Lecture 4 - Network RTK, GNSS Augmentation, GNSS Modernization (GUEST LECTURE)

Lecture 4: Network RTK, GNSS Augmentation, GNSS Modernization (GUEST LECTURE)

Structure

• Company Info (not covered)
• Principle GNSS
• Principles GNSS Augmentation(RTK, Network RTK, PPP, PPP-RTK)
• Improvements by new GNSS(Galileo, Beidou)

General Conclusions

• GNSS positioning is influenced by absolute and distance dependent errors
• DGNSS and local RTK systems can only handle absolute errors– > limited RTK (and DGNSS) range
• Network RTK models all error sources – > Therefore Network RTK (and DGNSS) offers:
• homogeneous accuracy, reliability and availability

1. Principal GNSS

• signal travel time = tst - trec
• distance = travel time light speed(3 10^8 m/s)

Figure 1: GNSS Constellation

Coordinates

1. RTK GNSS provides positions in:
   • European system (ETRS89)
   • Dutch national grid (RD)
2. RD system features:
   • Used for all objects in Netherlands
   • Uses absolute X-Y coordinates
   • Height measured relative to NAP (Amsterdam Ordnance Datum)
3. Technical procedures:
   • RDNAPTRANS2008™ implemented in GNSS receivers/software
   • Updated to RDNAPTRANS2018™ in October 2022
4. ETRS89 and WGS84
   • ETRS89 - derivation of WGS84 that is pinned to the European continent (so that it doesn’t change over time)
   • Need to use the same coordinate system for all applications
   • Receiver integrates transformation from ETRS to X,Y

Error sources

• Absolute positioning: one stand-alone GNSS receiver - suitable for navigation but not surveying (1 ~5m)
• Satellite orbit error
• Satellite clock error
• Ionosphere (100 -1000 km above earth - electrons are activated by solar activity and affect speed of GNSS signal)
• Troposphere: temp, humidity and pressure affect signal transmission
• Multipath: local, sat signal is reflected by the surface before reaching receiver (therefore distance measurement is affected)
• Receiver clock error

Figure 2: Influence error sources

Differential GNSS Positioning

1. Error Source Determination
   • calculates errors at known pos
   • generate corrections
2. Correction Transmition: from base station to mobile rover
3. Apply Corrections
   • rover uses corrections
   • reduces error effects
4. Position Determination using:
   • DGNSS: code observations
   • RTK: carrier phase observations(more accurate real time kinematic)
5. Distance dependency of DGNSS and RTK: the greater the distance is, the larger the error (the distance between reference and unknown place)
   
6. RTK Limitations: if too far from reference, the error is no longer acceptable
   
7. Network GNSS
   • Better Coverage: multiple reference stations from a network, better coverage than single based station, thus reliable error handling
   • Distance Benefits: overcome distance limitations of single based RTK, interpolate error between reference stations so rover accuracy can be determined

VRS Representation

This combines network reliability with single-base simplicity, ideal for static/slow-moving applications.

1. System Components

• Network of reference stations
• Computing center
• Virtual station created 1-2m from user

1. Process Flow

• Reference stations send data to computing center
• User connects to system
• Center interpolates network corrections
• Corrections delivered as if from nearby virtual station

1. Benefits

• RTK-level accuracy
• Compatible with standard rovers
• Minimal representation error
• Efficient data transmission

1. Limitations

• Not suitable for fast-moving objects over large distances (e.g., trains)
• Virtual reference can't maintain stability for long-distance movement

State Space and Observation Space

Figure 3: State and observation state

Explanations: Observation Space：RTK Services: raw measurements from satellites, code ranges, carrier phases, etc. direct observations State Space: SSR: Proccessed error components, troposhpheric delays, satellite orbits, etc. error sources and physical models

PPP-RTK

Figure 4: PPP-RTK

1. Direct OSR
   • Base measures satellite distance
   • Direct line-of-sight transmission to rover
2. Two-Way OSR
   • Rover sends position (GGA)
   • Base returns OSR corrections
   • Two potential error paths
   • Requires mobile network
3. SSR to OSR Conversion
   • Rover receives state space data
   • Converts to observation space internally
4. Pure SSR
   • Rover gets satellite data directly
   • Self-improves measurements
   • Independent position determination

2. Augmentation

Precise Point Positioning (PPP)

• GNSS satellite orbits and clock corrections broadcasted by geostationary satellite or internet; does not require bi-directional communication

Satellite-based augmentation systems (SBAS)

• EGNOS (which provides correction for GPS)

Ground-based augmentation systems (GBAS)

• sent via mobile internet (4G), or radio if own base is used.

Augmentation Comparisons

Figure 5: Techniques comparison

PPP and Network_RTK

Figure 6: PPP and Network RTK

3. Modern GNSS

Developments

• SBAS(EGNOS) developed
• GLONASS 24 satellites
• increase usage of rtk network
• mobile internet (5G/3GPP)
• GPS L2C: beter quality for civil user
• 3rd frequency GPS(L5)
• Full constellation of Galileo and Beidou

Figure 7: Satellite system

Improvements

• Availability of satellites, especially near obstacles:
  • More satellites means lower DOP values, more visible in sky plots
  • Skyplot affected by urban canyons, may not see minimum of 5 satellites for RTK, but combining systems could help
• Speed of initialisation (more=beter)
• Accuracy
• Robustness against solar activities

Conclusion:

1. more and larger constellations is better
   • Better Availability
   • Better Accuracy
   • Faster and more reliable fix of phase ambiguities over longer distances

• Resistance against atmospheric disturbances

1. Network RTK is still needed