Rtk Gps

How to use the Rtk Gps robot skill

An RTK GPS is a satellite receiver that is good to a centimetre or two instead of a metre or two. It gets there by comparing its own measurements with those of a base station a few kilometres away, which sends corrections over the internet or a radio. This robot skill reads an RTK receiver (a u-blox ZED-F9P, or any receiver that reports RTK in NMEA), feeds it the corrections, and hands the robot's position and heading to the rest of ARCx. The Outdoor Navigator robot skill can then drive the robot along a lawn's edge or between rows to within a few centimetres, and any other robot skill or script can read the position.

How RTK works, briefly

A receiver on its own works out its position from the time signals of the satellites. Those signals are delayed by the atmosphere by different amounts from minute to minute, which is why a plain GPS wanders by a metre or more. A base station that is not moving, a few kilometres away, sees almost the same delays. It sends what it measures to the robot's receiver as RTCM corrections, and the receiver cancels the delays out.

The receiver goes through three stages, shown in the window's status line:

State	Accuracy	What is happening
3D fix	A metre or two	No corrections yet, or none arriving
RTK float	Tens of centimetres	Corrections are being used, but the receiver has not yet worked out the exact number of radio wavelengths to each satellite
RTK fixed	One or two centimetres	It has, and holds on to them while the sky stays clear

Fixed usually comes within a minute of corrections arriving in an open field. Trees, buildings and the robot's own body block satellites and can drop it back to float; it comes back once the sky clears. The base station should be within about 20 km. Further away, fixed takes longer and is less certain.

Where the corrections come from

Source	What it is
NTRIP caster	The usual way: corrections over the internet from a caster, which relays many base stations as "mountpoints". Government CORS networks, commercial services, and RTK2go (free, run by volunteers' base stations) all work this way
Your own base station	A second RTK receiver on a roof or pole, running this skill in base station mode on another computer. It serves its corrections as an NTRIP caster that the robot connects to
Raw TCP stream	An RTCM stream from a server without NTRIP
The receiver gets them itself	A radio wired straight to the receiver. The skill just watches the fix

To use RTK2go, set the caster to rtk2go.com, port 2101, your email address as the user name and none as the password, then press Find mountpoints on the window's Corrections tab. The list is sorted by distance from the robot; press Use next to the nearest one.

The skill also sends the robot's position to the caster every few seconds. Network casters (VRS) need it to build corrections for where the robot is, and others ignore it.

The heading

RTK says where the antenna is, not which way the robot faces. The skill can take the heading from three places:

Heading from	How good it is
Dual antennas	Two antennas on the robot, on a moving base pair of ZED-F9Ps, a ZED-F9H, or a dual antenna receiver such as the UM982. The receiver measures the line between them to a fraction of a degree, even standing still. The best choice for a robot that turns on the spot
The direction of travel	Worked out by the fusion while the robot drives. Unknown while standing still or turning in place
A variable	A heading another robot skill publishes, such as the BN-880 GPS Compass skill's compass ($GpsCompassHeading) or an IMU

What it does with the readings

The fixes, the heading and the Movement Panel are combined in a Kalman filter and reported to the Navigation Messaging System (NMS) as the robot's pose, many times a second, so a navigator gets a smooth pose between fixes. While the Movement Panel is stopped, the robot is held still. The skill also ties the NMS map to the earth: it tells ARCx where the map's 0, 0 is, so any robot skill can turn a latitude and longitude into a point on the map. Every fix is published too, for scripts, other robot skills and the Robot Bus (/arcx/nms/fix).

If wheel encoders or a tracking camera already report the robot's pose, choose Align to another skill's pose instead. The skill then leaves the pose to that skill and keeps the map's tie to the earth sliding as the encoders drift, so latitudes and longitudes still land in the right place.

Wiring a ZED-F9P

A ZED-F9P board (SparkFun, ArduSimple simpleRTK2B, Holybro and others) plugged into the computer by USB appears as a serial port: COM5 on Windows, /dev/ttyACM0 on Linux. That is the easiest way, and the skill configures everything else. Wired by its UART instead (to a USB to serial adapter or an EZ-B v4 UART), a new F9P talks at 38400 baud; the skill finds it at any common rate and moves it to a faster one. Set Wired to the F9's to the receiver port you used, because the skill switches its messages on per port.

When it connects, the skill configures the receiver: the number of fixes a second, the motion model, the high precision position messages it reads, the dual antenna heading, and RTCM corrections accepted on that port. Untick Configure the receiver to leave a receiver configured with u-center alone.

The antenna wants a clear view of the sky and a metal ground plane under it (a 10 cm disc or the robot's lid). Measure where it sits from the robot's turning centre and enter it as Antenna ahead / left: at centimetre accuracy, a 20 cm offset matters.

The window

The status line shows the receiver link, the fix with how many satellites are used and the receiver's own error estimate, how old the corrections are and how far away the base station is, and the heading. Fixed is green and float amber.

Under it: Start and Stop, and a menu to move the map origin to the robot, reset the fusion, configure the receiver again, or clear the trails.

Map

North is up, over satellite photos (or a street map, or none). The orange dots are the raw fixes and the line is the fused path; the circle around the robot shows how sure the position is, green when RTK fixed and amber when float. The red line is the dual antenna heading. Distance sensors in the NMS (a lidar) are drawn around the robot. With the simulator, a white outline shows where the simulated robot really is.

The map is a view of what the skill is doing; waypoints and routes live in the Outdoor Navigator. Drag to pan, wheel to zoom, Center to find the robot again.

Corrections

Where the corrections come from, how old the last one is, the base station's position (from its RTCM 1005 message) and the baseline from the robot to it, and a table of every RTCM message type arriving: 1005 is the base position, 1074/1084/1094/1124 the GPS, GLONASS, Galileo and BeiDou observations, 1230 the GLONASS biases. A stream missing a constellation the receiver uses still works, with fewer satellites.

With an NTRIP caster, Find mountpoints asks the caster for its list and shows the nearest first.

Satellites

Where each satellite is in the sky (north up, the outer ring is the horizon) and its signal strength. Green is strong (35 dB-Hz and over), yellow usable, orange weak. RTK needs strong signals from many satellites.

Messages

The receiver's firmware version, the NMEA it sends, and the checksum errors. Useful when something is not working.

Base station mode

Choose Base station as the mode to make a computer with a second RTK receiver into your own base station. The receiver's antenna goes somewhere fixed with an open sky.

The receiver first surveys in: it averages its own position for a few minutes until it is sure enough of it. It then sends corrections, and the skill serves them as an NTRIP caster on this computer (port 2101, mountpoint ARCX by default). On the robot, choose An NTRIP caster as the correction source with this computer's address, port and mountpoint.

The robot's positions are then exact relative to the base, but only as right on the earth as the survey, which is good to about a metre after a few minutes. That does not matter for driving the same lawn every day, as long as the base position is the same every day: once the survey is done, choose Keep the surveyed base position from the menu. The base then starts at once next time from the same position, and maps and routes drawn today still line up tomorrow. If you know the base's position exactly (a surveyed marker), enter it under Use a known position instead.

Configuration

Receiver

Setting	What it does
Mode	Rover (on the robot) or base station
Receiver connection	Serial port (USB or an adapter), EZ-B v4 UART, TCP (ser2net, an Emlid receiver's TCP output, an ESP32 bridge), gpsd on Linux (which cannot pass corrections back), or the simulated RTK receiver
Receiver	A u-blox ZED-F9P/F9H, read in UBX at a tenth of a millimetre and configured by the skill, or any other RTK receiver that sends GGA with RTK quality (Unicore UM980/UM982, Septentrio, Emlid, Quectel LC29H), configured with its own tool
Wired to the F9's	USB, UART1 or UART2: the receiver's port the computer is on
Configure the receiver	Set it up when it connects
Move it to baud	For a UART: the faster rate to move the receiver to. 115200 carries 10 fixes a second with corrections
Fixes per second	1 to 20
Motion model	Pedestrian suits walking pace robots, Automotive car-like ones
Save in the receiver	Keep the configuration through a power cycle
Use fixes of at least	Any 3D fix, RTK float, or RTK fixed only. With fixed only, the pose holds still whenever RTK drops back: safer for a robot that must not leave a strip of lawn, but it stops more often
Ignore fixes worse than	The receiver's own error estimate, metres. 0 uses every fix

Corrections

Setting	What it does
Corrections from	An NTRIP caster, a raw RTCM TCP stream, the receiver itself, the simulator, or none
Caster, mountpoint, user name, password	The NTRIP caster's details
Send the position every	Seconds between positions sent to the caster, for network casters. 0 never sends it
RTCM server	For a raw stream: its host and port
Lost after	Report the corrections lost, and run the script, when none arrive for this long

Base station

Setting	What it does
Use a known position	Skip the survey and use this latitude, longitude and height (above the ellipsoid, not sea level)
Survey for at least / until it is within	The survey carries on until both are met. Longer is more accurate
Serve the corrections	Run the NTRIP caster on this computer. Allow its port through the firewall
Caster port, mountpoint, password	What rovers connect with. An empty password lets anyone on the network connect

Heading

Setting	What it does
Heading from	Dual antennas, the direction of travel, or a variable
Variable	The global variable holding a heading in degrees clockwise from true north
Heading offset	Added to the heading. For dual antennas, the direction of the second antenna from the main one: 0 straight ahead, 90 to the right, -90 to the left, 180 behind
Heading trust	The least uncertainty the heading is given, degrees

Fusion

Setting	What it does
Pose	Report the fused pose, align to another skill's pose, or neither
Map frame	North up (X north, Y west) or ROS's ENU (X east, Y north)
Poses per second	How often the pose is reported to the NMS
Map origin	Saved in the project, so saved navigator maps line up next time, or the first fix of each session
Antenna ahead / left	Where the main antenna sits from the robot's turning centre, metres
GPS error scale	Multiplies the receiver's error estimate. Raise it if the pose jitters, lower it if it lags
Hold still when stopped	While the Movement Panel is stopped the robot is not moving
Speed at full panel	The robot's speed at Movement Panel speed 255, so the filter can predict between fixes. 0 does not
Align over	For aligning to another skill: seconds of driving the alignment fits over
Publish fixes to the NMS	Every fix goes to the NMS and the Robot Bus

Scripts

Scripts run when RTK fixed is reached, when it is lost, and when corrections stop arriving. A lawn mower might stop its blade when RTK fixed is lost and start it again when it comes back.

Simulator and display

The simulated receiver drives wherever the Movement Panel says, from the start position and heading set here. With simulated corrections it reaches float after a quarter of Simulated time to fix and fixed after all of it, and drops back when the corrections stop, like a real receiver. In base station mode it surveys in and sends corrections, so two ARCx computers can try a base and a rover together without any hardware. The display settings choose the map's background and how many fixes are drawn. Untick Show the map on a small computer such as a Raspberry Pi to save the processor time the map takes while a browser shows the window: the Map tab and the map widget go away, and the fix, the corrections, the satellites and the pose that navigators drive by carry on as before.

The Window tab sets the robot skill's window title, its size on the workspace and notes about it.

Global variables

With the default prefix Rtk.

Variable	What it holds
RtkLatitude, RtkLongitude	The robot's fused position, degrees
RtkHeading	The fused heading, degrees clockwise from true north
RtkPositionError	How sure the position is, metres
RtkPoseX, RtkPoseY, RtkPoseYaw	The same position in the NMS map, metres and radians, as in ROS
Rtk	Fixed, Float or None
RtkCorrectionAge	Seconds since the last correction, -1 when none have arrived
RtkAntennaHeading	The heading from the dual antennas (or the heading variable) on its own
RtkRawLatitude, RtkRawLongitude, RtkAltitude	Straight from the receiver
RtkFix, RtkFixType, RtkSatellites, RtkHdop, RtkAccuracy	The fix and how good it is
RtkSpeed, RtkCourse	Speed over the ground (m/s) and direction of travel (degrees)
RtkUtcTime	The satellites' time
RtkRunning	Whether the receiver is being read

Custom UI widgets

RTK status shows fixed, float or plain in a colour readable from across the yard, with the accuracy, the correction age and the heading. RTK map is the live map, following the robot.

When it does not work

What you see	Usually means
The receiver link stays red	The wrong port or baud rate, or TX and RX swapped on a UART
"message(s) refused or not acknowledged"	Not a ZED-F9, firmware older than 1.13, or Wired to the F9's names the wrong port
Satellites in view but no fix	Indoors or near a building. A first fix outdoors takes up to a minute
Corrections never arrive	The mountpoint name is wrong (it is case sensitive), the caster wants a user name, or the base is offline. Try another mountpoint from Find mountpoints
Corrections arrive but it stays at 3D fix	The receiver is not accepting RTCM on that port: tick Configure the receiver, or switch RTCM input on with u-center
It stays at RTK float	The base is too far (over 20 to 30 km), the stream lacks a constellation, or trees and buildings block too much sky
Fixed comes and goes	Satellites blocked under trees or near walls. Mount the antenna higher, over a ground plane
Positions shift between days	Each day's base survey differs. Keep the surveyed base position
No heading while standing still	Heading from the direction of travel only works while driving. Use dual antennas or a compass variable
"Another robot skill is also reporting the robot's pose"	Wheel encoders or a tracking camera report the pose too. Choose Align to another skill's pose

Control Commands for the Rtk Gps robot skill

There are Control Commands available for this robot skill which allows the skill to be controlled programmatically from scripts or other robot skills. These commands enable you to automate actions, respond to sensor inputs, and integrate the robot skill with other systems or custom interfaces. If you're new to the concept of Control Commands, we have a comprehensive manual available here that explains how to use them, provides examples to get you started and make the most of this powerful feature.

Control Command Manual

This robot skill has 18 control commands, listed below. Send them from a JavaScript or Python script, from Blockly, or from another robot skill.

The first value is always the window title, which is the name in the title bar of the robot skill on your workspace. It starts out as Rtk Gps, which is what every example below uses. If you rename the robot skill, or add a second copy of it to the project, use the title as it appears on that window instead.

ControlCommand() can be shortened to CC(), and both spellings work in JavaScript and in Python. The examples are JavaScript; Python is the same call, except that true and false are written True and False.

BearingTo

The compass bearing from the robot to a latitude and longitude, degrees clockwise from true north.

Syntax

ControlCommand("Rtk Gps", "BearingTo", (Number)latitude, (Number)longitude)

Parameters

#	Name	Type	What to pass
1	latitude	Number	A number, without quotes
2	longitude	Number	A number, without quotes

Returns: Double [5.0e324 to 1.7e308]

Example

var result = ControlCommand("Rtk Gps", "BearingTo", 0, 0);

ConfigureReceiver

Configure the u-blox receiver again now.

Syntax

ControlCommand("Rtk Gps", "ConfigureReceiver")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "ConfigureReceiver");

DistanceTo

Metres from the robot to a latitude and longitude.

Syntax

ControlCommand("Rtk Gps", "DistanceTo", (Number)latitude, (Number)longitude)

Parameters

#	Name	Type	What to pass
1	latitude	Number	A number, without quotes
2	longitude	Number	A number, without quotes

Returns: Double [5.0e324 to 1.7e308]

Example

var result = ControlCommand("Rtk Gps", "DistanceTo", 0, 0);

GetCorrectionAge

Seconds since the last correction message, or -1 when none have arrived.

Syntax

ControlCommand("Rtk Gps", "GetCorrectionAge")

Returns: Double [5.0e324 to 1.7e308]

Example

var result = ControlCommand("Rtk Gps", "GetCorrectionAge");

GetHeading

The robot's heading in degrees clockwise from true north, from the fusion (dual antenna, travel or the heading variable).

Syntax

ControlCommand("Rtk Gps", "GetHeading")

Returns: Double [5.0e324 to 1.7e308]

Example

var result = ControlCommand("Rtk Gps", "GetHeading");

GetMountpoints

Ask the configured NTRIP caster for its mountpoints. Returns their names, nearest first.

Syntax

ControlCommand("Rtk Gps", "GetMountpoints")

Returns: Array of String

Example

var result = ControlCommand("Rtk Gps", "GetMountpoints");

GetPose

The robot's pose in the NMS map, as in ROS: X and Y in metres, yaw in radians counter-clockwise.

Syntax

ControlCommand("Rtk Gps", "GetPose")

Returns: Array of Double

Example

var result = ControlCommand("Rtk Gps", "GetPose");

GetPosition

The robot's position as an array: latitude and longitude in degrees, and the error estimate in metres.

Syntax

ControlCommand("Rtk Gps", "GetPosition")

Returns: Array of Double

Example

var result = ControlCommand("Rtk Gps", "GetPosition");

GetRtkStatus

The fix as text: "RTK fixed", "RTK float", "DGPS", "3D fix", "2D fix" or "No fix".

Syntax

ControlCommand("Rtk Gps", "GetRtkStatus")

Returns: String

Example

var result = ControlCommand("Rtk Gps", "GetRtkStatus");

HasFix

Does the receiver have a position good enough to use (the configured minimum fix quality and accuracy)?

Syntax

ControlCommand("Rtk Gps", "HasFix")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "HasFix");

IsRtkFixed

Is the solution RTK fixed (centimetres)?

Syntax

ControlCommand("Rtk Gps", "IsRtkFixed")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "IsRtkFixed");

IsRunning

Is the receiver being read?

Syntax

ControlCommand("Rtk Gps", "IsRunning")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "IsRunning");

ResetFilter

Forget the fused estimate and start again from the next fix.

Syntax

ControlCommand("Rtk Gps", "ResetFilter")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "ResetFilter");

SetMountpoint

Use another mountpoint of the configured caster, and reconnect.

Syntax

ControlCommand("Rtk Gps", "SetMountpoint", (String)mountpoint)

Parameters

#	Name	Type	What to pass
1	mountpoint	Text	Your text, in quotes

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "SetMountpoint", "mountpoint");

SetOriginHere

Move the map origin (0, 0 in the NMS) to where the robot is.

Syntax

ControlCommand("Rtk Gps", "SetOriginHere")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "SetOriginHere");

Start

Start reading the receiver, sending it corrections and reporting the pose.

Syntax

ControlCommand("Rtk Gps", "Start")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "Start");

Stop

Stop reading the receiver and the corrections.

Syntax

ControlCommand("Rtk Gps", "Stop")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "Stop");

UseSurveyedPosition

Base mode: keep the finished survey-in position, so the base starts at once next time.

Syntax

ControlCommand("Rtk Gps", "UseSurveyedPosition")

Returns: Boolean [true or false]

Example

var result = ControlCommand("Rtk Gps", "UseSurveyedPosition");

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This list is read out of the robot skill itself each time its package is uploaded, so it matches the published version (v2).