【Definitive Guide】Complete Steps to Set Up NTRIP on iPhone｜Confirm Required Items in 5 Minutes

With a combination of a smartphone and a compact GNSS receiver, centimeter-level high-precision positioning (cm level accuracy (half-inch accuracy)) that used to require dedicated equipment has become more accessible. To achieve high-precision positioning on an iPhone, it is common to use a network-based correction information service called "NTRIP." This article serves as a definitive guide, explaining all the steps in detail to configure NTRIP on an iPhone and begin high-precision GNSS positioning. So even beginners can check the required equipment and services in about five minutes, I have organized the items to prepare and summarized the actual setup steps in an easy-to-understand way.

First, I briefly explain the basics of NTRIP, then list what is needed to use NTRIP on an iPhone. After that, I introduce step-by-step the specific setup for NTRIP connections and explain operational cautions and key points. At the end of the article, I also touch on the iPhone-mounted GNSS high-precision device "LRTK," which makes high-precision positioning easier. If you are interested in RTK positioning using an iPhone, please refer to this.

Table of Contents

• What is NTRIP?

• Requirements for NTRIP configuration

• NTRIP setup procedure on iPhone

• Precautions when using NTRIP

• Summary

What is NTRIP?

NTRIP (Networked Transport of RTCM via Internet Protocol) is a communication protocol for exchanging RTK positioning correction data over the Internet. In conventional RTK (real-time kinematic) positioning, GNSS data from a base station (reference station) installed at a known position are transmitted by wireless communication to the rover (the mobile unit), correcting positioning errors in real time. As a result, errors that were several meters (several ft) in standalone positioning are reduced to several centimeters (several in), enabling high-precision positioning. However, with the conventional method, it was necessary to prepare one’s own base station at each site, which required effort and cost for installation and maintenance.

NTRIP is a network-type RTK mechanism devised to reduce such burdens. The base station's observation data is distributed via an Internet server (NTRIP caster), and the user's mobile station (rover) receives that data in real time and uses it for corrections. In other words, NTRIP is a technology that delivers correction information along the route base station — Internet — mobile station. The base station (NTRIP server) sends its own GNSS observation information (in formats such as RTCM) to the caster, and the caster manages multiple correction data streams. On the rover side, the device connects to the caster as an NTRIP client and selects and retrieves the required stream (specific base station data or virtual reference station data). In this process, connection requires the host name (server address), port number, mount point (identifier for the correction data), and user ID and password (required by some services). By correctly configuring these connection details, the rover can receive real-time correction data from the base station.

Many public and private RTK correction services are provided using the NTRIP method. Users must contract with the service provider in advance and obtain the connection information for the corresponding NTRIP caster (such as the aforementioned host name and mount point). By using NTRIP, RTK positioning is possible across the country without installing your own base station. For example, there are nationwide correction information services provided by telecommunications carriers, paid services from various surveying equipment manufacturers, and region-limited free services operated by local governments, and you can choose according to your needs (coverage areas and fees vary by service). In summary, the major advantage brought by NTRIP is that “as long as you have an Internet connection, centimeter-level positioning can be performed over wide areas.” However, conversely, because using NTRIP always requires a connection to a communication line, you cannot receive correction data in environments where mobile signal does not reach. There are countermeasures discussed later (such as the use of satellite augmentation), but basically it is assumed to be used within the coverage area of a smartphone's mobile network.

Requirements for NTRIP configuration

To perform RTK positioning via NTRIP with an iPhone, you need to prepare several pieces of equipment and environmental conditions in advance. The specific items to prepare are as follows.

First, prepare an iPhone (or an iOS device such as an iPad) that can access the internet. Because it is important to be able to use mobile data on-site, a cellular-capable device is preferred (if you only have a Wi‑Fi‑only iPad, you will need to connect to the internet separately via a pocket Wi‑Fi or tethering). The iPhone model itself does not need to be the latest, but a device with a relatively recent OS version is preferable to avoid issues when connecting to a high-precision GNSS receiver or using dedicated apps.

Next, prepare an RTK-capable high-precision GNSS receiver. These come in various types, such as small devices that attach externally to an iPhone or external GNSS modules that connect via Bluetooth/USB. The important point is that the receiver supports centimeter-level positioning (cm-level accuracy, half-inch accuracy) and can receive correction data via NTRIP. Recently, high-precision GNSS modules that support multiple satellite constellations such as GPS, GLONASS, Galileo, and Michibiki (QZSS), and can receive multiple frequencies such as L1/L2, have become available to the general public. Choosing a multi-GNSS, multi-frequency receiver like that makes it easier to obtain a stable RTK FIX solution (described below). Also, depending on the receiver's form factor, there are models with an integrated antenna and built-in battery, and cable-free types that can be mounted on an iPhone are very convenient to handle in the field. In any case, check the receiver's instruction manual in advance and familiarize yourself with the procedures for connecting and using it with an iPhone (such as Bluetooth pairing or connection via a dedicated app).

And a subscription agreement for an NTRIP correction service is required. As described in the previous chapter, you should pre-register and obtain an account with a service that provides correction information for network RTK (reference station data distribution service). Your provider will supply the information required for NTRIP connection (caster host name, port number, mount point name, user ID and password, etc.), so have these ready at hand. Because services differ in being free or paid and in their coverage areas, choose a service that matches your region of use and purpose (e.g., nationwide private services or region-limited public services). In Japan, correction services offered by telecommunications carriers and reference-station networks operated by surveying companies are typical, and some local governments also provide their own NTRIP distribution.

Finally, prepare a positioning app that runs on an iPhone. You need an application to display and record high-precision data obtained from an external GNSS receiver. In many cases, the GNSS receiver manufacturer provides a dedicated app (iOS-compatible), so using that is the easiest option. Dedicated apps typically let you handle the connection to the receiver, configure NTRIP information, and check positioning results all in one place. You can also use general-purpose NTRIP client apps or surveying apps (such as those that can plot points on a map). In any case, install the app you need from the App Store and skim its usage instructions. Some apps may be in English, but if you understand the meaning of settings like "Ntrip", "Correction", and "Mountpoint", you should not have trouble operating them.

Those are the basic items. Once you have them, it's time to configure the NTRIP connection on your iPhone. In the next chapter, we will go through the actual setup steps, step by step.

NTRIP Setup Procedure on iPhone

When you're ready, actually configure the NTRIP connection on the iPhone. Here, we will explain the general procedure step by step.

（1）Connection of the GNSS receiver – First, connect your high-precision GNSS receiver to your iPhone. Turn on the receiver, then either pair it with the iPhone via Bluetooth or physically attach it to the Lightning port (the connection method varies by model). For example, with a Bluetooth-connected receiver, pair the device in the iPhone’s Settings app and also select that device within the receiver’s dedicated app. If it is an integrated type that connects directly to the iPhone, make sure it is securely attached at this stage. Next, launch the GNSS receiver’s dedicated app (or an NTRIP-compatible positioning app) on the iPhone. Confirm that the app is communicating properly with the receiver (in most cases, the app screen will display the current positioning mode, number of satellites received, etc.). Once raw data from the receiver’s sensors can be obtained, proceed to configure the NTRIP correction information.

(2) Enter NTRIP connection information – Set the correction data via the network from the app’s settings menu. The menu name varies by app, but look for items such as "Network RTK", "Ntrip Settings", or "Correction Data Settings". There, enter the pre-prepared NTRIP connection information (host name, port number, mount point, username/password). When entering them, be careful of typos and enter uppercase and lowercase letters exactly as given (passwords and mount point names in particular are used for identification and must be exact). If the settings are split across multiple fields, make sure to enter the corresponding values for each field without omission. After you finish entering all the information, save the settings (or register the connection). The iPhone app is now prepared to connect to the correction data server.

(3) Start receiving correction data – Next, start the NTRIP connection. On the app screen, turn on buttons or switches such as "Start connection" and "Receive correction data." The app will then access the specified NTRIP caster over the Internet and begin receiving the correction data stream. If the connection is successful, data will start flowing in real time from the base station. Many apps display indicators or messages showing connection status (e.g., "Connecting", "Receiving corrections"). They may also update the base station name and the number of received bytes. Use these displays to confirm that correction information is arriving. Once corrections begin coming in, the positioning solution calculated by the receiver will improve in accuracy. What is important here is the RTK solution status. If the app displays statuses such as "Float" or "Fix," they indicate the quality of the RTK solution. Immediately after connecting it will probably show "Float." This means the integer ambiguities (the unresolved integer-valued phase-difference biases between satellites) have not yet been resolved, and accuracy is on the order of several tens of centimeters (several dozen in). However, if correction data continues to be applied and enough satellites are tracked, the solution should switch to "Fix" within tens of seconds to a few minutes. "Fix" means the ambiguities have been resolved; horizontal position is accurate to a few centimeters (a few in), and vertical accuracy is a few centimeters to several tens of centimeters (a few in to several dozen in). Always confirm that the status has become "Fix" before relying on the positioning results for serious use (be careful: accuracy is insufficient while still "Float"). During surveying, always keep an eye on this status and proceed while verifying that "Fix" is maintained.

(4) Checking and using positioning results – Once the RTK solution is Fix, you can finally put that high-precision position information to practical use. First, check in the app’s settings that the coordinate system is set appropriately. Especially when comparing against known control points for public surveying, you need to select a system suited to the purpose, such as a plane rectangular coordinate system based on the World Geodetic System (in Japan, JGD2011). Many NTRIP services distribute data in the WGS84 system or a system close to it, but strictly speaking, deviations of several tens of centimeters (several tens of in) may occur between it and regional coordinate systems. If necessary, configure the app to apply geoid heights or correct origin offsets, and convert the positioning results to the desired coordinate system. Once preparations are complete, observe and record survey points. Save the coordinates and heights of measured points within the app or record them by taking screenshots (with dedicated apps you may also be able to export positioning data in CSV or GeoJSON format). During work, continuously confirm that the RTK Fix solution is being maintained; if it temporarily reverts to Float, suspend data collection and wait for Fix recovery, or, if needed, disconnect and then reconnect the correction connection. Also, if there are known control points at the site, measuring them to confirm that errors fall within acceptable limits will give you peace of mind. If large errors occur, review whether the chosen mounting point is appropriate and whether there is any mismatch in the coordinate system settings, and reconfigure as necessary.

Through the above steps, real-time high-precision positioning using an iPhone and a GNSS receiver can be achieved. While a stable Fix solution is being obtained, the position coordinates obtained are ensured in real time with almost centimeter-level accuracy (cm level accuracy (half-inch accuracy)). After surveying work, be sure to back up the acquired data. It is safer to keep duplicate copies of the point data saved in the app, such as by uploading them to cloud storage or emailing them to yourself.

Important Notes When Using NTRIP

To stably operate RTK positioning using the NTRIP method, pay attention to the following points.

Securing the communication environment: As long as correction data is received via the Internet, the quality of the communication link is directly tied to the stability of positioning. If the rover's communication is unstable, correction data will be interrupted, during which the RTK solution may revert to Float, or in the worst case corrections may not be received and the system will fall back to regular standalone positioning. In urban areas, 4G/LTE or 5G connections are generally adequate, but NTRIP services cannot be used in environments where mobile phones are out of range, such as mountainous areas or underground. To perform high-precision positioning in such places, alternatives that do not rely on the Internet are necessary. One option is to use CLAS from Japan's Quasi-Zenith Satellite System (QZSS). With a CLAS-capable GNSS receiver, you can receive correction signals directly from satellites overhead even without cellular communication. Another option is to set up your own base station and transmit corrections via local radio (specified low-power radio or LoRa, for example). However, because preparing a base station adds effort, it is usually recommended to use NTRIP when you are within communication coverage and to switch to CLAS or local RTK when you are out of coverage. Note that using a VPN for the Internet connection can increase link latency. Because timing mismatches in applying correction information affect accuracy, try to keep the communication path as simple as possible and minimize latency.

Satellite reception environment: Good satellite visibility (line of sight) is essential for high-precision positioning. If there are tall buildings or forests nearby, satellite signals can be blocked or multipath interference from reflections can occur more easily. Place the antenna in as open a location as possible and perform positioning in an environment with a wide view of the sky. If the receiver has an elevation mask (an angle setting to exclude low-elevation satellite signals), setting it to about 15 to 20 degrees can be effective in reducing the impact of poor signals from low elevations. Also, if you feel RTK initialization is taking a long time, performing a reinitialization (reset) on the receiver once can sometimes lead to a faster Fix solution. If the receiver temporarily returns to Float during work, don’t panic; wait several tens of seconds or try disconnecting and reconnecting once to obtain a Fix again more easily. Always check the status (FIX/FLOAT) during positioning, and be careful not to use data obtained while in Float casually, as it has large errors.

Distance to the reference station: RTK correction data provided by a single base station becomes more difficult to maintain accurately as the distance (baseline length) to the rover increases. In general, a range of several km to about a dozen km is said to be a guideline for maintaining high accuracy. Network RTK services (e.g., VRS) mitigate this issue by using data from multiple reference stations, but even so, if the service area is extremely far from the base stations, initial FIXes can take longer and it becomes harder to maintain a solution. If possible, it is effective to choose a correction data stream (mount point) that is close to your area of use. If regional mount points are provided when you sign up for the service, specify the one closest to your site. Also, when surveying while moving, you can use a service that automatically switches to the optimal corrections according to position (for example, a virtual reference station that follows the user's current position). Note that this assumes the setting to transmit the rover's current position information (NMEA-format GGA message) to the caster is enabled. Some apps have an option such as "Send current position," and if this is turned off you may not receive corrections correctly from a virtual reference station service.

Equipment selection and management: To fully realize the performance of high-precision positioning, the receiver’s specifications are also important. Multi-GNSS (supporting multiple satellite systems) and multi-frequency equipment can maintain a FIX solution far more stably than single-GNSS, single-frequency units. Most modern RTK-capable receivers meet this specification, but if you are reusing older equipment, check its compatibility. Using a high-performance antenna also reduces noise and multipath effects, contributing to improved accuracy. In addition, managing equipment so that performance does not degrade on site is a basic but important point. Always monitor battery levels and provide backup power for long measurement sessions. In hot environments, devices can become unstable if they overheat, so measures such as keeping devices in the shade when in direct sunlight and cooling them as appropriate are effective.

By following the points above, you can maintain higher reliability for RTK positioning using NTRIP.

Summary

So far, we've gone through the workflow of using NTRIP corrections by combining an iPhone with an external GNSS receiver. Even without dedicated equipment, centimeter-level positioning is now possible with a smartphone, and the reach of high-precision positioning is steadily expanding. On the other hand, it's also true that for those tackling it for the first time, there are several hurdles such as preparing equipment and configuring connections.

Finally, as a solution to make high-precision positioning more accessible, we introduce LRTK. LRTK is a compact, all-in-one RTK-GNSS receiver device that can be retrofitted to smartphones (mainly iPhone and iPad). It is a pocket-sized unit with the antenna, high-precision GNSS chip, communication module, and battery all integrated, and can be easily attached to the back of a smartphone with a dedicated mount; it is lightweight as well, weighing about 125 g. By attaching this small device to an iPhone, entering the NTRIP correction service information in the dedicated app, and turning “Network RTK” on, a single operator can immediately begin centimeter-level positioning. Complex operations are abstracted within the app and are completed with simple steps such as “select correction information / start connection.” It is designed to be intuitive even without specialized knowledge, so those encountering RTK for the first time can use it with confidence.

High-precision positioning information obtained with LRTK can be tagged to photos taken on a smartphone and to point cloud data. For example, if you take site photos with an iPhone equipped with LRTK, accurate positioning coordinates will be attached to every image. This makes it easy to precisely overlay the captured photos on a map or to immediately utilize point cloud scan data in a GIS. Tasks that traditionally required expensive surveying equipment and skilled technicians can be performed with just one smartphone per person, dramatically improving on-site productivity. LRTK also integrates with cloud services, allowing positioning data and photos to be uploaded directly from the field to the cloud for sharing. By lowering the barrier to high-precision positioning and opening up "an era in which anyone can handle centimeter-level positioning on a smartphone (cm level accuracy (half-inch accuracy))," LRTK is a very compelling option for iPhone users to start using high-precision GNSS.