How to Choose a CORS Network for RTK Surveying in the United States

Table of Contents

• What is RTK surveying?

• What is a CORS network?

• Current state of RTK CORS networks in the United States

• Points to consider when choosing a CORS network

• Countermeasures when a CORS network is not available

• Simple surveying with LRTK

• FAQ

What is RTK surveying?

First, RTK surveying stands for Real-Time Kinematic positioning, a high-precision GNSS surveying method. RTK typically uses two GNSS receivers: one set up at a known point as a base station (reference station) and the other used as a rover carried around during surveying. The base station processes the GPS/GNSS signals it receives in real time (extracting observation errors) and continuously sends that correction information to the rover. By immediately applying the received corrections to its own observations, the rover can improve a position that would have meter-level errors in standalone positioning to centimeter-level accuracy (half-inch accuracy). Thanks to RTK’s real-time precision enhancement, surveyors can obtain accurate coordinates on-site and immediately perform tasks such as stakeout and as-built inspections.

Real-time communication from the base station to the rover is essential for RTK surveying. Historically, base-rover communication used short-range licensed/unlicensed radios or UHF radios, but recently methods that distribute corrections via the Internet have become widespread. In particular, the protocol called NTRIP is commonly used; if the rover has a cellular Internet connection, it can receive base station data from the network instead of via radio. With Internet-based RTK, users can utilize remote pre-existing base stations without setting up their own base. That network of pre-existing base stations is what is called a “CORS network.”

What is a CORS network?

A CORS network is a network that connects multiple GNSS reference stations located in various places via the Internet (or other means) and distributes positioning data. “CORS” stands for Continuously Operating Reference Station, referring to reference stations that operate 24 hours a day and provide high-precision observation data. In Japan, this corresponds to the Geospatial Information Authority’s “GEONET” of permanent reference stations, but similar reference-station networks are maintained in the United States and other countries.

CORS networks were originally built so that observation data recorded by each reference station could be used for post-processing (static positioning or PPK). However, in recent years services that provide these reference-station data in real time for RTK surveying have spread. Users can obtain reference-station data on the network via the Internet and apply them as corrections to their rovers. Advanced network-RTK services generate and distribute a virtual reference station (VRS) by combining multiple reference stations, which makes it easier to obtain stable accuracy over a wide area.

Simply put, by using a CORS network, you can perform RTK surveying without setting up your own base station. Think of borrowing public or private reference stations scattered across a region via the network; the user only needs to bring a rover to the field. Note, however, that many networks require prior registration or payment, and procedures and costs vary depending on which network you choose. So how should you choose a CORS network when conducting RTK surveying in the United States?

Current state of RTK CORS networks in the United States

There is no single nationwide RTK network that uniformly covers the entire United States; instead, various CORS networks operated by different entities are provided regionally. In Japan, the Geospatial Information Authority has installed about 1,300 permanent stations nationwide, but in the U.S. there is no federal real-time service deployed nationwide (the country’s vast area makes it difficult for a single company or agency to build a real-time network covering the entire nation).

A representative resource in the U.S. is the “National CORS network” managed by NGS (the National Geodetic Survey), an agency of NOAA. Over 2,000 GNSS reference stations are installed across the country, and observation data (such as RINEX files) are published on the Internet. However, this NOAA CORS network is mainly operated for maintaining geodetic reference frames and for post-processing data (OPUS, etc.), and it is not a service that provides real-time RTK correction streams. To use it for real-time positioning, you would need to access the data streams of individual stations separately; an integrated real-time distribution service is not generally provided. Therefore, NOAA CORS is more often used for static high-precision surveying (baseline analysis or establishing control networks) than for RTK.

In practice, real-time RTK surveying in the U.S. commonly relies on state-run CORS networks or commercial RTK services provided by private companies. Examples of free networks run by state governments or universities include Florida’s “FPRN,” Indiana’s “InCORS,” Iowa’s “IaRTN,” and Michigan’s “MDOT CORS.” These install numerous reference stations within their respective states and deliver correction information to registered users over the Internet (via NTRIP). Many state-operated networks are available free of charge upon registration, making them valuable infrastructure for surveyors and technicians.

Where free networks are not available or near state borders, it is also common to use paid commercial RTK network services provided by private companies. Major global surveying equipment manufacturers also offer their own RTK correction services, including networks usable nationwide (such as vendor-specific VRS networks or nationwide NTRIP services). These commercial services typically charge monthly or annual fees, but they often provide strong support and the advantage of being usable across state lines.

In summary, CORS networks used for RTK surveying in the U.S. are broadly divided into “public (state/government)” and “private commercial” categories. Public networks are generally free but geographically limited, while commercial networks are paid but offer broader coverage and potentially higher service quality. Multiple networks may overlap in some regions, so it is important to select the network best suited to your survey area.

Points to consider when choosing a CORS network

When multiple CORS networks are available in the United States, consider the following points when selecting one.

• Coverage area and reference-station density: First, confirm whether your survey site is within the network’s service area. If the site is too far from reference stations, RTK accuracy will degrade, so ideally choose a network with stations within 20 km (12.4 mi). Wide-area networks may provide coverage using virtual reference stations, but it is still desirable that real-time reference stations exist within several tens of kilometers (several tens of miles) around the survey point.

• Usage fees and registration procedures: Whether the service is free or paid is a major consideration. State-provided networks are often free but may require prior user registration or impose restrictions on the purpose of use. Paid services require a fee but often allow immediate use upon contract and provide support. Evaluate the cost-effectiveness according to project scale and duration (for short-term, small surveys, free networks may suffice; for long-term or wide-area work, consider paid subscriptions).

• Correction method and technology: Networks may provide correction information in different formats. Generally, RTCM 3.x is commonly distributed, but some networks offer proprietary virtual reference station (VRS) corrections or allow selection of corrections from the nearest single station. Confirm whether your GNSS receiver supports the network’s correction method (for example, multi-GNSS corrections including GLONASS or Galileo). Most modern receivers support standard formats, but older units may not be compatible.

• Reference coordinate system: U.S. CORS networks typically use the national geodetic datum NAD83 as their coordinate reference. Positioning results corrected by the network will often be in NAD83 coordinates (some services output IGS global coordinates or state-specific coordinate systems). If you bring equipment from Japan or prefer to work in WGS84, be aware of potential need for coordinate transformation. Some networks can output both NAD83 and IGS coordinates.

• Reliability and support: The reliability of the network operator is important. State-run services are generally stable but occasionally go offline for maintenance. Commercial providers usually offer support desks you can contact with questions. In any case, perform a test connection beforehand to verify that correction data can be received and applied properly. Avoid attempting first-ever connections on the day of a critical survey; allow time for verification.

• Field communication environment: Using a network requires mobile data connectivity (cellular). If the survey site is in mountainous areas or outside cellular coverage, even a contracted Internet-based RTK service will be unusable on site. In such locations, switch to local radio RTK (set up your own base station and broadcast corrections via radio) or consider offline correction methods described below. Check the cellular coverage of the site beforehand to determine if network RTK is feasible.

Based on these points, compare available networks and choose the optimal one. For example, you might “register and use the state’s free CORS network where the site is located,” “purchase a short-term subscription to a commercial nationwide network because you are outside coverage,” or “switch to a local base-station method because communications are unreliable.” Choose according to the situation.

Countermeasures when a CORS network is not available

There are cases where there are no suitable nearby CORS stations or where you cannot connect to a network. Here are some countermeasures to consider.

• Set up your own base station: In areas without a network, the standard approach is to set up your own GNSS base station. Place a high-precision GNSS receiver on a known point (a point whose coordinates were determined by prior static surveying) and use it as the base station, sending corrections to the rover via radio. Although this requires initial cost and effort, it enables real-time positioning even where there is no cellular coverage.

• Use alternative precise positioning techniques: If real-time operation is not required, consider PPK (post-processed kinematic) or PPP (precise point positioning). For example, record the rover’s GNSS logs in the field and later process them in the office using NOAA’s OPUS (Online Positioning User Service) or a commercial cloud PPK service to compute coordinates. You lose real-time results, but you can obtain high-precision outcomes without relying on communications. There are also satellite-based augmentation services that provide centimeter-level corrections directly from satellites (e.g., Japan’s CLAS), though such services may not be available in the U.S.; some regions offer similar commercial satellite augmentation services.

• Plan the survey workflow: If the network is only partially available, establish high-precision control points in areas with connectivity and then use those as local control to survey beyond the coverage. For example, obtain a reference point in an open area via network RTK, then use that point as a new local base station to survey within offline areas. Although this adds steps, it can ensure high precision across the entire site.

As shown, there are alternatives even when a CORS network cannot be used. The key is to be flexible and not insist on real-time methods when not necessary. Combine RTK and PPK as appropriate, or set temporary control points in advance to complete the job.

Simple surveying with LRTK

As described above, achieving high accuracy with RTK requires preparation such as selecting an appropriate reference-station network and ensuring communications. The LRTK series is a solution designed to greatly simplify those tasks and enable anyone to perform centimeter-level positioning easily. LRTK is a system developed by Lefixea Inc. consisting of a compact high-precision GNSS device, a smartphone app, and cloud services, engineered as an all-purpose surveying tool that allows easy high-precision positioning on site without specialized knowledge.

With LRTK, the previously complex tasks of base station setup and network selection are greatly simplified. It normally performs network RTK positioning via cellular communication and, in conjunction with a smartphone, provides centimeter-level positioning on site. For areas without cellular coverage, an optional antenna that receives the QZSS Michibiki CLAS (centimeter-level augmentation service) is available, enabling communication-free high-precision positioning via satellite augmentation. Users can seamlessly switch between real-time network RTK and offline positioning depending on field conditions, preventing interruptions in mountainous or underground environments.

The LRTK series also supports cloud integration for data management. Point coordinates and high-precision geo-tagged photos taken on site can be saved and shared to the cloud instantly, greatly improving office workflows for data consolidation and reporting. By sharing field data in real time through the cloud, all stakeholders can access the latest survey results, smoothing coordination for construction and inspections.

Adopting the LRTK series can dramatically improve productivity and precision in surveying work. It replaces expensive base-station equipment and complex configurations with a simple device, and is a modern solution compatible with Japan’s Ministry of Land, Infrastructure, Transport and Tourism initiatives like i-Construction. From construction, civil engineering, and surveying sites to infrastructure inspections, LRTK can significantly innovate field operations.

For more details on the LRTK series, please visit the [LRTK official site](https://lefixea.com/). Feel free to contact them with product questions or to request a demonstration. Take your company’s surveying work to the next stage with LRTK.

FAQ

Q: Is there a single nationwide RTK network covering the United States? A: No. There is no nationwide unified real-time RTK service in the U.S. similar to Japan’s permanent reference stations. While NOAA operates a national network of reference stations, it is mainly for data provision and post-processing. For real-time use, you need to rely on state-level CORS networks or private RTK services.

Q: How much does it cost to use a CORS network? A: Costs vary by provider. Networks run by states or universities are often free and available to registered users. Conversely, private nationwide services are paid, with pricing such as monthly subscriptions or time-based charges. If a free network is available in your area, start with that; if not, consider a paid service.

Q: How far from a base station can RTK surveying be performed? A: Generally, single-base RTK can achieve centimeter-level accuracy within 20 km (12.4 mi), but accuracy degrades beyond that due to increased ionospheric and tropospheric errors. Network RTK (VRS, etc.) interpolates data from multiple reference stations, so it can maintain stable accuracy even tens of kilometers away. However, when outside the network coverage (roughly beyond the edge of the reference-station array), accuracy guarantees become difficult. For very large-area surveys, you may need to move and reacquire corrections or establish your own base stations.

Q: If I’m in mountainous areas without cellular coverage, must I give up on RTK surveying? A: No. Even without communications infrastructure, there are options. You can set up your own base station and use radio modems for RTK, or record GNSS data for later PPK processing to obtain high-precision results. Additionally, satellite-based augmentation methods (e.g., Japan’s Michibiki CLAS or commercial satellite augmentation services) can provide centimeter-level positioning without terrestrial communications. In short, if real-time operation is not required, high-precision positioning is still achievable by choosing appropriate post-processing or augmentation strategies.