GNSS
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Questions and Answers

What is the primary purpose of delaying the pseudo-random code generated in a GPS receiver?

  • To reduce the computational load on the receiver's processor.
  • To correct for atmospheric interference and improve accuracy.
  • To encrypt the signal and prevent unauthorized access.
  • To synchronize the receiver clock with the satellite's signal and measure travel time. (correct)

Why is it necessary for a GPS receiver to receive signals from at least four satellites?

  • To enable the use of multiple frequencies for enhanced accuracy.
  • To improve the signal-to-noise ratio by averaging signals from multiple sources.
  • To correct for ionospheric and tropospheric delays using differential corrections.
  • To resolve the four unknowns: X, Y, Z coordinates and the receiver clock synchronization error. (correct)

Which atmospheric layer has the most significant impact on GPS signals due to its high density of electrically charged particles?

  • Ionosphere (correct)
  • Troposphere
  • Stratosphere
  • Mesosphere

What is the primary cause of multipath errors in GPS signal reception?

<p>Reflections of the GPS signal off surrounding surfaces. (B)</p> Signup and view all the answers

In which layer of the atmosphere do most meteorological events that contribute to GPS signal errors take place?

<p>Troposphere (C)</p> Signup and view all the answers

Why is measuring the distance from only three satellites insufficient for precise location determination?

<p>Receiver clocks are not perfectly synchronized, introducing error into the distance measurement. (C)</p> Signup and view all the answers

If a GPS signal takes 0.06 seconds to travel from a satellite to a receiver, approximately how far away is the satellite, assuming the signal travels at the speed of light (approximately $3 imes 10^8$ meters per second)?

<p>$1.8 \times 10^7$ meters (A)</p> Signup and view all the answers

What is the primary reason that GPS satellites are equipped with highly precise atomic clocks?

<p>To accurately measure the travel time of signals, which is crucial for distance calculation. (A)</p> Signup and view all the answers

What would be the resulting error in distance measurement if the receiver's clock is off by 1 microsecond ($1 \times 10^{-6}$ seconds)?

<p>300 meters (C)</p> Signup and view all the answers

The GPS receiver generates the same code simultaneously with the satellite. What is the main reason for doing so?

<p>To measure the time it takes for the signal to travel from the satellite. (B)</p> Signup and view all the answers

Why is it advantageous to determine the 3D vector between two GPS receivers in surveying, rather than the absolute coordinates of individual points?

<p>It allows for the elimination of systematic errors that are common to both receivers. (B)</p> Signup and view all the answers

What was the primary method used to intentionally introduce errors into GPS signals under Selective Availability (SA)?

<p>Transmitting erroneous ephemeris data and clock variations. (D)</p> Signup and view all the answers

How many GPS receivers are typically required to be operating simultaneously for conducting topographic surveys, and why?

<p>Two or more, to perform relative measurements. (D)</p> Signup and view all the answers

Which of the following parameters are most crucial for achieving high accuracy in topographic surveys using static data processing techniques?

<p>Network scheme, station occupation, and software for static data processing. (D)</p> Signup and view all the answers

What is the significance of 'dithering' in the context of Selective Availability (SA) applied to GPS signals?

<p>It involves artificially varying the clock data, thereby reducing the accuracy of positioning. (D)</p> Signup and view all the answers

Why is it necessary for a GNSS receiver to have multiple channels?

<p>To track satellites from different constellations, each using unique codes. (B)</p> Signup and view all the answers

What is the primary function of a GNSS receiver in the context of satellite signals?

<p>To generate replicas of the codes broadcast by each satellite. (C)</p> Signup and view all the answers

Which of the following is a GNSS system that is not yet fully operational?

<p>Galileo European system (C)</p> Signup and view all the answers

How does multi-constellation signal tracking enhance the performance of GNSS receivers?

<p>By increasing the number of visible satellites and improving accuracy. (B)</p> Signup and view all the answers

What is the significance of the codes broadcast by GNSS satellites?

<p>They allow receivers to differentiate between satellites and calculate position. (C)</p> Signup and view all the answers

Considering the global coverage intended for GNSS, systems like the Indian IRNSS and Japanese QZSS are classified as:

<p>Regional systems that enhance GNSS in specific areas. (A)</p> Signup and view all the answers

What is the purpose of studying GNSS in the context of Geomatics for Urban and Regional Analysis?

<p>To enable precise positioning and data collection for urban and regional planning. (A)</p> Signup and view all the answers

If a surveyor uses a multi-constellation GNSS receiver, what is one expected benefit compared to using a single-constellation receiver?

<p>Improved accuracy and reliability, especially in challenging environments. (D)</p> Signup and view all the answers

Why do GNSS receivers require multiple satellite tracking channels?

<p>To simultaneously track multiple satellites and improve accuracy. (D)</p> Signup and view all the answers

What is the purpose of having some channels set aside to receive specific code signals in a GNSS receiver?

<p>To enable the receiver to track signals from diverse satellite systems like Galileo. (C)</p> Signup and view all the answers

How does the tracking of multiple satellites by a GNSS receiver contribute to Location-Based Services (LBS)?

<p>By determining the precise location of people or objects. (C)</p> Signup and view all the answers

Which of the following best describes the role of generic tracking channels in a GNSS receiver?

<p>Tracking similar signals from various satellite systems such as GPS and Beidou. (B)</p> Signup and view all the answers

A GNSS receiver is designed to track signals from GPS, BeiDou, and Galileo. What is the most efficient channel configuration?

<p>Primarily generic channels capable of tracking similar signals from GPS and Beidou, with reserved channels for Galileo's specific codes. (D)</p> Signup and view all the answers

A new urban planning project requires highly accurate location data for asset mapping. How could Location-Based Services (LBS) utilizing multi-channel GNSS receivers improve this project?

<p>By providing precise positioning data for mapping assets in real-time, reducing time and costs. (D)</p> Signup and view all the answers

In designing a new GNSS receiver for use in autonomous vehicles, what aspect MOST directly benefits from the use of multiple tracking channels?

<p>Improved accuracy and reliability of positioning data in challenging environments. (B)</p> Signup and view all the answers

A surveying team is using GNSS technology in a densely forested area. What receiver configuration would provide the most reliable data?

<p>A receiver with multiple generic channels and dedicated channels to receive different satellite signals. (A)</p> Signup and view all the answers

Why are multiple satellites required to determine the position of a receiver's antenna on the ground?

<p>To apply trilateration, where distances from multiple satellites pinpoint a unique location. (A)</p> Signup and view all the answers

A GPS receiver calculates its position by measuring the distance to at least three satellites. What data is used to calculate these distances?

<p>The time it takes for the modulating codes (C/A, P1, P2) to travel from the satellite to the receiver. (D)</p> Signup and view all the answers

The NavData message transmitted by GPS satellites contains critical information. Which of the following is NOT a primary purpose of this message?

<p>Transmitting precise atmospheric pressure readings at the satellite's location. (B)</p> Signup and view all the answers

How does the use of two carrier frequencies (L1 and L2) enhance the accuracy of GPS positioning?

<p>It enables corrections for ionospheric delays that affect the signals differently. (B)</p> Signup and view all the answers

What is the fundamental difference between the C/A code and the P code in GPS?

<p>The C/A code is used for civilian purposes, while the P code was initially designed for military use. (A)</p> Signup and view all the answers

A GPS receiver obtains a position fix using signals from four satellites. What conditions will lead to the least accurate result?

<p>All four satellites are clustered together in one part of the sky. (C)</p> Signup and view all the answers

Suppose a GPS receiver is experiencing significant multipath interference. Which of the following strategies would be MOST effective in mitigating this issue?

<p>Relocating the antenna to an open area with a clear view of the sky. (D)</p> Signup and view all the answers

A GPS receiver is unable to decode the NavData message from one of the satellites it is tracking. How does this affect the receiver's ability to calculate its position?

<p>The receiver can still calculate a position, but it will be less accurate due to uncorrected satellite orbit and clock errors. (D)</p> Signup and view all the answers

Flashcards

GNSS

Global Navigation Satellite System; includes GPS, GLONASS, Galileo, Beidou, IRNSS, and QZSS.

GPS

US NAVSTAR Global Positioning System, a part of GNSS.

GLONASS

Russian global navigation satellite system, similar to GPS.

Galileo System

European global navigation satellite system, not yet fully operational.

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Beidou System

Chinese global navigation satellite system.

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IRNSS System

Indian Regional Navigation Satellite System.

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QZSS

Japanese Quasi-Zenith Satellite System.

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Multi-constellation Signal Tracking

A receiver's ability to track signals from multiple satellite constellations by generating replicas of broadcasted codes.

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Single GNSS Receiver

A receiver that needs multiple channels to track signals from different GNSS satellites.

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BeiDou Navigation Satellite System (BDS)

A Chinese satellite navigation system.

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Generic Tracking Channels

Channels in a receiver that can track similar signals from different satellite systems.

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Specific Code Channels

Channels specifically designed to receive unique code signals from certain satellites.

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Quasi-Zenith Satellite System (QZSS)

A Japanese satellite positioning system.

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NavIC (IRNSS)

Indian Regional Navigation Satellite System.

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Location Based Service (LBS)

Services that use location data to provide information.

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Receiver Tracking Channels

Tracking channels a receiver uses to process signals from individual satellites to determine location.

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Antenna Positioning

The antenna's ground location is found by measuring distances from satellites, whose positions (ephemeris) are known and transmitted.

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L1 Frequency

1575.42 MHz, with a wavelength of 19 cm. Used by GPS.

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L2 Frequency

1227.6 MHz, with a wavelength of 24 cm. Used by GPS.

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C/A Code

A code used on L1 for distance measuring and data transmission. Repeats more often than the P code.

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P Code

A precise code that is on L1 and L2 used for distance measurement.

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NAVDATA

Contains satellite status, orbit correction data, and satellite clock information, transmitted on both L1 and L2 at a low frequency.

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Satellite triangulation

To find a position, point distance needs to be measured to at least three satellites.

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Ephemeris

A table of data predicting the path of the satellite in the sky.

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Satellite Distance Calculation

Distance to a satellite is calculated by timing how long a signal takes to travel from the satellite to the receiver.

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Satellite Atomic Clocks

The satellites use precise atomic clocks synchronized by control stations for accurate timekeeping, known as GPS time.

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Receiver Clock Accuracy

Receivers have less accurate clocks compared to satellites and are not synchronized with GPS time.

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Clock Synchronization Errors

A synchronization error between satellite and receiver clocks can cause significant distance measurement errors.

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Code Generation by Receiver

The receiver generates the same code as the satellite simultaneously, in order to compare the time of arrival.

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Synchronization in GPS

Ensures the receiver's clock aligns with the satellite's signal timing.

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Pseudo-Random Code

Code emitted by satellites, unique to each and used for distance calculation.

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Signal Delay

The delay needed to synchronize the receiver's signal with the satellite's, indicating signal travel time.

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Atmospheric Error Sources

Ionosphere and Troposphere.

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Multipath Error

Occurs when signals bounce off surfaces, creating multiple paths to the receiver.

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Selective Availability (SA)

Intentional degradation of GPS accuracy by the U.S. government, removed in 2000.

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GPS Relative Measurements

Using two or more GPS receivers simultaneously to improve measurement accuracy.

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Static Data Processing Software

Software used to process static GPS data for high-accuracy topographic surveys.

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3D Vector Determination in Surveying

In surveying, determining the 3D vector between two receivers instead of absolute coordinates. Differentiating the measures obtained at the two receivers, the systematic errors - common to both - can be deleted.

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Systematic Errors (GPS)

Errors present in GPS signals that can be reduced by using differential measurements between receivers.

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Study Notes

  • GNSS stands for Global Navigation Satellite System
  • GNSS includes systems like GPS, GLONASS, Galileo, Beidou, IRNSS, and QZSS
  • Signals from different constellations require specific channels in the receiver
  • The receiver needs to replicate satellite codes for tracking
  • Many channels are needed to track signals from multiple constellations
  • Generic channels track similar signals, while others are dedicated to specific codes

Constellations

  • GPS has 30 space vehicles and 5 signals and is global
  • GLONASS has 24 space vehicles and 5 signals and is global
  • Galileo has 24 space vehicles and 5 signals and is global
  • BeiDou has 28 space vehicles and 6 signals and has both global and regional coverage
  • QZSS has 4 space vehicles and 6 signals and is regional
  • NavIC has 8 space vehicles and 2 signals and is regional

Applications of GNSS

  • Road
  • Maritime
  • Rail
  • Aviation
  • Agriculture
  • Surveying
  • Timing Synchronization

GPS System Basics

  • GPS was developed for military use by the US Department of Defense
  • GPS has been evolving since 1973
  • The first satellite was launched in 1978
  • The constellation was completed in 1994

GPS Satellite Constellation Design

  • 21 satellites + 3 spare satellites
  • Constellation was completed in 1994
  • There are 6 orbital planes at a 55° inclination
  • Satellites orbit at a distance of 20,200 km from Earth
  • The orbits are 12 hours long
  • At least 4 satellites are visible from any point on Earth 24 hours a day
  • Each satellite is visible for at least 5 hours

Control Stations

  • Control stations can send and receive data from satellites
  • Users can only receive data
  • All control stations receive signals transmitted by the satellites
  • Control stations contain very precise instruments, including atomic clocks
  • Satellites are controlled by the US DoD and must pass over US territory at least once a day

Control Station Functions

  • Atomic clocks stay synchronized
  • Orbits are under control through corrections
  • Satellites status is monitored for failures

Instrumentation Required

  • Receivers, antennas, batteries, and software

Differences Compared to Traditional Surveys

  • Data collection is continuous and weather-independent
  • Intervisibility between points is not required
  • Measurements taken from long distances
  • Positions are measured using a global datum of WGS84
  • High reliability

GPS Signal Details

  • Two carrier frequencies are in the band L (L1 and L2)
    • L1 has a frequency of 1575.42 MHz with a wavelength of 19 cm
    • L2 has a frequency of 1227.6 MHz with a wavelength of 24 cm
  • There are three modulating codes used: two for distance and one for data
    • C/A code: Only available on L1, with a 1.023 MHz frequency, a length of 293 meters, and 480 Watts
    • P code: (10.23 MHz, length 29.3 m) P1 on L1 (240 Watt), P2 on L2 (41 Watt)
    • NAVDATA: message on both frequencies, 50 Hz Satellites status Data for correction of orbits and satellite clocks

How GPS Works

  • The antenna position is determined by measuring the distance from at least 3 satellites
  • Satellite positions are known

Distance Calculation

  • Distance is determined by the time it takes a signal to travel from the satellite to the receiver at the speed of light
  • The code the satellite sends and the distance is calculated by synchronizing this with the receiver

Challenges in Measuring Distances

  • Satellites have the expensive, precise atomic clocks
  • Receivers have stable, but lower quality clocks
  • Clock errors can cause distance measurement errors
  • Four satellites (4 unknowns) are necessary to measure and receive distances from

Error Sources

  • Ionosphere, troposphere, satellite stability

Selective Availability

  • Introduced by the US government and phased out on May 2, 2000

GPS Surveys

  • Require two or more receivers operating simultaneously

Differential Correction

  • A technique used to improve GPS accuracy
  • Corrections can be applied in real-time or post-processing

Real-Time Transmission

  • Two options, a master station sends corrections, or differential correction is sent via L-band satellites

Survey Modes

  • There are kinematic, stop & go, and real-time modes

Precision Assessment

  • It can vary from meters to less than 1 cm
  • Absolute (one receiver) and relative (two or more receivers) mode affects precision
  • Type of observation, code C/A (rms of 3 m), P code. phase has mm
  • Two receivers working together are needed

Measurement Mode

  • There is static, kinematic and rapid-static

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