What Is the 208.2 mm (8.20 in) Offset Value of the Out-of-Coverage LRTK Antenna? How to Set It and Usage Scenarios

Table of Contents

• What is an out-of-coverage LRTK antenna

• What the 208.2 mm (8.20 in) offset value means

• How to set the offset value

• Usage scenarios for the out-of-coverage antenna

• Simple surveying achievable with LRTK

• FAQ

What is an out-of-coverage LRTK antenna

First, let’s explain what an “out-of-coverage LRTK antenna” is. The LRTK series are high-precision GNSS receivers designed to be attached to smartphones (mainly iPhones), providing a solution that enables centimeter-level positioning easily on site. These LRTK devices typically support network RTK positioning via the internet. However, in locations where cellular service is out of coverage—such as mountainous areas or construction sites where radio signals do not reach—you cannot receive RTK correction data via the internet. That’s where the “out-of-coverage LRTK antenna” comes in.

As the name implies, the out-of-coverage LRTK antenna is an optional antenna that enables high-precision positioning even when you are outside cellular coverage. By attaching this antenna to an LRTK unit, you can receive correction data directly from satellites that would normally be obtained via an internet connection. Specifically, it can receive correction signals from the Quasi-Zenith Satellite System (QZSS) in Japan, namely the Centimeter-Level Augmentation Service (CLAS). With an LRTK equipped with a CLAS-capable antenna, you can achieve real-time positioning with centimeter-level accuracy in environments without cellular service, so long as the sky is open and satellite signals can be received. The LRTK receiver itself supports triple-frequency GNSS (L1/L2/L5), and when used with the out-of-coverage antenna it can also receive the QZSS L6 band (CLAS signals). Multiple-frequency observations make it easier to obtain a stable, high-accuracy solution even in challenging environments.

The out-of-coverage antenna is a compact device integrating the antenna and battery, and it can be easily attached to an iPhone. It weighs a few hundred grams and is compact enough to hold in one hand. It’s easy to carry on site and can be attached and used whenever needed. Even when using the out-of-coverage antenna, you can still use conventional network RTK (correction services over the internet such as Ntrip). In other words, attaching the out-of-coverage LRTK antenna provides flexibility to switch between internet-based corrections and satellite-based corrections depending on site radio conditions. When cellular service is available, you can use conventional network RTK; in remote mountainous areas without coverage, the system can automatically switch to CLAS signals from satellites to continue positioning. This allows a single LRTK unit to perform high-precision surveying anywhere in Japan.

What the 208.2 mm (8.20 in) offset value means

So what exactly is the “208.2 mm (8.20 in)” offset value? Simply put, it is a value used to correct the height difference (offset) between the LRTK antenna and the survey point. In surveying, when there is a vertical difference between the GNSS antenna (receiver) and the ground point or control point you want to measure, you must correct for that difference to obtain accurate heights (elevations) or coordinates. To correctly correct this vertical difference, you need to set the antenna offset in advance.

With the LRTK series, the antenna is sometimes mounted on a pole (monopod) to measure ground control points. In that case, the antenna is raised above the ground. The out-of-coverage LRTK antenna is a small GNSS device that mounts on a smartphone, but in the field it is often attached to the tip of a dedicated pole to improve accuracy and to touch the ground measurement point. The setting that subtracts the height by the amount the antenna is raised above the ground is the “offset value.”

The value 208.2 mm (8.20 in) is the manufacturer-recommended offset for the out-of-coverage LRTK antenna. Specifically, it corresponds to the vertical distance from the antenna body of the out-of-coverage LRTK antenna to the bottom surface of the attached smartphone (LRTK Phone). This distance—just over approximately 20.82 cm (8.20 in)—means the antenna is positioned that much higher than the measured point, so by setting 208.2 mm (8.20 in) as the offset (the value to subtract from height) the surveying software will automatically subtract that amount.

In other words, when you set an offset of 208.2 mm (8.20 in) when surveying with the out-of-coverage antenna, about 20.82 cm (8.20 in) will always be subtracted from the measured height result. As a result, the output will closely match the actual ground elevation. The 208.2 mm (8.20 in) value is the precisely calculated vertical distance between the antenna center and the smartphone bottom surface determined by the manufacturer, so users can simply input this value to perform accurate height correction without complex calculations. Conversely, if you forget to set this offset, the reported height will be about 20 cm (7.9 in) higher than the actual value, so be careful.

How to set the offset value

Now let’s look at how to actually set the 208.2 mm (8.20 in) offset value in LRTK. The basic procedure is as follows.

• Attach the out-of-coverage LRTK antenna: First, attach the out-of-coverage LRTK antenna to your smartphone (such as an iPhone). It is designed to snap onto a dedicated smartphone case, so no complicated wiring is required. Once the antenna is securely fixed, mount it on a pole (monopod) or tripod as needed (attaching it to the pole tip makes it easier to touch the ground point).

• Launch the LRTK app and connect: Launch the LRTK app on your smartphone and confirm the connection to the antenna (GNSS receiver). When connected via Bluetooth or a Lightning connector, the app screen will show the receiver’s connection status. After connecting, check that the positioning mode is operating normally (i.e., it is in Float or Fix rather than noRTK).

• Enter the offset value in positioning settings: Open the antenna height (offset) item from the settings menu in the LRTK app. The item may be labeled “antenna height” or “offset value.” Enter the value 208.2 mm (8.20 in) there. App unit settings may differ; if the app accepts millimeters, enter “208.2” as is; if it requires centimeters, enter “20.82.” Save the settings or simply close the settings screen to apply the value.

• Verify results during measurement: With the offset set, perform surveying. For example, touch the pole tip (the ferrule) to the point to be measured and press the “position” button on the smartphone screen to acquire coordinates. Check the acquired height (elevation) to confirm that the offset has been automatically applied. In other words, even though the antenna was about 20.82 cm (8.20 in) higher than the ground, the reported value will have that amount subtracted.

The LRTK app also provides a one-touch toggle for offset settings. When switching between handheld measurement and pole measurement on site, you can enable or disable the pre-registered 208.2 mm (8.20 in) offset value with a single button, making adjustment smooth according to the situation. Once you input 208.2 mm (8.20 in) it will be remembered for subsequent sessions, so you don’t have to re-enter it each time (depending on app behavior, most apps save the last set value). If you change the pole length itself, add that difference as an additional “antenna height” value or use the app’s pole height adjustment feature if available. In any case, when using the out-of-coverage antenna, correctly setting the baseline offset value of 208.2 mm (8.20 in) is key to achieving high-precision positioning.

Usage scenarios for the out-of-coverage antenna

The combination of the out-of-coverage LRTK antenna and the 208.2 mm (8.20 in) offset setting enables easy high-precision surveying in environments that were previously difficult. Here are some specific scenarios where it is useful.

• Mountainous or forested areas outside cellular coverage: For sites like dam construction areas or forest surveys deep in the mountains where cellular signals don’t reach, the out-of-coverage antenna is reassuring. Because it can receive CLAS signals directly from QZSS, you don’t need to set up an independent base station or drive to the foothills to establish communication. It’s ideal for remote mountain surveys and infrastructure inspections.

• Offshore and remote island surveying: Offshore locations and remote islands often have unstable or no cellular service. A system with an out-of-coverage LRTK antenna can receive augmentation signals from satellites and perform high-precision positioning on a boat or on an island. This is helpful for coastal facility surveys or measuring positions from a ship or small boat during infrastructure inspections on remote islands.

• Agriculture, forestry, and rural areas: In rural or forested regions with limited cellular infrastructure, the antenna shines. It enables accurate positioning across large agricultural fields or for locating forest work roads, where standalone positioning used to have large errors. In agriculture it can assist autonomous tractors and field management; in forestry it can help confirm logging area boundaries.

• Emergency surveying after disasters: Immediately after disasters like earthquakes or heavy rains, communication infrastructure may be down. With an LRTK equipped with an out-of-coverage antenna, you can quickly survey affected areas. Using satellite corrections, you can immediately measure positions of collapsed structures or ground deformations to aid recovery planning. Having a surveying method that does not rely on communications is extremely valuable for disaster prevention and mitigation.

• Large sites and near-underground facilities: On large sites like golf courses or significant earthworks, there may be pockets with poor cellular reception. Underground facilities or tunnel entrances can also have unstable communication. The out-of-coverage antenna helps maintain positioning accuracy even when you enter areas where communication intermittently drops (provided satellite signals are receivable at the surface). It’s also effective for continuous surveying while moving across wide sites.

As illustrated above, the out-of-coverage LRTK antenna is a powerful tool for “measuring accurately anywhere.” Situations that previously required bringing a base station or costly mobile communication equipment can now be handled simply with an LRTK unit and this antenna. Additionally, if you correctly set the 208.2 mm (8.20 in) offset, you can obtain height information in the field that is immediately usable. This will significantly improve surveying efficiency for personnel on site.

Simple surveying achievable with LRTK

The biggest feature of the LRTK series is that it enables high-precision positioning to be performed easily by anyone. By attaching a dedicated device to a smartphone and using an intuitive app, on-site measurement is possible without specialized knowledge. Especially when combined with the out-of-coverage antenna, surveying that previously required surveyors or expensive equipment can be completed by a single person in a mobile manner.

Imagine one worker carrying an LRTK Phone walking around a site and collecting coordinates and elevations for required points one after another. The collected data can be uploaded to the cloud on site and shared immediately with staff in the office. There is no need to carry heavy total stations or conventional GNSS equipment; with a pocketable LRTK unit you can perform “simple surveying.”

Furthermore, LRTK supports photogrammetry and LiDAR scanning (point cloud acquisition using the LiDAR sensor on iPhone Pro models), allowing you to attach high-precision position information to the acquired data. This enables point cloud creation for as-built management and recording locations of buried utilities to be done efficiently by a small team. AR features also allow overlaying design drawings on the actual scene in the field. The scope for using measured data expands, and information sharing between contractors and clients becomes smoother.

By using LRTK, on-site surveying work is drastically streamlined. You can obtain high-precision position information in real time and immediately record and share it, greatly reducing time and labor compared to traditional methods. If you are interested in introducing a high-precision positioning solution, consider the new simple surveying style enabled by LRTK. With one LRTK per person, you can collect data faster and more accurately than before.

FAQ

Q. Is it absolutely necessary to set the 208.2 mm (8.20 in) offset for the out-of-coverage LRTK antenna? A. Yes—setting it is strongly recommended to obtain accurate positioning results. The 208.2 mm (8.20 in) value corrects the vertical difference between the antenna and the survey point. If you do not set this, the acquired height coordinates will be about 20 cm (7.9 in) higher than actual. When elevation or height information is important, entering the offset correctly will eliminate this error.

Q. How was the 208.2 mm (8.20 in) value determined? A. The 208.2 mm (8.20 in) value is based on the vertical distance from the antenna phase center to the smartphone bottom surface when the out-of-coverage LRTK antenna is attached to a smartphone. The manufacturer measured this precisely during the design phase, so users do not need to measure it themselves. Entering this specified value will apply the appropriate height correction.

Q. Can the out-of-coverage antenna be used anywhere? A. Basically, it can be used anywhere outdoors where the sky is visible within Japan. The out-of-coverage antenna receives CLAS signals from satellites, so it cannot be used indoors, in tunnels, or in deep valleys where GNSS and QZSS signals cannot be received. However, it works fine in mountainous areas or remote islands without cellular reception so long as the sky is open. Outside Japan, CLAS signals are not provided, so the antenna’s full capabilities are limited to domestic use (and nearby CLAS coverage areas).

Q. Are there additional fees or contracts to use CLAS? A. No, CLAS (the Centimeter-Level Augmentation Service) is provided by the Japanese government (Cabinet Office) and is free to use with a compatible receiver. Unlike commercial correction services that use cellular networks (subscription-based network RTK services), receiving CLAS signals incurs no fee. However, you do need to purchase compatible devices such as the LRTK unit and the out-of-coverage antenna.

Q. Do I need to set the offset when using a normal LRTK device? A. Even when not using the out-of-coverage antenna, you may need to set an offset depending on the surveying situation. For example, if you attach an LRTK receiver to the tip of a pole to measure a ground point, you need to enter the offset for the height the antenna is raised above the ground. Offset values for models other than the out-of-coverage antenna may differ from 208.2 mm (8.20 in) because distances vary with device shape and antenna position. As a rule, use the manufacturer-recommended value or measure the vertical distance from the antenna center to the survey point and set it accordingly. If you measure handheld directly to the ground point so the antenna is essentially at the measurement point, you can consider the offset to be 0.

Q. Can I make an existing LRTK unit out-of-coverage capable? A. It depends on your LRTK model. Current LRTK Phone 4 series (out-of-coverage models) can add CLAS reception by purchasing and attaching the optional out-of-coverage antenna. However, early LRTK models or unsupported models may not be able to use the out-of-coverage antenna because the hardware does not support CLAS signals. Check the manufacturer’s information to confirm whether your unit supports the out-of-coverage antenna.