Here’s a brief review of each product and what it does

IMU = Inertial Measurement Unit (senses motion in 6 degrees of freedom, has 3 gyroscopes to sense rotation, and 3 accelerometers to sense linear motion).

VG = Vertical Gyro = similar to the IMU, but has additional software loaded to help as an upfinding gyro system.  The vertical gyroscope system is between the IMU and the AHRS in the avionics lineup.

There are several core variants of vertical gyros.

The early vertical gyros were mechanical devices used as up finding indicators. The mechanical vertical gyro is a 2-degree of freedom gyroscope inside a gimbaled system.

The vertical gyro was used in conjunction with the directional gyro on the aircraft, 2 core parts of the navigation 6-pack.

Today, the vertical gyro is more likely a MEMS (Micro-Electro Mechanical) or FOG (Fiber Optic Gyro) based sensing system, and used for its up finding capability, not so much part of the navigation 6-pack on an aircraft.

AHRS = Attitude Heading and Reference System.  The AHRS takes the Vertical Gyro to the next level, adding a magnetometer (digital compass) input to help get heading and attitude direction information.

ADAHRS= Air Data system added onto the AHRS unit (basically has a pitot tube input system to find the airplane’s air speed and combines this with the AHRS data).

INS/GPS= Inertial Navigation System with GPS.  Like a IMU plus GPS, plus has a Kalman Filter, a piece of software that is basically like a weighted average that allows the unit to make tradeoff between using inertial data and GPS data, and what proportion of each should be relied on in any given situation.

the INS/GPS outputs very useful data, like position, velocity.  The integration of INS/GPS is often easier than IMU integration, due to the more user friendly data.

GPS/INS = like a INS/GPS, but more focused on the GPS, with weaker inertials.

Sensors level avionics

Gyro = Gyroscope

Accel = Accelerometer

GPS = Global Positioning System

Magnetometer =Digital Compass

Pitot Tube= input for air pressure

Doppler= undersea navigation based on sound.

Acoustic Sensor = like doppler but sound based