Source: NASA

Data Acquisition and Management�Unit 10 – Global Navigation Satellite Systems (GNSS)� � �

Based upon work supported by the National Science Foundation under Grants DUE 1304591, DUE 164409, DUE 1700496, DUE 1937177, Due 1938717 DUE 1937237, 2030206 and 2015927. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Author: Wing Cheung

Title: Professor, Palomar College

Assistant Director, GeoTech Center

Email: wcheung@palomar.edu

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Different GNSS

• GPS [US]
• GLONASS [Russia]
• Galileo [EU]
• BeiDou [China]
• IRNSS [India]
• QZSS [Japan]

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Source: GPS.gov, Wikipedia

Global positioning system (GPS)

• NAVSTAR (Department of Defense)
• US Air Force
• 30+ operational satellites
• 12-hour orbit
• Altitude of 12,550 miles

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Source: US Dept of Homeland Security

What’s in a GPS satellite signal?

• GPS date/time, health
• Almanac
• Constellation: collection of satellites
• Nearby satellites
• Acquisition code
• Pseudo random number (PRN)
• Identifier

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Source: EOS

What’s in a GPS satellite signal?

• Precision code
• Pseudo random number (PRN)
• Encrypted
• Distance / Range
• Shifts
• Greater shift = longer distance (R_x)
• Trilateration: narrow down location

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Source: (Been Im et al., 2013)

Source: gisgeography.com

R₁

R₂

R₃

What’s in a GPS satellite signal?

• A PRN example:
• Random number generated every 1/10 second
• Code streamed by satellite:

4685569589966336995233 …

• Code received by receiver:

123356894685569589966336995233 …

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Source: (Been Im et al., 2013)

What’s in a GPS satellite signal?

• A PRN example:
• Random number generated every 1/10 second
• Time difference = 8 digit shift * 1/10 second per digit shift = 8/10 seconds
• Distance / Range = 3,000,000 meter/second * 8/10 seconds = 2,400,000 meters

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Source: (Been Im et al., 2013)

4685569589966336995233

123356894685569589966336995233

8 digit shift

3,000,000 m/s

8/10 seconds

What’s in a GPS satellite signal?

• A PRN example:
• Random number generated every 1/10 second
• Time difference = 8 digit shift * 1/10 second per digit shift = 8/10 seconds
• Distance / Range = 3,000,000 meter/second * 8/10 seconds = 2,400,000 meters

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Common civil GPS frequencies

• Two commonly used frequencies
• L1: 1-10 meter accuracy
• L2: submeter accuracy, atmo. Info.
• Single vs. multi-frequency receivers
• Other frequencies
• L5: transportation focused
• L1C: common civil signal for GPS, Galileo, QZSS, BeiDou

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Source: agsgis.com

Single frequency receiver example (L1 only)

Multi-frequency receiver example (L1-L2-L5)

Sources of GPS range errors

Source: (Won et al., 2015)

Describing location

• Geographic coordinate systems
• Latitude, Longitude
• Angular unit of measurement (DD, DD MM SS)
• Projected coordinate systems
• X,Y Cartesian coordinates
• Linear unit of measurement (ft, m)

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Source: Esri

Describing elevation

• GPS gives the ellipsoidal height (h)
• Ellipsoidal height (h) ≠ Orthometric height (H)
• North American Vertical Datum 88 (H)
• Best available model of the Geoid
• Gravitational & tidal measurements
• Covert h to H with geoid height (N) provided by GEOID99 tool
• (https://www.ngs.noaa.gov/TOOLS/program_descriptions.html)

Source: (Jeffress, 2009)

REAL TIME KINEMATIC (RTK) GPS

CENTIMETER LEVEL ACCURACY FOR SURVEYING AND NAVIGATION

Source: DJI

Real time kinematic (RTK) GPS

• Signal correction modes:
• Real-time
• Post-processing
• Data collection types:
• Kinematic
• Stop-and-go
• Continuous
• Static

Real time kinematic (RTK) GPS components

I. GNSS satellites

• Carrier phase signals/waves
• Shift in waves
• Specialized receivers
• Sources of range errors
• Atmosphere
• Multipath
• Radio interference

Source: Switzerland Federal Office of Topography

Source: (Nasrullah, 2016)

Real time kinematic (RTK) GPS components

II. Reference base station and broadcast tower

• Continually receives satellite data
• Known pre-established hardcoded position (e.g. benchmarks)
• Compare its position according to the satellites vs. its known position = correction adjustments
• Broadcast correction adjustment data over cellular network to rover

Source: Switzerland Federal Office of Topography

Source: (Nasrullah, 2016)

Real time kinematic (RTK) GPS components

II. Reference base station and broadcast tower

• Single base RTK – your own base station
• Network RTK – fixed base stations
• CORS: continuously operating reference stations
• Pros: Don’t need two receivers; Don’t need known benchmark or location; Don’t need to leave device unattended
• Cons: CORS is down for maintenance; No cellular signal; Subscription fees

Source: Switzerland Federal Office of Topography

Source: (Nasrullah, 2016)

Real time kinematic (RTK) GPS components

III. Rover

• Anything gathering data in the field
• Apply correction data
• Distance from base station
• Less than 5 to 10 miles

Source: Switzerland Federal Office of Topography

Source: (Nasrullah, 2016)

Real time kinematic (RTK) GPS

• Data collection mode
• Autonomous/GPS mode
• No base station correction data (3-5m)
• RTK Float mode
• Receiving correction data, but rover is seeing different satellites than base station (<1m)
• RTK Fixed mode
• Receiving correction data and seeing same satellites as base station (<2cm)

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Float (FLT) vs Fixed (FIX) mode

A non-RTK vs RTK GPS comparison with a multi-frequency receiver (Arrow Gold)

Source: agsgis.com

See GeoTech Center website (https://geotechcenter.org) �for additional Model Courses and other curriculum resources.���Note: some content is a derivative of other CC authors�����

Author: Wing Cheung

Title: Professor, Palomar College

Assistant Director, GeoTech Center

Email: wcheung@palomar.edu

​