Aero 250 Flashcards
Wavespeed, c
fxlambda
Angular frequency, w
2πf
Difficulty of transmitting radio waves
Antenna must be similar size to wavelength
Channel
Transmission medium conveying the electrical signal. There’s two types: wired and unwired.
Distortion
Undesirable change in a signal that disappears when the signal is turned off.
Interference
Contamination of the channel by external signals.
Noise
Random, unpredictable and undesirable electrical signal from natural sources.
Transmitter
Modifies the input signal in order to cope with the limitations imposed by the channel.
Receiver
Processes the received signal by reversing the signal modifications made at the transmitter.
Analogue message
Data that varies continuously and smoothly over a continuous range of time.
Digital message
An ordered combination of finite symbols.
Reasons digital has replaced analogue
- Enhanced immunity to noise and interference. 2. Regenerative repeater stations can be set up for long-distance transmission.
SNR definition
The ratio of signal power to noise power. A certain min SNR is required at the receiver for successful communication.
SNR equation
SNR(dB) = 10Log(Ps/Pn)
Signal bandwidth, B, definition
The maximum range of frequencies a signal occupies; the amount of frequency needed to sustain the unmodified signal.
Signal bandwidth equation
B = fmax - fmin
Baseband bandwidth
The range of frequencies from zero to the highest frequency present in a signal.
Noise power
Equals the variance of the noise.
Channel bandwidth
The range of frequencies that a communication channel can transmit with reasonable accuracy. It limits the signal bandwidth that can pass through.
Channel capacity, C, definition
The amount of information that can be reliably transferred. In a digital system, this is the number of error-free bits that can be transferred per second.
Channel capacity, C, equation
C = BcLog2(1+SNR) = BcLog2(1+(P/N0Bc))
Features of a good communication system
High C, high SNR, low Ps, low Bs
Modulation
Uses the baseband signal to modify amplitude, phase or frequency of the carrier signal generated by the transmitter. Allows for the simultaneous transmission of multiple signals.
Demodulation
The opposite of modulation. The baseband signal is reconstructed at the receiver.
Amplitude modulation with suppressed carrier
Carrier signal, c(t) = cos(Wct), baseband message signal = m(t), so modulated signal, s(t) = m(t)c(t). The demodulator extracts m(t) from s(t). Also called DSB-SC.
Low-pass filter
Removes the high frequency component of a signal, usually around 2wc.
Amplitude modulation with large carrier
Modulated signal, s(t) = AcCos(wct) + kam(t)AcCos(wt). Also called DSB-LC.
Modulation factor, mu
mu = kaAm = Am/A
Envelope detection
An AM demodulation technique
Condition for envelope detection
0 < mu < 1
Upper sideband
Spectral content lies outside of the carrier frequency.
Lower sideband
Spectral content lies within the spectral frequency.
DSB-LC total power
Ptotal = Pcarrier + Plsb + Pusb
DSB-SC total power
Ptotal = Plsb + Pusb
Carrier power
Pcarrier = 0.5(Ac)^2
Relationship between Plsb and Pusb
They’re equal.
Power efficiency
nu = Ps/(Pc + Ps), where Ps is sideband power and Pc is carrier power.
Comparison of DSB-LC and DSB-SC
DSB-LC is less power efficient since the carrier components doesn’t convey any information. However, DSB-SC requires a more complex demodulator.
DSB disadvantage
DSB doubles the baseband bandwidth, which becomes a disadvantage for crowded frequency bands.
Single sideband, SSB
Only transmits either the upper or lower pair of sidebands. Requires energy gap at low frequencies.
SSB total power
Ptotal = Pusb or Ptotal = Plsb
SSB generation
Generate a DSB-SC signal, the suppress one of the sidebands by filtering.
SSB advantages
Saves power, and reduces the bandwidth by 50%.
SSB disadvantages
Complex circuits required for frequency stability and filtering.
Vestigial sideband, VSB
For signals with little/no energy gap at low frequencies, VSB transmits a pair of sidebands, plus a small amount of the other pair of sidebands.
If a generalised signal is Acos(wct), then what’s the generalised phase?
Generalised phase is wct = thetai(t)
Instantaneous angular frequency
wi(t) = dthetai(t)/dt
Power of an angle modulated signal
(A^2)/2
Max phase deviation
delta theta = (max(thetai(t) - wct) - min(thetai(t) - wct))/2
Max frequency deviation
delta w = (max(wi(t) - wc) - min(wi(t) - wc))/2
Bandwidth of an angle modulated signal
Infinite
Carson’s rule (angle modulation bandwidth)
Banglemod = 2(deltaf + B) = 2B(beta + 1), where beta = =deltaf/B
Zero-crossing demodulation
Only uses points of the modulated signal where it crosses zero.
Sample rate, fs
fs = 1/Ts, where Ts is how often the value of the signal is measured.
What are the three types of sampling?
Critical sampling, over-sampling and under-sampling.
Critical sampling
fs = 2B
Over-sampling
fs > 2B. Easy reconstruction, but generates more than necessary number of samples.
Under-sampling
fs < 2B. Data-reduction technique. Expensive. Normally an anti-aliasing filter is used.
Nyquist rate
2B
Aliasing
Spectral overlap that occurs in under-sampling.
How can a signal be recovered from its sample?
Use an ideal low-pass filter.
Quantisation
Maps samples of a continuous amplitude waveform to a finite set of amplitudes. This introduces quantisation noise.
Pulse amplitude modulation
Varies the amplitude of a constant width, constant position pulse, according to the position of the sample of the analogue signal.
Pulse position modulation
Varies the position of a constant width pulse within a prescribed time slot according to the amplitude of the sample of the analogue signal.
Pulse width modulation
Varies the width of a constant amplitude pulse, proportional to the amplitude of the analogue signal at the time the signal is sampled.
Line coding
Converts the bit stream produced by a source encoder to electrical pulses.
On-off line coding
1 is transmitted as a positive pulse, 0 by no pulse.
Polar line coding
1 is transmitted by a positive pulse, 0 by a negative pulse.
Bit number, N
N = Log2(L), where L is the quantisation level.
What does pulse code modulation do?
PCM transmits a message using a sequence of coded pulses representing the sampled, quantised and encoded signal.
For PCM, what is the required data rate to transmit a message signal?
2nB
For PCM, what’s the minimum channel bandwidth required to transmit a message signal?
nB
Digital band-pass modulation
Switching (keying) the amplitude, phase or frequency of a high frequency sine carrier.
Binary amplitude shift keying, BASK
Transmitting s(t) = Acos(wct) for binary symbol 1 and s(t) = 0 for binary symbol 0.
Binary phase shift keying, BPSK
Transmitting s(t) = Acos(wct) for binary symbol 1 and s(t) = -Acos(wct) for binary symbol 0.
Binary frequency shift keying, BFSK
Transmits zero by a pulse of frequency w0, and transmits a 1 by a pulse of frequency w1.
Differentially phase shift keying, DPSK
Transmitting data encoded as the phase change between consecutive symbols.
Which keying schemes have a quad-phase of zero?
BASK and BPSK
Multiplexing
Simultaneous transmission of multiple signals.
Two types of multiplexor
Frequency division and time division
Frequency division multiplexor
Several signals share the channel band - each signal is modulated by a different carrier frequency.
Time division multiplexor
Each signal is allocated a periodic time slot. This can either be done on a bit-by-bit basis or a word-by-word basis.
Spread spectrum
Expanding the signal to occupy a much broader spectrum than necessary, enabling lower SNR communications. This will appear as background noise to a receiver unless an authentication key is used.
Simplex
Communication flows in a single direction, e.g. radio.
Half-duplex
Communication in both directions, but not simultaneously, e.g. walkie-talkie.
Full duplex
Simultaneous communication in both directions, e.g. telephone.
Pulse radar system
Pulse is transmitted, then radar switches to receiver mode. Time taken for pulse to return gives distance, and echo approach angle gives object bearing angle.
Continuous wave radar
Two separate antennae for transmitting and receiving. Doppler shift is measured to give velocity information,
Range of object (radar)
R = 0.5ct, where t is the transit time.
Radar range resolution
deltaR = 0.5c/fBW, where fBW is the pulse bandwidth.
Radar pulse bandwidth, fBW
fBW = 1/T, where T is the pulse duration.
Problems with a short duration pulse in radar.
Requires a very large power to get sufficient radiant energy on the target, which requires large voltages that can be difficult to achieve. They can also damage the antenna.
Alternative to a short duration pulse in radar.
Long duration pulse that happens to have a large bandwidth, achieved by sweeping the frequency of the carrier frequency. Such pulse is called a chirp.
Pulse duty cycle, d
d = Tp/Tpri, where Tp is the pulse duration, and Tpri is the pulse repetition interval.
Max detection range (radar), Rmax
c/2fPRF, where fPRF is the pulse repetition frequency.
Radar average power, Paverage
dPpeak, where d is the duty cycle.
Radar SNRintegrated
GIxSNRperpulse, where GI is the integration gain, and ideally equals N, the number of pulses in a pulse train.
Parabolic antenna for radar
Feed antenna is at/near the parabolic dish focal point. Forms a radar beam with better directionality.
Radar beamwidth
deltaTheta = 1.22lambda/2a, where a is the radius of a circular aperture.
Radar cross-range resolution
deltaRc = 2Rsin(deltaTheta/2)
Radar polarisation
Often radars transmit one polarisation and receive both. The ratio of received power in each polarisation carries some information about the surface it was reflected off.
Pt in the radar equation
Peak transmission power.
Gt and Gr in the radar equation
Transmitter gain and receiver gain respectively.
Sigma in the radar equation
Radar cross-section.
Ls in the radar equation
A term that accounts for losses.
N in the radar equation
Noise power.
Two factors that radar noise depends on
Temperature of the electronics and bandwidth of the receiver.
F in the energy form of the radar equation
Noise figure of the electronics.
Radar range measurement accuracy, sigmaR
SigmaR = deltaR/root(2SNR) = c/2Broot(2SNR)
Radar angular measurement accuracy, sigmaA
SigmaA = deltatheta/1.6root(2SNR)
Bistatic radar
Radar that uses a transmitter and receiver that are spatially separated.
Disadvantages of bistatic radar
Complex communications infrastructure, because timing information needs to be transmitted between the radar transmitter and receiver. Also harder to deploy and more expensive.
Doppler shift for a moving target
fd = 2Vr/lambda, where Vr is the radial velocity from the radar.
Integration time needed to resolve two Doppler shifts, Tint
Tint = 1/deltafd
Doppler frequency error
sigmafd = ∆fd/Root(2SNR)
Radial velocity error
sigmaVr = lambda∆fd/2Root(2SNR)
Secondary surveillance radar, SSR
Radar signal from ground antenna is received by aircraft at one frequency, then rebroadcast at a different frequency.
One advantage and one disadvantage of secondary surveillance radar, SSR
Advantage is that signal power can be reduced, because signals go one way rather than two. Disadvantage is that each aircraft needs to be equipped with a transponder.
Radar cross section, RCS
The effective reflective area of a target. Normally very difficult to predict.
Total radar cross section
Sigma = Pradiated/Sincident = Pradiated/(Pincident/4pi), where Pradiated is the power of the re-radiated radiation, and Sincident is the power density of the incident radiation, and Pincident is the power of the incident radiation.
Three types of radar scattering regime
Low frequency (Rayleigh scattering), resonance region (Mie region) and high frequency (optical region).
Low frequency radar scattering regime
Rayleigh scattering. Wavelength much larger than the dimension of the object.
Resonance region radar scattering regime
Mie region. Resonances between the object and the radar waves.
High frequency radar scattering regime.
Optical region. Surface and edge scattering occur.
Four main contributions to RCS
Reflected waves, edge diffraction, surface waves and ducting.
Reflected wave RCS contribution
Direct reflection. The largest contribution to RCS.
Edge diffraction RCS contribution
Any discontinuity will cause diffraction, radiating perpendicular to the discontinuity. Common sources are wing leading/trailing edges and pitot tubes sticking out.
Surface waves RCS contribution
Since the airframe skin is conductive, the radar will generate currents within the skin. These surface currents can sustain EM waves propagating across the surface.
Ducting RCS contribution
Resonance between the radar and a cavity/duct, resulting in a large RCS lobe.
RCS combination formula
sigma = ∑((root(sigmam)e^ipsim)^2), where sigmam is the mth scatterer RCS and psim is the two-way phase difference for an EM wave reflecting off the structure.
Two ways stealth aircraft minimise RCS
Shaped to reflect radar energy in a direction away from the enemy radar. Minimised external carriage of weapons.
Advantage of using a phased radar antenna (E-scan)
Different parts of the array can be used in different ways.
Ground clutter
When tracking an aircraft close to the ground, complications arise. Radar reflections from ground objects clutter radar returns.
Synthetic aperture radar, SAR
Making use of the fact that the same object/target appears in multiple pulses, meaning you can improve the cross-range accuracy of the radar.
Beamwidth of an SAR pulse
ThetaB = lambda/2a, where a is the antenna radius.
Cross-range resolution of an SAR radar
dcr = a
Beamwidth of a synthetic array of length Le
ThetaB = 0.5lambda/Le, where Le = RthetaB
Two types of navigation system
Position fixing and dead reckoning.
Two types of inertial sensors
Accelerometer and gyroscope.
Accelerometer
Measures acceleration.
Gyroscope
Measures rotation/rotation rates.
Mechanical accelerator
Works based on detecting small mechanical movements in a mass of known size.
Solid state accelerometer example
Resonant silicon accelerometer. Two identical quartz crystals are set up so that an acceleration in one direction will cause one crystal to be compressed and the other to be stretched. This has the effect of increasing the resonant frequency of one of the crystals and decreasing that of the other. The system then detects the difference in these frequencies.
GPS
A satellite positioning system. One of four global satellite systems.
GPS operation
Measures the distance between the receiver and at least four satellite transmitters. These satellites are organised into six different orbital planes, forming a GPS bird-cage.
Acquisition problem (GPS)
A GPS position fix relies on having an accurate estimate for the positions of the satellites it’s going to use. These positions are broadcast at least once every 12.5 mins, but there’s no telling where in the cycle the receiver will be initialised. Consequently, the receiver may need to wait.
Jamming problem (GPS) and solution
The receiver signals are very weak, such that they sit below the background noise level, making them difficult to track. A phased array antenna can be used to direct the GPS antenna gain upwards, away from the jamming.
Ionosphere errors (GPS) and solution
The time taken for the radio signal to travel through the vacuum of space is constant, but it varies through the Earth’s atmosphere, especially the ionosphere. Two frequencies can be used to estimate this error.
Multipath errors (GPS) and solution
If a radio signal is reflected off a nearby object on its way from the satellite to the receiver, it’ll have travelled a greater distance than if it travelled directly. Signal processing can cancel these echoes.
Differential GPS
Two closely spaced points on the ground will have roughly the same ionospheric delay. If one of those points has a known location, then it can estimate this delay, and broadcast it to the mobile GPS receiver, which can correct its own position by removing the delay.
Non-directional beacon, NDB
Mainly used for airfield approaches. On-board radio equipment can determine which direction an NDB signal is coming from.
VHF omnidirectional range
The primary navigation system in most aircraft. Uses the phase relationship between a reference phase and a rotating phase signal to encode direction.
Distance measuring equipment, DME
Paired pulses sent out from aircraft, and received at ground station, which transmits paired paired pulses back to the aircraft at the same pulse spacing but at a slightly different frequency. The time for this all round trip is measure in the airborne DME unit, and converted to distance from the ground station to the aircraft.
Long range navigation (LORAN)
LORAN gives differences in distance. It’s a hyperbolic navigation system.
Instrumental landing system, ILS
Used to direct the final approach of an aircraft onto a runway. Consists of a localiser and glidescope, which provide lateral and vertical guidance respectively.
Two types of low-light imaging
Light-intensifiers and infrared imaging.
Light intensifiers
Use electronic amplification to turn a very dim picture into a bright one, but very sensitive to glare from light sources.
Two factors object detection is dependent on
Spatial resolution and contrast.
Spatial resolution
The ability to resolve objects in a certain dimension.
Avoiding limitations due to spatial resolution
Introduce colour by having imagers that use two or more wavebands.
Two factors that limit camera resolution
Aperture size and number of pixels
Rayleigh criterion equation
deltaTheta = 1.22lambda/D, where D is the aperture diameter.
Angular resolution equation
deltaTheta = Theta/N, where theta is the field of view and N is the number of pixels.
Johnson criteria
Detection - resolve 1 bar (3 pixels); Recognition - resolve 3 bars (7 pixels); Identification - resolve 5 bars (11 pixels), all with 50% probability. The number of bars is across the critical dimension.
Three types of aircraft display
Head down, head up and helmet-mounted.
Three key air data measurements
Dynamic pressure, static pressure and outside air temperature.
Electromagnetic compatibility, EMC
Every electrical will generate EM radiation, and will also be sensitive to the EM radiation produced by other systems. EMC ensures that components don’t produce too much stray EM radiation, and aren’t sensitive to EM radiation from other sources.
Safety critical system, SCS
A system that directly affects the safety of the people, equipment or environment.
Method to protect systems
Have current flowing or a voltage applied even when nothing is happening. This is because if nothing is being measured, you won’t necessarily realise when a circuit is broken or otherwise disrupted.
