The determination of a position from Global Navigation Satellite Systems (GNSS) depends on several factors, including satellite geometry, availability, signal quality, atmospheric delay and environment. The GNSS receiver must correctly estimate GNSS errors and account for these factors to deliver a precise positioning solution.
We combine GNSS and sensor fusion expertise, global corrections services infrastructure, authentication and integrity to mitigate GNSS limitations and deliver precision and reliable positioning. We bring together complementary technologies, solutions and services to deliver the most accurate positions in any environment.
Error Sources
GNSS Error Sources
A GNSS receiver calculates position based on data received from satellites. However, there are many sources of errors that, if left uncorrected, cause the position calculation to be inaccurate. Resolving errors is fundamental to the performance of a GNSS receiver. The more errors a receiver can eliminate, the higher the degree of positioning accuracy and reliability it can achieve.
GNSS System Errors
| Contributing Source | Error Range |
|---|---|
| Satellite Clocks | ±2 m |
| Orbit Errors | ±2.5 m |
| Ionospheric Delays | ±5 m |
| Tropospheric Delays | ±0.5 m |
| Receiver Noise | ±0.3 m |
| Multipath | ±1 m |
There are trade-offs between the different methods of removing errors in GNSS signals. The effectiveness of each method depends on the unique requirements of the application such as level of accuracy, system complexity, solution availability, reliability and cost. The type of error and how successfully it is mitigated is essential to calculating a precise position.
Multi-Constellation And Multi-Frequency
Processing multiple frequencies from multiple GNSS constellations is essential to accurately resolving GNSS system errors.
The Benefit of Multiple Frequencies
Multi-frequency GNSS is the most effective way to remove the largest system error, the ionospheric error, from the position calculation. A dual or multiple frequency receiver can compare the ionospheric delays of two GNSS signals and then correct for the impact of ionospheric errors. Conversely, single-frequency receivers must either rely on broadcast ephemeris or an alternative corrections source like space-based augmentation systems (SBAS) to model the ionospheric errors. Accuracy of these methods are limited, and in the case of SBAS, is regionally specific and can require additional infrastructure.
Multi-frequency receivers also provide more immunity to interference. If there is interference in one frequency band, a multi-frequency receiver will still track the other signals to ensure ongoing positioning.
The Benefit of Multiple Constellations
When we refer to multi-constellation, it means that the receiver can access signals from several constellations: GPS, GLONASS, BeiDou and Galileo, for example. The use of other constellations in addition to GPS, results in there being a larger number of satellites in the field of view, which has the following benefits:
- Reduced signal acquisition time
- Improved position and time accuracy
- Reduction of problems caused by obstructions such as buildings and foliage
- Improved spatial distribution of visible satellites, resulting in improved dilution of precision
When a receiver utilizes signals from a variety of constellations, redundancy is built into the solution. If a signal is blocked due to the working environment, there is a very high likelihood that the receiver can simply pick up a signal from another constellation — ensuring solution continuity.
Positioning Methods
Precise Point Positioning (PPP)
PPP is a GNSS positioning technique that yields sub-metre-level or better positions by combining global GNSS satellite and signal corrections with GNSS receiver error modelling and position estimation algorithms. The correction data required for a PPP solution includes GNSS satellite clock, orbit and signal-bias corrections generated from a network of global reference stations. Once the corrections are calculated, they are delivered to the end user over satellite or the Internet. A typical PPP solution requires a period of time to converge. This convergence period is necessary to estimate local measurement biases. The actual accuracy achieved and the convergence time required is dependent on the quality of the corrections, local-observing conditions, and the sophistication of the receiver algorithms.
Satellite Based Augmentation System (SBAS)
SBAS systems are geosynchronous satellite systems that provide services for improving the accuracy, integrity and availability of basic GNSS signals. Positioning accuracy is enhanced through the transmission of wide-area corrections for GNSS range errors. Integrity is enhanced by the quick detection of satellite signal errors and the sending of alerts to receivers that they should not track the failed satellite. Reference stations, which are geographically distributed throughout the SBAS service area, receive GNSS signals and forward them to the master station. Since the locations of the reference stations are accurately known, the master station can accurately calculate wide-area corrections. Corrections are uplinked to the SBAS satellite then broadcast to the GNSS receivers throughout the SBAS coverage area. User equipment receives the corrections and applies them to range calculations.