The advent of GNSS auto-steering systems has revolutionized modern agriculture, providing farmers with centimetre-level precision that significantly enhances efficiency and productivity. These systems, which relieve drivers from manual steering, have become increasingly popular in major agricultural regions worldwide. However, their reliance on GNSS positioning technologies poses a challenge when RTK correction signals are unavailable. This article delves into the transmission and reception of GNSS high-precision positioning correction data and explores three main methods to address this issue effectively.
### GNSS RTK Networks
One of the most prevalent solutions for high-precision GNSS RTK service is the establishment of Continuously Operating Reference Stations (CORS) networks. These networks consist of numerous base stations that synchronize and process GNSS data from multiple satellites in real-time, providing precise and continuous 3D corrections. Countries like China, the United States, Germany, and Japan have invested heavily in building extensive CORS networks. For example, China boasts over 6,000 CORS stations, while the United States has more than 2,000. Germany’s SAPOS network includes around 300 permanent GNSS reference stations, and Japan has about 1,400.
Agricultural GNSS auto-steering systems can typically connect directly to these CORS networks via 4G networks, Wi-Fi, or mobile hotspots to obtain RTK corrections. However, in areas with a lower density of CORS base stations, users might opt to build their own reference stations. A single GNSS reference station can provide RTK corrections within a range of 30-50 km, offering a viable solution for localized high-precision needs.
### Single GNSS Station
In mountainous or less developed areas where cellular networks are unreliable, receiving network RTK corrections can be challenging. In such scenarios, users can set up a mobile GNSS RTK reference station. These stations transmit RTK correction data over UHF and typically support protocols like Transparent TT450S and SATEL 3AS. The coverage of a mobile base station is generally 5-8 km, sufficient for most agricultural applications.
Major manufacturers of agricultural GNSS auto-steering systems often include radio modules in their receivers, providing users with alternative RTK correction sources beyond GNSS RTK networks. This flexibility reduces downtime and ensures continuous operation. However, radio signals can be attenuated by obstructions such as tall buildings or dense forests. For larger farms beyond the range of a mobile GNSS base station, combining the GNSS receiver with a radio modem can extend the range to 25-30 km, offering more stable signals.
### Satellite-based PPP
For GNSS auto-steering users, satellite-based Precise Point Positioning (PPP) technology provides a promising alternative. PPP offers global coverage without the need for ground-based reference stations by broadcasting corrections directly from satellites. This enables auto-steering systems to achieve centimetre-level positioning accuracy, with the potential to support an unlimited number of terminals, provided there is sufficient satellite visibility.
One of the significant advantages of PPP corrections for precision agriculture is free access. The Galileo navigation satellite system now offers a globally accessible High Accuracy Service (HAS) based on PPP technology. Companies like CHCNAV have developed a free H-PPP service for their precision agriculture solutions using Galileo HAS. With just one click, users can activate H-PPP when other RTK corrections are unavailable, a feature highly valued by farmers.
Moreover, PPP is easy to use. There is no need to set up a mobile GNSS base station, configure complex parameters, or pay for expensive RTK correction services. The GNSS auto-steering system can achieve high precision with H-PPP, saving users the cost of purchasing mobile RTK base stations or subscribing to RTK services.
Although traditional PPP requires a longer convergence time to achieve the required accuracy, the optimized H-PPP requires only 5-10 minutes to reach a positioning accuracy of 5-10 centimetres. While this is slightly less accurate than traditional RTK’s ±2.5 centimetres, advanced control algorithms ensure that the operational accuracy of agricultural vehicles in H-PPP mode meets most practical needs. This feature is especially valuable in remote areas with poor network coverage, significantly improving operational efficiency.
Satellite-based H-PPP offers a reliable solution for precise farm machinery auto-steering without the need for ground-based GNSS RTK corrections. By optimizing workflows and minimizing downtime, it enables farmers to make data-driven decisions that enhance their farming operations. Ultimately, H-PPP empowers farmers to adapt confidently to diverse agricultural environments, increasing efficiency in today’s competitive landscape.