Smartphones Become Survey Instruments on Site! Thorough Evaluation of External GNSS Receivers

Table of Contents

• Can a smartphone become a surveying instrument? A new era of high-precision positioning on site

• Accuracy comparison: built-in smartphone GPS vs. external GNSS receivers

• Benefits of adopting an external GNSS receiver

• How to choose an external GNSS receiver — comparison points

• Field test results: the real performance of external GNSS receivers

• Expanded use cases with smartphone RTK

• Simple surveying realized with LRTK

• FAQ

Can a smartphone become a surveying instrument? A new era of high-precision positioning on site

Traditionally, achieving centimeter-level high-precision positioning in surveying and civil engineering required dedicated, expensive surveying equipment. Large GNSS antennas mounted on tripods, base-station receivers, radio modems, batteries, and many other pieces of equipment had to be prepared, and initial investments on the order of several million yen were not uncommon. Operation was also predicated on surveying engineers with specialized knowledge and qualifications, making it difficult for general site workers to casually take advantage of high-precision positioning.

However, this situation has been rapidly changing in recent years. Thanks to technological advances and the emergence of new satellite positioning services, an era has arrived in which anyone can easily achieve centimeter-level positioning on site with just a smartphone and a compact external GNSS receiver. In Japan, the Quasi-Zenith Satellite System “Michibiki” and its centimeter-level positioning augmentation service (CLAS) are key. CLAS directly broadcasts augmentation signals from satellites for improved precision, and using a compatible GNSS receiver can correct GPS-only positioning errors that used to be around 5–10 m (16.4–32.8 ft) down to a few centimeters. Because this service allows high-precision positioning using only signals from satellites anywhere in Japan—even in mountainous areas outside mobile coverage—high-precision GNSS positioning has suddenly become much more accessible.

Solutions combining a smartphone with an external GNSS receiver, often called “smartphone RTK,” are also emerging. RTK (Real Time Kinematic) is a positioning method using two GNSS units—a base station and a rover—to cancel out errors in real time, typically reducing errors to within a few centimeters. Previously, it was necessary to set up your own base station or subscribe to a paid correction information service, but by leveraging Michibiki’s CLAS, you can achieve RTK-equivalent effects without additional infrastructure. Against this backdrop, simple high-precision positioning systems using smartphones and compact receivers are attracting attention, and a new era in which “smartphones become surveying instruments” is beginning.

Accuracy comparison: built-in smartphone GPS vs. external GNSS receivers

Smartphones come with built-in GNSS receivers, including GPS, but their positioning accuracy is generally said to be around 5–10 m (16.4–32.8 ft). You have probably experienced seeing your location on a map app shifted by several meters. This is partly because smartphone GPS chips are typically single-frequency and simple, and antennas are small, making them highly susceptible to reception environment effects. Single-point positioning cannot correct for atmospheric or clock errors in satellite signals, so there is a limit to how much accuracy can be improved. As a result, vertical positioning errors are also large, making it difficult to determine elevation precisely using only smartphone GPS.

On the other hand, connecting an external high-precision GNSS receiver to a smartphone can dramatically improve accuracy. High-precision GNSS receivers generally support multiple frequency bands (multi-band) and multiple satellite systems (not only GPS but also GLONASS, Galileo, Michibiki, etc.), designed to capture more satellite signals more stably. In addition, by using correction information from Michibiki’s CLAS or reference station networks to correct errors in real time, planar accuracy of a few centimeters and vertical accuracy of several centimeters to a dozen centimeters can be achieved. For example, performing RTK positioning with a dedicated external receiver can reduce errors that were over 5 m (16.4 ft) with a smartphone alone to only about 1–2 cm (0.4–0.8 in), enabling position determination approaching the accuracy of surveying instruments.

Stability of accuracy also improves greatly. While a smartphone’s built-in GPS may jitter by meters over time, a high-precision GNSS receiver shows very little variation in positioning results even when measuring the same point repeatedly. By removing error factors, measurements become stable and highly reproducible, significantly increasing on-site reliability. As such, there is a orders-of-magnitude difference in accuracy between built-in smartphone GPS and external GNSS receivers, and it is necessary to choose based on the intended use. Where precise positioning information is required, using an external GNSS receiver is indispensable.

Benefits of adopting an external GNSS receiver

A positioning solution combining a smartphone and an external GNSS receiver offers various benefits beyond simply increased accuracy. First, it is overwhelmingly smaller and lighter and easier to carry compared with traditional surveying equipment. All you need is a pocket-sized GNSS receiver and a smartphone, so you don’t have to carry heavy tripods or large hard cases. A single worker can walk around a site and survey, dramatically improving mobility.

Second, there are cost advantages. Traditional surveying equipment is expensive, and it was not easy to acquire multiple units. If a smartphone plus a small GNSS receiver can substitute, initial investment and maintenance costs can be greatly reduced. Especially when using Michibiki CLAS, there is no additional cost for obtaining correction information, keeping running costs low. What used to require equipment costing millions of yen can now be achieved with far more affordable gear, which is a major advantage.

Third, there is ease of operation and multifunctionality. Using a dedicated app on a smartphone, you can intuitively start and stop positioning and save data without having to worry about complicated settings. Measurement results can be displayed on a map on the spot, and you can save photos tagged with high-precision location information. You can add notes during positioning, compare with past data, and enjoy the many functions unique to digital devices. Cloud integration allows instant sharing of field data within the office. Compared with traditional methods of handwriting notes in a field notebook and bringing them back, work efficiency and the scope of data utilization expand dramatically.

Overall, the smartphone + external GNSS receiver solution represents a new surveying style that combines “high precision, low cost, and ease of use.” The future in which each field worker carries their own surveying instrument and can measure whenever needed is becoming a reality.

How to choose an external GNSS receiver — comparison points

When you look into introducing an external GNSS receiver for a smartphone, you’ll find several types and approaches. Here are the main points to consider when selecting a device.

• Measurement methods and supported services: Devices support different positioning methods. Some only support SBAS (satellite-based augmentation) and remain around 1 m accuracy, while others support RTK and achieve centimeter-level precision. If you plan to use the device in Japan, whether it supports Michibiki’s CLAS is an important point. A CLAS-compatible receiver can achieve cm-level accuracy standalone even outside communication coverage.

• Frequency bands and satellite support: For high precision, multi-band support is desirable. Receivers that support multiple frequencies such as L1/L2 or L1/L5 are better at removing ionospheric error and stabilizing positioning than single-frequency L1-only receivers. The more GNSS constellations supported—GPS plus GLONASS, Galileo, BeiDou, and QZSS (Michibiki)—the greater the number of satellites available for positioning, improving accuracy and availability.

• Connection method and usability: Check how the receiver connects to the smartphone. Wireless connections such as Bluetooth or Wi‑Fi are mainstream and offer better handling on site than cable connections. Also confirm whether a dedicated app or compatible apps are available for the smartphone and whether connection and setup are easy. Check if positioning data can be output in NMEA format or similar so you can use it with your existing apps or systems.

• Battery life: Many external receivers have built-in batteries, so continuous operating time is a comparison point. For long on-site use, a device that can operate for 5–6 hours or more provides peace of mind. USB charging or support for mobile battery operation allows you to continue operation with spare power.

• Size, weight, and ruggedness: Because the device is carried around, smaller and lighter is better. If it fits in a pocket and is under a few hundred grams, portability is good. For construction sites, consider dustproof/waterproof performance and shock resistance. Ruggedness that survives some rain or drops is reassuring on site.

• Support and price: Finally, manufacturer support and services are not to be ignored. If you are new to high-precision positioning, good manuals and responsive support are reassuring. Prices vary widely, so choose based on the balance of performance and budget. Cheaper devices may have limited accuracy or simpler features, while higher-performance models cost more. Still, recently surprisingly high-precision products have appeared at reasonable prices, increasing choices with good cost performance.

Considering these points will help you choose a GNSS receiver that fits your needs. Whether you prioritize accuracy or ease of use, you should be able to find the best device for your use case and budget.

Field test results: the real performance of external GNSS receivers

So how much accuracy can you actually get with a smartphone + external GNSS receiver? We tested positioning at known coordinate points using a smartphone and an external GNSS receiver to verify accuracy.

First, we connected a high-precision GNSS receiver to a smartphone and performed single-point positioning while receiving CLAS correction data from Michibiki. The resulting coordinates, compared with the true values determined from the Geospatial Information Authority of Japan’s electronic reference point data, showed horizontal errors within 1–2 cm (0.4–0.8 in) and vertical differences of about 3 cm (about 1.2 in). Compared with values measured by conventional surveying equipment (first-class GNSS surveying instruments), the difference between the two was less than 5 mm, an astonishing result confirming that positioning with an external GNSS receiver can match the accuracy of professional equipment.

We also examined accuracy stability by repeating measurements. At one point, we measured consecutively 10 times; the variation of single measurements (horizontal position standard deviation) was about 12 mm (0.47 in). However, when we used an averaging feature in the smartphone app to average measurements over 60 seconds, the standard deviation shrank to about 8 mm (0.31 in), showing that a more stable position can be obtained. That an accuracy under 1 cm (less than 0.4 in) can be achieved with just about one minute of measurement is a level of ease that was previously unthinkable.

We performed the same measurements with a smartphone’s built-in GPS, but the positions obtained were far from the true values and jumped around over time, making them unsuitable for precision surveying. Again, conventional GPS with meter-level errors shows its limitations, and the presence or absence of a high-precision receiver produces an overwhelming difference.

In our field test, measurements were made in open outdoor environments, and under such conditions external GNSS receivers almost always maintained stable cm-level positioning. When there are no tall buildings or trees nearby, positioning converges quickly and high accuracy is achieved. Of course, because signals are received directly from satellites, accuracy can degrade or solutions may not be obtainable in forests or urban canyons. But that is also true for traditional surveying instruments; portable GNSS receivers actually offer greater flexibility in selecting positioning points between obstacles, which is an operational advantage.

From these verifications, it is clear that smartphone + external GNSS receiver positioning can achieve accuracy comparable to professional equipment, provided correct usage and suitable environment. This easy method enabling immediate measurement and sharing on site is fully practical for real work.

Expanded use cases with smartphone RTK

High-precision positioning using a smartphone and an external GNSS receiver opens up many on-site use cases. Tasks that previously required specialized surveying teams or were abandoned because sufficient accuracy could not be obtained can increasingly be handled on the spot with smartphone RTK. Specific applications include:

• Single-point surveying and as-built management: You can measure coordinates of required points on site and compare them with design coordinates to check as-built conditions. Conversions to planar rectangular coordinate systems and geoid height transformations can be performed automatically on the smartphone, allowing on-site immediate confirmation of results.

• Setting-out (staking out): High-precision GNSS is useful for staking out positions specified on design drawings. By displaying the offset to target points in real time on the smartphone screen or visualizing design positions via AR (augmented reality), staking that previously required surveying equipment and two or more staff can now be done efficiently by one person.

• Photo records and inspections: Photos taken with the smartphone camera can be saved with high-precision location and orientation tags. For example, tagging pre- and post-construction photos with exact coordinates makes time-series comparisons and as-built documentation easier. Recording photos and coordinates of defects during equipment inspections also streamlines report preparation.

• 3D surveying (point cloud measurement): Combined with LiDAR-equipped smartphones, 360° cameras, or drones, you can place point cloud data or 3D models precisely in geographic space. By giving point clouds a coordinate reference obtained from positioning, you can perform advanced analyses such as 3D as-built management and earthwork volume calculations. 3D measurements that used to be outsourced to specialists are becoming accessible with smartphones and GNSS.

• Other applications: In agriculture, field boundary surveys and automated guidance for farm machinery; in disaster response, rapid surveying and mapping of affected areas—applications for smartphone RTK are expanding. The basic capability of obtaining high-precision location information in real time fuels new on-site solutions that improve productivity and reduce labor.

Thus, smartphone RTK is expected not only to improve surveying efficiency but also to accelerate digital transformation (DX) on site. If a smartphone surveying instrument for each worker becomes the norm, workflows on site will change dramatically, enabling faster and more accurate decision-making.

Simple surveying realized with LRTK

As a solution to make high-precision positioning with a smartphone and an external GNSS receiver even easier, we developed LRTK. LRTK is conceived as a “pocket-sized universal surveying instrument” that turns a smartphone into a centimeter-accuracy surveying device. A compact GNSS device weighing just about 125 g and about 13 mm (0.51 in) thick attaches to the back of an iPhone, and by launching a dedicated app anyone can immediately start high-precision positioning. With an optional monopod (pole) attached, you can take measurement points with a stable posture much like conventional surveying instruments.

LRTK’s strengths are its ease of use and accuracy. Its built-in battery runs for about 6 hours, and because it operates wirelessly with the smartphone, handling on site is comfortable. It uses Michibiki’s CLAS signals and our proprietary correction technology, enabling positioning with horizontal ±2 cm (±0.8 in) and vertical ±4 cm (±1.6 in) accuracy even in mountainous areas outside communication coverage. That accuracy rivals first-class GNSS surveying instruments defined by the Geospatial Information Authority of Japan, yet operation is completed within a smartphone app and requires no specialized knowledge. Positioning data are automatically saved to the cloud, allowing real-time confirmation of field survey results from the office.

The LRTK app includes features useful on site, such as single-point averaging, coordinate system conversion, and AR-based point navigation. For example, with one button you can observe your current position 60 times and calculate the average to record high-precision coordinates. If you specify the coordinates of the point you want to measure, the app guides you to it with an arrow on the smartphone screen, so even beginners can identify staking positions without difficulty. Acquired coordinates can be linked with photos and notes and managed in the cloud, where map display, distance and area calculations, and data sharing are all at your fingertips. It is truly the definitive solution for “simple surveying” that even those without surveying experience can handle.

The impact of LRTK on site is significant, making the vision of one smartphone surveying instrument per person a reality. If necessary surveying can be performed without expensive equipment or specialists, on-site productivity and autonomy will increase dramatically. In the coming era, with smartphones and LRTK, sites may increasingly measure and plan for themselves—ushering in a new mainstream for construction management.

FAQ

Q: Why is a smartphone’s GPS accuracy poor on its own? A: Smartphone-built-in GPS (GNSS) receivers are typically single-frequency and equipped with simple antennas, so they cannot sufficiently cancel error factors and their accuracy remains around 5–10 m (16.4–32.8 ft). In addition to satellite signals being distorted by the atmosphere and clock errors, smartphones are also more susceptible to multipath from buildings and terrain, which commonly results in position errors of several meters. For high-precision positioning, using an external GNSS receiver or augmentation information (RTK or CLAS) that can correct these errors is essential.

Q: Can an external GNSS receiver really achieve centimeter-level accuracy? A: Yes—if you have appropriate equipment and environment. For example, connecting a multi-band high-precision GNSS receiver to a smartphone and using Michibiki’s CLAS or RTK correction information from reference stations can yield planar accuracy of a few centimeters and vertical accuracy of several to a dozen centimeters. In our field tests, we obtained positioning results within about 1–2 cm (0.4–0.8 in) horizontally. However, accuracy does degrade in environments where satellite signals are disturbed, such as near tall buildings, so ideal accuracy is not guaranteed in all conditions.

Q: Is connecting and operating a smartphone with an external GNSS receiver difficult? A: It’s relatively easy even without special knowledge. Many external GNSS receivers connect to smartphones via Bluetooth or Wi‑Fi and perform positioning through a dedicated app. Once paired, measurement typically starts by following the app instructions and pressing a button, and results are displayed in real time. Positioning mode settings and coordinate system conversions are usually handled automatically by the app, so users don’t need to perform complex calculations. Many apps have intuitive UIs, and beginners can start surveying within minutes.

Q: What is LRTK? A: LRTK is our smartphone high-precision positioning solution. It consists of a pocket-sized RTK-GNSS receiver (LRTK Phone), a smartphone app, and cloud services, designed to enable anyone to perform centimeter-level surveying easily. Using Michibiki CLAS and proprietary correction technologies, LRTK achieves cm-level accuracy without a reference station, with positioning data managed and shared in the cloud. It combines professional-level accuracy with ease of use and low introduction cost.

Q: What precautions should be taken when using high-precision positioning on site? A: There are several points to keep in mind to achieve high-precision positioning. First, choose locations with as wide a sky view overhead as possible and few obstacles that block satellite signals. Near buildings or trees, satellites cannot be sufficiently acquired, which degrades accuracy or destabilizes positioning. Avoid being near metal construction machinery or strong sources of radio interference. Also, firmly fix the smartphone and receiver during positioning and avoid unnecessary movement. Using a monopod or pole for measurement helps prevent wobble and enables stable positioning. Finally, handle the obtained data carefully: back up important positioning results to the cloud and cross-check against other known points to confirm errors. Observing these points will help you maximize the benefits of high-precision positioning on site.