Synthetic Aperture Radar

A Synthetic Aperture Radar (SAR), or SAR, is a coherent mostly airborne or spaceborne sidelooking radar system which utilizes the flight path of the platform to simulate an extremely large antenna or aperture electronically, and that generates high-resolution remote sensing imagery. Over time, individual transmit/receive cycles (PRT's) are completed with the data from each cycle being stored electronically. The signal processing uses magnitude and phase of the received signals over successive pulses from elements of a synthetic aperture. After a given number of cycles, the stored data is recombined (taking into account the Doppler effects inherent in the different transmitter to target geometry in each succeeding cycle) to create a high resolution image of the terrain being over flown.

How does SAR works?

Figure 2: The synthesized expanding beamwidth

synthetic length of SAR

imagine phased array

Figure 2: The synthesized expanding beamwidth

The SAR works similar of a phased array, but contrary of a large number of the parallel antenna elements of a phased array, SAR uses one antenna in time-multiplex. The different geometric positions of the antenna elements are result of the moving platform now.
The SAR-processor stores all the radar returned signals, as amplitudes and phases, for the time period T from position A to D. Now it is possible to reconstruct the signal which would have been obtained by an antenna of length v · T, where v is the platform speed. As the line of sight direction changes along the radar platform trajectory, a synthetic aperture is produced by signal processing that has the effect of lengthening the antenna. Making T large makes the „synthetic aperture” large and hence a higher resolution can be achieved.
As a target (like a ship) first enters the radar beam, the backscattered echoes from each transmitted pulse begin to be recorded. As the platform continues to move forward, all echoes from the target for each pulse are recorded during the entire time that the target is within the beam. The point at which the target leaves the view of the radar beam some time later, determines the length of the simulated or synthesized antenna. The synthesized expanding beamwidth, combined with the increased time a target is within the beam as ground range increases, balance each other, such that the resolution remains constant across the entire swath.
The achievable azimuth resolution of a SAR is approximately equal to one-half the length of the actual (real) antenna and does not depend on platform altitude (distance).

shift register

arithmetic operation

Figure 3: Principle of SAR- operation

shift register

arithmetic operation

Figure 3: Principle of SAR- operation

The requirements are:

• stable, full-coherent transmitter
• an efficient and powerful SAR-processor, and
• exactly knowledge of the flight path and the velocity of the platform.

Using such a technique, radar designers are able to achieve resolutions which would require real aperture antennas so large as to be impractical with arrays ranging in size up to 10 m.
A Synthetic Aperture Radar was used on board of a Space Shuttle during the Shuttle Radar Topography Mission (SRTM).
SAR radar is partnered by what is termed Inverse SAR (abbreviated to ISAR) technology which in the broadest terms, utilizes the movement of the target rather than the emitter to create the synthetic aperture. ISAR radars have a significant role aboard maritime patrol aircraft to provide them with radar image of sufficient quality to allow it to be used for target recognition purposes.

Slant-range distortion

The slant-range distortion occurs because the radar is measuring the distance to features in slant-range rather than the true horizontal distance along the ground. This results in a varying image scale, moving from near to far range.

Figure 4: Foreshortening

Figure 4: Foreshortening

• Foreshortening occurs when the radar beam reaches the base of a tall feature tilted towards the radar (e.g. a mountain) before it reaches the top. Because the radar measures distance in slant-range, the slope (from point a to point b) will appear compressed and the length of the slope will be represented incorrectly (a' to b') at the image plane.

Figure 5: Layover

Figure 5: Layover

• Layover occurs when the radar beam reaches the top of a tall feature (b) before it reaches the base (a). The return signal from the top of the feature will be received before the signal from the bottom. As a result, the top of the feature is displaced towards the radar from its true position on the ground, and „lays over” the base of the feature (b' to a').

Figure 6: Shadowing

Figure 6: Shadowing

• The shadowing effect increases with greater incident angle θ, just as our shadows lengthen as the sun sets.

Side Looking Airborne Radar (SLAR)

The platform (aircraft or satellite) of an side-looking airborne radar (SLAR) travels forward in the flight direction with the nadir directly beneath the platform. The microwave beam is transmitted obliquely at right angles to the direction of flight illuminating a swath. Range refers to the across-track dimension perpendicular to the flight direction, while azimuth refers to the along-track dimension parallel to the flight direction.
Swath width refers to the strip of the Earth’s surface from which data are collected by a side-looking airborne radar. It is the width of the imaged scene in the range dimension. The longitudinal extent of the swath is defined by the motion of the aircraft with respect to the surface, whereas the swath width is measured perpendicularly to the longitudinal extent of the swath.
The SLAR is a real aperture radar primarily. This requires a reasonable large antenna for adequately angular resolution. The azimuth resolution, R_a, is defined as

Ra =	H · λ	H is the height of the antenna,    (height of the airplane) L is the geometric length of the antenna, λ is the wavelength of the transmitted pulses, and θ is the incidence angle	(1)
	L · cos θ

Figure 2: Resolution cell variation.

Figure 2: Resolution cell variation.

The equation shows, that with increasing altitude decreases the azimuthal resolution of SLAR. A very long antenna (i.e., large L) would be required to achieve a good resolution from a satellite. Synthetic Aperture Radar (SAR) is used to acquire higher resolution.
The size of the ground resolution cell increases on the side of the nadir as the distance between radar platform and the ground resolution cell increases. This means that the ground resolution cells are larger towards the edge of the image than near the middle. This causes a scale distortion, which must be accounted for.
At all ranges the radar antenna measures the radial line of sight distance between the radar and each target on the surface. This is the slant range distance. The ground range distance is the true horizontal distance along the ground corresponding to each point measured in slant range. The cross-track resolution, R_r, is defined as

Rr =	c0 · tp	c0 is the speed of light tp is the pulse duration of the transmitter, and θ = incidence angle	(2)
	2 sin θ

Example given:

For an SLAR with the following characteristics:
λ = 1 cm,
L = 3 m,
H = 6000 m,
θ = 60°, and
t_p = 100 ns,
has got a resolution of
R_a = 40 m and
R_r = 17.3 m

Note: The same SLAR on a platform in a height of 600 km would achieve an azimuth-resolution of R_a = 4000 m.

Orthophotos

Vertical photographs are often used to create orthophotos, photographs which have been geometrically "corrected" so as to be usable as a map. In other words, an orthophoto is a simulation of a photograph taken from an infinite distance, looking straight down from nadir. Perspective must obviously be removed, but variations in terrain should also be corrected for. Multiple geometric transformations are applied to the image, depending on the perspective and terrain corrections required on a particular part of the image.
Orthophotos are commonly used in geographic information systems, such as are used by mapping agencies (e.g. Ordnance Survey) to create maps. Once the images have been aligned, or 'registered', with known real-world coordinates, they can be widely deployed.
Large sets of orthophotos, typically derived from multiple sources and divided into "tiles" (each typically 256 x 256 pixels in size), are widely used in online map systems such as Google Maps. OpenStreetMap offers the use of similar orthophotos for deriving new map data. Google Earth overlays orthophotos or satellite imagery onto a digital elevation model to simulate 3D landscapes.

Types of aerial photographs

Oblique photographs

Photographs taken at an angle are called oblique photographs. If they are taken almost straight down are sometimes called low oblique and photographs taken from a shallow angle are called high oblique.

 Vertical photographs

Vertical photographs are taken straight down. They are mainly used in photogrammetry and image interpretation. Pictures that will be used in photogrammetry are traditionally taken with special large format cameras with calibrated and documented geometric properties.

 

Uses of imagery and Radio-controlled aircraft

Aerial photography is used in cartography (particularly in photogrammetric surveys, which are often the basis for topographic maps), land-use planning, archaeology, movie production, environmental studies, surveillance, commercial advertising, conveyancing, and artistic projects. In the United States, aerial photographs are used in many Phase I Environmental Site Assessments for property analysis. Aerial photos are often processed using GIS software.

 Radio-controlled aircraft

Advances in radio controlled models have made it possible for model aircraft to conduct low-altitude aerial photography. This has benefited real-estate advertising, where commercial and residential properties are the photographic subject. Full-size, manned aircraft are prohibited from low flights above populated locations.^[3] Small scale model aircraft offer increased photographic access to these previously restricted areas. Miniature vehicles do not replace full size aircraft, as full size aircraft are capable of longer flight times, higher altitudes, and greater equipment payloads. They are, however, useful in any situation in which a full-scale aircraft would be dangerous to operate. Examples would include the inspection of transformers atop power transmission lines and slow, low-level flight over agricultural fields, both of which can be accomplished by a large-scale radio controlled helicopter. Professional-grade, gyroscopically stabilized camera platforms are available for use under such a model; a large model helicopter with a 26cc gasoline engine can hoist a payload of approximately seven kilograms (15 lbs).
Recent (2006) FAA regulations grounding all commercial RC model flights have been upgraded to require formal FAA certification before permission to fly at any altitude in USA. Refer to http://www.dvinfo.net/forum/digital-video-industry-news/145993-rc-aerials-illegal-says-faa.html
Because anything capable of being viewed from a public space is considered outside the realm of privacy in the United States, aerial photography may legally document features and occurrences on private property

History of aerial photograph

Aerial photography was first practiced by the French photographer and balloonist Gaspard-Félix Tournachon, known as "Nadar", in 1858 over Paris, France. ^[1]
The first use of a motion picture camera mounted to a heavier-than-air aircraft took place on April 24, 1909 over Rome in the 3:28 silent film short, Wilbur Wright und seine Flugmaschine.
The first special semiautomatic aerial camera was designed in 1911 by Russian military engineer — Colonel Potte V. F.^[2] This aerial camera was used during World War I.
The use of aerial photography for military purposes was expanded during World War I by many others aviators such as Fred Zinn. One of the first notable battles was that of Neuve Chapelle.
With the advent of inexpensive digital cameras, many people now take candid photographs from commercial aircraft and increasingly from general aviation aircraft on private pleasure flights.

Aerial photography

Aerial photography is the taking of photographs of the ground from an elevated position. The term usually refers to images in which the camera is not supported by a ground-based structure. Cameras may be hand held or mounted, and photographs may be taken by a photographer, triggered remotely or triggered automatically. Platforms for aerial photography include fixed-wing aircraft, helicopters, balloons, blimps and dirigibles, rockets, kites, poles, parachutes,vehicle mounted poles . Aerial photography should not be confused with Air-to-Air Photography, when aircraft serve both as a photo platform and subject.

satellite

Satellites are used for a large number of purposes. Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites. Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low Earth orbit, polar orbit, and geostationary orbit.

Satellites are usually semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, attitude control and orbit control.

Rising Indian Satellites

Configuration of Chandrayaan-1 that lift off on the PSLV-C11 Operator	Indian Space Research Organisation
Mission type	Orbiter
Satellite of	Moon
Orbital insertion date	12 November 2008
Orbits	3400 orbits around the Moon.[1]
Launch date	22 October 2008 00:52 UTC
Launch vehicle	PSLV-C11[2]
Launch site	SDSC, Sriharikota
Mission duration	Intended: 2 years Achieved: 312 days
COSPAR ID	2008-052A
Homepage	Chandrayaan-1
Mass	1,380 kg (3,042 lb)
Orbital elements
Eccentricity	near circular
Inclination	polar
Apoapsis	initial 7,500 km (4,660 mi), final 100 km (62 mi), final (wef 19 May 2009) 200 km (124 mi)
Periapsis	initial 500 km (311 mi), final 100 km (62 mi), final (wef 19 May 2009) 200 km (124 mi)