GPS and Trilateration Quiz
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Questions and Answers

What is the nominal altitude of GPS satellites above the Earth?

  • 20,183 km (correct)
  • 25,000 km
  • 10,000 km
  • 15,000 km

    • What defines the passive nature of the GPS system?

  • Only ground controls transmit to users.
  • The system actively monitors user positions.
  • Users transmit signals to the satellites.
  • Only the satellites transmit signals. (correct)

    • What is the role of the clock in the GPS satellite?

  • To mark the time of signal departure. (correct)
  • To confirm signals have reached the receivers.
  • To measure the position of users.
  • To manage the satellite's orbit.

    • What is the primary function of trilateration in GPS?

    <p>To determine the position based on distance measurements.</p> Signup and view all the answers

    How does distance affect the measurement in GPS?

    <p>Distance depends on the speed of light and elapsed time.</p> Signup and view all the answers

    What describes the control points in GPS?

    <p>They are the satellites themselves.</p> Signup and view all the answers

    Which characteristic does the GPS signal not possess?

    <p>Requires user transmission.</p> Signup and view all the answers

    What is an aspect of GPS ranging?

    <p>It determines distance from satellites orbiting Earth.</p> Signup and view all the answers

    What information does a GPS signal communicate to its receiver?

    <p>Current time on the satellite</p> Signup and view all the answers

    What type of waves are used to carry GPS codes?

    <p>Radio waves</p> Signup and view all the answers

    What are the two fundamental frequencies of GPS?

    <p>L1 at 1575.42 MHz and L2 at 1227.60 MHz</p> Signup and view all the answers

    Which of the following correctly describes the modulation frequency of the Navigation code?

    <p>50 Hz</p> Signup and view all the answers

    What are the basic GPS codes used for communication?

    <p>Navigation code, Coarse acquisition code, and Precise code</p> Signup and view all the answers

    How many subframes does the Navigation message contain?

    <p>5 subframes</p> Signup and view all the answers

    Which frequency corresponds to the L1 band of GPS?

    <p>1575.42 MHz</p> Signup and view all the answers

    What is the wavelength associated with the L1 frequency of GPS?

    <p>19.0 cm</p> Signup and view all the answers

    What is the main purpose of the data provided by subframe 4?

    <p>To provide ionospheric corrections for single-frequency receivers.</p> Signup and view all the answers

    What information does the almanac contain regarding satellites?

    <p>Pseudorandom noise (PRN) numbers of satellites.</p> Signup and view all the answers

    Which frequencies do Control Segment monitoring stations analyze to determine GPS signal delays?

    <p>L1 and L2 frequencies.</p> Signup and view all the answers

    What does the flag in subframe 4 indicate to the receiver?

    <p>The activation of antispoofing security system.</p> Signup and view all the answers

    When can a receiver first utilize almanac data for satellite positioning?

    <p>After acquiring the satellite's signal.</p> Signup and view all the answers

    Which of the following statements about the Y-code and M-code is correct?

    <p>The M-code will become available in 2020.</p> Signup and view all the answers

    How often does the Control Segment generate and upload a new almanac to each satellite?

    <p>Every day.</p> Signup and view all the answers

    Which parameters related to the ionosphere are included in subframe 4?

    <p>Alpha values and their rates.</p> Signup and view all the answers

    How often does GPS time restart?

    <p>Every Sunday at midnight</p> Signup and view all the answers

    What is the total length of time it takes to broadcast the Navigation message from a cold boot?

    <p>12½ minutes</p> Signup and view all the answers

    Which subframe contains information essential for the receiver to synchronize its clock with that of the satellite clock?

    <p>Subframe 4</p> Signup and view all the answers

    How frequently are updates provided for the data in the ephemeris and clocks parameters?

    <p>Every 2 hours</p> Signup and view all the answers

    What does TLM indicate in the Navigation message?

    <p>The status of uploading the control segment</p> Signup and view all the answers

    Which GPS time standard is recognized as more stable than the rotation of the earth?

    <p>Coordinated Universal Time (UTC)</p> Signup and view all the answers

    What is the typical interval for renewing the almanacs provided in subframes 4 and 5?

    <p>Every 6 days</p> Signup and view all the answers

    What is the function of the HOW in the Navigation message?

    <p>Helps the receiver transition from C/A code to P(Y) code</p> Signup and view all the answers

    What is the primary function of the P code in GPS?

    <p>To prevent spoofing in military communications</p> Signup and view all the answers

    At what rate is the P code generated?

    <p>10.23 million bits per second</p> Signup and view all the answers

    How often is the entire P code renewed?

    <p>Every 37 weeks</p> Signup and view all the answers

    Which code is used for Standard Positioning Service (SPS)?

    <p>C/A code</p> Signup and view all the answers

    Which of the following frequencies does the C/A code broadcast on?

    <p>L1 only</p> Signup and view all the answers

    What technique do military users perform that is improved by the use of two frequencies in PPS?

    <p>Ionospheric correction</p> Signup and view all the answers

    Which of the following statements about the C/A code is true?

    <p>It is repeated every millisecond.</p> Signup and view all the answers

    What is the purpose of the M-code in GPS transmissions?

    <p>To enhance military communication capability</p> Signup and view all the answers

    Study Notes

    GPS Satellites Overview

    • Minimum of 24 operational GPS satellites available 95% of the time; currently 31 operational satellites.

    GPS and Trilateration

    • GPS utilizes trilateration to determine location by measuring distances (ranging) from satellites positioned 20,183 km above the Earth.
    • 'Range' denotes the distance measured in GPS terminology.

    Passive System Characteristics

    • GPS functions as a passive system; satellites transmit signals while receivers only receive.
    • Unlimited users can access the GPS without straining the system.

    Time and Signal Ranging

    • GPS distances are calculated based on the speed of light (c) and elapsed time (t) between signal transmission and reception.
    • A satellite clock records the signal's departure time; the receiver timestamps the arrival, aiding in distance calculation.

    Control Points in GPS

    • Satellites serve as control points, continuously moving; the GPS signal must relay time, position, atmospheric corrections, and satellite identification.
    • Information is transmitted using modulated carrier waves in the L-band (390 MHz - 1550 MHz), predominantly L1 (1575.42 MHz) and L2 (1227.60 MHz).
    • The Navigation Code operates at a frequency of 50 Hz and is modulated into both L1 and L2 carriers.
    • The complete navigation message consists of 37,500 bits, broadcast at a rate of 50 bits per second, taking approximately 12.5 minutes to receive after a cold start.

    GPS Time and Synchronization

    • GPS Time (or GPST) is a precise time standard, maintained within 1 microsecond of Coordinated Universal Time (UTC).
    • Subframe 4 conveys time synchronization information to help align the receiver's clock with the satellite clock.

    Atmospheric Correction

    • Signals are analyzed for delays caused by atmospheric conditions, specifically by examining the L1 and L2 frequency propagation rates to remove part of the error.

    Ionospheric Parameters

    • Ionospheric model parameters include specific Alpha and Beta values, crucial for understanding signal propagation and error correction.

    Antispoofing Mechanism

    • Antispoofing measures, effective since 1993, involve encrypting signals and using a new M-code for enhanced security.

    Almanac and Ephemerides

    • The almanac contains ephemeris data, updated daily, enabling receivers to determine satellite positions for tracking.

    P Code Characteristics

    • The P code, which is encrypted as the P(Y) code for security, operates at 10.23 million bits per second and is renewed every 37 weeks.

    C/A Code Features

    • The civilian C/A code, functioning at 1.023 million bits per second, is unique to each satellite and broadcast on the L1 frequency, also repeated every millisecond.

    SPS and PPS Services

    • Standard Positioning Service (SPS) is delivered via the C/A code, while the Precise Positioning Service (PPS) utilizes the P code, which allows military users to apply ionospheric corrections for improved accuracy.

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    Description

    Test your knowledge on GPS satellites and the trilateration process. This quiz covers key concepts related to the operation and availability of GPS satellites, specifically focusing on types such as Block II/IIA/IIR/IIR-M. Dive into the fascinating world of satellite navigation and learn how it impacts modern technology.

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