The world biggest decameter wavelength radio telescope UTR-2 and VLBI system URAN, as well as main observational programs carried out with the above mentioned instruments, are described.

An international Low Frequency VLBI Network (LFVN) has been organized for observations at 327 MHz and 1665 MHz frequencies with MARK-II and S2 recording terminals. Data on the interplanetary medium and spike-like solar bursts were obtained in first preliminary experiments.

Acting as a pilot of the Square Kilometer Array (SKA), the Five hundred meter Aperture Spherical Telescope (FAST) has been proposed for construction in the unique karst area of southwest China. FAST is of Arecibo-type with rather a number of innovations, among which the active main spherical reflector shows fascinating. Furthermore, the feed cabin is supported and moved by cables controlled by computer, which avoids a heavy and expensive focal pointing system. With the effective aperture of 300 m, a large sky coverage, and a broad bandwidth (200 to 2000 MHz), possible capability up to 5 ∼8 GHz, FAST will be the world's largest single dish and perform markedly role in radio astronomy.

We describe the nature of the interference challenges facing radio astronomy in the next decade. These challenges will not be solved by regulation only, negotiation and mitigation will become vital. There is no silver bullet for mitigating against interference. A successful mitigation approach is most likely to be a hierarchical or progressive approach throughout the telescope and signal conditioning and processing systems. We summarise some of the approaches, including adaptive systems.

Radio astronomy in the decameter to centimeter wavelength range is facing new challenges because of man made interferences due to increasing needs in telecommunications. At the Radioastronomy department of Paris Meudon Observatory, we have been working since four years on high dynamic range digital receivers based on Digital Signal Processors (DSP). The first achievement is a digital spectro- polarimeter devoted to spectroscopy of astrophysical radiation in decameter range, now in operation at the Nancay Decameter array. The block diagram of the receiver includes a high dynamic range analogue section followed by a 12 bits analogue to digital converter. The digital part makes use of high power, programmable digital circuits for signal processing, arranged in a dedicated parallel architecture, able to compute in real time the power spectrum and the correlation of the input signals. This receiver was also used, as spectrometer backend, at Nancay decimetric radiotelescope and has performed very well in the presence of very strong interferences. We are presently working on a new digital receiver with broader bandwidth. The objective is 2 × 25 MHz band with at least 60 dB dynamic range. This new receiver will use additional computation power in order to recognise and avoid man made interferences which corrupt the radio astronomical signal. At the Nancay Radioastronomy Observatory, we have started to develop a new digital configurable receiver with 8 times 25 MHz band and ten thousand channels. For low frequency radioastronomy, direct spectrum computation technique is really powerful and offers new capabilities for real time interferences excision. Fig. 1 shows pulsar observations in the presence of interference made with the DSP receiver on the UTR-2 radiotelescope. Fig. 2 shows the effect of satellite interfernce on OH observations made with the Nancay telescope. Fig. 3 shows the block diagram of the DSP system and demonstrates how offline excision of interference in the frequency time-domain enables recovery of the signal. The final spectrum had 960 minutes integration on and off source and took 8045 minutes of procession on a 450 MHz Pentium II.

Despite seemingly being poles apart, the two techniques of mosaicing and “polyhedron imaging” in radio interferometer do share some common aspects. This note shows some of these similarities. In particular, a simple method for producing geometrically correct images of large fields is described.

The Filter CLEAN algorithm has been developed for CLEANing images containing a mixture of extended and fine-scale sources. Filter CLEAN requires fewer iterations and the residual rms is much lower than results from Högbom CLEAN. Filter CLEAN is particularly good at recovering extended sources while maintaining good resolution on fine-scale sources. The Filter CLEAN algorithm is described and results presented.

The response of a pair of differently polarized antennas is determined by their polarization states and a phase between them which has a geometric part which becomes discontinuous at singular points in the parameter space. Some consequences are described.