RINEX 3.02

Aiding Data Examples and Use Cases for the Inertial Labs INS

Purpose: Learn how to use ading data to improve the INS operation in GNSS-denied conditions
Last Updated: January 2024

The purpose of this article is to consider the examples of using aiding data and possible use cases when operating an Inertial Labs INS.

The article information below is related to the following systems:

  • INS-B/BU
  • INS-P
  • INS-D/DU/DL
  • INS-DM
  • INS-U

The generalized name “INS” is used hereinafter for convenience and clarity.

The Inertial Labs GNSS-aided INS provides accurate and reliable position, velocity, and orientation. During a GNSS outage, the INS accuracy quickly degrades over time due to sensor errors. However, it is still possible to keep INS accuracy within acceptable limits by using aiding data from other external sensors. Regardless of the aiding data type, most data formats must be converted into the acceptable message type to be filtered by the INS Kalman Filter. This can be done by using a micro-computer to collect all aiding data being used and then outputting a packet of data to the INS. The structure of the external aiding data packet corresponds to Table 6.2 of the INS ICD with message identifier 0x62 and payload shown in Figure 1.

aiding data payload
Fig.1 Aiding data payload structure

The INS can be configured to accept aiding data from many sources. The following list shows the accepted types of aiding data:

  • Odometer (CANbus, OBD-II, Encoder, etc.) (not available for INS-U).
  • Air Speed
  • Wind Data
  • External position
  • Heading External Sensor (Gyro-compass, etc.)
  • DVL Data (or External Velocity Data)
  • GNSS antennas position
  • Ambient Air Data
  • Altitude external
  • External horizontal position
  • ADC static and dynamic pressure (not available for INS-U).

The following table shows the accepted aiding data types, their structure, and a description.

Aididng data table
Fig.2 Aiding data measurements structure

Odometer

Let’s consider an example: the INS is installed on the UAV with a wheel encoder. A wheel encoder is used to count the number of rotation times. This can be used to calculate the distance that the UAV has driven or turned. Accumulated distance in millimeters should be sent to the INS as aiding data.  Odometer aiding data type is used to correct the distance in the GNSS denied enviorement. The odometer aiding data type is used to correct the distance in the GNSS-denied environment and significantly reduce the error in the position determination. If the GNSS solution is available, the odometer aiding data is used for calibration of the scale factor and misalignment angles of the odometer and the INS axes.

Air Speed

The INS can be installed on different aerial application systems, but some devices don’t have embedded differential pressure sensors. If the customers` application has a sensor that can calculate air speed, it could be used as aiding data. This type of aiding data provides the INS velocity correction in the GNSS denied enviorement. It also decreases the error of the position determination. If the GNSS solution is available, the air speed aiding data is used for calibration of the scale factor and estimation of horizontal components of the wind speed.

An example of the aiding data packet with air speed:

AA 55 01 62 0A 00 01 02 B9 0B 31 04

Example Air speed
Fig.3 Air speed example

Wind data 

The area of application of this aiding data type is similar to the Air Speed aiding data. The data from wind sensor should be converted to the accaptable massage type and send to the INS algorithm. Wind data allows the INS algorithm to consider the wind speed in the calculation in cases of GNSS denied and using air speed. This aiding data type can accelerate and correct the estimation of the wind data in the INS Kalman filter, but it works when a GNSS solution is available.

An example of the aiding data packet with wind data:

AA 55 01 62 10 00 01  03  E9 03 0B FE 34 00 2A 00 CA 02

EXAMPLE WIND DATA
Fig.4 Wind data example

External position

The INS has an onboard GNSS receiver, which provides a highly accurate position using different GNSS systems such as GPS, GLONASS, Galileo, etc. But sometimes, the GNSS solutions can be absent due to jamming or spoofing on the receiver. If the customers` application has an external GNSS receiver with higher spoofing or jamming protections, the position data can be sent to the INS algorithm as aiding data. This aiding data type is used for the correction of all INS coordinates: latitude, longitude, and altitude. At the same time, it prevents the acumulation of INS errors in coordinates and velocity and estimates the wind data.

An example of the external position aiding data packet:

AA 55 01 62 1C 00 01 04 21 25 CF 17 B5 5E 04 D2 E2 51 02 00 7D 00 32 02 1E 04 2C 01 CE 05

External position example
Fig. External position example

Heading external

Using heading external data provides the heading correction to the INS. Applicable in the absence or invalidity of other internal heading sources. For example, the customer can use the gyrocompass as a source of external heading-aiding data. The gyrocompass is less sensitive to external magnetic fields and provides a more accurate heading. The decrease in heading errors reduces accumulation errors of the INS position and velocity in the absence of GNSS.

 DVL Data

The DVL data usually refers to marine application systems because it estimates velocity relative to the sea bottom. This aiding data type should be used in the MRU. Ground systems use external velocity data from different speed sensors. The ground speed should be measured in the object axes and send to the INS as aidind data. This type of aiding data provides the INS velocity correction in the GNSS denied enviorement. It also decreases the error of the position determination. If the GNSS solution is available, the external velocity aiding data is used for calibration of the scale factor and misalignment angles of the object and the INS axes

An example of the  External Velocity Data aiding data packet:

AA 55 01 62 20 00 01 07 CB 0D 00 00 F1 16 00 00 4F 05 00 00 F6 00 60 01 62 00 FA 00 00 00 00 00 71 05

DVL example
Fig.6 External Velocity Data example

GNSS antennas position

The INS uses a heading calculated by a single-antenna or a dual-antenna GNSS receiver for the correction. The GNSS antenna can be installed with virtually any orientation in the horizontal plane of the object. Still, the position of the antennas should be measured as accurately as possible relative to the accelerometer mass-center of the INS and stored in the INS memory. The INS can be installed on the gimbal in practice. The accelerometer mass-center of the device changes its location relative to the antenna since the gimbal moves and rotates. The antenna’s position change should be calculated using an onboard computer and sent to the INS as aiding data.

Ambient Air Data

Using Ambient Air Data can significantly increase the accuracy of barometric altitude calculation. If the true value of ambient temperature is set in this aiding data package, then temperature correction of the scale factor is applied for barometric altitude dependence from measured pressure. If true altitude and ambient pressure are set in the “Ambient Air Data” package, the pressure above mean sea level is re-calculated to increase the accuracy of the barometric altitude calculation. If only ambient temperature is known, set zero values for altitude and pressure in the “Ambient Air Data” fields; in such cases, the device will use these values calculated onboard.

Altitude external

This aiding data type is used for altitude correction in the GNSS-denied environment. It prevents the accumulation of INS errors in determining altitude and vertical speed. An altimeter can be used as a source of the Altitude external aiding data. We can consider aircraft terrain avoidance warning systems as an example. This system has a radar altimeter which measures altitude from the ground surface. The data from the radar altimeter can be used in the INS algorithm as aiding data.

External horizontal position

The INS can be installed on the UAV with the camera, which is a part of the visual navigation system. The visual navigation system processes and analyzes the images captured by a camera and can determine the horizontal position. External horizontal position can be used as aiding data in the INS algorithm.  This aiding data type corrects two INS coordinates: latitude and longitude. At the same time, it prevents the accumulation of INS errors in coordinates and velocity.

ADC static and dynamic pressure

Inertial Labs developed and announced an external Air Data Computer (ADC). The ADC can be integrate with the INS unit  to calculate various air data parameters. The external ADC has specific pressure sensors for measuring static and dynamic pressure. These measurements should be provided to the INS through the COM4 port as ‘ADC static and dynamic pressure’  aiding data. Static pressure is used for barometric altitude calculation. Dynamic pressure is used for air speed calculation in the same way as air speed aiding data. Please note, this aiding data type is not available for the INS-U.

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