LE/ESSE4360 - Payload Design - York · PDF fileLE/ESSE4360 - Payload Design ... K 18-27 GHz...

LE/ESSE4360 - Payload Design

3.4 Spacecraft Sensors - Radar Sensors

Dr. Jinjun Shan, Professor of Space Engineering

Department of Earth and Space Science and Engineering Room 255, Petrie Science and Engineering Building

Tel: 416-736 2100 ext. 33854 Email: [email protected]

Homepage: http://www.yorku.ca/jjshan

Earth, Moon, Mars, and Beyond

Radar Sensors 2Prof. Jinjun Shan

What is Radar?

n  RADAR = Radio Detection and Ranging n  RADAR is a system that uses radiowaves to detect,

determine the direction and distance and/or speed of objects such as aircraft, ships, terrain or rain and map them.

n  A transmitter emits radio waves, which are reflected by the target, and detected by a receiver, typically in the same location as the transmitter. Although the radio signal returned is usually very weak, radio signals can easily be amplified, so radar can detect objects at ranges where other emission, such as sound or visible light, would be too weak to detect.


History

n  Imaging from space – Fine resolution remote sensing of earth surface phenomena was first accomplished from space in 1960s on military and NASA satellites using cameras and camera-like image devices.

n  These early remote sensors in space were used to collect and discriminate radiated and reflected EM energy in the visible or infrared spectra (~0.4 to 20 micron).

n  In 1973, the NASA Earth Resource Technology Satellite (ERTS-1), later renamed LANDSAT, initiated a series of missions featuring fine resolution (10s of meters) optical imagers with many visible and infrared channels.

n  The LANDSATs continue operation to this day. n  Although their fine resolution and excellent multi-spectral

details, what are the drawbacks? n  Microwaves?


SEASAT Mission Parameters, June 1978

Satellite Altitude 800 kmRadar Frequency 1.275 GHz (L-band)Radar Wavelength 23.5 cmSystem Bandwidth 19 MHzTheoretical Resolution on the Surface 25 m (azimuth) x 25 m (range)Number of Looks 4Swath Width 100 kmAntenna Dimensions 10.74 m x 2.16 mAntenna Look Angle 20 degrees from verticalIncidence angle on the surface 23 degrees +/- 3 degrees across the swathPolarization Horizontal transmit, Horizontal receive (HH) Transmitted Pulse Length 33.4 microsecondsPulse repetition frequency (PRF) 1463-1640 HzTransmitted peak power 1.0 kWData recorder bit rate (on the ground) 110 Mbits/s (5 bits/word)


Space-Based Radar


Types of Space-Based Radars

n  Type I n  Small, short range rendezvous radars such

as those used on the shuttle, Apollo, and Gemini programs.

n  Type II n  Earth and planetary resources radars used

for mapping, scatterometers, altimeters, and subsurface probing.

n  Type III n  Large phased array surveillance radar

proposed for multimission defense, air traffic control, and so on.


Radar Introduction I

n  Wave generator creates a phase and amplitude of continuous wave (CW) or linear frequency modulation (LFM) type of waves.

n  An up-converter mixes a baseband signal up to the desired transmit frequency.

n  A transmitter amplifies the signal. n  An antenna directs the transmitted signal. n  The echoed signal is collected by the same antenna that runs it

to a low-noise amplifier which should be as close as possible to the front end.


n  The echoed signal is then mixed with a reference signal generated by a local oscillator to down-convert the signal to intermediate frequencies (IF) in order to fit into the bandwidth of the analog-to-digital converter.

n  A signal processing unit processes the echoed signal for target identification or range measurement. The processing level depends on the mission resources.

n  Often raw data or low-level processed data is down-linked to the ground for further processing since extensive processing time is often needed.

n  Note: The received signal is subject to many kinds of noise sources and interferences, therefore filters and amplifiers are placed in various steps to further enhance the SNR.

Radar Introduction II


n  A radiation source that is tuned to the target characteristics is radiated by an antenna that is designed to maximize the energy on a target.

n  Some of the reflected power reaches a receiver antenna that converts it to an electrical signal that is filtered, amplified and processed.

n  The signal undergoes a variety of changes in amplitude and form.

n  After the travel is complete, the signal is dramatically weakened, but still could be detected in the receiver.

Radar Introduction III


Radar Introduction IV

Pt , Gt , Lt , Pr , Gr , Lr , Lp: transmit, receiver, propagation


Frequency Bands I Band Name Frequency Range Wavelength Range Notes

HF 3-30 MHz 10-100 m coastal radar systems, over-the-horizon (OTH) radars; 'high frequency'

P < 300 MHz 1 m+ 'P' for 'previous', applied retrospectively to early radar systems

VHF 50-330 MHz 0.9-6 m very long range, ground penetrating; 'very high frequency'

UHF 300-1000 MHz 0.3-1 m very long range (e.g. ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency'

L 1-2 GHz 15-30 cm long range air traffic control and surveillance; 'L' for 'long'

S 2-4 GHz 7.5-15 cm terminal air traffic control, long range weather, marine radar; 'S' for 'short'

C 4-8 GHz 3.75-7.5 cm Satellite transponders; a compromise (hence 'C') between X and S bands; weather

X 8-12 GHz 2.5-3.75 cmmissile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the USA the narrow range 10.525 GHz ±25 MHz is used for airport radar. Named X band because the frequency was a secret during WW2.

Ku 12-18 GHz 1.67-2.5 cm high-resolution mapping, satellite altimetry; frequency just under K band (hence 'u')

K 18-27 GHz 1.11-1.67 cm

from German kurz, meaning 'short'; limited use due to absorption by water vapor, so Ku and Ka were used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.

Ka 27-40 GHz 0.75-1.11 cmmapping, short range, airport surveillance; frequency just above K band (hence 'a') Photo radar, used to trigger cameras which take pictures of license plates of cars running red lights, operates at 34.300 ± 0.100 GHz.

mm 40-300 GHz 7.5 mm - 1 mm

Millimeter band, subdivided as below. The letter designators appear to be random, and the frequency ranges dependent on waveguide size. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.

Q 40-60 GHz 7.5 mm - 5 mm Used for Military communication.

V 50-75 GHz 6.0 - 4 mm Very strongly absorbed by the atmosphere.

E 60-90 GHz 6.0 - 3.33 mm

W 75-110 GHz 2.7 - 4.0 mm used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.


Frequency Bands II

n  Ka, K, and Ku bands: very short wavelengths used in early airborne radar systems but uncommon today.

n  X-band: used extensively on airborne systems for military reconnaissance and terrain mapping.

n  C-band: common on many airborne research systems (CCRS Convair-580 and NASA AirSAR) and spaceborne systems (including ERS-1 and 2 and RADARSAT).

n  S-band: used on board the Russian ALMAZ satellite.

n  L-band: used onboard American SEASAT and Japanese JERS-1 satellites and NASA airborne system.

n  P-band: longest radar wavelengths, used on NASA experimental airborne research system.


Frequency Bands III

n  The choice of the appropriate frequency of operation is intrinsic to the mission design and target of interest nature, resolution and attenuation.

n  Once the frequency and bandwidth are determined, a certification has to be granted from the ITU.

n  Why? n  The purpose of this certification is to ensure that there is no

impact of the frequency chosen on existing radar, communication or any other sources that may use similar frequencies.

n  Interferences with other sources could be very harmful to both systems, especially for radars that are in the receiving mode. In this mode, amplifiers and filters are both active. An unanticipated high power signal in the appropriate frequency reaching the receiver can dramatically damage the receiver.


Radar Equation I

n  The amount of power Pr returning to the receiving antenna is given by the radar equation:

where Pt = transmitter power Gt = gain of the transmitting antenna Gr = gain of the receiving antenna Ar = effective aperture (area) of the receiving antenna σ = radar cross section, or scattering coefficient, of the target F = pattern propagation factor Rt = distance from the transmitter to the target Rr = distance from the target to the receiver.

222

4

)4( rt

rrttr RR

FAGGPPπ

σ=


Radar Equation II

n  In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the term Rt

2 Rr2 can be replaced by R4,

where R is the range. This yields:

n  This shows that the received power declines as the fourth power of the range, which means that the reflected power from distant targets is very, very small.

n  The equation above with F = 1 is a simplification for vacuum without interference. The propagation factor accounts for the effects of multipath and shadowing, and depends on the details of the environment. In a real-world situation, pathloss effects should also be considered.

n  Other mathematical developments in radar signal processing include time-frequency analysis, as well as the chirplet transform which makes use of the fact that radar returns from moving targets typically "chirp" (change their frequency as a function of time, as does the sound of a bird or bat).

)4(

)4( 43

2

42 RGGPP

RAGGPP rtt

rrrtt

r πσλ

πσ

==>=


Radar Equation III

n  A radar antenna receives a portion of the re-radiated signal. Antenna theory explains the relationship between the gain of a lossless antenna as related to its aperture A or effective area by

n  There are many sources of dissipation, or noise, between the sensor and the target that would lead to a degradation or attenuation of the signal. They could eventually be captured together and denoted by L where L<1. The reduced received power is thus

4 ,422 λπ

λπ r

rt

tAGAG ==

43

22

)4( RGPP t

r πλσ

=

43

22

)4( RLGPS t

πλσ

=


The Radar Range Equation I

n  There are many factors that affect radar range. n  On the transmitter side, range is affected by transmitter power,

pulse length, antenna gain and its beamwidth and pattern. n  On the receiver side, it is affected by the receiver threshold, its

noise figure and the fact that the receiver antenna is the same as the transmitter antenna.

n  The target cross section and the wavelength also affect the radar range.

n  Using the radar equation, the maximum detectible range Rmax is

n  which is determined by the minimum detectable echo signal power Psmin required by the receiver to detect and distinguish the target.

n  The detection range and most parameters in radar area are expressed in decibels (dB).

4/1

min3

22

max )4( ⎟⎟⎠

⎞⎜⎜⎝

⎛=

s

t

PGPR

πλσ


The Radar Range Equation II

n  Therefore,

n  For the design requirements, this equation by itself is not sufficient. An SNR requirement is needed in order to compute Rmax.

n  Methods to improve the SNR for a fixed radar frequency? n  A typical method to maintain similar radar resolution while

increasing the radar pulse length is to modulate the signal. FM chirps is the most popular scheme..

dBmin3

2

10dBdBdBmax10 )4(log102log40 st PGPR −⎟⎟

⎠

⎞⎜⎜⎝

⎛+++=

πλ

σ

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=

θτρλσ

πλ

sin2)4( 43

22 cLBkTfLLR

GPSNRannrt

t


Radar Pulse

n  Target range is measured by noting the time difference between the transmission of a pulse and the reception of an echo.

n  Then the range is given by

n  In order to be able to distinguish a target’s echo from each pulse without mixing responses from secondary pulses, targets should lie within the range from 0 to T/2. The range is

where Runamb is the range for unambiguity and fr is the pulse repetition frequency (PRF). The inverse of PRF is called the pulse repetition interval (PRI).

n  The range resolution δR, which is the minimum range separation of two targets, is a function of the transmitted signal bandwidth β. For a point target, we have

2/tcR Δ=

rfcR 2/unamb =


Example

n  Use the radar range equation to determine the required transmit power for the a radar given

Psmin =10-13 Watts, G=2000, λ=0.23 m, PRF=524, σ=2.0 m2


Polarization - I

n  In the transmitted radar signal, the electric field is perpendicular to the direction of propagation, and this direction of the electric field is the polarization of the wave. Radars use linear (horizontal and vertical) and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces, and help a search radar ignore rain.


Polarization - II

n  HH - horizontal transmit and horizontal receive VV - vertical transmit and vertical receive HV - horizontal transmit and vertical receive VH - vertical transmit and horizontal receive

n  The first two polarization combinations are

referred to as like-polarized because the transmit and receive polarizations are the same. The last two combinations are referred to as cross-polarized because the transmit and receive polarizations are opposite of one another.


Radar Cross Section - I

n  Radar cross section (RCS) is a description of how an object reflects an incident electromagnetic wave. For an arbitrary object, the RCS is highly dependent on the radar wavelength and incident direction of the radio wave. The usual definition of RCS differs by a factor of 4π from the standard physics definition of differential cross section at 180 degrees. Bistatic radar cross section is defined similarly for other angles.

n  Quantitatively, the RCS is an effective surface area that intercepts the incident wave and that scatters the energy isotropically in space. For the RCS, σ is defined in three-dimensions as

where σ is the RCS, Pi is the incident power density measured at the target, and Ps is the scattered power density seen at a distance R away from the target.

i

s

PPR24πσ =


Radar Cross Section - II

n  In electromagnetic analysis this is also commonly written as

where Ei and Es are the incident and scattered electric field intensities, respectively. In the design phase, it is often desirable to employ a computer to predict what the RCS will look like before fabricating an actual object.

2

224

i

s

EE

Rπσ =


Resolution - I

§  PW = Pulse Width. PW has units of time and is commonly expressed in ms. PW is the duration of the pulse.

§  RT = Rest Time. RT is the interval between pulses. It is measured in ms. §  PRT = Pulse Repetition Time. PRT has units of time and is commonly

expressed in ms. PRT is the interval between the start of one pulse and the start of another. PRT is also equal to the sum, PRT = PW + RT.

§  PRF = Pulse Repetition Frequency. PRF is commonly expressed in Hz (1 Hz = 1/s) or as Pulses Per Second (PPS). PRF is the number of pulses transmitted per second and is equal to the inverse of PRT.

§  RF = Radio Frequency. RF has units of Hz and is commonly expressed in GHz or MHz. RF is the frequency of the carrier wave which is being modulated to form the pulse train.


Resolution - II

n  Bandwidth - is a measure of frequency range and is typically measured in hertz. B = 1/PW

n  Beamwidth - The beam-width of an antenna is a measure of the angular extent of the most powerful portion of the radiated energy. For our purposes the main portion, called the main lobe, will be all angles from the perpendicular where the power is not less than ½ of the peak power. The beam-width is the range of angles in the main lobe, so defined. Usually this is resolved into a plane of interest, such as the horizontal or vertical plane. The antenna will have a separate horizontal and vertical beam-width. For a radar antenna, the beam-width can be predicted from the dimension of the antenna in the plane of interest by

where:

θ is the beam-width in radians, λ is the wavelength of the radar, and L is the dimension of the antenna, in the direction of interest

Lλ

θ =


Resolution - III

n  Radar imaging resolution is determined by the pulse length (which affects the range resolution), the beamwidth (which affects the azimuth resolution), the size of the antenna and the orbit of the platform.


Resolution - IV


Resolution - V

n  Range resolution (Cross-track)

n  Azimuth resolution (Along-track)

2cos2cos2τ

γγτ c

BccR

wr ⇒==

)sin/(/ γλλβ LHLSRSRRa =⋅=⋅=


Example

n  S/C at 1000 km, antenna size 1 m, incident angle is 30°, frequency 13 GHz, and bandwidth of 20 MHz, compute resolution.


Synthetic Aperture Radar


Introduction

n  Several methods can be used to improve the azimuth resolution.

n  Use the radar’s echo Doppler history that results from the forward motion of the spacecraft to synthesize a long antenna equal to the distance the satellite traveled during the integration time.

n  Basically, SAR uses a coherent microwave signal to synthesize a long antenna using a short one.

n  The process in principle is simple but the implementation requires a complex integrated system, spacecraft orbital position and attitude, and antenna pointing control loops.

)sin/(,cos2cos2

γλγγ

τ LHRBccR a

wr ===


SAR Principles - I (Resolution)


SAR Principles - II (Ambiguity Relationships)

n  Range Ambiguity Limit

n  Along-Track Doppler Ambiguity Limit

n  For SBR and single look case,

cRRTPRF

nfhigh /)(22

1−+

=

ATATATlow

VDV

DVPRF

δ===

22/

VcWg

WgcPRFV

ATAT

<⇒<<δδ2

2


SAR Principles - III


Example

n  For a LEO SAR satellite, determine the best along-track resolution and the aperture length in this direction given velocity of S/C 7.0 km/s, swath width requirement is 210 km.


Minimum Antenna Area for SAR

n  Minimum antenna area for SAR

n  Area Cover Rate (ACR): Product of swath width and

platform velocity

αλ

αλ tan4tan4

min ⋅=⇒⋅⋅=cRVA

cRV

PRFPRF

Alow

highR

ATgcVWACR δ⋅<⋅=2


RAR and SAR resolution

n  The key to converting theoretical groundwork into a “full-bodied” system is an appropriate signal-processing scheme.

n  Fundamental to the SAR concept is the realization that SAR is a marriage of radar and signal processing technologies.


n  Real aperture radar (RAR)

Synthetic aperture radar (SAR) n  Ambiguity conditions place certain limits on the assumption.

n  Radar range equation

n  SAR aperture are determined by the desire to achieve good along-track resolution and the need to avoid range and doppler ambiguity. Ambiguity conditions establish a lower bound on SAR antenna area.

)sin/(,cos2cos2

γλγγ

τ LHRBccR a

wr ===

2/,cos2 ATar DRcR ==

γτ

ATAT

cVWgACRWgcPRFV

δδ 22

<⋅=⇒<<

PRFcR ⋅= 2/unamb

System Design & Technology Considerations I


n  Important spacecraft systems that are significantly inf luenced by SAR include thermal (heat dissipation), data collection (the most limiting SAR support system), attitude stability, power generation, structure, and orbit maintenance (required to maintain performance altitude, especially significant for low orbits ≈ 300 km).

n  Limiting Technologies n  Analog-to-digital conversion n  On-board image processing n  On-board data recording n  Data transmission to the ground

System Design & Technology Considerations II

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