Centimeter-Level Positioning with Single-band RTK

Recent commercial developments in GNSS receivers have begun to make the dream of centimeter-level positioning with single-band RTK a reality for certain applications. These receivers use Real-Time Kinematic (RTK) or Differential GNSS (DGNSS or DGPS) methods to reduce the impact on the positioning accuracy of atmospheric and similar effects. Watch the video or read on to learn more…

These developments bring up questions about the applicability of Taoglas GNSS antennas for RTK applications:

  • Will they work?
  • How well might they work?
  • Which antennas can be recommended?
  • Which antennas are not recommended?

In an attempt to answer these questions, a study began (and is on-going) to test a multitude of Taoglas and competitive antennas in an RTK GPS positioning system.
This page presents the results of this study and will be updated when further data is available. The video is a walkthrough of the comparative study between a Quadfilar helical antenna, the Taoglas AQHA.11 and an Active Patch antenna. The executive summary including a number of other antennas is below.

Executive Summary

A number of antennas were connected in succession to standard, off-the-shelf receivers from u-blox. These receivers are the same receivers which Taoglas customers may use in an IoT RTK system (u-blox M8P modules).
To provide RTK (and thus centimeter-level positioning), at least two receivers are required: a base/reference and rover. The base receives the same GNSS signals and sends “correction” data to the rover, which then calculates a precise differential position from the base. For these tests, a Taoglas AQHA.11 was used for the base/reference antenna.

During the test, various types of data were collected, including:

  • Relative position (of the rover from the base), in meters
  • Signal strength (C/N0) from each satellite
  • Success in calculating the RTK position

The latter data type deserves more explanation: even though the base station may be sending proper “correction” data to the rover, the rover might not be able to fully use that data. Things like noisy signals, inconsistencies between satellites, or multipath may pollute the rover’s signal to the point that it cannot calculate the “corrected” position.
Modern receivers (like the u-blox) actually have three levels of correction: Fixed, Floating, and No Correction. Fixed is the best and provides the highest precision but is the most sensitive to the antenna or environment. Floating is an in-between correction that provides some improvement. “No Correction” means that no correction can be calculated and the receiver acts as a “standard precision” GNSS receiver.

How accurate can GPS coordinates be?

Examples of test results are shown below

Figure 1 Taoglas AGGP.18F Deviation Map – an example of a “standard precision” antenna. This plot shows all of the positions collected as a scatter plot.
Figure 2 Taoglas AQHA.11 deviation map – the same plot as the previous figure, but for a high-precision antenna.
Figure 3 Taoglas AQHA.11 deviation map – the same plot as the previous figure, but with a smaller scale.

 

Figure 4 Taoglas AGGP.18F C/N0 vs Elevation – this plot shows the signal strength of GPS and GLONASS satellites (C/N0) against elevation, where 90 (the right side) is zenith (straight up) and the 0 (the left) is looking directly at the horizon. This is a plot for a “standard” grade antenna. Note the inconsistent C/N0 at higher elevations.
Figure 5 Taoglas AQHA.11 C/N0 vs elevation –“Better” and “Best” antennas typically show similar patterns to the above – consistently high C/N0 above ~40° and degrading down to 0°.

 

Figure 6 Taoglas AGGP.18F RTK Availability – a percentage of time spent on each correction type. Note the absence of any “Fixed” corrections.
Figure 7 RTK availability, Taoglas AQHA.11. Note the absence of anything except “Fixed” correction

 

 

 

 

 

 

 

 

 

 

Results

Table 1 provides a comparison between a number of different antennas from a range of different product types. To explain the columns:

  • DRMS: all of the positions collected during the test are collected and boiled down to a single metric, Distance Root Mean Squared (DRMS). Simply know that larger the number, the more spread-out the positions; the smaller the number, the tighter the grouping.
    • A great antenna for RTK should allow a DRMS of < 1 cm.
  • % RTK: a higher number means that a RTK correction was computed for a larger portion of the test.
    • A great antenna for RTK should provide for RTK 100% of the time.
  • Grade: the above two results and other factors were considered to attempt to more simply compare the tested antennas. Briefly:
    • Best: 1-cm or better DRMS; 100% RTK; and “premium” features such as high rejection. These antennas can be references to which other antennas can be compared.
    • Better: 2-cm or better DRMS; 100% RTK. These antennas are solid choices for RTK systems.
    • Good: 1-m or better DRMS; >90% RTK. These antennas provide some RTK but not consistently enough to be recommended for all RTK applications.
    • Standard: these antennas do not provide for consistent RTK and/or positioning and should not be considered for RTK applications. They should be targeted strictly for standard-precision applications.
AntennaPart NumberDRMS%RTKRTK GradeRecommended for RTK
RTKAQHA.110.7 cm100%BestYes
AA.109 on 15cm GP1.0 cm100%BetterYes
AA.160 on 15cm GP0.7 cm100%BetterYes
AA.162 on 15cm GP1.1 cm100%BetterYes
AA.170 on 15cm GP0.7 cm100%BetterYes
AA.171 on 15cm GP1.0 cm100%BetterYes
GPDF.47.8.A.021.0 cm100%BetterYes
FXP.6115.7 cm100%GoodYes
AP.25E65 cm99%GoodYes
AP.25J70 cm99%GoodYes
AP.35E55 cm100%GoodYes
AGGP.25F4.9 cm100%GoodYes
AGGBP.25A68 cm98%GoodYes
AGGBP.25B73 cm93%GoodYes
AGPSF.36B (L1+L5)7.2 cm100%GoodYes
SGGP.18.2.A.022.0 cm100%GoodYes
SGGP.25.4.A.02 GPS/GLONASS SMD Mount Patch (front)SGGP.18.4.A.0824 cm100%GoodYes
SGGP.25.4.A.0230 cm100%GoodYes
MA.22022 cm100%GoodYes
MA.10415 cm100%GoodYes
A.80210 cm88%StandardNo
AP.10E380 cm42%StandardNo
AP.12F280 cm37%StandardNo
AP.17E110 cm94%StandardNo
AP.17F230 cm71%StandardNo
AP.25F320 cm30%StandardNo
AP.35A210 cm73%StandardNo
AGGP.18F230 cm36%StandardNo
AGGP.35F220 cm14%StandardNo
CGGP.35.3.A.02120 cm98%StandardNo
GGBLA.01130 cm100%StandardNo
MA.203120 cm99%StandardNo
AQHA.50112 cm100%StandardNo
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