Ionospheric limitations to time transfer by satellite

The ionosphere can contribute appreciable group delay and phase change to radio signals traversing it; this can constitute a fundamental limitation to the accuracy of time and frequency measurements using satellites. Because of the dispersive nature of the ionosphere, the amount of delay is strongly frequency-dependent. Ionospheric compensation is necessary for the most precise time transfer and frequency measurements, with a group delay accuracy better than 10 nanoseconds. A priori modeling is not accurate to better than 25%. The dual-frequency compensation method holds promise, but has not been rigorously experimentally tested. Irregularities in the ionosphere must be included in the compensation process

Nurses\u27 Alumnae Association Bulletin - Volume 3 Number 5

Jefferson Marches On Christmas Packages The United States Cadet Nurse Corps and Jefferson Medical College Hospital Attention Promotions Welcome, Miss Hopkins Student Nurse Activities Scholarship Girls Taking Anaesthesia Our Office and Teaching Staff Message from the President News Bulletins Information Please! Coming Events Graduate Nurses\u27 Chorus Condolences Emergency Nursing Penicillin Organized Staff - 1943-1944 New Positions New Alumnae News of Our Doctors Doctors in Army Army Nurse Corps Navy Nurse Corps Engagements Marriages New Arrival

Experiments and numerical simulations of the dynamics of an R.O.V. thruster during maneuvering

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996Propeller dynamics have typically been ignored in controller design, lumped into the category of 'unmodeled dynamics.' This is acceptable for propellers operating at constant speed in relatively uniform flows. Operational parameters of small remotely operated vehicles and autonomous underwater vehicles require a great deal of transient operation of the propellers. This and the small mass of the vehicles make the dynamics of the propellers a significant factor in vehicle control. Expanding roles of these vehicles require improved control and therefore improved understanding of the dynamics of the thrusters during maneuvering. In this thesis, the dynamics of maneuvering thrusters were explored through numerical simulation and experiments. Vortex lattice propeller code developed for use with nonuniform inflow was adapted to incorporate varying propeller speed and inflow velocity. Test runs were made using a three bladed propeller. Experiments were preformed on a thruster from the ROV Jason using the water tunnel at the Massachusetts Institute of Technology. The thruster incorporated a ducted three bladed propeller. Runs were made using step changes in shaft velocity as well as sinusoidal perturbations on top of steady state velocities. Runs were also made incorporating fully reversing propeller operation. Experiments were done with and without the duct in place. The numerical simulation and experimental results showed that accelerating propeller angular velocity created higher thrust values than steady state propeller operation at the corresponding instantaneous shaft velocity. Decelerating angular velocities created lower thrust values. This is attributed to a lag in the local flow velocity due to the momentum of the fluid. For the case of the accelerating propeller, the angle of attack at the blade is higher, resulting in higher lift force and greater thrust. Errors in the numerical code at low advance coefficients prevented direct comparison of numerical code results to experimental results

Slow transit constipation: clinical and aetiological studies.

PhDConstipation is the second most commonly self-reported gastrointestinal symptom. On the basis of anorectal physiological investigations and colonic transit studies, a subgroup of patients with several intractable symptoms, but without organic disease will be found to have slow transit constipation (STC). STC is a condition of gut dysmotility which predominantly affects young women, and may result in surgical intervention with variable, often unsatisfactory results. The aetiology remains elusive. New aetiological hypotheses for STC were examined following full clinical and pathophysiological characterisation of a large cohort of 130 patients referred to our institution over the last 10 years. Aspects of nerve and muscle dysfunction were studied. A new scoring system demonstrated some ability of multiple symptoms to discriminate STC from other forms of constipation. Detailed clinical and gastrointestinal physiological studies confirmed the heterogeneity of STC patients. Some significant physiological differences were detectable between clinically defined sub-groups of patients and refuted previous assumptions based on smaller numbers. Detailed neurophysiological studies, including quantitative peripheral sensory and autonomic testing, provided evidence of a small fibre neuropathy in a proportion of patients with STC. Mutational screening of some early-onset cases for a possible congenital pathogenetic mechanism, based on the observation that some STC patients had relatives with Hirschsprung's disease demonstrated that mutation of 2 important genes now implicated in this disorder were not a frequent cause of STC. Serum immunoprecipitation assays showed that anti-neuronal ion channel autoantibodies may have an as yet unrecognised role in the development of STC in a small proportion of acquired cases. An inclusion body myopathy was identifiable in colonic tissue of patients with STC, and this appeared to arise secondary to denervation. Further knowledge of the single or multiple pathogenetic mechanisms leading to this clinical condition may allow more rational or directed therapies aimed at the correction of the disease process or processes themselves