This thesis is a study of multiple access schemes for satellite land mobile systems that provide a domestic or regional service to a large number of small terminals. Three orbit options are studied, namely the geostationary, elliptical (Molniya) and inclined circular orbits. These are investigated for various mobile applications and the choice of the Molniya orbit is justified for a U. K. system. Frequency, Time and Code Division Multiple Access (FDMA, TDMA and CDMA) are studied and their relative merits in the mobile environment are highlighted. A hybrid TDMA/FDMA structure is suggested for a large system. Reservation ALOHA schemes are appraised in a TDMA environment and an adaptive reservation multiple access protocol is proposed and analysed for a wide range of mobile communication traffic profiles. The system can cope with short and long data messages as well as voice calls. Various protocol options are presented and a target system having 100,000 users is considered. Analyses are presented for the steady state of protocols employing pure and slotted ALOHA and for the stabilty of the slotted variant, while simulation techniques were employed to validate the steady state analysis of the slotted ALOHA protocol and to analyse the stability problem of the pure ALOHA version. An innovative technique is put forward to integrate the reservation and the acquisition processes. It employs the geographical spread of the users to form part of the random delay in P-ALOHA. Finally an economic feasibility study is performed for the spacesegment. For costs of capital (r) less than 23 % the discounted payback period is less than the project's lifetime (10 years). At r- 8% the payback period is about 5.6 years, while the internal-rate-of-return is 22.2 %. The net present value at the end of the projects lifetime is £M 70 at r-8%.

This thesis is a study of multiple access schemes for satellite land mobile systems that provide a domestic or regional service to a large number of small terminals. Three orbit options are studied, namely the geostationary, elliptical (Molniya) and inclined circular orbits. These are investigated for various mobile applications and the choice of the Molniya orbit is justified for a U. K. system. Frequency, Time and Code Division Multiple Access (FDMA, TDMA and CDMA) are studied and their relative merits in the mobile environment are highlighted. A hybrid TDMA/FDMA structure is suggested for a large system. Reservation ALOHA schemes are appraised in a TDMA environment and an adaptive reservation multiple access protocol is proposed and analysed for a wide range of mobile communication traffic profiles. The system can cope with short and long data messages as well as voice calls. Various protocol options are presented and a target system having 100,000 users is considered. Analyses are presented for the steady state of protocols employing pure and slotted ALOHA and for the stabilty of the slotted variant, while simulation techniques were employed to validate the steady state analysis of the slotted ALOHA protocol and to analyse the stability problem of the pure ALOHA version. An innovative technique is put forward to integrate the reservation and the acquisition processes. It employs the geographical spread of the users to form part of the random delay in P-ALOHA. Finally an economic feasibility study is performed for the spacesegment. For costs of capital (r) less than 23 % the discounted payback period is less than the project's lifetime (10 years). At r- 8% the payback period is about 5.6 years, while the internal-rate-of-return is 22.2 %. The net present value at the end of the projects lifetime is £M 70 at r-8%.

Squeeze Film Dampers (SFD) provide viscous damping in rotor bearing systems to reduce lateral vibration amplitudes and to isolate mechanical components. Aircraft engine shafts, often supported on roller bearings, operate at high rotational speeds and are susceptible to large amplitude shaft whirl due to rotor imbalance. SFDs aid to reduce such large whirl amplitudes while also eliminating rotor instabilities. he current work quantifies experimentally the forced performance of two parallel squeeze SFDs separated by a central groove. Force coefficients are identified in a specialized SFD test rig constructed to undergo similar operating and loading conditions as in jet engines. Of interest is to quantify the effect of a central feed groove on the forced performance of SFDs and to validate predictions from a computational tool. The test rig comprises of an elastically supported bearing structure and one of two journals. Tests are conducted on two open ends SFDs, both with diameter D and nominal radial clearance c; each damper with two parallel film land lengths L= 1/5 D and 2L, separated by a feed groove of width L and depth 3/4 L. ISO VG 2 grade lubricant oil flows into the central groove via 3 orifices, 120 degrees apart, and then through the film lands to finally exit to ambient. In operation, a static loader pulls the bearing to various static off center positions with respect to the stationary journal, and electromagnetic shakers (2,200 N) excite the test system with single frequency loads over a frequency range to generate rectilinear, circular and elliptical orbits with specified motion amplitudes. A frequency domain method identifies the SFD mechanical parameters, viz., stiffness, damping, and added mass coefficients. The long damper generates 7 times more direct damping and 2 times more added mass compared to the short length damper. The damping coefficients are sensitive to the static eccentricity (up to 50 percent c) while showing lesser dependency on the amplitude of whirl motion (up to 20 percent c). On the other hand, added mass coefficients are nearly constant with static eccentricity and decrease with higher amplitudes of motion. The magnitudes of identified cross-coupled coefficients are insignificant for all imposed operating conditions for either damper. Large dynamic pressures recorded in the central groove demonstrate the groove does not isolate the film lands by merely acting as a source of lubricant, but contributes to the generation of large added mass coefficients. The recorded dynamic pressures in the film lands and central groove do not evidence lubricant vapor or gas cavitation for the tested static eccentricities and amplitudes of motion. The direct damping coefficients for both dampers are independent of excitation frequency over the frequency range of the tests. Predictions derived from a novel SFD computational tool that includes flow interactions in the central groove and oil supply orifices agree well with the experimental force coefficients for both dampers. The current work advances the state of the art in SFDs for jet engines.