What is SLAS? Explaining the Differences from RTK and CLAS

In fields such as surveying, construction, agriculture, and disaster response, improving the accuracy of GNSS (satellite-based positioning) is becoming increasingly important. The term "SLAS," which you may have heard recently, is one of the keywords related to high-precision positioning. This article explains clearly what SLAS (Sub-meter Level Augmentation Service) is and outlines the differences from other positioning technologies such as RTK and CLAS. It covers how SLAS works and is delivered, compares its accuracy and characteristics with RTK and CLAS, discusses on-site advantages and limitations, and presents real-world use cases. Finally, it introduces tools that make surveying easy by combining SLAS with other augmentation information.

What is SLAS (Sub-meter Level Augmentation Service)?

SLAS stands for Sub-meter Level Augmentation Service, provided by Japan’s Quasi-Zenith Satellite System "Michibiki." Standalone GPS positioning typically has errors of about 5–10 m (16.4–32.8 ft) due to satellite signal errors. SLAS, however, transmits satellite error correction information (augmentation signals) to receivers, dramatically improving positioning accuracy and reducing position errors to about 1 m or less (3.3 ft), i.e., sub-meter level. For example, a location that previously had nearly 10 m (32.8 ft) of error can be pinpointed to under 1 m (3.3 ft) when using SLAS.

SLAS primarily corrects one of the GPS error sources: ionospheric delay. Single-frequency GPS receivers cannot accurately correct ionospheric effects, resulting in errors of several meters, but SLAS transmits dedicated augmentation signals (L1S signal) from the Quasi-Zenith Satellite "Michibiki" to reduce ionospheric delay errors. The L1S signal uses the same frequency band and signal format as the common GPS signal (L1 C/A), which means it can be supported by existing single-frequency GNSS receivers (with firmware or setting updates on some models). This contrasts with CLAS, a higher-precision augmentation service that uses a dedicated frequency band (L6) and advanced receivers; SLAS is therefore easier to use with relatively inexpensive, compact equipment.

No additional communication lines or base stations are required to use SLAS. Because augmentation information is received directly from Michibiki satellites, SLAS can be used in mountainous areas or at sea where cellular networks do not reach. The service coverage is essentially nationwide Japan and surrounding areas; SLAS signals can be received across the Japanese archipelago and adjacent seas. The Japanese government (Cabinet Office, Office for the Promotion of Space Policy) provides SLAS as part of social infrastructure as a free service, so no special application or fee is required. However, you need a GNSS receiver that supports the SLAS augmentation signal (L1S-capable device).

What is RTK? Differences from SLAS

Let’s compare SLAS with RTK (Real Time Kinematic), a representative high-precision positioning technology. RTK, called "real-time kinematic positioning" in Japanese, is a relative positioning technique using two GNSS receivers: a rover (mobile unit) and a base station. The base station is a receiver at a known precise location that computes GNSS errors in real time and sends correction information to the rover (the receiver at the point being measured). By applying these corrections, the rover can cancel out errors and determine position with very high accuracy.

RTK characteristics:

• Positioning accuracy: With RTK, horizontal positioning can be within a few centimeters (cm-level accuracy (half-inch accuracy)). While standalone positioning typically has errors of 5–10 m (16.4–32.8 ft), RTK can reduce errors by about a factor of 100 to the centimeter level. In widely used network RTK implementations, positioning errors of about 3–5 cm (1.2–2.0 in) are commonly reported.

• Mechanism: RTK requires a two-receiver setup (base station + rover). The base station can be self-installed or can be an existing reference such as the Geospatial Information Authority of Japan’s continuously operating reference stations or commercial correction data services (network RTK). Correction data are delivered to the rover via radio or the Internet (e.g., Ntrip).

• Operational range: With a privately installed base station, accuracy improves when the base and rover are close; centimeter-level accuracy is maintained within a few km. Network RTK (VRS and similar) can utilize wide-area reference data via cellular networks, making RTK positioning practically available nationwide in Japan (provided communication is available).

• Advantages: Because errors are corrected in real time, RTK is suitable for dynamic measurements and can track moving objects (vehicles, drones, construction machines) at centimeter accuracy. In surveying and construction, RTK enables immediate precision tasks such as stake setting and as-built control, greatly improving efficiency.

• Challenges: Achieving centimeter-level accuracy requires high-performance receivers (typically multi-frequency) and antennas, plus communication means to transmit corrections. There are equipment and operational costs (communication fees, service subscription), and the system setup and operation are more complex than standalone positioning or SLAS. In environments with lots of obstructions, communication with the base may be interrupted or multipath may occur, degrading accuracy.

Main differences between SLAS and RTK:

• Accuracy: RTK achieves centimeter-level (≈1–5 cm) accuracy (cm-level accuracy (half-inch accuracy)), while SLAS remains at meter-level accuracy (≈0.5–1 m (1.6–3.3 ft)). Therefore, for millimeter- or centimeter-level precision tasks, RTK is indispensable; for applications where tens of centimeters to about 1 m are sufficient, SLAS can be adequate.

• Required equipment: RTK requires two receivers and a communication environment, whereas SLAS can be used with a single receiver. An SLAS-capable GNSS device alone can receive satellite augmentation signals and improve accuracy, making the system configuration simpler.

• Cost and operation: SLAS is free and its augmentation signals are delivered from satellites, so no service fees apply. RTK may incur equipment costs for a private base station or communication and subscription fees when using network RTK. RTK also requires more specialized setup and operational knowledge; SLAS is relatively easy to adopt once the device is obtained, making it beginner-friendly.

• Real-time responsiveness: RTK corrections are applied nearly instantaneously, so RTK supports real-time positioning of moving objects. SLAS also provides real-time augmentation, but there is a small time lag in correction generation and delivery (a few seconds to several tens of seconds). Rapidly changing error sources (e.g., sudden ionospheric disturbances) may not be fully tracked by SLAS, so it is less reliable than RTK for latency-sensitive applications such as autonomous vehicle control. For pedestrian walking speeds or agricultural machine operating speeds, SLAS latency is generally not problematic.

• Coverage: RTK can be used anywhere in principle, but with private base stations the coverage is limited to the vicinity of the base. Network RTK enables wide-area use but requires communication coverage. SLAS can be received anywhere Michibiki signals reach (Japan and surrounding areas) without relying on ground communications, but it is not available outside Japan.

What is CLAS? Differences from SLAS

Next, let’s explain CLAS, another high-precision augmentation service provided by Michibiki. CLAS (Centimeter Level Augmentation Service, commonly called “CLAS”) delivers centimeter-level positioning accuracy as the name suggests. CLAS offers even higher accuracy than SLAS, enabling positioning errors within a few centimeters. What sets CLAS apart?

How CLAS works:

• CLAS uses data from about 1,300 Geospatial Information Authority of Japan continuously operating reference stations (GNSS continuous observation system) nationwide to estimate satellite orbit and clock errors, ionospheric and tropospheric errors, and other biases with high precision. This error information is broadcast via Michibiki’s L6-band signals, allowing receivers to apply corrections to their observations and achieve centimeter-level positioning. Technically, CLAS uses a PPP-RTK hybrid approach (precise point positioning combined with RTK), similar in concept to RTK but applicable over wide areas.

• CLAS transmits correction signals known as L6D, using a dedicated frequency not found in conventional GPS. Therefore, CLAS requires high-performance GNSS receivers that support L6 signals (typically multi-band receivers). Antennas also tend to be larger than those used for sub-meter services, so CLAS is currently mainly used with professional equipment.

• CLAS began service in 2018 and was initially refined as an experimental service, but multiple receivers are now commercially available, and CLAS integration into construction machinery, agricultural machines, and surveying equipment is progressing. CLAS-capable devices can obtain centimeter-level corrections almost anywhere within Japan.

Main differences between SLAS and CLAS:

• Positioning accuracy: The biggest difference is accuracy—CLAS achieves almost RTK-equivalent centimeter-level accuracy, while SLAS is meter-level (several meters to about 1 m (3.3 ft)), representing more than an order-of-magnitude difference. For example, in precision agricultural tasks like seeding and planting where a few centimeters of error are unacceptable, CLAS or RTK is required; for field-level position awareness or rough straight-line guidance, SLAS may suffice.

• Use cases: CLAS is suited for applications demanding high accuracy such as surveying, intelligent machine control in construction, and precision agriculture (auto-steering). SLAS suits uses where “a certain level of accuracy is sufficient,” such as pedestrian or bicycle navigation, vessel navigation assistance, simple guidance for farm machinery, and GIS data collection. SLAS is also being considered for improving trajectory logs for pedestrians and vehicle dashcam GPS logs.

• Real-time behavior and stability: Both SLAS and CLAS receive augmentation unilaterally from satellites, but CLAS correction data are very advanced and voluminous, requiring processing time that can cause delays of about 10–20 seconds (from correction generation to broadcast). For real-time control such as autonomous driving, CLAS alone may not keep up, so local sensors or other methods are typically used in combination to ensure safety. SLAS also has delays of a few seconds but is generally intended for less dynamic use cases, so delays are less critical.

• Equipment and compatibility: SLAS is broadcast in the L1 band and is comparatively easy to support with existing inexpensive GNSS receivers. Some handheld GPS units and smartwatches now advertise SLAS support. CLAS-compatible receivers are more limited and are currently dominated by surveying and high-precision GNSS manufacturers; these devices are typically expensive and aimed at professional users.

• Provision and cost: Both SLAS and CLAS are provided free of charge by the Japanese government. There is no usage fee for either service (once you have compatible equipment). As with WAAS in the U.S. or EGNOS in Europe, satellite-based augmentation services are public infrastructure. However, because CLAS demands higher device performance, associated equipment costs tend to be higher. In practice, SLAS serves as an inexpensive way to try augmentation, while CLAS is an investment for specialized applications.

SLAS Advantages and Limitations

Summarizing SLAS characteristics reveals the following advantages and limitations (disadvantages).

Main SLAS advantages:

• Ease of use and low cost: SLAS is a free satellite service and does not require complex equipment or communication contracts. Anyone with a compatible GNSS receiver can use it at no additional cost. Compared to RTK, which requires expensive base stations or network subscriptions, SLAS has a lower initial barrier.

• Wide coverage and no communications required: Because augmentation is broadcast from satellites nationwide, SLAS can be received in mountains, remote islands, and at sea. In disasters where ground infrastructure (cellular networks, etc.) is down, SLAS can still provide augmentation from space, making it useful for emergency positioning.

• Improved accuracy: SLAS significantly improves over standalone positioning. Specifically, errors of several meters to about 10 m (32.8 ft) can be reduced to about 1 m (3.3 ft) or less with SLAS. This enables tasks that were previously impractical with GNSS alone, such as rough surveying and GIS data collection.

• Use of existing assets: Because SLAS uses the L1 band, many widely used GNSS chips and modules can support it via software updates. Some commercial GPS receivers have gained SLAS reception via firmware updates, allowing hardware reuse—an advantage for industry.

• Small impact on battery: No communication to a base station is required, so the mobile unit only needs to receive signals. Since satellite reception is continuous anyway, receiving SLAS does not cause a significant additional power draw. Compared with corrections delivered over cellular data, SLAS can be more power efficient for continuous operation.

SLAS limitations and cautions:

• Not centimeter-level accuracy: Although called sub-meter level, SLAS is not suitable for tasks requiring centimeter accuracy. For boundary stake setting, precise structural positioning, or surveying with strict accuracy control, a 1 m error is insufficient. SLAS should be understood as “better than standalone positioning,” not equivalent to RTK or CLAS.

• Dynamic use and latency: Because there is a time lag in correction generation, SLAS is weak in situations where satellite signal errors change rapidly. Sudden ionospheric disturbances, for example, may cause correction updates to lag behind and degrade positioning accuracy. High-speed vehicle precision positioning or latency-critical control applications are generally not appropriate; SLAS is considered suitable for low- to moderate-speed or stationary use.

• Dependence on satellite visibility: SLAS augments GPS and Quasi-Zenith Satellite signals, but to receive augmentation you need to both detect Michibiki’s augmentation signals and have sufficient GPS satellites in view. In urban canyons or dense forests where sky visibility is limited, the number of observable satellites can be insufficient, and the required accuracy may not be achieved (this is a general GNSS limitation, not unique to SLAS).

• Compatible equipment required: Older GPS receivers or smartphones may not recognize SLAS signals. The number of products explicitly supporting SLAS is growing but still limited. You must prepare and select a compatible receiver, so SLAS is not entirely plug-and-play without considering device support.

• Accuracy guarantees: As a free public service, SLAS does not provide guaranteed accuracy levels like RTK (for example, “always within X cm”). In years with strong solar activity causing large ionospheric disturbances, SLAS accuracy can exceed its performance targets (e.g., 95% within 1 m (3.3 ft)) and degrade. For critical operations, do not rely solely on SLAS; have backup positioning means and consider risk management.

Main Scenes where SLAS is Utilized

SLAS’s ease of use and adequate accuracy make it promising across various fields. Below are some anticipated use cases and examples.

• Surveying and mapmaking: When full-precision surveying is unnecessary but meter-level errors are too large, SLAS is effective. For instance, for field surveys to collect GIS data, an SLAS-capable receiver can obtain much more accurate position data than before. Municipal infrastructure inspections, site supervisors needing approximate location references, and other tasks benefit from 1 m-class accuracy to improve work efficiency.

• Agriculture: GPS guidance and autonomous tractors have become more common in agriculture. High-precision auto-steering requires RTK or CLAS centimeter accuracy, but SLAS is useful for work-route guidance and tracking heavy machinery positions. In Hokkaido, examples exist where multiple tractors equipped with GNSS trackers recorded and shared SLAS-based positions every second, improving team operations in the field. Without physical markers, operators could maintain straight lines and reduce overlapping fertilizer application.

• Construction and civil engineering: As high-precision GNSS use expands in construction machinery and surveying, SLAS serves as a simple positioning tool to support on-site tasks. For example, during earthworks, operators or workers with handheld GNSS devices can check SLAS-augmented positions for rough elevation checks or as-built inspections without calling a surveyor. Tasks where a 1 m error is acceptable—temporary structure placement checks, material yard layout, or routine site management—can use SLAS.

• Disaster response and rescue: Ground communications may be disrupted in disaster zones, but SLAS enables direct satellite augmentation and helps maintain positioning accuracy in emergencies. Rescue personnel with SLAS-capable GPS devices can know their positions on maps with greater accuracy than usual. The difference between 1 m and 10 m accuracy is significant for recording search areas and mapping damage. Drones using SLAS for aerial imagery can improve the positional accuracy of geotagged photos, streamlining subsequent analysis and map creation.

• Everyday positioning improvements: Beyond professional use, SLAS is gradually appearing in consumer products. Golf GPS rangefinders and smartwatches with SLAS support are emerging, improving outdoor leisure navigation and user experience. If smartphones adopt SLAS augmentation in the future, map apps could display more precise current locations.

Thus SLAS is a handy tool that provides one step better accuracy than conventional GPS and can be useful in many situations where RTK or CLAS-level precision is unnecessary but reduced position error is desired.

How to Easily Use SLAS

To benefit from SLAS and other satellite augmentation services, you need GNSS receivers that support those services. Fortunately, user-friendly GNSS positioning tools exist that allow non-experts to leverage augmentation like SLAS for easy high-precision positioning.

One example is the LRTK series. LRTK is a compact high-precision GNSS terminal that pairs with smartphones and is designed to support SLAS and CLAS satellite augmentation signals as well as network RTK correction data. No complicated setup is required: start the LRTK device on site and it will automatically obtain correction information from Michibiki, and the dedicated app will present positioning results on your smartphone. Stable, higher-precision coordinates that were not obtainable with standalone positioning become available to anyone on site in real time.

For example, using LRTK, simple surveying in areas without control points or high-precision geotagging of construction photos can be achieved easily. While SLAS alone yields about 1 m accuracy, LRTK can switch to CLAS or network RTK corrections as needed, allowing a balance of accuracy and convenience tailored to site conditions. With beginner-friendly interfaces and all-in-one systems, high-precision positioning is becoming an ordinary part of daily operations rather than something special.

Summary: SLAS fills the gap between conventional GPS and cutting-edge RTK positioning, offering a mix of convenience and improved accuracy. By choosing among RTK, CLAS, and SLAS according to the accuracy needs of each site, the range of possible location-based applications expands. Solutions like LRTK that combine SLAS and other augmentation services make it possible for anyone to obtain and use high-precision location data easily. If you are considering adopting high-precision positioning, start with SLAS and step up as needed. Use SLAS as an accessible entry point and experience the potential of positioning technology in the field.