01 Inertial Navigation System (INS)

Question 1

What is the principle of operation of an INS?

Answer 1

Uses integration to find position based on acceleration

Integration is multiplying by time

Question 2

With regards to an INS, what are the two integrations which can be performed?

Answer 2

1. Acceleration x Time = Speed
2. Speed x Time = Distance

I.e. It takes 2 integrations to go from an acceleration to a distance

Question 3

With regards to an INS, what is required to find:

• Instantaneous speed
• Position

Answer 3

• Instantaneous speed requires the initial speed to be known

- Position requires the initial position and the initial speed to be known

Question 4

Describe the two types of errors that an INS/IRS can suffer from?

Answer 4

Bounded Errors:

• Constant, known errors
• Can be compensated out
• E.g. Schuhler Period

Unbounded Errors:

• Vary with time and are unknown
• Can not be compensated for
• Caused by incorrect sensing (e.g. Turbulence)
• Displayed as a residual ground speed/drift parameter after flight

Question 5

How long is a Shuhler Period?

Answer 5

84.4 mins

Question 6

What is an Inertial Navigation System (INS) also known as?

Answer 6

Stabilised Platform

Question 7

What are the key components of an INS and what are their functions?

Answer 7

• 3 x Rate Gyros for stabilisation

- 2 x Accelerometers for sensing North/South and East/West travel

Question 8

What are the steps for alignment for an INS?

Answer 8

• Gyros spin up to speed
• The gyro oil is heated up to the required temp
• Accelerometers identify local vertical (gravity)
• Level platform
• Gyro drift is measured to sense latitude
• Align platform with true North

Question 9

What outputs does an INS have?

Answer 9

• Latitude
• Longitude
• Ground speed
• True heading
• True track
• W/V (If TAS is an input)
• Limited ATT info

Question 10

What does DSRTK / STS mean?

Answer 10

Desired Track / Status

Question 11

With regards to an INS, what can be retrieved if alignment is lost during the flight?

Answer 11

• Attitude information can be retrieved
• Only if the AC has remained straight and level
• Navigation data can NOT be retrieved

Question 12

What is the average position error for an INS?

Answer 12

1.5 nm per hour of flight time

Question 13

What does XTK / TKE mean?

Answer 13

• Cross Track Position: Distance in nm left or right of the track
• Tracking Error: Angular difference between current track and desired track

Question 14

If the control switch for an INS is set to ATT what data can be displayed?

Answer 14

• Attitude

- Heading

Question 15

What is the ‘normal’ alignment time (i.e. In mid altitudes) for an INS?

Answer 15

3 to 10 mins