Week 1

Question 1

Surveillance radar

Answer 1

Scans the horizon looking for targets

Air traffic control, air defense, ships

Question 2

Tracking radar

Answer 2

Locks onto target and follows it

Question 3

Law enforcement radar

Answer 3

Measures vehicle velocity

Question 4

Radar acronym

Answer 4

RAdio Detection And Ranging

Question 5

A basic type of radar can

Answer 5

Detect the presence of a target

Measure the range

Find the azimuth and elevation

Question 6

Range

Answer 6

The distance between the radar and the target

Question 7

Azimuth and Elevation

Answer 7

The direction to the target using a directional antenna

Question 8

Synthetic aperture radar

Answer 8

Maps the ground with high resolution for earth resources and military purposes

Question 9

Weather radar

Answer 9

Ground based - maps precipitation over wide area

Aircraft based - warns of dangerous rain areas

Question 10

Radar Applications

Answer 10

Ground probing - looking for land mines
Movement sensing - security, door openers
Terrain avoidance - low flying military aircraft
Radar altimeter - accurate measurement of aircraft height (essential for blind landing)
Space radar - detect satellites, debris, space weather
Autonomous vehicles - key sensor technology (lidar - light radar)

Question 11

Radar definition

Answer 11

An electrical system that transmits RF EM waves towards a region of interest and receives and detects these EM waves when reflected from objects in that region

Question 12

Pulse radar

Answer 12

Sends out a short burst of RF energy

For a point target, the received echo is spread in time by the width of the pulse.
The echo from the point target has the appearance of the transmitted pulse but is greatly reduced in magnitude

Series of pulses, typically 10 to 50

Standard radar processing integrates over all the pulses and echoes in the series to produce a single pulse-echo combination

Question 13

Mono static radar

Answer 13

TX and RX are co-located and often share an antenna

Question 14

Bistatic radar

Answer 14

Tx and Rx are in separate locations

Question 15

Tx power

Answer 15

kW to MW

Question 16

Rx power

Answer 16

nW levels and lower

Question 17

Measuring range

Answer 17

Measure time for radar pulse to go to target and back

Radar measures time from transmitted pulse to received echo

Question 18

Pulses repetition interval

Answer 18

Tp

Time between pulses

Question 19

Range Ambiguity

Answer 19

Pulsed radars process the echoes (or returns) from many pulses collectively

When a successive pulse is transmitted before the echo from the previous pulse is received, the range measurement for the echo will be wrong

Question 20

Measured range is correct only for

Answer 20

R < Rua

Rua - maximum unambiguous range

Question 21

Range resolution

Answer 21

If two targets are close together, the echoes will overlap in time —> the radar will report a single target

Minimum spacing at which two targets can be separated by radar

Point targets can be separated provided their echoes are received with time delay greater than one pulse width

Question 22

If we want to resolve targets that are closer

Answer 22

We must use a shorter pulse

Question 23

CW radar

Answer 23

Sends out a continuous wave of RF energy

Tx and Rx operate continually

Because of the continual transmission, a mono static CW radar must be low power (to protect the receiver) —> limits CW radars to near range operations

Police radars

Question 24

Can we measure the range to a target with a CW radar

Answer 24

No, not if we use a simple waveform

Question 25

Radar transmitters are rated by

Answer 25

Pulse power

Question 26

Pulse power

Answer 26

Pt - rms output power of the transmitter while it transmits a pulse

Question 27

Time-range radar correspondences

Answer 27

1 ms - 150 km 1 us - 150 m 1 ns - .150 m = 15 cm

Question 28

Early radars operated in the bands:

Answer 28

HF: 3 to 30 MHz VHF: 30 to 300 MHz UHF: 300 to 1000 MHz

Question 29

Most radars today operate at

Answer 29

Microwave frequencies | - 1000 to 100,000 MHz = 1 GHz to 100 GHz

Question 30

HF Band

Answer 30

3 to 30 MHz

Question 31

VHF band

Answer 31

30 to 300 MHz

Question 32

UHF band

Answer 32

300 to 1000 MHz

Question 33

L-band

Answer 33

1-2 GHz

Question 34

S-band

Answer 34

2 to 4 GHz

Question 35

C-band

Answer 35

4 to 8 GHz

Question 36

X-band

Answer 36

8 to 12 GHz

Question 37

Ku-band

Answer 37

12 to 18 GHz

Question 38

K-band

Answer 38

18 to 27 GHz

Question 39

Ka-band

Answer 39

27 to 40 GHz

Question 40

W-band

Answer 40

75 to 110 GHz

Question 41

MHz / m radar systems

Answer 41

f[MHz]λ[m] = 300

Question 42

GHz / cm radar systems

Answer 42

f[GHz]λ[cm] = 30

Question 43

Motion of a wave

Answer 43

In the direction of the wave vector k, k = 2π/λ [rad/m]

Question 44

Coherent radar

Answer 44

Detect the amplitude of returned signal Provides info on the phase of the returned signal, measured relative to the transmitted signal LO, STALO - listening time

Question 45

Stable local oscillator

Answer 45

LO, STALO Used to generate the transmit signal and to process the returned signal

Question 46

Pulse coherent radar

Answer 46

Phase ca be measured on the signal returned from successive pulses Can measure the difference in phase between the echo and the LO reference If the phase changes between pulses, gives rise to Doppler shift

Question 47

If the phase is changing between pulses

Answer 47

The distance to the target is changing (target is approaching or receding)

Question 48

Doppler shift

Answer 48

``` ωd = dφm/dt fd = ωd/2π = 2vr/λ ```

Question 49

Sign of the Doppler shift

Answer 49

Positive for motion toward radar Negative for away

Question 50

Resolving the Doppler frequency

Answer 50

Ambiguity can arise If phase changes by more than π rad between pulses, can’t tell where phase is increasing or decreasing If phase change is 2π between pulses, then dφ/dt and the apparent speed is zero Must sample at least twice as fast as fd -sample the phase once per pulse (sampling rate = PRF)

Question 51

Doppler shift / velocity measurement only correct for

Answer 51

|fd| < fd_max | |vr| < vr_max

Question 52

Ambiguity and PRF regimes

Answer 52

Resolving an ambiguity in range - lower PRF Resolving an ambiguity in Doppler shift or velocity - higher PRF

Question 53

Low PRF

Answer 53

Unambiguous in range Ambiguous in Doppler shift / velocity

Question 54

High PRF

Answer 54

Ambiguous in range Unambiguous in Doppler shift / velocity

Question 55

Medium PRF

Answer 55

Ambiguous in both range and Doppler shift/velocity

Question 56

Radar range equation

Answer 56

Calculates the power received from a target at a given range Calculates the max range at which a target of known radar reflectivity can be detected

Question 57

Gain generally refers to

Answer 57

The gain in the direction of peak antenna performance

Question 58

Radar cross section

Answer 58

RCS, expresses the relationship between the power density incident on the target and the power density received at the antenna from the target as a result of reflection (scattering)

Question 59

Power density at the radar receiver

Answer 59

Target has become its own transmitter, an isotropic radiator with Prefl = Pint

Question 60

Effective collective area

Answer 60

Receiving antenna scoops radio wave power out of the air over this area

Question 61

Smin

Answer 61

Minimum power needed in the receiver to reliably detect the target Usually determined by noise in the receiver

Question 62

Max range of detection of a given target is a measure of

Answer 62

Radar sensitivity

Question 63

Losses in a radar

Answer 63

Propagation effects, losses in waveguides and components May be as high as 20 dB Max range of the radar will be (10^-20/10)^1/4 = 31.6% of the range predicted by the idealized equation