How RTK works, and what the GPS badge is telling you

Ordinary GPS, like the one in a phone, can be several feet out, and its elevation drifts up and down. Grade work needs the cutting edge within a fraction of an inch, every pass, every day. RTK (short for real-time kinematic) is how a GPS receiver gets that close.

Ditch Assist® endorses only one kind of fix for surveying and grading: RTK Fixed. This page explains what that means, and how to tell when you have it.

RTK in plain words

1. Satellite signals reach the ground with small errors. Two receivers close to each other get almost the same errors at the same moment.
2. A second receiver, the base, sits still on one spot. It knows it isn't moving, so it can work out how far off the satellites are right now.
3. The base sends that out about once a second. These messages are called corrections. They reach your machine by radio, or over the internet.
4. The receiver on your machine, the rover, takes the errors away. With the corrections it knows where it is to within a centimetre or two (under an inch).

The three parts

The rover then tells Ditch Assist where it is, in standard text messages called NMEA. On the Full Automation Kit, the Guidance Only Kit and DIG™, they go down the RTK GPS cable to the control module or DIG module. With Ditch Assist Lite they go to the tablet over the receiver's own Wi-Fi. What the receiver must send is on What your receiver must send.

How the app shows your fix: the GPS badge

On every map screen there's a white bar across the top. In it, to the right of the numbers, is the GPS badge: a rounded box with a target symbol, a word for the fix (here RTK), the number of satellites, and a small ⓘ.

Its word and colour tell you the fix at a glance. Tap it for the full details: see The GPS Info window.

1

The fix ladder

Every position the receiver sends says how good it is. That's called the fix. After you switch on, the rover climbs from no fix to RTK Fixed, one step at a time. Only the top step is good enough to survey or grade.

Three rarer words: PPP (amber: a precise point positioning solution from a satellite service, with no base), SIM (grey: the receiver itself is in a simulation or test mode) and N/A (the receiver reports an estimated or typed-in position). None of them is RTK: no survey points, grading holds.

The app's own Simulate button is different. It climbs from NO GPS through GPS and FLOAT to RTK in about seven seconds, and the GPS Info window names Simulator as the source.

The System Check shows the same steps

When you connect from a workflow card, the window that opens ends with a System Check. Its bottom row, GPS Accuracy, shows the fix in words. You should see it climb like this, from top to bottom:

What the app does when you lose RTK Fixed

A switch called RTK Filter controls this. It's in Settings, in the SURVEY box, and it's on unless someone turned it off. With it on:

• Surveying: the app records points only while the fix is RTK. Points on a lesser fix are skipped.
• Automatic grading: the app holds the blade straight away, and an orange status label on the grade screen reads HOLD — No RTK Fix. If the fix stays bad for 5 seconds, a red banner adds RTK LOST - AUTO HOLD (…s) and counts the seconds. When RTK comes back and settles, grading carries on by itself. You don't need to tap anything. In Ditch Assist Lite the banner reads RTK LOST - GUIDANCE PAUSED (…s).
• Waiting to carry on: if RTK is back but hasn't settled yet, an amber banner says what the app is waiting for, for example AUTO HOLD - WAITING FOR RTK FIX TO SETTLE - SENSITIVITY MID. That's the app making sure the elevation has settled again. It isn't a fault.

A second setting under it, RTK Sensitivity (Low, Mid or High; Mid to start with), sets how quickly the hold kicks in. Low holds only when RTK Fixed is lost or positions stop. Mid and High also hold when the satellites are poorly spread across the sky (a high HDOP) and when the fix keeps dropping in and out. See Survey settings.

GPS Stale

This is different from a poor fix: the positions stop arriving altogether. If no new position comes in for longer than the GPS Delay Timeout (5 seconds unless someone changed it; it's in the DEBUG box of Settings), the GPS is called stale:

• the GPS badge fades;
• automatic grading holds (HOLD — GPS Stale), whatever the RTK Filter is set to;
• Start is refused with GPS: STALE — position not updating.

After 10 seconds with no data at all, the app treats the connection as lost. Stale almost always means a Wi-Fi or cable problem, not the sky. See No GPS in the app.

After you switch on: getting to RTK

When the rover starts, it climbs the ladder: NO GPS, then GPS, then FLOAT once corrections arrive, then RTK once it has locked on. That last step is called convergence. How long it takes depends on how many satellites the rover sees, how open the sky is, and how far it is from the base.

Wait for RTK before you survey, set blade height or calibrate to a benchmark.

The base has its own start-up. An Emlid Reach base set up by the app averages its position for two minutes every time it's switched on, and sends nothing until it's done. Give it that time.

Sky view

Both receivers need open sky. The rover's antenna should see everything from about 15° above the horizon upward, all the way round, in every working position, fully lowered too. Things that cost you the fix:

• Blockage: trees, buildings, grain bins, the cab, or the implement's own frame above the antenna.
• Bounced signals: near metal sheds and bins, signals bounce and give wrong positions (called multipath).
• Distance and radio: the further from the base, the weaker the corrections link. Stay within 2 miles of your own base, with nothing blocking the view between them.
• Vibration: a loose mast or mount on rough ground can drop the fix.
• Space weather: strong solar activity can upset RTK for hours. NOAA's Space Weather Prediction Center (swpc.noaa.gov) publishes forecasts.

Antenna placement is covered on Mount the GPS antenna.

Why the base must not move

The rover's position is worked out from the base's. If the base stands in a different spot, or averages its position again after a restart, every elevation the rover reports shifts by the same amount. Your saved surveys and designs were recorded against the old base position, so they no longer line up with today's readings.

The answer is a benchmark: a spot in the field you drive back to and calibrate on. Calibrating puts today's readings back in line with your saved work. Calibrate whenever the base has been moved, replaced or restarted, or when you load work from an earlier day. See Benchmarks and calibration and Same base, same spot.

If it doesn't work

• Stuck on FLOAT: corrections are arriving but the fix won't lock on. Check the sky view, the distance to the base and the correction age. Stuck on Float, or RTK keeps dropping.
• GPS, never FLOAT: no corrections are reaching the rover. Check the base is on and sending, the radio frequency matches, or the NTRIP login is right. Corrections troubleshooting.
• DGPS on a John Deere receiver you know is on RTK: Shows DGPS instead of RTK.
• NO GPS, or the badge has faded: no positions are reaching the app. No GPS in the app.
• Ditch Assist Lite: Fix Ditch Assist Lite problems.