GNSS

What is Global Navigation Satellite Systems

  • A GNSS is a network of satellites and supporting systems that provide locational data for geolocation
  • The modern tool for answering the question: Where Am I?

Global Positioning System

  • GPS is the American GNSS:
    • First operational GNSS - first satellite launched in 1978
    • Created for the military but available to civilians
    • Selective Availability
      • Only lower resolution available to citizens until 2000/2001
      • Military could limit availability as needed but as of 2007 new GPS satellites (GPS III) do not have SA ability

Components of a GNSS

  • Space Segment
  • Control Segment
  • User Segment

GNSS Control Segment

Monitoring (Base station)

  • Track GPS satellites as they pass overhead
  • Collect navigation signals, range/carrier measurements, and atmospheric data
  • Feed observations to the master control station
  • Utilize sophisticated GPS receivers

Processing

  • Master Control station
    • Provides command and control of the GPS constellation
    • Uses global monitor station data to compute the precise locations of the satellites
    • Generates navigation messages for upload to the satellites
    • Performs satellite maintenance and anomaly resolution, including repositioning satellites to maintain optimal constellation
    • Monitors satellite broadcasts and system integrity to ensure constellation health and accuracy

Communicating

  • Ground Antennas
    • Send commands, navigation data uploads, and processor program loads to the satellites
    • Communicate via S-band and perform S-band ranging to provide anomaly resolution and early orbit support

GNSS Satellite Segment

  • The satellites in GNSS contain atomic clocks that measure time down to approximately 20-30 nanoseconds (nanosecond = 1 billionth of a second)
  • They constantly send out radio signals with their location and the time

Orbit Comparisons

  • Satellites farther from earth have longer orbits (distance and time)
    • Geosynchronous orbits match earth’s spin (24-hour orbits); return to same spot in sky every 24 hours
    • Geostationary orbits are equatorial geosynchronous orbits - stay in same spot in sky always
    • Most GNSS satellites are in medium earth orbits (approx. 12-hours; return to same spot in sky twice a day)

GNSS User Segment

Receivers

  • Receivers are devices that receive communication from satellites
  • They are passive, provide for one-way communication from satellite to receiver only
  • Receivers can communicate with various GNSS satellite systems

Satellite Location and Travel Time

  • Receivers compare the time of message against its own clock to arrive at travel time of the message
  • With location and travel time information from four satellites, it can determine its own location

Smartphones are GNSS Receivers

  • Smartphones calculate location using a blend of trilateration based on cell towers and trilateration based on GNSS

The Process of Geolocation with GNSS

Trilateration

  • Trilateration is the determination of a location using s series of distance measurements to other known locations
  • If we do not know the location of R, we can figure it out if we know three locations (S1,S2,S3) and R’s distance to them (d1,d2,d3)

The Concept of the Range

  • Trilateration is thus all based on the distance measurements between the known locations and the unknown location
  • The range is the distance between a receiver and a satellite
  • Trilateration with a series of ranges yields a GNSS receiver’s location

Calculating the Range

  • With GNSS, we know the time message left a satellite
  • The receiver knows the time it received the message
  • do a subtract we can get travel time
  • The messages travel at the speed of light
  • Range = speed of light x travel time

Trilateration in 3D

  • We reference the range calculations against a chosen datum (and its ellipsoid)
  • Three Ranges yields two points, only one will be on the ellipsoid
  • Four ranges yields one location, the receiver is fully geolocated, not just on ellipsoid, but with altitude as well

Uncertainty with GNSS

  • The calculations of the range is subject to error
  • The possibility of error in the range calculations is called uncertainty
  • The position of the receiver according the GNSS system at right is somewhere within the blue circle due to uncertainty in the range calculations(red)
  • Sources of Uncertainty
  • Ionosphere (5-7m): changed particles can cause refraction of radar signal
  • Troposphere (.5m): Changing density of the atmosphere affect the signal
  • Components of GNSS: satellite clocks or orbits or receiver noise
  • Multipath Error: Ground structures can block signals entirely or reflect signals
  • Range uncertainty from each satellite combine to form the area of possible location
  • Positional Dilution of Precision (PDOP) is rated using tetrahedron formed by four most clear satellites. Lower numbers are better

Differential Correction

  • Range measurements are compared with simultaneous range measurements from a base station with a known location
  • Reading from nearby stations will have suffered similar timing errors, and data is corrected accordingly.

Post-Processed Differential Correction

  • Receiver and base station data are transferred to a computer and corrections from base station are applied to receiver data

Real-Time Correction with DGPS or RTK

  • DGPS
    • User receivers correction information and automatically update position
    • Digital correction broadcast over ground-based transmitters
    • Good for sea navigation near coast
  • RTK
    • User receive correction information and automatically updates position from a base station

Satellite-Based Augmentation Systems

  • User receives correction information and automatically updates position
  • Digital correction broadcast over ground-based transmitters

Remote Sensing: Aerial and Satellite Image

Remotely Sensing Defined

  • The science of measuring or inferring the physical properties of an object or medium, using a sensor that is at some distance from the object or medium
  • Typically associated with satellites, but many other sensors

Why is Remote sensing Important?

  • Provides both high-quality local and global data
  • Can show change over time
  • Can provide consistent data
  • Can identify objects
  • Has created a huge archive of geospatial data
  • Provides data and measurements not otherwise possible of collecting
  • Can provide rapidly updated data

Modern Remote Sensing Systems

  • Surface-based: cameras sensing visual light, but also other radiation
  • Aerial: high-quality imagery of small areas
  • Earth observation orbit: most remote sensing satellites

Components of a Remote Sensing System

  • A source of radiation
  • Object
  • Sensor
  • Ground Station
  • Analysis and visualization platform

Electromagnetic Radiation (EMR)

Electromagnetic Spectrum

Sensors and Light Energy

  • Sensors detect energy in different portions (wavelengths) of the spectrum
  • Bands = range of wavelengths; how the light energy is measured and organized
  • Cameras and our eyes measure reflected light energy in the red, green and blue bands
  • Imaging spectrometers measure light energy within and beyond the visible spectrum
  • Most important parts of the EM spectrum for remote sensing are:
    • Visible
    • Infrared
    • Microwave
  • All object above 0 degree K emit radiation and can be sensed

EMR and Object Identification

  • Objects of the same type yield similar measurements of types and amounts of radiation
    • “Objects of the same type” = objects with similar physical and chemical properties
    • “Types of radiation” = radiation of particular wavelengths
    • “Amounts of radiation” = how much radiation is reflected
  • A particular pattern of radiation can therefore be used to identify objects, called its Spectral Signature

Sensors and Systems

  • Active Sensors
  • Passive Sensors

Altitude of Remote Sensing Systems

  • Surface-based: cameras sensing visual light, but also other radiation
  • Airborne: high-quality imagery of small areas
    • Unoccupied Aerial Vehicles (UAV) are commonly referred to as drones. Allow very low altitude data collection
    • Unoccupied Aircraft Systems (UAS) refers to the vehicle as well as the ground control technology and people
  • Low Earth orbit
  • Satellites: variety of types of imagery over large areas

Imaging vs. Non-Imaging Systems

  • For each input, imaging systems record a range of values across the extent of the sensor’s field
  • Each input in a non-imaging systems is a single value - i.e. a point - along with the angle at which it was received
    • The values of many of the inputs can be put combined to create images
    • These include RADAR, LiDAR, SONAR

Light Detection and Ranging(LiDAR) & RAdio DEtection And Ranging & SONAR

  • RADAR is primarily active - Passive only when third party transmitter available
  • LiDAR is always active - measures three dimensional distance between object and sensor -UV, visible, and NIR
  • SONAR is active

Comparison of LiDAR & RADAR

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Remote Sensing and Resolution

Three Types of Resolution

  • Spectral Resolution
    • Range of wavelength
  • Spatial Resolution
    • Size of unit
  • Temporal Resolution
    • Frequency of readings

Spectroscopy

  • Each pixel of a remotely sensed image contains many bands of light information from across the spectrum
  • Simultaneous acquisition of multispectral bands
  • Direct identification of surface materials on a picture-element basis
  • But we can also gain more information about features than just what they look like

Spectral Resolution

  • Visible light is one portion of the electromagnetic spectrum
  • Infrared, NIR, and thermal are also measured with RS sensors

Multispectral vs. Hyperspectral

Spatial Resolution

  • Area on the ground covered by each pixel

Temporal Resolution

  • Interval between flights over the same area
  • Determined by altitude and orbit of the satellite

High Spatial Resolution

  • Under 2m = dimension of each pixel
  • Limited spectral resolution
  • Varying temporal resolutions
  • Commercially available: previously just UAV and Aerial platforms

Medium Spatial Resolution

  • 2m - 30m
  • Global observations of land surfaces
  • Revisit 15-20 days (medium temporal resolution)

Low Spatial Resolution

  • greater than 30
  • Large regions or global
  • Daily revisits (high temporal resolution)

Aerial Imagery

Why use Unoccupied Aerial Systems/Vehicles?

  • Developments in sensors and miniaturization
    • Multispectral, hyperspectral
    • Handheld/ease of use
  • Researchers/industry need imagery that is flexible, affordable, and high resolution
  • UAS have filled this mission space
    • Achieve high resolution with increased affordability and flexibility

Aerial Imagery Variations

  • Camera Angle
  • Sensor

Modern Photogrammetry

  • Aerial Photogrammetry - UAVs
    • Making measurements from photographs
    • Used for survey and mapping
  • Close-Range (Terrestrial) Photogrammetry
    • Potential for 3D models

Satellite Imagery

  • Geosynchronous (or sun-synchronous)
    • orbit in synch with the Earth’s rotation
    • Meterological purposes
    • passes over the same place on the Earth at the same time of day

Constellations

  • A group of orbiting satellites
  • Global or regional coverage
  • Everywhere on earth is visible by at least one satellite OR certain places can be seen very frequently
  • Some constellations are steerable
  • No longer a trade-off between spatial and temporal resolution and spatial extent