Device and method for protecting electronic units in a satellite from electrical disturbance

Device and method of protecting a satellite from undesired electrical disturbances without significantly increasing the satellite's weight. The device includes two faraday cage structures around the electronic units of the satellite. The two structures are electrically connected via shielded wires to create one continuous faraday cage. Signals from components located outside the faraday cage to the electronic units are made through filter assemblies. The filter assemblies electronically filter undesired signals before routing desired signals to their destination.

BACKGROUND OF THE INVENTION 
A satellite is a type of spacecraft that orbits either the earth or another 
body of the solar system. There are two main classes of satellites: 
information satellites and communications satellites. Information 
satellites transmit signals related to atmospheric and meteorological 
data, infrared, ultraviolet, gamma and X-ray studies of celestial objects, 
and surveys of the earth's shape, surface, and resources. Communications 
satellites receive radio frequency signals from earth by means of highly 
directional aerials and return them to another earth location for purposes 
such as long-distance telephony and TV broadcasting. 
A satellite typically includes a payload module and a platform module. For 
a communications satellite, the payload is the communications equipment 
necessary to perform the mission. The platform typically includes the 
following subsystems: (1) power; (2) attitude and control; (3) propulsion; 
(4) on-board telemetry tracking and command (TTC); (5) thermal control; 
and (6) structure. These subsystems play a vital role throughout the 
satellite's operational life. 
The structure of the satellite platform module includes a configuration of 
rigid panels capable of withstanding intense mechanical stresses imposed 
during various phases of the mission. The panels are typically 
manufactured from strong lightweight materials such as aluminum honeycomb 
combined with aluminum or carbon fiber face sheets. Panel weight is 
further minimized by providing holes through the solid material. 
An electrical shield called a faraday cage is commonly used to protect the 
platform electronic units (e.g. power distribution unit and power 
controller unit) from unwanted electrical disturbances such as 
electrostatic discharge. Previous satellites have been built with the 
electronic units and propulsion components (e.g. propulsion tanks and 
thrusters) dispersed throughout the platform. In order to protect the 
electronic units, the faraday cage must surround the entire platform. One 
method of accomplishing this is to wrap the platform in thermal blankets 
containing layers of metallized material. The blankets are then connected 
together by blanket ground wires to form a faraday cage around the 
platform. An alternative approach is to incorporate within the platform 
panels a material such that, when the panels are connected, a faraday cage 
is created. 
The previous methods suffer from drawbacks. In the first method, numerous 
wires are required to completely connect the blankets together. The wire 
connections, which are often missed, cause penetrations in the faraday 
cage. In the second method, the solid panels add weight and increase 
satellite cost. In both methods, multiple penetrations of the faraday cage 
are required in order to make the necessary connections from propulsion 
components located outside the platform to propulsion components located 
inside the platform. 
Accordingly, there exists a need for a device that provides adequate 
shielding for the electronic components of a satellite while minimizing 
satellite weight. 
SUMMARY OF THE INVENTION 
The present invention provides a device and method which can be implemented 
in the structure subsystem of a satellite. Specifically, the device and 
method of the present invention are capable of protecting a satellite from 
unwanted electrical disturbance without increasing the satellite's weight. 
According to a preferred embodiment of the present, the electronic units 
and propulsion components are separated. Further, the electronic units are 
grouped closely together to minimize the size of the structure needed to 
house them. Preferably, there are two sets of electronics units located on 
the satellite. The structure that houses the electronic units is 
preferably made from solid panels containing a sufficient amount of 
aluminum to create a faraday cage around the units. The faraday cage 
protects the units from electrical disturbances such as the electrostatic 
discharge formed on the satellite during its mission. 
The device and method of the present invention provides several advantages 
to the structural design of a satellite. The modular placement of 
electronic units and propulsion components separates critical electronics 
from propulsion components, thereby allowing a structure with solid panels 
to house the electronic components without significantly increasing the 
satellite's weight. The modular placement also allows the faraday cage to 
be broken into discrete structures connected via a cluster of wires called 
a harness. Because the propulsion components are located outside the 
faraday cage, and the electronic units are centrally located, the number 
of wire connections that penetrate the cage are minimized. Thus a faraday 
cage constructed according to the present invention is penetrated only 
where the harness penetrates the two structures. In a first preferred 
embodiment, the harness is covered by a braided metal in the form of a 
tubular mesh that wraps 360 degrees around the harness and connects to the 
two structures via connectors. In a second preferred embodiment, two 
harnesses are used and are covered by a shielded cable tray made from an 
aluminum material to provide additional protection from unwanted 
electrical disturbances. 
In another aspect of the invention, modular placement of electronic units 
allows a novel method of shielding the wires that connect the electronics 
units to components located outside of the faraday cage. In the presently 
preferred embodiment, wires from the electronics units are connected to a 
first side of a filter assembly which is located on a panel of the 
electronic unit housing. Wires connected to the second side of the filter 
assembly are routed to the components located outside the faraday cage. 
The filter assembly electronically filters undesired signals originating 
from components outside the faraday cage before routing the desired 
signals to electronics components inside the faraday cage. The new 
technique alleviates the problems caused by previous methods of 
individually wrapping and grounding each wire connecting the electronics 
units to components outside the faraday cage. 
The invention itself, together with further objects and attendant 
advantages, will be understood by reference to the following detailed 
description, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
A spacecraft or satellite 10 capable of utilizing the present invention is 
shown in FIG. 1. The satellite 10 has a spacecraft body 12 which includes 
a lower bus module or platform 14 and an upper payload module 16. Attached 
to the aft end of the lower bus module 14 are a plurality of engines. 
These engines include a centrally positioned liquid orbital thruster (not 
shown), four chemical propulsion engines 24 located at the corners of the 
bus module 14 and two pairs of xenon ion propulsion engines 26 (one pair 
shown). Lower bus module 14 contains fuel tanks (not shown) and various 
power and control modules that operate the engines and power the payload 
module 16. Bus module 14 further includes a pair of solar panels 18 that 
convert sunlight into electricity. The electricity is sent to batteries 
(not shown) located on the bus module 14. Bus module 14 also has a pair of 
antennae 20, which receive signals from an earth ground station. The 
antennae 20 reflect the received signals into reflectors 22, which in 
turn, reflect the signals into receivers (not shown). The antennae 20 are 
used to control the satellite 10 and to send signals to the ground 
station. 
Payload module 16 is attached to the bus module 14 and contains a variety 
of electronic equipment which may contain a number of sensors (not shown). 
The electronic equipment processes information gathered by the sensors and 
sends the processed information back to the ground station via antennae 
20. The gathered information may concern for example, communications, 
weather observation, and navigational information. 
FIG. 2a illustrates a first preferred embodiment of the Faraday cage 
configuration 30 of the present invention. The configuration 30 preferably 
includes two shielded faraday structures 32, 34 which are located on the 
north panel 28 and south panel (not shown) of the bus module 14 of FIG. 1. 
The shielded faraday structures 32, 34 house the electronic units on board 
the satellite. 
The panels of the structures 32, 34 are constructed of a honeycomb core 
adhesively affixed between two solid aluminum face sheets. In the 
presently preferred embodiment, the cell size of the core is 3/16 of an 
inch, the thickness of the core is 3/4 to 1 inch, and the thickness of the 
aluminum face sheets is 10 to 20 mils. The panels are connected to form a 
faraday cage around the electronic units using known RF sealing materials. 
The exact dimensions of the structures 32, 34 should be chosen according 
to the particular application. 
In the present example, the structures 32, 34 are rectangular boxes. The 
faraday cages of the boxes 32, 34 are electrically connected via a bundle 
of wires or a harness (not shown) surrounded by a braided metal mesh 64. 
The braided metal mesh 64 is connected to the boxes 32, 34 via connectors 
66 (only one shown) commonly known in the art. A suitable metal mesh 64 is 
mil-std space qualified commonly known in the art. 
In a second preferred embodiment shown in FIG. 2b, the faraday cages of the 
boxes 32, 34 are electrically connected via two bundles of wires or 
harnesses (not shown) surrounded by shielded cable trays 36, 38. The 
bundles of wires are preferably 1 inch thick. Suitable wires are mil-std 
space hardware wires commonly known in the art. The shielded cable trays 
36, 38 preferably comprise two rectangular sheets of aluminum 15 to 20 
mils thick. The two rectangular sheets surround the harness and are held 
together by screws (not shown). The shielded cable trays 36, 38 provide 
continuity of the faraday cage of structures 32, 34. 
Connections between electronics units and units located outside the faraday 
cage are made through filter assemblies 40. FIG. 3a illustrates a 
perspective view of a filter assembly 40 which includes a box structure 
46, an aluminum lid 42, and an RF gasket 44 commonly used in the art to 
ensure electrical conductivity. The box structure 46 includes two flanges 
48 for mounting the assembly 40 to a panel of the faraday cage structure 
32, 34. The panels of the box structure 46 are preferably made from 
aluminum. One panel 50 of the assembly 40 is configured to accept 66 pin 
circular input connectors 52 (FIG. 3b). Another panel (not shown) of the 
assembly 40 is configured to accept 66 pin circular output connectors 54 
(FIG. 3b). The connectors 52, 54 are off-the-shelf devices commonly known 
in the art. 
FIGS. 3b and 3c illustrate further details of the filter assembly 40. 
Inside the filter assembly box structure 46, the wire end of the input 
connector 52, which is represented by filter pin 52a, is coupled to a 
first end of a ferrite 56. The other end of the ferrite 56 is connected to 
the first end of a ferrite 58. The second end of the ferrite 58 is coupled 
to the first end of a ferrite 60. The second end of the ferrite 60 is 
coupled to the first end of a diode 62. The second end of the diode 62 is 
coupled to the wire end 54a of the output connector 54. The ferrites 56, 
58, 60 and diode 62 are chosen according to mil-std specifications and 
should be approved for space applications. The particular size of ferrites 
and diode should be chosen according to the type and size of signals that 
are to be filtered. 
The components located outside the faraday cage structures 32, 34 are 
coupled to the pin ends of the 66 pin circular input connectors 52. The 
electronic units located inside the faraday cage structures 32, 34 are 
coupled to the pin ends of the 66 pin circular output connectors 54. 
Signals originating from components outside the faraday cage are 
electronically filtered through the ferrites 56, 58, 60 and diode 62 
before reaching the electronic units inside the faraday cage structures 
32, 34. 
The device and method of the present invention provide several advantages 
to the structural design of a satellite. Modular placement of units and 
propulsion components allows a structure with solid panels to be used to 
house the electronic units without significantly increasing the 
satellite's weight. Modular placement minimizes penetrations of the 
faraday cage, thereby eliminating damage to the electronic units. Modular 
placement also significantly decreases the probability of digital 
electronics equipment randomly changing states due to electrical 
disturbances. Connections between electronic units and components outside 
the faraday cage through filter assemblies eliminate the problems 
associated with individual connections such as bad wraps, bad grounds and 
forgotten grounds. 
It should be understood that a wide range of changes and modifications can 
be made to the preferred embodiment described above. For example, although 
rectangular faraday boxes are illustrated, the invention can be 
implemented using various configurations. It is therefore intended that 
the foregoing detailed description be regarded as illustrative rather than 
limiting and that it be understood that it is the following claims, 
including all equivalents, which are intended to define the scope of this 
invention.