Everything important!!..except Flight Planning Flashcards
Generic INS components
- Accelerometers (A)
- Computers (C)
- Control Display Unit (CDU) (C)
- Stable platform (S)
Advantages of INS
- Self-contained (S)
- Undetectable (U) (doesn’t radiate)
- Unjammable (U)
- All WX operational (W)
- Worldwide operational (W)
- Accurate Px and very accurate attitude (A)
How INS calculates positions?
- Acceleration is measured (in 3 direction)
- Newton’s 2nd Law: for a constant mass, force and acceleration vary inversely
- Integrate a (acceleration) to get v (velocity)
- Integrate v(velocity to get d (displacement)
Mech gyros have been replaced by Ring Laser Gyros (RLG). Reduction in? & Increases in?
Reductions in: 1. Weight/hardware 2. Power consumption 3. Maintenance 4. Cost Increases in: 1. Reliability 2. Service life
Desired characteristics of Accelerometer
- Low threshold of sensitivity
- Wide range of sensitivity
- Linear output
- High resolution
Parts of a simple Gyro
- Rotor (spinning wheel)
- Spin Axis (about which rotor turns)
- Gimbal (support that provides spin ais with a DOF)
Properties of a Gyro
- Rigidity: dependent on angular momentum
2. Precession: movement of the spin axis in response to an external force
2 Main Advantages of INS integration/hybridization
o Improved Accuracy
o ↑ mission completion
2 Main disadvantages of INS integration/hybridization
- Loss of complete self-containment
* Loss of covertness (dependence on external radiation)
Parts of RLG (CT-142 Honeywell)
- Cervit block
- 3 mirrors
- Prism
- Cathode
- Anode
- Inert gas (Ar)
- Photo cells
2 types of error for the RLG?
Error 1: LOCK IN
o Caused by non-perfect optics (back-scattering)
o Can lead to drift rates of 40-50°/hr if left undetected. Largest source of error: mirror quality
o Fixed with dithering
Error 2: DIFFERENTIAL PATH LENGTH
o T° difference between cervit cavities causes a time/phase
o Eliminated by using one cervit cavity for both lasers.
Initialization (10x initial conditions):
- 2 initial posn cords (Lat/Long)
- 2 initial velocities (N & E)
- 3 initial orientations (x,y,z gyros)
- 3 orientations rates
Types of Alignement :
- Self-Alignment (most popular)
- Reference alignment (Rwy heading)
- Moving alignment (carrier based A/C)
- In-flight alignment (ability to align Airborne)
Self-Alignment Sequence:
- Initialization
- Warm-up (alignment performed by computer)
- Coarse levelling
- Coarse alignment
- Fine levelling
- Fine alignment (aka gyro compassing)
High latitude Problems:
- Undetectable tilt preventing initiation of gyro compassing
* Inability to accurately resolve True North
Types of Errors during the alignment
Unbounded (3x): increase with time
• Leveling gyro drift(largest single error in INS)
• Initial Azimuth misalignment (due to operator input)
• Azimuth gyro drift (2nd largest source of total INS error)
Bounded (3x): oscillate about a mean value with time
• Initial levelling (Platform tilt-computer error)
• Accelerometers (acceleration error)
• First integration errors (velocity errors)
Schuler Theory
- Platform with initial tilt error, thinks it’s moving and is torqued to what it ‘’thinks’’ is level.
- Due to this ‘’correction’’ torque applied to the platform, the accelerometer feel a ‘’real’’ component of acceleration due to gravity.
- The platform retilts in the original direction and goes back to where it was and the cycle repeats itself every 84.4 min.
2 Components of Honeywell IRS
Inertial Reference Unit (IRU) & Mode Selector Unit (MSU)
Electrical Requirements of IRS
- Receives AC & DC power from the aircraft and provides switching to primary AC or secondary DC.
- Can operate with either:
- 115V AC ; or
- 28V DC.
Cooling Requirements of IRS
- Cool air provided via A/C air conditioning
* Loss of cool air = shut down of IRU after a specified time.
Modes of Operation of IRS
OFF
ALIGN
NAV
ATTITUDE
Describe ALIGN mode in IRS
- Computes Hdg and latitude by measuring horizontal earth rate components
- Equator (0°) = 2.5 min, 60-70° = 10 min, 70°+ = longer or may fail
Describe NAV mode in IRS
(Can’t do external update in Nav mode) • Provides output to FMS • A/C attitude • Body rates • Body accelerations • True Heading • Velocity vectors • Wind data • Lat/Long • Inertial altitude
Nav components calculate: • Present location • Velocity • Heading • Attitude
Describe ATTITUDE mode in IRS
• NO Nav output provided
Used when:
• MSU fault enunciator illuminates
• After the IRS temporary loss all power
Describe the System Performance Test
- Entered LAT must pass system performance test at end of alignement phase
- LAT entered must be within 30 min of arc of LAT computed by IRU during alignment
- If entered LAT is good to go, alignment is complete and it allows entry into NAV MODE.
IRU feeds ATS. ATS maintains data base of what ?
- True heading
- True Track
- Present Lat/Long
- Inertial G/S
- Wind velocity & direction
- System status
What does ALERT & WARNING sign means in CT-142 Emulated INS
ALERT: 90 sec to next waypoint if on track or 90 sec to abeam position if off track
WARNING:Malfunction is preventing IRU alignment OR a latitude that is inaccurate by more than ½ a degree is entered.
What are the 2 types of Apparent Precession in the GPS?
- Earth Rate: apparent precession of a stationary gyro due to the earth’s rotation wrt space.
- Transport Wander: apparent precession due to the movement of the gyro from one point on the earth to another.
Unique Capabilities of the GPS
3 Very Gross Children Puked Up Red Ants • Highly accurate 3D position • Provides velocity and time • Global coverage • Continuous availability • Passive service (receiver) • Unlimited # of users • Resistant to interference & jamming • Allows common grid reference
What are the 3 distinct parts of the GPS?
Space segment
Control segment
User segment
Describe the Space segment
- 6 to 11 satellites are always in view
- Orbit period ~12hr
- Orbit à 10 900m
Describe the Control segment
- Monitor the health and operating parameters of the satellites
- 1 master control station @ Colorado Springs
- Monitor stations look for errors caused by gravitational pulls (moon & sun) & solar radiation pressure.
- Master Control Station (MCS) periodically sends data to satellites via Nav Data msg
Describe the User segment
• Military & civilian GPS receivers decode and process the GPS satellite signals. Consists of: • Antenna • Computer • Oscillator • Signal processing equipment
What are the 2 navigation services? Describe them.
SPS (Standard Positioning Service): • L1: coarse acquisition • L2: interim for carrier phase • 60-100nm accuracy when SA (selective availability) active PPS (Precise Positioning Service): • L1: coarse acquisition & P/Y code • L2: P/Y code • 16m accuracy • Requires decryption
Describe the C/A code
- Available to all users
- Code is 1023 bits long
- Tx @ 1.023 mbits/sec
- Accuracy = 100m (95% of the time)
- Each satellite has its own unique code
Describe the P code
- 267 days long code (each satellite gets a portion of the code (7 days)
- 10.23mbits/sec
- Accuracy = 16m (95% of time)
- Available to US mil & limited allies
Describe the Y code
• P Code that has been encrypted