graphic courtesy Ball Aerospace
When we first started this project in 1992, we assumed that we could acquire multiple Landsat and SPOT images of the region to conduct mapping, change detection, and vegetation analysis. We quickly learned that cloud cover was to be a very serious problem. Virtually all of the archived data were cloud covered.Our interest quickly turned to satellite radar data, as these systems, unlike passive Landsat and SPOT, can "see" through cloudless and operate at night. This is because the sensors send their own burst of electromagnetic energy down to the target (Earth) and measure the reflected energy, unlike passive systems that rely upon the Sun's light.
Radar Fundamentals Radar stands for Radio Detection and Ranging. It refers to electronic equipment that detect the presence, composition, direction, height, and distance of objects by using reflected electromagnetic energy. The electromagnetic energy wavelengths used in imaging radar can penetrate clouds (and even dry sand in some cases), and the system can be operated day or night, as it uses its own energy and not the Sun.
graphic courtesy JPL
The Shuttle Imaging Radar (SIR-C) sends busts of energy from the antenna in the payload bay of Endeavour down to the ground and measures the reflected energy with the same antenna.
Different types of terrain will reflect differing amounts of energy. Steep mountains will reflect energy towards the radar, but not on the "lee" side. Metal objects like bridges will be highly reflective, while water or other smooth surfaces will not. Different types of vegetation communities will reflect differently.
graphic courtesy JPL
Radar Bands (wavelengths)
There are several wavelengths of energy that radar remote sensing satellites can use. These are all in the centimeter range. The following is a chart showing the various 'bands' of radar:
The terms P band, L band, S band, etc. are derived from secret World War II code names, and can be confusing.
Before the SIR-C mission, radar imaging satellites only used a single band and polarization (see below) for each image. This meant that each image was a black and white picture. SIR-C allowed for multiple simultaneous bands and polarizations, which could be combined to create color images, and that provided for much better understanding of what was down on the ground.
SIR-C operates in the C and L bands, and the X-SAR operates in the X band.
Radar Polarization
In addition to the wavelengths, there are several ways that we can differentiate what we receive back from the ground. Since radar energy is coherent, meaning that we send out a single wavelength, we can decide to send and receive either horizontal or vertical polarization. We can also receive either horizontal or vertical, giving four possible combinations:
HH (horizontal out, horizontal return)
HV (horizontal out, vertical return)
VH (horizontal out, vertical return)
VV (vertical out, vertical return)The latest SARs, including SIR-C, take advantage of these different combinations of transmission and reception, or polarimetry. These polarimeters can vary the polarization of their signals, thereby taking advantage of the unique reflectance signatures of different target materials. SIR-C/X-SAR provided increased capability by acquiring images with quad-polarization in C-band and L-Band. The X-band acquired VV polarization only.
Visit the NASA/Jet Propulsion Laboratory's excellent introduction to Radar imagery
Search online for other Radar images around the world.
Brute Force and Synthetic Aperture Radar (SAR) In real aperture radar, backscatter is received from the same location as the initial transmitted microwave signal. A synthetic aperture radar sends out a signal, but uses the forward motion of the satellite to receive the backscatter over a distance, thus simulating a larger aperture. This simulated aperture, and the more sophisticated signal processors required, allow for higher resolution than a standard aperture sensor. Typical spatial resolution is 18 to 30 meters, with a swath width of 75-100km. This resolution is similar to some high resolution visible/IR sensors, but SAR sensors can collect data independent of weather or light conditions. With such fine resolution, SARs are the sensor of choice for research and applications in the tropics where cloud cover is a constant problem.
The search for data
A quick search was conducted of working Radar satellites, and the news was not good. There were no existing SeaSat or SIR-A, or SIR-B data of our region. The Europeans had recently launched a Radar satellite, ERS-1, but it did not have on-board data recorders (radar satellites acquire much more data than passive systems), and had to be in view of a receiving station in order to acquire data. There were no receiving stations operating in central Africa at the time, so this was not an option.
The Russians were selling data commercially from their Almaz (Diamond) radar satellite, and we arranged to acquire data from it. Unfortunately, Almaz suffered a failure, and de-orbited before our data could be acquired. It seemed that we were out of luck.
Our only hope was the upcoming NASA Space Shuttle Imaging Radar (SIR-C) mission that was planned to fly in spring and fall of 1995. As I had previously worked at the NASA Stennis Space Center, I contacted NASA and requested that data be acquired in our area. This was not a simple task, as it was very late in the planning process and the data acquisition schedule was very full. But through perseverance and good luck, we had our application to become guest investigators on the mission approved. Two data takes were scheduled for the Virunga region for each mission, and both were successful.
SIR-C X-SAR
The SIR-C/X-SAR instrument
The overall size of the SIR-C antenna is 12.0 x 3.7 meters and consists of three panels each divided into four subpanels. It is the largest piece of hardware ever created by the Jet Propulsion Lab (JPL).
SIR-C/X-SAR (The Spaceborne Imaging Radar - Version C and the X-band Synthetic Aperture Radar) was the most advanced space radar imaging system when it was developed. The SIR-C/X-SAR antenna structure actually consists of three individual antennas, one operating at L-band (23.5cm wavelength), one at C-band (5.8cm wavelength) and the third at X-band (3cm wavelength). The L-band and C-band antennas are constructed from separate panels that can measure both horizontal and vertical polarizations.
SIR-C/X-SAR (Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar) is a joint project of the National Aeronautics and Space Administration (NASA), the German Space Agency (DARA) and the Italian Space Agency (ASI). Data are collected in L-Band and C-Band by SIR-C, and X-Band by X-SAR. An imaging radar system launched aboard the NASA Space Shuttle twice in 1994, SIR-C/X-SAR's unique contributions to Earth observation and monitoring are its capability to measure, from space, the radar signature of the surface at three different wavelengths and make measurements for different polarizations at two of those wavelengths.
It was designed to fit in the payload bay of the Space Shuttle. SIR-C/X-SAR also has a variable look angle, and could image at incidence angles between 20 and 65 degrees.
SIR-C/X-SAR System Characteristics
|
|
|
|
Wavelength |
0.235 m |
0.058 m |
0.031 m |
Swath Width |
15 to 90 km |
15 to 90 km |
15 to 40 km |
Pulse Length |
33.8, 16.9, 8.5 us |
33.8, 16.9, 8.5 us |
40 us |
Data Rate 90 Mbits/s |
90 Mbits/s |
90 Mbits/s |
45 Mbits/s |
Data Format |
8,4 bits/word |
8,4 bits/word |
8,4 bits/word |
The L band has a longer wavelength and is more penetrating than the C band. Hence, it is very useful in forest and vegetation study as it is able to penetrate deeper into the vegetation canopy and also reflects trunk/ground interaction. The C band wavelength interacts with the upper canopy, while X band, being shorter still, interacts with the very top of the canopy, similar to optical wavelengths used in Landsat and SPOT. The fact that SIR-C acquires data in three wavelengths at the same time provides superior data over most imaging radar systems that only acquire a single wavelength.
canopy penetration with different wavelengths graphic courtesy JPL For more information on application examples of SIR-C, go here.
http://www.jpl.nasa.gov/radar/sircxsar/
For specific information about the 1994 mission, go here.
http://www.jpl.nasa.gov/radar/sircxsar/sirc-pkt.html
Click on the Shuttle mission patches below (or the blue next arrow) to see the images we acquired over the Virunga region.
STS-59
SRL-1 (Shuttle Radar Lab 1)
Click on the mission patch above to view the images acquired
STS-68
SRL-2 (Shuttle Radar Lab 2)
Click on the mission patch above to view the images acquired
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