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Tandem Operation of ERS satellites
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Joint operation of ERS-1 and ERS-2


Computer illustration of ERS-2

When it was decided in 1990 to build ERS-2, the prime concern was to provide a follow-on satellite to ERS-1. Its main task is therefore to ensure the necessary continuity of data supply to scientists and applications-oriented users. ENVISAT will then take over from ERS-2 in 1999, offering even more powerful and sophisticated instruments than either of the ERS craft.

When it became clear towards the end of 1993 that ERS-1 would remain technically operational well beyond its 30-month design life, a great many users approached ESA with proposals for the combined operation of ERS-1 and ERS-2 for a period of six to nine months. Since then ESA has been instructed by its Member States to keep ERS-1 "alive" and, during the ERS-2 commissioning period of about three months, to use it to maintain the regular supply of data to users.

So what are the benefits of an ERS-1/ERS-2 tandem mission? Three main areas can expect to gain from it:

SAR interferometry

This is a technique for extracting highly precise information about the topography of the Earth's surface from the phase information provided by two or more SAR images acquired in the course of successive passes on the same orbit.

Only radar satellites are suitable for interferometry and although as little as three years have passed since the launch of ERS-1 turned SAR interferometry from theory into scientific reality there are already large numbers of scientists worldwide working with this technique. The most impressive results have been the production of "digital elevation models" - high-value cartographic products accurate to a few metres - and the measurement of shifts in the Earth's crust to within a few centimetres, for example following earthquakes, prior to volcanic eruptions and as a result of glacier flows.

The quality of the products does however depend greatly on the time interval between passes - from 3 to 35 days - and on the distance between satellite trajectories from one pass to another, a typical distance being several hundred metres. These two parameters can be systematically optimised using two satellites.

Scientists and also firms specialising in digital elevation models are hoping that the tandem mission will generate a raw data set covering as much of the Earth's land surface as possible that could be used in the years to come, as and when required, to compute elevation models.

Agriculture and hydrology


Tierra del Fuego, Argentina (multitemporal SAR image).

For agriculture and hydrology the most important ERS-1 products are "multitemporal images", that is to say combinations of two and sometimes three frames acquired on different dates. The images, each in a different primary colour, are then digitally analysed or simply superimposed. This technique allows the SAR's stability and its sensitivity to the structure and humidity of the Earth's surface to be used to best effect to detect changes over time. If during the observation period no changes have occurred on the surface area concerned the superimposition produces a white or grey output. In multitemporal SAR images this is the typical "signature" of densely populated areas. The surface structure of the natural landscape, on the other hand, changes with the seasons - the melting of the snows, foliage growth, harvesting and also deforestation and erosion all have their own colour code, their own signature.

Here again two satellites working in tandem can be used effectively to adapt the time interval between two passes to local conditions. This is particularly true when it comes to monitoring snowbreak and natural disasters, when the situation generally changes quickly.

Oceanography

Modern oceanography can scarcely be imagined without satellites. The structure and dynamics of ocean currents are recorded by radar altimeters (not only on ERS-1) and then immediately used to investigate global climate change. So it was that data from ERS-1 allowed the last occurrence of the El Ničo climatic phenomenon in 1991/2 in the South Pacific to be monitored virtually in real time.

The relatively long ERS orbit repeat cycle of 35 days makes it possible to scan the ocean surface more densely than other satellites. In summer 1994 the ERS-1 orbit cycle was extended to 168 days to give an even more detailed map of the ocean surfaces. The joint operation of two satellites combines the advantage of a dense network of measurements and that of a comparatively short repeat cycle, which is of great value for the investigation of ocean currents and changes in them.

The areas considered above - SAR interferometry, agriculture, hydrology and oceanography - do not of course constitute an exhaustive list. Other fields of scientific and applications research could also benefit from a tandem mission.