Under contract of the European Space Agency a system study for a spaceborne gravity field recovery mission was performed, covering as a secondary mission objective geodetic point positioning in the cm range as well. It was demonstrated that under the given programmatic constraints including dual launch and a very tight development schedule, a six months gravity field mission in a 200 km near polar, dawn-dusk orbit is adequate to determine gravity anomalies to better than 5 mgal with a spatial resolution of 100 x 100 km half wavelength. This will enable scientists to determine improved spherical harmonic coefficients of the Earth gravity field equation to the order and degree of 180 or better

Benz, R.

Faulks, H.

Langemann, M.

English

NASA Technical Reports Server

ARISTOTELES
-" N90-20559
A EUROPEAN APPROACH FOR AN EARTH GRAVITY FIELD RECOVERY MISSION
R. Benz, H. Faulks, M. Langemann
Dornier-System GmbH, 7990 Friedrichshafen, FRG
Abstract
Under contract of the European Space Agency a
system study for a spaceborne gravity field
recovery mission has been performed, covering as a
secondary mission objective geodetic point
positioning in the cm range as well, It was
demonstrated that under the given programmatic
constraints including dual launch and a very tight
development schedule, a six months gravity field
mission in a 200 km near polar, dawn-dusk orbit is
adequate to determine gravity anomalies to better
than 5 mgal with a spatial resolution of
100 x 100 km half wavelength. This will enable
scientists to determine improved spherical
harmonic coefficients of the Earth gravity field
equation to the order and degree of 180 or better.
on the type of payload accommodation. The payload
is a gradiometer comprising up to 8 ultrasensitive
electrostatic accelerometers, here named GRADIO.
As an optional mission part it is intended to
raise the orbit after completion of the gravity
mission to about 700 km altitude in order to
perform point positioning for another 3 years.
The ARISTOTELES satellite is Earth oriented,
3-axis stabilized and shall be launched by an
ARIANE 44 as lower passenger within SPELDA I0
together with ERS-2 (ESA Remote Sensing Satellite)
or another Earth observation satellite. The
nominal launch date is 1994. Kiruna ground station
will be used for telemetry and telecommand on a
time sharing basis with ERS-2. The gravity field
requires an orbit restitution of better than ID m
in the radial direction.
Introduction
A detailed knowledge of the Earth gravity field is
of great interest in geophysical sciences and
their applications. In addition orbit
determination of satellites will benefit
significantly from an improved gravity field
model. Although a large amount of local data are
available, on a global scale a large part of the
Earth has not so far been covered, because of
obvious natural, political and cost constraints. A
spaceborne system can overcome these constraints
and can provide consistent and precise data on a
global scale. A comprehensive outline of the
benefits _f such a mission was given by the SESAME
workshop (Solid Earth Science & Application
Mission for Europe). -
In the framework of the Earth Observation
Preparatory Programme (EOPP) of the European Space
Agency a system study has been performed to
demonstrate the feasibility of such a programme
under special constraints including dual launch, a
tight development schedule and a limited Financial
budget. The name of the mission ARISIOTELES stands
for 'Application and Research Involving Space
Techniques Observing The Earth fFeld from a- Low
_arth Orbit-Satellite _ an_ is a reminder of _he
great philosJpher Aristotle who was the first to
speak of gravity forces.
Scientific Evaluation
Within the present study a gravity field recovery
analysis has been performed in order to establish
the system requirements based on the inter-
relations between the instrument accuracy, the
orbit altitude, the operational gravity mission
duration, the tensor components to be measured and
the gravity field accuracy to be met. The results
indicate that on the one hand a maximum orbit
altitude of 200 km is allowed, assuming a two
dimensional instrument accuracy of lO "2 E.U. and
half a year operational life time (see Figure l).
On the other hand this altitude is the lower limit
for a fixed GRADIO accommodation on the
non-drag-free satellite because of the maximum
allowed acceleration tolerance requirement of
S, I0-_ m/s 2 for the accelerometers. A better
performance of between 3 and 4 mgal can be
expected in a lower altitude of about 180 km or
less if a suspended GRADIO accommodation on a
drag-free satellite were selected. For this option
only a 3 months gravity field mission is required.
However, Figure I shows that even with the
measurement of the two tensor components across
track and radial Tyy and Tzz respectively, the
scientific requirement can be just met from a
200 km orbit. Figure 2 underlines the importance
of the instrument accuracy.
System Requirements
The principal scientific objective of the mission
is a global gravity field determination to an
accuracy better than 5 mgal in terms of gravity
anomalies with a spatial resolution of lO0 km half
wavelength, where 1 mgal is equivalent to
lO -s m/s 2. The operational mission duration for
gravity measurements is between 3 and 6 months.
The orbit shall be quasi-circular, near polar with
a mean altitude between 160 and 240 km, dependent
Mission Analysis
Using the dual launch with ERS-2, ARISTOTELES will
be brought into an 780 km sun synchronous orbit
with 10.30 h descending node local time. Within a
drift period of about 9 months completed by final
descent to 200 km the satellite moves into the
operational orbit of the gravity mission. The
drift period can also be used for point
positioning measurements and is needed to
calibrate the GRADIO instrument in space. Because
180
ORIGINAL PAGE IS
OF POOR QUALITY
https://ntrs.nasa.gov/search.jsp?R=19900011243 2020-03-19T22:53:04+00:00Z
6me4_
4
3
2
I
o
Fig. I
• ii m,onthTvv Tel
• gmonthTyy,T_I
)20 )gO +g'O I)00 2)0 _
o,tm Ait,t_
Relationship between System Performance
and Main System Parameters
(Instrument Accuracy 10"I E.U.)
+T
6 •
• Tyy • O_Eu
ijz .ool £u
• Tyy -oo) (u
Tzz - oo_6( u
o i • _yy * Oor ( u
izl • OO5 £ u
OOO6 OO) 001_ 002
Immru_mAccura_.v | u
Fig. _' : Infl_nce of Instrument
Accuracy on System Performance
of the Earth's gravity the GRADIO instrument
cannot be fully tested on ground, therefore a
relatively long period with low drag forces is
very attractive to calibrate the instrument under
low disturbance conditions.
Because of the large fuel consumption for the
transfer and operational phase the originally
required polar orbit must be shifted to a near
polar, dawn-dusk orbit, advantageous for
spacecraft design, but not covering a 600 km
circle around the poles.
The optional point positioning mission for another
3 years or more is only possible when flying
during the gravity mission above 200 km and using
bipropellant fuel without driving the cross
section area of the satellite to an unacceptable
size and consequently to a higher fuel consumption
during the gravity mission phase. A full mission
scenario sketch is given by Fig. 3.
by the air drag must be compensated by nearly
continuous thrusts with considerable amounts of
fuel. Bipropellant fuel is then mandatory, because
of the lack of qualified thrusters for hydrazine
in this operational mode.
The alternative instrument accommodation option is
the so-called non-drag-free version, with the
GRADIO mounted isostatically on the satellite
structure. Consequently, the maximum acceleration
of the satellite has to be below 5.10 "s m/s 2
demanding an orbit altitude above 200 km. In
addition via cross coupling effects the air drag
variations now impair the measurement and must be
compensated by aerodynamic flaps. The proposed
design requires that drag variations within the
measurement bandwidth are damped out by a factor
of one hundred.
Both instrument accommodation options have strong
system impacts, and therefore a comprehensive
system level trade-off had to be conducted.
Instrument Design
The primary instrument of ARISTOTELES is a
gradiometer formed by a set of up to
ultrasensitive accelerometers and a calibration
Oevice. These accelerometers with a maximum
resolution in the pico-g range and sensitive
either in 2 or 3 axes, have to be grouped as
symmetrically as possible in a 2D or 3D framework.
Each accelerometer itself consists of a proof mass
and a caging with 6 pairs of electrodes. Figure 4
shows a laboratory model of a 2D electrostatic
accelerometer. The theoretical resolution limit of
the proposed device is
IO-'Eu /V_T
and the required bandwidth (5.10 -_ Hz to 0.25).
The maximum allowed acceleration to avoid
saturation of the accelerometers is 5.10 -S m/s 2.
In principle, two different ways of GRADIO
accommodation have been studied: A drag-free
accommodation, avoiding to the maximum extent the
impact of surface forces on the GRADIO+ The
instrument is then magnetically suspended within
the satellite, affected only by gravity forces,
while the satellite flies around the instrument,
accurately controlled by the attitude control
system. In particular this accommodation option
avoids air drag forces acting on the
accelerometers anO therefore allows flight below
200 km. However, the deceleration of the satellite
Major System Options
The dominant system options are related to the
GRADIO instrument:
Drag-free or non-drag-free
GRADIO accommodation,
2D or 3D GRAD[O,
2D or 3D accelerometers.
A trade-off dealing with more than 20 different
GRADIO designs has resulted in two preferred
solutions: Four 2D accelerometers located at the
corners of a plate (planar solution) or either 6
or 8 3D accelerometers located at the corners of
an octahedron or a cube. While the first solution
is associated with a fixed GRADIO accommodation on
the satellite, the second should be accommodated
suspended to enable achievement of the high
theoretical performance of this instrument. Based
on the scientific evaluation mentioned above and
the very tight schedule, the planar gradiometer
was selected as baseline (see Figure 6). For
clarification it must be said that the 2D
accelerometers measure in all 3 axis, but with
degraded sensitivity in flight direction which is
then of no use for gravity gradient measurements.
However, this signal can support the attitude
control System.
181
°1
J/ " ,r " Li -1
I>
<,<I,
6,
7_
tt
• el ell
-" _,la}_
e_ _
:i0
• Ill
7_i
7 _ • Ill Oil
, o;_--x
\
:c u_
g,
182
ORIGINAL PAGE IS
OF POOR QUALITY
Orbit Determination
The restitution of the gravity gradient data
requires an accurate knowledge of the orbit
position. The 'a posteriori' orbit determination
requirement is therefore
lO m in radial direction and
1.5 km across and along track.
This requirement cannot be met by normal S-band
tracking, but requires special equipment. For
ARISTOTELES a Precise Range and Range Rate
Microwave Tracking Equipment named MTS/PRARE
similar to the PRARE system to be flown on ERS-}
will be used. Optionally the use of the Global
Positioning System (GPS) is under discussion.
When using MTS/PRARE for orbit determination, with
the same on-board equipment, geodetic point
positioning can be performed by scientists,
requiring only additional dedicated PRARE ground
stations, to be operated independently by the
scientists themselves.
Satellite Configuration
The GRADIO instrument has dictated all of the
principal features incorporated in the satellite
configuration. The need to fly GRADIO at the
minimum possible altitude results in a significant
drag force. To provide a mission of long enough
duration for recovery of adequate scientific data,
the effect of the drag must be minimised. The
cross sectional area presented to the 'airflow' is
thus reduced resulting in a long slender satellite
body limited by accommodation constraints of the
AR44 SPELDA ]0 payload carrier assembly (see
Figure 6). The electrical energy demand is large
enough to require solar array wings as well as the
body mounted solar arrays, but these must be edge
on the airflow again not only to minimise drag,
but also to avoid large disturbance torques on the
satellite and thereby affecting GRADIO. In
consequence it is only possible to fly in near
dawn-dusk orbits, placing a minor restriction on
scientific return in relation to tidal effects
from the sun.
That the GRADIO instrument measures the gravity
field of the satellite as well, has unusual
consequences for the satellite internal layout in
addition to the above mentioned drag minimisation.
]he instrument must be placed not only at the
centre of gravity of the vehicle, but this centre
of gravity must also be the neutral point for the
gravitational attractions of all components to
minimise their influence. Symmetry is thus of
major importance, and the heavier units should be
placed as far away from GRADID as possible. The
supporting electronic subsystems contained in pods
on the sides of the body meet this requirement in
principle, while also assisting thermal stability
close to GRADIO since significant variable heat
generation is remote from the instrument.
The largest masses are however the fuel (approx.
one ton in total} in the 16 tanks. Normally under
manoeuvring accelerations the fuel in the tanks
moves or 's)oshes'. To almost totally remove this
effect, tanks with stable metal diaphragms are
used, restraining the fuel to one end of the tank.
The fuel used during the period of GRADIO
measurements also is contained only in the outer
tanks, i.e. furthest from GRADIO. The fuel in the
inner tanks is used during initial manoeuvres to
place ARISTOTELES in the correct orbit.
In addition, the symmetry requirement can only be
met by positively monitoring fuel consumption from
the individual tanks, in order to maintain the
centre of gravity within fractions of a millimetre
of the GRADIO centre.
Tab. ) : ARISTOTELES Satellite Properties
o Satellite Mass dry 960 kg
launch 1990 kg
o GRADIO Mass 65 kg
o Power at Solar Array 1080 W
o GRADIO Power Consumption 60 W
o S-Band Telemetry I Mbps
o Data Storage (36h) 260 Mb
SubsTstem Characteristics
The majority of the subsystems for ARISTOTELES are
conventional in character with special attention
being paid to satellite structural and thermal
effects on GRADIO. However, the performance of the
attitude and orbit control system is intimately
linked with the orientation and control of GRADIO
and thus to the quality of the instrument data
output. Because the instrument is attached to the
spacecraft structure, it is directly affected by
drag forces and spacecraft disturbances, placing
high demand on control accuracy.
Conclusions
The ARISTOTELES system study 2 proved that a
spaceborne gradiometer can meet an Earth gravity
field determination requirement of better than
5 m<jal with a spatial resolution of lOO km half
wavelength. The required orbit altitude is about
200 km or less with an instrument accuracy of
better than lO "2 E.U. given that at least the
tensor components Tyy (across track) and Tzz
(radial) were measured. This is achieved by means
of a 2D GRADIO directly attached to the satellite
structure. Although a suspended accommodation of a
3D GRADIO with at least 6 3D accelerometers
promises better results, the fixed GRADIO solution
has been selected as baseline to meet the tight
schedule which may not allow the more challenging
approach. The lower instrument performance of the
_D fixed GRADIO can be partly compensated for by a
gravity mission of six n_nths instead of three and
a high performance AOCS subsystem.
ReferenCes
l) SESAME ESA Special Workshop, ESA SP-I080,
Ising, FRG, March 1986
2) Benz, R. et al., ARISTOTELES Phase A Study,
Final Report, Dornier-System GmbH, Friedrichs-
hafen, FRG, 1988.
183


ARISTOTELES: A European approach for an Earth gravity field recovery mission

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server