In construction, just a fraction of a centimeter can directly affect a project’s budget, timeline, safety, and more. And with autonomous vehicles moving at highway speeds, a few centimeters can be enough to avoid a wreck.
That precision is a tall order for GPS, Galileo, GLONASS, Beidou, and other GNSS, whose accuracy is limited to about 1.5-10 meters. The only way to achieve decimeter-, centimeter-, or even millimeter-level accuracy is by augmenting standard GNSS with correction services that provide additional location information.
Correction services are necessary because a host of factors undermine GNSS availability and accuracy. For starters, GNSS satellites orbit at between 19,000 to 36,000 kilometers, so their signals are weak by the time they reach the Earth. That makes them vulnerable to atmospheric interference.
| Item | Description | Typ. Error |
|---|---|---|
| Satellite Clock Errors | Position depends on clocks. Each satellite’s clock can wander. Correction information is sent down from each satellite. Without correction, this error can be up to 300 km. | 0.4 – 1 m |
| Satellite Position Errors | Position also depends on knowing the position of each satellite. The satellites transmit their own position (ephemeris) but this isn’t perfect. | 0.3 – 1 m |
| Ionospheric Delay | The upper layers of the atmosphere are “ionized” by the sun, which interacts with signals sent between satellites and the Earth. Stand-alone receivers can use a mathematical model to provide some correction. Without correction, this can be 7 m. | 1 – 3 m |
| Tropospheric Delay | The Troposphere is the lowest layer of the atmosphere and where we live. Rain, fog, and other water in the air delays the signal. This delay varies by location, height, and angle to the satellite. Models can be used to reduce the error. | 0.2 m |
| Receiver & Antenna Biases | Receivers have biases that introduce errors. These are typically small, on the order of cm. Antennas can also introduce biases (phase center and group delay). | 0.2 m |
| Multipath | As signals travel from the satellite to the Earth, they bounce, reflect, and distort. | 0.2 m |
| Total | Single-Frequency Receiver | 2.3 ~ 5.6 m |
| Dual-Frequency Receiver | 1.5 ~ 2.8 m |
There are three ways to categorize correction services:
- Some require a subscription, such as real-time kinematic (RTK), precise point positioning (PPP), and L-Band. This can be an issue for highly price-sensitive applications.
- Some use an internet connection to deliver correction data, such as RTK. This means the device will need a cellular module and subscription, which would be two additional costs. And if the device is used in places where cellular service is unavailable, then it won’t have access to the correction data.
- Some deliver correction data over satellite, such as L-Band services. One drawback is that their signals are susceptible to the same vulnerabilities as standard GNSS, including attenuation by dense foliage and concrete canyons and jamming and spoofing.
Also known as precise point positioning (PPP), L-Band correction services achieve accuracy of about 20 centimeters. (For more information, see “How to Leverage the L-Band to Balance Accuracy and Affordability for GNSS Applications.”)
If the application requires accuracy as low as 1 centimeter, the best solution is a correction service that uses terrestrial reference stations broadcast location data. One example is RTK, which also can be combined with PPP in a hybrid approach. (For more information, watch the on-demand webinar “High Precision GNSS and RTK Positioning” and read “Does Your Positioning Application Require Centimeter-Level Accuracy?”)
Galileo’s New High Accuracy Service
As the need for correction services grows, so do the options. The latest example is the Galileo High Accuracy Service (HAS), a PPP service that provides real-time orbit, clock, and bias data to achieve up to decimeter-level accuracy. Here are the top six things to know about the first phase of HAS, which launched in 2023:
- It’s free.
- It’s global.
- It enables horizontal accuracy <20 cm and vertical accuracy of <40 cm.
- Its correction data covers orbits, clocks, and biases.
- It covers multiple constellations and frequencies: Galileo E1, E5a, E5b, and E6 signals, as well as GPS L1 and L2C.
- It delivers correction data via satellite (in the Galileo E6-B signal at 1278.75 MHz), unlike RTK, which typically requires cellular connectivity to receive internet-delivered data. Operating independently of ground-based networks enables simpler, lower cost devices, which need only a GNSS receiver and antenna to receive correction data. As a result, HAS is a good fit for applications that require both high accuracy and low cost.
Phase 1 provides HAS Service Level 1 capabilities, albeit with some coverage and performance limitations. Phase 2 will upgrade it to full operation capability and provide additional features.
