would have got an overview on the spatial and temporal framework to receive solutions for spatially related questions. We would also be able to appreciate the fact that spatial dimensions help us in getting better solutions to a problem.

 

2. Concept of space and time

 

The concept of space can be related to the concept of ‘geo’ in ‘geography’ – geographical entities (physical features) and phenomena (processes). The geographical entities vary over scale, ranging from a small house to the entire planet itself. The phenomena occurs at varied spatial scales and changes over a period of time. Thus, geographical phenomenon encompasses the concept of both ‘space’ and ‘time’.

 

Real world is the physical environment around us that we perceive according to the five senses- sight, touch, taste, smell or hearing. Real world phenomenon may be natural or man-made. Any process that takes place without any human intervention is a natural phenomenon, such as topography, weather, natural disasters; while, those which require human intervention are man-made processes, e.g., construction activities, industrial processes etc.

 

Everything in the world has a spatial dimension or pattern, for example, location of ATM in a city, telephone directories, maps, location of civic amenities, travel route to work place, picnic destinations and so on. The perception of the world around us falls in the domain of spatial cognition; a field that explores the spatial properties of the world and its changes over time.

 

3. Properties of space

 

The concept of space broadly encompasses the following properties:

 

3.1 Location – Every object in a space can be characterized by its location. Location exemplifies ‘spatial’ data, implying that it is in some way referenced to locations on the earth. This location may be represented by Cartesian geometry (x,y plane) or geographical coordinates (latitude, longitude). The location of an object can provide answers to the questions ‘where’, ‘next to what’, ‘close to …. or how far…… from another object’ and may be answered in terms of latitude/longitude, precise address or in relation to other object. The precise location based on latitude and longitude is called as ‘absolute location’; while, a location defined on the basis of a frame of reference or on the basis of a place whose absolute location is known is called as ‘relative location’.

 

3.2 Size – Size refers to the geometric dimensions of any object in space. Further, size also involves the concept of relative size rather than absolute size; thus, emphasizing the concept of scale. Any object on large scale will provide higher details about the object or area under investigation; while that in a small scale will provide lesser details but larger area.

 

3.2.1 Scale

 

Scale refers to the ratio of distance on the map to the actual distance on the ground. This has a direct bearing on the details that can be observed on the data; for example, if the scale is fine, higher details can be observed and vice-versa. Spatial scale involves the concept of grain and extent. Grain represents the size of the pixel which is the smallest resolvable unit; while extent represents the size of the study area and is thus, the largest resolvable unit.

Figure 1: Wetland representing bigger grain sizes

 

(http://gif.berkeley.edu/documents/Scale_in_GIS.pdf)

 

The series of pictures in Figure 1 shows a section of wetland at progressively bigger grain sizes. It can be observed clearly that for better details; higher computational power is required. An example of scale is given in Table 1; which provides an overview of different scales employed at various levels of planning.

 

Table 1 gives the information at various scales and levels of planning.

Table 1. Information Contents at Various Levels and Scales of Planning

(http://www.isprs.org/proceedings/XXXIII/congress/part7/127_XXXIII-part7s.pdf)

 

3.2.2 Resolution: It refers to the size of the smallest possible feature that can be detected. Most remote sensing images are composed of a matrix of picture elements, or pixels, which are the smallest units of an image. Image pixels are normally square and represent a certain area on an image. It is important to distinguish between pixel size and spatial resolution – they are not interchangeable. If a sensor has a spatial resolution of 20 metres and an image from that sensor is displayed at full resolution, each pixel represents an area of 20m x 20m on the ground. In this case the pixel size and resolution are the same. However, it is possible to display an image with a pixel size different than the resolution. Many posters of satellite images of the Earth have their pixels averaged to represent larger areas, although the original spatial resolution of the sensor that collected the imagery remains the same.

 

Types of Resolution

 

3.2.2.1 Spatial

 

3.2.2.2 Spectral

 

3.2.2.3 Radiometric

 

3.2.2.4 Temporal

 

 

3.2.2.1 Spatial Resolution

 

It is the measure of smallest angular or linear separation between two objects that can be resolved. Spatial resolution of sensors depends on their Instantaneous Field of View (IFOV); which is defined as the angle in the form of cone from sensor to the ground at a given altitude at any instant of time. The dimensions of the area viewed on the ground is determined by multiplying the IFOV by the distance from the ground to the sensor.-

Figure 2: Spectral Resolution and IFOV

 

In images that have coarse or low resolution, such as various commercial satellites, larger details are not available in the image. However, in fine or high resolution images, very minute objects can be detected. Examples include sensors deployed for military purpose.

Figure 3: Coarse vs Fine Spatial Resolution

 

(http://gis.humboldt.edu/club/Images/Documents/lecture7.pdf)

3.2.2.2. Spectral Resolution– It considers number and width of wavelength intervals (spectral bands). It is believed that different features in an image respond differently to different wavelengths. For example, water and vegetation can be distinguished by visible and near infrared wavelengths. However, it would be difficult to distinguish different rock types (Figure 4) based on wide wavelength ranges; rather it requires narrow wavelength bands, implying the sensor should have high resolution.

Figure 4: Spectral resolution (http://www.nrcan.gc.ca/earth-sciences/geomatics/satellite-imagery-air-photos/satellite-imagery-products/educational-resources/9393)

 

Multi-spectral sensors record energy over different wavelength ranges at various spectral resolutions. Besides, hyperspectral sensors detect hundreds of very narrow spectral bands throughout the visible, near-infrared, and mid-infrared portions of the electromagnetic spectrum.

Figure 5: Spectral Resolution

 

3.2.2.3 Radiometric resolution

 

It refers to sensitivity to differences in radiance; i.e., the number of brightness levels which a sensor can detect. It determines the information content of the image. The more the number of digital values, the more detail can be expressed.

 

Figure 6: Radiometric Resolution

 

3.2.2.4 Temporal Resolution

 

Temporal resolution is defined as the amount of time needed to revisit and acquire data for the exact same location. When applied to remote sensing, this amount of time depends on the orbital characteristics of the sensor platform as well as sensor characteristics. The temporal resolution is high when the revisiting delay is low and vice-versa. Temporal resolution may vary from hours to days; and is usually expressed in days.

Figure 7: Temporal Resolution

 

3.2.2.3 Accuracy: Accuracy is the degree to which information on a map or in a digital database matches true or accepted values. Thus, data accuracy is a statement of how closely a bit of data represents the real world. Accuracy conceptually includes statistical measures of uncertainty and variation, as well as how and when the information was collected. Spatial data accuracy is independent of map scale and display scale, and should be stated in ground measurement units.Accuracy, however, differs from Precision. Precision refers to the level of measurement and exactness of description in a database.

 

3.3 Distance – Distance refers to the length between two objects in space. It has a linear dimension measured in terms of kilometres, meters or sub-units of metres. Distance generally provides answers to questions such as ‘how far’, ‘how near’ and so on.

 

3.4 Direction- When the concept of angle is included in defining the location of an object in space, it is called as direction. It is generally measured in degrees or radians; and is useful in defining as to one object may lie in what direction to the other object.

 

4. Models

 

When we try to provide an abstract representation of the real world phenomena, the representation is called a model. For example, a globe is a physical model of the earth; weather forecasting is carried out using simulation models to predict the weather conditions of an area.

 

Various disciplines have tried to develop tools and techniques to provide answers pertaining to objects and processes in space through the spatial and temporal analysis of data about geographical entities and phenomena. Quite a many of these concepts have been incorporated in geospatial studies by means of static and dynamic models. Often the most useful of these applications have to do with the complex interactions between relatively static geographical entities and the dynamic phenomena through which these entities themselves evolve. They also involve the notion of space and time. While, the notion of space involves the answers to ‘where’ question; that of time involves the answer to the question of ‘when’.

 

5. Types of Satellites

 

A Satellite is any object man made or natural that revolves around a planet in a circular or elliptical path. Based on its applications, satellites are of varied types; such as weather satellites, communication satellites, scientific satellites, navigational satellites, military satellites and earth observation satellites.

 

5.1. Weather satellites

 

Weather satellites help in predicting the weather of a place or region; and are of significance interest to meteorologists for weather forecasting. Typical weather satellites include the TIROS (Television InfraRed Observational Satellite), COSMOS and GOES Geostationary Operational Environmental Satellites (GOES) satellites.

 

With the advent of space missions of India in 1970s, India launched its weather satellites with assistance from Soviet Union. These satellites include:

 

5.1.1. Aryabhatta – This satellite was launched on 19 April, 1975 from Kapustin Yar Missile and Space Complex using Interkosmos-II launch vehicle with a period of 96.5 minutes. The decay date of the satellite was 11 February, 1992. The satellite provided data in the field of space physics and earth sciences.

5.1.2. Bhaskara Sega-I : This satellite was launched on 7 June, 1979 from Kapustin Yar Missile and Space Complex using Modified SS-5 (SKean IRBM) plus Upper Stage launch vehicle with a period of 95.2 minutes. The decay date of the satellite was 17February, 1989. The satellite provided data in the disciplines of astronomy, communications, engineering and earth sciences.

 

5.1.3. Rohini

 

Rohini Technology Payload: It is an experimental satellite with a payload of 35kg , launched on 10 August 1979 from Satish Dhawan Space Centre, Sriharikota, Andhra Pradesh. The satellite was intended for measuring in-flight performance of first experimental flight of SLV-3, the first Indian launch vehicle; however, it did not achieve its orbit.

 

Rohini RS-1: This satellite with a payload of 35kg, was launched on 10 August 1979 from Satish Dhawan Space Centre, Sriharikota, Andhra Pradesh. This was India’s first indigenous satellite launch and used for measuring in-flight performance of second experimental launch of SLV-3. The launch of Rohini RS-1 made India the seventh nation to possess the capability to launch its own satellites on its own rockets. The decay date was 20 May, 1981.

 

Rohini RS D-1: This satellite was used for conducting remote sensing technology studies using a landmark sensor payload. It was launched on 31 May, 1981 from Satish Dhawan Space Centre, Sriharikota, Andhra Pradesh; with a payload of 35kg. The decay date was 8 June, 1981.

 

5.2. Communication satellites – These satellites find their applications in transmission of TV and radio signals through the satellite. Examples include Telstar and Intelsat.

 

Figure 9: TELSTAR satellite (http://www.nasa.gov/topics/technology/features/telstar.html)

 

Telstar: Telstar is the world’s first active communication satellite and an international collaboration between AT&T, Bell Labs, NASA of U.S.A., French National Post, Telegraph, and Telecom Office of France, and British broadcasting agencies- British General Post Office. The satellite was launched on a Delta rocket on July 10, 1962. It enabled broadcasting of television signals, telephone calls, and fax images through space. Telstar 1 was placed in low Earth orbit with a period of two and a half hours.

APPLE: India launched its first experimental communication Satellite APPLE on 19 June 1981 with a payload of 670kg. INSAT-1A was the first multipurpose communication and meteorological satellite that was procured from USA; however, it worked only for six months. INSAT-1B was launched on 1 June, 1983 which was identical to INSAT-1A; and served for more than the design life of seven years. Both INSAT-1A and INSAT-1B was launched from Air Force Eastern Test Range, Florida.

 

5.3. Scientific satellites – These perform a variety of scientific missions such as Hubble Space Telescope and Hiten scientific. In India, examples include Astrosat, Mars Orbiter Mission, Chandrayaan-1 and Chandrayaan-2.

 

5.3.1 AstroSat: AstroSat is the first dedicated Indian astronomy mission launched on September 28, 2015 into a 650 km orbit by PSLV-C30 from Satish Dhawan Space Centre, Sriharikota. It was aimed at studying celestial sources in X-ray, optical and UV spectral bands simultaneously. It enables the simultaneous multi-wavelength observations of various astronomical objects with a single satellite. The minimum useful life of the AstroSat mission is 5 years.

 

5.3.2 Mars Orbiter Mission: Mars Orbiter Mission is ISRO’s first interplanetary mission to planet Mars with an orbiter craft designed to orbit Mars in an elliptical orbit of 372 km by 80,000 km. The primary driving technological objective of the mission is to design and realize a spacecraft with a capability to perform Earth Bound Manoeuvre (EBM), Martian Transfer Trajectory (MTT) and Mars Orbit Insertion (MOI) phases and the related deep space mission planning and communication management at a distance of nearly 400 million Km.

 

5.3.3. Chandrayaan

 

5.3.3.1 Chandrayaan-1: Chandrayaan-1, India’s first mission to Moon, was launched successfully on October 22, 2008 from SDSC SHAR, Sriharikota. The spacecraft was orbiting around the Moon at a height of 100 km from the lunar surface for chemical, mineralogical and photo-geologic mapping of the Moon. The spacecraft carried 11 scientific instruments built in India, USA, UK, Germany, Sweden and Bulgaria.

 

5.3.3.2 Chandrayaan-2: Chandrayaan-2 is an advanced version of the previous Chandrayaan-1 mission to Moon. Chandrayaan-2 is configured as a two module system comprising of an Orbiter Craft module (OC) and a Lander Craft module (LC) carrying the Rover developed by ISRO.

 

5.4. Navigational satellites – These satellites help in navigation of vehicles, ships and planes. Typical example includes NAVSTAR GPS (Navigational Satellite Timing and Ranging Global Positioning System) developed by US Department of Defence and later released for civilian purposes. India has developed its own navigational satellite system, known as IRNSS (Indian Reional Navigational Satellite System), a constellation of seven satellites, the last one of these launched on April 28, 2016.

 

5.5. Military satellites – These satellites find applications in defence and intelligence services for military strengthening. Examples include COSMOS, SKYMED, ELISA, RISAT, OPTSAT 3000 etc.

 

GSAT-7 Rukmini: Rukmini was the first military communication satellite developed by the Indian Space Research Organisation (ISRO) for the Indian Defence forces, with the Indian Navy being the primary user. Rukmini was launched early on August 30, 2013 atop an Ariane 5 ECA rocket from Kourou in French Guiana. Rukmini was successfully placed into a geosynchronous orbit, around 36,000 km above Earth. This gave India a major push in maritime security. Rukmini is also the last of ISRO’s seven fourth-generation satellites.

 

5.6. Earth observation satellites – They observe the planet earth for mapping, detecting the changes as well as a variety of environmental applications. In general, satellites are placed in one of three types of orbits around the Earth: Geostationary, polar or sun-synchronous. The type of orbit determines the design of the sensor, its altitude with respect to the Earth, and its instantaneous field of view (the area on the Earth which can be viewed at any particular moment in time). The details about these satellites would be dealt elsewhere (in Module 11, where we would study the types of orbits).

 

6.  Conclusions

 

Thus, at the end of this module, you would have gained a general insight about the concept of space and time, the spatial and temporal dynamics of models, properties of space and the various types of satellites present in the world along with their applications. This module will be of great help in laying the basic foundation of introduction to geospatial technology, that includes remote sensing, GPS and Geographical Information System.

 

References