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Chapter:
1. Explain Field Astronomy and GPS
ASTRONOMY:
Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). Some astronomical terms are:
SPHERICAL TRIANGLE:
A triangle formed by the intersection of three arcs of great circles is known as spherical triangle.
SPHERICAL EXCESS:
The quantity by which the sum of the three angles of a spherical triangle exceeds `180^0` is known as spherical excess of the triangle.
CELESTIAL SPHERE:
An imaginary sphere of infinite radius with the earth at its centre and other celestial bodies lying on the inside surface is known as celestial sphere.
GREAT CIRCLE:
The imaginary line of intersection of an infinite plane, passing through the centre of the earth and the circumference is called great circle.
ZENITH(Z):
If a plumb line through an observer is extended upward, the imaginary point at which it appears to intersect the celestial sphere is called zenith. The imaginary point at which it appears to intersect downward in the celestial sphere is known as Nadir (N).
VERTICAL CIRCLE:
Great circle passing through zenith and nadir is called vertical circle.
HORIZON:
Great circle perpendicular to the line joining the xenith and the nadir is known as horizon.
POLES:
If the axis of rotation of the earth is imagined to be extended infinitely in both directions, the points at which it meets the celestial sphere are known as poles.
The point of intersection in the northern hemisphere is known as north celestial pole and that in the southern hemisphere is south celestial pole.
EQUATOR:
The line of intersection of an infinite plane passing through the centre of the earth and perpendicular to the line joining celestial poles with the celestial sphere is known as the equator.
HOUR CIRCLE:
Great circle passing through the celestial poles is known as hour circle (declination circle).
MERIDIAN:
The hour circle passing through observers zenith and nadir is known as meridian. It represents the North-South direction at observer station.
ALTITUDE:
The altitude of a celestial body is the angular distance measured along a vertical circle passing through the body. It is considered positive if the angle measured is above the horizon and below horizon is considered as negative.
AZIMUTH(A):
The azimuth of a celestial body is the angular distance measured along the horizon from the observer meridian to the foot of the vertical circle passing through the celestial body.
DECLINATION:
The declination of a celestial body is the angular distance measured from the equator to the celestial body along the arc of an hour circle. It is considered positive in North direction and negative in South.
ECLIPTIC:
The great circle along which the sun appears to move round the earth in a year is called the ecliptic.
EQUINOCTIAL POINTS:
The point of intersection of the ecliptic circle with the equilatorial circle are known as equinoctial points.
The point at which sun transits from Southern to Northern hemisphere is known as point of Aeries.
RIGHT ASCENSION:
The right ascension of the celestial body is the angular distance along the arc of the celestial equator measured from the first point of Aeries to the foot of hour circle.
PRIME MERIDIAN:
Reference meridian that passes through the Royal Naval Observatory in Greenwich, England is known as prime meridian. It is also known as the Greenwich meridian.
LONGITUDE:
The longitude of an observers station is the angular distance measured along the equator from the prime meridian to the observer meridian. It varies from `0^0` to `180^0` E and `0^0` to `180^0` W.
LATITUDE:
The latitude of an observers station is the angular distance measured along the observers meridian from the equator to the zenith point. It varies from `0^0` to `90^0` N and `0^0` to `90^0` S.
HOUR ANGLE:
The hour angle of the celestial body is the angle at the equatorial plane measured westward from meridian to the hour circle passing through the celestial body.
LOCAL HOUR ANGLE (LHA):
The angular distance of a celestial body measured westward from the point of intersection of the equator and the meridian of the observer to the foot of the hour circle passing through the celestial body is called local hour angle.
GREENWICH HOUR ANGLE (GHA):
The angle at the equatorial plane measured westward from the prime (Greenwich) meridian to the hour circle through the celestial body is called Greenwich hour angle.
GEOGRAPHIC COORDINATE SYSTEM:
A geographic coordinate system (GCS) uses a three-dimensional spherical surface i.e, latitude and longitude to define locations on the earth.
Latitude is defined with respect to an equatorial reference plane. This plane passes through the center C of the sphere, and also contains the great circle representing the equator. The latitude of a point P on the surface is defined as the angle , passing through both P and C, subtending with respect to the equatorial plane. If P is above the reference plane, the latitude is positive (or northerly); if P is below the reference plane, the latitude is negative (or southerly). Latitude angles can range up to +90 degrees (or 90 degrees north), and down to -90 degrees (or 90 degrees south). Latitudes of +90 and -90 degrees correspond to the north and south geographic poles on the earth, respectively.
The line of latitude midway between the poles is called the equator. It defines the line of zero latitude.
Longitude is defined in terms of meridians, which are half-circles running from pole to pole. A reference meridian, called the prime meridian , is selected, and this forms the reference by which longitudes are defined. On the earth, the prime meridian passes through Greenwich, England; for this reason it is also called the Greenwich meridian. The longitude of a point P on the surface is defined as the angle that the plane containing the meridian passing through P subtends with respect to the plane containing the prime meridian. If P is to the east of the prime meridian, the longitude is positive; if P is to the west of the prime meridian, the longitude is negative. Longitude angles can range up to +180 degrees (180 degrees east), and down to -180 degrees (180 degrees west). The +180 and -180 degree longitude meridians coincide directly opposite the prime meridian.
The origin of the graticule (0,0) is defined by where the equator and prime meridian intersect.
Although longitude and latitude can locate exact positions on the surface of the globe, they are not uniform units of measure. Only along the equator does the distance represented by one degree of longitude approximate the distance represented by one degree of latitude. This is because the equator is the only parallel as large as a meridian. (Circles with the same radius as the spherical earth are called great circles. The equator and all meridians are great circles.)
Above and below the equator, the circles defining the parallels of latitude get gradually smaller until they become a single point at the North and South Poles where the meridians converge. As the meridians converge toward the poles, the distance represented by one degree of longitude decreases to zero. On the Clarke 1866 spheroid, one degree of longitude at the equator equals 111.321 km, while at 60° latitude it is only 55.802 km. Because degrees of latitude and longitude don't have a standard length, you cant measure distances or areas accurately or display the data easily on a flat map or computer screen.
USES OF ASTRONOMY IN SURVEYING AND MAPPING:
To determine the azimuth of the starting base of the triangulation series,
To determine the azimuth of starting and closing sides of precise traverse.
To determine the latitude and longitude of at least one of the triangulation stations so as to locate its position on the earth
To check the accuracy of triangulation series at suitable intervals independently
To carry out exploratory triangulations
To demarcate the international boundaries
GLOBAL POSITIONING SYSTEM:
The Global Positioning System (GPS) is a satellite-based radionavigation system owned by the United States government and operated by the United States, which is a network of 24 satellites that orbit the earth at an altitude of 20200 km, constantly emitting GPS coded signal to provide geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.
WORKING PRINCIPLE OF GPS:
Any GPS receiver (say, your smartphone) is constantly visible to at least four satellites at any given instant. Each of these satellites transmits a signal that contains information about where the satellite was and what time it was when the signal was transmitted. This signal is then received by your smartphone, which uses the information contained in the signal to calculate its distance from each of the four satellites by using a basic formula.
`D=V * T`
Where,
`D=`distance between the satellite and receiver
`V=`Velocity of the signal (radio wave)
`T=`Time taken by the signal to travel from the satellite to the receiver.
Thus, the distance between one satellite and receiver (say smartphone) is calculated. The same process is repeated to calculate the distance of receiver from three other satellites.
Once the GPS receiver (i.e. your smartphone) calculates how far it is from these satellites, it can pinpoint its exact location on Earth through a process called trilateration and display it on the screen in the form of coordinates involving latitude and longitude.
Although distances from at least four satellites is calculated, the receiver only uses the distances from three satellites to pinpoint location; the fourth is used to confirm the results obtained from the first three.
COMPONENTS OF GPS:
The GPS system has three components:
The space segment,
Control segment and,
User segments.
SPACE COMPONENT:
The space component consists of about 31 GPS satellites. The United States Air Force operates these 31 satellites, plus three to four decommissioned satellites that can be reactivated if needed. At any given moment, a minimum of 24 satellites are operational in a specially designed orbit, ensuring that at least four satellites are in view at the same time from almost any point on earth. The complete coverage that satellites offer makes the GPS system the most reliable navigation system in modern aviation. Some basic features of space segment are:
Composed of satellites that transmit signals from space, on the basis of which time and position of the user is measured.
Set of satellites is called as constellation.
GPS uses two satellite constellations i.e. NAVSTAR and GLONASS.
NAVSTAR (Navigation satellite timing and ranging)
NAVSTAR composed of 24 satellites, arrayed in 6 orbital planes, inclined 55 degrees to the equator and with a 12 hours period.
They orbit at altitudes of about 20,200km each.
Each satellite contains four precise atomic clocks, only one of which is in use at a time.
CONTROL SEGMENT:
The control segment is made up of a series of ground stations used to interpret and relay satellite signals to various receivers. Ground stations include a master control station, an alternate master control station, 12 ground antennas, and 16 monitoring stations.
Control segment consists of a group of 5 ground based monitor stations, three antennas and a master control station.
The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado.
The monitor stations measure signals from the SVs continuously and provides data to the master control station.
The master control station calculates satellite ephemeris and clock correction coefficients and forwards them to an antenna.
The antenna transmit the data to each satellite at least once a day. The SVs then send subsets of the orbital ephemeris to GPS receivers over radio signals.
USER SEGMENT:
The user segment of the GPS system involves various receivers from all different types of industries. National security, agriculture, space, surveying, and mapping are all examples of end users in the GPS system.
GPS User Segment consists of the GPS receivers and the user community.
The typical receiver is composed of an antenna and pre-amplifier, radio signal microprocessor, control and display device, data recording unit, and power supply.
GPS receivers convert SV signals into position, velocity, and time estimates. A minimum of four satellites are required to compute the four dimensions of X, Y, Z (position) and Time.
GPS FIELD PROCEDURES:
The field work of GPS can be divided into 3 periods:
Orientation period,
Data collection,
Field work wrap up
ORIENTATION PERIOD:
The orientation period includes the various activities that takes place prior to the begining of the actual data collection like training, arrival of the research team along with various equipments etc. Regardless of the intensity of the field work, there is an orientation period that apply to almost everyone entering the field for the first time in which they have to perform the following tasks:
Local contacts and initial meeting
Training
Lodging and supplying
DATA COLLECTION:
This component of field work includes the practicalities and troubleshooting of GPS operation, as well as data capture, download, backup, correction, data entry etc. At the end of each day, the field technician should download all data files to his computer. If the file is missing, it should be recollected later in the data collection.
FIELD WORK WRAP UP:
This component of field work includes concluding steps such as data verification and storage, equipment inventory and preparing both data and equipment to be returned from the field.
USES OF GPS:
Global positioning system applications generally fall into 5 major categories:
Location - determining a position
Navigation - getting from one location to another
Tracking - monitoring object or personal movement
Mapping - creating maps of the world
Timing - bringing precise timing to the world
Based on the above five factors, some practical applications of GPS are:
Aviation
Most of the modern aircraft use GPS receivers to provide the pilots and passengers with real-time aircraft position.
Public Safety and Disaster Relief
The best thing about GPS is that it can be used in any weather or environmental condition which is the reason why is preferred for use during disaster management. The emergency vehicles and supplies are tracked using GPS.
In the Robotic field
GPS enhances navigation of mobile robots and makes them applicable to diverse fields such as outdoor industrial work, mapping, and agriculture.
Business Tracking
. GPS technology provides business owners with real-time information about their business and other relevant information.
Live Recording
Live streaming video and tracking are other uses of GPS app, and the best thing about it is that it offers real-time data and you can record activities as they happen and through the transmission, through the satellite, you can view the happenings million miles away.
Tracking Luggage and Laptops
One can install a GPS tracking system on their luggage or laptop which will help you locate them in case they are stolen.