Day 1 Flashcards
How do you convert a real number into a dB?
LOG the number, x 10
How do you convert a dB into a real number?
Divide by 10, put number to power of 10
What dB value would equate to half the power?
-3
What dB power would equate to double?
3
What is a decibel?
Logarithmic scale for a huge numerical range
What is Bandwidth?
The difference between upper and lower frequencies of the signal. Too small will cut off pulse, too large will allow more noise.
What is Bandwidth?
The difference between upper and lower frequencies in a continuous set of frequencies typically measured in Hz. Too small will cut off pulse, too large will allow more noise.
What is signal to noise ratio? SNR
Shown in dB, it is a measure of a radars ability to see targets in the presence of noise.
SNR > 1 = detectable signal
SNR
What are the sources of noise in a system?
Internal
External
What are the internal sources of noise in a system?
Equipment, transmission and antenna
What are the external sources of noise?
Interference, Jamming, Weather and Ground clutter
What is EW broken into?
EA - Jamming, deception & neutralisation
ES - SIGINT - COMINT/ELINT & Warning
EP - EMCON, masking, hardening, WRM
What is an electronic wave?
Is a sinusoidal wave that consists of electric (E) and magnetic (H) fields always at 90 degrees to each other. No need for a medium, obey same laws of wave motion.
What is the speed of light?
3 x 10^8 metres per second
162,000 nm per second
186,000 miles per second
What does radar stand for?
Radio detection and ranging
What is frequency?
The number of cycles that occur in 1 second in Hz
What is wavelength?
The physical distance one complete cycle of the wave occupies
What is the relationship between wavelength and frequency?
Inversely proportional
What is the formula for calculating wavelength and frequency?
C = 300,000,000 microseconds (ms) λ = C / F F = C / λ
What is phase?
A wavelength rotates through 360 degrees. 1st quarter = 90 degrees Halfway = 180 degrees 3rd quarter = 270 degrees Full wavelength = 360 degrees
What is the difference between an analogue and digital signal?
Analogue - variable quantity which changes continuously with time
Digital - Discrete permitted values with time
What are the military’s main uses of the EM spectrum?
Radio communications
Primary and Secondary Radar
IR Detection
Ultraviolet Seekers
What is polarisation?
The orientation of the radio wave in respect to the earth. EM energy is free to vibrate in any of the infinite planes at 90 degrees to the direction of propagation.
Polarisation is determined by the orientation of which field?
The Electric Field
What are the types of Polarisation?
Random (Static, Light)
Linear: Horizontal, Vertical, Slant
Circular and Elliptical: Left hand and Right hand
Rotating: Clockwise and Anti-Clockwise
Explain Circular & Elliptical Polarisation
RHCP = Right Hand Circular Polarisation
From the point of origin, away from you.
i.e. RHCP = Clockwise
LHCP = Anti-clockwise
What is propagation?
The process of how EM waves travel through the different layers of the atmosphere.
All EM waves, regardless of frequency, propagate in the same way.
In a vacuum they travel in straight lines, indefinitely, at the speed of light, however in the earth’s atmosphere they are subject to losses.
What is Isotropic Distribution?
The spread of EM waves equally in all directions, as if coming from a point at the centre of a sphere
What is Power Density?
The amount of energy per unit area in an EM wave
Decreases as range from the source increases
Measured in Watts per square metre
What is the power density formula?
Pt / 4 π R^2
Pt = transmitted power R = range from transmitter
What is the reflected power density formula?
Reflected Pd = Pt / 16 π ^2 R^4
What is the ESM Range Advantage?
Probability of Interception (POI)
For intercepting equipment, the advantage occurs because signals only have to travel one way from the emitter to the intercept platform.
Power entering an airborne ELINT antenna is about 4 times greater than the reflected power returning to a Radar antenna.
What is Atmospheric Attenuation?
The weaking of signal as it propagates through atmosphere.
Caused mainly by:
Absorption
Scattering
Explain Atmospheric Absorption
Occurs when the energy from the EM wave dissipates as it interacts with gase molecules which absorb the radiation
Most of it occurs due to oxygen and water molecules.
As altitude increases, attentuation decreases as lower level of water molecules
At altitude the RF windows are more pronounced allowing better propagation
Explain Atmospheric Scattering
Occurs when EM waves hit molecules in the atmosphere and change their direction as a result
If the circumference of the sphere is smaller than the wavelength, little energy is scattered.
If larger, the sphere will scatter a constant high-level of energy
If similar, the sphere will scatter large amounts of energy that varies over large values
What is diffraction?
The bending of waves around edges of objects such as building or mountains.
The longer the wavelength, the greater the amount of diffraction
Can generate surface waves, increasing operational range
At higher frequencies, diffraction is negligible
What is reflection?
State the different wave types
Ground Wave (Surface and Space) Sky Waves
State the factors affecting a Space Wave
Attenuation = Absorption and Scattering
Average Path length is restricted to around 25-40km
For longer distances relay stations are required
Which layers of the Earth’s atmosphere have a major effect on the performance of EM waves?
Ionosphere
Troposphere
