title: Synthetic-aperture radar

text: Synthetic-aperture radar determines the 3D reflectivity from measured SAR data. It is basically a spectrum estimation, because for a specific cell of an image, the complex-value SAR measurements of the SAR image stack are a sampled version of the Fourier transform of reflectivity in elevation direction, but the Fourier transform is irregular. Thus the spectral estimation techniques are used to improve the resolution and reduce speckle compared to the results of conventional Fourier transform SAR imaging techniques.


title: Synthetic-aperture radar

text: FFT (Fast Fourier Transform i.e., periodogram or matched filter) is one such method, which is used in majority of the spectral estimation algorithms, and there are many fast algorithms for computing the multidimensional discrete Fourier transform. Computational "Kronecker-core array algebra" is a popular algorithm used as new variant of FFT algorithms for the processing in multidimensional synthetic-aperture radar (SAR) systems. This algorithm uses a study of theoretical properties of input/output data indexing sets and groups of permutations.


title: Synthetic-aperture radar

text: A branch of finite multi-dimensional linear algebra is used to identify similarities and differences among various FFT algorithm variants and to create new variants. Each multidimensional DFT computation is expressed in matrix form. The multidimensional DFT matrix, in turn, is disintegrated into a set of factors, called functional primitives, which are individually identified with an underlying software/hardware computational design.


title: Synthetic-aperture radar

text: The FFT implementation is essentially a realization of the mapping of the mathematical framework through generation of the variants and executing matrix operations. The performance of this implementation may vary from machine to machine, and the objective is to identify on which machine it performs best.


title: Synthetic-aperture radar

text: The Capon spectral method, also called the minimum-variance method, is a multidimensional array-processing technique. It is a nonparametric covariance-based method, which uses an adaptive matched-filterbank approach and follows two main steps:


title: Synthetic-aperture radar

text: The adaptive Capon bandpass filter is designed to minimize the power of the filter output, as well as pass the frequencies (formula_1) without any attenuation, i.e., to satisfy, for each (formula_1),


title: Synthetic-aperture radar

text: where "R" is the covariance matrix, formula_8 is the complex conjugate transpose of the impulse response of the FIR filter, formula_9 is the 2D Fourier vector, defined as formula_10, formula_11 denotes Kronecker product.


title: Synthetic-aperture radar

text: Therefore, it passes a 2D sinusoid at a given frequency without distortion while minimizing the variance of the noise of the resulting image. The purpose is to compute the spectral estimate efficiently.


title: Synthetic-aperture radar

text: where "R" is the covariance matrix, and formula_13 is the 2D complex-conjugate transpose of the Fourier vector. The computation of this equation over all frequencies is time-consuming. It is seen that the forward–backward Capon estimator yields better estimation than the forward-only classical capon approach. The main reason behind this is that while the forward–backward Capon uses both the forward and backward data vectors to obtain the estimate of the covariance matrix, the forward-only Capon uses only the forward data vectors to estimate the covariance matrix.


title: Synthetic-aperture radar

text: The APES (amplitude and phase estimation) method is also a matched-filter-bank method, which assumes that the phase history data is a sum of 2D sinusoids in noise.


title: Synthetic-aperture radar

text: Empirically, the APES method results in wider spectral peaks than the Capon method, but more accurate spectral estimates for amplitude in SAR. In the Capon method, although the spectral peaks are narrower than the APES, the sidelobes are higher than that for the APES. As a result, the estimate for the amplitude is expected to be less accurate for the Capon method than for the APES method. The APES method requires about 1.5 times more computation than the Capon method.


title: Synthetic-aperture radar

text: SAMV method is a parameter-free sparse signal reconstruction based algorithm. It achieves super-resolution and is robust to highly correlated signals. The name emphasizes its basis on the asymptotically minimum variance (AMV) criterion. It is a powerful tool for the recovery of both the amplitude and frequency characteristics of multiple highly correlated sources in challenging environment (e.g., limited number of snapshots, low signal-to-noise ratio. Applications include synthetic-aperture radar imaging and various source localization.


title: Synthetic-aperture radar

text: SAMV method is capable of achieving resolution higher than some established parametric methods, e.g., MUSIC, especially with highly correlated signals.


title: Synthetic-aperture radar

text: This subspace decomposition method separates the eigenvectors of the autocovariance matrix into those corresponding to signals and to clutter. The amplitude of the image at a point (formula_19 ) is given by:


title: Synthetic-aperture radar

text: where formula_21 is the amplitude of the image at a point formula_22 , formula_23is the coherency matrix and formula_24 is the Hermitian of the coherency matrix,formula_25 is the inverse of the eigenvalues of the clutter subspace, formula_26 are vectors defined as


title: Synthetic-aperture radar

text: MUSIC detects frequencies in a signal by performing an eigen decomposition on the covariance matrix of a data vector of the samples obtained from the samples of the received signal. When all of the eigenvectors are included in the clutter subspace (model order = 0) the EV method becomes identical to the Capon method. Thus the determination of model order is critical to operation of the EV method. The eigenvalue of the R matrix decides whether its corresponding eigenvector corresponds to the clutter or to the signal subspace.


title: Synthetic-aperture radar

text: The MUSIC method is considered to be a poor performer in SAR applications. This method uses a constant instead of the clutter subspace.


title: Synthetic-aperture radar

text: In this method, the denominator is equated to zero when a sinusoidal signal corresponding to a point in the SAR image is in alignment to one of the signal subspace eigenvectors which is the peak in image estimate. Thus this method does not accurately represent the scattering intensity at each point, but show the particular points of the image.


title: Synthetic-aperture radar

text: Backprojection Algorithm has two methods: "Time-domain Backprojection" and "Frequency-domain Backprojection". The time-domain Backprojection has more advantages over frequency-domain and thus, is more preferred. The time-domain Backprojection forms images or spectrums by matching the data acquired from the radar and as per what it expects to receive. It can be considered as an ideal matched-filter for synthetic-aperture radar. There is no need of having a different motion compensation step due to its quality of handling non-ideal motion/sampling. It can also be used for various imaging geometries.


title: Synthetic-aperture radar

text: In GEO-SAR, to focus specially on the relative moving track, the backprojection algorithm works very well. It uses the concept of Azimuth Processing in the time domain. For the satellite-ground geometry, GEO-SAR plays a significant role.


title: Synthetic-aperture radar

text: Capon and APES can yield more accurate spectral estimates with much lower sidelobes and more narrow spectral peaks than the fast Fourier transform (FFT) method, which is also a special case of the FIR filtering approaches. It is seen that although the APES algorithm gives slightly wider spectral peaks than the Capon method, the former yields more accurate overall spectral estimates than the latter and the FFT method.


title: Synthetic-aperture radar

text: FFT method is fast and simple but have larger sidelobes. Capon has high resolution but high computational complexity. EV also has high resolution and high computational complexity. APES has higher resolution, faster than capon and EV but high computational complexity.


title: Synthetic-aperture radar

text: MUSIC method is not generally suitable for SAR imaging, as whitening the clutter eigenvalues destroys the spatial inhomogeneities associated with terrain clutter or other diffuse scattering in SAR imagery. But it offers higher frequency resolution in the resulting power spectral density (PSD) than the fast Fourier transform (FFT)-based methods.


title: Synthetic-aperture radar

text: The backprojection algorithm is computationally expensive. It is specifically attractive for sensors that are wideband, wide-angle, and/or have long coherent apertures with substantial off-track motion.


title: Synthetic-aperture radar

text: SAR requires that echo captures be taken at multiple antenna positions. The more captures taken (at different antenna locations) the more reliable the target characterization.


title: Synthetic-aperture radar

text: Multiple captures can be obtained by moving a single antenna to different locations, by placing multiple stationary antennas at different locations, or combinations thereof.


title: Synthetic-aperture radar

text: The advantage of a single moving antenna is that it can be easily placed in any number of positions to provide any number of monostatic waveforms. For example, an antenna mounted on an airplane takes many captures per second as the plane travels.


title: Synthetic-aperture radar

text: The principal advantages of multiple static antennas are that a moving target can be characterized (assuming the capture electronics are fast enough), that no vehicle or motion machinery is necessary, and that antenna positions need not be derived from other, sometimes unreliable, information. (One problem with SAR aboard an airplane is knowing precise antenna positions as the plane travels).


title: Synthetic-aperture radar

text: For multiple static antennas, all combinations of monostatic and multistatic radar waveform captures are possible. Note, however, that it is not advantageous to capture a waveform for each of both transmission directions for a given pair of antennas, because those waveforms will be identical. When multiple static antennas are used, the total number of unique echo waveforms that can be captured is


title: Synthetic-aperture radar

text: The antenna stays in a fixed position, and may be orthogonal to the flight path or squinted slightly forward or backward .


title: Synthetic-aperture radar

text: When the antenna aperture travels along the flight path, a signal is transmitted at a rate equal to the pulse repetition frequency (PRF). The lower boundary of the PRF is determined by the Doppler bandwidth of the radar. The backscatter of each of these signals is commutatively added on a pixel-by-pixel basis to attain the fine azimuth resolution desired in radar imagery.


title: Synthetic-aperture radar

text: where formula_31 is the angle formed between the beginning and end of the imaging, as shown in the diagram of spotlight imaging and formula_32 is the range distance.


title: Synthetic-aperture radar

text: The spotlight mode gives better resolution albeit for a smaller ground patch. In this mode, the illuminating radar beam is steered continually as the aircraft moves, so that it illuminates the same patch over a longer period of time. This mode is not a traditional continuous-strip imaging mode; however, it has high azimuth resolution.


title: Synthetic-aperture radar

text: While operating as a scan mode SAR, the antenna beam sweeps periodically and thus cover much larger area than the spotlight and stripmap modes. However, the azimuth resolution become much lower than the stripmap mode due to the decreased azimuth bandwidth. Clearly there is a balance achieved between the azimuth resolution and the scan area of SAR. Here, the synthetic aperture is shared between the sub swaths, and it is not in direct contact within one subswath. Mosaic operation is required in azimuth and range directions to join the azimuth bursts and the range sub-swaths.


title: Synthetic-aperture radar

text: Radar waves have a polarization. Different materials reflect radar waves with different intensities, but anisotropic materials such as grass often reflect different polarizations with different intensities. Some materials will also convert one polarization into another. By emitting a mixture of polarizations and using receiving antennas with a specific polarization, several images can be collected from the same series of pulses. Frequently three such RX-TX polarizations (HH-pol, VV-pol, VH-pol) are used as the three color channels in a synthesized image. This is what has been done in the picture at right. Interpretation of the resulting colors requires significant testing of known materials.


title: Synthetic-aperture radar

text: New developments in polarimetry include using the changes in the random polarization returns of some surfaces (such as grass or sand) and between two images of the same location at different times to determine where changes not visible to optical systems occurred. Examples include subterranean tunneling or paths of vehicles driving through the area being imaged. Enhanced SAR sea oil slick observation has been developed by appropriate physical modelling and use of fully polarimetric and dual-polarimetric measurements.


title: Synthetic-aperture radar

text: SAR polarimetry is a technique used for deriving qualitative and quantitative physical information for land, snow and ice, ocean and urban applications based on the measurement and exploration of the polarimetric properties of man-made and natural scatterers. "Terrain" and "land use" classification is one of the most important applications of polarimetric synthetic-aperture radar (PolSAR).


title: Synthetic-aperture radar

text: SAR polarimetry uses a scattering matrix (S) to identify the scattering behavior of objects after an interaction with electromagnetic wave. The matrix is represented by a combination of horizontal and vertical polarization states of transmitted and received signals.


title: Synthetic-aperture radar

text: where, HH is for horizontal transmit and horizontal receive, VV is for vertical transmit and vertical receive, HV is for horizontal transmit and vertical receive, and VH – for vertical transmit and horizontal receive.


title: Synthetic-aperture radar

text: The first two of these polarization combinations are referred to as like-polarized (or co-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 orthogonal to one another.


title: Synthetic-aperture radar

text: The three-component scattering power model by Freeman and Durden is successfully used for the decomposition of a PolSAR image, applying the reflection symmetry condition using covariance matrix. The method is based on simple physical scattering mechanisms (surface scattering, double-bounce scattering, and volume scattering). The advantage of this scattering model is that it is simple and easy to implement for image processing. There are 2 major approaches for a 3formula_343 polarimetric matrix decomposition. One is the lexicographic covariance matrix approach based on physically measurable parameters, and the other is the Pauli decomposition which is a coherent decomposition matrix. It represents all the polarimetric information in a single SAR image. The polarimetric information of [S] could be represented by the combination of the intensities formula_35 in a single RGB image where all the previous intensities will be coded as a color channel.


title: Synthetic-aperture radar

text: For PolSAR image analysis, there can be cases where reflection symmetry condition does not hold. In those cases a "four-component scattering model" can be used to decompose polarimetric synthetic-aperture radar (SAR) images. This approach deals with the non-reflection symmetric scattering case. It includes and extends the three-component decomposition method introduced by Freeman and Durden to a fourth component by adding the helix scattering power. This helix power term generally appears in complex urban area but disappears for a natural distributed scatterer.


title: Synthetic-aperture radar

text: There is also an improved method using the four-component decomposition algorithm, which was introduced for the general polSAR data image analyses. The SAR data is first filtered which is known as speckle reduction, then each pixel is decomposed by four-component model to determine the surface scattering power (formula_36), double-bounce scattering power (formula_37), volume scattering power (formula_38), and helix scattering power (formula_39). The pixels are then divided into 5 classes (surface, double-bounce, volume, helix, and mixed pixels) classified with respect to maximum powers. A mixed category is added for the pixels having two or three equal dominant scattering powers after computation. The process continues as the pixels in all these categories are divided in 20 small clutter approximately of same number of pixels and merged as desirable, this is called cluster merging. They are iteratively classified and then automatically color is delivered to each class. The summarization of this algorithm leads to an understanding that, brown colors denotes the surface scattering classes, red colors for double-bounce scattering classes, green colors for volume scattering classes, and blue colors for helix scattering classes.


title: Synthetic-aperture radar

text: Although this method is aimed for non-reflection case, it automatically includes the reflection symmetry condition, therefore in can be used as a general case. It also preserves the scattering characteristics by taking the mixed scattering category into account therefore proving to be a better algorithm.


title: Synthetic-aperture radar

text: Rather than discarding the phase data, information can be extracted from it. If two observations of the same terrain from very similar positions are available, aperture synthesis can be performed to provide the resolution performance which would be given by a radar system with dimensions equal to the separation of the two measurements. This technique is called interferometric SAR or InSAR.


title: Synthetic-aperture radar

text: If the two samples are obtained simultaneously (perhaps by placing two antennas on the same aircraft, some distance apart), then any phase difference will contain information about the angle from which the radar echo returned. Combining this with the distance information, one can determine the position in three dimensions of the image pixel. In other words, one can extract terrain altitude as well as radar reflectivity, producing a digital elevation model (DEM) with a single airplane pass. One aircraft application at the Canada Centre for Remote Sensing produced digital elevation maps with a resolution of 5 m and altitude errors also about 5 m. Interferometry was used to map many regions of the Earth's surface with unprecedented accuracy using data from the Shuttle Radar Topography Mission.


title: Synthetic-aperture radar

text: If the two samples are separated in time, perhaps from two flights over the same terrain, then there are two possible sources of phase shift. The first is terrain altitude, as discussed above. The second is terrain motion: if the terrain has shifted between observations, it will return a different phase. The amount of shift required to cause a significant phase difference is on the order of the wavelength used. This means that if the terrain shifts by centimeters, it can be seen in the resulting image (a digital elevation map must be available to separate the two kinds of phase difference; a third pass may be necessary to produce one).


title: Synthetic-aperture radar

text: This second method offers a powerful tool in geology and geography. Glacier flow can be mapped with two passes. Maps showing the land deformation after a minor earthquake or after a volcanic eruption (showing the shrinkage of the whole volcano by several centimeters) have been published.


title: Synthetic-aperture radar

text: Differential interferometry (D-InSAR) requires taking at least two images with addition of a DEM. The DEM can be either produced by GPS measurements or could be generated by interferometry as long as the time between acquisition of the image pairs is short, which guarantees minimal distortion of the image of the target surface. In principle, 3 images of the ground area with similar image acquisition geometry is often adequate for D-InSar. The principle for detecting ground movement is quite simple. One interferogram is created from the first two images; this is also called the reference interferogram or topographical interferogram. A second interferogram is created that captures topography + distortion. Subtracting the latter from the reference interferogram can reveal differential fringes, indicating movement. The described 3 image D-InSAR generation technique is called 3-pass or double-difference method.


title: Synthetic-aperture radar

text: Differential fringes which remain as fringes in the differential interferogram are a result of SAR range changes of any displaced point on the ground from one interferogram to the next. In the differential interferogram, each fringe is directly proportional to the SAR wavelength, which is about 5.6 cm for ERS and RADARSAT single phase cycle. Surface displacement away from the satellite look direction causes an increase in path (translating to phase) difference. Since the signal travels from the SAR antenna to the target and back again, the measured displacement is twice the unit of wavelength. This means in differential interferometry one fringe cycle − to + or one wavelength corresponds to a displacement relative to SAR antenna of only half wavelength (2.8 cm). There are various publications on measuring subsidence movement, slope stability analysis, landslide, glacier movement, etc. tooling D-InSAR. Further advancement to this technique whereby differential interferometry from satellite SAR ascending pass and descending pass can be used to estimate 3-D ground movement. Research in this area has shown accurate measurements of 3-D ground movement with accuracies comparable to GPS based measurements can be achieved.


title: Synthetic-aperture radar

text: SAR Tomography is a subfield of a concept named as multi-baseline interferometry. It has been developed to give a 3D exposure to the imaging, which uses the beam formation concept. It can be used when the use demands a focused phase concern between the magnitude and the phase components of the SAR data, during information retrieval. One of the major advantages of Tomo-SAR is that it can separate out the parameters which get scattered, irrespective of how different their motions are.


title: Synthetic-aperture radar

text: On using Tomo-SAR with differential interferometry, a new combination named "differential tomography" (Diff-Tomo) is developed.


title: Synthetic-aperture radar

text: Tomo-SAR has an application based on radar imaging, which is the depiction of Ice Volume and Forest Temporal Coherence (Temporal coherence describes the correlation between waves observed at different moments in time).


title: Synthetic-aperture radar

text: Conventional radar systems emit bursts of radio energy with a fairly narrow range of frequencies. A narrow-band channel, by definition, does not allow rapid changes in modulation. Since it is the change in a received signal that reveals the time of arrival of the signal (obviously an unchanging signal would reveal nothing about "when" it reflected from the target), a signal with only a slow change in modulation cannot reveal the distance to the target as well as a signal with a quick change in modulation.


title: Synthetic-aperture radar

text: Ultra-wideband (UWB) refers to any radio transmission that uses a very large bandwidth – which is the same as saying it uses very rapid changes in modulation. Although there is no set bandwidth value that qualifies a signal as "UWB", systems using bandwidths greater than a sizable portion of the center frequency (typically about ten percent, or so) are most often called "UWB" systems. A typical UWB system might use a bandwidth of one-third to one-half of its center frequency. For example, some systems use a bandwidth of about 1 GHz centered around 3 GHz.


title: Synthetic-aperture radar

text: The two most common methods to increase signal bandwidth used in UWB radar, including SAR, are very short pulses and high-bandwidth chirping. A general description of chirping appears elsewhere in this article. The bandwidth of a chirped system can be as narrow or as wide as the designers desire. Pulse-based UWB systems, being the more common method associated with the term "UWB radar", are described here.


title: Synthetic-aperture radar

text: A pulse-based radar system transmits very short pulses of electromagnetic energy, typically only a few waves or less. A very short pulse is, of course, a very rapidly changing signal, and thus occupies a very wide bandwidth. This allows far more accurate measurement of distance, and thus resolution.


title: Synthetic-aperture radar

text: The main disadvantage of pulse-based UWB SAR is that the transmitting and receiving front-end electronics are difficult to design for high-power applications. Specifically, the transmit duty cycle is so exceptionally low and pulse time so exceptionally short, that the electronics must be capable of extremely high instantaneous power to rival the average power of conventional radars. (Although it is true that UWB provides a notable gain in channel capacity over a narrow band signal because of the relationship of bandwidth in the Shannon–Hartley theorem and because the low receive duty cycle receives less noise, increasing the signal-to-noise ratio, there is still a notable disparity in link budget because conventional radar might be several orders of magnitude more powerful than a typical pulse-based radar.) So pulse-based UWB SAR is typically used in applications requiring average power levels in the microwatt or milliwatt range, and thus is used for scanning smaller, nearer target areas (several tens of meters), or in cases where lengthy integration (over a span of minutes) of the received signal is possible. However, that this limitation is solved in chirped UWB radar systems.


title: Synthetic-aperture radar

text: The principal advantages of UWB radar are better resolution (a few millimeters using commercial off-the-shelf electronics) and more spectral information of target reflectivity.


title: Synthetic-aperture radar

text: Doppler Beam Sharpening commonly refers to the method of processing unfocused real-beam phase history to achieve better resolution than could be achieved by processing the real beam without it. Because the real aperture of the radar antenna is so small (compared to the wavelength in use), the radar energy spreads over a wide area (usually many degrees wide in a direction orthogonal (at right angles) to the direction of the platform (aircraft)). Doppler-beam sharpening takes advantage of the motion of the platform in that targets ahead of the platform return a Doppler upshifted signal (slightly higher in frequency) and targets behind the platform return a Doppler downshifted signal (slightly lower in frequency).


title: Synthetic-aperture radar

text: The amount of shift varies with the angle forward or backward from the ortho-normal direction. By knowing the speed of the platform, target signal return is placed in a specific angle "bin" that changes over time. Signals are integrated over time and thus the radar "beam" is synthetically reduced to a much smaller aperture – or more accurately (and based on the ability to distinguish smaller Doppler shifts) the system can have hundreds of very "tight" beams concurrently. This technique dramatically improves angular resolution; however, it is far more difficult to take advantage of this technique for range resolution. (See pulse-doppler radar).


title: Synthetic-aperture radar

text: A common technique for many radar systems (usually also found in SAR systems) is to "chirp" the signal. In a "chirped" radar, the pulse is allowed to be much longer. A longer pulse allows more energy to be emitted, and hence received, but usually hinders range resolution. But in a chirped radar, this longer pulse also has a frequency shift during the pulse (hence the chirp or frequency shift). When the "chirped" signal is returned, it must be correlated with the sent pulse. Classically, in analog systems, it is passed to a dispersive delay line (often a surface acoustic wave device) that has the property of varying velocity of propagation based on frequency. This technique "compresses" the pulse in time – thus having the effect of a much shorter pulse (improved range resolution) while having the benefit of longer pulse length (much more signal returned). Newer systems use digital pulse correlation to find the pulse return in the signal.


title: Synthetic-aperture radar

text: In a typical SAR application, a single radar antenna is attached to an aircraft or spacecraft such that a substantial component of the antenna's radiated beam has a wave-propagation direction perpendicular to the flight-path direction. The beam is allowed to be broad in the vertical direction so it will illuminate the terrain from nearly beneath the aircraft out toward the horizon.


title: Synthetic-aperture radar

text: Resolution in the range dimension of the image is accomplished by creating pulses which define very short time intervals, either by emitting short pulses consisting of a carrier frequency and the necessary sidebands, all within a certain bandwidth, or by using longer "chirp pulses" in which frequency varies (often linearly) with time within that bandwidth. The differing times at which echoes return allow points at different distances to be distinguished.


title: Synthetic-aperture radar

text: Image resolution of SAR in its range coordinate (expressed in image pixels per distance unit) is mainly proportional to the radio bandwidth of whatever type of pulse is used. In the cross-range coordinate, the similar resolution is mainly proportional to the bandwidth of the Doppler shift of the signal returns within the beamwidth. Since Doppler frequency depends on the angle of the scattering point's direction from the broadside direction, the Doppler bandwidth available within the beamwidth is the same at all ranges. Hence the theoretical spatial resolution limits in both image dimensions remain constant with variation of range. However, in practice, both the errors that accumulate with data-collection time and the particular techniques used in post-processing further limit cross-range resolution at long ranges.


title: Synthetic-aperture radar

text: The total signal is that from a beamwidth-sized patch of the ground. To produce a beam that is narrow in the cross-range direction, diffraction effects require that the antenna be wide in that dimension. Therefore, the distinguishing, from each other, of co-range points simply by strengths of returns that persist for as long as they are within the beam width is difficult with aircraft-carryable antennas, because their beams can have linear widths only about two orders of magnitude (hundreds of times) smaller than the range. (Spacecraft-carryable ones can do 10 or more times better.) However, if both the amplitude and the phase of returns are recorded, then the portion of that multi-target return that was scattered radially from any smaller scene element can be extracted by phase-vector correlation of the total return with the form of the return expected from each such element.


title: Synthetic-aperture radar

text: The process can be thought of as combining the series of spatially distributed observations as if all had been made simultaneously with an antenna as long as the beamwidth and focused on that particular point. The "synthetic aperture" simulated at maximum system range by this process not only is longer than the real antenna, but, in practical applications, it is much longer than the radar aircraft, and tremendously longer than the radar spacecraft.


title: Synthetic-aperture radar

text: Although some references to SARs have characterized them as "radar telescopes", their actual optical analogy is the microscope, the detail in their images being smaller than the length of the synthetic aperture. In radar-engineering terms, while the target area is in the "far field" of the illuminating antenna, it is in the "near field" of the simulated one. Careful design and operation can accomplish resolution of items smaller than a millionth of the range, for example, 30 cm at 300 km, or about one foot at nearly .


title: Synthetic-aperture radar

text: The conversion of return delay time to geometric range can be very accurate because of the natural constancy of the speed and direction of propagation of electromagnetic waves. However, for an aircraft flying through the never-uniform and never-quiescent atmosphere, the relating of pulse transmission and reception times to successive geometric positions of the antenna must be accompanied by constant adjusting of the return phases to account for sensed irregularities in the flight path. SAR's in spacecraft avoid that atmosphere problem, but still must make corrections for known antenna movements due to rotations of the spacecraft, even those that are reactions to movements of onboard machinery. Locating a SAR in a crewed space vehicle may require that the humans carefully remain motionless relative to the vehicle during data collection periods.


title: Synthetic-aperture radar

text: Returns from scatterers within the range extent of any image are spread over a matching time interval. The inter-pulse period must be long enough to allow farthest-range returns from any pulse to finish arriving before the nearest-range ones from the next pulse begin to appear, so that those do not overlap each other in time. On the other hand, the interpulse rate must be fast enough to provide sufficient samples for the desired across-range (or across-beam) resolution. When the radar is to be carried by a high-speed vehicle and is to image a large area at fine resolution, those conditions may clash, leading to what has been called SAR's ambiguity problem. The same considerations apply to "conventional" radars also, but this problem occurs significantly only when resolution is so fine as to be available only through SAR processes. Since the basis of the problem is the information-carrying capacity of the single signal-input channel provided by one antenna, the only solution is to use additional channels fed by additional antennas. The system then becomes a hybrid of a SAR and a phased array, sometimes being called a Vernier array.


title: Synthetic-aperture radar

text: Combining the series of observations requires significant computational resources, usually using Fourier transform techniques. The high digital computing speed now available allows such processing to be done in near-real time on board a SAR aircraft. (There is necessarily a minimum time delay until all parts of the signal have been received.) The result is a map of radar reflectivity, including both amplitude and phase.


title: Synthetic-aperture radar

text: The amplitude information, when shown in a map-like display, gives information about ground cover in much the same way that a black-and-white photo does. Variations in processing may also be done in either vehicle-borne stations or ground stations for various purposes, so as to accentuate certain image features for detailed target-area analysis.


title: Synthetic-aperture radar

text: Although the phase information in an image is generally not made available to a human observer of an image display device, it can be preserved numerically, and sometimes allows certain additional features of targets to be recognized.


title: Synthetic-aperture radar

text: Unfortunately, the phase differences between adjacent image picture elements ("pixels") also produce random interference effects called "coherence speckle", which is a sort of graininess with dimensions on the order of the resolution, causing the concept of resolution to take on a subtly different meaning. This effect is the same as is apparent both visually and photographically in laser-illuminated optical scenes. The scale of that random speckle structure is governed by the size of the synthetic aperture in wavelengths, and cannot be finer than the system's resolution. Speckle structure can be subdued at the expense of resolution.


title: Synthetic-aperture radar

text: Before rapid digital computers were available, the data processing was done using an optical holography technique. The analog radar data were recorded as a holographic interference pattern on photographic film at a scale permitting the film to preserve the signal bandwidths (for example, 1:1,000,000 for a radar using a 0.6-meter wavelength). Then light using, for example, 0.6-micrometer waves (as from a helium–neon laser) passing through the hologram could project a terrain image at a scale recordable on another film at reasonable processor focal distances of around a meter. This worked because both SAR and phased arrays are fundamentally similar to optical holography, but using microwaves instead of light waves. The "optical data-processors" developed for this radar purpose were the first effective analog optical computer systems, and were, in fact, devised before the holographic technique was fully adapted to optical imaging. Because of the different sources of range and across-range signal structures in the radar signals, optical data-processors for SAR included not only both spherical and cylindrical lenses, but sometimes conical ones.


title: Synthetic-aperture radar

text: The following considerations apply also to real-aperture terrain-imaging radars, but are more consequential when resolution in range is matched to a cross-beam resolution that is available only from a SAR.


title: Synthetic-aperture radar

text: The two dimensions of a radar image are range and cross-range. Radar images of limited patches of terrain can resemble oblique photographs, but not ones taken from the location of the radar. This is because the range coordinate in a radar image is perpendicular to the vertical-angle coordinate of an oblique photo. The apparent entrance-pupil position (or camera center) for viewing such an image is therefore not as if at the radar, but as if at a point from which the viewer's line of sight is perpendicular to the slant-range direction connecting radar and target, with slant-range increasing from top to bottom of the image.


title: Synthetic-aperture radar

text: Because slant ranges to level terrain vary in vertical angle, each elevation of such terrain appears as a curved surface, specifically a hyperbolic cosine one. Verticals at various ranges are perpendiculars to those curves. The viewer's apparent looking directions are parallel to the curve's "hypcos" axis. Items directly beneath the radar appear as if optically viewed horizontally (i.e., from the side) and those at far ranges as if optically viewed from directly above. These curvatures are not evident unless large extents of near-range terrain, including steep slant ranges, are being viewed.


title: Synthetic-aperture radar

text: When viewed as specified above, fine-resolution radar images of small areas can appear most nearly like familiar optical ones, for two reasons. The first reason is easily understood by imagining a flagpole in the scene. The slant-range to its upper end is less than that to its base. Therefore, the pole can appear correctly top-end up only when viewed in the above orientation. Secondly, the radar illumination then being downward, shadows are seen in their most-familiar "overhead-lighting" direction.


title: Synthetic-aperture radar

text: The image of the pole's top will overlay that of some terrain point which is on the same slant range arc but at a shorter horizontal range ("ground-range"). Images of scene surfaces which faced both the illumination and the apparent eyepoint will have geometries that resemble those of an optical scene viewed from that eyepoint. However, slopes facing the radar will be foreshortened and ones facing away from it will be lengthened from their horizontal (map) dimensions. The former will therefore be brightened and the latter dimmed.


title: Synthetic-aperture radar

text: Returns from slopes steeper than perpendicular to slant range will be overlaid on those of lower-elevation terrain at a nearer ground-range, both being visible but intermingled. This is especially the case for vertical surfaces like the walls of buildings. Another viewing inconvenience that arises when a surface is steeper than perpendicular to the slant range is that it is then illuminated on one face but "viewed" from the reverse face. Then one "sees", for example, the radar-facing wall of a building as if from the inside, while the building's interior and the rear wall (that nearest to, hence expected to be optically visible to, the viewer) have vanished, since they lack illumination, being in the shadow of the front wall and the roof. Some return from the roof may overlay that from the front wall, and both of those may overlay return from terrain in front of the building. The visible building shadow will include those of all illuminated items. Long shadows may exhibit blurred edges due to the illuminating antenna's movement during the "time exposure" needed to create the image.


title: Synthetic-aperture radar

text: Surfaces that we usually consider rough will, if that roughness consists of relief less than the radar wavelength, behave as smooth mirrors, showing, beyond such a surface, additional images of items in front of it. Those mirror images will appear within the shadow of the mirroring surface, sometimes filling the entire shadow, thus preventing recognition of the shadow.


title: Synthetic-aperture radar

text: The direction of overlay of any scene point is not directly toward the radar, but toward that point of the SAR's current path direction that is nearest to the target point. If the SAR is "squinting" forward or aft away from the exactly broadside direction, then the illumination direction, and hence the shadow direction, will not be opposite to the overlay direction, but slanted to right or left from it. An image will appear with the correct projection geometry when viewed so that the overlay direction is vertical, the SAR's flight-path is above the image, and range increases somewhat downward.


title: Synthetic-aperture radar

text: Objects in motion within a SAR scene alter the Doppler frequencies of the returns. Such objects therefore appear in the image at locations offset in the across-range direction by amounts proportional to the range-direction component of their velocity. Road vehicles may be depicted off the roadway and therefore not recognized as road traffic items. Trains appearing away from their tracks are more easily properly recognized by their length parallel to known trackage as well as by the absence of an equal length of railbed signature and of some adjacent terrain, both having been shadowed by the train. While images of moving vessels can be offset from the line of the earlier parts of their wakes, the more recent parts of the wake, which still partake of some of the vessel's motion, appear as curves connecting the vessel image to the relatively quiescent far-aft wake. In such identifiable cases, speed and direction of the moving items can be determined from the amounts of their offsets. The along-track component of a target's motion causes some defocus. Random motions such as that of wind-driven tree foliage, vehicles driven over rough terrain, or humans or other animals walking or running generally render those items not focusable, resulting in blurring or even effective invisibility.


title: Synthetic-aperture radar

text: These considerations, along with the speckle structure due to coherence, take some getting used to in order to correctly interpret SAR images. To assist in that, large collections of significant target signatures have been accumulated by performing many test flights over known terrains and cultural objects.


title: Synthetic-aperture radar

text: A technique closely related to SAR uses an array (referred to as a "phased array") of real antenna elements spatially distributed over either one or two dimensions perpendicular to the radar-range dimension. These physical arrays are truly synthetic ones, indeed being created by synthesis of a collection of subsidiary physical antennas. Their operation need not involve motion relative to targets. All elements of these arrays receive simultaneously in real time, and the signals passing through them can be individually subjected to controlled shifts of the phases of those signals. One result can be to respond most strongly to radiation received from a specific small scene area, focusing on that area to determine its contribution to the total signal received. The coherently detected set of signals received over the entire array aperture can be replicated in several data-processing channels and processed differently in each. The set of responses thus traced to different small scene areas can be displayed together as an image of the scene.


title: Synthetic-aperture radar

text: In comparison, a SAR's (commonly) single physical antenna element gathers signals at different positions at different times. When the radar is carried by an aircraft or an orbiting vehicle, those positions are functions of a single variable, distance along the vehicle's path, which is a single mathematical dimension (not necessarily the same as a linear geometric dimension). The signals are stored, thus becoming functions, no longer of time, but of recording locations along that dimension. When the stored signals are read out later and combined with specific phase shifts, the result is the same as if the recorded data had been gathered by an equally long and shaped phased array. What is thus synthesized is a set of signals equivalent to what could have been received simultaneously by such an actual large-aperture (in one dimension) phased array. The SAR simulates (rather than synthesizes) that long one-dimensional phased array. Although the term in the title of this article has thus been incorrectly derived, it is now firmly established by half a century of usage.


title: Synthetic-aperture radar

text: While operation of a phased array is readily understood as a completely geometric technique, the fact that a synthetic aperture system gathers its data as it (or its target) moves at some speed means that phases which varied with the distance traveled originally varied with time, hence constituted temporal frequencies. Temporal frequencies being the variables commonly used by radar engineers, their analyses of SAR systems are usually (and very productively) couched in such terms. In particular, the variation of phase during flight over the length of the synthetic aperture is seen as a sequence of Doppler shifts of the received frequency from that of the transmitted frequency. Once the received data have been recorded and thus have become timeless, the SAR data-processing situation is also understandable as a special type of phased array, treatable as a completely geometric process.


title: Synthetic-aperture radar

text: The core of both the SAR and the phased array techniques is that the distances that radar waves travel to and back from each scene element consist of some integer number of wavelengths plus some fraction of a "final" wavelength. Those fractions cause differences between the phases of the re-radiation received at various SAR or array positions. Coherent detection is needed to capture the signal phase information in addition to the signal amplitude information. That type of detection requires finding the differences between the phases of the received signals and the simultaneous phase of a well-preserved sample of the transmitted illumination.


title: Bye Bye Birdie (1963 film)

text: Bye Bye Birdie is a 1963 American musical romantic comedy film directed by George Sidney from a screenplay by Irving Brecher, based on Michael Stewart's book of the 1960 musical of the same name. It also features songs by composer Charles Strouse and lyricist Lee Adams, and a score by Johnny Green. Produced by Fred Kohlmar, the film stars Janet Leigh, Dick Van Dyke, Ann-Margret, Maureen Stapleton, Bobby Rydell, Jesse Pearson, and Ed Sullivan. Van Dyke and featured player Paul Lynde reprised their roles from the original Broadway production. It was also Van Dyke's feature film debut.


title: Bye Bye Birdie (1963 film)

text: The story was inspired by the phenomenon of singer Elvis Presley being drafted into the United States Army in 1957. Jesse Pearson plays the role of teen idol Conrad Birdie, whose character name is a word play on country singer Conway Twitty, who was, at that time, a teen idol pop artist. Presley himself was the first choice for the role of Birdie, but his manager, Colonel Tom Parker, rejected the idea as he did not want Presley in any roles that were parodies of himself or his career. Ed Sullivan appears as himself, host of the popular long-running CBS variety show. The film is credited with making Ann-Margret a superstar during the mid-1960s, leading to her appearing with Presley himself immediately after in "Viva Las Vegas" (shot in the summer of 1963, but only released in 1964).


title: Bye Bye Birdie (1963 film)

text: In 2006, the film was ranked number 38 on "Entertainment Weekly" list of the 50 Best High School Movies.


title: Bye Bye Birdie (1963 film)

text: In 1958, popular rock and roll star Conrad Birdie receives an Army draft notice, devastating his teenage fans nationwide. Despite his doctorate in biochemistry, unsuccessful songwriter Albert Peterson schemes with his secretary and long-suffering girlfriend Rosie DeLeon to have Conrad sing a song Albert will write. Rosie convinces Ed Sullivan to have Conrad perform Albert's song "One Last Kiss" on "The Ed Sullivan Show" and then kiss a randomly chosen high school girl goodbye before joining the Army. After this succeeds, Albert will feel free to marry Rosie, despite his widowed, meddlesome mother Mae's long history of interfering with her son's life.


title: Bye Bye Birdie (1963 film)

text: Columbus, Ohio, is chosen as the location for Conrad's farewell performance. The random lucky girl chosen, Kim MacAfee, is thrilled, unlike her high school sweetheart, Hugo Peabody. The teenagers of nearby Sweet Apple, blissfully unaware of their town's impending fame, are spending the "Telephone Hour" discussing the latest gossip: Kim and Hugo have just gotten pinned (a tradition where a boy gives a girl his fraternity pin, indicating a serious commitment to each other) and Kim feels grown up ("How Lovely to Be a Woman").


title: Bye Bye Birdie (1963 film)

text: Upon Conrad's arrival, the teenaged girls sing their anthem, "We Love You Conrad", but the boys despise him for stealing their girls' attention ("We Hate You Conrad!"). Sweet Apple becomes very popular, but some local adults are unhappy with the sudden celebrity, especially after Conrad's song "Honestly Sincere" and his hip-thrusting moves cause every woman, including the mayor's wife, to faint.


title: Bye Bye Birdie (1963 film)

text: Pressured by the town's notable citizens, Kim's father Harry declines to allow her to kiss Conrad on television, until Albert placates him by promising that his "whole family" will be on Sullivan's TV show ("Hymn for a Sunday Evening"). Albert reveals to Harry that he is actually a biochemist who has developed a miracle supplement for domestic animals that will make a hen lay three eggs a day; they test it on the family's pet tortoise, which speeds out the door. Harry, a fertilizer salesman, sees a great future for himself marketing this pill with Albert.


title: Bye Bye Birdie (1963 film)

text: Hugo feels threatened by Conrad, but Kim reassures him that he is the "One Boy" for her. Rosie, meanwhile, feels unappreciated by Albert, who persuades her to "Put on a Happy Face". Albert's mother Mae shows up, distressed to find the pair together; Harry is also agitated about Conrad's monopoly of his house and Kim's behavioral changes. Both lament the problems with "Kids" today.


title: Bye Bye Birdie (1963 film)

text: During rehearsal for the broadcast, an impatient Conrad kisses Kim (who swoons). Hugo is hurt and Kim and Hugo break up, with all three asserting that they have "A Lot of Livin' to Do". Informed the Russian Ballet has switched to a different dance requiring extra time, therefore eliminating Conrad's song and farewell kiss to Kim, Albert unsuccessfully attempts to convince the Ballet's manager to shorten its performance, and he dejectedly decides to drown his sorrows at Maude's Madcap Café.


title: Bye Bye Birdie (1963 film)

text: Surprisingly, he finds Mae there, playing canasta with the café's owner Mr. Maude, also a widower. Rosie, fed up with Albert and his mother, also goes to the café for "a night to remember". After ordering three drinks (but only gulping down one), Rosie goes into another room where the Shriners convention is taking place. She starts dancing and flirting with the men ("Sultans' Ballet"), but when the scene becomes too wild, Albert rescues her from the crazed Shriners.


title: Bye Bye Birdie (1963 film)

text: The next day, Rosie formulates how to get back Conrad's spot on "The Ed Sullivan Show" that evening. She slips one of Albert's pills into the orchestra conductor's milk, which speeds up the ballet, amusing the audience, offending the Russians and placing Conrad back on the show to sing "One Last Kiss". However, just as Conrad is about to kiss Kim, Hugo runs onstage and punches him out on the live telecast, which shocks Albert and Rosie.


title: Bye Bye Birdie (1963 film)

text: Kim and Hugo reunite. Albert is free to marry now ("Rosie") and his mother agrees, revealing her own marriage to Mr. Maude. All three couples live happily ever after. Kim, now wiser, bids Conrad a fond goodbye in "Bye Bye Birdie (Reprise)".


title: Bye Bye Birdie (1963 film)

text: Several significant changes were made in the plot and character relationships in the film from the stage version. The film was rewritten to showcase the talents of rising star Ann-Margret, adding the title song for her and dropping songs by certain other characters.


title: Bye Bye Birdie (1963 film)

text: According to Ann-Margret, she was cast when director George Sidney saw her dancing while on a date at the Sands Casino on New Year's Eve 1961.


title: Bye Bye Birdie (1963 film)

text: Sidney was so smitten with the rising new star that Janet Leigh was "very upset that all the close-ups were going to Ann-Margret", as Leigh herself was the lead star of the film.


title: Bye Bye Birdie (1963 film)

text: Sidney says originally he was only going to produce and Gower Champion would direct, but Champion told Sidney he could not see it as a film, so Sidney stepped in. "That was a great deal of fun," said Sidney. "It was a young people's picture, with a lot of bright, gay noisy cast members yelling and screaming."


title: Bye Bye Birdie (1963 film)

text: As of July 2019, "Bye Bye Birdie" holds a Rotten Tomatoes rating of 89% based on 28 reviews, with an average of 6.7/10. The critics' consensus states: "A poppy satire on pop music, "Bye Bye Birdie" is silly, light, and very, very pink."


title: Bye Bye Birdie (1963 film)

text: "Bye Bye Birdie" grossed $233,825 in its opening week at Radio City Music Hall in New York, a house record. It was the 13th highest-grossing film of 1963, grossing $13.1 million domestically, earning $6 million in rentals in the United States and Canada.


title: Bye Bye Birdie (1963 film)

text: The film was given a Royal Charity Premiere when released in the U.K. on 7 November 1963, at the Odeon Marble Arch in the presence of H.R.H. The Duke of Edinburgh.


title: Ross McCall

text: Ross McCall (born 13 January 1976) is a Scottish actor best known for his roles as T-5 Joseph Liebgott in the HBO miniseries "Band of Brothers" (2001) and Matthew Keller in the hit series "White Collar". He played the child version of Freddie Mercury in the music video for the Queen single "The Miracle".


title: Ross McCall

text: McCall was born in Port Glasgow to John and Maggie McCall but was raised in England from the age of 7, when his family relocated to Kent. His older brother, Stuart, is a policeman in Sussex. His father was a fire chief and his mother was a nurse. His parents later amicably divorced. Both live in England. His first notable screen role was at the age of 13 when he appeared as Freddie Mercury in the 1989 music video for the Queen song "The Miracle". In 1993, he was featured in BBC children's television series "The Return of the Borrowers".


title: Ross McCall

text: In 2001, McCall acted in the HBO miniseries "Band of Brothers". Years later, in 2005, Ross co-starred in the independent drama film "Green Street" (2005) and later reprised his role as Dave in the 2009 straight-to-video sequel "". He then played Kenny Battaglia in the television series "Crash", on the Starz network.


title: Ross McCall

text: In 2008, McCall co-starred in an After Dark Horrorfest film, "Autopsy". On 23 February 2010, he appeared as a guest star on "White Collar" in the episode "Bottlenecked" as Matthew Keller, an old rival of lead character Neal. He reprised the role in Episode 14 of Season 2, Episodes 9-11 of Season 3 and Episodes 2-6 of Season 6. On 1 June 2010, he appeared as a guest star on "Luther". "Serving Up Richard", shot at the Sunset Gower studios in LA, was released in 2011. He also had a recurring role as Ron Clark on "".


title: Ross McCall

text: In June 2016, McCall released a short documentary detailing his trip to the Faroe Islands, and his support of Sea Shepherd's campaign, 'Operation Sleppið Grindini'. In the documentary, he interviewed several locals about their views on the controversial traditional pilot whale drive hunt known as the 'Grindadrap'. He also interviews several Sea Shepherd volunteers stationed in the Islands monitoring the activities of the hunters.


title: Ross McCall

text: McCall was engaged to American actress Jennifer Love Hewitt, with whom he guest starred in an episode of "Ghost Whisperer". On 5 January 2009, "People" magazine reported that Hewitt called off their engagement in late 2008.


title: Rendlesham Forest incident

text: The Rendlesham Forest incident was a series of reported sightings of unexplained lights near Rendlesham Forest in Suffolk, England in late December 1980 which became linked with claims of UFO landings. The events occurred just outside RAF Woodbridge, which was used at the time by the United States Air Force (USAF). USAF personnel, including deputy base commander Lieutenant Colonel Charles I. Halt, claimed to see things they described as a UFO sighting.


title: Rendlesham Forest incident

text: The occurrence is the most famous of claimed UFO events to have happened in the United Kingdom, and is among the best-known reported UFO events worldwide. It has been compared to the Roswell UFO incident in the United States and is sometimes referred to as "Britain's Roswell".


title: Rendlesham Forest incident

text: The UK Ministry of Defence has stated the event posed no threat to national security, and therefore, it was never investigated as a security matter. Sceptics have explained the sightings as a misinterpretation of a series of nocturnal lights: a fireball, the Orfordness Lighthouse and bright stars.


title: Rendlesham Forest incident

text: Around 03:00 on 26 December 1980 (reported as 27 December by Halt in his memo to the UK Ministry of Defence) a security patrol near the east gate of RAF Woodbridge saw lights apparently descending into nearby Rendlesham Forest. These lights have been attributed by astronomers to a piece of natural debris seen burning up as a fireball (meteor) over southern England at that time. Servicemen initially thought it was a downed aircraft but, upon entering the forest to investigate they saw, according to Halt's memo, what they described as a glowing object, metallic in appearance, with coloured lights. As they attempted to approach the object, it appeared to move through the trees, and "the animals on a nearby farm went into a frenzy". One of the servicemen, Sergeant Jim Penniston, later claimed to have encountered a "craft of unknown origin" while in the forest, although there was no publicised mention of this at the time and there is no corroboration from other witnesses.


title: Rendlesham Forest incident

text: Shortly after 04:00 local police were called to the scene but reported that the only lights they could see were those from the Orford Ness lighthouse, some miles away on the coast.


title: Rendlesham Forest incident

text: After daybreak on the morning of 26 December, servicemen returned to a small clearing near the eastern edge of the forest and found three small impressions on the ground in a triangular pattern, as well as burn marks and broken branches on nearby trees. At 10:30 the local police were called out again, this time to see the impressions, which they thought could have been made by an animal. Georgina Bruni, in her book "You Can't Tell the People", published a photograph of the supposed landing site taken on the morning after the first sighting.


title: Rendlesham Forest incident

text: The deputy base commander, Lieutenant Colonel Charles Halt, visited the site with several servicemen in the early hours of 28 December 1980 (reported as 29 December by Halt). They took radiation readings in the triangle of depressions and in the surrounding area using an AN/PDR-27, a standard U.S. military radiation survey meter. Although they recorded 0.07 milliroentgens per hour, in other regions they detected 0.03 to 0.04 milliroentgens per hour, around the background level. Furthermore, they detected a similar small 'burst' over half a mile away from the landing site. Halt recorded the events on a micro-cassette recorder (see § The Halt Tape).


title: Rendlesham Forest incident

text: It was during this investigation that a flashing light was seen across the field to the east, almost in line with a farmhouse, as the witnesses had seen on the first night. The Orford Ness lighthouse is visible further to the east in the same line of sight.


title: Rendlesham Forest incident

text: Later, according to Halt's memo, three star-like lights were seen in the sky, two to the north and one to the south, about 10 degrees above the horizon. Halt said that the brightest of these hovered for two to three hours and seemed to beam down a stream of light from time to time. Astronomers have explained these star-like lights as bright stars.


title: Rendlesham Forest incident

text: Rendlesham Forest is owned by the Forestry Commission and consists of about of coniferous plantations, interspersed with broadleaved belts, heathland and wetland areas. It is located in the county of Suffolk, about east of the town of Ipswich.


title: Rendlesham Forest incident

text: The incident occurred in the vicinity of two former military bases: RAF Bentwaters, which is just to the north of the forest, and RAF Woodbridge which extends into the forest from the west and is bounded by the forest on its northern and eastern edges. At the time, both were being used by the United States Air Force and were under the command of wing commander Colonel Gordon E. Williams. The base commander was Colonel Ted Conrad, and his deputy was Lieutenant Colonel Charles I. Halt.


title: Rendlesham Forest incident

text: The main events of the incident, including the supposed landing or landings, took place in the forest, which starts at the east end of the base runway or about 0.3 miles (0.5 km) to the east of the East Gate of RAF Woodbridge, from where security guards first noticed mysterious lights appearing to descend into the forest. The forest extends east about one mile (1.6 km) beyond East Gate, ending at a farmer's field at Capel Green, where additional events allegedly took place.


title: Rendlesham Forest incident

text: Orfordness Lighthouse, which sceptics identify as the flashing light seen off to the coast by the airmen, is along the same line of sight about further east of the forest's edge. At that time it was one of the brightest lighthouses in the UK.


title: Rendlesham Forest incident

text: The first piece of primary evidence to be made available to the public was a memorandum written by the deputy base commander, Lieutenant Colonel Charles I. Halt, to the Ministry of Defence (MoD). Known as the "Halt memo", this was made publicly available in the United States under the U.S. Freedom of Information Act in 1983. The memorandum, was dated "13 Jan 1981" under the title "Unexplained Lights". The two-week delay between the incident and the report might account for errors in the dates and times given. The memo was not classified in any way.


title: Rendlesham Forest incident

text: David Clarke, a consultant to the National Archives, has investigated the background of this memo and the reaction to it at the MoD. His interviews with the personnel involved confirmed the cursory nature of the investigation made by the MoD, and failed to find any evidence for any other reports on the incident made by the USAF or UK apart from the Halt memo. Halt has since gone on record as saying he believes that he witnessed an extraterrestrial event that was then covered up.


title: Rendlesham Forest incident

text: In 1984, a copy of what became known as the "Halt Tape" was released to UFO researchers by Colonel Sam Morgan, who had by then succeeded Ted Conrad as Halt's superior. This tape chronicles Halt's investigation in the forest in real time, including taking radiation readings, the sighting of the flashing light between trees, and the starlike objects that hovered and twinkled. The tape has been transcribed by researcher Ian Ridpath, who includes a link to an audio download and also a step-by-step analysis of the entire contents of the tape.


title: Rendlesham Forest incident

text: In 1997, Scottish researcher James Easton obtained the original witness statements made by those involved in the first night's sightings. One of the witnesses, Ed Cabansag, said in his statement: "We figured the lights were coming from past the forest since nothing was visible when we passed through the woody forest. We would see a glowing near the beacon light, but as we got closer we found it to be a lit-up farmhouse. We got to a vantage point where we could determine that what we were chasing was only a beacon light off in the distance." Another participant, John Burroughs, also stated: "We could see a beacon going around so we went towards it. We followed it for about two miles [3 km] before we could [see] it was coming from a lighthouse."


title: Rendlesham Forest incident

text: Burroughs reported a noise "like a woman was screaming" and also that "you could hear the farm animals making a lot of noises." Halt heard the same noises two nights later. Such noise could have been made by Muntjac deer in the forest, which are known for their loud, shrill bark when alarmed.


title: Rendlesham Forest incident

text: In June 2010, retired Colonel Charles Halt signed a notarised affidavit, in which he again summarised what had happened, then stated he believed the event to be extraterrestrial and it had been covered up by both the UK and US. Contradictions between this affidavit and the facts as recorded at the time in Halt's memo and tape recording have been pointed out.


title: Rendlesham Forest incident

text: In 2010, base commander Colonel Ted Conrad provided a statement about the incident to Clarke. Conrad stated that "We saw nothing that resembled Lieutenant Colonel Halt's descriptions either in the sky or on the ground" and that "We had people in position to validate Halt's narrative, but none of them could." In an interview, Conrad criticised Halt for the claims in his affidavit, saying "he should be ashamed and embarrassed by his allegation that his country and Britain both conspired to deceive their citizens over this issue. He knows better." Conrad also disputed the testimony of Sergeant Jim Penniston, who claimed to have touched an alien spacecraft; he said that he interviewed Penniston at the time and he had not mentioned any such occurrence. Conrad also suggested that the entire incident might have been a hoax.


title: Rendlesham Forest incident

text: A 1983 Omni article says "Colonel Ted Conrad the base commander... recalls five Air Force policemen spotted lights from what they thought was a small plane descending into the forest. Two of the men tracked the object on foot and came upon a large tripod-mounted craft. It had no windows but was studded with brilliant red and blue lights. Each time the men came within 50 yards of the ship, Conrad relates, it levitated six feet in the air and backed away. They followed it for almost an hour through the woods and across a field until it took off at 'phenomenal speed.' Acting on the reports made by his men, Colonel Conrad began a brief investigation of the incident in the morning. He went into the forest and located a triangular pattern ostensibly made by the tripod legs. ...he did interview two of the eyewitnesses and concludes, 'Those lads saw something, but I don't know what it was'."


title: Rendlesham Forest incident

text: Suffolk police were called to the scene on the night of the initial sighting and again the following morning but found nothing unusual. On the night of the initial incident they reported that the only lights visible were from the Orford lighthouse. They attributed the indentations in the ground to animals. The Suffolk constabulary file on the case was released in 2005 under the UK's Freedom of Information Act and can be accessed on their website. It includes a letter dated 28 July 1999 written by Inspector Mike Topliss who notes that one of the police constables who attended the scene on the first night returned to the site in daylight in case he had missed something. "There was nothing to be seen and he remains unconvinced that the occurrence was genuine," wrote Topliss. "The immediate area was swept by powerful light beams from a landing beacon at RAF Bentwaters and the Orfordness lighthouse. I know from personal experience that at night, in certain weather and cloud conditions, these beams were very pronounced and certainly caused strange visual effects."


title: Rendlesham Forest incident

text: Evidence of a substantial MoD file on the subject led to claims of a cover-up; some interpreted this as part of a larger pattern of information suppression concerning the true nature of unidentified flying objects, by both the United States and British governments. However, when the file was released in 2001 it turned out to consist mostly of internal correspondence and responses to inquiries from the public. The lack of any in-depth investigation in the publicly released documents is consistent with the MoD's earlier statement that they never took the case seriously. Included in the released files is an explanation given by defence minister Lord Trefgarne as to why the MoD did not investigate further.


title: Rendlesham Forest incident

text: One proposed theory is that the incident was a hoax. The BBC reported that a former U.S. security policeman, Kevin Conde, claimed responsibility for creating strange lights in the forest by driving around in a police vehicle whose lights he had modified. However, there is no evidence that this prank took place on the nights in question.


title: Rendlesham Forest incident

text: Other explanations for the incident have included a downed Soviet spy satellite, but no evidence has been produced to support this.


title: Rendlesham Forest incident

text: The most plausible sceptical explanation is that the sightings were due to a combination of three main factors. The initial sighting at 03.00 on 26 December, when the airmen saw something apparently descending into the forest, coincided with the appearance of a bright fireball over southern England, and such fireballs are a common source of UFO reports. The supposed landing marks were identified by police and foresters as rabbit diggings. No evidence has emerged to confirm that anything actually came down in the forest.


title: Rendlesham Forest incident

text: According to the witness statements from 26 December, the flashing light seen from the forest lay in the same direction as the Orfordness Lighthouse. When the eyewitnesses attempted to approach the light they realised it was further off than they thought. One of the witnesses, Ed Cabansag, described it as “a beacon light off in the distance” while another, John Burroughs, said it was “a lighthouse” (see Statements from eyewitnesses on 26 December,


title: Rendlesham Forest incident

text: Timings on Halt's tape recording during his sighting on 28 December indicate that the light he saw, which lay in the same direction as the light seen two nights earlier, flashed every five seconds, which was the flash rate of the Orfordness Lighthouse.


title: Rendlesham Forest incident

text: The star-like objects that Halt reported hovering low to the north and south are thought by some sceptics to have been misinterpretations of bright stars distorted by atmospheric and optical effects, another common source of UFO reports. The brightest of them, to the south, matched the position of Sirius, the brightest star in the night sky.


title: Rendlesham Forest incident

text: In his 6 January 2009 Skeptoid podcast episode titled "The Rendlesham Forest UFO," scientific sceptic author Brian Dunning evaluated the original eye-witness reports and audio recordings, as well as the resulting media reporting of this incident. After a lengthy analysis Dunning concluded:


title: Rendlesham Forest incident

text: In 2005, the Forestry Commission used Lottery proceeds to create a trail in Rendlesham Forest because of public interest and nicknamed it the "UFO Trail". In 2014, the Forestry Service commissioned an artist to create a work which has been installed at the end of the trail. The artist states the piece is modelled on sketches that purportedly represent some versions of the UFO claimed to have been seen at Rendlesham.


title: Rendlesham Forest incident

text: In 2010, Jenny Randles, who first reported the case in the "London Evening Standard" in 1981 and co-authored with the local researchers who uncovered the events, the first book on the case in 1984, "Sky Crash: A Cosmic Conspiracy", emphasised her previously expressed doubts that the incident was caused by extraterrestrial visitors. Whilst suggesting that a UAP, an unidentified atmospheric phenomenon of unknown origin, might have caused parts of the case, she noted: "Whilst some puzzles remain, we can probably say that no unearthly craft were seen in Rendlesham Forest. We can also argue with confidence that the main focus of the events was a series of misperceptions of everyday things encountered in less than everyday circumstances."


title: Rendlesham Forest incident

text: In December 2018, David Clarke, a British UFO researcher, reported a claim that the incident was a set-up by the SAS as a revenge plot on the USAF. According to this story, in August 1980, the SAS parachuted into RAF Woodbridge to test the security at the nuclear site. The USAF had recently upgraded their radar and detected the black parachutes of the SAS men as they descended to the base. The SAS troops were interrogated and beaten up, with the ultimate insult that they were called "unidentified aliens". To enact their revenge, the SAS "gave" the USAF their own version of an alien event; "...as December approached, lights and coloured flares were rigged in the woods. Black helium balloons were also coupled to remote-controlled kites to carry suspended materials into the sky, activated by radio-controls." However, Clarke's investigation concluded that the story was itself a hoax.


title: Thomas Ravenel

text: Thomas Jonathan Jackson Ravenel (born August 11, 1962) is an American politician and reality television star. He is the son of former Representative Arthur Ravenel Jr. from South Carolina.


title: Thomas Ravenel

text: He starred for five seasons on the Bravo reality-television series "Southern Charm," was state treasurer for six months, an Independent candidate for the United States Senate in the 2014 election in South Carolina, and a Republican candidate for U.S. Senate in the 2004 primary election. He is also a member of the French Huguenot Church in Charleston.


title: Thomas Ravenel

text: Prior to running for office, Ravenel founded the Ravenel Development Corporation, a commercial real estate company.


title: Thomas Ravenel

text: Ravenel ran for the U.S. Senate in 2004, seeking the seat left open when Fritz Hollings retired. He came in third in the Republican primary election. After Ravenel endorsed Republican Jim DeMint, DeMint went on to win the Republican runoff and the general election. Subsequently, Ravenel was fined over $19,000 by the Federal Election Commission for improperly filing the required forms for the election. Ravenel ran for South Carolina Treasurer in 2006, defeating nine-term state treasurer Democrat Grady Patterson in the 2006 general election. Ravenel endorsed Ron Paul in the 2012 Republican primaries.


title: Thomas Ravenel

text: Ravenel was previously in the cast of Bravo’s "Southern Charm". Ravenel lived at Brookland Plantation, an estate he purchased in 2006, until he sold it in 2021.


title: Thomas Ravenel

text: Ravenel graduated from The Citadel in Charleston, receiving a Bachelor of Science in Business Administration (B.S.B.A.) in 1985. He later received an M.B.A. from the University of South Carolina in 1991. His M.B.A. studies were focused on finance and real estate.


title: Thomas Ravenel

text: In June 2007, Ravenel was indicted on federal cocaine distribution charges by US Attorney Reginald Lloyd. The filed charges allege that Ravenel purchased less than 500 grams of cocaine through an intermediary in Charleston, South Carolina, in 2005, prior to his election as treasurer but after his abortive entry in the 2004 Senate race.


title: Thomas Ravenel

text: Governor Mark Sanford suspended Ravenel from his duties as state treasurer upon learning of the charges, and appointed Columbia attorney Ken Wingate to replace Ravenel on an interim basis. Meanwhile, Ravenel decided to enter a rehabilitation facility, according to prosecutors.


title: Thomas Ravenel

text: Ravenel had also been volunteering for the Rudolph Giuliani presidential campaign, serving as his South Carolina state chairman. Ravenel resigned from that post after the indictment, according to Giuliani campaign officials. The campaign subsequently replaced Ravenel with Barry Winn, who was co-chair of South Carolina finance committee.


title: Thomas Ravenel

text: On July 24, 2007 Ravenel resigned as Treasurer. The South Carolina General Assembly elected state Rep. Converse Chellis to serve out the remainder of his term.


title: Thomas Ravenel

text: Ravenel underwent a rehabilitation program in Arizona, and then returned to plead guilty to "conspiring to buy and distribute less than 100 grams of cocaine" in September 2007.


title: Thomas Ravenel

text: On Friday, March 4, 2008, Thomas Ravenel was sentenced to 10 months in federal prison for drug charges against him, but was not immediately ordered to report to begin his sentence.


title: Thomas Ravenel

text: On Monday, July 14, 2014, Thomas Ravenel submitted more than 17,000 signatures, officially placing him on the ballot as an independent candidate against incumbent Senator Lindsey Graham. He finished in third place, garnering 3.9% of the vote.


title: Thomas Ravenel

text: Ravenel and his ex-girlfriend, Kathryn Calhoun Dennis, have two children, a girl and a boy. His third child, a son, was born on June 29, 2020 with ex-girlfriend Heather Mascoe.


title: Thomas Ravenel

text: In April 2018, Ashley Perkins, a Florida-based real estate agent, claimed that Ravenel sexually battered her mother, Debbie Holloway Perkins, after meeting him on the dating app Tinder in December 2015. Perkins claims her mother later told her that Ravenel "stuck his penis in her face" and held her arms back while sticking his fingers inside her vagina and anus. Perkins said she did not witness the battery, but that her mother seemed "very withdrawn" and finally confided in her. According to Perkins, her mother is unable to publicly discuss the case due to a non-disclosure agreement negotiated by attorney Gloria Allred as part of an alleged $200,000 settlement with Ravenel. However, Perkins, who claims to be a witness to the mediation process between her mother and Ravenel, was not bound to such an agreement and therefore able to publicly disclose the details of the case.


title: Thomas Ravenel

text: Richard P. Terbrusch, Ravenel's attorney, denied Perkins' allegations, stating that his "client enjoys a certain degree of fame and unfortunately has become—unfairly—a target for an individual who has, in my opinion, dubious motivations." Bravo and "Southern Charm" production company, Haymaker, announced on May 4, 2018 that it was taking the "allegations very seriously" and launching its own investigation into their validity.


title: Thomas Ravenel

text: In May 2018, a second accuser—Dawn Ledwell, former nanny of Ravenel's children—came forward (Dawn admitted in court filings Kathryn Dennis pushed Dawn to come forward) with new allegations of sexual battery against Ravenel. "Nanny Dawn" (as she was known to "Southern Charm"s viewers), and close friend of Kathryn Dennis, claims that in January 2015, Ravenel assaulted her after a night out with his friends: "I went upstairs to get my jacket which had the key to my vehicle in the pocket. I was corralled into the master bedroom, and he shut the door behind me. I turned around, and he had his pants dropped, not wearing underwear," she said. "He was blocking the door. I was mortified, embarrassed, and scared." Charleston police opened an investigation into the alleged assault after the nanny filed an incident report in early May.


title: Thomas Ravenel

text: In September 2018, Ashley Perkins published on her official website that Ledwell had first contacted her that spring, prior to going to the police. At that time, Ledwell shared with Perkins "very intimate details of her life and events with Thomas". By May, however, Perkins said she had "serious doubts" about Ledwell's accusations, stating that Ledwell's police report and public accounts of "forcible rape" differed from what Ledwell had told her privately. As a result, Perkins cut off her relationship with Ledwell, but "wished her the best". February 2019, Dawn Ledwell amended her original summons and complaint through the courts where the sexual contact is no longer listed.


title: Thomas Ravenel

text: On September 11, 2019, Ravenel pled guilty to third-degree assault and battery in connection with an attack on Ledwell. He will pay a $500 fine, but serve no jail time. He also settled a civil suit with Ledwell, agreeing to donate $80,000 in her name to a charity for sexual assault survivors in lieu of paying her damages.


title: Dál Riata

text: Dál Riata or Dál Riada (also Dalriada) () was a Gaelic kingdom that encompassed the western seaboard of Scotland and north-eastern Ireland, on each side of the North Channel. At its height in the 6th and 7th centuries, it covered what is now Argyll ("Coast of the Gaels") in Scotland and part of County Antrim in Northern Ireland. After a period of expansion, Dál Riata eventually became associated with the Gaelic Kingdom of Alba.


title: Dál Riata

text: In Argyll, it consisted of four main kindreds, each with their own chief: Cenél nGabráin (based in Kintyre), Cenél nÓengusa (based on Islay), Cenél Loairn (who gave their name to the district of Lorn) and Cenél Comgaill (who gave their name to Cowal). The hillfort of Dunadd is believed to have been its capital. Other royal forts included Dunollie, Dunaverty and Dunseverick. Within Dál Riata was the important monastery of Iona, which played a key role in the spread of Celtic Christianity throughout northern Britain, and in the development of insular art. Iona was a centre of learning and produced many important manuscripts. Dál Riata had a strong seafaring culture and a large naval fleet.


title: Dál Riata

text: Dál Riata is said to have been founded by the legendary king Fergus Mór (Fergus the Great) in the 5th century. The kingdom reached its height under Áedán mac Gabráin (). During his reign Dál Riata's power and influence grew; it carried out naval expeditions to Orkney and the Isle of Man, and assaults on the Brittonic kingdom of Strathclyde and Anglian kingdom of Bernicia. However, King Æthelfrith of Bernicia checked its growth at the Battle of Degsastan in 603. Serious defeats in Ireland and Scotland during the reign of Domnall Brecc (died 642) ended Dál Riata's "golden age", and the kingdom became a client of Northumbria for a time. In the 730s the Pictish king Óengus I led campaigns against Dál Riata and brought it under Pictish overlordship by 741. There is disagreement over the fate of the kingdom from the late 8th century onwards. Some scholars have seen no revival of Dál Riatan power after the long period of foreign domination ( to ), while others have seen a revival under Áed Find (736–778). Some even claim that the Dál Riata usurped the kingship of Fortriu. From 795 onward there were sporadic Viking raids in Dál Riata. In the following century, there may have been a merger of the Dál Riatan and Pictish crowns. Some sources say Cináed mac Ailpín (Kenneth MacAlpin) was king of Dál Riata before becoming king of the Picts in 843, following a disastrous defeat of the Picts by Vikings. The kingdom's independence ended sometime after, as it merged with Pictland to form the Kingdom of Alba.


title: Dál Riata

text: Latin sources often referred to the inhabitants of Dál Riata as "Scots" ("Scoti"), a name originally used by Roman and Greek writers for the Irish Gaels who raided and colonized Roman Britain. Later, it came to refer to Gaels, whether from Ireland or elsewhere. They are referred to herein as "Gaels" or as "Dál Riatans".


title: Dál Riata

text: The name ' is derived from Old Irish. ', from Proto-Celtic "*dālom", means 'portion' or 'share' (as in 'a portion of land'); "Riata" or "Riada" is believed to be a personal name. Thus, the name refers to "Riada's portion" of territory in the area.


title: Dál Riata

text: The Dalradian geological series, a term coined by Archibald Geikie in 1891, was named after Dál Riata because its outcrop has a similar geographical reach to that of the former kingdom.


title: Dál Riata

text: Dál Riata spanned the North Channel and included parts of western Scotland and northeastern Ireland. In Scotland, it corresponded roughly to Argyll (from "Airer Goídel", "coast of the Gaels") and later grew to include Skye. In Ireland, it took in the northeast of County Antrim, roughly corresponding to the baronies of Cary and Glenarm.


title: Dál Riata

text: The modern human landscape of Dál Riata differs a great deal from that of the first millennium. Most people today live in settlements far larger than anything known in early times, while some areas, such as Kilmartin, and many of the islands, such as Islay and Tiree, may well have had as many inhabitants as they do today. Many of the small settlements have now disappeared so that the countryside is far emptier than was formerly the case, and many areas that were formerly farmed are now abandoned. Even the physical landscape is not entirely as it was: sea levels have changed, and the combination of erosion and silting will have considerably altered the shape of the coast in some places, while the natural accumulation of peat and man-made changes from peat-cutting have altered inland landscapes.


title: Dál Riata

text: As was normal at the time, subsistence farming was the occupation of most people. Oats and barley were the main cereal crops. Pastoralism was especially important, and transhumance (the seasonal movement of people with their livestock between fixed summer and winter pastures) was the practice in many places. Some areas, most notably Islay, were especially fertile, and good grazing would have been available all year round, just as it was in Ireland. Tiree was famed in later times for its oats and barley, while smaller, uninhabited islands were used to keep sheep. The area, until lately, was notable for its inshore fisheries, and for plentiful shellfish, therefore seafood is likely to have been an important part of the diet.


title: Dál Riata

text: The "Senchus fer n-Alban" lists three main kin groups in Dál Riata in Scotland, with a fourth being added later:


title: Dál Riata

text: The Senchus does not list any kindreds in Ireland, but does list an apparently very minor kindred called Cenél Chonchride in Islay descended from another son of Erc, Fergus Becc. Another kindred, Cenél Báetáin of Morvern (later Clan MacInnes), branched off from Cenél Loairn about the same time that Cenél Comgaill separated from its parent kindred. The Morvern district was formerly known as Kinelvadon, after the Cenél Báetáin. The Cenél Loairn may have been the largest of the "three kindreds", as the Senchus reports it being divided further into Cenél Shalaig, Cenél Cathbath, Cenél nEchdach, Cenél Murerdaig. Among the Cenél Loairn it also lists the Airgíalla, although whether this should be understood as being Irish settlers or simply another tribe to whom the label was applied is unclear. Bannerman proposes a tie to the Uí Macc Uais. The meaning of Airgíalla 'hostage givers' adds to the uncertainty, although it must be observed that only one grouping in Ireland was apparently given this name and it is therefore very rare, perhaps supporting the Ui Macc Uais hypothesis. There is no reason to suppose that this is a complete or accurate list.


title: Dál Riata

text: Four sites in Dál Riata may have had royal associations: Dunadd, Dunollie, Dunaverty and Tarbert. Among them, Dunadd appears to have been the most important. It has been partly excavated, and weapons, quern-stones and many moulds for the manufacture of jewellery were found in addition to fortifications. Other high-status materials included glassware and wine amphorae from Gaul, and in larger quantities than found elsewhere in Britain and Ireland. Lesser centres included Dun Ollaigh, seat of the Cenél Loairn kings, and Dunaverty, at the southern end of Kintyre, in the lands of the Cenél nGabráin. The main royal centre in Ireland appears to have been at Dunseverick ("Dún Sebuirge").


title: Dál Riata

text: Dál Riata had a strong seafaring culture. It was an archipelago with many islands and peninsulas. This, and the difficulty of overland travel, meant that travel by sea was the easiest means of moving any distance. As well as long-distance trade, local trade must also have been significant. Currachs were probably the most common seagoing craft, and on inland waters dugouts and coracles were used. Large timber ships, called "long ships", perhaps similar to the Viking ships of the same name, are attested to in a variety of sources. Dál Riata had a large war fleet manned by skilled sailors, capable of undertaking far-reaching expeditions. It had an organized system for manning the fleet. Houses were grouped into twenties for the purpose of naval recruitment, with each group having to provide a quota of 28 oarsmen.


title: Dál Riata

text: No written accounts exist for pre-Christian Dál Riata, and the earliest known records come from the chroniclers of Iona and Irish monasteries. Adomnán's "Life of St Columba" implies a Christian Dál Riata. Whether this is true cannot be known. The figure of Columba looms large in any history of Christianity in Dál Riata. Adomnán's "Life", although useful as a record, was not intended to serve as history, but rather as hagiography. Because the writing of the lives of the saints in Adomnán's day had not reached the stylised formulas of the High Middle Ages, the "Life" contains a great deal of historically valuable information. It is also a vital linguistic source indicating the distribution of Gaelic and P-Celtic placenames in northern Scotland by the end of the 7th century. It famously notes Columba's need for a translator when conversing with an individual on Skye. This evidence of a non-Gaelic language is supported by a sprinkling of P-Celtic placenames on the remote mainland opposite the island.


title: Dál Riata

text: Columba's founding Iona within the bounds of Dál Riata ensured that the kingdom would be of great importance in the spread of Christianity in northern Britain, not only to Pictland, but also to Northumbria, via Lindisfarne, to Mercia, and beyond. Although the monastery of Iona belonged to the Cenél Conaill of the Northern Uí Néill, and not to Dál Riata, it had close ties to the Cenél nGabráin, ties which may make the annals less than entirely impartial.


title: Dál Riata

text: If Iona was the greatest religious centre in Dál Riata, it was far from unique. Lismore, in the territory of the Cenél Loairn, was sufficiently important for the death of its abbots to be recorded with some frequency. Applecross, probably in Pictish territory for most of the period, and Kingarth on Bute are also known to have been monastic sites, and many smaller sites, such as on Eigg and Tiree, are known from the annals. In Ireland, Armoy was the main ecclesiastical centre in early times, associated with Saint Patrick and with Saint Olcán, said to have been the first bishop at Armoy. An important early centre, Armoy later declined, overshadowed by the monasteries at Movilla (Newtownards) and Bangor.


title: Dál Riata

text: As well as their primary spiritual importance, the political significance of religious centres cannot be dismissed. The prestige of being associated with the saintly founder was of no small importance. Monasteries represented a source of wealth as well as prestige. Additionally, the learning and literacy found in monasteries served as useful tools for ambitious kings.


title: Dál Riata

text: The illuminated manuscript Book of Kells was probably at least begun at Iona, although not by Columba as legend has it, as it dates from about 800 (it may have been commissioned to mark the bicentennial of Columba's death in 597). Whether it was or not, Iona was certainly important in the formation of Insular art, which combined Mediterranean, Anglo-Saxon, Celtic and Pictish elements into a style of which the book of Kells is a late example.


title: Dál Riata

text: For other arts, a number of sculptures remain to give an impression of Dál Riatan work. The St. Martin's Cross on Iona is the best-preserved high cross, probably inspired by Northumbrian free-standing crosses, such as the Ruthwell Cross, although a similar cross exists in Ireland (Ahenny, County Tipperary). The Kildalton Cross on Islay is similar. A sculpted slab at Ardchattan appears to show strong Pictish influences, while the Dupplin Cross, it has been argued, shows that influences also moved in the opposite direction. Fine Hiberno-Saxon metalwork such as penannular brooches is believed to have been created at Dunadd.


title: Dál Riata

text: In addition to the monastic sites, a considerable number of churches are attested, not only from archaeological evidence, but also from the evidence of place names. The element "kil", from Gaelic "cill", can be shown in many cases to be associated with early churches, such as at Kilmartin by Dunadd.


title: Dál Riata

text: The 11th century "Duan Albanach" (Song of the Scots) tells that the three sons of Erc—Fergus Mór, Loarn and Óengus—conquered "Alba" (Scotland) in around 500 AD. The 8th century writer Bede offers another, and probably older, account wherein Dál Riata was conquered by Irish Gaels led by a certain "Reuda". Old Irish "dál" means 'portion' or 'share', and is usually followed by the name of an eponymous founder. Bede's tale may come from the same root as the Irish tales of Cairpre Riata and his brothers, the Síl Conairi (sons/descendants of Conaire Cóem and Conaire Mór). The story of Dál Riata moves from foundation myth to something nearer to history with the reports of the death of Comgall mac Domangairt around 540 and of his brother Gabrán around 560.


title: Dál Riata

text: The version of history in the "Duan Albanach" was long accepted, although it is preceded by the fictional tale of Albanus and Brutus conquering Britain. Traditionally, the presence of Gaelic in Scotland has been seen as the result of either a migration from Ireland, or a takeover by Irish Gaelic elites. However, in his academic paper "Were the Scots Irish?", archaeologist Dr Ewan Campbell says that there is no archaeological or placename evidence of a migration or takeover. This lack of archaeological evidence was previously noted by Professor Leslie Alcock. Campbell suggests that Argyll and Antrim formed a "maritime province", united by the sea and isolated from the rest of Scotland by the mountains of the Highlands, historically called the "Druim Alban". This hypothetical separation allowed a shared language to be maintained through the centuries; Argyll remained Gaelic-speaking while the rest of Scotland spoke either Pictish or another Brittonic language. Campbell suggests that the medieval accounts were a kind of dynastic propaganda, constructed to bolster a dynasty's claim to the throne and to bolster Dál Riata claims to territory in Antrim. Although this view of the medieval accounts is shared by other historians, his theory has been challenged.


title: Dál Riata

text: Irish scholar Eoin MacNeill postulated that Scottish Dál Riata came about in two stages. He conjectured that Irish settlements were founded in Argyll at the time of Irish raids on Britain, during the end of Roman rule. Later, as these settlements became economically and politically more significant than the home territory, its rulers moved from Ireland to Argyll.