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RTK Precision Positioning

Overview

Real Time Kinematic (RTK) is a precision satellite positioning technique which utilizes a base station to transmit position corrections to a receiver. The Inertial Sense RTK solution provides centimeter level position accuracy.

To use RTK, a base station, arover (receiver), and a method to send corrections from the base to the rover are required.

See the multi-band GNSS section for details on using our multi-frequency ZED-F9 GNSS system.

RTK Hardware Setup

Base Station Options

Any of the following devices can be used as a RTK base station. All Inertial Sense base station options require a GPS antenna.

  • Inertial Sense EVB 2 - Sends corrections using the onboard 915 MHz radio, the onboard WiFi module, either serial port, or USB.
  • Inertial Sense µINS module, EVB 1 or Rugged - Sends corrections on either serial port or USB that can then be forwarded to a rover using a communication method of choice.
  • 3rd Party Base Station - e.g. Emlid Reach Receiver.
  • Public NTRIP Caster - e.g. CORS Network.

Rover Options

The following configurations can be used for the RTK rover:

  • Inertial Sense EVB 2 - Can receive corrections via the onboard 915 MHz radio, onboard WiFi module, serial ports, or USB.
  • Inertial Sense µINS module, EVB 1 or Rugged - Can receive corrections via either serial port or USB.

Base to Rover Communication

  1. Direct Serial - Using USB, RS232, RS422/485, or TTL to pass corrections from Base to Rover.
  2. Radio Link - Inertial Sense EVB 2 uses the Digi Xbee Pro SX module to send RTK corrections. Other communication methods such as Bluetooth may also work for the chosen application.
  3. NTRIP - Transmits RTK correction data over the Internet. To receive messages with NTRIP, the user must supply a URL, port number, and mount point . Often a username and password are also required.
  4. TCP/IP - A protocol for communicating directly between computers. In order to receive messages using TCP/IP, an address (IP Address or DNS) must be suppled to the Base where the corrections will be transmitted.

How to Know RTK is Working

Using the EvalTool

  1. Connect the µINS Rover to a computer with the EvalTool running. Open the comport for the unit in the Settings > Serial Ports.
  2. Navigate to Settings > RTK.
  3. Under the Status section, RTK functionality can be verified in 3 ways:
    • Status field will show Single. Over the course of several minutes this status will change to Float then Fix.
    • The Differential Age will show a timestamp that increments and resets back to zero about every second. This shows that the Rover is receiving Base messages.
    • The Accuracy: H, V will show a large number at first. This number will decrease over time as the system acquires RTK Fix. Once in Fix, this number will average at +- 0.08, 0.14 m.

EvalTool RTK

Using the CLTool

  1. Connect the µINS Rover to a computer with the CLTool running.
  2. Include the argument -msgPresetPPD in the CLTool command.
  3. Observe the DID_GPS_RTK_NAV message, Status: 0x******** (Single) over the course of several minutes this will change to (Float) then (Fix).

RTK Fix Status

LED Indicators

The LEDs on the IMX will indicate RTK fix status.

LED Behavior Status Description
RTK_Float 3D Fix, RTK Float Allows improved accuracy up to ~1m
RTK_Fix RTK Fix Allows increased accuracy up to ~3cm

RTK Positioning Valid Flags

The RTK precision positioning fix status can be identified using the valid bit in the INS and GPS status flags.

// INS status
INS_STATUS_NAV_FIX_STATUS(DID_INS_1.insStatus) == GPS_NAV_FIX_POSITIONING_RTK_FIX

// GPS status
DID_GPS1_POS.status & GPS_STATUS_FLAGS_GPS1_RTK_POSITION_VALID

RTK precision positioning fix is indicated is indicated when the RTK-Pos radio button turns purple in the EvalTool INS tab.

RTK fix in EvalTool

Progress and Accuracy

The ambiguity resolution ratio, arRatio, is a metric that indicates progress of the solution that ranges from 0 to 999. Typically values above 3 indicate RTK fix progress. 

DID_GPS1_RTK_POS_REL.arRatio                        // Ambiguity resolution ratio

The DID_GPS1_RTK_POS_REL status can be monitored in the EvalTool GPS tab.

RTK status in EvalTool

RTK Base Messages

The IMX RTK solution accepts both RTCM3 and uBlox raw GNSS base correction messages. See the RTK Base or NTRIP pages for details on using base stations.