GNSS Augmentation Systems
The precision and integrity of a GNSS positioning solution can be improved significantly with augmentation. Augmentation is achieved by accessing an external source of corrections and/or warning messages. This information is usually generated from tracking data originating from a GNSS reference station or network.
Figure 1 presents an overview of the continuum of GNSS augmentation services under three main categories, distinguished mainly by end-user requirements and service performance. While sharing a number of common features, the service categories can be significantly different in terms of the density of their tracking networks and the methods used to estimate and disseminate corrections. The main categories of augmentation services are usually referred to as Real-Time Kinematic (RTK), Precise Point Positioning (PPP) or Differential/Wide-Area GPS (DGPS/WAGPS).
Figure 1: Continuum of GNSS augmentation services
Accuracy is a prime consideration in the choice of an augmentation service as it is often critical to meeting application requirements. GNSS augmentation systems deliver centimetre- to metre-level accuracy depending on factors such as the number of frequencies tracked, the types of signals measured (code or carrier), the density of stations in the reference network and the correction update, latency and transmission rates. The effective use of carrier-phase measurements and the ability to recover integer ambiguities may also have a significant impact on accuracy.
The purpose for GNSS augmentation should also be well understood as it determines whether access to a post-mission or real-time service is required, which can significantly affect cost. To improve the coordinate estimates of a station or the trajectory determination of a moving object after a period of data collection, post-mission access to correction messages is usually sufficient. Alternatively, when precise positions are required instantly to stake out a property boundary or guide machinery for precision farming or mining, access to a real-time stream is essential.
Linking to reliable and affordable communication channels is critical to access real-time streams of GNSS corrections for navigation. In areas where cellular coverage is available, corrections can often be made available using wireless internet services at reasonable cost. In remote areas, the use of satellite communications is a more costly option that becomes less reliable at higher latitudes when delivered from geostationary satellites. Deploying dedicated short-wave ground transmitters and radios locally is always a possibility, although covering larger areas where line of sights are obstructed by terrain can become too complex and cost-prohibitive.
While accuracy is often the main factor that determines the selection of a particular GNSS correction stream, overall performance is also measured in terms of reliability and integrity. For instance, auto-steering machinery may require centimetre-level accuracy but tolerate a correction service outage of a few hours overnight, with no impact on productivity. For marine or air navigation, metre-accuracy may be satisfactory, but unexpected outages of a few seconds or undetected errors can seriously compromise safety. In such situations, reliability and integrity become key performance factors.
As providers consider user requirements and operational costs, tradeoffs are usually made to optimize performance for a particular user community. As the density of reference stations and choice of a communication channel (ground- or satellite-based) have significant impact on system cost, providers may offer a variety of user solutions. A good understanding of different aspects of augmentation methods is essential to select the most appropriate solution for an application.