RTK corrections and NTRIP casters
Real-Time Kinematic (RTK) is a technique used to improve the accuracy of a standalone GNSS receiver. The technique uses two receivers: one stationary and one moving freely. They are called the base station and the rover. The base station provides correction data to the rover, which uses this data to calculate centimeter-precise position.
You do not necessarily need a second unit for RTK all the time. Usually, there are local services that share base corrections over the internet. This technology is called NTRIP.
Reach RX is a network rover that receives corrections using your phone's internet connection. The standard way to do this is through the NTRIP protocol.
You can learn more about the RTK technology in the How RTK works article.
NTRIP protocol overview
NTRIP (Networked Transport of RTCM via Internet Protocol) is a communication protocol used to transmit RTK correction data over the internet. The protocol is based on the client-server model, where a reference station (the server) sends correction data to one or more rovers (the clients) using an internet connection. The data is sent using the RTCM format, which is a standard format for GPS correction data.
NTRIP has 3 main roles: a server, a caster, and a client.
The NTRIP protocol also includes a mechanism called "mount points" that allows multiple streams of correction data to be sent from a single server and allows the rovers to select the stream they want to receive by specifying the mount point in the request.
You can find full description of the NTRIP protocol can be found in the official documentation.
Reach RX and NTRIP casters
When working with Reach RX, mobile apps and data collectors act as an NTRIP clients, passing the correction feed to the receiver via Bluetooth. The apps act as a bridge that passes corrections from a network connection to a Bluetooth connection.
NMEA feedback
In many cases, NTRIP casters require the approximate position of rover to function. They may need the position to automatically determine the appropriate base station or for VRS operation. This is achieved by sending the approximate position of your rover (usually the SINGLE solution) as an NMEA GGA string to the caster after the connection is established. In the case of Reach RX, this is done by the mobile application as well.
NMEA GGA messages can be formed by using the receiver's status information that includes the position coordinates.
In-app NTRIP
Managing NTRIP casters in the application is straightforward. To connect to an NTRIP caster, you typically need an address, a port, a mount point name, a username, and a password.
You can learn more about setting up RTK via NTRIP in Emlid Flow in the Working with NTRIP service guide.
There's also an optional step to determine the available mount points for this particular caster. It is possible to download and parse a source table that contains information about active mount points. That simplifies the process for the end user and allows for neat tricks, like recommending the closest base station.
You can find an example of the source table on this page.
Emlid Flow workaround
Emlid Flow for iOS supports sending corrections in the background. The user can configure corrections in Emlid Flow and proceed working in your app. Using Emlid Flow for corrections is free and available to all integrators.
NTRIP client inside your app
Alternatively, you can implement the NTRIP client functionality inside your app.