Answer Scheme - June 2015

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  JUN 2015 Question 1 a)   Basic principles of positioning using GPS/GNSS technology The basic concept of GNSS positioning is that of  positioning-by-ranges . The geometrical principles of positioning can be demonstrated in terms of the intersection of locii. In the two-dimensional case, a measured range to a known point constrains the position to lie on circle with the measured range as radius. In three dimensions a measured range to a known point constrains the position in 3-D space to lie on the surface of a sphere centered at the known point, with radius being the measured distance. In the case of GPS, the distance measurement is made to a satellite with known position (coordinates are obtained from the satellite ephemeris data transmitted within the navigation message), however the principle applies to any range measuring positioning system, terrestrial or satellite-based. b)   Based on Kepler’s 1 st  law of planetary motion, six orbital Keplerian elements are used to describe the position of a satellite in space. Show and define clearly all these elements with the aids of a suitable diagram. The traditional orbital elements are the six Keplerian elements : The main two elements that define the shape and size of the ellipse: Eccentricity (℮ ) and Semimajo r axis ( α )  Two elements define the orientation of the orbital plane in which the ellipse is embedded: ã   Inclination (i) -  vertical tilt of the ellipse with respect to the reference plane, measured at the ascending node ã  Longitude of the ascending node - horizontally orients the ascending node of the ellipse (where the orbit passes  upward through the reference plane) with respect to the reference frame's vernal point (angle Ω in diagram).  And finally: ã    Argument of periapsis (w  ) - defines the orientation of the ellipse in the orbital plane, as an angle measured from the ascending node to the periapsis (the closest point )t). ã   Mean anomaly at epoch (M◦)  defines the position of the orbiting body along the ellipse at a specific time. c)   The GPS system can be divided into 3 segments namely; space, control and user. Explain the function of each segments in detail. Space Segment : ã   24 + 3 svs placed in MEO ã   6 orbital plane, inclination 55 degree ã   20,200 km altitude ã   Orbital period 11:58 hrs ã   At least 6 Svs always above horizon ã   Atomic clock ã   3 carrier waves, L1, L2 & L5 Control Segment: ã   Master control in Colorado Spring (main), alternate master control ã   6 monitor stations ã   A network of monitoring stations distributed around the world ã   Tracking, monitoring and maintainance ã   Determines data to be uploaded to satellites ã   Control and maintaining the position of the svs in the orbit User Segment: ã   Military and civilian ã   Receivers hardware, firmwares and assessories ã   Including more than 300 million receivers in cell phone ã   Data processing and software Land, air and sea transportations    1. Daily test 2. Real Time Test (Berkala) 3. Static Test (Berkala) Daily Test ã   This test conducted in real time instrument test and after processing instrument test. ã   Conducted daily to control the quality of the observe data through testing/checking the GNSS instrument used. ã   This test must be conducted at : -   Any point/control station that have been observed before and the coordinates are acceptable. -   Control station that specially created at the project area for daily checking purposes. -   Any PMPGN station that near the project area. ã   This test must be conducted through connecting GNSS unit to control unit and communication unit ã   This test must be done with 2 epoch observation for every station/pilar with separated initialization process for every epoch. ã   Conducted with at least 5 satellite tracking. ã   The coordinate difference for every epoch must be less than 3cm for N/E and 6 cm for heighting. o   Satellite clock bias o   Ephemeris error o   Receivers clock bias o   Ionosphere refraction delay o   Trophosphere refraction delay o   Antenna phase center variation o   Multipath signal o   Receiver noise o   Satellite perturbations o   Selective Availability (SA) o   Anti spoofing (AS)   

Auditoria RH 2015

Mar 17, 2019
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