Naval Weapons Exam 2

electronic combat (EC)

military action involving use of electromagnetic energy to determine, exploit, reduce (or prevent) an enemy’s use of EM spectrum, and actions which retain friendly use of EM spectrum

commonly referred to as electornic warfare (EW)

EC and EW are interchangeable

three components of EW

1. electronic support (SW)

2. electronic attack (EA)

3. electronic protection (EP)

electronic support (ES)

passive surveillance of EM spectrum

surveillance of enemy

• strength

• intention

• warning of target/homing

Signal Intelligence (SIGINT) (ES)

electronic intelligence (ELINT)

• info derived from emissions other than comms (emphasis on radar)

communications intelligence (COMINT)

• info derived from communications

bandwidth

range of frequencies detectable by a receiver

receiver sensitivity

minimum power signal needs to be detected

bandpass filted

passes only certain range of frequences through

probability of intercept

probability signal will be detected

signal analysis

manually or electronically examining a received signal to categorize it

parametric data

characteristics of received signal (RF, PRF, PW, scan rate/type, etc.)

ES receiver design requirements

• wide spectrum surveillance- frequency spectrum being searched must be large

• wide dynamic range - receiver must be able to receive and process very weak and very strong signals

• unwanted signal rejection: discriminate against other signals that are similar in frequency to signal of interest

• angle of arrival measurement capability: locate transmitter by triangulation

• signal analysis capaility: identify signal based on its parameters and associate it with specific threat

• display: type of display dictated by way receiver is used (audio, warning lights, alphanumeric, etc.)

direct detect receiver

designed to indicate (via alarm) presence of a signal within its sensitivity and bandpass

• reliable, small in size, lightweight, cheap

radar warning receiver (RWR) in aircraft indicates presence of and bearing to a contact that has been predetermined hostile based on parametric data

limitation of these systems: they typically incorporate a wide bandpass. why is that a problem?

• excess noise, especially in dense EM environments

• high false-alarm rates

electronic attack (EA)

prevent or reduce enemy use of EM spectrum

• can be employed against enemy sensors and communication systems

• interferes with operation of air and surface defense system sensors to reduce info content and induce errors

4 major classes of EA

1. jamming

• denial (noise)

• deception (range, angular, velocity)

2. medium modifiers

• change electrical properties of medium between sensor and target

3. improving own-platform stealth

• apply radar-absorbing material

• use decoys

4. destruction of enemy sensor systems

• physical destruction via blast or fragmentation warheads

denial jamming tactics

stand-off jamming

self/escort jamming

deception jamming tactics

angular deception

range deception

denial jamming

designed to overload opposition’s receiver so that its use is denied to enemy

• creates noise signal powerful enough to mask signal receiver is trying to detect

advantages

• simple to implement

disadvantages

• effectiveness is limited by max radiated power

• countermeasures are simple and widely employed

• vulnerable to home-on jam-type missile systems

jamming tactics - stand-off jamming

jamming platform stays outside lethal range of enemy weapons, but doesn’t escort other units

• safer for jammer (GOOD)

• lower jamming power on target (BAD)

• jamming power may go off-target (BAD)

jamming tactics - self/escort jamming

jamming aircraft escorts strikers in, or the strike aircraft provide their own jamming

• easier to ensure jamming energy paints target site (GOOD)

• higher jamming power on target (GOOD)

• susceptible to energy home-on-jam (HOJ) missiles (BAD)

deception jamming

instead of trying to “mask” targets, the jamming creates synthesized (fake) targets to overwhelm tracking logic and/or confuse system

angular deception jamming

radar antennas are not “perfect” and do not reflect only a single beam

this results in prevalence of “sidelobes” about main beam

transmit upon radar sidelobe detection

• any energy received by radar system is “assumed” to have originated in main lobe

if jamming platform’s transponder can “sense” presence of a sidelobe, it can transmit strong pulse toward radar and “spoof” radar into thinking target is present in main lobe

other angular deception methods

“cross eye”

• very effective against active monopulse missile radar

• missile using active radar for guidance finds its target by aligning itself perpendicularly to wavefront of receive echo

cross eye jamming changes orientation of echo wavefront

wingtip transponders receive radar energy, one immediately retransmits, other waits to be “interrogated” by immediate transponder before it responds

transponders are essentially transmitting out of phase with one another

result is reflected waveform is “distorted” and missile will attempt to align itself with distorted wave, missing target

range deception jamming

principle victims are fire control radars

spoils fire-control solution by giving multiple targets at different ranges

jammer sends multiple copies of return echo with time delays to confuse range calculation (makes it appear further away)

expendable decoys

decoys are thruway devices designed to imitate a real target

chaff - radar countermeasure

• aluminum-coated fiberglass filaments released into atmosphere to produce larger radar cross section than the target

flare - infrared countermeasure

• chemical infrared source that mimics target signature at a higher power

reflector - radar reflector

and mobile or static off-board active transmitters

target masking and modification

“you can’t hit what you can’t see” - an undetected target has highest probability of survival

after platform shape

• use angles that reflect in directions that are not towards radar

• avoid flat, cylindrical, parabolic, or conical surfaces perpendicular to transmission

destructive interference to reflect only part of incident radiation

coat surface with a radar absorbing material

stealth: radar absorbent material

surface absorbs energy (heats up)

stealth: destructive interference

reflected waves (destructive interference) off semi-transparent coating

electronic protection (EP)

protect friendly combat capabilities against undesirable effects of enemy electronic attack

• most understand various forms of EA that radar is likely to encounter to protect against them

• must train operator to recognize various countermeasures used and to select appropriate combination of options to counter each

radar design for EP has 3 areas:

• radar parameter management

• signal-processing techniques

• radar design philosophy

radar parameter management

power

• EA is power battle- whoever is stronger and more powerful will win

frequency

• some radars are designed to change frequency, making deception and jamming difficult

pulse repetition frequency (PRF)

• changing PRF in a random fashion is effective because EA depends on constants

antenna design

• can prevent jammer or deceiver from affecting radar at many bearings (i.e. low sidelobe levels)

scan pattern

• influences amount of energy directed toward radar target (random fashion instead of circular), making it difficult to predict

electronic protection table

electro-optic spectrum

electro-optic systems operate in the optical region of the E-M spectrum, between x-rays and microwaves, and includes ultraviolet, visible, and infrared regions

• E-O in millions of MHz, in wavelength in μm

EO system units

by convention use wavelength rather than frequency

• standard unit of measurement for EO spectrum wavelength is micron

• micron is 10^-6 m (μm or μ or microns)

IR portion of Em spectrum

6 “bands” of the EO spectrum

1. UV (0-0.4 micrometers)

2. visible (0.4-0.7 micrometers)

3. near IR (0.7-3)

4. mid IR (3-6)

5. far IR (6-15)

6. extreme IR (15-20)

EO system types

visible imaging systems

• 0.4-2.0µm

• detects/amplifies visible energy

• example: night vision goggles

infrared imaging systems

• 3-14µm

• detect radiated or reflected IR

• example: thermal imagers

EO system: visual imaging systems

amplify very low levels of ambient light

EO system: IR systems

detect heat from a target

EO system vs. RADAR

ADV

• passive (covert)

• precision imaging/tracking

• VID possible - FLIR/NVG

DISADV

• susceptible to atmospheric conditions (smoke, fog, rain)

• shorter max range

• range harder to determine

radiometric quantities - radiant energy

Q

• total energy transported in form of EM waves from a source

• entire amount of energy radiated from a source during a given time interval

• unit: Joules (J)

joule = watt * second

radiometric quantities - radiant flux (power)

Φ (“phi”)

• rate of flow of radiant EM source

• energy per time radiated from a source

• equation

Φ = dQe / dt

unit: J/s = Watts (W)

radiometric quantities- radiant exitance

M

• power put per unit area leaving a surface and radiated into space

• exitance characterizes a self-emitting source that is producing energy

• equation:

M = dΦ / dA (surface area)

if power is uniform across surface, M = Φ / A

units : watts/area = W/m²

radiometric quantities- irradiance

E

• radiant power per unit area incident upon a surface

• irradiance characterizes a passive receiving surface

• equation

E = Φ / A

unit: W/m²

radiometric quantities - radiant intensity

I

• brilliance of a source of EM energy over entire eletromagnetic spectrum

• flux emitted per solid angle from an isotropic emitter (point source)

• think power density measured from a point source

• equation

I = dΦ / dΩ

Ω = solid angle = 4π sr

unit: W/sr

sr = steradian

radiometric quantities - radiance

L

• power radiated per source area per solid angle

• think power density across an extended source

• equation:

L = Φ / AΩ

L = d²Φ / dA * cosθ * dΩ

unit: W / (sr * m²)

sources of EO energy

EVERYTHING IS AN EO SOURCE

conservation of energy

when EO energy hits an object it either

• absorbs - absorptivity (α)

• reflects - reflectivity (ρ)

• passes through - transmissivity (T)

blackbodies vs. greybodies

blackbody

• an ideal (theoretical) substance that absorbs ALL incident energy and then reradiates that absorbed energy with complete efficiency

• perfect absorber, perfect radiator/emitter

• stars are approximated as blackbodies

greybody

• all other real objects

• EO energy may either absorb, reflect, or pass through

blackbody

absorptivity = reflectivity = transmissivity = EO energy

all incident energy absorbed

greybody

most objects absorb some fraction of energy and reflect the rest

no energy transmit through

reflected radiation

reflect EO radiation is important for military operations

• allows eyes to detect and differentiate between objects during the day

  • at night, however, ambient EO radiation goes down, making it difficult for humans to see without an aid (such as NVGs)

types of illumination

natural illumination

• solar illumination - the sun, our best illuminator, but is not available at night

• lunar illumination - the moon, primary illuminator at night; reflects about 7% of sunlight

• night sky illumination - at night, sky gives off near-IR energy (“air glow”) that can be useful for NVGs to intensify; starlight; auroras

artificial illumination

• lights from cities, vehicles, weapons, flares, etc.

two types of radiators

selective radiators

• emit energy as discrete wavelengths

• ex: exhaust gasses, laser energy

thermal radiators

• emit energy over a span of wavelengths

• ex: hot metal, terrain, people

light amplification (low-level-light (LLL) systems)

enhances existing light

• require some background illumination

  • do not work in total darkness

  • saturate if too much light is present

• resolution dictated by # / size of elements

• amplifies visible and near-IR light

NVDs in combat

huge tactical advantage against combatants without the technology

can communicate visual signals to friendlies covertly - IR signals, lasers

lasers

is light, a form of EM radiation

like radars, lasers can operate in pulsed or continuous wave modes (blinking vs. solid)

laser will have nearly-constant wavelength over its entire life

reflected laser energy can be used for targeting purposes

can be used for ranging (from operator)

infrared systems

targeting / ranging lasers

thermal imagers

IR seekers

thermal imagers

see both emitted radiation and reflected radiation

rely on M_contrast to discern objects

infrared sensor basics

require no visible light

detect IR energy emitted from targets

• passive (target is source)

• no direct ranging capability

• covert

limitations

• background limited - predominant source of noise is the environment

• noise limited - predominant source of source is the detector itself masking the target signal

passive IR detectors: exitance contrast

ability to discern target is based on DIFFERENCE in energy detected from target vs. background

IR imager performance

spatial resolution

• imaging system’s ability to distinguish separate objects

thermal resolution

• ability to detect small variations in temperature differences

• changes too small to be distinguished from background noise will not be detected

background limited

• lack of contrast between target and background

noise limited

• sensor’s internal heat masks signature of target

EO countermeasures

laser dazzlers (visual defense system)

IR flares

IR staring sensors

requires many detecting elements and produces output in parallel (sends entire image from each detector element simultaneously)

fast but expensive

IR scanning sensors

serial scan

• rotating mirror scans sequentially

• one piece of the picture at a time

less expensive than staring sensors

prone to mechanical failure

good when fast update rate not needed

thermal signatures: aircraft

nose cone

leading edges of wings

engine inlets and nozzles

exhaust plume

thermal signatures: armored vehicles

engine and exhaust

mechanical tracks from friction

metal body if in sun

thermal signatures: ships

engine exhaust

masts when used

metal body in sun

SONAR

sound navigation and ranging

SONAR - why?

options for finding things underwater

• RADAR

  • radio frequencies don’t propagate well in water

  • limited range and resolution

• Light (EO)

  • very limited range (10s of meters)

  • may be useful at shallow depths)

• Magnetic Signatures

  • limited range (100s of meters)

• Sound

  • speed of propagation is fast enough

  • tactically significant ranges are possible

  • while lower resolution than light, resolution is good enough for our applications

SONAR vs. RADAR 3 main similarities

• just as with RADAR (S_min), there is a min threshold that defines when SONAR contact is possible (detection threshold, DT)

• same signal losses as RADAR when SONAR energy propagates: spreading, absorption, scattering

• like RADAR, there is a set range beyond which your system (and/or operator) doesn’t register a contact because the incoming energy is less than S_min or DT

SONAR is a new sensor, but one that acts a lot like what we’ve seen already

SONAR vs. RADAR 2 major differences

• where RADAR systems use EM energy, SONAR systems use sound energy

• where RADAR systems operate in air, SONAR is used in water

two ways to exploit sound first one

active SONAR (analogous to RADAR)

two ways to exploit sound second one

passive SONAR (analogous to EO sensors)

wave propagation

sound originates as wave motion from a vibrating source and requires a medium (air or water) to move through

• two forms of wave propagation:

  • transverse - disturbance perpendicular to direction of propagation

    • E-M waves (previously discussed in RADAR)

  • longitudinal - disturbance in same direction as propagation

    • vibrations - causes series of compressions and rarefactions

    • amplitude measured at a single point will look like a sine wave

    • wave represents pressure differences around a static level

sonic concepts, 3 elements required to tactically exploit sound energy:

source - any

medium - an elastic medium (air or water)

detector/receiver

λ = v / f

speed of sound in water

v = f λ

v / c = ~1500m/s (v is speed of sound in water)

• speed of sound in water varies from point to point in ocean

• speed of propagation, frequency and wavelength depend upon 3 factors

  • temperature

  • pressure

  • salinity

temperature is single most influential factor in affecting speed of sound in water

as temperature increases,

speed increases

predominant factor affecting speed

Δ 1°C = Δ 3 m/s v

as pressure increases,

speed increases

predominant factor at very large depths

Δ 3 ft depth = Δ .017 m/s v

as salinity increases,

speed increases

Δ 1ppt of salt = Δ 1.3 m/s v

sound speed =

pressure, salinity, temperature

typical deep ocean sound velocity profile (SVP)

surface layer (shallow - changes daily)

seasonal thermocline (temp and v change with seasons)

main thermocline

• temperature decreases w/ depth, causing decrease in v

• independent of surface layer temp changes

deep isothermal layer

• temperature is constant

• pressure has most significant effect on v

sonic layer depth (SLD) → deep sound channel (DSC) (sound will get trapped in region) → critical depth → depth excess

ray propagation theory

• sound speed gradient with respect to horizontal changes of location can be assumed to be zero

  • major gradient of interest therefor is vertical (depth) gradient

• if ship at surface of ocean makes a sound, it radiates in all directions, transferring its energy from particle to particle, forming a propagating wave

  • ray is created when you draw a line from source in direction of energy propagation

• when ray enters a part of water that has different characteristics, it chantes direction and speed (bends)

Snell’s law

rays will bend when passing between 2 different mediums (refraction)

sound will always bend toward region of slower speed (sound is lazy)

tactical significance of snell’s law

why do we care how sound bends?

• we use sound to detect, localize, classify, track our TOIs, so we need to know where sound will go so we can exploit it

sound will bend toward where it can go slowest

how do we know what sound velocity profile looks like?

• historic data updated with real time readings

why do we care?

• this info will inform our tactical game plan

how do we measure speed of sound in real time?

• expendable bathythermograph (XBT)

sound propagation in isovelocity gradient

occurs in isothermal (constant temp) - constant SVP

long ranges, straight line rays with little to no angle change

sound propagation in negative gradient

temperature decreases w/ depth, causing sound speed to decrease w/ depth

sound bends down

shadow zones formed - region where sound intensity is 0 (common near sea surface)

sounds propagation in positive gradient

temperature increases w/ depth, causing sound speed to increase w/ depth

sound bends up, then reflect off sea surface, causing continued “bouncing” - causes sound to travel farther

possible shadow zones

layer depth phenomenon (positive over negative)

layer of warm isothermal water over water with a negative gradient

critical ray defines sonic layer boundary - this is where sound speed is highest (sonic layer depth)

rays split at sonic layer - one half bends up, other half bends down, forming SZ

subs use SZ to escape detection

SZ = shadow zone

sound propagation paths

1. direct path

2. surface duct

3. half channel

4. sound channel

5. convergence zone (CZ)

6. bottom bounce (BB)

direct path

simplest propagation path

straight line path btwn source and receiver with no reflections

only seen at close ranges

surface duct

positive velocity gradient - refracts sound back towards surface, where it is repeatedly reflected at surface

surface duct can produce long ranges

both source and receiver must be in sonic layer

only ray paths trapped are considered surface duct

half-channel

cold isothermal water causes positive velocity gradient from surface to bottom

essentially surface duct that exists all the way down to the bottom

• produces very long ranges

no shadow zones

sound channel

in deep ocean, negative speed gradient overlays a positive speed gradient, forming sound channel

sound rays are refracted back and forth in a horizontal channel

• longest range transmission path available

only loss is absorption

convergence zone

negative gradient over positive gradient in extremely deep water

• need >200m depth excess

sound bends intially towards greater depths due to decreasing temp, until it goes so deep that it bends upward back to surface

a CZ is typically about 50km from source

• beyond CZ is a zone of silence as all rays diffract downward, followed by another CZ about 50kms later

amount of depth excess determines zone width

bottom bounce

ray penetrates down to ocean bottom, hits, and reflects back towards surface

ray continues to bounce between surface and bottom until it dissipates

needs relatively flat, smooth, hard bottom

detection threshold (DT)

minimum received signal strength above the noise floor required for a (human) operator, using installed equipment, to detect a target 50% of the time

smaller DT = better detection ability

S - N >(or equal to) DT

passive sonar equation

S - N = DT (Source - noise = detection threshold)

S = SL - TL (source level - transmission loss)

N = NL - DI (noise level - directivity index)

Signal (SL - TL)

source level (SL)

• for PASSIVE sonar operations

  • noise generated by target (self-noise)

  • could be operating machinery, propeller noise, hull flow noise, etc.

transmission loss (TL)

• passive SONAR (like EO):

  • target puts out specific amount of sound. as it travels through the water, it is delayed, distorted, and weakened, resulting in transmission loss

  • three categories of loss: spreading, scattering, absorption

noise level (NL)

other sources of sound make it more difficult to hear targets of interest “noise” can come from 2 sources:

• your ship → self noise, NL_s (dB)

• other “stuff” → ambient noise, NL_a (dB)

NL = NL_s + NL_{a (power sum)}

kinds of noise - broadband vs. narrowband

broadband signal - gives off wide range of frequencies (think white light)

narrowband signal - gives off specific frequency/frequency ranges (think laser)

in general, you are listening for man-made (narrowband) sources; competing noise may or may not obscure your target based on the frequency/frequencies at which they broadcast

self noise, NL_s

machinery noise

• originates from propulsion plan (pumps, reduction gears, generators, etc.)

• narrowband (specific frequency)

flow noise

• caused by relative motion between an object (hull) and water around it

• broadband (wide range of frequencies)

• high speed or hull fouling (animal life on hull) causes turbulent flow which causes increased flow noise

cavitation

• caused by ships propeller generating local pressure drops where steam bubbles form and subsequently collapse causing sharp hissing noise

• broadband

ambient noise, NLa (bosh)

biological ‘

• produced by marine life

• might be narrowband (depends on frequency emitted)

ocean traffic

• at close ranges, noise is over a wide spectrum of frequencies

• at long ranges, only low frequencies are present

seismic

• movement of earth (earthquakes)

hydrodynamic

• caused by movement of water

• includes tides, current, storms, wind, rain, etc.

NL_a = B + O + S + H