LE/ESSE Payload Design

Transcription

1 LE/ESSE Payload Design 3.4 Spacecraft Sensors - Radar Sensors Earth, Moon, Mars, and Beyond Dr. Jinjun Shan, Professor of Space Engineering Department of Earth and Space Science and Engineering Room 255, Petrie Science and Engineering Building Tel: ext jjshan@yorku.ca Homepage:

2 What is Radar? RADAR = Radio Detection and Ranging 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. 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. Radar Sensors 2

3 History 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. 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). 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. The LANDSATs continue operation to this day. Although their fine resolution and excellent multi-spectral details, what are the drawbacks? Microwaves? Radar Sensors 3

4 SEASAT Mission Parameters, June 1978 Satellite Altitude 800 km Radar Frequency GHz (L-band) Radar Wavelength 23.5 cm System Bandwidth 19 MHz Theoretical Resolution on the Surface 25 m (azimuth) x 25 m (range) Number of Looks 4 Swath Width 100 km Antenna Dimensions m x 2.16 m Antenna Look Angle 20 degrees from vertical Incidence angle on the surface 23 degrees +/- 3 degrees across the swath Polarization Horizontal transmit, Horizontal receive (HH) Transmitted Pulse Length 33.4 microseconds Pulse repetition frequency (PRF) Hz Transmitted peak power 1.0 kw Data recorder bit rate (on the ground) 110 Mbits/s (5 bits/word) Radar Sensors 4

5 Space-Based Radar Radar Sensors 5

6 Types of Space-Based Radars Type I Small, short range rendezvous radars such as those used on the shuttle, Apollo, and Gemini programs. Type II Earth and planetary resources radars used for mapping, scatterometers, altimeters, and subsurface probing. Type III Large phased array surveillance radar proposed for multimission defense, air traffic control, and so on. Radar Sensors 6

7 Radar Introduction I Wave generator creates a phase and amplitude of continuous wave (CW) or linear frequency modulation (LFM) type of waves. An up-converter mixes a baseband signal up to the desired transmit frequency. A transmitter amplifies the signal. An antenna directs the transmitted signal. 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. Radar Sensors 7

8 Radar Introduction II 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. A signal processing unit processes the echoed signal for target identification or range measurement. The processing level depends on the mission resources. Often raw data or low-level processed data is down-linked to the ground for further processing since extensive processing time is often needed. 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 Sensors 8

9 Radar Introduction III 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. Some of the reflected power reaches a receiver antenna that converts it to an electrical signal that is filtered, amplified and processed. The signal undergoes a variety of changes in amplitude and form. After the travel is complete, the signal is dramatically weakened, but still could be detected in the receiver. Radar Sensors 9

10 Radar Introduction IV P t, G t, L t, P r, G r, L r, L p : transmit, receiver, propagation Radar Sensors 10

11 Frequency Bands I Band Name Frequency Range Wavelength Range Notes HF 3-30 MHz 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 MHz m very long range, ground penetrating; 'very high frequency' UHF MHz m very long range (e.g. ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency' L 1-2 GHz cm long range air traffic control and surveillance; 'L' for 'long' S 2-4 GHz cm terminal air traffic control, long range weather, marine radar; 'S' for 'short' C 4-8 GHz cm Satellite transponders; a compromise (hence 'C') between X and S bands; weather X 8-12 GHz cm missile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the USA the narrow range GHz ±25 MHz is used for airport radar. Named X band because the frequency was a secret during WW2. K u GHz cm high-resolution mapping, satellite altimetry; frequency just under K band (hence 'u') K GHz cm K a GHz cm mm GHz 7.5 mm - 1 mm from German kurz, meaning 'short'; limited use due to absorption by water vapor, so K u and K a 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 ± GHz. mapping, 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 ± GHz. 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 GHz 7.5 mm - 5 mm Used for Military communication. V GHz mm Very strongly absorbed by the atmosphere. E GHz mm W GHz mm used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging. Radar Sensors 11

12 Frequency Bands II Ka, K, and Ku bands: very short wavelengths used in early airborne radar systems but uncommon today. X-band: used extensively on airborne systems for military reconnaissance and terrain mapping. C-band: common on many airborne research systems (CCRS Convair-580 and NASA AirSAR) and spaceborne systems (including ERS-1 and 2 and RADARSAT). S-band: used on board the Russian ALMAZ satellite. L-band: used onboard American SEASAT and Japanese JERS-1 satellites and NASA airborne system. P-band: longest radar wavelengths, used on NASA experimental airborne research system. Radar Sensors 12

13 Frequency Bands III The choice of the appropriate frequency of operation is intrinsic to the mission design and target of interest nature, resolution and attenuation. Once the frequency and bandwidth are determined, a certification has to be granted from the ITU. Why? 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. 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 Sensors 13

14 Radar Equation I The amount of power P r returning to the receiving antenna is given by the radar equation: where P P t = transmitter power 4 t t r r ( 4π ) Rt Rr G t = gain of the transmitting antenna G r = gain of the receiving antenna A r = effective aperture (area) of the receiving antenna σ = radar cross section, or scattering coefficient, of the target F = pattern propagation factor r = PG G A σf R t = distance from the transmitter to the target R r = distance from the target to the receiver. Radar Sensors 14

15 Radar Equation II In the common case where the transmitter and the receiver are at the same location, R t = R r and the term R 2 t R 2 r can be replaced by R 4, where R is the range. This yields: P r PG t tgr Arσ = 2 4 (4π ) R => PG t tg (4π ) 2 rλ σ 3 4 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. 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. 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). P r = R Radar Sensors 15

16 Radar Equation III 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 4πA t 4πA G t =, G 2 r = 2 λ λ 2 2 σpt λ G Pr= 3 4 ( 4π ) R 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 σpλ G S = L 2 2 t 3 4 ( 4π ) R r Radar Sensors 16

17 The Radar Range Equation I There are many factors that affect radar range. On the transmitter side, range is affected by transmitter power, pulse length, antenna gain and its beamwidth and pattern. 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. The target cross section and the wavelength also affect the radar range. Using the radar equation, the maximum detectible range R max is R max = 1/ σp 3 ( 4 ) tλ G π Psmin which is determined by the minimum detectable echo signal power P smin required by the receiver to detect and distinguish the target. The detection range and most parameters in radar area are expressed in decibels (db). Radar Sensors 17

18 The Radar Range Equation II Therefore, (4π ) 2 λ 40 log10 Rmax = σ db + Pt db + 2GdB + 10 log 10 P 3 smindb For the design requirements, this equation by itself is not sufficient. An SNR requirement is needed in order to compute R max. PG t SNR = 3 (4π ) R λ L L t r σ ktn B Methods to improve the SNR for a fixed radar frequency? f 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.. n ρλ cτ La 2sinθ Radar Sensors 18

19 Radar Pulse Target range is measured by noting the time difference between the transmission of a pulse and the reception of an echo. Then the range is given by R = cδt / 2 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 R c / 2 unamb = where R unamb is the range for unambiguity and f r is the pulse repetition frequency (PRF). The inverse of PRF is called the pulse repetition interval (PRI). 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 f r Radar Sensors 19

20 Example Use the radar range equation to determine the required transmit power for the a radar given P smin =10-13 Watts, G=2000, λ=0.23 m, PRF=524, σ=2.0 m 2 Radar Sensors 20

21 Polarization - I 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. Radar Sensors 21

22 Polarization - II HH - horizontal transmit and horizontal receive VV - vertical transmit and vertical receive HV - horizontal transmit and vertical receive VH - vertical transmit and horizontal receive 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 crosspolarized because the transmit and receive polarizations are opposite of one another. Radar Sensors 22

23 Radar Cross Section - I 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. 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 Ps σ = 4πR 2 P where σ is the RCS, P i is the incident power density measured at the target, and P s is the scattered power density seen at a distance R away from the target. i Radar Sensors 23

24 Radar Cross Section - II In electromagnetic analysis this is also commonly written as σ = 4πR 2 where E i and E s 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. E E s i 2 2 Radar Sensors 24

25 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. Radar Sensors 25

26 Resolution - II Bandwidth - is a measure of frequency range and is typically measured in hertz. B = 1/PW 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 θ = λ L 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 Radar Sensors 26

27 Resolution - III 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. Radar Sensors 27

28 Resolution - IV Radar Sensors 28

29 Resolution - V Range resolution (Cross-track) R r = c τ 2cos γ = 2B w c cos γ c τ 2 Azimuth resolution (Along-track) R = SR β = SR λ / L = Hλ /( Lsinγ ) a Radar Sensors 29

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

31 Synthetic Aperture Radar Radar Sensors 31

32 Introduction cτ c Rr = =, Ra = 2cosγ 2B cosγ Hλ /( Lsinγ ) Several methods can be w used to improve the azimuth resolution. 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. Basically, SAR uses a coherent microwave signal to synthesize a long antenna using a short one. The process in principle is simple but the implementation requires a complex integrated system, spacecraft orbital position and attitude, and antenna pointing control loops. Radar Sensors 32

33 SAR Principles - I (Resolution) Radar Sensors 33

34 SAR Principles - II (Ambiguity Relationships) Range Ambiguity Limit PRF high = 2T + 1 2( R f R n ) / c Along-Track Doppler Ambiguity Limit PRF low = D V AT / 2 = 2V D AT = V δ AT For SBR and single look case, V δ AT < PRF < c 2Wg 2Wg δ AT < c V Radar Sensors 34

35 SAR Principles - III Radar Sensors 35

36 Example For a LEO SAR satellite, determine the best alongtrack resolution and the aperture length in this direction given velocity of S/C 7.0 km/s, swath width requirement is 210 km. Radar Sensors 36

37 Minimum Antenna Area for SAR Minimum antenna area for SAR A R = PRF PRF high low 4VλR c tanα A min = 4VλR c tanα Area Cover Rate (ACR): Product of swath width and platform velocity ACR = W g V < c 2 δ AT Radar Sensors 37

38 RAR and SAR resolution The key to converting theoretical groundwork into a fullbodied system is an appropriate signal-processing scheme. Fundamental to the SAR concept is the realization that SAR is a marriage of radar and signal processing technologies. Radar Sensors 38

39 System Design & Technology Considerations I Real aperture radar (RAR) cτ c Rr = =, Ra = 2cosγ 2B cosγ Synthetic aperture radar (SAR) w cτ R r =, Ra = D 2cosγ Hλ /( Lsinγ ) / 2 Ambiguity conditions place certain limits on the assumption. V c c < PRF < ACR = Wg V < δ δ 2Wg 2 AT AT AT Radar range equation R unamb = c / 2 PRF SAR aperture are determined by the desire to achieve good alongtrack resolution and the need to avoid range and doppler ambiguity. Ambiguity conditions establish a lower bound on SAR antenna area. Radar Sensors 39

40 System Design & Technology Considerations II Important spacecraft systems that are significantly influenced 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). Limiting Technologies Analog-to-digital conversion On-board image processing On-board data recording Data transmission to the ground Radar Sensors 40