Acceptance Data Package of Consert orbiter antenna system.

This document is a compilation of all necessary information that are needed for the Structural and Thermal Model delivery of the Consert instrument (Part 2: Antenna)

1.3. Applicable documents

EID A, Issue 1, Rev 1, Date 15/6/98, Ref RO-EST-RS-3001/EID A.

EID B, Issue 1, Rev 0, Date 15/1/99, Ref RO-EST-RS-3001/EID B.

EID C, Issue 1, Rev 2, Date 1/9/98, Ref RO-EST-RS-3001/EID C.

Payload Units STM Build Standard Description, Rev 0, Date 18/12/98, Ref TOS-MMS/1998/550/ln/JCS.

1.4. Reference documents

1.5. Abbreviations

2. Required Documentation

 

Test Procedures:

Properties procedure RO-OCN-PR-3002

Deployment procedure RO-OCN-PR-3003

Vibration procedure RO-OCN-TR-3004

Centrifuge procedure RO-OCN-PR-3005

Thermal Vacuum procedure RO-OCN-PR-3006

 

Test Reports:

Properties Inspection report RO-OCN-TR-3002

Deployment Test report RO-OCN-TR-3003.1-4

(The test that was done 15/07/99)

Centrifuge Test report RO-OCN-TR-3005

Thermal Vacuum Test report RO-OCN-TR-3006

Shock Test RO-OCN-TR-3007

Vibration Test 2 report RO-OCN-TR-3021

 

Technical reports:

ITMM RO-OCN-TN-3009

CONSERT Orbiter antenna RO-OCN-TN-3017

Manual for handling the Orbiter antenna RO-OCN-TN-3019

 

 

 

 

Lists:

Components RO-OCN-LI-3001

Materials RO-OCN-LI-3002

Processes RO-OCN-LI-3003

 

Drawings:

Collapsed antenna RO-OCN-DW-3001, issue 2, rev E, page 1of3

Materials and surface treatments RO-OCN-DW-3001, issue 2, rev E, page 2of3

Deployed antenna RO-OCN-DW-3001, issue 2, rev E, page 3of3

 

 

3. Description of the STM Antenna Unit

CONSERT Orbiter antenna RO-OCN-TN-3017

http:\\www.linmpi.mpg.de/english/projekte/consert

For a frequency of 90 MHz the length of half wave antenna is about 1.5m. Including deployment booms the linear dimensions of an antenna will be between 1.5 and 2 meters. The antenna therefore has to be collapsed

during launch, and later deployed on command. The use of thin elements, which can be placed parallel to each other in the collapsed state, is therefore probably a necessary condition.

During the mission the spacecraft will move between 1 and 5.25AU. The large variations in thermal environment will be controlled by surface treatment of the antenna and support structures, and by inserting a

thermal resistance between the antenna and the spacecraft, so as to keep the energy flow between the antenna and spacecraft below the allowed 2W.

Two dipoles placed perpendicular to each other and fed 90 degrees out of phase constitute a circular polarized antenna. For example the input signal is lead directly to one dipole and through a quarter wave cable to the other dipole. Such a crossed dipole antenna has improved impedance properties (relative to a single dipole) across a wide bandwidth. The impedances of the dipoles are equal to each other. The transformation of one impedance in a quarter wave cable, corresponds to mirroring the impedance in the center of the Smith diagram. The resulting impedances are said to be complementary. This means that the combination of these two impedances is close to 50 Ohm across the bandwidth.

Actually the crossed dipole antenna is only ideally circular polarized along the normal to the plane formed by the two dipoles, the increasing deviation from circular polarization associated with increasing angular distance from the normal is not a problem for the experiment.

The Antenna is of so large dimensions that it must be folded during launch, and then deployed after the spacecraft has separated from the rocket. The antenna support structure consists of two masts, which in deployed state makes an angle of 90 degrees to each other. One mast separates the dipoles and the reflectors levels, and the other place the antenna away from the side of the spacecraft. The masts are connected to each other with a spring, and one mast is connected to the spacecraft with a spring. Each of the dipole- and reflector elements are connected to the mast with a spring. This spring loaded mounting of the antenna parts, allows the antenna to be folded to a size suitable for launch conditions, and the springs provide the forces needed to deploy the antenna. In the stored position the dipole- and reflector elements are placed along the

masts, and the two masts are placed parallel to each other along the spacecraft edge, and are lashed to a support structure on the spacecraft with a single wire . When the wire is cut by activating a pyro all the

springs start into action and deploy the antenna.

Essential for the antenna location on the spacecraft is of course the orientation of the spacecraft to the comet: the Rosetta spacecraft is stabilized relative to the comet such that one side of the orbiter (hereafter 'side-z') will be kept perpendicular to the direction to the comet. This direction is also the z-axis of the spacecraft coordinate system.

Several positions of the antenna on the spacecraft has been considered. It is tempting to use the side-z as a ground plane for the crossed dipoles; however, the shielding and possible electromagnetic disturbances of experiments on the side-z has lead to a denial of that position. Another suggestion was to place the antenna away from side-z along an extended diagonal of that side. A crossed dipole with a ground plane (a simple one consisting of two wires parallel to the dipoles) in that position is predicted to perform well, however, the possibility of the antenna could cast a shadow on a solar panel, has lead to denial of kind of that position.

A schematic of the antenna in the position finally selected is shown in Figure 1. The dipole center is placed outside side-z (about 1.1m from the side), with the plane of the dipoles parallel to the z-side and containing the axis of the solar panels. The antenna ground plane is parallel to the side-z, and located about 5cm above. The crossed dipoles are place about a quarter wavelength over the ground plane along the z-axis. In this position the antenna is not shielding other experiments. Since the antenna is place near the center plane of the spacecraft, it can not throw shadow on a solar panel (the sun will move near the plane parallel to the Z-axis and perpendicular to the solar panel axis, and therefore the shadow thrown by the antenna can not fall on the solar panels).