Synthetic aperture RADAR (SAR) principles/instruments October 31, 2018

Transcription

1 GEOL 1460/2461 Ramsey Introduction to Remote Sensing Fall, 2018 Synthetic aperture RADAR (SAR) principles/instruments October 31, 2018 I. Reminder: Upcoming Dates lab #2 reports due by the start of next week s lecture lab #3 is two week s from tonight (Nov. 14 th ) three weeks (Nov. 21 st ): no class (Thanksgiving Holiday) II. Radar Principles very large topic o we will only touch on the basic principles here very long history o around since WWII o also known as microwave remote sensing basics o the microwave atmospheric window is nearly 100% clear o now examining much longer wavelengths: cm to m scale o primarily an active form of remote sensing o energy return is dominated by surface roughness and measured as a function of the travel time of the radar pulse radar components o many components are similar to other remote sensing systems o differences are pulse generator, duplexer, and the antenna o duplexer controls the timing of the pulse release and reception benefits o all time / all-weather capability

2 o information on surface roughness at the human scale centimeters rather than microns o great deal of sensor development and technology o deeper penetration of soil function of the dielectric constant rule of thumb is that for dry soils, penetration depth (cm) = 10λ for hyper-arid environments, radar can penetrate 3-5 meters o vegetation may or may not be a concern limitations o very costly o imagery is complex and typically hard to interpret o little to no information on composition of the surface materials microwave atmospheric window radar bands o radar pulses are sent and received in discrete wavelength regions (designated with letters) o controlled by the federal government so as not to interfere with commercial broadcasting and emergency frequencies o most commonly used: Ka-band: cm (1.0 cm) used in local news weather forecasting C-band: cm (5.3 cm) L-band: cm (23.5 cm) o less common: X-band: cm (3.0 cm) S-band: 12 cm used on the Magellan radar instrument that mapped Venus P-band: cm (68 cm) airborne X-band (Harrisburg, PA)

3 Two primary radar modes 1. passive same principles as emitted energy in the Thermal IR however, energy is a function of the surface dielectric constant as well as the temperature the dielectric constant is greater for metals and soils with higher moisture content 2. active most common form of radar remote sensing ~ 90% of all data collected known as SLAR (side-looking airborne radar) SLAR can be either real aperture radar (RAR) or synthetic aperture radar (SAR) active remote sensing controls the source as well as the data collection resolution is primarily a function of the radar antenna length and the pulse length of the energy sent SLAR o most common form of radar o energy is transmitted and received by an antenna looking off at an angle to the surface typically mounted to the side of a plane s fuselage for airborne systems side-looking geometry affects how the signal interacts with the surface also causes unique geometric distortions that must be corrected o nadir-viewing radar systems are known as radar altimeters used for mapping topography we won t examine these here terminology (see figure on next page): 1. ground range (Rg) = distance away from the nadir point (perpendicular to the flight direction) 2. slant range (Rs) = distance along the beam path 3. azimuth = distance along the flight direction 4. look angle (γ) = angle from the vertical to the beam 5. depression angle (θ) = complement to the look angle 6. swath width = illuminated surface on the ground 7. pulse duration (τ) = time of the pulse

4 III. Surface Roughness background o the amount of backscatter is a function of the statistical variations of the random surface heights from the reference surface o larger wavelengths will not be affected by a small statistical surface variation also depends greatly on the local incidence angle o Raleigh Criteria equations which relate approximate surface roughness to amount of backscatter in other words, a surface is rough if the criteria is exceeded 1. smooth if: h < λ / 25 sin θ ex., for a θ between degrees, you would get a roughness of ~ cm 2. intermediate if: λ / 25 sin θ < h < λ / 4.4 sin θ 3. rough if: h > λ / 4.4 sin θ ex., for a θ between degrees, you would get a roughness of ~ cm example: for SeaSat (θ = 70, λ = 23.5 cm) smooth < 1.0cm & rough > 5.7cm

5 sea surface roughness with varying depression angle IV. Radar Polarization details o SLAR systems can commonly transmit and receive in different polarization planes (horizontal and vertical) o results in image designations such as: Chv (C-band; horizontal send, vertical receive) Phh (P-band; horizontal send, horizontal receive) o interaction with surface features can depolarize the beam the physical process of depolarization is not always well understood horizontal send and receive is the strongest most objects on the surface have a vertical orientation therefore they scatter back most of the energy Chv Lhv Chv Chh R,G,B example: Phoenix, AZ (Shuttle Imaging Radar)

6 o depending on the surface properties of the surface under study, vertical send/receive may be important V. Radar Instruments 1. Seasat: 1978 o the first commercial SAR satellite o designed to measure sea surface roughness o large depression angle (θ = 70 ) reduced shadow effects but induced notable layover o L-band, HH-polarization o swath width of 100 km o repeat time = 24 days o spatial resolution = m o only lasted 99 days questionable reason as to failure of the satellite Yakutat, Alaska (SeaSat) Selma sand-sheet, Sudan (SIR-A) 2. Shuttle Imaging Radar (SIR) -A, -B, -C o SIR-A: 1982 L-band, HH-polarization 40 m spatial resolution near to far range depression angles varied from 43 to 37 diminished foreshortening and layover effects o SIR-B: 1984 L-band, HH-polarization 40 m spatial resolution variable depression angle from 15 to 55 provided tunable capability to diminished foreshortening and shadow effects in different areas

7 o SIR-C: 1994 L- and C-band each capable of HH, VV, HV, and VH polarization also had an X-band, VV polarization instrument variable depression angle between 20 and 65 spatial resolutions between 10 and 25 m Kliuchevskoi Volcano, Kamchatka (Russian Siberia) visible photograph SIR-C (Lhh, Lhv, Chv = B,G,R) 3. ERS-1 (European Space Agency): 1991 o C-band SAR o VV-polarization o resolution = 30m o repeat = 35 days o swath width = 500 km o fixed depression angle extending from 64 to 70 o launched ERS-2 in 1995 same instrument as ERS-1 4. JERS-1 (Japanese National Space Agency): o repeat = 35 days o contained a seven band optical sensor and a SAR o L-band SAR o HH polarization o fixed depression angle view between 52 and 58 o swath width of 75 km o resolution of 18 m

8 Mekong Delta, Vietnam (RadarSat) Mt. Fuji, Japan (JERS-1) Maui, Hawaii comparison RadarSat (C-band, θ = 43 ) ERS-2 (C-band, θ = 67 ) JERS-1 (L-band, θ = 55 ) SIR-C (L-band, θ = 32 ) 5. RadarSat 1 (Canada): 1995 o C-band SAR o HH-polarization o depression angle can range between 32 and 80 o variable resolution centering around 25 m o swath width = km

9 6. RadarSat 2 (Canada): 2003 o C-band SAR o spatial resolution = 28 m and 11 m o quad polarization o 24-day repeat cycle 7. Magellan: May 4, 1989 (space shuttle launch) o arrived: Venus on Aug. 10, 1990 o collected radar images of 98% o resolution of ~100m/pixel o S-band radar o measured the surface topography 8. ALOS (Japanese National Space Agency) o launched in 2006 o repeat time = variable o quad-band o swath width = m o resolution = m o different scan modes o ALOS (Japanese National Space Agency) 9. many others recently o by numerous countries:

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