Does Your Positioning Application Require Centimeter-Level Accuracy?
Here’s How to Achieve That with High-Precision GNSS.
Autonomous vehicles and drone light shows are two examples of how just 1 decimeter can mean the difference between success and failure. They’re also examples of why high-precision global navigation satellite system (GNSS) technology is increasingly becoming a key requirement for positioning applications.
To operate safely on crowded highways and tight city streets, autonomous vehicles need highly accurate location information. To create realistic-looking images, thousands of drones need to be packed as tight possible for the same reason that higher pixel density makes a 4K TV look more lifelike than a 1080p HD set.
Those requirements are a tall order for GPS, 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 technology that provides additional location information.
The challenge isn’t that standard GNSS is inherently limited, but rather that these systems are all subject to a host of error sources that undermine overall performance. GNSS systems engineers and scientists have created an “error budget” to highlight where error creeps in (or, where the accuracy leaks away), much like how you might need to write down where your money goes every month.
The graphic and table below illustrate GNSS positioning errors in further detail.
| 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 |
High-precision GNSS technologies mitigate these challenges by using a second receiver nearby to provide the primary receiver with correction data. These systems are also known as differential GNSS (DGNSS or DGPS), whose name refers to the “differencing” that the two receivers do with the data. The primary, high-precision receiver analyzes the correction data and its own data to find the difference between the two, which enables it to remove sources of error common to both receivers.
Differential receivers can remove or reduce the following sources of error:
- Satellite clock errors
- Satellite position errors
- Ionospheric delay
- Tropospheric delay
- Antenna biases (under specific circumstances)
Additionally, dual- or multi-frequency receivers can significantly reduce the effect of Ionospheric Delay (down to ~0.2 m) by taking advantage of the fact that different frequencies travel at different rates through the Ionosphere. This is reflected in the error budget.
There are multiple methods, which can be used individually or combined. Each method has its owns pros and cons:
Differential GPS (DGPS) or Differential GNSS (DGNSS):
- Summary: DGPS/DGNSS specifically uses the differences of the “code phase” (also called “pseudorange”) between two receivers to remove many of the errors in the budget.
- Advantage: Simpler to implement in the receiver, this method is more widely available than other types
- Disadvantage: Because this method uses the code rather than the carrier, the ultimate precision available is much more limited
Post-Processed Kinematic (PPK) or Real-Time Kinematic (RTK):
- Summary: Kinematic methods use both the code phase and carrier phase to find the position. These methods can be used after the fact (post-processed, as in PPK) or in real-time (as in RTK).
- Advantages: The ultimate precision of Kinematic methods is excellent, achieving centimeter or decimeter accuracy even for devices in motion.
- Disadvantages: Because Kinematic methods use carrier phase, they are more susceptible to multipath and antenna biases. There’s also no guarantee that a Kinematic solution can be found, so in practice there will be times (especially for moving receivers) where the accuracy is no better than a stand-alone receiver.
Precise-Point Positioning (PPP):
- Summary: PPP isn’t a traditional differencing approach like DGNSS or PPK/RTK. Instead, it uses a variety of methods (dual frequency; carrier phase; substitute satellite clock and position information) to provide precision positioning.
- Advantages: Although a data link is still required, this method does not require a second receiver. This method provides the ultimate in precision, allowing for centimeter- or even millimeter-level positioning.
- Disadvantages: This method still requires a data link (generally to the internet or via satellite). This method also can take a substantial amount of time to “converge” on the desired position —easily tens of minutes.
The bottom line is that drone OEMs, Tier 1 automotive suppliers, and other companies have a lot of options for achieving decimeter-, centimeter-, or millimeter-level accuracy. Taoglas’ team of Engineering experts are available to help navigate those options and choose the ideal solution. We’ve developed a helpful workflow to explore our extensive range of antenna design and testing services, accessible via the button below.
