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Part C. InSAR processing: a mathematical approach
- Marlene Robinson
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1 Part C InSAR processing: a mathematical approach
3 Statistics of SAR and InSAR images 1. Statistics of SAR and InSAR images 1.1 The backscattering process Introduction The dimension of the resolution cell of a SAR survey is much greater than the wavelength of the impingeing radiation. Roughly speaking, targets that have small reflectivity and are more distant than a wavelength, backscatter independently. The amplitude of the focused signal corresponds to the algebraic combination of all the reflections from independent scatterers within the cell, with their proper amplitudes and phases. This superposition of effects is only approximate as one should consider not only the primary reflections (i.e. satellite target satellite), but also multiple ones (say satellite tree trunk ground satellite). Anyway, the focused signal is the combination of many independent reflections, with the possibility that some of them are much higher than the others. Therefore, statistics is the main tool to describe the backscattered signal: the probability density of the returns will be approximately Gaussian as the probability density of the sum of several independent complex numbers tends to be Gaussian, for the central limit theorem. The power of the reflections is additive, as usual. In this section we shall consider the amplitudes of the returns, first in the cases of artificial and then of natural back scatterers. Obviously, as the artificial reflectors of interest are those highly visible from the satellite, they will correspond to scatterers made in such a way as to concentrate the incoming energy back towards the receiver; as the receiver is far away, the curvature of the surfaces will be small, and the scatterer will look like a mirror or a combination of mirrors. These scatterers are characterised by their Radar Cross Section (RCS), i.e. the effective surface area that intercepts the incident wave and that scatters the energy isotropically into space. The behaviour of natural scatterers, on the other hand, is better described by using the superposition principle and statistics Artificial backscatterers Radiation coming from a satellite impinges on all sorts of backscatterers on the terrain under observation. If the surface of the scatterer is flat, the radiation is in general mirrored away from the satellite. For this reason, flat surfaces (lakes, quiet seawater, etc.) appear mostly black in the intensity images. However, it may also happen that a flat surface mirrors back towards the satellite (say, a flat roof with the right inclination and azimuth) and in this case the pixel appears very bright, as the RCS is high. Other manmade scatterers that appear bright are long cylinders (wires) aligned along the azimuth direction or vertically (the polarisation of the radiation in the case of ERS is VV). Dihedral scatterers (e.g. the corners formed by C-3
4 TM-19 InSAR Principles buildings) parallel to the azimuth direction are also bright. The directivity of a dihedral filling a resolution cell is very high, since this cell is generally about 5 metres long. Any change in the azimuth direction that makes the ends of the dihedral move ±λ/4 in the satellite direction would eliminate the backscatter, and this corresponds to a beam-width of about 0.33 degrees. Finally, metallic mirrors pointed precisely in the direction of the satellite ensure a bright reflection with a very high directivity if the radar cross section is high enough. Taking into account the two-way path and the directivity, the radar cross section of a mirror pointed towards the satellite is: where A is the area of the mirror. A λ S S = 4π A = Equation 1.1 λ π Assuming a square mirror with side length l, in the case of ERS we have: S = 393 l 4 Equation 1. If we wish a mirror to appear much brighter than the surroundings, its reflectivity should be, say, 100 times that of the neighbouring pixels. Now, the area of a pixel for Single Look Complex data is about 90 m. If we suppose that the normalised local reflectivity of the terrain is σ 0 = 6dB, then: 0.6 S > Equation 1.3 Figure 1-1: Trihedrons The area of the mirror should be greater than 0.76 m. The ensuing directivity is rather high (about 1.9 degrees between the zeros of the radiation pattern in azimuth and range directions, if the mirror is square). In order to decrease this directivity and simplify the positioning of the reflector, trihedral scatterers (corner reflectors) are often used. Three mirrors together form the corner of a trihedron. The multiple reflection of the incoming radiation is scattered exactly back, notwithstanding a low directivity (tens of degrees). The phase of the return is equivalent to that of a reflection from a point located at the corner of the trihedron. The effective area of the corner reflector is two thirds of that of the area of its triangular mouth, if the corner is pointed exactly; otherwise, it decreases gently with the angle. In order to have the same 0.76 m area derived from the metallic mirror, the side of the trihedron corresponding to the corner reflector should be greater than 1.14 m. Other corners have a cubic shape, rather than triangular. The RCS then increases by a factor of Natural backscatterers: the spectral shift principle If we have natural (non-metallic) scatterers, then their reflectivity is usually rather small and we can neglect multiple reflections. We can use the principle of superposition of effects to calculate the total backscatter by combining the contributions of series of hypothetical idealised elementary C-4
5 Statistics of SAR and InSAR images backscatterers. Two types of elementary backscatterers are of interest in interferometry: the grating (a spatial sinusoid) the point scatterer a) The grating that we refer to lies in a horizontal plane, is uniform along the azimuth direction, has the length δ g of the ground range resolution cell. We define Λ/ as its period along the range direction. Considering the two-way travel path, in order to have a reflection the returns from the elements of the grating should arrive in phase. If θ is the incidence angle, the spatial period of the grating should be in the interval: c c Λ min = Λ max = Equation 1.4 f sinθ f sinθ max min where: f max = f min + B Equation 1.5 and B = is the bandwidth (for the ERS case). Any extended scatterer visible to the radar can be considered as superposition of gratings at these different wave numbers, or of point scatterers. b) The point scatterer is wide band and has a very wide directivity function, i.e. the amplitude of its return changes very slowly with the wavelength and with the incidence angle. In the case of the grating, the amplitude of the return of the extended scatterer changes rapidly both with the frequency and the direction of arrival of the radiation. In fact, as is the case for a mirror or a dihedral, the amplitude of the return is cancelled out if the contributions of the elementary scatterers that delimit the grating move ±λ/4 in the direction of the satellite. Indicating the slant and ground range resolutions with δ s and δ g respectively, we get: c δ s δ s = ; δ g = Equation 1.6 B sinθ Let us consider now the return from a grating that occupies the entire range resolution cell and is tuned to the radar central frequency. If the radar frequency changes from f to f ± B/, i.e. at the edges of the band, the return will reduce to zero. Again the return is zero if the off-nadir direction changes from θ to θ +Δθ; to calculate Δθ we impose that the two-way travel path change between the nearest and the farthest elementary point scatterers (with the same amplitude) composing the grating is equal to λ. Then: ( θ + Δθ ) δ g sin + δ g sinθ = λ λsinθ B Δθ = = tanθ δ cosθ f g Equation 1.7 C-5
6 TM-19 InSAR Principles The change Δθ is obtained if the satellite orbit shifts in the sky by the socalled critical baseline B cr : B cr where h is the height of the satellite orbit. Δθ h B h sinθ = = Equation 1.8 cosθ f cos θ Notice however, that if both the off-nadir angle increases by Δθ and the radar central frequency shifts (decreases) by B the grating is fully visible, since the perspective change compensates for the frequency change (the wave number shift concept). If incident angle and observation frequency change at the same time by the correct amount, the same ground grating is observed. If only one grating existed in the image, say at the wave number Λ 0, it would generate a return only if illuminated with different temporal frequencies f 1 in one image and f in the other image. The interferogram would show fringes at the frequency f f 1 = Δf. f Δθ fbn cosθ Δ f = = Equation 1.9 tanθ h tanθ Now, the observation that the reflectivity of the terrain is the combination of the effects of different gratings is equivalent to the expansion of the terrain reflectivity in a Fourier series. Distributed scatterers have independent Fourier components and therefore the amplitudes of the gratings that compose the reflection will be independent random variables. In interferometric imaging, multiplying two images corresponds to multiplying the amplitudes of the component gratings. The average of the product of the amplitudes of two independent gratings is zero. If the reflecting grating is the same for the two images (because it was illuminated with different temporal frequencies in the two images) the amplitude of the product is non-zero, and the beat will have the frequency Δf (the fringe). The progressive increase of the baseline and therefore the progressive change in viewing angle implies that the fringe frequency Δf increases to be finally greater than the signal bandwidth thus making the fringe not observable. Larger baselines thus produce fringes with increasing frequency. The gratings that do not find corresponding elements with which to interfere in the other image (the noncooperating components) contribute to the noise. Finally, no grating at all will be visible in both images for a baseline greater than the critical one: in this case, distributed scatterers will be unable to generate interferograms. If the gratings that do not appear in both images are removed with proper filtering, the resolution of the interferogram is unchanged even if the single images have a reduced resolution. However, the level of the noise in the interferogram will be lower. Isolated point scatterers interfere properly until the baseline exceeds the directivity of the scatterer itself. If the scatterer is small, this directivity is very low, i.e. it will scatter over a wide range of directions. In conclusion, natural scatterers are the superposition of point scatterers located at random positions within the resolution cell with random amplitudes; using band limited radar, we see only their spatial frequency C-6
7 Statistics of SAR and InSAR images components (the gratings) with wave numbers in the interval corresponding to (Λ min Λ max ), depending on the incident angle and the illumination frequency. To have a good interferogram with distributed scatterers and high baselines, suitable range filtering should be applied Statistics of the return For the central limit theorem to hold, the phase and quadrature components of the return, and superposition of many independent elementary scatterers, gratings or point scatterers, are independent Gaussian random variables, with variance dependent on the terrain reflectivity. The amplitude of a single pixel in a SAR image (the modulus of the return) then has a Rayleigh distribution. Its amplitude squared (or intensity) has a Laplacian distribution with mean I [Bamler98A]. The value of I in a pixel of horizontal coordinates r (range), a (azimuth) is dependent on the local reflectivity of the terrain characterised by a non-dimensional parameter, σ 0, times the inverse of the sine of the slope of the terrain, to incorporate foreshortening effects that brighten any surfaces that verge towards the satellite. The value of σ 0 decreases for increasing values of the off-nadir angle θ, and depends on the terrain cover [Laur98]. In order to correctly estimate σ 0, it is necessary to average the value of the intensity (the amplitude squared) over several pixels that should have the same statistics. N 0 1 = N i= 1 u i σ Equation 1.10 The amplitude of each pixel being a random variable, the SAR images are covered in speckle. However, it has to be understood that for repeated acquisitions of a stationary object the speckle noise remains the same, which is different from other kinds of random noise. To remove the randomness of the amplitudes, in order to have a good estimate of σ 0, the amplitude squares of several neighbouring pixels should be averaged. The Parseval relation tells also that the same results of speckle reduction can be achieved by reducing the azimuth and/or the range resolution of each image, by a factor of say N r N az (where N r is the number of range looks and N az the number of azimuth looks) with as many disjoint frequency and/or wave number filters and then averaging incoherently the amplitudes of the N r N az different images thus obtained. The resulting average estimate of 0 0 σ, will have a reduced variance by a factor of N r N az compared with σ. The formula that gives the dispersion of the estimate, however, depends also on the random noise superposed on the data that increases the dispersion of the estimate. With very noisy data (high signal-to-noise ratio, or SNR), the amplitude is due to noise more than to signal, and therefore the dispersion of the combined estimates increases. It is usually given in terms of the Equivalent Number of Looks (ENL): C-7
8 TM-19 InSAR Principles ENL = 0 0 E σ E σ 0 0 ( σ ) ( σ ) = 0 E σ E σ 0 N r N = SNR az Eq Radiometric resolution is another parameter used to characterise the image quality and therefore the amount of speckle on the data: R db 1 1 = 10log SNR N r N az = 10log ENL Equation 1.1 ENL 1. Interferometric images: coherence In the case of interferometric images, the amplitude v of each pixel of the interferogram is proportional to the product of the amplitudes u 1, u of the two initial images, and its phase φ is equal to their phase difference. We have seen that SAR image pixels are the realisation of random processes and therefore we can expect the amplitudes of the interferogram to fluctuate severely even in the most favourable case of no temporal decorrelation (i.e. the target did not change between the two takes) and zero baseline (i.e. there is absolutely no change in the off-nadir angle). Therefore, the phase noise changes from pixel to pixel due to the different impact of the random noise superposed on the random amplitudes of the pixels. Pixels with weak returns will show more dispersed interferometric phases; strong and stable scatterers will yield more reliable phases. In addition, there are important changes between the two acquisitions: temporal, due to the change in the off-nadir angle, and due to random noise. We now define the measure of this change γ, the coherence of the two SAR images (also called the complex correlation coefficient): 1 1 ] [ v] * E[ u u ] E γ = = ; γ < 1 ; γ = φ0 E[ u E[ u ] I where E is the expected value of the random variable x Eq The argument of γ, φ 0, is equal to the expected value of the interferometric phase. Obviously γ is a function of the expected values that we have to estimate from the data. Using N independent image samples, the usual estimator is: γˆ = N i= 1 u N i= 1 1i u 1i u * i N i= 1 u i Equation 1.14 C-8
9 Statistics of SAR and InSAR images Examples of tandem coherence images of the Etna volcano are shown in Figure 1-. Figure 1-: Tandem coherence of Mount Etna in different seasons 1..1 Statistics of coherence estimators Estimation of the coherence is particularly difficult when its value is low; in fact, the estimator in Equation 1.15 is biased away from 0, as it is clear that if N=1, the estimate of the coherence is 1. For N > 4, and if γ = 0: π [ ˆ ] > 0 E γ Equation N The probability density of this estimate has been found [Touzi99]; the Cramer Rao Lower Bound (CRLB) for the dispersion is: var ( ) ( 1 ) ˆ γ γ CR N = Equation 1.16 C-9
10 TM-19 InSAR Principles The interferometric phase can be estimated as: N ˆ * φ = γˆ = u1 i ui Equation 1.17 We notice that using this estimate, weaker, noisier pixels have less influence on the final estimate. This estimate also shows that the interferometric phase is usually based on local averages. The value of N, i.e. the number of independent pixels generally used to estimate the coherence, ranges from 16 to 40. This limits the bias, but has the disadvantage of making local estimates of the coherence impossible since it is averaged on areas of thousands of metres square. This implies that the spatial resolution of an interferogram is intrinsically lower than that of a single SAR image, since the phases corresponding to single pixels are not necessarily reliable. In cases of low coherence (say 0.1), the number of looks to be averaged should increase up to 100 [Eineder99]. The Cramer Rao bound on the phase variance is: σ ˆ φ i= γ = Equation 1.18 γ N This bound can be assumed as a reasonable value of the phase estimator variance for N big enough; say, greater than 4 [Rosen00]. Figure 1-3: Argument of the coherence (estimated interferometric phase) as a function of the number of looks (N = 1,, and 5) for a sequence of 1000 pixels with random amplitude. The actual phase to be estimated is 0; the image SNR is 0 db. Due to the randomness of the amplitude of the pixel, the dispersion of the phase estimates (for N=1, ), is unacceptable even if the image SNR is very high. For N > 4, the phase dispersion decreases approximately with the square root of N. C-10
11 Statistics of SAR and InSAR images For small N, as seen in Figure 1-3, it may happen that the amplitude of the pixel is low and thus small amounts of noise can create wide phase deviations. This effect decreases sharply by increasing the number of looks [Lee94]. Notice also the presence in Equation 1.18 of the term 1/ γ ; this term accounts for the interaction of the noise in one image with the noise in the other and becomes important as soon as the coherence gets low. Then, to reduce the phase variance to acceptable values, N should increase and then the resolution will drop. It is important to stress that, when the dimensions of the window used to estimate the coherence increase, a phase factor could be considered to compensate for volumetric effects within the window, that are unaccounted for using the current topographical model. This phase correction, if unknown, could be determined in such a way as to maximise the resulting coherence; hence, this correction would increase the coherence bias ([Dammert99]). To avoid this problem we can use another estimator of the coherence, which is phase independent but also biased and with higher variance. This estimate is based on the intensities of the two images, rather then on their complex values [MontiGuarnieri97]: I i 1i I i ˆ γ = 1 q Equation 1.19 I I i 1i The advantage of the use of amplitudes is that the argument of the coherence, i.e. the interferometric phase, is spatially varying and therefore it has to be removed before averaging. This entails that in rugged areas the coherence will be systematically underestimated if the above formula is used without corrections or optimisation. On the other hand, we have seen before that if we insert a phase correction in that formula in order to optimise the coherence value, we get a bias. The joint probability density of amplitude and phase of an interferogram is [Lee94]: pdf ( v, φ) ( 1 γ ) i i ( φ φ ) v cos 0 ( ) 1 γ = πi v γ exp I v K 0 I ( 1 γ ) Equation 1.0 where γ is the coherence K 0 is the modified Bessel function C-11
12 TM-19 InSAR Principles The marginal distributions of the phase and of the amplitudes are: 1 γ pdf (( φ) = π 1 γ 1 cos ( φ φ ) γ cos( φ φ0 ) ar cos( γ cos( φ φ0 )) 1+ 1 γ cos ( φ φ0 ) 0 Eq. 1.1 = 4 v v γ v pdf (( v ) I K I Eq ( ) ( ) 0 1 γ I 1 γ I ( 1 γ ) 1.. Impact of the baseline on coherence The coherence has an important diagnostic power. Remember that two acquisitions at baselines equal to or greater than B cr will entail a complete loss of coherence in the case of extended scatterers. This effect has been discussed in the previous sections and was intuitively shown in Part A as corresponding to the celestial footprint of an antenna as wide as the ground resolution cell. Then, unless the non-cooperating wave number components (the useless parts of the spectrum of the signal) are filtered out, the coherence of the two acquisitions will decrease linearly, becoming zero when the baseline reaches B cr. In other words, if we do not eliminate the non-cooperating components we have, on flat terrain: B = n γ b 1 Equation 1.3 B cr The components at each wave number of the illuminated spectrum will be imaged at frequencies f 1 in one take and f 1 + Δf in the other take. Δf f Δθ fbn cosθ = = tan( θ α) h tan ( θ α) Equation 1.4 where: B n is the component of the baseline in the direction orthogonal to the range h is the satellite height B n cosθ Δ θ = Equation 1.5 h These formulas show that the situation becomes more complex when the slope greatly changes with range and/or azimuth, since then the filter necessary to remove the useless components becomes space varying. Finally, when the change of slope is so severe as to become significant within a resolution cell, then coherence loss is unavoidable, due to the volume effect as will be discussed in section 1.4. [Fornaro01]. C-1
13 Statistics of SAR and InSAR images 1.3 Power spectrum of interferometric images The power spectrum of an interferometric image is composed of two parts: the coherent part generating the fringe, and the noise. In order to evaluate it, first we oversample both images in range by a factor :1; the spectrum of each image is then a rectangle, occupying half of the available frequency range. Then, we cross-multiply the two images and thus we convolve the two spectra along the range axis. If the two images are totally incoherent, the resulting spectrum convolution of the two rectangles is triangular and centred in the origin. If there is a coherent component in the two images after cross multiplication, it generates a peak at zero frequency, if the baseline is zero. If the baseline is non-zero, the peak is located at the frequency Δf seen in a previous paragraph Equation 1.4. If we carry out the so called spectral shift filtering, removing the signal components corresponding to wave numbers visible in one image but not in the other (i.e. the components that do not contribute to the spectral peak at frequency Δf), the spectrum is still triangular, but centred at Δf. 1.4 Causes of coherence loss Noise, temporal change There are several causes that limit coherence in interferometric images: the most immediate to appreciate is the effect due to the random noise added to the radar measurement. From the definition of coherence, we get: 1 γ SNR = 1 Equation SNR The changes with time of the scattering properties of the target should also be taken into account, which is done in the corresponding section in level of this manual Volumetric effects Another important effect that reduces the coherence with increasing baselines is the volumetric effect [Gatelli94, Rott96]. If we have stable scatterers having uniform probability density within a box of length δ g along ground and height Δz, then the radar returns will combine with different phases as the off-nadir angle changes. The height of the box that entails total loss of coherence at baseline B n is equal to the altitude of ambiguity h a : Δz λ h tanθ 0 = ha Bn 9400 B where B n = 50 m, Δz 0 = 38 m (for ERS/Envisat) n Equation 1.7 C-13
14 TM-19 InSAR Principles For small values of Δz, the coherence loss due to volume is approximately [Gatelli94] : π Δ z γ = 1 6 z Equation 1.8 ha Similarly, for a Gaussian distribution of the terrain heights over a plane, with dispersion σ z within the resolution cell, we get a phase dispersion of the returns equal to σφ = πσ z /h a. The coherence is: σ φ σ φ γ exp z = 1 = 1 π σ z ha Equation 1.9 showing that the loss of coherence depends on the vertical dispersion of the scatterers. Finally, as we have pointed out previously, the coherence γ is dependent on the baseline, unless proper filtering actions are taken. In the case of extended scatterers the coherence is always zero, when the baseline is greater than B cr. = 1 B ; < γ n b Bn Bcr B Equation 1.30 cr In conclusion, taking into account all the causes of coherence loss (noise, temporal change, volume, baseline) we get: 1 B n γ = γ = SNRγ tγ zγ b γ t exp( σ φ / ) 1 Eq / SNR Bcr Further coherence reductions could be due to the imperfect alignment of the pointing of the antenna (Doppler centroid differences), that would cause a mismatch of the two azimuth spectra, similar to the effect of baseline decorrelation. The consequences could be reduced with a proper azimuth filtering of the two images. C-14
15 Focusing, interferometry and slope estimate. Focusing, interferometry and slope estimate In this chapter we study the effect of range and azimuth focusing on the final InSAR quality. We then derive an optimal technique for estimating the terrain topography, namely the slope. Finally, we give hints for optimising the processor, in order to get the best quality image, and at the same time we evaluate the impact of non-ideal processing..1 SAR model: acquisition and focusing The design of an optimal SAR focusing kernel for interferometric applications is based on the linear model for SAR acquisition and processing drawn in Figure -1. Figure -1: Equivalent simplified model of SAR acquisition and focusing The source s(t,τ), is the scene reflectivity in the range, azimuth SAR reference (defined here by the fast-time, slow-time coordinates t,τ) of the Master image. The SAR acquisition is modelled by the linear, space-varying impulsivetarget response, h SAR (t,τ; t 0,τ 0 ), that depends on the target location t 0,τ 0. The noise term, w(t,τ), accounts for all the noise sources, mainly thermal, quantisation, alias and ambiguity. The SAR focusing is represented by the space-varying impulse response h FOC (t,τ; t 0,τ 0 ). The output, Single Look Focused (SLC) SAR image, is represented by the signal s ˆ( t, τ )..1.1 Phase preserving focusing The role of the SAR processor is to retrieve an estimate of the complex source reflectivity: s ˆ( t, τ ) that is the closest as possible to the true one: sˆ ( t, τ ) s( t, τ ). C-15
16 TM-19 InSAR Principles The optimisation would depend greatly upon the characterisation of the sources, of the noise, and the output. As an example, a processor optimised for measuring backscatter on a detected image would not necessarily be optimal for interferometry; likewise a processor optimised for interferometry with a distributed scene would not necessarily be optimal for applications on permanent scatterers. Here we assume that the processor is intended for classical interferometry, where the input is a distributed target modelled as a Gaussian circular complex process. We make here two further assumptions, which are quite accepted in most SAR systems: 1) We approximate the SAR acquisition impulse response as stationary within the D support of the focusing operator ) We assume that noise is also stationary, at least in the same signal support These assumptions allows us to optimise the processor on a local base, and thus to approach the problem in the continuous-time domain or, equivalently in the frequency domain. In practice we ignore border effects. The estimate of the reflectivity, s(t,τ), can be achieved by minimisation of the mean squared error (MMSE) in Equation.1. h FOC [ sˆ( t, τ ; t0, τ 0 ) s( t, τ; t0, 0 ] ( t, τ ; t0, τ 0 ) = arg min E τ ) Equation.1 The corresponding frequency domain expression is in Eq.., where we have defined f a the azimuth frequency and we have used capital letters H and S for the Fourier Transforms of h( ) and s( ) respectively. H FOC ( f, f ; t = arg min E Sˆ( f, f a 0, τ ) 0 a ; t, τ ) S( f, f ; t 0 0 a 0, τ ) 0 Eq.. The minimisation involved in Equation Eq.. leads to the incorrelation between error and data, hence the optimal focusing in MMSE sense is the solution of Equation.3 (we have dropped the arguments for simplicity). E H S S S = [( ) ] 0 FOC RAW RAW E[ S SRAW ] [ S ] H FOC = Equation.3 E S The optimisation would require knowledge of the power spectrum of the sources. According to our assumptions, source reflectivity is white (over a very large bandwidth) with Power Spectrum Density (PSD) RAW RAW * [ S( f, f a ) S ( f, f a )] N s E = Equation.4 We then compute the PSD of the raw data by exploiting the model in Figure -1. The final expression of the optimal MMSE focusing kernel estimate is in Equation.5, where N w (f,f a ) is the PSD of the noise. C-16
17 Focusing, interferometry and slope estimate H FOC = H SAR H SAR Nw ( f, fa ) + N s Equation.5 The result in Equation.5 is the well known MMSE Wiener inversion, that leads either to the matched filter, when N w << N s, or to the inverse filter, when N s >> N w. In any case, the focusing reference is phase matched to the SAR acquisition, so that the overall end-to-end transfer function has zero phase (linear, if we add the proper delay): that is the reason why SAR focusing for interferometry is usually defined as Phase Preserving. In a conventional SAR, we expect a constant PSD for both thermal and quantisation noise, whereas the azimuth ambiguity is weighted by the antenna pattern giving a coloured noise. A conventional rectangular antenna of length L causes the spectral weighting: G L sinc a Equation.6 v ( f ) ( f f ) a = DC The azimuth ambiguity is due to the folding of the spectra due to PRF: the PSD of SAR data, the first left and right ambiguities, and the expected ambiguity-to-signal PSD are plotted in Figure -. Figure -: Azimuth Antenna Pattern in the frequency domain (db) (continuous line) together with its left and right replica, folded by azimuth sampling (PRF). The ambiguity-to-signal ratio is plotted as the dotted line. We notice that for most of the azimuth bandwidth the signal is much larger that the ambiguity, so that the optimal focusing in Equation.5 is very close to the inverse filter. This is quite different form the matched filter used in RADAR detection and communications; in fact the matched filter comes out from the maximum likelihood optimisation for isolated targets, and not for a continuous distribution of scatterers. Finally, we observe that in most commercial Phase Preserving processors, proper spectral shaping windows are applied in order to reduce the side lobes in presence of non-stationary C-17
18 TM-19 InSAR Principles targets (like strong point scatterers or man made artefacts). These shapes should then be equalised for interferometric processing on a nearly homogenous scene..1. CEOS offset processing test The CEOS phase preserving test has been introduced to check the quality of the space-domain varying implementation of the processor. SAR processors usually approach a space-varying transfer function by block-processing, and adapting the processing parameters in each block. The CEOS offset tests the self consistency of such approach, in particular checking that the same transfer function is implemented at a specific range and azimuth, disregarding the relative position with respect to the block [Bamler95A, Rosich96]: Process two SLCs from the same raw data set and with the same orbit, but offset by l00 lines in azimuth and l00 sample in range. The interferogram formed from these two properly coregistered SLCs should ideally have a constant phase of zero and thus reveals processor induced artefacts. The test must be passed with the following results: Mean of interferogram phase 0.1 o Standard deviation of interferogram phase 5.5 o (corresponding to a reduction in coherence 0.1 %) No obvious phase discontinuity at the boundaries of processing blocks The test does not provide a reference Doppler, however it is wise to perform it at the largest Doppler foreseen for the mission. Moreover, the azimuth shift should not be given when testing ScanSAR modes. Clearly the test provides only a necessary condition, for example any range invariant operator would pass it, and the same for a one-dimensional operator. Hence, it is necessary to combine it with a measure of the actual focused quality. We also notice that further self-consistency tests could be implemented, such as comparing the processing of blocks of different sizes.. Interferometric SAR processing So far we have considered the design of an optimal focusing processor as the one that provides the most consistent reconstruction of the source reflectivity. However, in processing a specific interferometric pair, some further processing specifically matched to that image pair can be performed to provide the best quality final interferogram. The simplified design of the system and the processing in the interferometric case is shown in Figure -3. Here again we assume a distributed target, as we refer to conventional interferometry. We observe that the source reflectivity is affected by a noise contribution, namely scene decorrelation noise that will not be noticed in processing a single channel (see Figure -1). With this warning in mind, we represent this contribution as an additive C-18
19 Focusing, interferometry and slope estimate noise source. Scene decorrelation may arise from change in the scattering properties between the two acquisitions, from volumetric effects or baseline decorrelation [Zebker9]: for repeat-pass interferometry this may be indeed the strongest noise. Figure -3: Schematic block diagram for the generation of a complex interferogram in the single-baseline case. The scene decorrelation noise is not noticeable on the single image. The SAR acquisition channels for master and slave are not identical to the single channel shown in Figure -1, as here we must account for the different acquisition geometry that interferometry looks for. Following the discussion in section we need to replace the reflectivity s(t,τ) by its modulated version: Δf sm ( t, τ ) = s( t, τ ) exp + jπ t Δf ss ( t, τ ) = s( t, τ ) exp jπ t where M and S refer to the master and slave respectively. Equation.7 We have approximated the case of a constant sloped terrain, leading to the total frequency shift Δf expressed in Equation 1.19 (notice that the equal repartition of the total spectral shift into two contributions of Δf/ in each channel is purely conventional). The case of space-varying slopes will be approached in a later section...1 Spectral shift and common band filtering (revisited) The object of the processing block named: spectral shift and common band filtering in the diagram of Figure -3 is to extract only those contributions correlated in both channels, and likewise to remove all the decorrelated contributions. Let us refer to the master and slave phase-preserving complex focused images as s M (t,τ) and s S (t,τ) respectively. C-19
20 TM-19 InSAR Principles We again have to face the problem of an optimal estimate in the MMSE sense, but now the task is to filter the master instead of focusing, thus getting an image as close as possible to the slave: sˆ S M ( t, τ ) = hm * sm ss ( t, τ ) Equation.8 where s ˆ S M is the reconstruction of the slave image given the master, and h M is the reconstruction filter to be designed (the spectral shift and common band block in Figure -3). In the same way, a slave reflectivity would then be reconstructed from the master. The design of filter h M is accomplished by minimising the MMSE in the bidimensional frequency domain: Sˆ H S M M = H M S M = arg min E Sˆ H S M S M Equation.9 This minimisation leads to the incorrelation between the error and data: [( ˆ E S ) ] = S 0 M S M S M Equation.10 Eventually we derive the formulation of the reconstruction filter: E S H = E S [ S SM ] [ S ] M M Equation.11 This filter keeps the most correlated information. However, Equation.11 does not hold for non-zero spectral shifts (Δf 0), i.e. it does not hold for different modulation of the sources that come out of the interferometric acquisition geometry (see section 1.1.3). In this case we have to shift the spectra of the two signals prior to correlating them, and this leads to the following result: H = = H H E EM EM [ S M ( f + Δf /, f a ) S S ( f Δf /, f a )] E[ S S ] ( f Δf ( f, f a ) /, f a 1 + ) H H S ES FOCM S ( f + Δf /, ( f, fa ) N N s wm ( f, f ) a f a ) Eq..1 where: H EM and H ES represent the end-to-end transfer functions (acquisition + focusing, as shown in Figure -1) of the master and slave channels respectively N wm (f,f a ) is the equivalent noise PSD on the raw master image H FOCM is the focusing transfer function of the master Be warned that the inversion involved in Equation.16 is meaningful only in the bandwidth of the end-to-end transfer function (out of this band there is C-0
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"What's in a name? That which we call a rose, By any other word would smell as sweet." -- William Shakespeare in Romeo and Juliet
Does an old discarded penny stock traded under a flashy new name smell sweeter to investors? Apparently so. Shares of Repros Therapeutics (NASDAQ:RPRX) have risen sharply, more than doubling since September of last year. However, I wonder if many shareholders in Repros have forgotten, or maybe never learned, that Repros is the same corporation as the discredited and oft-sued Zonagen. In 2006, after numerous failures, Zonagen changed its name to Repros and hoped to shove two decades of failure under the rug. Well, it's past time to go looking in the closet to see what sort of things Zonagen and its long-time CEO Joesph Podolski might not want you to know.
At this point, I should credit Manuel Asensio and his book Sold Short for opening my eyes to the story at Zonagen. Manuel is a notable short-seller who issued a flurry of negative reports on Zonagen as its stock fell from a climax of more than $40 per share down to under a buck. Here's a chart of Zonagen from 1995 through 2003 that shows the complete pump and dump of its shares.
What led Zonagen's shares from under $5 to up to $45 before completely collapsing? Zonagen was founded more than twenty years ago to try to develop birth control methods based on modification of the zona pellicula—a part of the ovum. The company bought the rights to the research of Dr. Bonnie Dunbar and attempted to create a contraceptive vaccine. Her research, which had been progressing well, ground to a complete halt according to Brian Wallstin of the Houston Press newspaper in his article "Biological Disaster." In it, Brian explains how Zonagen stole her work. She ended relations with the company and sued them. Zonagen would not develop any commercial product from research relating to the zona pellicula—despite naming itself after it.
After this, Zonagen needed something new with which to excite investors. Its next project would be to create an adjuvant. These are products modify the effects of the active ingredient in a drug without having any direct effects of their own. However, Zonagen's attempt, ImmuMax, was merely an attempt to patent and cash in off a widely researched generic product: shrimp shells. The compound, Chitosan, was indeed capable of causing some desirable effects within the immune system, but it also caused permanent scarring at injection point which made it useless for the sort of medicinal purposes Zonagen hoped to use it for.
After abandoning ImmuMax, Zonagen moved on to its third, and most exciting, failed drug: Vasomax. Realizing that Pfizer's Viagra was going to be a hit product, Zonagen hoped to ride its coattails with an erectile-dysfunction drug of its own. However, unlike Pfizer, Zonagen repeated its experience from ImmuMax; it got a forty year old generic drug, phentolamine, dressed it up as something new, and acted as if it was a genuine Viagra competitor. While phentolamine was never proven to create erections by itself, Zonagen claimed that its formulation, simply phentolamine administered orally instead of in pill form, would work magic.
When there was already a working drug for ED, Viagra, it is baffling to try to figure out why Zonagen would bring out an old generic drug and attempt to repackage it as something useful. However, Vasomax's inability to perform didn't stop the stock, investors bid it up sharply. Despite a failed Phase II trial in 1996 for Vasomax, the company proceeded to Phase III trials anyway and initially got investors excited. Once Schering-Plough (SGP) pulled out of its partnership with Zonagen for Vasomax, Zonagen abandoned its New Drug application for Vasomax. Investors were left feeling limp as shares skidded into the low-single digits.
The company was left with little more than a corporate shell and a bunch of lawsuits, several from former employees. Several of the ex-employees had less than flattering things to say about the company. For instance, Brian Wallstin interviewed Zonagen's former head of immunology Balbir Bhogal. Wallstin reported:
Bhogal says the repeated failure to conduct a proper [test] reflected the overall scientific environment at Zonagen. The staff took long lunches. Many researchers didn't come to work until 10:30 or 11 in the morning. On Fridays, most of Bhogal's co-workers would join Podolski at Molly's, a local pub, and not return to the lab until midafternoon. When Bhogal questioned the research methods or the work ethic at Zonagen, he says he was accused of being bossy and overly aggressive.
Though Zonagen was little more than a corporate shell after the Vasomax failure, the above-mentioned Joesph Podolski would not abandon ship. Instead, the man who led Zonagen through two decades of failure decided it was time for a name change. Zonagen was newly christened Repros and began work on its fourth lead drug, Proellex. I'm not a biologist or chemist, so I'm not going to comment specifically about the viability of Proellex as it isn't my area of expertise. That said, it's alarming to realize that under Podolski's leadership Zonagen/Repros has had a history of trying to take other people's generic/failed drugs, renaming them, and then hoping for the best. Remembering that, it is important to note that Proellex is very similar to Asoprisnil. Asoprisnil was developed by Schering to treat Endometriosis. Development was discontinued because the drug was causing thickening of the endometrium. Note that Asoprisnil appeared to be effective but was unsafe. Enter Proellex; Repros has frequently trumpeted how effective Proellex has been at relieving pain, but has been less clear on the safety issues.
However, a March 7th, 2008 press release from Repros sheds light on the subject. It states, "The clinical data showed that progressive endometrial thickening as measured by ultrasound occurs with all three doses of Proellex(R) that have been used in clinical trials to date." Well, if that's what stopped Schering's Asoprisnil from being approved, I've got to wonder what's different here. Repros claims that its time on the drug/off the drug approach will make a big difference in safety versus Asoprisnil; however, could this be a minor change, a red herring to throw investors off the real story? Remember, Vasomax was little more than someone's drug with a different method of ingestion.
Finally, one has to wonder about the leadership of Repros. For starters, why, after more than two decades of failure, is Joesph Podolski still the CEO of this company? How many failed drugs, share dilutions, and years without profit must pass before the board finally kicks him out? Also of interest, why are bulls on this stock so excited about Mark Lappe who was recently elected to Repros's board? While Lappe is a big investor in Repros and a hedge fund operator at Efficacy Capital, I have to question his credentials. The only other traces of him or Efficacy Capital I could find was Efficacy's investment in several small biotech companies, most notably the British firm Osmetech where he is also a director. The chart of Osmetech isn't exactly reassuring:
While I hope Proellex ends up being a successful drug that relieves the symptoms of millions of suffering women, I am doubtful. The long track record of shoddy research and the repackaging of old or generic drugs into hot new items smell more of a stock promotion than of a successful biotech firm. With twenty years devoid of profits but full of share dilution, I have to assume that investors buying into Repros near its 52-week high will find only more untreated pain.
Disclosure: Short RPRX.

subset: commoncrawl


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Much More To Love 16Favorites [+]
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Running Time: 106 minutes
Released: 10/2007
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Series: Much More To Love
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Interested in Mexico City?
We'll send you updates with the latest deals, reviews and articles for Mexico City each week.
Get away. You know you want to. A weekend in Mexico City will entice the senses, charm the soul, and please the mind
Like most of Mexico City's beautiful and well-located business hotels, the Four Seasons is way underbooked on the weekends, giving you a tremendous chance to get five-star comfort at three-star rates.
Any trip to Mexico City has to start in the Zocalo, the ancient hub of the colonial city. Filled with throngs of people, it still maintains a deep resonance with the centuries. Visit the awesome national cathedral and the Palacio Nacional to see Mexico past and present. The Aztec past can be seen at the Templo Mayor, just around the corner. This ancient temple was only recently rediscovered after 500 years. Well worth gaping in amazement.
For dinner, be sure to call ahead or you might not get a seat at Izote, one of Mexico City's more outstanding restaurants. Located in Polanco (the Beverly Hills of Mexico City), the restaurant perfectly updates the classic Mexican recipes to mouth-watering luxuries. The ceviche is particularly awe-inspiring.
Spend the next morning shuffling through this incredible museum, filled with the relics of all of Mexico's ancient cultures, from the Aztecs to the Zapotecs. It can get very crowded late in the day, so the earlier you can go, the better. It's honestly one of the world's truly great museums.
Take a break from ancient artwork by shopping for new stuff. The Saturday market in San Angel in the southern outskirts of town is filled with arts, crafts, furniture, music, and taco vendors. While the prices aren't as rock-bottom low as other markets in the city, the quality is excellent, and coming from New York or LA, everything will seem like a bargain. Find something cute for your home, but be careful. I met an armoire here that nearly caused me to redo my entire bedroom.
All that shopping will work up an appetite. Fuel yourself with dinner at another of Mexico City's truly gourmet establishments. This is updated Mexican soul food. I had a filet mignon that was as tender as any I have had in any country, and it was perfectly matched with a chipotle sauce that sent my taste buds into orbit (in a good way). The hibiscus flower margaritas will make anyone weep with joy. Again, be sure to call in advance.
Before you catch your flight back to the States, you do need your exercise, if for no other reason than to burn some of those delicious calories. Take a half-day bus tour to Teotihuacan, a mysterious ruined city of pyramids north of town. The city has been abandoned centuries before the Aztecs arrived. They called it the "city where men became gods" and you'll feel the same way when you climb to the top of the Pyramid of the Sun and stare out across the valley. It's a magical place.


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Definition Edit
A cost-plus award fee contract (CPAF) is
|a cost reimbursement contract that provides for a fee consisting of a base amount fixed at inception of the contract plus an award amount that may be given based upon a judgmental evaluation by the customer of contract performance. The theory behind this type of contract is that although the customer assumes most of the cost risk, it retains control over most or all of the contractor's potential fee as leverage.[1]
U.S. government Edit
"The CPAF contract should be used when the work to be performed is neither feasible nor effective to devise predetermined objective incentive targets applicable to cost, schedule or technical performance. In cost reimbursement contracts when it is not possible to establish pre-negotiated objective targets, it may be necessary to incentivize subjective areas of the contractor's performance, thus a cost-plus-award-fee contract may be appropriate.
"The CPAF contracts contain attributes that often result in better communication than other types of contracts between the Government and the contractor and greater contractor motivation to achieve exceptional contract performance. These attributes are normally associated with the process of monitoring and evaluating contractor performance.
"The philosophy of providing contractors an award fee is based on the premise that the potential improvement in quality of contract performance offsets an additional cost to the contract. How one establishes and allocates fee on a CPAF contract is critical to obtaining the best possible motivation for excellent performance at the most significant times. . . .
"The combination of contractor motivation and evaluation flexibility can prove advantageous in the situation making necessary use of a cost reimbursement type contract. It also can encourage more effective communications between the parties and foster a kind of management discipline that is often difficult to sustain in other than an award fee environment. For this reason, many believe the award fee approach is as much a management tool as an incentive contract type."[2]
References Edit
- ↑ Information Management: Challenges in Implementing an Electronic Records Archive, at 4 n.4.
- ↑ Defense Acquisition University, ACQuipedia, Cost Plus Award Fee Contracts (full-text).
External resources Edit
U.S. government contracting:
- FAR Subpart 16.4 Incentive Contracts
- DFARS Subpart 216.4 Incentive Contracts
- PGI 216.4 Incentive Contracts


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Doctoral Degree in Music and Dramatic Art
Doctoral Degree in Music and Dramatic Art
Doctoral studies were introduced at the EAMT in 1996 (during the Soviet era, the best graduates of the Tallinn Conservatory usually continued their studies at post-graduate level in Moscow or Leningrad). The first dissertation was defended in 2004. In the beginning only musicologist were accepted to the PhD programme, whereas in 2000 specific curricula were designed for performers and composers. In 2006 a new programme in dramatic art was added, which also has artistic emphasis. 43 doctoral students are enrolled in doctoral studies at present (June 1, 2011) and 3 curricula are open for applicants: Musicology, Music (artistic emphasis), Dramatic Art (artistic emphasis). Doctoral studies are led by the Doctoral Committee that includes both scholars and professors of artistic specialities. Since 2009 EAMT coordinates the Graduate School of Culture Studies and Arts. This is a joint venture of four Estonian universities: Estonian Academy of Music and Theatre, University of Tartu, Tallinn University, and Estonian Academy of Arts. The graduate school is led by professor Jaan Ross.
The examination is organised in two rounds. Round one is based on documents. In this round, a student candidate should submit to the admissions secretary, by 2 June 2014:
- a video recording of a performed program of 45-60 minutes in performance specialties, or ca 45 min of music recordings (with sheet music) in composition specialties, or a list of major creative achievements with written sources and/or recordings in dramatic art;
- a plan of prospective doctoral studies, which should include four concert programs (or other creative tasks depending on specialty), and a research paper proposal on the required form;
- an overview of previous creative activities and research (CV).
Up to 10 candidates will be admitted to the second round, combined for both doctoral curricula. The candidates will be notified of the results of the first round at least 5 days before the second round.
In round two, a student candidate should:
- at the request of the examination panel, perform a requested part of the program submitted previously as a recording, or answer the questions of the panel on his or her previous creative activities (a live performance is not required for candidates in the specialties of conducting and dramatic art);
- justify and defend his or her plan of prospective doctoral studies.
As a rule, candidates are required to attend the second round in person. In exceptional cases, the interview can also be conducted via electronic communication channels.
The examination panel evaluates and considers the following:
- persuasiveness and originality of artistic ideas of a candidate;
- specialty-specific level of technical mastery;
- scope and quality of previous creative activities;
- creative potential and qualities required for creative activities at international level;
- relevance of the research subject and its connection to creative activities;
- clarity of the research problem, adequacy of selected methodology, and familiarity with reference literature and sources on the subject.
The examination is graded on a scale of 1-10. Based on the examination results, a single ranking list will be established for candidates of both doctoral curricula. Five highest-ranked candidates will be admitted to the Academy. The doctoral council can additionally suggest other candidates for doctoral studies as an external student.
This school offers programs in:
- English
A Doctoral Degree generally takes 4 years to complete.


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Plus Size Woman's Guide To Evening Dresses
It comes as a shock to most people after they learn that garage, backyard and storage shed contents take the longest to pack before moving. Not solely this, A2zShoes additionally ships your merchandise proper to your doorstep and accepts returns if you end up buying the fallacious measurement or shade. As every Sarong is handcrafted and distinctive there shall be slight differences in sample and coloration with every sarong. I was utterly relaxed watching six enthusiastic young dancers wearing costumes of the Netherlands and performing four plus size abs folk dances to the tunes of the Golden Angel Street Organ. Pick a costume with a v-neck line or a deep neck line, then praise it with a smashing necklace to create an impression of slimness. Your one cease locations for online buying catering big selection of plus measurement clothing for plus dimension girls on affordable prices. Tankini bathing suit pieces are normally offered separately, making them perfect for the lady who's a unique measurement on top than on the bottom.
If you are if you're afraid of giving this pattern a go as a result of you're a plus measurement girl, try carrying tremendous power underwear comparable to Spanx or girdle to smoothen your silhouette. And some styles of Hawaiian clothing could make nice plus measurement clothes due to its unique design. Longer hem lines, greater v necks and looser fitting enable plus sized individuals plus size abba costume to feel comfortable in these costumes. If you will have a special dimension which is not accessible in the stock, you possibly can always get a personalized apron sink. Some girls find they'll put on one cup dimension smaller than they normally purchase once they put on Bali bras.
In basic, ladies with just a few additional rolls round their waistlines should look for clothes with vertical, slightly than horizontal patterns. Nowadays all the plus size salwar kameez things may be purchased in journey dimension they usually take up quite a bit less area then a jumbo bottle of shampoo.
But even when you don't wish to go for that full morning look of glamour there are still some very nice plus size sleep shirts for ladies out there for quick in the meanwhile. The Corpse Groom Barry D'Alive accompanied by the Corpse Bride costume or the Lady Nightshade, the Ghost Bride additionally appears to be like cute for young couples. The 2010 World Cup Official Match Ball from Adidas is being sold plus size salwar kameez all over the world, principally online, although some soccer outlets carry one or two. Lack of creative designers till a couple of decades ago doubled with high price of costume supplies prevented corset high to develop into potential wedding apparel.
Fortunately, yow will discover what you want online, with lots of the new launch Halloween costumes are also accessible within the the plus dimension format, which means you get the identical plus size satin robe variety and choice as each other person. Up-to-date Halloween costumes for 2010 provide selection, together with attractive and authentic Halloween costumes from the 2010 Tim Burton model of Alice in Wonderland.
Author: Dilan BeliThis author has published 2445 articles so far. More info about the author is coming soon.


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|06 Jul 2011
Windows 7 Home Premium x86
Cannot access a certain website | Please help
I am unable to access this website - BleepingComputer Discussion Forums
I am getting an error saying -
The connection has timed out
The server at Bleeping Computer - Computer Help and Discussion is taking too long to respond.
I have tried on FF and IE but no luck what so ever.
Just a few minutes ago I was on that site and now I cannot get on it.
Is the site working for everyone else?
|My System Specs
|06 Jul 2011
Windows 7 pro x64
I tested website and working. Can you try to reset your browser on menu TOOLS\INTERNET OPTIONS\ADVANCED TAB then reset your browser with personel setting and password then restart your browser not a computer.
|My System Specs
|Thread Tools
|Similar help and support threads
Can't access a website
I have a strange problem: I cannot access websites on a certain server (Firefox gives me "Network Timeout", Windows network diagnostics says "website is online but isn't responding to connection attempts"). The problem is quite annoying since that happens to be the server that hosts my own...
|Browsers & Mail
Win 7 64 bit : Can't access a particular website
Im not sure why, but i cant access this website even though all my friends can. Blox Sound - Sound of Revolution Unable to connect bloxearphone.com:80. Connection timed out. Pinging bloxearphone.com with 32 bytes of data: Request timed out. Request timed out.
|General Discussion
Can't access certain website
Up until a few days agao everything was working fine but all of a sudden I have not been able to access a certain webiste on any of our computers at home or through using our internet but if i turn off wireless on my phone i can access it through 3G and if we take the laptop to our neighbours house...
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404 no access to this website.......?
i have account with myhpmini.com for a while, a few days ago, clicked desktop short cut, and tried to go to myhpmini.com, and error message showed up, something was like '404 no access to this website.....' and next day, error message did not show up.
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firefox can't access website
Ever since 2 days ago, I haven't be able to get to onemanga while all other website are still perfectly normal. At first I thought the site was down but siteknocker confirmed the opposite and my friends said it's still up and running for them. In addition, when I switched to IE, it still load...
Can't access one website and only the one website
The website is Gen-R website I was on it earlier today and also using the ftp account but know I can't even access the site. I am using Windows Seven 64 bit Microsoft Windows Copyright (c) 2009 Microsoft Corporation. All rights reserved. C:\Users\Watson>TRACERT generationr.co.uk Tracing...
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This set of large duplications covers >90% of the second chromosome. These duplications are particularly useful in screening for dosage-dependent enhancers and suppressors of mutant phenotypes. In addition, because most of the Cam duplications are nested within inverted chromosomal segments, they act as balancer chromosomes for the duplicated regions, making them ideal for recovering and maintaining haplo-lethal and haplo-sterile mutations. To order the whole kit copy this list of stock numbers and paste them into the order form.
Return to the Duplications home page.


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Poster – Charting the Beatles: Schedule
What did the band's calendar look like during during Beatlemania?
There are parallels in timing between each year, but the similarities between 1964 and 1965 are particularly notable.
"[1965] was a curious year for the Beatles, one in which they consolidated all the successes and excesses of 1964 by virtually repeating everything already achieved. They made a second feature-film, Help!, they toured North America...and Britain again...John even had a second book published."
-Mark Lewisohn, Beatles historian
Posters are printed on thick, durable, matte paper (archival and acid-free).