This thesis describes the thermal design and thermal testing of the development model radiative cooler for the Composite Infra-Red Spectrometer (CIRS) due for launch on the Cassini spacecraft in 1997. The radiative cooler is used to cool the instrument's Focal Plane Assembly (FPA) to approximately 80K. The FPA holds two arrays of HgCdTe detectors for the mid infra-red spectrometer of the instrument which covers the wavelength range 7μm to 17μm. The FPA is mounted from the optics on a titanium alloy tripod and is cooled conductively by the radiator via a flexible link and a cold finger. A range of thermal models of the system have been developed ranging from a simple, analytical model to a finite difference numerical model. A calorimeter was designed to perform heat leak measurements on samples of Multi- Layer Insulation (MLI) blankets to determine the number and type of shields required for the MLI blanket covering the back of the cooler radiator. A test facility incorporating a vacuum system, a space simulator target, and a simulator for the CIRS instrument was designed and constructed for testing the assembled cooler. Various configurations of the Development Model (DM) CIRS cooler were tested as components became available and the results obtained compared to the thermal model predictions. It was found that the cooler will attain a temperature of 80K in operation, but with less excess cooling power than predicted by the thermal models.

During the past few years, a worldwide interest and unanimous consensus has arisen on personal communication services (PCS), where satellites can play a crucial role in a global scenario for the provision of PCS's all over the world. While for maritime and aeronautical communication services, the mature technologies of geostationary earth orbit (GEO) satellite systems seem the most suitable for present and future enhanced systems, other orbital configuration such as low earth orbit (LEO) and medium earth orbit (MEO) are being considered for the provision of satellite personal communication services to hand-held mobile terminals. One of the main objectives of personal communication services is the capability to provide personal (or continuous) mobility, communication anytime, anywhere. In general, satellite systems can provide a limited capacity with respect to terrestrial networks, nevertheless they are particularly suited in order to cover large terrestrial areas offering a scarce amount of traffic. The problem of radio frequency management for mobile applications has been addressed by World Administrative Radio Conference (WARC-92). One of the most relevant decisions taken at WARC-92 was to allocate the radio determination satellite system (RDSS) 1610-1626.5 MHz (L-Band) and 2483.5-2500 MHz (S-Band) slots to LEO satellite services on a worldwide, primary basis. This enables "big LEO's" to have a reasonable amount of spectram (i.e. capacity) to serve a substantial number of subscribers. However, the available amount of spectrum at L-band and S-band out of WARC-92 appears hardly sufficient to permit several systems to operate. Hence, to enable all the systems to operate and fulfil the capacity demand, S-PCNs need to share the limited available frequency spectrum. In this thesis we have proposed and evaluated a new fully overlapped band sharing scheme for mobile satellite systems operating in a land mobile satellite channel environment. The results show that the mobile satellite systems can share the limited available bandwidth. However, the overall system capacity of the MSS has been reduced due to excess intersystem interference. In order to reduce the excess inter-system interference a new enhanced overlapped band sharing protocol is proposed. The performance of this optimised band sharing scheme outperforms the conventional band segmentation scheme. Thus, achieving superior overall system capacity.