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Avionics And Instruments > Chapter 7 - Navigation Systems and Aids 3 > Flashcards

Chapter 7 - Navigation Systems and Aids 3 Flashcards

1
Q

What does INS stand for?

A

Inertial Navigation System

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2
Q

What is an INS?

A

A SELF-CONTAINED navigation system using inputs from accelerometers and gyroscopes to determine and track position, orientation and velocity.

Given a known start point, an INS can measure angular and linear changes to calculate position and velocity.

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3
Q

What does a typical INS comprise of?

A

Frame mounted gimbals driven by motors.

Accelerometers and gyroscopes (motion sensors) mounted on a gyro-stabilised platform or module.

A computer

Aircrew interface

Cockpit displays (MFD moving map, HUD, EHSI, etc)

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4
Q

How many accelerometers does an INS typically have?

A

3

One measures North-South

One measures East-West

One measures in the vertical plane

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5
Q

How many gyros does a typical INS have?

A

3 RATE gyros

One gyro detects rotation about the North (longitudinal) axis (roll).

Another detects rotation about the East (lateral) axis (pitch).

The third detects rotation about the vertical.

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6
Q

How does an INS platform prevent displacement?

A

Signals from the gyros are fed to their associated gimbal motor to correct any changes in orientation.

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7
Q

Signals from the accelerometers are sent via an amplifier to two integrators. These integrators are time multiplication devices.
What values do these integrators provide?

A

First multiplies the acceleration signal by time to get velocity.

Second multiplies the velocity by time to get distance.

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8
Q

What must be done to an INS before use to ensure it is accurate?

A

Must be aligned, i.e. told where it is and what direction it is facing.

Aircraft must remain stationary during alignment.

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9
Q

What are two methods by which the INS can be provided with a starting point?

A

1 - Manual input of lat and long coordinates

2 - Drawing data from another system, e.g. GPS

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10
Q

What are the stages of INS alignment?

A

Warm up

Coarse Levelling

Fine Levelling

Gyro Compassing

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11
Q

Explain the ‘Warm Up’ stage of INS alignment?

A

INS systems are designed to operate at an optimum temperature.

The optimum temperature for the gyro and accelerometer fluid is approx. 70 degrees celsius.

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12
Q

Explain the ‘Gyro Compassing’ stage of INS alignment?

A

East-West gyro detects Earth’s rotation.

Azimuth gimbal is torqued until the output from the East-West gyro reduces to zero.

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13
Q

What INS systems do most modern military aircraft use?

A

Strapdown INS

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14
Q

What is Strapdown INS?

A

Strapdown INS is fixed to the aircraft, i.e. there are no gimbals.

Uses modern computing/electronics, optics and solid state tech instead, e.g. Ring Laser Gyroscopes (RLGs).

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15
Q

What are the advantages of strapdown INS?

A

Cheaper

More reliable

More rugged

(than than gimballed systems)

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16
Q

When is Schuler Tuning error at a minimum?

A

84.4 mins

and the initial 0 mins

17
Q

When is Schuler Tuning error at a maximum?

A

21.1 mins and 63.3 mins

18
Q

What errors can affect INS systems?

A

Gimbal Lock

Strapdown INS electronics require careful alignment and cross checking with other navigation systems

NOTE - INS should be regularly cross checked with the visual navigation picture and other navigation instruments/systems

19
Q

What is NAVSTAR?

A

The US space-based radio positioning system which provides GPS service.

20
Q

What information does the GPS constellation of satellites provide?

A

PVT - Position, Velocity, Time

21
Q

How high is the orbit of the GPS satellites, what is their speed and how often do they orbit the Earth?

A

12500 miles above Earth’s surface

7000 mph

Orbit Earth twice per day
(approx. 12 hour orbital period)

22
Q

How many satellites are in the GPS constellation and how many are in use at any one time?

A

27 airborne satellites

24 in use at any one time

23
Q

How many orbital planes are the GPS satellites in and how many satellites are in each plane?

A

6 orbital planes

4 to 5 satellites in each plane

24
Q

How many GPS satellites are normally observable anywhere on Earth?

A

5

25
Q

What makes up the GPS Control segment?

A

Master Control Station (MCS) - Hub is in Colorado Springs

Monitoring Stations (MSs)

Ground Antenna (GA)

26
Q

What is the role of the GPS Control segment?

A

Monitors and maintains the GPS system via the Master Control Station (MCS), Monitoring Stations (MSs) and Ground Antenna (GA).

27
Q

How many MSs are there in the GPS Control segment and what is their role?

A

5 primary MSs

Passively track all GPS satellites in view.
Ranging data is passed to the MCS where each satellite ephemeris (position info) and clock parameters are estimated and predicted.

28
Q

Some MSs are paired with a Ground Antenna, what is the role of a GA?

A

To communicate error corrections to the satellites.

29
Q

What are the 3 GPS segments?

A

Control Segment

User Segment - Mil or civi with GPS receivers which can decode and process the satellite signals

Space Segment

30
Q

What are the two frequencies GPS satellites transmit on?

A

Link 1 (L1) - 1575.42MHz

Link 2 (L2) - 1227.6MHz

31
Q

What are the two GPS services provided?

A

C/A - Coarse Acquisition - Transmitted on L1

P - Precision Code - Transmitted on L1 and L2

32
Q

What are the two GPS services available?

A

Standard Positioning Service (SPS)

Precise Positioning Service (PPS)

33
Q

What is the accuracy of the SPS?

A

+/- 8m

34
Q

What is the accuracy of the PPS?

A

+/- 4m

Max accuracy obtained from the P(Y) code on L1 and L2

35
Q

What is the purpose of the Selective Availability (SA) feature on the PPS?

A

Can be used to reduce the accuracy to unauthorised users by introducing controlled errors into the signals

36
Q

What is ‘first fix’ with a GPS receiver?

A

The receiver determines which satellites are visible/available and selects one to track.

It can then work on receiving data from the 3 other satellites required to provide full info.

37
Q

What are the 4 types of GPS receiver?

A

Continuous - Must have at least 4 hardware channels in order to track 4 satellites simultaneously

Sequential - Tracks the necessary satellites by using one or two hardware channels at any given time. Takes one second to receive position of best places satellites.

Multiplex - Reads from all available satellites (of the 4 being tracked) every 20 milliseconds

All-in View - Designed to use all available satellites

Continuous, Sequential, Multiplex, All-in View

38
Q

What GPS errors can occur?

A

Insufficient satellite availability

Ionospheric effect

External reference and masking - LoS (e.g. in a valley)

Multipath effects - Caused by signal reflection, i.e. you receive the reflected signal, not the one directly from the satellite

39
Q

How can some of the potential GPS errors be minimised?

A

Use “Differencing”, aka Differential GPS (DGPS)

Details in course manual if required.

Avionics And Instruments (9 decks)

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