Friday 19 October 2018

Global Positioning System (GPS)

Global Positioning System (GPS)Engineer interview questions.
  1. Answer :
    GPS is a satellite-based radionavigation system developed and operated by the U.S. Department of Defense (DOD). GPS permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time 24 hours a day, in all weather, anywhere in the world with a precision and accuracy far better than other radionavigation systems available today or in the foreseeable future.
    GPS consists of three segments: space, control, and user.
    The Space Segment consists of a minimum of 24 operational satellites in six circular orbits 20,200 km (10,900 NM) above the earth at an inclination angle of 55 degrees with an 11 hour 58 minute period. Although it is not a stated requirement, typically the satellites are spaced in primary orbital slots so that at any time a minimum of 6 satellites will be in view to users anywhere in the world.
    The Control Segment consists of a master control station in Colorado Springs, with five monitor stations and three ground antennas located throughout the world. The monitor stations track all GPS satellites in view and collect ranging information from the satellite broadcasts. The monitor stations send the information they collect from each of the satellites back to the master control station, which computes extremely precise satellite orbits. The information is then formatted into updated navigation messages for each satellite. The updated information is transmitted to each satellite via the ground antennas, which also transmit and receive satellite control and monitoring signals.
    The User Segment consists of the receivers, processors, and antennas that allow land, sea, or airborne operators to receive the GPS satellite broadcasts and compute their precise position, velocity and time.
    The GPS concept of operation is based upon satellite ranging. Users figure their position on the earth by measuring their distance from the group of satellites in space. The satellites act as precise reference points.
    Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver, which is the direct measure of the apparent range to the satellite. Measurements collected simultaneously from four satellites are processed to solve for the three dimensions of position, velocity and time.
  2. Answer :
    GPS receivers collect signals from satellites in view. They display the user's position, velocity, and time, as needed for their marine, terrestrial, or aeronautical applications. Some display additional data, such as distance and bearing to selected waypoints or digital charts.
  3. Answer :
    GPS is used to support land, sea, and airborne navigation, surveying, Geophysical exploration, mapping and geodesy, vehicle location systems, and a wide variety of additional applications.
  4. Answer :
    Jamming cell phones and GPS equipment is against the law!
    In recent years, the number of websites offering “cell jammers” or similar devices designed to block communications and create a “quiet zone” in vehicles, schools, theaters, restaurants, and other places has increased substantially. While these devices are marketed under different names, such as signal blockers, GPS jammers, or text stoppers, they have the same purpose. We remind and warn consumers that it is a violation of federal law to use a cell jammer or similar devices that intentionally block, jam, or interfere with authorized radio communications such as cell phones, police radar, GPS, and Wi-Fi. Despite some marketers’ claims, consumers cannot legally use jammers within the United States, nor can retailers lawfully sell them.
  5. Answer :
    Use of jamming devices can place you or other people in danger. For instance, jammers can prevent 9-1-1 and other emergency calls from getting through or interfere with law enforcement communications (ambulance, fire, police, etc). In order to protect the public and ensure access to emergency and other communications services, without interference, the FCC strictly prohibits the use, marketing, manufacture, and sale of jammers.
    1. Answer :
      Operation of a jammer in the United States is illegal and may subject you to substantial monetary penalties, seizure of the unlawful equipment, and criminal sanctions including imprisonment.
    2. Answer :
      To file a complaint alerting the FCC’s Enforcement Bureau to illegal cell, GPS, or other jamming devices, please visit by emailing the Enforcement Bureau.
    3. Answer :
      GPS will be free as stated in the Presidential Decision Document (29 March 1996) and by Congress in the 1998 Public Law (105-85)
      Both state that the U.S. "will continue to provide the GPS Standard Positioning Service for peaceful civil, commercial and scientific use on a continuous, worldwide basis, free of direct user fees.
    4. Answer :
      The Global Positioning System reached full Operational Capability (FOC) on July 17, 1995.
    5. Answer :
      GPS provides two levels of service -- a Standard Positioning Service (SPS) for general public use and an encoded Precise Positioning Service (PPS) primarily intended for use by the Department of Defense. SPS signal accuracy is intentionally degraded to protect U.S. national security interests. This process, called Selective Availability (SA), controls the availability of the system's full capabilities. The SPS accuracy specifications, given below, include the effects of SA.
      SPS provides accuracies of (for position, the accuracy with respect to geographic, or geodetic coordinates of the Earth) within:
      100 meters (2 drms) horizontal 156 meters (2 Sigma) vertical 300 meters (99.99% prob.) horizontal 340 nanoseconds time (95% prob.)
      SPS Coverage is continuous and worldwide, with a position dilution of precision (PDOP) of 6 or less.
      1. Answer :
        Effective as of Midnight 01 May 2000, Selective Availability has been set to zero.
      2. Answer :
        This document contains information about GPS Week 1024 Rollover.
      3. Answer :
        This document contains information about GPS Week 1024 Rollover.
      4. Answer :
        Interim Standard 95 (IS-95) is the first CDMA based digital cellular standard by Qualcomm. The brand name for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA-95. 
        It is a 2G Mobile Telecommunications Standard that uses CDMA, a multiple access scheme for digital radio, to send voice, data and signaling data (such as a dialed telephone number) between mobile telephones and cell sites.
      5. Answer :
        NMEA 2000 is a combined electrical and data specification for a marine data network for communication between marine electronic devices such as depth finders, nautical chart plotters, navigation instruments, engines, tank level sensors, and GPS receivers. It has been defined by, and is controlled by, the US based National Marine Electronics Association (NMEA).
      6. Answer :
        NMEA 0183 (or NMEA for short) is a combined electrical and data specification for communication between marine electronic devices such as echo sounder, sonars, anemometer (wind speed and direction), gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the U.S.-based National Marine Electronics Association
      7. Answer :
        The 2d Space Operations Squadron (2 SOPS) is a unit of the United States Air Force at Schriever Air Force Base, Colorado. Its mission is to manage the Navstar Global Positioning System satellite constellation for global navigation, time transfer, and nuclear detonation detection.
      8. Answer :
        The Interagency GPS Executive Board (IGEB) was an agency of the United States federal government that sought to integrate the needs and desires of various governmental agencies into formal Global Positioning System Planning. GPS was administered by the Department of Defense, but had grown to service a wide variety of constituents. The majority of GPS uses are now non-military, so this board was fundamental in ensuring the needs of non-military users.
      9. Answer :
        The United States Global Positioning System (GPS), having reached Fully Operational Capability on July 17, 1995 has completed its original design goals. However, additional advances in technology and new demands on the existing system led to the effort to modernize the GPS system. Announcements from the Vice President and the White House in 1998 initiated these changes. In 2000, U.S. Congress authorized the effort, referred to as GPS III.
      10. Answer :
        Three distinct parts make up the Global Positioning System. The first segment of the system consists of 24 satellites, orbiting 20,000 km above the Earth in 12 hour circular orbits. This means that it takes each satellite 12 hours to make a complete circle around the Earth. In order to make sure that they can be detected from anywhere on the Earths surface, the satellites are divided into six groups of four. Each group is assigned a different path to follow. This creates six orbital planes which completely surround the Earth.
      11. Answer :
        Although any small global positioning system could be referred to as a portable GPS, the term generally refers to those units which give driving directions and are meant to be mounted in cars. Others, used for outdoor activities, are generally referred to as handheld GPS units. Still, there could be some times when the terms are confused and a portable GPS and handheld GPS are synonymous.
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