Fundamentals of GIS Exam 2
Questions and All Correct Answers.
GNSS - Answer Satellite-based technologies that give precise positional information
-Day or night
-Most weather and terrain conditions
-Inexpensive, accurate, and easy to use.
-Most common method of field data collection for use in GIS.
GNSS Terminology - Answer -The generic acronym is GNSS.
-The acronym GPS actually applies to the U.S. NAVSTAR system.
-GPS is widely used as a generic acronym, in a similar fashion as...Jacuzzi : hot tubs :: GPS: GNSS
NAVSTAR (aka GPS) - Answer -First deployed
-Most widely used system
-Military/civilians applications
-Designed to operate with 24 satellites.
-Satellites in orbit: 31
-First launch: 1978
-Total launches: 72
-Orbital height: 20,180 km (12,540 mi)
Components of GNSS - Answer 1. Satellite Segment
-Constellation of satellites orbiting and transmitting signals (21+ satellites, 6 orbital planes, 12 hr
return interval for each satellite)
-4 to 8 usually visible
2. Control Segment
-Tracking, communications, data gathering, integration, analysis, and control facilities.
-Five stations for the GPS system
-Master Control Station (Colorado)
-The satellites have to be maintained!
3. User Segment
-GNSS receivers - any device that records data transmitted by each satellite and then processes
these data to obtain 3D coordinates
-Manufacturers have developed receivers capable of receiving both GPS and GLONASS.
, -Receivers specializing in GIServices vs. specialized GIS data collection receivers
GNSS Positioning - Answer GNSS positioning is based on radio signals broadcast by each
satellite.
-Receivers decode each signal to identify: Satellite, Transmission time, Satellite position
-This in combination with signals from other satellites is what is needed for positioning.
Range Distance Measurements - Answer Range - distance between two objects
-E.g. distance between satellite and receiver
-Range distance: range = speed of light * travel time, where travel time is (t1 - t0)
-Simultaneous range measurements from multiple satellites are used to estimate a receiver's
location.
How does your smartphone know your location? - Answer https://youtu.be/70cDSUI4XKE
Positional Uncertainty - Answer -Uncertainty in range measurements leads to position errors
in GNSS measurements.
-Measured travel time errors (timing errors) are often the largest sources of positional
uncertainty.
Positional dilution of Precision (PDOP) - Answer -High PDOP = satellites close together, less
accurate
-Low PDOP = satellites wide apart, smaller area of uncertainty
Differential positioning/correction - Answer -Perhaps the greatest improvement for ensuring
high accuracy.
-Based on simultaneous GNSS measurements.
-We use the known base station to estimate the timing errors and hence range errors.
-Determine "true" location of satellite based on base station.
-Difference between true distance and GNSS-measured distance used to estimate timing error,
which are then used in timing corrections.
Selective Availability - Answer -Selective Availability (SA) was an intentional degradation of
public GPS signals implemented for national security reasons.
-In May 2000, at the direction of President Bill Clinton, the U.S government discontinued its use
of Selective Availability in order to make GPS more responsive to civil and commercial users
worldwide.
-"The United States has no intent to ever use Selective Availability again."
Questions and All Correct Answers.
GNSS - Answer Satellite-based technologies that give precise positional information
-Day or night
-Most weather and terrain conditions
-Inexpensive, accurate, and easy to use.
-Most common method of field data collection for use in GIS.
GNSS Terminology - Answer -The generic acronym is GNSS.
-The acronym GPS actually applies to the U.S. NAVSTAR system.
-GPS is widely used as a generic acronym, in a similar fashion as...Jacuzzi : hot tubs :: GPS: GNSS
NAVSTAR (aka GPS) - Answer -First deployed
-Most widely used system
-Military/civilians applications
-Designed to operate with 24 satellites.
-Satellites in orbit: 31
-First launch: 1978
-Total launches: 72
-Orbital height: 20,180 km (12,540 mi)
Components of GNSS - Answer 1. Satellite Segment
-Constellation of satellites orbiting and transmitting signals (21+ satellites, 6 orbital planes, 12 hr
return interval for each satellite)
-4 to 8 usually visible
2. Control Segment
-Tracking, communications, data gathering, integration, analysis, and control facilities.
-Five stations for the GPS system
-Master Control Station (Colorado)
-The satellites have to be maintained!
3. User Segment
-GNSS receivers - any device that records data transmitted by each satellite and then processes
these data to obtain 3D coordinates
-Manufacturers have developed receivers capable of receiving both GPS and GLONASS.
, -Receivers specializing in GIServices vs. specialized GIS data collection receivers
GNSS Positioning - Answer GNSS positioning is based on radio signals broadcast by each
satellite.
-Receivers decode each signal to identify: Satellite, Transmission time, Satellite position
-This in combination with signals from other satellites is what is needed for positioning.
Range Distance Measurements - Answer Range - distance between two objects
-E.g. distance between satellite and receiver
-Range distance: range = speed of light * travel time, where travel time is (t1 - t0)
-Simultaneous range measurements from multiple satellites are used to estimate a receiver's
location.
How does your smartphone know your location? - Answer https://youtu.be/70cDSUI4XKE
Positional Uncertainty - Answer -Uncertainty in range measurements leads to position errors
in GNSS measurements.
-Measured travel time errors (timing errors) are often the largest sources of positional
uncertainty.
Positional dilution of Precision (PDOP) - Answer -High PDOP = satellites close together, less
accurate
-Low PDOP = satellites wide apart, smaller area of uncertainty
Differential positioning/correction - Answer -Perhaps the greatest improvement for ensuring
high accuracy.
-Based on simultaneous GNSS measurements.
-We use the known base station to estimate the timing errors and hence range errors.
-Determine "true" location of satellite based on base station.
-Difference between true distance and GNSS-measured distance used to estimate timing error,
which are then used in timing corrections.
Selective Availability - Answer -Selective Availability (SA) was an intentional degradation of
public GPS signals implemented for national security reasons.
-In May 2000, at the direction of President Bill Clinton, the U.S government discontinued its use
of Selective Availability in order to make GPS more responsive to civil and commercial users
worldwide.
-"The United States has no intent to ever use Selective Availability again."
