What’s inertial? What’s an inertial navigation system? What does INS stand for? These are good questions to ask as they are relevant to everyday life. For example, when trying to navigate to work by car. In this blog we’ll help you answer them but we’ll start, sensibly enough, at the beginning…
What’s an INS?
Inertial navigation systems come in all shapes and sizes. One thing they have in common though is their use of multiple inertial sensors, and some form of central processing unit to keep track of the measurements coming from those sensors.
An INS uses a range of technologies but typically include:
+ Inertial Measurement Unit (IMU)
– Accelerometers
– Gyroscopes
+ GNSS Receiver
+ Central Processing Unit (CPU)
We’ll look at how an INS actually works in a moment, but for now, the most important thing to realise is how they differ from GPS—which you’re probably more familiar with.
Switch on a GPS receiver and, assuming everything works correctly, after a short time it will generate a position measurement. Ignoring the inaccuracies GPS has, the position measurement the receiver generates is quite specific. It says ‘you are at this latitude and this longitude‘—in other words, it gives us an absolute position using a known co-ordinate system. Inertial navigation systems don’t work like that. In their case, the measurement they generate is relative to their last known position. So even after an inertial navigation system has been turned on for several minutes, it can’t say ‘you are at this latitude and this longitude‘, but what it can say is, ‘you haven’t moved from where you started‘.
So why do people use inertial navigation systems at all? If they can’t tell you where you are, how were they able to navigate man to the moon, why don’t submarines crash all the time and how do aeroplanes and missiles find their way? Thankfully the answer to these question is simple. An inertial navigation system works out where it is in relation to where it started—so if you tell the INS where it started, it can easily work out where it is now, based on its own measurements. That is how spaceships, submarines, aircraft and missiles all successfully navigate using an INS—because they know where they started from.
Now we’ve discussed what an inertial navigation system is, the next logical question is ‘How does an INS actually work?’
To fully understand inertial navigation systems, its useful to also know about INS frames of reference, so that you can accurately interpret the values registered on your x-, y- and z-axis. You’ll also want to know more about the sensor types used in most inertial navigation systems – accelerometers and gyroscopes. To understand how an inertial navigation system keeps track of where it is in three-dimensional space, you also need to know about dead reckoning. And then to appreciate the strengths and weaknesses of INS, you also need to know about drift.
Need an INS for Automotive Testing? – Read more
Need an INS for Mapping? – Read more
Need an INS for an Autonomy application? – Read more
Inertial Navigation Technologies, Sensors and more
As mentioned already, an Inertial Navigation System is made up of several different technologies and sensors. The Inertial Measurement Unit (IMU) contains a number of accelerometers and gyroscopes that provide a wide range of inertial measurements such as pitch/roll and acceleration. A GNSS receiver is also included which provides position updates and a central processing unit that manages the data. In this section we’ll take a closer look at the technologies and sensors that make up an INS.
GNSS Receivers
A GNSS-aided Inertial Navigation System uses position updates from a GNSS receiver to supplement the measurements created by the onboard Inertial Measurement Unit. A GNSS receiver tracks satellite signals from multiple satellite constellations. The four main satellite constellations are Galileo, GLONASS, GPS and BeiDou.
By blending the satellite inputs with data from the IMU, an OxTS GNSS/INS is able to provide real-time centimetre-level position accuracy at high update rates and with orientation and acceleration in all three axes.
Learn more about GNSS receivers here
INS Frames of Reference
Before a data collection project, it’s important for an Inertial Navigation System to understand how it is orientated in 3D space or its ‘frame of reference’. The onboard IMU will provide information pertaining to the INS’ motion and orientation, however before it can do this accurately it first needs to know which way is up, down, left and right.
An OxTS Inertial Navigation System is able to convert movement from one frame of reference to another, as long as it knows which frame of reference it is in to begin with. This is generally done at the configuration stage.
Learn more about INS Frames of Reference here
Accelerometers
Accelerometers are one of the sensor types used in all OxTS Inertial Navigation Systems. As the name suggests, they only measure the rate of acceleration. However, other measurements, such as velocity, can be calculated by multiplying the acceleration by other factors, like time.
An accelerometer is useful in an inertial navigation system because it can help give the user important information about a vehicle’s speed, distance travelled and importantly in an automotive testing setting, time until impact.
There are a range of different types of accelerometers but OxTS Inertial Navigation Systems use micro-electromechanical system (MEMS) accelerometers.
Learn more about accelerometers here
Gyroscopes
A gyroscope, or gyro, is a sensor used in an inertial navigation system that measures the the rotation of the device.
OxTS Inertial Navigation Systems use micro-electromechanical system (MEMS) gyroscopes. There are many different types of MEMS gyroscope available, however OxTS Inertial Navigation Systems use those that measure angular velocity in °/s.
In a similar way to how an accelerometer works, a gyroscope doesn’t immeditely tell the INS which way it is orientated, but as long as it has this information begin with it can work out its orientation from then on.
Learn more about gyroscopes here
Dead Reckoning
The inertial measurement unit within an OxTS Inertial Navigation System contains three accelerometers and three gyroscopes. Using three of each allows the device to understand and keep track of its position in 3D space using a process known as dead reckoning.
The inertial navigation system will calculate its position in 3D space by simply taking the measurements from whichever onboard gyroscopes and accelerometers are subjected to a force and adding them to its last known position. From that, it will understand where it is now.
Learn more about how dead reckoning works here
Drift
Like most things, an inertial navigation system has its strengths and weaknesses. An inertial navigation system works by using position updates from a GNSS receiver to aid the measurements from the IMU. When an additional aiding source isn’t available the INS is ‘un-aided’ and uses only the IMU measurements. When ‘un-aided’ position drift occurs due to the accumulation of small errors in the gyroscope and accelerometer measurements.
In the absense of GNSS – other aiding sources can be integrated into the navigation engine to constrain position drift. You can learn more about the work OxTS is doing to take advantage of additional sources of aiding here – OxTS Generic Aiding Data (GAD) Interface
How do Inertial Navigation Systems work?
So we’ve discussed what an INS is, but how does it work? An inertial navigation system is a complex device. It comprises multiple technologies to enable its user to accurately understand its position in 3D space. Central to the INS is the IMU. A collection of accelerometers and gyroscopes that provide the user with accurate measurements about the device’s motion and orientation.
Through integration with a GNSS receiver, the system is able to ensure the systems doesn’t drift. To further constrain position drift in the absense of GNSS, other technology and sensors can be used.
In the most simplest terms, an INS works by understanding the force which acts upon it. It will then provide a new relative position based on a known starting point, by calculating the speed and distance travelled.
The INS is then capable of providing updates on attitude, position and velocity.
Attitude
Attitude measurements are improtant for positioning in many applications, including Automotive Testing and Mapping. Attitude provides information about an objects position in a 3D environment, namely roll, pitch and yaw. The measurements are calculated with respect to a horizontal plane and due north reference frame.
Velocity and Position
As mentioned earlier in this post an INS doesnt directly measure velocity as the accelerometers will only measure acceleration. That isn’t to say however, that an INS cannot measure velocity. By keeping track of how much acceleration there is on an object, and how long it lasts, the INS can easily work out what the velocity is by multiplying the acceleration by time.
Through a process known as dead reckoning an un-aided INS can still accurately calculate its position. By understanding its last known position and having a knowledge of where it’s heading and how long it takes to get there the INS can calculate its new estimated position. Its important to take into account however, that an un-aided INS will drift over time.
OxTS Inertial Navigation Systems
OxTS manufacture inertial navigation systems each with varying levels of accuracy and form factor that make them ideal for a wide range of applications.
Each device comes with our free NAVsuite software that allows you to configure, monitor, post-process and analyse your INS data. The devices can be supplemented with a number of optional software tools and features that enable you to build a device tailored to your specific requirements.
