GPS accuracy has come a long way since the system was opened to non-military applications in 1983. Civilian GPS receivers initially were limited to an accuracy of about 330 feet (100 meters) until 2000, when that restriction was eliminated.
Another improvement was additional signals. GPS initially had just one signal for civilian applications: L1 at 1575.42 MHz, which enabled location accuracy of 3 meters. Today, L1 can be combined with the newer L2 (1227.60 MHz) and/or L5 (1176.45 MHz) signals to increase accuracy down to 1.5 meters. That’s because L2 mitigates ionosphere-related errors by providing a second signal to augment L1. The L5 signals also are more powerful than L2 signals.
But even 1.5 meters isn’t granular enough for many business applications, such as autonomous material handlers at ports and logistics parks, surveying, semi-autonomous vehicles, and precision agriculture. In some cases, the solution is to subscribe to an L-Band service, whose data augments GPS and other GNSS constellation signals to achieve accuracy of about 20 centimeters. (For more information, see “How to Leverage the L-Band to Balance Accuracy and Affordability for GNSS Applications.”)
Other applications require accuracy as low as 1 centimeter, which only a real-time kinematic (RTK) correction service subscription can achieve. Here’s how RTK works, the implementation options and limitations, how it’s evolving, and how to add it to IoT devices and other products.
Correcting GNSS Errors to Achieve Centimeter-Level Precision
Whether it’s GPS, Galileo, GLONASS, or another constellation, all GNSS signals are susceptible to errors caused by orbit deviations, satellite clock drift, and atmospheric latency. RTK services use terrestrial base stations that apply sophisticated statistical methods to the received GNSS signals to eliminate the most common errors.
Each RTK base station has a coverage area of about 10-20 kilometers. The base stations transmit the corrected signal data to nearby subscriber devices, typically once per second. For example, the GNSS module in a vehicle receives the RTK data to correct its position in real time.
Geodnet, Point One Navgation, and RTK FIX are examples of RTK subscription services. Some providers charge a fee, while others are free. See https://ntrip-list.com for a list of services available in a particular area, such as a state, country, or continent.
Depending on the application, another option is to build an RTK system. For example, an agriculture co-op could install one or more base stations to create an RTK service for its members.
Coverage and Other Caveats
There are several factors to consider when deciding whether to use RTK:
- Cellular Coverage: An internet connection is necessary to receive the corrected data. That can be a challenge in rural areas and other places where cellular service is spotty, slow, or unavailable. If cellular service is available but slow — such as because it’s only 3G — users could choose to receive updates every 5, 10 or 30 seconds rather than in 1-second intervals. However, that would undermine positional accuracy. A better option is to choose a high-gain cellular antenna to make the most of weak signals. Another option is to build a local “reference” station that uses Wi-Fi or LoRa to send the correction data.
- Base Station Coverage: A base station typically has a range of up to 20 kilometers. Thus the density of base stations in the application’s area — such as a county or state — determines whether the mobile devices will always be in an location served by at least one base station. This can be a challenge for private, small-scale RTK services such as the agriculture co-op example. In those cases, a subscription to a commercial RTK service may be necessary to ensure blanket coverage.
- Signal Degradation: GNSS signals are relatively weak by the time they reach the Earth. Dense foliage and tall buildings can attenuate them even more. All of this can undermine RTK performance.
The good news is that GPS and other GNSS systems have modernization programs that are continually adding signals, including ones with higher power, all of which will help mitigate attenuation. (For more information about GNSS modernization programs, see “How the GNSS Evolution Enables Future Proofing.”)
Some other helpful trends are:
- RTK base station networks are steadily expanding thanks to collaborations between private companies, government agencies, and research institutions.
- Research is underway to use 5G cellular to augment RTK services, including in challenging locations such as urban canyons.
- RTK is increasingly leveraging artificial intelligence (AI), LiDAR, and other technologies to maximize accuracy.
How to Add RTK Quickly and Cost-Effectively
Taoglas developed the EDGE Locate platform to streamline the process of adding RTK functionality to devices. This helps OEMs reduce the cost and lead time of developing centimeter-level positioning applications.
EDGE Locate integrates the Taoglas AGPSF.36G Embedded Active Dual-band GNSS L1/L2 Stacked Patch Antenna with the u-blox ZED-F9P GNSS receiver, all in a single package. This combination enables concurrent reception of GPS, GLONASS, Galileo, and BeiDou. EDGE Locate achieves position accuracy of up to 0.01 m + 1 ppm CEP with RTK and up to 0.05 m without RTK. A blue and silver electronic components
Edge Locate is ideal for device OEMs, systems integrators, and end users that want to future-proof their IoT applications. For example, multi-constellation support provides both flexibility and redundancy, while RTK support allows devices and applications to accommodate market changes such as increased accuracy requirements.
To learn more about Taoglas EDGE Locate, visit https://www.taoglas.com/product/edge-locate. And for more information about RTK, check out the “High Precision GNSS and RTK Positioning” webinar.
High-Precision Alternatives to RTK and L-Band
RTK and L-Band are not the only options for maximizing accuracy. Depending on the application’s requirements, a high-precision GNSS antenna solution can be a viable alternative for optimizing accuracy. Taoglas offers a broad, deep selection of high-precision GNSS antennas, along with engineering services such as:
