Centimeter-level accuracy (half-inch accuracy) even without network coverage! Smartphone GNSS supporting mountain-area mega-solar sites

If the latest GNSS positioning technology could enable centimeter-level accuracy (half-inch accuracy) even at remote mountain sites where cellphone signals do not reach――this article explains the benefits of smartphone GNSS that works outside network coverage and the technologies that make it possible, using a mountain-area mega-solar construction site as an example.

Site conditions of mountain-area mega-solar construction and communication infrastructure challenges

Large-scale photovoltaic power plants (so-called mega-solar) installed by clearing slopes and forests in mountainous areas face unique challenges due to their locations. One such challenge is inadequate on-site communications infrastructure. In mountain regions far from urban areas, there are often “no-coverage” zones where mobile phone and Internet signals do not reach. Even within vast development sites, attempts to use positioning equipment or work terminals can be hindered by the inability to secure connectivity, causing data transmission to stall.

The terrain features typical of mountainous regions also affect on-site conditions. In valleys and highly undulating terrain, sight lines are poor and signals from base stations can be blocked. Communication-poor environments make tasks such as coordinating among construction staff and remotely monitoring heavy equipment operations difficult to carry out smoothly. A particularly problematic issue is that in work requiring high-precision positioning, network connectivity needed to receive real-time positioning correction information can be interrupted. Mega-solar construction requires consistently accurate location information for tasks such as site-wide surveying and stake verification for panel installation, but constraints in communication infrastructure can impede these processes.

Obviously, options such as installing temporary relay antennas or using satellite communications are considered to supplement connectivity, but these approaches are often costly or impractical. Ultimately, establishing a system that allows work to proceed regardless of communication infrastructure is an urgent need at such sites.

Importance of high-precision positioning and limitations of conventional methods (RTK dependence, base station deployment)

On large development sites like mega-solar projects, even errors of several meters can cause serious problems. For example, if the installation position of solar panels shifts, spacing between adjacent panels can be thrown off, hindering racking work or obstructing sunlight and affecting power generation efficiency. Therefore, from site development to equipment installation, surveying and staking must be performed with centimeter-level accuracy (half-inch accuracy).

However, conventionally achieving such high-precision positioning on site has involved various constraints. Standard GNSS (GPS, etc.) positioning alone yields errors of about 5-10 m (16.4-32.8 ft), so it cannot be used standalone without augmentation. The RTK (real-time kinematic) method has therefore been used for corrections, but RTK positioning requires a pair of devices: a reference station and a rover. When within network coverage, network RTK can be used to receive public reference station data via NTRIP over mobile networks. In places like mountainous areas where internet connectivity is unreliable, the method has been to deploy a private base station on site and communicate with the rover via radio.

When deploying a private base station, constraints include the need to determine precise reference coordinates in advance and the limited radio range between base station and rover. RTK equipment itself is also expensive and requires specialized setup, making operation difficult without personnel skilled in surveying. Deploying a base station or dispatching a surveying team at each mountain-site mega-solar project is inefficient and costly. Despite the importance of high-precision positioning, conventional methods present significant hurdles in both communications infrastructure and equipment/personnel. There is also the approach of recording observational data for later analysis in the office (post-processed positioning), but this does not provide immediate on-site results and reduces work efficiency.

Network-off GNSS positioning technologies (CLAS and evolution of smartphone receivers)

Recently, new GNSS positioning technologies have emerged to solve these issues. One keyword is “CLAS (centimeter-class augmentation service).” CLAS is a positioning augmentation service provided by Japan’s Quasi-Zenith Satellite System “Michibiki,” which transmits correction information calculated from the Geospatial Information Authority of Japan’s electronic reference station network via satellite. In simple terms, instead of receiving correction information from a base station over the Internet, users can receive correction data directly from satellites overhead. This enables real-time centimeter-level positioning (half-inch accuracy) even in no-coverage mountain areas, provided the sky is sufficiently open. Currently four quasi-zenith satellites are in operation, and the system is scheduled to expand to seven satellites by the late 2020s, which should provide more stable augmentation signals even in mountainous areas.

Another keyword is the evolution of smartphone receivers. Traditionally, high-precision positioning using CLAS required dedicated, bulky receivers. Now, compact high-performance GNSS antennas and receivers can be combined with smartphones. Modern smartphones support multiple frequency bands such as L1/L5 and multiple GNSS constellations including GPS, GLONASS, Galileo, and QZSS. Reductions in ionospheric error and increased satellite visibility have dramatically improved stability and accuracy. Moreover, using external receivers that connect to smartphones can compress surveying equipment that once weighed several kg into pocket-sized devices. In other words, the “smartphone + GNSS” combination is making an era in which centimeter-level accuracy can be achieved without a base station even outside network coverage a reality.

Practical on-site use cases (staking, point clouds, as-built measurements, etc.)

How exactly can these network-off compatible smartphone GNSS solutions be useful at mountain-area mega-solar construction sites? Here are several use cases.

• Positioning for staking: When installing foundations or supports for solar panels, piles must be driven at precise positions. Smartphone GNSS can immediately navigate workers to stake-out positions based on design coordinates. By checking the current position displayed on the smartphone screen, workers can mark the designated spot and drive the pile on the spot. Traditionally, a surveyor would use a total station to stake out positions and then instruct workers, but smartphone GNSS allows a single person to intuitively carry out staking. In addition, height information obtained by GNSS can be used on site to check pile driving depth and level, contributing to quality control of foundation work.

• Topographic recording by point cloud surveying: For mountain developments, it is important to record and manage terrain changes caused by cut-and-fill in detail. Combining a smartphone camera or LiDAR sensor with GNSS makes it possible to obtain site terrain as point cloud data. For example, walking around scanning the ground with a smartphone can complete wide-area 3D surveying in a short time. Because the acquired point cloud is tied to geographic coordinates (absolute coordinates), it can be directly used for as-built management and volume calculations. Being able to digitally record terrain with just a smartphone without hauling heavy laser scanners up is a major advantage. For distant areas that a smartphone LiDAR sensor cannot capture directly, photogrammetry using photos taken from multiple positions can supplement the data. Combining LiDAR and cameras as needed allows precise digital measurement of even vast mountainous terrain.

• As-built measurement and quality control: High-precision GNSS is also powerful for as-built measurement to verify that completed earthworks match design shapes and dimensions. Distances and height differences between two distant points can be measured immediately on site and compared with design values on the smartphone. For instance, the height or slope of a racking structure for panel installation can be checked on the spot using GNSS positioning, and any deviations can be corrected immediately. Real-time measurement and verification help prevent rework and ensure quality. By measuring multiple points in sequence, the overall finish of the site can be understood in a short time. Measurement results are automatically recorded digitally, simplifying the preparation of inspection reports and contributing to more efficient quality management.

As described above, smartphone GNSS usable outside network coverage supports various processes from surveying to construction management. The fact that tasks once requiring separate equipment and procedures—such as guiding pile locations, acquiring terrain point clouds, and verifying finished accuracy—can now be almost entirely completed with a single smartphone greatly increases on-site productivity.

Portability and simplicity unique to smartphone GNSS

The portability and ease of operation of smartphone-based GNSS positioning are major reasons for its growing attention. Conventional high-precision GNSS surveying equipment requires tripods and mounting fixtures, and transporting and setting them up on site consumes time and effort. By contrast, smartphone GNSS equipment fits in the palm of the hand, making it easy to carry into mountainous sites on foot. Devices can be handled single-handedly even on steep slopes, allowing light movement to measurement points. Because the equipment is lightweight, the burden of transporting gear is greatly reduced even on steep slopes without trails.

Also, user-friendly smartphone app interfaces mean that non-specialists can operate them intuitively. Functions such as real-time display of one’s position on a map that navigates to the target point, and cloud-linked features that save and share measurement results on the spot, provide conveniences unique to smartphones that support fieldwork. For example, arrows or bearings displayed on the smartphone screen guide users to specified measurement points so they can reach targets even in complex terrain. Photos and notes can be recorded during positioning, eliminating the need to carry paper drawings or multiple instruments.

In addition, smartphone GNSS systems have the advantage of easy power supply. Conventional devices required batteries or generators, but combinations of smartphones and small receivers consume little power even with long operation times, and can be conveniently charged with mobile batteries. This portability and simplicity enable continuous surveying and monitoring operations across vast mega-solar development sites.

Spillover effects: labor reduction, safety, and real-time management

Introducing smartphone GNSS usable outside network coverage brings not only direct on-site efficiency improvements but also various positive effects in related areas. One such effect is contributing to labor reduction. Surveying and staking traditionally required multiple people, but smartphone GNSS makes it possible for a single person to complete more tasks. This not only reduces headcount but also helps cover for shortages of skilled technicians, enabling limited personnel to handle more sites.

Improvements in safety are also notable. The risk of falls and accidents is reduced because fewer people need to set up surveying equipment or physically enter hazardous slopes and unstable areas for measurements. Utilizing smartphone camera functions and AR displays makes it possible to position or monitor required points from a distance, allowing workers to assess situations from safe areas.

Furthermore, if positioning data is shared to the cloud in real time, site supervisors and remote managers can immediately grasp the situation. This enables real-time progress and quality management and quick implementation of countermeasures when problems arise. Remote specialists can also provide immediate advice by viewing cloud data, enabling collaborative real-time work.

Moreover, efficiency gains from such digitalization can shorten construction schedules and reduce costs. Accumulated surveying and construction data stored in the cloud can be analyzed to improve future planning and facilitate knowledge sharing across sites. The on-site introduction of smartphone GNSS thus contributes not only to improved positioning accuracy but also to work-style reform in construction and the promotion of smart construction. The future in which bringing just a smartphone and a small GNSS device into a remote mountain site enables everything from high-precision surveying to construction management is already within reach.

Field deployment proposal: LRTK for network-off operation, cloud linkage, and AR guidance

As described above, there are many benefits to high-precision positioning and smartphone GNSS use outside network coverage at mountain-area mega-solar sites. Finally, we introduce one solution that realizes these capabilities: LRTK. LRTK is a high-precision GNSS positioning system designed to work with smartphones that supports RTK positioning outside network coverage, data linkage with cloud services, and AR-based visual guidance.

One feature of LRTK is its compatibility with the CLAS signal from the Quasi-Zenith Satellite Michibiki. This enables centimeter-level positioning (half-inch accuracy) without internet connectivity even at sites outside mobile coverage such as mountainous areas. Positioning data, acquired point clouds, and photos can be stored and shared in the cloud, allowing field information to be delivered to the office instantly and promoting collaborative work. In addition, AR technology can overlay design drawings and measurement points on the smartphone screen to provide intuitive guidance to workers. For example, visualizing pile positions or excavation limits for heavy equipment through the screen can reduce on-site mistakes and enhance safety.

LRTK offers these cutting-edge technologies in an all-in-one solution, lowering the barriers to field deployment. By simply attaching a dedicated compact device to a smartphone, anyone can handle survey-grade accuracy, enabling even untrained workers to perform on-site positioning tasks. The assurance of centimeter-level accuracy (half-inch accuracy) outside network coverage, together with smart on-site management using cloud and AR, means LRTK can make mountain-area mega-solar construction more efficient, safer, and more reliable. LRTK has already begun to be introduced in mountain civil surveying and solar power plant construction sites, making high-precision positioning that was once difficult increasingly practical. These efforts align with the Ministry of Land, Infrastructure, Transport and Tourism’s i-Construction initiative and construction DX (digital transformation) policies, and the adoption of smartphone GNSS for smart construction is expected to accelerate. A future where bringing only a smartphone and a small GNSS device into a remote mountain enables everything from high-precision surveying to construction management is right around the corner.