Surveying, Civil Engineering, and Construction Sites — Reliable Even Out of Coverage with an LRTK Antenna That Works Offline (208.2 mm Offset)

Table of contents

• Introduction

• Conventional RTK positioning and the challenges of being out of coverage

• RTK without communication infrastructure: How the CLAS method works

• What is an offline-capable LRTK antenna?

• Reliable vertical accuracy with a 208.2 mm offset

• Use cases where it shines out of coverage (mountains, disaster sites, etc.)

• Easy surveying via smartphone pairing

• Towards simple surveying with LRTK (conclusion)

• FAQ

Introduction

RTK (Real Time Kinematic) technology, which can measure position in real time with centimeter-level precision, has become indispensable at surveying, civil engineering, and construction sites. RTK is used for many tasks such as establishing control points, as-built management, guiding heavy machinery, confirming property boundaries, and earthwork volume calculations. However, conventional RTK positioning relies on radio from base stations or correction information transmitted over the internet, so it cannot perform at its best in areas without communication infrastructure such as mountainous regions, remote islands, or disaster sites. If an “RTK that works even out of coverage” could be realized, centimeter-accurate positioning would be possible even where radio is out of range, dramatically improving productivity and peace of mind on site. This article focuses on such a solution — the new “offline-capable LRTK antenna” — explaining how it differs from conventional technology, how it works, and the benefits of its use.

Conventional RTK positioning and the challenges of being out of coverage

First, let’s summarize how conventional RTK achieves high accuracy and why being out of coverage poses a problem. RTK is a method in which a reference station (base station) with a precisely known position and a mobile station (rover) at the point to be measured simultaneously receive GNSS satellite signals, and centimeter-level positioning is achieved by canceling errors through differential processing. Historically, two main means have been used to exchange correction information in real time:

• Local base station method (standalone RTK): The user sets up a GNSS reference station near the survey site and continuously transmits correction data to the rover via radio (e.g., low-power radio). This simple one-to-one setup requires preparing and installing base station equipment and is limited to the radio transmission range (a few km to about 10 km (a few mi to about 6.2 mi)), making it unsuitable for wide-area surveys. Moreover, correction accuracy degrades as the rover moves away from the base station, and beyond about 10 km it becomes difficult to maintain centimeter-level accuracy.

• Network RTK (VRS/Ntrip): Correction information distributed from networks of continuously operating reference stations (CORS) managed by the Geospatial Information Authority of Japan or private operators is obtained via the internet. By equipping the rover with a communication modem and connecting to the service via the Ntrip protocol, high-accuracy positioning over wide areas becomes possible. Distance-related degradation can be compensated by virtual reference station (VRS) technology. However, this method assumes a connection to a communication line, so it cannot be used where cellular signals are out of range. In many cases it also requires a paid service subscription (monthly or annual), posing a cost barrier.

As described above, conventional RTK positioning requires either radio from a base station or internet communication. Therefore, in regions where cellular signals do not reach — such as deep mountains or dense forests — or in situations where base station equipment and communication networks are down due to a large-scale disaster, real-time centimeter-level positioning was often impossible. In such environments, operators had to give up immediate positioning and bring data back for post-processing (PPK), which made rapid response difficult. The need for dedicated equipment and paid services also hindered RTK adoption and dissemination.

RTK without communication infrastructure: How the CLAS method works

A trump card gaining attention to solve the above issues is the Centimeter-Level Augmentation Service (CLAS) provided by Japan’s Quasi-Zenith Satellite System “Michibiki.” By using CLAS, real-time centimeter-level positioning becomes possible without ground base stations or communication lines that were previously essential. CLAS aggregates error information observed by the Geospatial Information Authority of Japan’s continuous GNSS observation network (GEONET) — including satellite orbit and atomic clock errors, ionospheric and tropospheric delays — and transmits that information directly to users from the Quasi-Zenith Satellites. Specifically, correction data are superimposed on the Michibiki satellite’s L6-band signal, and CLAS-compatible GNSS receivers receive and analyze this signal to apply real-time corrections to their positioning. In other words, it is a satellite-communication-type RTK, technically classified as PPP-RTK (Precise Point Positioning-RTK).

With the CLAS method, a standalone receiver can achieve centimeter-level accuracy (half-inch accuracy) without relying on base stations or the internet. The augmentation signal from the Quasi-Zenith Satellites covers most of Japan, and as long as there is a view of the sky, it can be received even in mountainous regions, remote islands, or offshore areas that are otherwise out of coverage. It is truly an innovative technology that realizes “RTK usable out of coverage.” CLAS is provided publicly, so there is no usage fee, and anyone with a compatible receiver can benefit for free. Another great advantage is the simplicity: even in areas without communication infrastructure, turning on the receiver is enough for the satellite to automatically deliver correction information.

What is an offline-capable LRTK antenna?

The LRTK series by Refixia Co., Ltd. brings the CLAS method to the field in an easy-to-use form. Among them, the “offline-capable LRTK antenna” is a high-precision GNSS receiving device specialized for operation out of coverage, configured by combining an external antenna with the smartphone-integrated positioning terminal, LRTK Phone. The LRTK Phone itself weighs only approximately 165 g and is ultra-compact and lightweight, with a thickness of about 1 cm (0.4 in), yet it achieves centimeter-level positioning accuracy of about horizontal ±1–2 cm (±0.4–0.8 in) and vertical ±3 cm (1.2 in) in actual measurements. It is attached to the back of an iPhone or iPad, powered by an internal battery, and connects wirelessly to the smartphone via Bluetooth. By installing the dedicated “LRTK app” on the smartphone and turning on the device, positioning can begin after an initialization of tens of seconds.

The LRTK starter kit that includes the offline-capable antenna contains, in addition to the device itself, a high-sensitivity external antenna and field-use accessories such as a dedicated pole and bipod stand. Attaching the high-performance antenna to the pole tip makes it easier to capture satellite signals even in locations with poor sight lines caused by surrounding trees, enabling stable positioning accuracy in forested mountain areas. Furthermore, LRTK supports network RTK (Ntrip) using the Geospatial Information Authority’s station network when communication is available, allowing flexible switching between conventional base-station-assisted methods and satellite-only operation. In other words, whether or not communication infrastructure exists, this single solution enables high-precision positioning anywhere in the country — an all-in-one positioning system.

Reliable vertical accuracy with a 208.2 mm offset

For high-precision positioning, it is also important to correct for how far the antenna is from the measured point (instrument height). Traditionally, users had to measure the pole length and manually input the correction value each time, but LRTK simplifies that process. When using the offline-capable LRTK antenna, the height from the bottom surface of the LRTK Phone to the antenna is fixed at 208.2 mm (8.20 in). If you set this value as the offset in the app, the instrument height is automatically subtracted and reflected in the measured values. For example, if you place the pole tip (the bottom surface of the LRTK unit) on a benchmark and measure, the GNSS data received at the antenna 208.2 mm above will automatically have 208.2 mm subtracted, so the benchmark’s coordinates are recorded directly. Since this offset is set with a single tap in the app, even beginners can perform height correction without mistakes, ensuring reliable vertical accuracy control.

Use cases where it shines out of coverage (mountains, disaster sites, etc.)

The LRTK antenna’s independence from communication infrastructure makes it particularly powerful in the following situations:

• Mountainous and forest surveying: In deep-forest surveys or forestry investigations where cellular signals are out of range, LRTK can provide real-time high-precision positioning so long as the sky is visible. There is no need to carry heavy base station equipment into rugged mountains, allowing single operators to survey nimbly. It is also advantageous for safely surveying steep terrain where heavy machinery cannot be brought in, using only the minimum necessary equipment.

• Work on remote islands and isolated sites: On islands or remote construction sites lacking communication infrastructure, LRTK enables accurate position acquisition on the spot. Even in geographically isolated locations, surveying can begin immediately upon arrival, greatly improving work efficiency. With conventional equipment, securing communication means could be difficult, but LRTK removes that preparatory burden.

• Disaster response sites: In areas where cellular networks and power supplies are cut off due to earthquakes, heavy rain, or other disasters, LRTK can continue to position autonomously. It can quickly measure terrain changes and damage immediately after a disaster, supporting restoration planning and rescue operations. Continuous observations from immediately after a disaster enabled by LRTK contribute to faster understanding of overall damage and quicker decision-making.

• Large facilities and port areas: For extensive port facilities, large plants, tunnels, and other sites where maintaining stable communications across the entire work area is difficult, LRTK allows surveys across the entire area with stable accuracy. Because coverage is not affected by communication conditions, it brings reassurance to as-built management and stakeout work over wide areas. It also reduces rework and variability typically encountered when covering large areas manually, contributing to improved quality control.

Because LRTK receives one-way augmentation signals from satellites, a single signal can serve multiple receiver users simultaneously. Even on large projects where numerous survey teams work in parallel, LRTK enables efficient operation without increasing the number of base stations or worrying about communication bandwidth.

Thus, in situations where operators previously had to give up real-time positioning or compromise on accuracy due to being out of coverage, LRTK allows immediate acquisition of accurate data on site. Being able to survey anywhere contributes significantly to rapid decision-making and safety management on site. Moreover, the abundance of high-precision field data supports advanced construction management and improved as-built quality.

Easy surveying via smartphone pairing

Traditional GNSS surveying equipment tended to be large, heavy, and required specialized knowledge, typically operated by multiple people. Less experienced staff found them difficult to handle, and site setup took time. LRTK, by pairing with a smartphone, achieves intuitive simplicity that anyone can use.

The LRTK Phone connects wirelessly to a smartphone, and the dedicated app handles everything from starting and stopping positioning to saving data. Tapping the “Start Positioning” button on the phone’s screen instantly acquires the current coordinates, and recording survey points or displaying them on drawings can be completed with a fingertip. Troublesome initial setup and coordinate system adjustments are unnecessary, so you can start measuring as soon as you arrive on site. Combining the smartphone camera with AR (augmented reality) technology makes it possible to visualize survey points on the ground in the video or to check design-versus-field deviations on the spot. Survey tasks that once relied on veteran technicians can be managed by anyone with minimal training using LRTK, enabling safe and efficient solo surveying.

Towards simple surveying with LRTK (conclusion)

Finally, let us summarize the new value LRTK brings to the field. In the past, many small-scale surveys and simple field checks were done with tape measures or handheld GPS devices as “simple surveying,” often with some compromise on accuracy. By introducing LRTK — a surveying device that can fit in a pocket — you can dramatically increase positioning accuracy while maintaining ease of use. This enables even small construction or investigation tasks to obtain high-precision position data immediately, vastly expanding the potential uses of survey data. Once used, users are likely to be surprised by the accuracy and ease, and to appreciate the usefulness of the information obtained on site.

For first-time users of high-precision positioning equipment, LRTK is an ideal entry tool. It operates on the familiar smartphone screen and requires only minimal training to start, significantly lowering the hurdle of “just give it a try.” The experience of achieving high-precision surveying without specialized knowledge can broaden the base of surveying work and improve productivity across the industry. Easy acquisition and sharing of accurate field data also support DX (digital transformation) in construction. Centimeter-level positioning by LRTK may well become the new norm for future field work. As precise positioning becomes accessible to everyone, the conventions of fieldwork are poised for major change.

FAQ

Q: Can you really achieve centimeter accuracy in remote mountains with no radio coverage? A: Yes. The LRTK series supports the CLAS signal from the Michibiki Quasi-Zenith Satellite System, allowing receivers to obtain correction information directly from satellites even in sites without communication infrastructure. Therefore, in locations with an open sky where satellites can be tracked, LRTK can maintain real-time centimeter-level positioning accuracy even when cellular coverage is unavailable.

Q: What is the 208.2 mm offset? A: It is the fixed height dimension from the bottom surface of the LRTK Phone to the external antenna. When using the offline-capable antenna, setting this 208.2 mm (8.20 in) as the offset value in the app causes that amount to be automatically subtracted from GNSS-derived heights to compute accurate elevations. In other words, the device always corrects for the instrument height so you don’t have to worry about complicated height adjustments.

Q: Do I need a base station or a paid correction service to use LRTK? A: No, generally not. LRTK devices can directly receive CLAS satellite augmentation signals, allowing centimeter-level positioning without a private base station or internet-based correction service. Additionally, when cellular communication is available the system also supports network RTK via Ntrip, offering flexibility to switch methods as needed.

Q: Which smartphones or tablets are supported? A: Currently, the LRTK series supports iOS devices (iPhone and iPad). The LRTK Phone is attached to the back of a compatible iPhone or iPad and pairs via Bluetooth with the dedicated app. Newer-generation iPhones typically offer better processing performance and AR capabilities, enabling a more comfortable experience.

Q: Can positioning data be shared and managed? A: Yes. Coordinate and point cloud data acquired with the LRTK app can be shared and managed in real time via the cloud service “LRTK Cloud.” This allows remote offices to immediately check field positioning results, overlay survey data on drawings for comparison, and strengthen coordination between field and office.

Q: How long can it be used continuously (battery runtime)? A: The LRTK Phone runs on an internal battery and can perform continuous positioning for several hours on a full charge (actual time varies with usage). For extended work, it is possible to operate while charging from a mobile battery, allowing flexible extension of field operating time.