Reverse Path Research Articles

Kinetics of transport systems dependent on periplasmic binding proteins

Rate equations are derived for a transport model involving a water-soluble binding protein outside the plasma membrane. On addition of the substrate, the conformation of the binding protein changes, the complex then combines with the membrane carrier, transferring the substrate to the carrier site. The free binding protein leaves and the carrier shifts inward, releasing the substrate inside the cell. Exit follows the reverse path. The predicted behaviour is as follows. (i) Uptake does not necessarily conform to Michaelis-Menten kinetics. (ii) In both the energized and de-energized states, the maximum rate of exit is far lower than that of entry; the asymmetry is determined by the conformational change in the binding protein, which is independent of the energy state of the system. (iii) Exchange transport is inhibited by external substrate and is extremely slow; consequently counter-transport is not expected. (iv) The half-saturation constant in uptake can differ from the dissociation constant of the binding protein. (v) The maximum rate of uptake depends on the intrinsic substrate affinity of the membrane carrier relative to that of the binding protein. (vi) The maximum rate of uptake and the substrate half-saturation constant depend on the concentration of the binding protein.

Thermohaline forcing of the Indian Ocean by the Pacific Ocean

An idealized box model of the Indian Ocean forced by steady winds and Haney-type surface heat fluxes is used to examine the importance of the warm, fresh throughflow from the equatorial Pacific on the climate of the Indian Ocean. In particular, the hypothesis proposed by Godfrey and Weaver (1991, Progress in Oceanography, 27, 225–272), that the anomalous poleward buoyancy-forced Leeuwin current off the west coast of Australia is a manifestation of a basinwide thermohaline circulation driven by the Indonesian throughflow, is examined. The stronger Sverdrup circulation dominates the thermohaline circulation in most of the model ocean except near the eastern boundary. The effects of the throughflow, however, can be determined by comparing two runs forced by a Pacific Ocean with either the warm, fresh profile of the western equatorial Pacific or a cooler, more saline profile more typical of the eastern equatorial Pacific (where a more usual equatorward wind-driven boundary current is found). It is found that heat imported from the Pacific is transported zonally across the Indian Ocean to the western boundary by the South Equatorial Current. The enhanced meridional steric height gradient south of the SEC drives an eastward return flow back to the eastern boundary, where it turns south to form the poleward Leeuwin Current. The reverse path is traced out by the waters immediately below the thermocline. None of these features are observed when the model Pacific has the cooler profile typical of the eastern boundaries of other oceans. Indonesian throughflow significantly affects the surface heat fluxes and the meridional heat transport in the Indian Ocean. The importance of the throughflow in maintaining the very warm climate of the Indian Ocean (a net exporter of heat) is noted. In the model, the poleward western boundary current along the coast of Africa south of about 27°S appears to play only a very minor role in the basinwide thermohaline circulation described above. This differs from the “warm water route” proposed by Gordon (1986, Journal of Geophysical Research, 91, 5037–5046) where heat is returned to the South Atlantic past the Agulhas Retroflection. Large-scale, fairly long period (>100 days) barotropic eddies are found in the western portion of the basin for some solutions. The generation mechanism for these eddies appears to be barotropic instability in the model South Equatorial Current.

A LISREL 8 model with constrained parameters for twin and adoptive families

Constrained optimization recently has been implemented in the LISREL software package, allowing formulation of parent-offspring transmission models in a simple and efficient manner. A reverse path model of parental transmission is described within the LISREL framework for application to twin and/or adoptive family data. The model incorporates genetic and environmental parameter constraints arising from assortative mating and cultural transmission. An illustration of the LISREL model is given using measures obtained from twins and parents involving fear of social criticism.

OPTIMAL IDENTIFICATION OF MUSKINGUM ROUTING COEFFICIENTS1

ABSTRACTTraditionally, identification of the Muskingum routing coefficients has been based on observations of the linearity of a loop formed by graphically plotting a forward and a reverse path. This graphical procedure is time‐consuming and may not minimize the error of estimation.A procedure was developed to improve the drawbacks of the graphical method. This procedure calls for (a) the use of least square regression on the forward and reverse paths to determine their respective slopes, and (b) the use of statistical t‐test to evaluate the hypothesis that these two slopes are equal. The computational procedure is repeated, using incremental values of the flow weighting coefficient, x. A graph of the computed t‐value versus x can be constructed. The optimal value of x, as read from the graph, occurs at the minimum computed t‐value. The procedure has been demonstrated superior to the graphical method for three illustrative examples, resulting in a reduction of the error squares by factors ranging from 5 to 6.

Orchestra pit design considerations

The first design priority is to provide good intra‐pit communication, permitting the orchestra to coordinate tuning, timing, and loudness. Secondly, performers on stage need clear signals from the pit, to coordinate their efforts with those of the orchestra (the reverse path is important, but the conductor can usually mitigate problems that arise). Finally, the good overall performance achieved by these means must be properly transmitted to the audience. These goals are chiefly met by providing an adequate supply of early reflections (t ≲ 30 ms), each of which is a reasonably coherent version of an originally radiated sound. Designer's resources include reflection, quasicoherent scattering, and absorption via properly oriented surfaces whose dimensions correspond to wavelengths in the musically critical 250 to 1500‐Hz range. Electronic aids are risky if attention is not given to the need to assure multiple signal paths. [Work supported by NSF.]

Precision phase comparison via communication satellites using loop‐back tones

Phase comparison of remote oscillators by tone transmission via communication satellites is considered. By introducing loop‐back tones at each station to produce a virtual downlink frequency equal to the uplink frequency, the path phase between two stations is made independent of the direction of transmission. All nondispersive path effects are then canceled by using reverse path transmissions. The effective period for phase comparison is the mean period of the uplink and downlink transmissions. This represents a 2–order of magnitude increase in effective frequency over the previously used two‐tone method. The precision of phase comparison via satellites is now limited by uncertainties of ionospheric effects.

Reverse Combustion Instabilities in Tar Sands and Coal

Abstract A linear stability analysis shows that reverse combustion in coal and tar sands is only conditionally stable for mobility ratios less than one. However, high air-flow rates and gas generation at the combustion front can be stabilizing influences. For unstable operation, an estimate of the size of the reverse combustion channel may be obtained from the curve for the most highly amplified wave length. This provides a method for calculating the air flux, combustion front velocity, and rate of progress of the burn front. Recently the U.S. DOE Laramie Energy Technology Center (LETC) and Sandia Laboratories obtained experimental data about reverse combustion from a field test of in-situ coal gasification at Hanna, WY. These data show that 9.7 days were required for the development of a reverse combustion path 68 to 70 ft in length. The stability theory developed in this work predicts a length of 64 ft for this same 9.7-day period. In addition to quantitative predictions, stability theory provides an explanation of certain puzzling qualitative observations concerning reverse combustion.

Photoreceptor optics II: application to angular sensitivity and other properties of a lens-photoreceptor system.

The formalism of photoreceptor optics, developed in an earlier paper, is applied to a system consisting of a small receptor, supporting a few wave-guide modes, which is excited by light focussed onto it by a lens. The effects of various lens parameters on the angular and absolute sensitivities of this system are described. Receptor variations in the form of caps and tapers are introduced and their effects on the excitation process evaluated. The results obtained are also related to the approach to vision research based on the receptor as a sampling element. Finally, an analysis is presented of the deduction of receptor properties by means of reverse light path observations. The emphasis is on results and graphs are presented so that trends and magnitudes are readily appreciated.

Reverse path forwarding of broadcast packets

A broadcast packet is for delivery to all nodes of a network. Algorithms for accomplishing this delivery through a store-and-forward packet switching computer network include (1) transmission of separately addressed packets, (2) multidestination addressing, (3) hot potato forwarding, (4) spanning tree forwarding, and (5) source based forwarding. To this list of algorithms we add (6) reverse path forwarding, a broadcast routing method which exploits routing procedures and data structures already available for packet switching. Reverse path forwarding is a practical algorithm for broadcast routing in store-and-forward packet switching computer networks. The algorithm is described as being practical because it is not optimal according to metrics developed for its analysis in this paper, and also because it can be implemented in existing networks with less complexity than that required for the known alternatives.

The meteorological jet stream as a source of medium scale gravity waves in the thermosphere: an experimental study

Results are reported of a co-ordinated experimental study of medium scale gravity waves involving use of a multistatic incoherent scatter system together with two networks of polarimeters monitoring transmissions from a geostationary satellite during a six-day period in July 1974. The latter observations give access to the horizontal propagation parameters of the waves while the former technique yields information on the vertical structure of both waves and the ambient atmosphere. A reverse ray tracing analysis, through the atmosphere, for each gravity wave is described and its validity and accuracy discussed in detail. For the majority of the waves the reverse group path can be followed down to tropopause level and comparison with meteorological data has shown that many of the possible source regions of the observed waves appear to lie in proximity to the jet stream, with some evidence to suggest a preference for the polar side of the jet. Other wave sources are found to be close to regions of convectively unstable cold polar air. Wave energetics are discussed together with possible generation mechanisms, the latter in the form of non-linear interaction of shear flow instabilities in the jet stream and penetrative convection.

Effect of soil burning on microfungi

The qualitative and quantitative nature of microfungi in soil after burning was studied for seven months. The numbers of fungal species and their colonies in soil were reduced immediately after burning but returned to their normal state after about a couple of months. Species of Aspergillus and Penicillium were the first to colonize the burnt soil. Trichoderma viride appeared after two months. Penicillium spp. decreased consistently with time whereas T. viride followed the reverse path. Several other species appeared in due course as and when ecological conditions became suitable for them. re]19760720

Estimating reverse path loss from oblique ionospheric sounder signals

Tests are performed over a 4300‐km path to determine the accuracy of reverse path loss estimates using the step‐frequency sounder of the Navy Tactical Sounder System. The objective is to develop computational techniques for real‐time estimates of communication channel performance. Experimental results are presented which indicate the statistical performance of several path loss estimation techniques. It is concluded that, for Navy applications, reverse path loss can be predicted to within 6 db for 90% of the time.

Special microscope microspectrophotometer: optical design and application to the determination of waveguide properties of frog rods.

A special-purpose microscope microspectrophotometer permits the light distribution from photoreceptor waveguides to be separately isolated and masked both at the waveguide termination and at the far field. Single units or small groups of photoreceptors can be studied, and the patterns analyzed densitometrically or spectrophotometrically. The light patterns can be tracked from near to far field in order to follow the relationship between the two distributions. Frog photoreceptor cells were isolated for study and their near-and far-field mode-pattern distributions were photographed with light passing through the retina by normal path and by reverse path. Six to nine minutes after interdicting retinal blood flow, individual fibers of the preparation appeared to be separating into multiple smaller fibers within the cell-membrane envelope. Individual frog rods were shown to be directionally transmissive, and the far-field pattern of groups of receptors appeared to be a simple combination of the individual far-field patterns. Evidence was found that the input plane of the frog receptor-fiber optic bundle may not coincide with the external limiting membrane, contrary to what has been frequently assumed.

Experiments concerning the measurement of floating-emitter potential

Floating-emitter potential measurement indicates the surface condition and therefore is used for the detection of defects on the surface. So far the cause of excess floating-emitter potential has been given due to channel formation only. The present paper deals with experiments which have revealed that external conducting paths provided by the adsorbed or condensed layer of water on the encapsulation of the transistor is also equally or even more responsible for performance degradation. A mathematical equation has been formulated which correlates floating potential under humid conditions to the initial floating potential, applied collector reverse bias and leakage path resistances. Variation of floating-emitter potential with collector reverse bias and temperature have been shown graphically. The effect of 100 per cent relative humidity at 40°C on this potential has been studied.

Reflection of a Sound Ray Bundle from a Sloping Ocean Bottom

The theory for calculating the intensity of sound in a refracting medium after reflection from a sloping bottom was first formulated by Einstein and Eby [L. T. Einstein and E. S. Eby, USL Tech. Memo. No. 1210-103-60 (1960)]. However, they did not pursue their development to the point of evaluating certain angle derivatives which are necessary to obtain quantitative results. The present paper describes such a computation for the particular case of up- and down-slope reflection from a flat inclined bottom bounding a medium in which the sound speed gradient is constant. The results have an important bearing on the sound intensities obtained with most ray tracing programs, since these usually trace rays from receiver to source, and a correction must be applied to obtain the intensity for the reverse path. For the flat bottom or for RSR or refracted transmission, this correction factor is the well-known ratio Cs2/Cr2, where Cs and Cr are the sound speeds at the source and receiver, respectively. This factor is not modified in the presence of reflection from a sloping bottom. [This work was supported by the U. S. Navy Electronic Systems Command.]

Average gain of feedback system with one time-varying gain

The feedback system considered has a transfer function in the forward path and a time-varying gain in the reverse path. Bounds on the average of the system output will be established for a constant input.

Path maps and continuity

A function f: X-> Y will be called a path map if it maps the paths of X to paths of Y. That is, if u: I->X is a path, then so is f: I-?Y. Any continuous function is a path map. Conversely, I shall show that with mild restrictions on the space X, all path maps f: X-* Y are continuous. A duality consideration yields some more information about the situation. In this case, the object of interest is the following: g: X-> Y is a reverse path map if whenever q: Y->I is a continuous (real-valued) function, then so is qg: X->I. If certain conditions are placed upon Y, then every reverse path map g: X-> Y is continuous. The aim of the following theorems is to reveal some of the interplay between paths, continuous functions, and continuous real-valued functions.