目次

• 開けた環境ではRTKが威力を発揮する

• 障害物が多い環境ではRTKが苦手

• 都市部（高層ビル街）でのRTK測位の課題

• 山間部・通信不良エリアでのRTK運用の注意点

• RTK測量が困難な現場での対策と工夫

• おわりに：LRTKによる簡易測量への展望

• FAQ

開けた環境ではRTKが威力を発揮する

RTK surveying (real-time kinematic positioning) is a method that uses differential corrections to GNSS positioning errors to achieve centimeter-level accuracy. Its performance is maximized in environments with a wide, open sky. On sites where there are no nearby obstructions and the whole sky is visible, many GNSS satellites can be tracked simultaneously and satellite signals can be received stably. With a good view, receiving signals from more than 10 satellites at once is easily achievable, allowing a Fix solution (fixed solution) to be obtained quickly and stably. With a sufficient number of satellites the geometry is also favorable, keeping the DOP values (dilution of precision) low, which otherwise degrade positioning accuracy. For these reasons, in open environments RTK’s Fix rate (the proportion of time a fixed solution is maintained) is high, and the accuracy and reliability of positioning results are excellent.

For example, locations with few obstructions such as wide plains, riverbanks, fields, coastlines, and development sites under construction are where RTK’s strengths—immediacy and high accuracy—are easily realized. If sky visibility is secured, the rover can receive correction information from the base station and obtain an integer fixed solution in a short time to perform positioning on the order of centimeters. In open sites, multipath (radio wave reflections) errors are almost nonexistent, and atmospheric and satellite clock errors can be canceled out by relative positioning to the base station, so the resulting positional errors are typically confined to a few centimeters horizontally and a few centimeters to about 10 cm (3.9 in) vertically. Communication is also easier in locations with clear line-of-sight; radio from the base station or cellular signals reach more readily, stabilizing correction data transmission. Overall, sites with an open sky and good communication are the situations RTK surveying handles best.

障害物が多い環境ではRTKが苦手

On the other hand, RTK surveying’s accuracy and stability tend to deteriorate in environments with many surrounding obstructions. In forests and wooded areas, in the shadows of buildings and structures, or on sites surrounded by large objects like heavy machinery or vehicles, GNSS signals can be physically blocked. When satellite signals are interrupted the number of satellites that can be tracked simultaneously decreases, and the probability of obtaining a Fix solution drops significantly. If even the minimum of four satellites cannot be tracked, not only RTK but even ordinary positioning becomes difficult; and even if calculations are possible with four or five satellites, poor satellite geometry can make the solution unstable and increase the risk of remaining a Float solution (float solution).

Also, in environments with many obstructions, signals that cannot reach directly may arrive after being reflected by surrounding objects. In forests, in addition to attenuation by trunks and foliage, radio waves may be reflected or refracted by trunks causing delays. Building façades or metal surfaces of machinery also cause signal reflections, and when these multipath errors mix into positioning the reported location can be off by several tens of centimeters in some cases. In heavily obstructed places, for these reasons RTK accuracy is hard to obtain, and in the worst cases a Fix solution may not be obtained at all.

Communication also requires caution in obstructed environments. When receiving corrections from a base station via UHF radio, intervening obstructions can block signals and cause interruption of correction data. Even when using NTRIP over cellular networks, signal conditions may be poor deep inside forests or near building basements, making communication unstable. If correction information is delayed or missing, the RTK solution can revert to Float or contain large errors, so ensuring communication is a challenge on obstructed sites.

When conducting RTK surveying in such environments, the basic countermeasure is to find locations with even a little open sky. For example, in forestry sites use gaps between trees or logged areas, and around buildings select points where the sky is visible away from corners—i.e., carefully choose antenna placement points. Installing the antenna as high as possible (on poles or tripods) and keeping it away from nearby obstructions can increase the number of satellites received and improve Fix rate. Additionally, attaching a ground plane to the antenna to block reflections from below, or raising the minimum elevation angle used for positioning (for example setting it to 15° or higher) to exclude low-angle signals are effective multipath countermeasures. If those measures are still insufficient, consider combining methods—for example, establish a control point at a location where positioning is possible and then use a total station to survey areas beyond obstructions.

都市部（高層ビル街）でのRTK測位の課題

In urban centers and other areas where high-rise buildings are clustered, RTK positioning presents particular challenges. In city streets the sky is covered by surrounding buildings and the visible sky is limited to small portions. This situation, often called an “urban canyon,” can make it difficult to track a sufficient number of GNSS satellites, causing satellite shortages. Generally, high-precision RTK positioning is desirable with five to six or more satellites, but in the gaps between buildings there may be cases where fewer than four satellites are visible, or even when four are tracked the geometry is poor (satellites clustered in the same direction) and a Fix solution is difficult to obtain. As a result, in urban areas the Fix rate can drop dramatically, and it is common for solutions to remain Float and fail to meet accuracy targets.

Furthermore, multipath interference from concrete structures and glass façades is severe in urban environments. Reflected waves from buildings combine with direct waves and create pseudorange errors that introduce errors into RTK solutions. Especially in building shadows the direct satellite signals may be blocked and only reflected waves are received, producing NLOS (non-line-of-sight) conditions. In such cases the position solution can be greatly disturbed or offset by meters. In cities, multipath and NLOS are among the major causes of RTK accuracy degradation and solution instability.

Although communication infrastructure in cities is often well developed, RTK requires measures to receive correction information reliably. If broadcasting UHF from a private base station in a city, building blockage may prevent the radio from reaching. Therefore, urban surveying typically uses network RTK (e.g., VRS corrections via NTRIP) over cellular networks. Via the internet there are fewer line-of-sight radio obstructions, and virtual reference station (VRS) methods effectively keep the distance to a reference station within a few kilometers even in urban areas. However, city-specific issues such as radio noise from surrounding Wi‑Fi and other transmitters must be considered. Approaching high-voltage lines or communication antennas can interfere with GNSS reception, so it is desirable to position the measurement away from strong local radio sources.

When performing RTK surveying in dense urban areas, keep the following points in mind:

• Look for open sky: Even in building canyons choose locations like intersections or plazas where a relatively large patch of sky is visible overhead for positioning.

• Be clever about antenna placement: If possible place the antenna on rooftops or other high points; if not, extend the pole to get the antenna as close to the sky as possible. Use a ground plane and an elevation mask to eliminate low-angle reflections.

• Choose the right time: Satellite geometry changes over the day, so use a GNSS planner to identify times when satellite count increases and DOP values improve (when satellite geometry is more dispersed), and schedule surveying during those windows.

• Use high-performance equipment: Use multi-GNSS, multi-frequency receivers to obtain signals from as many satellites as possible. Modern receivers and antennas may include multipath reduction features that perform well in urban settings. Devices with built-in IMUs can help maintain position during short interruptions in Fix by using inertial measurements.

山間部・通信不良エリアでのRTK運用の注意点

In mountain and depopulated areas, RTK surveying faces the dual challenges of restricted satellite visibility and insufficient communication infrastructure.

At points in valleys or under mountain shadows, parts of the sky are blocked by surrounding terrain, and as in urban areas the number of visible satellites drops significantly. In deep valleys you may only receive satellites near zenith, causing biased satellite geometry and worsening the PDOP (position dilution of precision). When satellite count and geometry are poor, RTK solutions become unstable and accuracy degrades. In terrain with large elevation differences, pay attention that vertical accuracy tends to degrade more than horizontal accuracy.

Moreover, mountain areas often contain many locations that are out of cellular coverage. With network RTK, if you cannot secure an internet connection you cannot receive corrections and RTK cannot be established. Also, in areas far from national geodetic control stations or private base station networks, the available reference stations may be distant, causing long baseline issues. When the base station distance exceeds 50 km or more, ionospheric and tropospheric error differences become difficult to correct, so even if communication is possible it may take a long time to obtain a Fix or accuracy may deteriorate to several tens of centimeters.

Operating RTK in mountainous and communications-poor environments requires different countermeasures. First, if possible set up your own base station near the survey area to generate correction information locally. Placing a base station antenna on a ridge or other high point where radio propagates and shortening the distance to the rover can allow coverage of a certain area even in narrow valleys (consider relay stations to route radio around obstacles if needed). In Japan, one can also utilize the QZSS “Michibiki” CLAS augmentation signal. The CLAS centimeter-class augmentation service can be received directly from satellites by compatible receivers, enabling reception of correction information without internet even in mountainous areas. Using equipment that supports CLAS makes it possible to achieve RTK-equivalent positioning accuracy without a base station or other communication.

Understanding satellite geometry on site is also important. In mountain areas, track when satellites rise over ridgelines and choose times with more satellites available to improve accuracy. Wait until enough satellites are available; if PDOP temporarily worsens suspend surveying and wait for geometry to improve. If real-time positioning is impossible, switch to recording observation data and post-process with base station data later using PPK (post-processed kinematic). In remote mountain areas balancing real-time capability and accuracy requires ingenuity and flexible operation.

RTK測量が困難な現場での対策と工夫

As described above, RTK-unfavorable sites pose various problems, but there are several basic countermeasures to mitigate them. Here we summarize practices effective regardless of environment.

• Optimize satellite count and geometry: Plan in advance and use GNSS planning software to check satellite visibility predictions at the survey site. Working during times when more satellites are visible (when GDOP is low) helps prevent accuracy degradation and reduced Fix rate due to limited satellites. Choose receivers that support not only GPS but also GLONASS, Galileo, BeiDou, QZSS—i.e., multi-GNSS—to maximize the number of observable satellites. Multi-frequency receivers improve correction of ionospheric delays and shorten initial Fix times in difficult environments. Satellite selection via mask settings is also important. Extremely low-elevation satellites (e.g., under 10°) are prone to noise and reflections and should be excluded, but set the elevation mask so that remaining satellite count does not fall too low—for example using around 15° depending on the site.

• Antenna placement and device settings: Place the antenna away from obstructions on a stable, level surface. If possible raise the antenna height to 1.5 m (4.9 ft)–2 m (6.6 ft) or higher to reduce ground reflections and shielding effects. Securely fasten the pole mount so wind does not cause sway. Check receiver settings: in RTK mode select the correct correction data format (e.g., MSM4 vs MSM7) and ensure rover/base roles are correctly assigned. When starting a measurement, remain still long enough to obtain a Fix before moving; operational tips like staying stationary until a Fix is obtained are important for maintaining accuracy. If the receiver will not fix in the field, try restarting it or reacquiring correction data to reset the system.

• Shorten the baseline: As noted above, RTK accuracy worsens as the distance between base and rover increases. Therefore, keep the distance to the reference point as short as possible. If you can provide your own base station, install it close to the work area. If that is difficult, using national control stations or commercial correction services (VRS, etc.) can virtually place a reference station nearby. Utilizing network RTK services helps maintain accuracy over longer distances by applying regional corrections. Keeping baselines short speeds initial Fix acquisition and improves solution stability during measurements.

• Ensure communications: RTK requires continuous reception of correction data in real time, so communication stability is crucial. When using cellular networks, survey the site’s signal conditions in advance; if signal is weak consider pocket Wi‑Fi or alternative carriers to build redundancy. In mountains go to higher ground before measuring or use longer antennas to improve reception. If communication is impossible, consider satellite-based augmentation like CLAS or log base station data locally and merge with rover data later (post-processing). Communication delays directly affect solution quality, so monitor status indicators such as Age of Diff (age of differential corrections) on the receiver or app; if latency becomes large switch connections or pause until conditions improve.

• Other noise and error countermeasures: Avoid RF interference from strong radio sources. GNSS signals can be overwhelmed under high-voltage transmission lines or very near construction radios and cellular base station antennas. Avoid such locations when possible, or if measurements must be taken nearby use noise filters on the receiver or choose times when interference is minimal. Also verify measured results against known control points when possible. If you have a point with known accurate coordinates on site, observe it with RTK to check errors. Repeatedly measuring the same point and obtaining results within a few centimeters indicates the system is functioning properly. Large scatter in results suggests environmental factors or configuration errors; systematically eliminate causes one by one.

おわりに：LRTKによる簡易測量への展望

As seen so far, RTK surveying performance is heavily influenced by site environmental conditions. Whether satellite reception and communications infrastructure are adequate dramatically changes achievable accuracy and Fix rate, and in some cases conventional RTK equipment cannot cope. However, recently new solutions have emerged that compensate for RTK’s weaknesses and enable high-precision positioning more easily. One of these is a system called LRTK.

LRTK is a next-generation surveying device that combines a smartphone with an ultra-compact RTK‑GNSS receiver. A pocket-sized receiver attaches to a smartphone and, using a dedicated app to receive network RTK or CLAS augmentation signals, allows immediate acquisition of global coordinates on site. LRTK can perform RTK surveying even in areas without cellular coverage by receiving correction information directly from Michibiki (QZSS) with three-frequency GNSS support. This enables centimeter-level positioning in mountain areas and other places without cellular service. LRTK also operates through an intuitive smartphone app and includes features that simplify fieldwork such as cloud data sharing, embedding coordinates into photos, and AR-based staking out. Tilt compensation is implemented so the coordinates of the pole tip are automatically corrected even when the pole is inclined, which helps when obstacles prevent placing the pole vertically.

In this way LRTK is attracting attention as a tool that addresses situations where RTK has struggled and enables simple surveying without relying exclusively on traditional equipment. It is not a panacea, but it can be an effective option for surveying efficiently across various environments including open sites, urban areas, and mountain regions. Going forward, using smart, easy-to-use positioning technologies like LRTK is expected to promote a surveying style that is less constrained by site limitations.

FAQ

Q1. なぜRTK測量には衛星の見通しが重要なのですか？ A. RTK surveying requires receiving signals from multiple GNSS satellites simultaneously. If satellites are not adequately visible the number of tracked satellites becomes insufficient and the calculations that determine position cannot be performed. Narrow satellite visibility also degrades geometry, leading to accuracy loss and lower Fix rate, so an open sky is a prerequisite for high-precision positioning.

Q2. 都市部でRTKの精度が出にくいのはなぜですか？ A. In cities high-rise buildings block satellite signals, reducing the number of satellites that can be received. Additionally, reflected waves from buildings and glass surfaces (multipath) enter the receiver and create ranging errors that contaminate RTK solutions. This blocking and multipath interference make it difficult to obtain a Fix and cause unstable accuracy in urban areas.

Q3. RTK基準局からどれくらい離れて測位できますか？ A. Generally, it is desirable for the base station (reference point) to be within about 10 km for RTK. Beyond that distance ionospheric and tropospheric errors vary between the baseline and are harder to correct, so acquiring a Fix may take longer and accuracy can degrade from several centimeters to several tens of centimeters. Using wide-area network RTK services can mitigate accuracy degradation to some extent by creating virtual reference points.

Q4. RTKでFix解が得られない場合、どう対処すべきですか？ A. First check the number of tracked satellites and DOP values, and if necessary move to a location with open sky or adjust the antenna position and height. If it still will not fix, check the reception state of correction information: confirm communication is not interrupted and that the base station settings are correct. If those are normal, try resetting and reinitializing the system. Trying at a different time when satellite geometry is better is also effective.

Q5. 携帯電話の圏外でもRTK測量は可能ですか？ A. RTK surveying is not impossible where cellular service is unavailable. Besides setting up your own base station and sending corrections by radio, in Japan you can receive CLAS signals from the QZSS Michibiki satellites to use for corrections and achieve centimeter-class positioning without the internet. Alternatively, if real-time operation is not required, record data on site and perform PPK processing later at the office.

Q6. LRTKとは何ですか？ A. LRTK is a compact RTK‑GNSS positioning system intended for use with smartphones. It consists of a handset-mountable receiver device and a dedicated app, using network RTK corrections and satellite augmentation to provide real-time high-precision positioning. Because it can receive Michibiki augmentation directly even outside cellular coverage, it is gaining attention as a new solution that makes surveying easier in locations that were previously difficult.