External GNSS Receivers for Smartphones: Accuracy & Use Cases | LRTK

Table of Contents

• Accuracy and Limits of Smartphone Internal GPS

• What Is an External GNSS Receiver for Smartphones?

• How Does an External GNSS Receiver Improve Surveying Accuracy?

• Benefits of Smartphone GNSS Surveying

• Key Points for Introducing Smartphone GNSS Surveying

• Use Cases for Smartphone GNSS

• Simple Surveying with LRTK

• FAQ

Accuracy and Limits of Smartphone Internal GPS

Recent smartphones come standard with GNSS (Global Navigation Satellite Systems), including GPS, and are widely used for everyday map navigation and location-based services. However, their positioning accuracy is generally on the order of several meters and is insufficient for applications like surveying, where errors of a few centimeters are unacceptable. You have likely experienced your position being shown off the road in a smartphone map app or your travel trace being recorded offset from the actual route. These errors are caused by satellite signal errors and reflections from buildings (multipath), and cannot be avoided by the smartphone’s built-in GPS alone.

Of course, smartphone positioning performance has improved, and the latest models support multi-GNSS—using multiple satellite systems—and reception of multiple frequencies such as L1 and L5. Also, by incorporating augmentation signals from Japan’s QZSS “Michibiki,” errors when stationary can in some cases be reduced to around 1 m (3.3 ft). Nevertheless, they still do not reach centimeter-level accuracy, so surveying tasks such as boundary confirmation, installation of structures, and as-built measurements remain beyond their capability. Vertical errors on smartphones are often tens of centimeters or more, making them unsuitable where strict height measurement is required. Against this backdrop, “external GNSS receivers for smartphones” have emerged to leverage the convenience of smartphones while achieving surveying-level high accuracy.

What Is an External GNSS Receiver for Smartphones?

An external GNSS receiver for smartphones is, as the name implies, a high-precision GNSS receiving device used by connecting it to a smartphone or tablet. By attaching a small antenna-integrated GNSS module to the smartphone or wirelessly connecting via Bluetooth, you can obtain high-precision positioning information instead of relying on the phone’s internal GPS. Traditional surveying GNSS equipment (RTK kits) required a substantial setup—stationary base station and rover plus dedicated controller and radio equipment. In contrast, a smartphone external GNSS receiver allows positioning with just a pocket-sized receiver and a smartphone. The receiver contains a high-sensitivity antenna and positioning chip, while the positioning computations and logging are handled by a dedicated app on the smartphone.

The spread of GNSS receivers that integrate with smartphones has begun only in recent years, drawing attention in the surveying and GIS industries. The advantages of external smartphone GNSS receivers start with overwhelming portability. They are lightweight—about 100–200 g—and do not burden the smartphone when mounted. They are easy to carry on a surveying pole without feeling heavy, making equipment transport to the field significantly easier. Connecting to a smartphone also enhances operability and extensibility. You can start/stop positioning and enter point names on a familiar smartphone screen, and perform intuitive tasks linked to maps and camera images. Moreover, using the smartphone’s connectivity you can retrieve correction information over the internet or instantly upload measurement data to the cloud, enabling digital integration not available with conventional instruments. In short, an external GNSS receiver for smartphones is an attachment that transforms a smartphone into a high-precision positioning device, offering the functions of traditional dedicated equipment in pocket size.

How Does an External GNSS Receiver Improve Surveying Accuracy?

So how much can surveying accuracy actually improve by using an external GNSS receiver with a smartphone? In short, it can dramatically reduce positioning errors from meter-level to centimeter-level. Typical smartphone internal GPS errors are around 5–10 m, but using a high-precision GNSS receiver that supports RTK (Real-Time Kinematic), you can achieve survey-grade accuracy such as 2–3 cm (0.8–1.2 in) in horizontal position and a few centimeters in vertical direction. For example, when walking around a park track, the smartphone’s internal GPS trace may deviate from the sidewalk at certain points, but an external GNSS receiver plus RTK will keep the trace on the sidewalk consistently, allowing you to determine exactly where within a road width of several m (several ft) you were walking. This is because the rover (smartphone side) receives real-time correction data from a base station installed at a known position, canceling out errors.

RTK corrects satellite signal errors (such as ionospheric delay and clock errors) to achieve high accuracy that standalone positioning cannot provide. In Japan, network RTK services utilizing networks such as the Geospatial Information Authority of Japan’s GEONET are available, and by connecting the smartphone to the internet you can easily obtain correction information. In practice, starting a smartphone plus external GNSS receiver and connecting to network RTK can yield a “fixed solution” in as little as about 1 minute, after which centimeter-level position updates continue. Furthermore, receivers that support Michibiki’s centimeter-level augmentation service (CLAS) can maintain high-precision positioning by receiving correction signals directly from satellites even in mountain areas without cellular coverage. These technologies allow a smartphone plus external GNSS receiver to achieve accuracy comparable to conventional dedicated surveying instruments. In good open conditions, standalone observations can be very stable, and if you take multiple measurements on the spot and average them you may even approach millimeter-level precision. In other words, using an external GNSS receiver for a smartphone, one can confidently say that surveying accuracy rises to at least the level of a few centimeters in both horizontal and vertical directions.

Benefits of Smartphone GNSS Surveying

Combining a smartphone with an external GNSS receiver offers many benefits. The main advantages are summarized below.

• Lightweight, compact, and highly mobile: External GNSS receivers for smartphones are compact and lightweight enough to fit in a pocket. They weigh less than a few hundred grams and can be carried with a smartphone in one hand. There is no need to carry large tripods or stationary base stations, and you can easily collect points while moving around the site.

• Simple and intuitive operation: With a dedicated app, you can record and save survey points with a tap while checking your current location on a map. Familiar touchscreen operation and Japanese input make entering point names and notes smooth. Complex settings typical of professional equipment are guided step by step, making the devices easy to use even for those with little surveying experience.

• Real-time data management and sharing: Positioning data is instantly saved on the smartphone and can be automatically synced to the cloud via cellular networks. You can share measurement results with the office on the spot and multiple people can view the data simultaneously. There’s no need to record in a paper field book and transcribe later, reducing transcription errors. You can directly compare acquired coordinates with design drawings or decide immediately if additional measurements are needed, reducing rework.

• Multifunctionality through smartphone integration: Combining the smartphone’s camera and sensors enables diverse applications. For example, adding high-precision geotags to photos lets you accurately understand site conditions later. You can use AR to display virtual stakes or design lines on-site (layout guidance), or synchronize GNSS positions with point clouds obtained by the smartphone’s LiDAR for 3D measurements—completing surveying, recording, and visualization on a single smartphone.

• Significant reduction in introduction cost: The cost of performing high-precision GNSS surveying drops dramatically. Traditionally, an RTK-capable GNSS kit required an initial investment of several million yen, but external GNSS receivers for smartphones are increasingly available for a fraction of that—depending on the product, many are obtainable within several hundred thousand yen. Since you can use an existing smartphone, there’s no waste and no need to buy dedicated controllers or large batteries. As a result, it becomes realistic for each field staff member to have a high-precision positioning terminal “one per person,” eliminating inefficiencies due to waiting for equipment.

• Addressing labor shortages and promoting DX: Since operation is simple and easy to learn, not only veteran surveyors but also younger or cross-discipline technicians can perform high-precision positioning. This makes it easier to share surveying tasks even where experienced personnel are scarce, contributing to labor-saving and efficiency. Furthermore, it enables a shift from paper-based workflows to digital and automated workflows, offering significant value as part of construction DX (digital transformation). Real-time accurate data can be applied immediately to site management and quality control, improving overall productivity and safety.

Key Points for Introducing Smartphone GNSS Surveying

Here are several points and precautions to keep in mind when introducing and operating an external GNSS receiver for a smartphone.

• Check compatible smartphones and apps: Supported smartphone OS and models may be limited depending on the device. Confirm before purchase whether your smartphone (iOS/Android) is supported. High-precision positioning typically requires the manufacturer’s dedicated app or specific surveying apps. When using it for the first time, you can start positioning by following the app’s instructions, but it’s reassuring to read the manual or tutorials in advance.

• How to obtain RTK correction information: Correction data from base stations is essential to achieve centimeter accuracy. Generally, you connect over the internet to a regional base station network (e.g., GEONET or commercial VRS services) and receive data using the Ntrip protocol. Commercial services may require monthly subscriptions, but in some regions administrative correction information is available free of charge. In areas where network access is difficult, receiving corrections directly from satellites—such as Michibiki’s CLAS—can be effective (device support and service area are limited). Prepare the correction method that fits your area and use case.

• Measures for satellite reception environment: GNSS positioning is affected by the surrounding environment. In urban canyons or dense forests satellite signal reception degrades and accuracy drops, so observing in as open sky as possible is ideal. If you must measure in challenging environments, accuracy can be improved by repeating short observations and averaging, choosing times when satellite geometry is favorable, or using a longer pole to receive signals as high as possible. In urban shadows, neither smartphone GNSS nor conventional equipment is perfect, but multi-GNSS and multi-frequency receivers can capture more satellites and stabilize positioning even under some reflective conditions.

• Stable equipment setup and battery: For high-precision positioning, it is important to keep the receiver stable and still. For point-by-point measurements rather than while moving, attach the smartphone and receiver to the supplied monopod or pole and hold it vertically on the ground. Height offsets can be easily set in the app. Using these accessories prevents hand shake and yields stable measurements. Also, manage batteries for both smartphone and receiver. Receivers have internal batteries lasting a few hours, so carry a spare mobile battery for long workdays. Since GNSS and communications drain the smartphone battery, having spare power is advisable.

• Use in official surveying: If you intend to use positioning data from an external GNSS receiver for official survey results, confirm that the equipment meets required accuracy standards. Recently, compact smartphone-mountable devices have been registered as “Class 1 GNSS surveying instruments” by the Geospatial Information Authority of Japan, and models usable for control point surveys and public surveying have appeared. With such equipment and proper measurement procedures, results can be accepted for official surveying. However, observational methods and durations must comply with regulations, so when producing official results be sure to conduct sufficient accuracy control under the supervision of a licensed surveyor. In any case, small GNSS receiver performance improves year by year, and under the right conditions they can produce results comparable to conventional large instruments.

Use Cases for Smartphone GNSS

Here are some concrete scenarios where external GNSS receivers for smartphones are useful.

• Urban surveying: In cities with many high-rise buildings, signal reflection and blockage are challenges, but modern smartphone GNSS receivers with multi-frequency support reduce errors compared to before. In urban areas with good cellular coverage, obtaining network RTK corrections in real time is easier, enabling stable centimeter-level positioning. The compact, lightweight smartphone GNSS equipment is also highly maneuverable in narrow alleys and around buildings, making it suitable for recording locations of infrastructure while walking. Urban infrastructure inspection, which used to require time and manpower, can be streamlined.

• Use in mountainous or no-coverage areas: In remote mountains or islands without cellular coverage, previously you had to bring your own base station and radio communication. While smartphone GNSS receivers generally require network connection, CLAS-compatible models can receive augmentation directly from Michibiki satellites and maintain high-precision positioning even outside network range. For example, in forestry or mountain surveying where communications are unavailable, if the sky is open you can achieve centimeter accuracy on your own—an important advantage. Of course, deep forests or narrow valleys may prevent satellite detection, but the area where high-precision positioning is possible has expanded dramatically.

• Rapid surveying in disaster sites: After earthquakes or landslides, rapid surveying to grasp damage is required. Compact and portable smartphone GNSS receivers are very useful in such situations. At collapse sites where you cannot bring large tripods or power supplies, pocket-sized GNSS devices let you move lightly and immediately record coordinates or terrain changes where needed. Even when communications infrastructure is down, CLAS-compatible devices can continue positioning, making them useful in isolated areas. There are reports of smartphone RTK devices being used in disaster surveys, enabling a single person to perform surveying under aftershocks. In emergencies, this portability and reliable accuracy are especially powerful.

• Tight sites and near-indoor positioning: GNSS positioning is difficult indoors or underground, but smartphone GNSS can be surprisingly useful in semi-outdoor environments or measurements directly under structures. For example, in underpass construction where it’s not fully indoors but conventional equipment is hard to set up, smartphone GNSS allows measurements from narrow scaffolding. For single-family house foundation work in small plots where checking heights is needed, smartphone + GNSS is more manageable than deploying large equipment. Where GNSS is wholly unusable, it cannot be forced, but the new technology often allows easy measurement in places you previously thought impossible. With creative field methods, smartphone GNSS can support positioning and checks in small spaces once considered unmeasurable.

Simple Surveying with LRTK

A representative product among external GNSS receivers for smartphones is LRTK Phone (hereinafter LRTK), developed by Refexia, a startup spun out of Tokyo Institute of Technology. LRTK is an ultra-compact RTK-GNSS receiver that attaches to an iPhone or iPad with one touch and turns the smartphone into a centimeter-class versatile surveying instrument. It weighs about 150 g, is pocket-sized with a thickness of about 1 cm (0.4 in), and supports multi-GNSS such as GPS, GLONASS, Galileo, and Michibiki, as well as multi-frequency positioning on L1/L2/L5. It achieves horizontal and vertical errors within a few centimeters and runs for about 6 hours on its internal battery. It connects to the smartphone via Bluetooth or Lightning connector and starts positioning via a dedicated app.

LRTK’s strength is providing an all-in-one surveying experience that integrates the smartphone’s camera and the cloud. The app can record and manage survey points, and also includes functions useful for continuous distance measurement, area calculation, and as-built management. Acquired coordinates can be uploaded to the LRTK cloud with one tap, allowing office staff to instantly check them on web maps. Optional poles make stakeout (layout) work more efficient, and height offset corrections can be performed with a single button. AR functionality can also overlay construction positions and design lines on the smartphone screen to guide work—an advanced use case.

Despite these features, LRTK is offered as a very easy and reasonably priced solution. It eliminates the need to assemble dedicated instruments individually. With a package model combining hardware and cloud services, it achieves a price point where everyone on site can realistically have one device each. Operations are simple enough that anyone can handle them after brief training, so non-specialists can perform on-site positioning themselves. LRTK pioneers the era of “simple surveying,” enabling high-precision surveying anytime, anywhere with just a smartphone. The introduction of LRTK is transforming surveying workflows that once required two-person teams or heavy equipment. Leveraging smartphone RTK, which combines mobility and accuracy, directly contributes to improved productivity, labor savings, and safety on site. If you’re skeptical that such accuracy is possible with a smartphone, try this new surveying experience in the field—you’ll likely be surprised by how easy and accurate it is. Simple surveying with LRTK can be a powerful partner supporting not only professional surveyors but everyone involved on site.

FAQ

Q: Why can’t high-precision surveying be done with the smartphone’s built-in GPS alone? A: Smartphone-built GNSS provides convenient positioning but typically has errors on the order of several meters. Satellite signals suffer slight deviations due to the atmosphere and buildings, and a smartphone alone cannot correct these. While a few meters of error is acceptable for navigation, centimeters matter in surveying, so internal GPS accuracy is insufficient. RTK technology using correction data from a base station and stable reception with a high-performance antenna are necessary. Using an external GNSS receiver enables such corrections, allowing a smartphone to achieve survey-level accuracy.

Q: What is RTK? How does it achieve centimeter accuracy? A: RTK (Real-Time Kinematic) is a high-precision positioning method that exchanges measurement data between a rover (the receiver at the measurement point) and a base station placed at a known accurate location to perform real-time error correction. Since the base station knows its exact location, it can compute error components from the received satellite signals and send correction information to the rover, which then computes a position with the errors removed. This narrows a standalone error of several meters down to a few centimeters. RTK can be implemented by directly linking a base station and rover via radio or by obtaining correction data from an existing network of base stations over the internet (network RTK). For smartphone GNSS receivers, the latter—network RTK—is commonly used; connecting to a correction service via a dedicated app will automatically enable RTK.

Q: How do I obtain correction information? Are additional devices or costs required? A: There are several ways to obtain correction information. The most common is connecting over the internet to a base station network service provided by government or private entities. For example, real-time services using the Geospatial Information Authority of Japan’s GEONET or paid VRS services from private companies are available. If the smartphone has network connectivity, you can input account information for these services in the external GNSS receiver’s app and receive correction data via Ntrip. Fees vary by service; some regions offer free administrative services while private services may charge monthly fees from a few thousand to tens of thousands of yen. In areas without network access, satellite-based corrections can help—Japan’s Michibiki CLAS is one such service that, with a compatible receiver, provides centimeter-level augmentation directly from satellites without extra cost. In summary, as long as you have internet access and a correction service subscription (if required), you don’t need special radio equipment; choose the correction source that fits your region and budget.

Q: How much does it cost to introduce this? Is it really cheaper than conventional surveying equipment? A: Prices for external GNSS receivers for smartphones vary by manufacturer and performance, but in general they are available from tens of thousands to a few hundred thousand yen. This is far less expensive than professional conventional GNSS surveying equipment, which can cost several million yen. Equipping multiple field staff with total stations or high-precision GNSS kits used to be very costly, but with smartphone GNSS you only need to supply small receivers to employees’ smartphones, greatly reducing initial investment. Additionally, license fees for dedicated software and annual calibration costs are often unnecessary, lowering maintenance costs. For correction services, public free services can eliminate running costs, and even paid services are minor compared to leasing large equipment. Overall, the cost performance is overwhelmingly better than traditional methods.

Q: Is device operation difficult? Can non-experts use it? A: Operation is very simple—basic flows are just pressing buttons in a smartphone app. There is no need for users to perform complex initial settings or manual calculations like with dedicated equipment. For instance, start positioning by tapping “Start Positioning,” and when accuracy stabilizes tap “Save” to record a point. Point names and notes can be entered from the smartphone keyboard. Apps provide clear Japanese guidance, so you can use them without understanding technical terms. While knowledge of RTK and geodetic systems helps, recent products are designed for intuitive use by non-experts. There are many cases where construction site workers learned to use smartphone RTK after short training and performed as-built measurements or stakeout checks themselves.

Q: Can centimeter accuracy really be achieved in places with poor signal, such as deep mountains or urban canyons? A: It depends on whether satellite signals can be received. In valley bottoms, dense forests, or places where buildings make the sky narrow, the number of visible satellites decreases and multipath increases, making it difficult to maintain centimeter accuracy. This limitation applies to both smartphone GNSS and expensive conventional GNSS equipment. Some smartphone GNSS receivers employ multi-GNSS and high-sensitivity antennas to capture as many satellites as possible and reduce errors. Still, in extreme environments accuracy may temporarily degrade and errors can worsen to several tens of centimeters. Regarding communications, CLAS-compatible devices can continue receiving corrections from satellites even when the network is out, so accuracy does not instantly drop to zero when connectivity is lost. However, where satellites themselves cannot be observed (indoors, tunnels, etc.), GNSS positioning is impossible. In such cases, smartphone GNSS cannot be used and you must rely on other surveying methods like total stations. In short, centimeter accuracy is achievable where the sky is visible, but when satellite signals cannot reach the device the same limitations as traditional GNSS apply.