Introduction

After the first success of helioseismology, it has been shown than new results could only be obtained from long set of continuous observations. Therefore different groups intented to set-up worldwide networks in order to observe the Sun 24 hours a day. This is the case of GONG and IRIS. Both are six-stations networks. Figure 1 shows the different sites of the two networks. They have one common site in Izaña.

IRIS has already 5 stations installed and running. The last site, in Australia, is under selection, and very probably will be in Culgoora, more accessible than the Western site selected by GONG team, in Learmonth. It will be set-up at the beginning of 1994. The operation, already started since 1990, will continue untill at least 2000. So far, the best piece of data obtained covers a period of three monthes, during Summer 1991, with a duty cycle of 60%, with only 3 instruments working. With 6 sites, a duty-cycle of at least 85% is expected.

The six instruments of the GONG network are constructed and under tests. The deployement is supposed to take place during the second half of 1994. The network will be fully operational in 1995. A duty-cycle higher than 90% is expected.

IRIS provides only full-disk velocity measurements, whereas GONG is designed to record images of the Sun, providing a spatial resolution. GONG will measure non-radial modes, from 1=1 to 1=200, when IRIS is only sensitive to radial and low degree non-radial modes (1=0 to 3).