Choose and check your RTK corrections

Corrections are the messages a base receiver sends to the receiver on your machine (the rover), about once a second. They tell the rover how far off the satellites are right now, so it can get to RTK Fixed. If they stop, the rover soon drops back to Float. (New to these words? Start with How RTK works.)

Ditch Assist® never handles corrections itself. The receiver gets them and works out its position; the app reads the result. So every correction setting (radio channel, network address, login) lives in the receiver, never in the Ditch Assist app.

Your options

Your own base over radio

A base you own gives you the most control: you choose where it stands and how far away it is. With two Emlid Reach receivers, the base sends corrections to the rover over the Reach's built-in LoRa radio.

• Base and every rover use the same LoRa frequency, between 902 and 928 MHz, and the same air data rate, 18.23 kb/s.
• The corrections go out in a standard format called RTCM3. You don't have to set these numbers by hand; the app does it. The base sends messages 1006 (base position, every 10 seconds) and 1074, 1084 and 1094 (GPS, GLONASS and Galileo, once a second each).
• An RS4 base transmits at 500 mW, or 1000 mW for more range. An RS3 always transmits at 20 dBm, its maximum.
• The rover's correction input reads Waiting for corrections in Emlid Flow until the base is on the air.

The app's Emlid Diagnostics button, at the bottom of Settings, sets all of this up on both receivers. See Set up an RTK base and Check and program a Reach with Emlid Diagnostics. For another brand, set up the base and radio the way its maker describes.

NTRIP over cellular (network RTK)

An NTRIP service is a network of bases run by a state, a province or a company. It sends corrections over the internet. You need an account with the service, and a mobile data connection on the receiver.

The service gives you four things to type into the receiver (on an Emlid Reach, in the Emlid Flow app; on other brands, in the receiver's or display's correction settings):

• Caster address (a web address or number) and port
• Mount point: the name of the correction stream to use, usually the nearest base, or a stream the service builds for your area (often called VRS)
• Username and password

There's nowhere to type these in Ditch Assist, and the app never reads or saves them, even when it reads a Reach's settings in Emlid Diagnostics.

On an Emlid Reach (RS3 or RS4), in Emlid Flow

1. Put a nano-SIM in the receiver

   A SIM with a data plan. A data-only or IoT SIM with some prepaid credit is ideal: corrections use very little data.

2. Turn on Mobile data

   In Emlid Flow, open the receiver's Settings and turn Mobile data on. Type the SIM's PIN if it asks.

3. Open Correction input

   It's in the receiver's main list in Emlid Flow.

4. Choose NTRIP via Reach

   Then type the service's caster address, port, mount point, username and password.

Pictures of each Emlid Flow screen are in the receiver setup guide.

Emlid Corrections over the receiver's SIM

A Reach RS4 or RS4 Pro can take Emlid's own correction service over a SIM card in the receiver, with no base at all. It suits Ditch Assist Lite, where one receiver on the blade can be all you need.

1. Put the nano-SIM in the receiver

2. Turn on Mobile data

   In Emlid Flow, open the receiver's Settings and turn Mobile data on. Type the SIM's PIN if it asks. Most SIMs connect without anything else; some also need Data roaming on.

3. Check the network bars

   Bars appear next to the battery symbol once the SIM is connected.

4. Open Correction input

5. Choose Get corrections via Reach

   The receiver now gets Emlid Corrections over its own SIM.

Correction age: are corrections arriving?

Correction age is how many seconds since the rover last got a correction. It should stay low and steady. When it climbs, corrections aren't getting through, and the fix will drop to Float.

Two places show it:

• Pre-Flight Checks (the orange button at the bottom of the right-hand column), row Corrections: green up to 2 seconds, amber up to 10 seconds, red beyond that. If the receiver doesn't report an age, the row reads Active while you have RTK Fixed, and Not reported otherwise.
• The GPS Info window (tap the GPS badge), row Correction Age: the same figure, to a tenth of a second, without colour. See The GPS Info window.

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Correction age comes from the GGA message, so only sources that send NMEA straight to the app show a number: the DIG module, an Emlid Reach in Ditch Assist Lite, and Bluetooth receivers. The control module doesn't pass it on.

If it doesn't work

• Rover stays on GPS, never Float: no corrections at all. Own base: is it switched on, finished averaging its position (two minutes on a Reach set up by the app), and on the same frequency as the rover? NTRIP: does the receiver have mobile data, and are the caster, port, mount point and login right?
• Correction age climbing, fix dropping to Float: the link is breaking up. Move the base higher or closer, raise an RS4 base to 1000 mW, or check the mobile signal for NTRIP. Stuck on Float, or RTK keeps dropping.
• An Emlid base won't turn its LoRa radio output on: the radio can't carry all three satellite systems once a second. Setting up the base from the app slows them to 0.5 Hz one at a time, by itself. How the app lowers the rates.
• The badge says RTK, but the elevation drifts while you stand still: you may be on a satellite service reported as RTK. Check which correction source the receiver is really using.
• Ditch Assist Lite: Fix Ditch Assist Lite problems.