Naval Weapons Exam 1

weapon

a tool of combat which is ultimately designed with the express purpose of incapacitating personnel and/or destroying equipment

• artillery guns

• small arms

• rocket and missiles

• swords/knives

• torpedoes

• mines

• depth changes

ordnance

explosives, projectiles, fuses, propellants, etc. (as opposed to launching and fire control system)

• bullet/casing/powder

• torpedo as opposed to torpedo tube

• rocket

• missile

weapons system

a collection of components, both hardware and software, with single or multiple destructive weapons, plus the sensors and operators, which make the entire unit function as designed

• person w/ a gun and a round of ammunition that contains gunpowder and a bullet

guns

expanding gases, separate from projectile, provide propulsion

• projectile cannot move w/out external force

non-propelled, upguided

• once it’s fired, cannot change where it will go

rockets

expanding gases in projectile itself provide propulsion

• projectile produces the force needed for it to move

self-propelled, unguided projectile

missiles

guided rocket

• expanding gas in projectile itself provides propulsion, but has a known path that is has been intentionally told to travel

path determined by internal and/or external mechanisms

• some missiles allow path to be updated mid-flight (like tomahawks) while others do not (trident II D5 missiles)- steering

bombs

dropped ordnance with no self-propulsion

classified by weight

• “dumb”/”iron” are unguided

• “smart” are guided

torpedo

self-propelled, guided underwater explosive

heavyweight (submarines), lightweight torpedoes (p-8)

mines

self-contained, underwater explosives designed to be placed and remain until contact with an enemy ship

detonation occurs with contact, magnetic field interaction and/or pressure transient

detect to engage sequence

three distinct phases

• detection

  • target detection

  • localization

  • classification

• tracking

  • target tracking

• engagement

  • weapon selection

  • target neutralization

detection

1. surveillance and detection

• search a predetermined area for a target and detect its presence

• accomplished actively or passively

  • actively: sending energy out, waiting for it to bounce off something and return (radar)

  • passively: receiving energy being emitted by a target (passive sonar)

2. localizing the target’s position

• determine a target’s range, bearing, and depth or elevation

3. classify the target

• estimate the target’s type, number size, identity

tracking

to successfully engage a target, we need continuously updated position and velocity of target (where is it going, how fast)

tracking allows us to:

• evaluate how much of a threat the target is

• predict target’s future position

  • determines intercept point for our weapon to ensure weapon is accurately aimed

engagement

effective engagement and neutralization of the target requires that a destructive mechanism be delivered to the target’s vicinity or impact the target

must consider aiming, launch, type of propulsion system of the weapon, and forces subjected to en route

1. detect

2. localize

3. classify

4. track

5. weapon selection

6. neutralize

electromagentic (EM) wave generation

characteristics

• transverse

• no medium

accelerated charged particles

• electric field includes magnetic field

• magnetic field induces electric field

two types of waves

mechanical

• requires medium for propagation (SONAR)

electromagnetic

• requires no medium for propagation

two types of motion

longitudinal

• motion of medium moves from left to right

transverse

• motion of medium moves up and down

frequency

(f )

rate at which the source oscillates through one complete cycle- cycles per second (Hertz or 1/sec)

wavelength

(λ)

distance between 2 identical points on adjacent waves or distance traveled by wave in one cycle (cm, mm, m)

velocity

( c)

speed that wave travels

c = (λ)(f)

(for EM energy in vacuum, c = 3×10^8 m/s)

amplitude

(a)

maximum displacement of wave from zero (cm, mm, m)

period

(T)

time to complete one cycle (sec)

T = 1/f

spherical wave (wave propagation)

undisturbed wave

omni directional from source

only works as a model when close to the source

think about ripples on a pong

plane wave (wave propagation)

comes into effect far from the source

spreads out to appear to have same amplitude everywhere on plane perpendicular to direction of travel

think of entire wave traveling in one direction

phase shift (delta phi)

identical sinusoidal waves that’re shifted either ahead or behind due to distance separations or time delay

measured in either degrees or radians

radians = (2pi/360) x degrees

degrees = (360/2pi) x radians

advanced =

positive phase shift

retarded =

negative phase shift

principle of superposition

overlapping waves add algebraically to produce a resultant wave

happens when 2 or more waves pass simultaneously through the same region

reflection

when an electromagnetic wave encounters a conducting surface of energy from that surface occurs

specular reflection

smooth surface reflects at equal but opposite angle

diffuse reflection

irregular surface reflects over a broad range of angles

refraction

the bending of an electromagnetic ray when it transmits from one medium to another

• when a wave strikes a boundary/surface, not all of wave is reflected

  • some is transmitted into medium interface at some angle

• the angle can be determined using snell’s law

• n1 sinθ1 = n2 sinθ2

(n = index of refraction for that medium)

refractive index

n = c0 / c

c0 = speed of light (in vacuum)

c = speed of light (in medium)

c = fλ

bend more towards medium with higher index of refraction (waves are lazy)

diffraction

causes plane waves traveling in a straight path to bend around a boundary or obstruction

• allows radar waves to see “behind” an object like a mountain

also refers to the phenomenon of a wave passing thru an opening (aperture) and spreading out from that opening

• amount wave spreads depends on size of the opening

interference

if 2 waves with same frequency are combined, it will cause an interference pattern

• interference can be constructive or destructive

constructive interference

overall field strength has increased (the two wave points get added together)

destructive interference

the overall field strength has decreased (the two wave points get subtracted

electromagnetic signal loss

spreading

• energy distributed over increasingly larger area (energy) at a point has to go down as the area increases

scattering

• energy bouncing off suspended particles within a medium

absorption

• energy dissipated into medium (molecules of medium absorb some of the energy as it passes thru)

scattering and absorption both cause attenuation (loss) of energy

modes of propagation

ground waves (<30kHz)

• vertically polarized waves, travel between ground and ionosphere

  • very long range is possuble

  • ELF, VLF, LF, can be MF

sky waves (30kHz-30MHz)

• waves refracted back to the earth by inososphere

  • changes btwn night/day due to atmospheric conditions

  • travel far due to refraction and relfection

  • MF, HF, can be LF

space waves (>30MHz)

• signal frequency is too high to be refracted by ionosphere

  • short range comms

  • VHF, UHF, SHF, EHF

max range line of sight (LOS)

at earth’s surface, radar line-of-sight horizon is the maximum allowable separation (R) based on transmitter (h_t) and receiver (h_r) height

• radio waves bend

• optical waves don’t bend (much)

magnetic fields induce electric fields

electric fields induce magnetic fields

fields are perpendicular to each other

optimum dipole length (antenna length) for free space antenna is λ/2

optimum length for grounded antenna is λ/4 (reflection from ground)

fine the optimum antenna size for a ground-plane (quarter wave) dipole used to broadcast commercial radio (~1MHz)

λ/4 f=10^6 Hz C = fλ → c/f = λ

3×10^8m/s / 10^6 1/sec = (cross out sec) = 300m

λ/4 = 300/4= 75m

polarization

antenna is said to be polarized in the direction of the electric field (electric field is parallel to dipole axis)

when electric lines of force are horizontal,

the wave is horizontally polarized

if transmitting antenna is close to the ground,

vertically polarize

if transmitting antenna is high above the ground,

horizontally polarize

for most efficient communication (polarization),

transmit and receive antennas should have same polarization

circular polarization is used for satellite comms to eliminate the needy to try to match the receiving antenna to the orientation of the satellite

DIPOLE Antenna Distribution

energy radiates out the sides most strongly at the perpendicular and decreases in intensity such that no energy comes out the ends

results in donut shape, with antenna at “hole”

due to this, dipole antenna have some preferred direction (directionality)

Directional Antennas

antennas can be made even more directional via beamforming

directionality allows us to be more efficient with our energy (send it where we want it to go)

two ways to achieve directionality

antenna arrays

quasi-optical reflectors

bea forming

for radar systems, its a desirable to concentrate radiated energy into directional beam (flashlight)

this can “illuminate” a specific area in space to determine location from which reflected energy is returned

this helps us to know where someone is

Antenna Arrays

linear antenna arrays are used to concentrate radar energy to obtain directivity

• arrange two or more antennas in such a way that their fields add together in specific directions and cancel in others

  • form of interference

• two most common types of linear arrays are broadside and endfire arrays

difference between broadside and endfire arrays

what axis the constructive interference occurs on, which determines where the field is strongest (what direction the wave will travel in)

endfire and broadside arrays picture

linear arrays- directional beams

beamwidth can be predicted theoretically: θ = kλ/L

θ= beamwidth (radians)

k = depends on antenna shape; 0.88 for linear antenna

λ = wavelength

L = overall length of array

narrow beamwidth is desirable in order to improve resolution and therefore target identification and weapon targeting. if USAF C130 gunship requires new targeting radar with 2.2 Ghz frequency to have a beamwidth of .005 radians

with beam width, θ=kλ/L, what would be overall length of linear radar antenna array?

1st. find wavelength

λ= c/f = 3×10^8/2.2×10^9 = 0.136m

2nd. apply equation to beamwidth

θ = kλ/L → 0.005rad → 0.88 × 0.136m / L

L = 24m

quasi-optical reflectors

reflecting EM waves can be used to narrow the beamwidth, which increases directivity

• parabolic reflectors primarily used

• reflectors are made of sheet metal or wire mesh

Parasitic Reflector

broadside and endfire arrays are still not optimum because their main beams go in 2 directions

parasitic element is a conductor that creates EM wave that will somewhat cancel on one side of the antenna and reinforce on the opposite side

beam width target position accuracy

wide beam width

target can maneuver within beam without noticeable bearing/alt change

beam width target position accuracy

narrow beamwidth

target positional accuracy increased dramatically. any movement results in new bearing/alt

interference and 1-d arrays

single dipole radiation pattern

interference and 1-d arrays

2-dipole radiation pattern:

green = constructive interference

interference and 1-d arrays

10-dipole radiation pattern

from 1-d to 2-d arrays

1-dimensional linear arrays can only focus the main lobe in azimuth or elevation

it takes a 2-dimensional array to focus a beam in both azimuth and elevation

moving the beam: MSA vs. ESA

MSA: mechanically steered array (physically moving array)

ESA: electronically steered array (“phased array”)

antenna array beams can be steered either way

phased array antennas=

electronic scanning

SPY-1A

electronic scanning adv and disadv

ESA advantages

• beam can move instantaneously

• fewer moving parts to break

• high data rates

• instantaneous beam positioning

• no mechanical breakdown

• increased flexibility

  • “multimode” operation

    • air/sea search

    • fire control

• multi-target tracking

ESA disadvantages

• lost efficiency at high angles off array boresight

steer beaming main point

for an array (bunch of emitters spread out in real physical space) you can adjust each angle to cause destructive or constructive interference at different spots or in 1 direction

if you change delay, the emitter might be more spread out

RADAR

radio detection and ranging

extend the environment beyond our own eyes and ears which therefore extends detection ability

adv of RADAR

• can see through conditions that impair visual detection (night, fog, clouds)

• longer than visual LOS range

• gives accurate measurement of range and relative motion (more so than just eyes)

• gives accurate measurement of azimuth and elevation

• accurate info fed into fire control systems much more efficiently than human capable of doing

disadv of RADAR

• poor target resolution (2 targets can look like one if close together)

• poor ID capability

basic pulsed radar

most widely used technique, “conventional” radar

“on” for a short period and “off” for a long period

based on electromagnetic pulse that’s sent out into the environment and waited on for ‘echo’ (reflection)

utilizes speed of light to measure range of echo

pulse repetition time (PRT)

elapsed time btwn beginning of one pulse transmission and beginning of the next

must be long enough to allow echo pulse to return from system’s maximum range

• this is to avoid missing an echo due to it being drowned out by next pulse

we commonly use pulse repetition frequency (PRF), expressed in Hz, instead of PRT- number of pulses per second   

PRF = 1/PRT

pulse width (PW)

active transmit time or duration of the EM pulse (μsec)

minimum range at which target can be detected is determined by pulse width

rest time (RT)- nontransmit time

• interval btwn pulses

pulse transmission relationships

pulse repetition time (PRT) = PW + RT

pulse repetition frequency (PRF) = 1/PRT

duty cycle = (PW)(PRF) = PW/PRT - Pavg/Ppeak

pulse transmission example problem

a 50 MHz RADAR has rest time of 18 milliseconds and pulse width of 2 milliseconds. calculate duty cycle

PRT = PW + RT = 2ms + 18ms

duty cycle =

PW / PRT = 2ms / 20ms = 0.1

basic radar components

transmitter

receiver

power supply

synchronizer

duplexer

antenna

display

transmitter

generates high-power pulses under control of a timer

receiver

converts incoming EM waves into electrical signals to be used by processor

power supply

furnishes all electrical voltages required for operation and interaction of system components

synchronizer

timer that supplies controlling signals to determine timing of the transmitted pulses

duplexer

switch that allows antenna to go from transmitting to receiving

antenna

receives radio frequency energy from transmitter and radiates it in a directional beam

display

produces visual indications of received pulses

basic pulse RADAR components

minimum range

while transmitting, RADAR is unable to listen for echo

if target’s echo returns while RADAR transmitting, target is undected

range resolution

minimum distance btwn 2 targets, in direction of range, resolved

max unambiguous range, R_unamb

max range a target can be that provides accurate calculation of range

beyond R_unamb, target returns arrive after subsequent pulses are sent out such that RADAR uses the wrong ∆t to calculate range

unambiguous return, ambiguous return

return of echo prior to transmittal of next pulse (w/ unambiguous range)

echo from first pulse arrives after 2nd pulse sent

R_unamb in real time

a tanker at R_unamb — at the distance where its return arrives just as the next pulse is sent

exceeding R_unamb

tanker is so far away that its return arrives AFTER subsequent pulse is sent

calculated range will be much closer than actual range, since RADAR is using the wrong (∆t )

PRT / PRF

PRT (μs) — time delay btwn successive pulses

PRF )Hz) —1/ PRT

lower PRT (higher PRF) gives

• more updates per second (good)

• higher probability of detecting targets (good)

• reduced R_unamb (bad)

PRT / PRF fire control RADARS and search RADARS

fire control radars

• use high PRT for fast update rates

search radars

• use low PRF

• compensate through slower scan or wider beam

HIGH PRF will improve resolution BUT decrease R_unamb

beamwidth vs. accuracy

search radar

• wide beamwidths

• sacrifice accuracy

targeting radar

• narrow beamwidths

• more accuracy

pulse radar effectiveness three key ranges

R_max

• assumes nothing blocks propagation

• depends on RADAR characteristics, target characteristics, spreading

R_LOS

• assumes signal is strong enough to propagate out and back

• depends on receiver height, target height, curvature of earth and bending of radio waves

R_unamb

• dependent on PRF only

pulsed radar effectiveness, in order to receive a detectable return:

R ≤ R_max

R ≤ R_LOS

pulsed radar effectiveness in order to get range to target correct:

R ≤ R_unamb

signal reception

the return energy from a distant target is only a fraction of the transmitted energy

• therefore, the function of a radar receiver is to receive weak target returns and amplify them to provide the desired target info

the effective range of a radar is therefore correlated to how weak of a signal the receiver can use

THE WEAKER THE SIGNAL THE RECEIVER CAN PROCESS, THE GREATER THE EFFECTIVE RANGE

“simplified” range equation

R_max = max range of radar system

P_xmit = transmitted power

G = power gain

σ = radar cross section of target

A_e = antenna effective area

S_min = minimum signal for detection

transmitter power

high peak power is desirable to achieve max ranges

but lower power supports being covert

sometimes power is a design parameter

sometimes power is a design constraint

→ …power output @ high-power amplfier → P = P_xmit

power out

if pulse radiates uniformly in all directions, the power density at any given point would be:

“power density”