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%.

The number of man-made space objects is dramatically growing nowadays. The continuous monitoring and studying of these objects are necessary to keep their number under control and ensure safe space operations. With respect thereto, international guidelines recommend decongesting the most populated space regions from satellites arriving at the end of their operational lifetime by performing post-mission disposal strategies. In general, a satellite is considered to be functional if it is still performing periodic manoeuvres to stay within the orbital operation configuration. This study presents a promising method to detect historical in-plane manoeuvrers of satellites on a geostationary orbit (GEO). Since a manoeuvrer changes the orbital state of the spacecraft, its effect can be detected by comparing the observed data to a reference evolution. In this case, the  model is represented by the dynamical model STELA  based on a semi-analytical theory. The observed data is provided by the public American space object catalogue. The Two-line element (TLE) database contains the orbital state of each tracked object, however, not all six orbital parameters are interesting to study in terms of in- plane manoeuvrers. The evolution of the longitude and of the eccentricity vector is immediately affected by a manoeuvre that changes the shape or the size of an orbit. Within the longitude analysis, the manoeuvre epoch is estimated by focusing on the manoeuvre strategy. An operational spacecraft usually performs a manoeuvre as soon as the longitude motion threatens to violate the operational deadband. Consequently, the longitude evolution follows a parabolic motion. Two polynomial curves of second degree are laid over the observation: the first curve is derived from a simplified dynamical model and the second curve is directly obtained through a Least Squares (LS) fitting method. The discrepancy between the LS and physical fitted parabolas gives an indication on the quality of the input data, that is to say, of the TLEs. The detected manoeuvre epoch must be companioned by a confidential parameter that denotes the time range around the estimated epoch in which the manoeuvre is expected to have happened. The manoeuvre interval is then forwarded to the eccentricity analysis where the manoeuvrer epoch is estimated more precisely by studying the divergence between the observed and expected eccentricity vector evolution. The latter is propagated with STELA after having estimated the area-to-mass ratio that is needed in order to model the perturbation effects accurately upon which the performance of the dynamical reference model strongly depends. As soon as the observed eccentricity vector deviates significantly from the expected evolution, the epoch and the velocity ΔV of the manoeuvre can be recovered, too.