GNSS has drastically changed the way both humans and machines navigate on Earth, leading many organizations to harness positioning data as they create innovative applications and products. For instance, GNSS technology is extensively used by autonomous vehicles, robots, logistics fleets, and emergency response systems to determine precise locations of people, places, and things – and then use the data to optimize routing.
GNSS signals are often challenged by atmospheric and technological factors that require correction after they’ve been transmitted from a satellite. Various correction methods have been developed to counter these inaccuracies and avoid errors in GNSS-powered solutions. Each method comes with its own set of considerations depending on accuracy requirements and application scenarios. In this blog post, we break down the strengths and weaknesses of the top three GNSS signal correction methods to help developers choose the right solution for their specific use case.
A quick overview of GNSS functionality
GNSS is a navigation system utilizing satellites to provide accurate positioning and timing information anywhere on Earth. A constellation of satellites circling the planet, ground control stations, and GNSS receivers comprise the system. GNSS satellites continuously communicate through signals containing data about their positions and precise time, which are then received and interpreted by GNSS receivers on Earth.
To establish its location on the ground, a GNSS receiver captures these signals and calculates the time taken for them to travel from the satellites. The receiver then uses this time information and the speed of light as inputs to determine the distance between itself and the satellite. Finally, the receiver trilaterates its precise position on Earth’s surface by continuously performing this process and intersecting distance measurements from multiple satellites.
TL;DR: A lot of math happens that can be quickly derailed by even the slightest signal inaccuracies.
Explaining the top 5 reasons for GNSS signal inaccuracies
When choosing the most suitable GNSS correction method for your specific use case, it is essential to understand signal errors and their underlying causes. Ephemeris inaccuracies, discrepancies in satellite clocks, ionospheric disturbances, tropospheric conditions, and biases among various satellite systems are the main factors that cause errors in GNSS signals. Understanding these challenges in detail is helpful when choosing a correction method for a particular application or product.
Ephemeris data errors
Many factors contribute to ephemeris data errors, which must be corrected for precise positioning.
Ephemeris data, an input used by GNSS to calculate location, is information about a satellite’s orbital parameters and its position in space. When ephemeris data is inaccurate, it can impact the overall accuracy of GNSS positioning calculations that determine the satellite’s location – and as a result, the receiver’s position on Earth.
In simpler terms, if the GNSS lacks precise knowledge of the satellite’s actual position or uses incorrect location information, the intersection of signals will not align correctly. As a result, the accuracy of the position and signal transmission/reception time will be compromised, leading to less accurate results.
Satellite clock inconsistencies
Major positioning inaccuracies can result due to minor differences in atomic satellite clocks.
GNSS Satellites are equipped with highly accurate atomic clocks, but these clocks aren’t free from sources of error. Over time, subtle variations in the clock rate can introduce significant timing errors. The tremendous speed at which GNSS satellites travel, reaching approximately 7,000 mph, introduces relativistic differences in time on the ground and in space – which are predictable. There are also other errors which are random. Although the ground segment attempts to correct for both predictable and random variability in clock rate, these updates only occur every few hours, and clock errors inconsistencies do occur.
Accurate distance measurement in GNSS requires precise clock synchronization because GNSS receivers utilize these timestamps to calculate their distance from a satellite. A tiny discrepancy of merely a nanosecond can lead to substantial positioning errors, as this directly affects the receiver’s perception of the time taken for the signal to travel from the satellite to Earth.
Ionospheric conditions
Charged particles in the ionosphere must be accounted for in positioning calculations due to their impact on GNSS signals.
The ionosphere constitutes a layer in the Earth’s upper atmosphere composed of charged particles capable of influencing the speed of light and radio signals. Fluctuations in solar radiation and other conditions can delay or distort GNSS signals, resulting in measurement errors that need correction to achieve accurate positioning on Earth.
Although the ionosphere’s impact can cause significant errors in signal interpretation, the good news is that these errors can be effectively modeled and corrected from a relatively large distance. With the right technology, the adverse effects of the ionosphere can be mitigated in GNSS signal calculations.
Tropospheric conditions
Tropospheric elements must be corrected from a close distance because of their hyper-local impact on GNSS signals.
The troposphere, the atmospheric layer responsible for weather phenomena, is another factor that can influence GNSS signals. Fluctuations in temperature, humidity, and atmospheric pressure within the troposphere can lead to errors in GNSS measurements by modifying the speed of light and any subsequent signal interpretation. While these tropospheric effects have a milder impact on signal interpretation compared to other factors, they are highly localized and require modeling and correction from a relatively close distance.
Code bias and group delay
Disparate time references and frequencies from various satellites can contribute to inaccurate positioning calculations.
Lastly, it’s worth noting that GNSS satellite systems are operated by different countries or organizations, each utilizing a distinct time reference and frequency. The disparities between signals originating from these various satellite systems, known as code bias or group delay, can also give rise to errors in signal measurement and impact the accuracy of GNSS positioning.
Top 3 GNSS correction methods – and how to choose the best one for your use case
When deciding on a GNSS signal correction method, understanding these sources of error and their impact is critical. RTK, PPP, and SSR all have their own advantages and disadvantages depending on your use case, so knowing their specific methodologies can help you optimize for your particular objectives.
Real-time kinematic positioning (RTK)
RTK requires an extensive network of base stations across a large geographic area, but results in efficient and highly precise GNSS signal corrections.
RTK correction is widely considered the gold standard in high-precision GNSS signal correction. This approach involves setting up a base station in close proximity to the target area (typically within 30-50 kilometers). The base station then transmits a reference signal to the GNSS receiver, enabling precise positioning calculations. Many of the previously mentioned signal effects have minimal impact because the reference signal and receiver are physically close together on Earth.
It becomes possible to calculate the positional discrepancy between the reference station and the GNSS receiver by comparing these subtle signal differences. Given the high accuracy of the reference station’s position, adding this discrepancy to the reference station’s coordinates results in extremely accurate GNSS receiver positions. Any signal inaccuracies that do exist can be corrected by measuring the difference between the reference station position and the GNSS receiver.
Despite its reputation for remarkable precision, the primary setback of classical RTK solutions lies in the need for an extensive network of base stations to facilitate global-scale signal correction. Establishing and maintaining such a dense infrastructure can quickly become cost-prohibitive for some organizations.
Most ideal for: Autonomous vehicle and consumer navigation in developed areas
Less ideal for: Positioning applications in remote areas
Precise point positioning (PPP)
PPP works at a much slower speed than other solutions to create accurate signal corrections.
PPP uses a limited number of highly precise and accurate stations for signal correction. Essentially, the PPP algorithm divides the correction calculations between these stations and GNSS receivers.
PPP stations model various known error sources within the GNSS system, including ephemeris inaccuracies, clock discrepancies, and group delay. This information is then transmitted to GNSS receivers, which perform further calculations based on local conditions to close the error gap. By combining the signal data collected over time with the known error sources from PPP stations, GNSS receivers measure both universal and localized errors (such as ionospheric and tropospheric effects), ultimately determining the necessary signal corrections.
While PPP offers exceptional accuracy, the scarcity of PPP stations leads to slower signal correction times. For instance, it may take about 20-25 minutes for signal correction (compared to a matter of seconds with other methods). Correction may take even longer in extremely challenging conditions, as the receiver needs to independently calculate both ionospheric and tropospheric effects.
Most ideal for: Heavy equipment or similar applications that can wait for accurate positioning information, or that operate in areas where RTK service may not be possible, like in water or remote locations.
Less ideal for: Consumer GNSS, autonomous vehicles, and other applications requiring almost instantaneous positioning information.
State space representation (SSR)
SSR is the latest innovation in signal correction, and delivers precise corrections at a relatively high speed.
SSR is the latest innovation in signal correction, and represents the forefront in GNSS technology. It not only provides essential data like ephemeris, clock, and code bias discrepancies, similar to what PPP offers, but also includes information about localized ionospheric and tropospheric interferences.
Nevertheless, a significant challenge lies in the fact that many GNSS receivers are not equipped to effectively process and utilize all the data provided by SSR, making it difficult to convert it into meaningful positions. To address this issue, SSR data can be transformed into a virtual base station (VBS). Essentially, this VBS acts as a simulation of an RTK base station for conventional receivers, allowing them to tap into the benefits of SSR data. This approach enables the broader adoption of SSR and makes high-precision positioning more accessible to a wider range of GNSS receivers.
Most ideal for: Automotive and robotics use cases, or users with low-level access to the GNSS hardware that can integrate the needed support software.
Less ideal for: Teams using generic receivers.
Choosing a GNSS correction solution
Each method for correcting GNSS signals offers different levels of precision and suitability for specific applications. However, the rising demands of organizations seeking the highest precision in their positioning requirements mean all correction methods must be scalable, efficient, and accurate.
Point One operates an extensive network made of more than 1,440 global base stations, with over 900 located in the US alone. This network continues to expand regularly in response to user demand, ensuring the delivery of centimeter-level GNSS signal corrections wherever they are required, including areas without cellular coverage.
Leveraging this robust network, Point One has developed a comprehensive suite of GNSS correction products, catering to the diverse needs of various organizations. These offerings range from classic RTK to SSR and everything in between, enabling users to choose the most suitable solution to achieve precise positioning.
Polaris RTK
Point One’s Polaris RTK solutions enable developers with high-precision GNSS signal corrections globally without needing to manage their own base stations. By using any RTK-compatible GNSS receiver, centimeter-level corrections can be achieved within seconds, providing rapid and accurate positioning data for applications that demand both speed and precision.
Positioning accuracy: centimeter-level
Correction speed: <5 seconds
Receiver required: Any RTK-compatible receiver
Polaris SSR
Polaris SSR solutions from Point One merge their vast network of global RTK base stations with cutting-edge SSR technology, bridging the gaps that typically require the creation of a VBS. The world’s most comprehensive RTK network can be utilized with any RTK-compatible receiver, allowing for 5-10 centimeter corrections in under 30 seconds. This speed is over 80 times faster than traditional PPP solutions, delivering highly efficient and precise positioning capabilities.
Positioning accuracy: 5-10 centimeters
Correction speed: <30 seconds
Receiver required: Any RTK-compatible receiver
Polaris VBS
Point One’s Polaris VBS solutions empower generic receivers to make accurate signal corrections for teams who aren’t able to set up their own VBS but are looking for a blend of traditional and cutting-edge technology. Point One’s VBS network communicates with any GNSS receiver to make 5-10 centimeter level corrections in approximately 15 seconds. This makes it ideal for teams looking to improve speed with a legacy receiver.
Positioning accuracy: 5-10 centimeters
Correction speed: <15 seconds
Receiver required: Generic receiver
Learn more about Polaris correction solutions
GNSS signal correction methods are continuously evolving alongside technological advancement, resulting in more accessible high-precision positioning and increased reliability across a wide range of applications. Point One offers a comprehensive suite of correction solutions that empower developers to build location-based applications and products with the highest possible precision.
To determine the best GNSS signal correction method for your application, get in touch with a Point One expert.
