Chapter 7 - Navigation Systems and Aids 3 Flashcards
What does INS stand for?
Inertial Navigation System
What is an INS?
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.
What does a typical INS comprise of?
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)
How many accelerometers does an INS typically have?
3
One measures North-South
One measures East-West
One measures in the vertical plane
How many gyros does a typical INS have?
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.
How does an INS platform prevent displacement?
Signals from the gyros are fed to their associated gimbal motor to correct any changes in orientation.
Signals from the accelerometers are sent via an amplifier to two integrators. These integrators are time multiplication devices.
What values do these integrators provide?
First multiplies the acceleration signal by time to get velocity.
Second multiplies the velocity by time to get distance.
What must be done to an INS before use to ensure it is accurate?
Must be aligned, i.e. told where it is and what direction it is facing.
Aircraft must remain stationary during alignment.
What are two methods by which the INS can be provided with a starting point?
1 - Manual input of lat and long coordinates
2 - Drawing data from another system, e.g. GPS
What are the stages of INS alignment?
Warm up
Coarse Levelling
Fine Levelling
Gyro Compassing
Explain the ‘Warm Up’ stage of INS alignment?
INS systems are designed to operate at an optimum temperature.
The optimum temperature for the gyro and accelerometer fluid is approx. 70 degrees celsius.
Explain the ‘Gyro Compassing’ stage of INS alignment?
East-West gyro detects Earth’s rotation.
Azimuth gimbal is torqued until the output from the East-West gyro reduces to zero.
What INS systems do most modern military aircraft use?
Strapdown INS
What is Strapdown INS?
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).
What are the advantages of strapdown INS?
Cheaper
More reliable
More rugged
(than than gimballed systems)
When is Schuler Tuning error at a minimum?
84.4 mins
and the initial 0 mins
When is Schuler Tuning error at a maximum?
21.1 mins and 63.3 mins
What errors can affect INS systems?
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
What is NAVSTAR?
The US space-based radio positioning system which provides GPS service.
What information does the GPS constellation of satellites provide?
PVT - Position, Velocity, Time
How high is the orbit of the GPS satellites, what is their speed and how often do they orbit the Earth?
12500 miles above Earth’s surface
7000 mph
Orbit Earth twice per day
(approx. 12 hour orbital period)
How many satellites are in the GPS constellation and how many are in use at any one time?
27 airborne satellites
24 in use at any one time
How many orbital planes are the GPS satellites in and how many satellites are in each plane?
6 orbital planes
4 to 5 satellites in each plane
How many GPS satellites are normally observable anywhere on Earth?
5
What makes up the GPS Control segment?
Master Control Station (MCS) - Hub is in Colorado Springs
Monitoring Stations (MSs)
Ground Antenna (GA)
What is the role of the GPS Control segment?
Monitors and maintains the GPS system via the Master Control Station (MCS), Monitoring Stations (MSs) and Ground Antenna (GA).
How many MSs are there in the GPS Control segment and what is their role?
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.
Some MSs are paired with a Ground Antenna, what is the role of a GA?
To communicate error corrections to the satellites.
What are the 3 GPS segments?
Control Segment
User Segment - Mil or civi with GPS receivers which can decode and process the satellite signals
Space Segment
What are the two frequencies GPS satellites transmit on?
Link 1 (L1) - 1575.42MHz
Link 2 (L2) - 1227.6MHz
What are the two GPS services provided?
C/A - Coarse Acquisition - Transmitted on L1
P - Precision Code - Transmitted on L1 and L2
What are the two GPS services available?
Standard Positioning Service (SPS)
Precise Positioning Service (PPS)
What is the accuracy of the SPS?
+/- 8m
What is the accuracy of the PPS?
+/- 4m
Max accuracy obtained from the P(Y) code on L1 and L2
What is the purpose of the Selective Availability (SA) feature on the PPS?
Can be used to reduce the accuracy to unauthorised users by introducing controlled errors into the signals
What is ‘first fix’ with a GPS receiver?
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.
What are the 4 types of GPS receiver?
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
What GPS errors can occur?
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
How can some of the potential GPS errors be minimised?
Use “Differencing”, aka Differential GPS (DGPS)
Details in course manual if required.
