Weeks 9-11

Question 1

What do “stealth’ aircraft do?

Answer 1

Stealth aircraft will reflect radar energy due to uneven and sporadic contours on its design that distorts the radar energy.

They are designed to deflect radar energy in another direction (except not back towards the enemy)

Designed to minimise the external carriage of weapons and to reduce the effect on the radar cross section of any internal systems

Question 2

Explain the three main radar scattering regimes

Answer 2

1. Low frequency (λ&raquo_space; L): When the wavelength is much larger than the dimension of the object, the
   dominant scattering process is Rayleigh scattering.
2. Resonance region (λ ≈ L): Also called the Mie region, the most complicated region is where there are resonances between the object and the radar waves
3. High frequency ( λ&laquo_space;L): When the wavelength is small relative to the object, the RCS is very close to the actual area and it is insensitive to variations in orientation and frequency. This is called the Optical region. Surface and edge scattering occur. Specular (or, regular mirror-like) reflection

Question 3

What are specular waves? (Reflected waves)

Answer 3

• Largest contribution to radar cross section (In the optical region, this is the only contrbution)
• Main contribution is present when the surface is facing the radar – otherwise the energy is reflected away from the radar (snells law)

Question 4

Explain multiple reflections as a form of reflected waves. (Reflected waves)

Answer 4

Radar energy is reflected away after contact with first surface but may reflect off another surface and back towards the radar.

(Multiple reflections are much more difficult to predict than specular reflection)

Question 5

What are surface/creeping waves? (Reflected waves)

Answer 5

If the surface is smooth (electrically and physically) the surface wave can propagate a long way along or around the airframe – those waves propagating around the airframe and back again towards the radar are known as ‘creeping waves’.

If the surface wave encounters a discontinuity (such as the end of the wing or fuselage), the wave can be reflected back along the transmission path and be scattered back toward the radar – known as ‘travelling waves’

Question 6

What is edge diffraction? (Reflected waves)

Answer 6

• Any discontinuity (an edge or a tip) will cause diffraction, radiating (approximately) perpendicular to the discontinuity
• Often cause by leading and trailing edges of wing and other surfaces.

Question 7

What is ducting? (Reflected waves)

Answer 7

• Any electrical conductive cavity will have certain frequencies, corresponding to electromagnetic modes, at which it will resonate.

– These modes will typically have wavelengths which are of the same order as the size of the
cavity.

– The effect is commonly seen in waveguides, which are electrical cavities which are used to carry certain frequencies and act as transmission lines.

• When there is a resonance between the radar and a cavity or duct, the result is a large, broad RCS lobe

Question 8

Whats one major advantage of a phased array radar antenna?

Answer 8

Half of the array can do one job and the other half to do another will increase the beam width of each part of the radar, but each part can be swept in different ways.

Question 9

How do we improve the cross-range accuracy of a radar?

Answer 9

• We can use the fact that the same object/target appears in multiple pulses
• Different pulses from the same antenna can be combined to form a ‘synthetic’
  array.

– The synthetic array is equivalent to a real array as long as the phase information from different pulse
returns is saved and integrated coherently, as they would be from a real antenn

Question 10

How do accelerometers work?

Answer 10

• Accelerometers work based on detecting small mechanical movements in a mass of known size (proof mass)
• Accelerations along the axis of the system tend to displace the proof mass. The displacement, or the force needed to avoid displacement, is proportional to the
  applied acceleration (Accuracies of 50mg-1𝜇g)
• Solid State Accelerometers: Two identical quartz devices are set up so that an acceleration in one direction will cause one crystal to be compressed and the other stretched. This has the effect of increasing the resonant frequency of one of the oscillators and reducing that of the other. The system then detects the
  difference in these frequencies

Question 11

How do momentum wheel gyroscopes work?

Answer 11

• Spinning gyroscopes measure rotation based on the conservation of angular momentum
• Every point on a spinning rotor has an angular momentum vector along the spin axis which,
  in the absence of an external torque, will be preserved acting as an “intertial reference.
• Displacement gyroscopes are isolated from the airframe via low friction gimbals and maintain orientation when the platform rotates. Picking off the angles that the gimbals have rotated gives the rotation of the airframe

Question 12

What is the Sagnac effect (Used in optical gyroscopes)?

Answer 12

• The Sagnac effect occurs when two light beams travel in opposite directions around a ring that is rotating starting at the same point at the same time. It takes a finite amount for light to travel around a ring, and in that time the ring will have rotated by a small amount.
• It therefore takes slightly longer for the light travelling with the rotation to reach the same place on the ring.
• This creates a measurable phase difference between the two beams coming out from the ring

Question 13

How do ring laser gyroscopes work?

Answer 13

• RLGs use an optical ring cavity that resonates at a given (laser) frequency. This frequency will be set so that the optical path length around the ring is an integer number of wavelengths of the light (giving rise to a resonance).
• Two light beams are used, one travelling clockwise and one travelling anti-clockwise.
• As the ring rotates, the effective path length for each beam changes due to the Sagnac effect.
• As the path length changes for each beam, the frequency of the resonances changes so that each beam has a different frequency.
• The difference between the frequencies of the clockwise and anti- clockwise beams is proportional to the rate of rotation about an axis that is perpendicular to the plane plane of the ring:

Question 14

How to fibre optical gyros work?

Answer 14

• Fibre optical gyroscopes use the Sagnac effect more directly than the RLG
• The Sagnac effect is normally too small to observe directly over small optical paths, so longer optical paths are generated by using a fibre optical coil consisting of a large number of turns
• Two beam splitters divide the source beam into two equal portions that travel in opposite
  directions in the coil and divert outcoming beams to the detector.
• The clockwise and anti-clockwise beams have the same frequency, but differ by a phase
  difference that is proportional to the rate of rotation of the gyro:

Question 15

How does an inertial element compensate for gravity?

Answer 15

Any inertial system must allow for gravity effects so that it only detects accelerations related to non-uniform motion of the airframe.

The forces/accelerations need to be resolved into earth axes to subtract the effect of gravity. This requires accurate knowledge of the airframe attitude.

Question 16

How does a global national satellite system work (GNSS)?

Answer 16

• GNSS works by measuring the distance between the receiver and at least four satellite
  transmitter systems
  – The distance is measured by working out the time that it takes for a radio signal to travel from the
  satellites to the receiver.

Question 17

What are the main two problems with GPS and possible solutions to them.

Answer 17

Acquisition: A GPS position fix relies on having an accurate estimate for the positions of all of the satellites
that it is going to use. These positions are broadcast by each the satellites at least once every 12.5 minutes, but there is no telling where in this cycle the receiver will be initialised, so it may be that the receiver needs to wait for a significant period of time until it has collected enough data to generate a position update.

(Solution: For a GPS system that is being initialised quickly, it is common to transfer an approximate
almanac of ephemeris data on initialisation – although the accuracy will depend upon how old
this data is)

Jamming: Signals received from a GPS satellite are extremely weak so they are extremely difficult to track and more difficult to acquire (requiring extra 6-10 db SNR)

(Solution: use phased array antenna to direct GPS antenna gain upwards and away from jamming)

Question 18

Explain possible GPS errors

Answer 18

• Timing errors at the satellites/receivers: Clock errors and time-of-arrival estimation errors at the receivers.
• Ionospheric errors: To calculate the distance to the satellite, the GPS receiver determines the time it took for the radio signal to travel from the satellite. But the speed of a radio signal, though constant in the
  vacuum of space, is affected by the Earth’s atmosphere, particularly the ionosphere. Most GPS receivers have built in an “ionospheric correction”, but this is based on a fixed model of the behaviour of the ionosphere. Since the characteristics of the ionosphere change, the
  signal from a satellite may take more or less time to reach the receiver than anticipated.

Multipath: If a radio signal is reflected off a nearby object on its way from the satellite to the receiver, it
will have travelled a longer distance than if it reached the receiver directly. If the receiver locks
onto this instead of the original signal, it will calculate an erroneous position of the satellite.
Unfortunately, there are not many ways to help eliminate or reduce this effect other than by
using more expensive receivers that are less prone to multipath.

Question 19

How does a differential GPS work?

Answer 19

• Differential GPS works because the ionospheric delay for two closely spaced points on the ground will be approximately the same
• If one of those points has a known location (normally generated by integrating the GPS
  location over a long period of time), the it can estimate the time delay caused by the
  atmosphere.

– If it then broadcasts the time delays to the mobile GPS receiver, it can correct it’s own
position by removing the time delay biases.
Ionospheric delays

Question 20

How does Loran Work?

Answer 20

• The LORAN system uses a chain of transmitters: one master and a number of slave transmitters (X, Y, Z,…):
  – The master transmits the signal (a series of shaped pulses separated by
  1000 μm at a frequency of 100kHz and 20kHz bandwidth).
  – Each slave receives the master signal and retransmits it (in turn) after a
  fixed processing delay (X before Y before Z, etc.).
  – The aircraft receiver gets the signals from the master and each slave within
  range and (knowing the fixed delays between the retransmissions) it can
  calculate the effective time delay between the master and the slaves.

Question 21

What is scene matching?

Answer 21

Scene-matching and area correlation (SMAC) uses imagery from a camera and a database of ground features to produce a position estimate.

If a database of ground features is available then it is possible to predict what would be seen by a camera at a particular position and orientation

Question 22

What are the use of NDB’s and errors?

Answer 22

• Aircraft on-board radio equipment can determine in which direction
  from the aircraft an NDB signal is coming.
  – The on-board aerial consists of a simple metal loop which is rotatable.
  – The radio signal induces a current in the loop, as in a normal aerial, but this current is weaker or stronger depending on the orientation of the loop. When the loop is flat-on to the origin of the signal, the signal is strongest Problems:

– First, erroneous signals. Radio beacons are subject to disturbances that may result in erroneous bearing information. Such disturbances result from such factors as lightning, precipitation static, etc.

– Second, you can only tell the relative bearing of your aircraft to the NDB - that is, the direction in which the NDB lies. Only by comparing this against the aircraft compass heading can a pilot determine at which (magnetic or true) bearing the NDB lies from the aircraft.

Question 23

What are the use of VOR’s and errors?

Answer 23

The VOR antenna is similar to the NDB loop antenna, but it is rotated continuously and it has no null point.
– The loop and sense antennas are combined in such a way that it has a ‘limacon’ antenna pattern – similar to the cardioid NDB pattern but with a smaller dip and no true null point.
* The accuracy of course alignment of the VOR is excellent, being
generally 1o (although the requirement is +/-5 degrees 95% of the
time).
– VOR’s operate within the 108.0 to 117.95 MHz frequency band and
have a power output necessary to provide coverage within their assigned operational service volume. They are subject to line-of-sight
restrictions, and the range varies proportionally to the altitude of the receiving equipment.
– There is also a ‘cone of confusion’ where bearing readings are unreliable, typically 6 nautical miles radius extending up to 30,000 feet.

Question 24

What are the three different bands for infrared cameras?

Answer 24

– Near Infrared: 0.7 𝜇m to about 3 𝜇m.
Little thermal emission in this band, but can be used with laser illumination systems.
Atmosphere not transparent between 2-3 𝜇m.

– Mid-wave Infrared: 3 𝜇m to 8 𝜇m.
The mid-wave band is good for hot objects, not much background thermal emission.
Although it is used for imaging background scenes because the electronics are better.
Atmosphere not transparent between 5-8 𝜇m.

– Long wave Infrared: 8 𝜇m to 15 𝜇m.
The long wave band is good for room temperature scenes since the thermal emission peaks in this band for
300K. Also known as the “thermal infrared” region

Question 25

What are the two factors impacting the field of view of the pilot?

Answer 25

• Optical flow fields: pilots tend to use visual cues in the scene to guide their manoeuvres. Often the pilot’s perceived motion is more important that their actual motion

– Peripheral vision: if the field of view is too narrow, it gives the pilot an ‘unreal’
feeling of motion – like looking down a cardboard tube.

Question 26

What two factors is the ability to detect objects in imagery

Answer 26

1. Spatial resolution is the ability to resolve objects of certain dimension – if an object is smaller than a pixel in the image it cannot be resolved and no information can be obtained.
2. The contrast of an object is important because if the
   contrast between the object and its immediate background is small relative to other things in the image, the object will blend into the background

Question 27

What is the Johnston criteria?

Answer 27

Detection (with 50% probability) = resolve 1 bar (3 pixels).
– Recognition (with 50% probability) = resolve 3 bars (7 pixels).
– Identification (with 50% probability) = resolve 5 bars (11 pixels

Question 28

What is the critical dimension based on Johnston Criteria?

Answer 28

smallest ‘significant’ dimension of the object that provides useful information

Question 29

How does an instrumented landing system work?

Answer 29

An instrumented landing system (ILS) or precision approach aid is used to direct the final approach of an aircraft into a runway, often supplemented by range beacons or distance measuring equipment (DME)

One beam is (amplitude) modulated at 90Hz and one at 150 Hz, when the approach is on the centre line, the strengths of these two modulations will be equal, more of one will indicate that the aircraft is too far to one side and can correct.

Question 30

What two things are critical for the design and testing of electrical/electronic components (electronic compatibility)

Answer 30

1) They do not produce too much stray EM radiation,
2) They are not sensitive to the EM signals from other sources