This article gives an introduction for the GPS keywords DGPS, multi-frequency and RTK and shows performance demos of some specific RTK GPS systems.
The earth ionosphere leads to reflections of the satellite signal, and that changes (increases) the time-of-flight (ToF) of the signal leading to error-prone distances (wrong measurements) at the GPS receiver.
The solution to this problem is relative measurement (differential GPS): measuring time-of-flight at a 1st near-by static GPS receiver (the ‘base‘) and then using the time difference of the 2nd GPS receiver (the ‘rover‘) measured time-of-flight to compute the relative distance to the static base (see above triangle). DGPS will bring you into the 1cm precision-level (Note: precision at the alone is often not enough, you may also need robustness – more on this further below).
So for a robust RTK GPS system one typically need:
- 2x RTK capable, multi-frequency GPS receivers (one for ‘base’ and one for ‘rover’) with good GPS antennas
- 2x radios to send the correction signal continuously from base to rover (a correction signal every few seconds is sufficient)
Where should the base station be located?
The base GPS system should be located not more than several kilometers away from the rover and it should be located on a fixed point with good sky view so it receives the best signal.
Using multiple GPS frequencies (L1,L2,L5 etc.) allows to filter out a signal that did not come directly from a satellite but instead was reflected at a building etc (multi-path). The higher the frequency the more defined the peaks, and the easier to filter out the unwanted multi-path signals. A multi-frequency GPS receiver will make your GPS robust.
A RTK GPS system will do the computation of the precise GPS position based on the correction signal automatically (integrated into the receiver) and in realtime.
RTK tries to calculate the exact number of radio wavelengths between the satellites and the base station antenna (aka ambiguity resolution) and yields to either a fixed or a float solution. In a fixed solution, the number of wavelengths is a whole number, or integer, and the algorithm is constrained to yield a whole number (a fixed solution typically generates precise coordinates down to 1 cm). In a float solution, the the ambiguity is allowed to be a decimal or floating point number. If the sky view is reduced, the number of satellites is reduced and a float solution is only possible. A float solution typically generates precise coordinates up to approx. 45cm.
Available ‘low-cost’ RTK GPS systems
This table shows some low-cost RTK GPS receivers (up to approx. 500 EUR) and the supported satellite frequencies (L1, L2, L5 etc). Generally speaking, the more frequencies supported, the more robust the system is against multi-path.
Summary of available RTK GPS systems GPS L1 L2 L5 GLONASS G1 G2 BDS B1 B2 B3 Galileo E1 E5a E5b ----------------------------------------------------------------------------------------- reach rs+ (ublox m8p) x x x x piksi multi x x x x ublox f9 x x x x x x x x unicorecomm ub4b0 x x x x x x x x x x x
Depending on the used frequencies of the GPS receiver (L1, L2, L5 etc.) the antennas have to support all the required frequencies too. Additionally, a good antenna also helps to filter out multi-path signals (example: a signal coming from the ground cannot be a direct signal).
Some selected antennas:
- Swift Navigation Mini survey antenna GPS500 (L1/L2, B1/B2/B3, SBAS)
- Tallysman TW3870 / TW3872 (L1/L2, G1/G2, B1/B2/B3, SBAS)
- ANTCOM OmniSTART/CRPA
- Maxtena M1227HCT-A2-SMA (L1/L2, G1/G2)
Use case: Piksi Multi performance demos
- Swift Navigation Piksi Multi (Firmware v2.0) directly near a building (see test points 1, 2 in photo above)
- Swift Navigation Piksi Multi (Firmware v1.4.5) in the backyard
RTK GPS demo applications
1) Robotic lawn mower
DIY box with Piksi Multi GPS receiver evaluation board and 2.4 Ghz radio