Objectives of the Tandem Mission
The ERS Tandem Mission
encompasses the following combinations of instrument data:
Figure 1. Unwrapped interferogram superimposed on a SAR intensity
image from the Bern area, Switzerland (ERS-1: 9 July; ERS-2: 10
July 1995). Bern is indicated by a square at the double oxbow in
the river. The lake seen as a dark area at the bottom right
corner is the Thunersee. (Image produced by C.H. Buck, ESTEC-
XRI).
The Operational Scenario
The Tandem Mission's objectives are met
by correct adjustment of the orbital state of the two satellites.
During the ERS-2 Commissioning Phase and the Tandem Operations
Phase, the two satellites have been maintained in the same
orbital plane, ERS-2 following thirty minutes behind ERS-1. This
means that the same swath on the ground is acquired by ERS-2 one
day later than by ERS-1.
While ERS-1 still provided the nominal high-bit-rate (HBR) and low-bit-rate (LBR) services to respond to user requests, all SAR acquisition requests were copied and scheduled also for ERS-2 acquisition. In coordination with the global network of national and foreign receiving stations, a background acquisition plan was set up matching (within the constraints of satellite resources) the availability period of the station with specific orbit maintenance procedures.
For example, during the period 15 January to 25 March (Polar Campaign), the orbit was maintained to meet baseline requirements (cross-track separation of 50-250 m) at latitudes above 60 N, while orbit cross-over points were at equatorial latitudes. During this period, maximum acquisition was scheduled over the stations of O'Higgins, McMurdo, Syowa, Alaska Prince Albert and Kiruna. In contrast, full coverage of South America (Cuiaba) was acquired in April/May, when the orbit maintenance was focussed on meeting a 50-250 m cross-track separation at the equator.
Loss of SAR tandem coverage due to late availability of certain receiving stations and conflicts caused by the completion of the ERS-2 WSC commissioning activities during the Tandem Phase could be compensated for by extremely precise orbit maintenance. The high frequency of in-plane manoeuvres, especially towards the end of the mission, ensured that nearly every acquired data pair met the specifications in terms of cross-track separation.
Acquired SAR Tandem Pairs
About 110 000 ERS SAR pairs have been
acquired during the last nine months, covering nearly the total
global land surface. Whilst over South America and parts of
Southeast Asia just one data pair was acquired, as many as five
or six interferometric pairs (1-d offset) are available for
Europe and North America. 73% of the acquired data meet the
optimal baseline requirements for DEM generation of 50 to 300 m
and cover the global land mass (Fig. 2). This is a much higher
percentage than originally expected, thanks to the precise orbit
maintenance during the Tandem Mission. Another 6% of the acquired
data with baseline values between 300 and 600 m are still usable
for DEM generation. The remaining 21% of the data with baseline
values below 50 m are still usable for other applications like
change detection and differential interferometry.
Figure 2. ERS-1/ERS-2 SAR tandem acquisition pairs with optimum
baseline values for DEM generation (status of 1 June 1996).
Applications
The first results of ERS SAR interferometry using
data from both satellites have been summarised in a document
prepared by ESRIN and presented to the DOSTAG delegates. A
preliminary investigation was performed of the potential of
tandem interferometry for a variety of application areas,
including glacier motion, forest and land cover mapping, land-
surface motion related to volcanism and neo-tectonic forcing, as
well as DEM production.
In order to further strengthen the exploitation of the ERS Tandem Mission, an Announcement of Opportunity dedicated to the scientific exploitation of the data collected was released in January. It resulted in some 60 projects focusing on ERS INSAR techniques and complementary use of ERS-1 and ERS-2 instruments.
One of the challenges for the near future is the development of INSAR processors with a sufficiently high throughput to process the huge amount of ERS tandem data on a continental or even global scale. Various initiatives such as the one to generate a DEM of the Antarctic Continent for scientific exploitation using the data acquired over stations such as O'Higgins, McMurdo and Syowa, are currently being defined and set up.
Post-Tandem Operations
After the completion of the ERS Tandem
Mission, from June 1996 onwards, ERS-2 will ensure the nominal
services for ERS data. In view of its good health, the ERS-1 SAR
will initially (during the Post-Tandem Campaign Phase) be
operated jointly with that of ERS-2 to meet limited requirements
for monitoring ice melting and vegetation growth in Europe and
to support acquisition of tandem pairs over new acquisition
stations (Usbekistan, Malindi, Israel). The initial post-tandem
campaigns will be followed by a 'Parallel Operations Phase',
which will permit users to request acquisition and products from
the SAR instruments on both satellites, reducing however the
average number of acquisitions and instrument operation time on
both satellites. This scenario is expected to prolong the
lifetime of the ERS satellite system and the services to users
until the end of the Envisat Commissioning Phase.
A workshop dedicated to ERS SAR interferometry will be held in Zurich from 30 September to 2 October 1996.
ESA EOQ 52