Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric "radio window": 10-240 MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR.

Aims. Given their photospheric origin and refractive nature, SiS molecules can provide major constraints on the relative roles of dust condensation and non-equilibrium processes in regulating the chemistry in circumstellar envelopes around evolved stars. Methods. New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented. A combination of low-and high-energy lines are important in constraining the circumstellar distribution of SiS molecules. A detailed radiative transfer modelling of the observed SiS line emission is performed, including assessment of the effect of thermal dust grains in the excitation analysis. Results. We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical models. The carbon stars (C/O > 1) have a mean fractional abundance of 3.1 × 10 −6 , about one order of magnitude higher than that found for the M-type AGB stars (C/O < 1) where the mean value is 2.7 × 10 −7 . These numbers are in reasonable agreement with photospheric LTE chemical models. SiS appears to behave similarly to SiO in terms of photodissociation in the outer part of the circumstellar envelope. In contrast to previous results for the related molecule SiO, there is no strong correlation of the fractional abundance with density in the CSE, as would be the case if freeze-out onto dust grains were important. However, possible time-variability of the line emission in the lower J transitions and the sensitivity of the line emission to abundance gradients in the inner part of the CSE may mask a correlation with the density of the wind. There are indications that the SiS fractional abundance could be significantly higher closer to the star which, at least in the case of M-type AGB stars, would require non-equilibrium chemical processes.

Abstract. The Sub-millimetre and Millimetre Radiometer (SMR) is the main instrument on the Swedish, Canadian, Finnish and French spacecraft Odin. It consists of a 1.1 metre diameter telescope with four tuneable heterodyne receivers covering the ranges 486−504 GHz and 541−581 GHz, and one fixed at 118.75 GHz together with backends that provide spectral resolution from 150 kHz to 1 MHz. This Letter describes the Odin radiometer, its operation and performance with the data processing and calibration described in Paper II.

Two of the major second generation standards, GSM and TDMA/136 1 , have built the foundation to offer a common global radio access for data services. Through use of a common physical layer, EDGE (Enhanced Data rates for GSM and TDMA/136 Evolution), both standards will have the same evolutionary path towards providing third-generation services.EDGE is currently subject to standardization in TIA TR45.3 and ETSI SMG, a process which will be finalized in the end of 1999. Compared to the existing data services in GSM and TDMA/136, EDGE will provide significantly higher user bit rates and spectral efficiency. EDGE can be introduced in these systems in a smooth way, using existing frequency plans of already deployed networks. This paper gives the rationale behind the development of the EDGE concept, presents the EDGE technology and addresses performance by means of system simulations.1. TDMA/136 is also known as D-AMPS and IS-136.

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