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MIMOSA

MIMOSA (Microaccelerometric Measurements of Satellite Accelerations) is a microsatellite of the Czech Republic, designed and developed at the Astronomical Institute/Academy of Sciences (ASU/CAS) of Ondrejov, Czech Republic. The project is funded by the Grant Agency of the Czech Republic. The overall mission objective is to obtain total density distributions in space and time of the upper ionosphere by sensitive measurements of the non- gravitational orbital perturbations (atmospheric drag, solar radiation pressure). The data obtained will serve for analytical model improvements of the upper atmosphere.

The project builds on non-gravitational research conducted with the instrument MACEK in previous short-duration missions on the Russian satellite Resurs-F1 (launch June 23, 1992) and on Shuttle flight STS-79 (Sept. 16 - 26, 1996). An improved version of MACEK is the only science instrument onboard MIMOSA. 1) 2) 3) 4) 5) 6)

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Figure 1: Illustration of the MIMOSA spacecraft (image credit: ASU/CAS)

The design of the MIMOSA spacecraft structure, built by Space Devices Ltd. of Prague as prime contractor, employs a 26-sided symmetrical polyhedron (maximum diameter of 617 mm, the shape is nearly spherical to eliminate the effects of a possible lift) with a high area- to-mass ratio for overall measurement sensitivity (smooth surface for drag measurements). The outer surface is covered with 17 solar panels of 200 mm x 200 mm in size. The inside of the S/C features six subsystems: Balancing masses, ACS (Attitude Control Subsystem), PDU (Power Distribution Unit), Data Management and Store Unit, Telemetry and Telecommand System, and Service Module.

· The balancing mass system consists of three movable masses (1.5 kg each) in each of the main S/C axes with a moving range of ±20 mm. The objective is to aline the center of S/C gravity with that of the proof-mass cavity to better than 0.5 µm.

· For all gravity measurement operation periods, the S/C is not stabilized, it is orbiting and rotating freely. However, the tumbling or spin of the spacecraft can be readjusted/retarded by magnetic coils (a telecommand action by ground control) whenever the spin of 0.01 radians/s is exceeded. ACS employes eight solar sensors to determine the sun direction to an accuracy of 0.5º. In addition, there is a fluxgate magnetometer and for attitude measurement. Actuation is provided by three magnetic coils to eliminate the initial spin rate. MACEK measurement periods require a residual spin below 0.01 radians/s. In addition, a GPS receiver \[MPE-1 (Miniature PLGR Engine) model, where PLGR (Precision Lightweight GPS Receiver)\] in PPS mode is being used for orbit determination.

A S/C power of 11 W is provided. Lithium-ion batteries with a capacity of 14 Ah are used during orbital eclipse phases. The S/C mass is 55 kg.

RF communication. The uplink is in UHF-band at 450 MHz the data rate is 1.2 kbit/s. The dual downlink employs S-band (2.2 GHz, data rate at 20/80 kbit/s, bi-phase FM modulation) and VHF-band (137 MHz, data rate at 5 kbit/s). Data of up to 2 MByte can be stored on-board in solid-state memory. The spacecraft control center and ground station is located at the Astronomical Institute. The ground station is in Panska Ves, Czech Republic (the Panska Ves station served already as the ground station for the MAGION satellites).

A launch of MIMOSA (multiple launch of payloads) took place on June 30, 2003 on Rockot KS of Eurockot from Plesetsk, Russia. Co-passengers were MOST (Microvariability and Oscillations of Stars) a microsatellite of CSA, Canada, Monitor-E mock-up of GKNPT Khrunichev, Russia, and 6 Cubesats of various universities.

Orbit: Sun-synchronous orbit, perigee = 326 km, apogee = 839 km, inclination = 98.8º. The MIMOSA spacecraft mission is subjected to a decaying orbit, measuring the orbital and attitudinal accelerations (non-gravitational forces) with an expected life time of about nine years. The elliptical orbit selection emphasizes the predominance of drag forces in the lower altitudes, as well as the overriding influence of the solar radiation pressure in regions near the apogee. The expected lifetime 4.5 years.


 

Sensor complement:

MACEK (Mikroakcelerometr) is the Czech name for the electrostatically compensated microaccelerometer instrument. MACEK is an accelerometer with six degrees of freedom and a cubic proof-mass. The objective is to detect accelerations to a magnitude of 10-10 m/s2.

Parameter

SI value of acceleration

Value expressed in "g - notation"

Input range

±4 x 10-4 ms-2

±40 µg (micro g)

Input frequency

1 mHz to 100 mHz (milli-Hz)

1 mHz to 100 mHz

Scale factor

25 kV/ms-2

0.25 V/µg

Bias

<1 x 10 -5 ms-2

<1 µg

Bias temperature coefficient

2.6 x 10-8 ms-2 /º C

2.6 x ng/º C

Measurement resolution

10-10 ms-2

0.01 ng

Axis misalignment

5 µrad

5 µrad

Table 1: Performance parameters of the MACEK instrument

The major element of the MACEK accelerometer is the cubic proof-mass (29.96 mm) moving in a cubic cavity of 30 mm size. The cavity is created by six prism blocks. The cube and prism blocks are made of a special quartz glass and slightly chrome-plated such that appropriate patterns on the prism blocks and the cube make up the electrodes. The cube is kept within the geometric center of the cavity. During periods of measurement, the cube position is detected by capacitive sensors and stabilized in all six degrees of freedom. The signal of the stabilizing forces provides the magnitude and direction of the acceleration measured.

The total instrument mass is 5.61 kg (2.67 kg sensor head, 2.94 kg electronics box). The sensor head power is 0.6 W, the power for the electronic box is 5.9 W. The sensor head dimensions are: 15 cm x 15 cm x 18 cm. The electronic box dimensions are: 14 cm x 20 cm x 23 cm.

Mission status: The spacecraft is operational as of June 2004. The MACEK instrument measures the accelerations in three axes; however, due to technical difficulties, the proof mass is freely moving in two axes only (it should do this in three axis). Therefore, only the collection of some technological data is possible. For instance, the stability derived from the measurement data samples shows its value being of the order of 10-9 m/s2. The efforts towards the complete loosening of the proof mass are still continuing. 7)

Note: An earlier version of the MACEK instrument was flown in the SPACEHAB module of Shuttle mission STS-79 (Sept. 16-26, 1996), Atlantis. During this demonstration flight, MACEK operated successfully for a period of more than 8 days.

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Figure 2: Illustration of MACEK prior to SPACEHAB integration (image credit: ASU/CAS)


1) R. Peresty, L. Sehnal, M. Chvojka, P. Dostal, "MIMOSA Satellite," Acta Astronautica, Vol. 46, No 2-6, 2000, pp. 345-349

2) L. Sehnal, R. Peresty, L. Pospisilova, P. Dostal, "Software Features for the Orbital Dynamics of the MIMOSA Satellite," Proceedings of the 51st IAF Congress, Oct. 2-6, 2000, Rio de Janeiro, Brazil, IAF-00-A.5.03

3) L. Sehnal, R. Peresty, L. Pospisilova, A. Kohlhase, "Mission Analysis of the MIMOSA Satellite," Proceedings of the 49th IAF Congress, Sept. 28 - Oct. 2, 1998, Melbourne, Australia

4) L. Sehnal, L. Pospisilova, R. Peresty, P. Dostal, A. Kohlhase, " MIMOSA - A Satellite Measuring Orbital and Attitudinal Accelerations caused by Non-Gravitational Forces," Advances in Space Research, Vol. 23, No 4, 1999, pp. 705-714

5) http://www.asu.cas.cz/~macek/nep.html

6) L. Sehnal, R. Peresty, L. Pospisilova, "Project MIMOSA - Final Stage of the Satellite Fabrication," 3rd International Symposium of IAA, Berlin, April 2-6, 2001, pp. 241-244

7) The information was kindly provided by Ladislav Sehnal of the Astronomical Institute of Ondrejov, Czech Republic.


This description was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments and corrections to this article are welcomed by the author.