ABSTRACT. The problem of the orbital situation analysis are discussed in this paper as a foundation of the multi-satellite missions and campaign long-term planning. INTERBALL mission and its participation in the IACG/ISTP campaigns are presented as examples (see World Wide Web site http://www.iki.rssi.ru/vprokhor/amisan.htm.)

The modern space research is based on close international cooperation. In frame of the International Solar- Terrestrial Physics (ISTP) Science Initiative there exist many scientific trends like the Global Geophysical Science (GGS) and the International Auroral Study (IAS) which are based on the acting space missions, such as Wind, Polar (NASA), Soho (ESA), Geotail (ISAS, NASA), Interball (IKI-RKA).

The Inter-Agency Consultative Group (IACG) coordinates international campaigns aiming at understanding of solar-terrestrial correlation. For example, in October 1995-January 1996 there was successfully conducted the Campaign No.1 devoted to problems of Magnetotail Energy Flow and the Role of Non-Linear Dynamics. This campaign combined 4 missions: Geotail, Interball Tail, IMP-8 (NASA), Wind (NASA).

In scheduling global measurements in multi-satellite missions and campaigns the most important element is the selection of location strategy and based on it the situation analysis of orbits with the main task to predict, select and visualize the missions and campaigns key situations in space and time.

Major contribution into this activity was made by the Satellite Situation Center (SSC) and the National Space Science Data Center (NSSDC) of the Godard Space Flight Center (GSFC) distributing empirical models of magnetosphere, varied information of orbits of space vehicles operating under the above mentioned programs, and results of the scientific measurements through the Internet service.

Russian scientific community contributed into the above mentioned international programs by the multi-satellite international project INTERBALL developed at the IKI RAN in cooperation with 18 countries. This project comprises two couples of spacecraft - Tail Probe (TP) and Auroral Probe (AP), each supplied with its own subsatellite. The Tail pear is launched in August, 3 1995, with its subsatellite (Magion 4) separated in August, 4, 1995. The Magion 4 was provided with thrusters for maintaining proper distance from the satellite to the subsatellite (up to 10,000 km). The apogee altitude of the Tail pear initial value is 192,000 km, the perigee altitude - 790 km, the orbital inclination - 62.9 . The Auroral pear is planned to be launched in late August, 1996. The apogee-perigee altitude is due 20,000-700 km, respectively, the orbit inclination - 62,9 ; the scheduled satellite-subsatellite separation - up to 500 km.

According to the main goal of the Interball mission - to study active processes in the magnetospheric tail and magnetosphere-ionosphere coupling the Interball orbits configuration is selected to conduct simultaneous measurements in the magnetic conjugate regions - tail plasma sheet and auroral region. The scientific equipment includes instruments measuring plasma, plasma waves, magnetic and electric fields, and particles. Aboard the Auroral Probe ultraviolet auroral imagers are installed.

Scientific teams from 18 countries participate in the Interball project. Comprehensive investigations performed under this project are founded on the wide net of ground-based stations.

During preparation of the Interball mission the hardware and software were developed at IKI intended for informational support of the mission. One of the aspects of this activity is the situation analysis of orbits originating from the empirical models of the regions under study and with due consideration of the tasks of the long-term strategic and routine control of the mission.

Much efforts in the present work were made to adequately present the global situation picture in space and time at the prolonged time intervals (year, month) and to analyze distinct situations in full details. We tried to keep to the maximal visibility and unification (standardization) of the ways of showing orbital situation data for each spacecraft.

Such an approach yields much benefits for correlating orbital situation pictures of different spacecraft or missions when planning coordinated measurements in accordance with the location strategy of the corresponding mission or campaign.

Since numerous scientific teams are involved into the process of making decisions at that international missions, IKI made much efforts to develop modern means (including software) for distributing orbital information. For this purpose the Internet service is widely used.

Some results of the situation studies under the mission Interball (Interball Tail - Auroral Probes) and IACG/ISTP Campaign 1 (Interball Tail - Geotail) are given below as examples.

The global situation picture for the TP orbit is shown in the Fig. 1 as a satellite passage time through regions of interest of the magnetosphere. Here, we are dealing with thin boundaries of the near-earth bowshock (BS) and magnetopause (MP) and the regions like radiation belts (RB), tail neutral sheet (NS), plasma sheet (PS) and cusp region (CUSP). For each region a corresponding symbol is used. The date is plotted on the X axis, and the passage time from the node (in hours) for each revolution (draconian period of the orbit) is laid off on the Y axis.

For each month individually the standard set of pictures is prepared containing the information like time distribution of crossing magnetospheric regions, visibility zones for radio-control stations, geocentric distance and geomagnetic local time at the satellite site. Fig. 2a,b show this kind of information for TP and AP orbits in Oktober 1996 as an example. Fig. 2a shows time schedule of the TP passage through regions, where the date is plotted on the X axis and the universal time - on the Y axis. Fig. 2b shows the same information for AP. Correlation of the situation pictures for two orbits is helpful in choosing proper time for fulfillment of coordinated measurements in conjugate regions: plasma sheet (PS) of magnetosphere tail - auroral region (AUR). Another couple of figures Fig. 2c,d shows in the northern polar diagram the position of footprints (projections onto the ionosphere received with the help of tracing magnetic force lines making their way through the satellite location). The polar diagram shows the footprints geomagnetic co-latitude - geomagnetic local time dependence. The passage through regions is specified by corresponding symbols. The Fig. 2c shows the corresponding picture for AP, and the Fig. 2d - for TP footprints, respectively.

In the course of the situation analysis of the orbits the modern models of magnetosphere and is different zones are used. At the state of long-term forecasts average statistical models are being employed. When getting into detailed analysis of the selected situations the parametric models are used making it possible to give due consideration to real geophysical conditions. For more information see World Wide Web Home Page http://www.iki.rssi.ru/vprokhor/amisan.htm) .

The Interball TP and Geotail orbits during IACG/ISTP Campaign No.1 (November, 5 - January, 1996) were located at the tail of the magnetosphere. These orbits well complemented each other: the Geotail orbit lies nearby the ecliptic plane, whereas the Tail Probes orbital plane is almost perpendicular to it. One can judge of interrelation between orbital positions in November, 1995 by the figures Fig. 3a,b show projection of the orbits on the ecliptic plane in the Solar-Ecliptic system of coordinates. One-hour step of each orbit is marked by symbols of those regions, the satellites pass through. The periods of the orbits are twenty-four hours apart from each other (period of the TP orbit - 4 days, and of the Geotail orbits - about 5 days).

For the selection of preferable time intervals to perform coordinated measurements the depictions given in the Fig. 4a,b are used. The Fig. 4a shows geocentric distances of both probes in the function of time and marked by one-hour-step symbols of the corresponding regions (those a satellite makes its way through). Fig. 4b shows the geomagnetic local time along the orbit at the satellite location for both satellites.

For more information welcome to the World Wide Web Home page http://www.iki.rssi.ru/vprokhor/camp.htm.

The above presented tools and experience can be successfully used during other missions and campaigns.