Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech IV Year GLOBAL POSITIONING SYSTEM Previous Question Paper (September - 2020 - R16)
Answer any five questions. All questions carry equal marks.
Main
Draw the functional block diagram of the Master Control Station. Also explain the functions of each block.
Answer
The Master Control Station (MCS) is the primary ground segment facility responsible for the overall monitoring and control of the GPS satellite constellation. It ensures the health, integrity, and orbital accuracy of the satellites, which is critical for maintaining precise positioning services.
The functional block diagram of the MCS consists of the following key components:
- Data Processing System: This is the central computing hub that collects tracking data from Monitor Stations. It performs complex orbital determination and clock synchronization algorithms to generate the navigation message.
- Satellite Command and Control System: This unit formats and transmits commands to the satellites. It manages station-keeping maneuvers and satellite health monitoring.
- Ground Antenna (GA): A high-gain antenna used to upload the updated navigation data, ephemeris parameters, and clock corrections to the satellites during their visibility window.
- Communication Interface: A robust network interface that facilitates the exchange of data between the MCS, the Monitor Stations, and the Ground Antennas.
- Operations and Maintenance Console: Provides the human-machine interface for system operators to monitor constellation status and intervene in case of anomalies.
The functions of the MCS are summarized as follows:
- Orbit Determination: By processing range measurements from multiple monitor stations, the MCS calculates the precise orbital parameters (ephemeris) for each satellite.
- Clock Synchronization: The MCS determines the drift of the atomic clocks on board each satellite relative to GPS Time and generates the necessary correction coefficients.
- Navigation Message Generation: It compiles the collected data into a standardized navigation message, which is then uploaded to the satellites to be broadcast to users.
- Satellite Health Monitoring: Continuous analysis of telemetry data allows the MCS to detect malfunctions or degradation in satellite hardware, triggering corrective commands or switching to redundant systems.
- Station Keeping: The MCS calculates and commands the firing of thrusters to maintain satellites in their designated orbital slots, compensating for gravitational perturbations and solar radiation pressure.
The overall process can be represented by the data flow loop:
Answer the following questions regarding GPS positioning and systems:
Explain the trilateration method to estimate GPS receiver position in 3D.
Answer
The trilateration method is the fundamental geometric principle used by GPS to determine the position of a receiver in 3D space. It relies on measuring the distance (range) between the receiver and multiple satellites whose precise orbital positions are known.
- Range Measurement: The receiver calculates its distance from a satellite by measuring the time it takes for a signal to travel from the satellite to the receiver. This is known as the pseudorange, calculated as: ρ = c × (t_r - t_s)where ρ is the pseudorange, c is the speed of light, t_r is the time of reception, and t_s is the time of transmission.
- Geometric Intersection: Each satellite distance defines a sphere centered at the satellite's coordinates. The intersection of two spheres forms a circle, and the intersection of this circle with a third sphere results in two points. One of these points is typically on the Earth's surface and represents the receiver's position.
- Clock Bias Correction: Because the receiver clock is not perfectly synchronized with the highly accurate atomic clocks on board the satellites, the measured distance is a 'pseudorange' containing a time bias. To solve for four unknowns (x, y, z coordinates and clock bias Δt), a minimum of four satellites is required.
- Mathematical Formulation: The system of equations is: ρ_i = √((x_i - x)^2 + (y_i - y)^2 + (z_i - z)^2) + cΔtwhere (x_i, y_i, z_i) are the known coordinates of the i-th satellite.
Compare GPS and GALILEO system with respect to satellite constellation and signal structure.
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Explain the characteristics of C/A code.
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Derive the equation for ionospheric delay for phase range measurement starting from the refractive index.
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Draw the schematic functional block diagram of the GPS receiver. List the signal processing functions of the GPS receiver.
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Explain the following errors in GPS receivers:
Ionospheric errors
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Tropospheric errors
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SA errors
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With the help of a neat diagram explain Wide Area DGPS.
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Compare GEO uplink and down link systems.
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How the GEO orbit can be determined by geometric analysis.
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Explain the RINEX format of observation and navigation data files.
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Describe the steps involved in receiver position estimation using Lease Squares Approximation method.
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