ERS-2 goes gyro-less
ESA engineers have saved the life of a crucial environmental spacecraft,
enabling the decade-long European Remote Sensing programme to continue on
track. The orbital rescue took place on terra firma, as teams from ESTEC,
ESOC and Astrium came up with new navigation techniques so the ERS-2
satellite could outlast failed gyroscopes.
The team wrote software to make the new methods work, proceeding as
cautiously as surgeons operating on a frail human patient. As with any
terrestrial PC, an input of indigestible code risked a crash. Control of
the van-sized spacecraft — in polar orbit 780 km up — could have been
lost for good.
“It was risky,” said Miguel Canela of the ERS-2 gyro-less project. “We had
to rearrange onboard memory to free space for the new software, written in
an antiquated language from 15 years ago, when ERS-2 was first designed.”
The software was exhaustively tested on simulators before being uplinked
to the satellite. Even then, they only dared run it in pieces, over the
course of a week.
The first European Remote Sensing spacraft was ERS-1, launched in 1991 but
currently inactive. Four years later its near-twin ERS-2 was flown. It has
a suite of environmental sensors onboard including cloud-piercing radar to
observe the Earth in unprecedented detail.
But to maximise data yield ERS-2 needs to be kept steady across three axes
(roll, pitch and yaw). Twin gyroscopes are dedicated to each axis. If they
detect a positioning error — to less than a fraction of one degree —
onboard reaction wheels are set rotating to generate compensating momentum.
Alternatively electro-magnetic coils called magneto-torquers are charged
up to interact with the Earth’s magnetic field and move the satellite that
way.
But you can’t make corrections if you don’t know where you are. By January
2000, ERS-2 was down from six gyroscopes to limping along with one.
Complete failure would make the otherwise functional spacecraft useless.
Fast-spinning gyroscopes keep their initial orientation, like a child’s
top. This property means they are used as navigational tools inside guided
missiles and submarines as well as spacecraft. Except gyroscopes don’t
spin forever.
“Most Earth or space observation satellites have their operational lifetime
determined by their onboard gyroscopes,” said Miguel. “These frictionless
wheels perform billions of turns. But eventually they stop or are no longer
reliable.”
With this in mind, Miguel and the ESA team worked out a method of operating
the ERS-2 equipment (sensors and actuators) in a new way, to permit
gyroscope-free ERS-2 positioning. When the last gyroscope failed on 13
January this year, they were ready. Part of their design involved a device
called the Digital Earth Sensor (DES), set to our planet’s horizon for
extremely basic positioning checks.
“After five years of use we knew we could get a lot more precise data out
of the DES than just the horizon line,” Miguel explained. “We cleaned up
the DES signal to filter out noise, then used it to estimate pitch and
roll errors.”
That still left the final yaw (or downward) pointing error, which could
no longer be measured by the gyro-less spacecraft. The team realised they
could check yaw drift themselves, by analysing Doppler frequency shifts in
the ERS-2 radar instrument signals. Turning raw signal into useful data
takes up to three hours however, too long to keep the spacecraft correctly
orientated in real time.
So instead they monitored ERS-2 over a 105-day ‘shakedown cruise’,
totalling three 35-day repeat tracks over the Earth’s surface. Recurring
patterns of spacecraft ‘depointing’, caused mainly by terrestrial magnetic
field variations as well as pressure from the solar wind, were rendered
into a detailed model uplinked to the satellite. This enables depointing
to be anticipated and compensated for.
“We’re about to finish fine-tuning the model,” Miguel said. “We operate
the reaction wheels more than before, but we keep navigation errors within
the limits required by all ERS-2 instruments.”
The gyro-less technique should extend the lifespan of numerous other ESA
missions, and preserves ERS-2 to operate with its scheduled successor
Envisat and to provide wind measurement until Metop-1 takes the relay (2003).
Related articles
* About ERS
http://www.esa.int/export/esaSA/GGGYXW7RVDC_earth_0.html
* A decade of ERS Earth-watching
http://www.esa.int/export/esaCP/ESAI6C0VMOC_index_0.html
Related links
* ERS homepage
* Astrium Space
IMAGE CAPTIONS:
[Image 1:
http://www.esa.int/export/esaCP/ASEOTTNW9SC_index_1.html]
ERS-2 gyro-less operations — Lisbon, 16 Sept. 2001.
Portugal: The southern coast and the Tejo river estuary, close to Lisbon.
Quite visible across the river, at the harbour entrance, is the suspended
bridge named ’25th of April’. Imaged by ERS 2 during an ascending pass,
the bridge is marked by three distinct bright stripes, due to the multiple
reflection effects. The port of Barreiro can also be detected. Remarkable
throughout the estuary is the bottom topography revealed by the SAR
instrument. ERS-2 gyro-less operations ensured by the joint ESA/European
industry efforts continue supporting a wide range of environmental
applications through the provision of routinely and timely available ERS-2
SAR products of good quality.
[Image 2:
http://www.esa.int/export/esaCP/ASEOTTNW9SC_index_1.html#subhead1]
ERS-2 gyro-less operations showing oil slick in the North Sea.
The North Sea: Oil Slick between the Netherlands and the UK. ERS-2 SAR
Precision Image generated from ERS-2 data acquired by the Troms¯ Satellite
Station in Norway on 28th August, 2001 over the North Sea. It shows a long
oil slick occurred between the UK and The Netherlands. A thinner oil slick,
to the right, can also be detected together with the ship that has probably
caused it. ERS-2 gyro-less operations ensured by the joint ESA/European
industry efforts continue supporting a wide range of environmental
applications through the provision of routinely and timely available ERS-2
SAR products of good quality.
[Image 3:
http://www.esa.int/export/esaCP/ASEOTTNW9SC_index_1.html#subhead2]
ERS-2 gyro-less operations — Corsica.
Corsica: Cape Corse and the Island of Capraia. ERS-2 SAR Precision Image
generated from ERS-2 data acquired over Corsica by Matera Ground Station,
Italy, on 6th September 2001. On top: the Island of Capraia (part of
Livorno Province). Bottom left part of the image: the town of Bastia,
facing the Tyrrhenian sea and the remarkable ship traffic (white dots)
in all directions. ERS-2 gyro-less operations ensured by the joint
ESA/European industry efforts continue supporting a wide range of
environmental applications through the provision of routinely and timely
available ERS-2 SAR products of good quality.
