GNSS-INS

An overview of GNSS + Inertial navigation and its applications in UAV-based remote sensing

What is GNSS?

Global Navigation Satellite System (GNSS) refers to a system of spaceborne satellites that provide signals transmitting geolocation and timing data to GNSS receivers. The receivers then use this data to determine their real-time 3D location and date/time at a relatively low frequency (1-10Hz) and meter-scale accuracy. Raw GNSS observations from a roving GNSS receiver can be stored and post-processed with updated satellite geolocation (ephemeris), atmospheric corrections, and fixed base station reference data to produce a more accurate trajectory estimate than possible in real-time.

GNSS systems provide global coverage by definition, but component satellite constellations are operated by different entities. The most well known GNSS, Global Positioning System (GPS), is operated by the USA. Other constellations are listed below.

GNSS Constellations and Operating Countries

GNSS Constellation	Operating Entity
Global Positioning System (GPS)	US Government
Global Navigation Satellite System (GLONASS)	Russian Government
Galileo	EU/GSA
BeiDou	Chinese Government

What is an INS?

An Inertial Navigation System (INS) integrates measurements from an Inertial Measurement Unit (IMU) over time to estimate position, velocity, and orientation when given an initial starting condition. Because IMU data is available at a relatively high frequency, Inertial Navigation Systems estimate position and orientation at a much higher rate than GNSS, around 200Hz.

The main components of an IMU- gyroscopes and accelerometers, determine the change in orientation and velocity of an object. Some IMUs also employ a magnetometer to measure orientation relative to the Earth's magnetic field.

GNSS-INS

In order to achieve the best position and orientation estimate, a combined GNSS-Inertial Navigation System (GNSS-INS) is used. The fusion of GNSS and IMU data is essential to create an advanced GNSS + Inertial Navigation System that offers high precision, accuracy, and reliability. This fused system leverages the strengths of both GNSS and IMU technologies to overcome their respective weaknesses, providing more accurate position and orientation in real-time, and survey-grade accuracy when post-processed.

Benefits of GNSS-Inertial Navigation

• Continuous Positioning: INS can maintain positioning when GNSS signal is momentarily lost.

• High Update Rate: The high-frequency data from the IMU complements the slower GNSS update rate, ensuring smooth and continuous motion tracking.

• Reduced Drift: GNSS helps to correct the long-term drift that is inherent in the IMU data.

• Better Accuracy: Combining both GNSS and IMU data can provide centimeter-level positioning when post-processed.

GNSS-INS for Direct Georeferencing

The GNSS-INS is the key component allowing orthorectification and reconstruction of image and LiDAR data on GRYFN systems. In a process called Direct Georeferencing (DG), sensor exterior orientation parameters (EOPs) are measured directly by the GNSS-INS and are used to project data from an initial sensor reference frame into a common "real world" mapping frame. These EOPs describe a sensor's position ($X, Y, Z$) and orientation ($\omega, \phi, \kappa$) with respect to Earth.

Line-scan hyperspectral and LiDAR sensors rely purely on DG to process raw data and produce orthorectified mosaics and point clouds. Frame camera data processed in GRYFN's software, however, follows a more traditional photogrammetric approach, but is augmented with GNSS-INS position and orientation to remove the need for Ground Control Points (GCPs) and improve weak image feature matching often experienced when processing data over a homogenous scene, such as large agricultural fields.

Orthorectification and Boresight

Because the GNSS-INS is not co-located and perfectly aligned with the reference coordinate system of each sensor, translation and rotation offsets must be applied to correctly position pixel and point data on Earth's surface or in 3D space. The process of determining the geometric relationship between the GNSS-INS and each sensor is called Boresight Calibration and is discussed in more detail in the Boresight Calibrationsection of this wiki.

FAQ

How accurate are the GNSS-INS solutions in GRYFN systems?

GNSS-INS solution accuracy depends heavily on positioning mode and other factors. Typical performance of real-time and post-processed solutions is given below for a UAV in flight conditions with good GNSS reception.

Typical GNSS-INS Performance

	Real-Time (C/A)	Post-Processed
Position (m)	1.2 - 3.0	0.01 - 0.05
Roll & Pitch (deg)	0.03 - 0.04	0.015 - 0.025
Heading (deg)	0.1 - 0.3	0.035 - 0.080

Why don't GRYFN systems use the GNSS receiver or IMU on my UAV?

GRYFN chooses to use independent GNSS-INS hardware to ensure the highest quality data is available for post-processing. By using a survey-grade GNSS-INS and post-processing software, much higher accuracy and repeatability can be achieved, resulting in the best possible data products and alignment.

What can cause issues with the GNSS-INS?

Here are some things to do when using a GNSS-INS:

• Ensure the GNSS antenna has a clear view of the sky

• Avoid long periods of static data

• Ensure the GNSS-INS is aligned before starting data collection

• Avoid Electromagnetic Interference (EMI) or unnatural magnetic fields that may interfere with the GPS or IMU