Interactive fixed and mobile satellite network

A communications system includes at least one low earth orbit first satellite (10), at least one second satellite (11) in other than a low earth orbit, and a ground segment (12) that includes a plurality of user transceivers (78, 80, 82, 84) and at least one gateway (76) coupled to a publicly-accessible terrestrial communications system, such as a PSTN and/or a fiber optic network. The first satellite includes a first transceiver for communication with the at least one gateway, a second transceiver for communication with at least one user transceiver, and a third transceiver for communication with the at least one second satellite. The first, second and third transceivers are switchably coupled together on-board the first satellite by on-board processors and a switching matrix for relaying a user communication between the at least one gateway and the at least one user transceiver via the at least one second satellite. The plurality of user transceivers can include a plurality of data processors which are interconnected into a network through the at least one first satellite. This network can be considered as a virtual network, and can have a mesh, star, or other topology. The user transceivers can be adapted to transmit and receive direct sequence, code division/multiple access communications. Transmission of signals to and from the user transceivers is accomplished by spreading a digital data stream (e.g., voice, data, image, video) with assigned spreading codes.

FIELD OF THE INVENTION 
This invention relates generally to communications systems and, in 
particular, to communications systems that employ one or more satellites 
to direct user communications through the system. 
BACKGROUND OF THE INVENTION 
Satellite delivered individual services are emerging as a new global 
enterprise. These systems utilize or are proposed to utilize many 
individual circuits routed through one satellite or a constellation of 
many satellites to effect communications. One value of the satellite 
system is that it provides ubiquitous coverage of large areas of the earth 
without the construction of ground-based infrastructure. Since the recent 
availability of portions of the frequency spectrum for these services, 
several proposals have been advanced by a number of organizations. One 
proposal would use Time Division Multiple Access (TDMA), while several 
others would employ Code Division Multiple access (CDMA). A feature of the 
CDMA systems is an ability to share the available frequencies by 
co-frequency operation, while experiencing only a percentage decrease in 
the capacity of each system. 
Furthermore, Low Earth Orbiting Satellite (LEOS) systems, also referred to 
as Non-GSO (geosynchronous orbit) satellite systems, offer a new dimension 
in communications. For example, the LEOS systems can provide diversity, as 
described in U.S. Pat. No. 5,233,626, issued Aug. 3, 1993, entitled 
"Repeater Diversity Spread Spectrum Communication System", to Stephen A. 
Ames. Another capability provided by the LEOS systems is an ability to 
interconnect users to a fixed point, typically referred to as a Public 
Switched Telephone Network (PSTN). 
High capacity, fiber optic-based communications is currently being deployed 
world-wide, and in particular in the United States, to directly connect to 
subscribers in their homes. In addition to providing conventional voice 
communication capability, the fiber optic-based networks can also provide 
video and high speed data capabilities. The proliferation of networked 
personal computers having multimedia capabilities can take advantage of 
the increased speed and capacity provided by the fiber optic based 
networks. However, the significant costs involved in providing fiber optic 
lines is not economical in every locale, and it can be expected that large 
non-urban areas will not be in a position to benefit from the advantages 
provided by fiber optic networks within a reasonable period of time. 
OBJECTS OF THE INVENTION 
It is a first object of this invention to provide a system and a method for 
providing communications services to regions which are not currently 
economical to serve with fiber optics. 
It is a further object of this invention to provide a satellite-based 
communications system that provides, in addition to mobile and fixed voice 
and data service, a capability to provide high speed video and data 
service. 
SUMMARY OF THE INVENTION 
The foregoing and other problems are overcome and the objects of the 
invention are realized by a communications system that is constructed and 
operated in accordance with this invention. The communications system 
includes at least one low earth orbit first satellite, and preferably a 
constellation of multiple low earth orbit repeater satellites. The 
satellites of the constellation are preferably in inclined circular orbits 
operating at an altitude of less than 2000 kilometers. The communications 
system also includes at least one, and preferably a plurality of second 
satellites in other than a low earth orbit, such as a geosynchronous 
orbit. The communications system also includes a ground segment having a 
plurality of user transceivers and at least one gateway coupled to a 
publicly-accessible terrestrial communications system and/or to various 
private networks, such as a PSTN and/or a fiber optic network. The first 
satellite(s) include a first transceiver for communication with the at 
least one gateway, a second transceiver for communication with at least 
one user transceiver, and a third transceiver for communication with the 
at least one second satellite. The first, second and third transceivers 
are switchably coupled together on-board the first satellite for relaying 
user communications, such as voice, data, image, and video, between the at 
least one gateway and the at least one user transceiver via the at least 
one second satellite. 
The at least one first satellite further includes a first on-board 
processor that is bidirectionally coupled to the first transceiver; a 
second on-board processor that is bidirectionally coupled to the second 
transceiver; and a switching network that is bidirectionally coupled to 
the first and second on-board processors and to the third transceiver for 
selectively establishing communication paths between the first and second 
on-board processors and the third transceiver. 
The plurality of user transceivers can include a plurality of data 
processors which are interconnected into a network through the at least 
one first satellite. This network can be considered as a virtual network, 
and can have a mesh, star, or other topology. In a presently preferred, 
but not limiting embodiment of this invention, the user transceivers are 
adapted to transmit and receive direct sequence, code division/multiple 
access (DS-CDMA) communications, wherein the transmission of signals to 
and from the user transceivers is accomplished by spreading a digital data 
stream (e.g., voice, data, image, video) with predetermined spreading 
codes.

DETAILED DESCRIPTION OF THE INVENTION 
Reference is made to FIG. 1 for illustrating an exemplary embodiment of 
this invention. At least one and preferably a plurality of satellites 10 
are provided in earth orbit. The satellites 10 may form a constellation of 
low earth orbit satellites (LEOS), such as a constellation of 48 
satellites orbiting at less than 2000 kilometers, such as about 1400 
kilometers, in several inclined orbital planes. The orbits may be 
circular, although the teaching of this invention is not limited for use 
only with circular orbits. Coupled to the satellites 10 via uplink and 
downlink RF signals and associated transceivers is a terrestrial or ground 
segment 12. The satellites 10 operate so as to interconnect various 
elements of the ground segment 12 through different portions of the 
frequency spectrum via a plurality of RF transmitters and receivers 
(transceivers), on-board processors, and a switching matrix capable of 
interconnecting any one of the on-board processors to another. A provision 
is also made for coupling to other satellites of the same and/or a 
different constellation though inter-satellite links (ISL), such as RF or 
optical links. 
In the presently preferred embodiment of this invention the satellites 10 
include circuitry 14 and antennas 16 and 18 for providing inter-satellite 
links with other satellites 11, such as a higher-orbiting geosynchronous 
orbit (GSO), medium earth orbit (MEO), or Molniya constellation of 
satellites. In this manner a given communication signal can be uplinked 
from a portion of the ground segment 12 to one of the satellites 10, and 
can then be routed through one or more other satellites 11 before being 
downlinked back to the ground segment 12, either directly or through 
another LEO satellite 10. This link may be bidirectional (e.g., full 
duplex). 
The antennas 16 and 18 may be either non-deployed phased arrays or deployed 
reflectors with multiple beam feed assemblies located in a reflector focal 
plane. 
Describing FIG. 1 now in greater detail, the satellite 10 includes an 
S-band receive antenna 20, an S-band transmit antenna 22, an L-band 
receive antenna 24 and an S-band transmit antenna 26. S-band antennas 20 
and 22 may operate at frequencies of 2.2 and 1.9 GHz, respectively, with a 
bandwidth of 30 MHz. The L-band antenna 24 may operate at 1.6 GHz, while 
the downlink S-band antenna 26 may operate at 2.5 GHz. The antennas 20 and 
22 may be either non-deployed phased arrays, deployed phased arrays, or 
deployed reflectors with multiple beam feeds located at the focal plane of 
the reflector. The bandwidth of the L-band and S-band transmissions 
through antennas 24 and 26 may be 16.5 MHz. Coupled to antennas 20-26 are 
respective RF circuit blocks 28-34 respectively. The RF circuit blocks 
28-34 include suitable signal modulators and demodulators, as appropriate. 
A presently preferred access technique employs a direct sequence (DS), 
code division/multiple access (CDMA) technique. This invention is not, 
however, limited to only a DS-CDMA approach. By example, a suitable time 
division/multiple access (TDMA) technique can also be used. 
In the case of DS-CDMA, each RF circuit block includes circuitry for phase 
demodulating and despreading received communications using user-assigned 
pseudo-noise (PN) spreading codes to separate a plurality of user signals 
that occupy a same portion of the bandwidth of the uplinked RF signal. The 
result is a plurality of digital data streams that are input to an 
on-board processor (OBP) 36 for processing and routing. Transmission of 
signals to the users is accomplished by spreading a digital data stream 
(e.g., voice, data, image, video) that is received from the OBP 36 with 
assigned spreading codes, and then phase modulating the spread 
communications prior to transmission. 
Bidirectionally coupled to the S-band antenna/RF block pair 20, 22, 28 and 
30 is the first on-board processor (OBP) 36. Coupled to the L-band, S-band 
antenna/RF block pair 24, 26, 32 and 34 is a second on-board processor 38. 
As was indicated above, the on-board processors 36 and 38 receive 
communications signals that have been down-converted to baseband and 
demodulated (i.e., taken down to bits) within the respective RF blocks 28 
and 32. Routing and other information within the communications, for 
example destination addresses associated with data packets of speech, 
video, or data, is examined by the OBP for destination and other 
information, and is thence routed through an interconnector-router (ICR) 
block 70 to another OBP for completing a required circuit. The ICR block 
70 can be comprised of a cross-bar or similar switching arrangement that 
is programmed by the OBPs so as to establish and maintain non-blocking 
communication paths between its various input and output (I/O) ports 
70a-70f. The ICR block 70 is thus able to controllably route communication 
signals to and from the various ones of the OBPs and also, if provided, 
other satellites 10 via the inter-satellite links block 14 and its 
associated antennas 16 and 18, via the satellite(s) 11. This 
interconnection capability enables a variety of ground segment terminal 
and equipment types to be coupled together, and to be coupled to an 
underlying communications infrastructure (e.g., PSTN and/or fiber optic 
network) through one or more satellites 10 and/or 11. 
The satellite 10 further includes a first Ka-band (user-link) transmit 
antenna 40, a first Ka-band (user-link) receive antenna 42, a second 
Ka-band (feederlink) transmit antenna 44, and a second Ka-band 
(feederlink) receive antenna 46. The Ka-band antennas may operate at about 
19 GHz (receive) and about 28 GHz (transmit), bandwidth 400 MHz, and 
provide the high speed, high capacity user links that are a feature of 
this invention. These antennas may be either non-deployed phased arrays, 
deployed phased arrays, or deployed reflectors with multiple beam feeds 
located at the focal plane of the reflector. Coupled to antennas 40-46 are 
respective RF circuit blocks 48-54 respectively. The RF circuit blocks 
48-54 include suitable signal modulators and demodulators, as appropriate. 
OBPs 56 and 58 are bidirectionally connected to the RF circuit blocks 48, 
50 and 52, 54, respectively, and also to the ICR 70. 
The satellite 10 also includes, by example, a Ka-band or a C-band 
(feederlink) transmit antenna 60 and a Ka-band or a C-band receive 
(feederlink) antenna 62. For a presently preferred C-band embodiment the 
feederlinks operate in the range of 3 GHz to 7 GHz. Coupled to antennas 60 
and 62 are respective RF circuit blocks 64 and 66, respectively. The RF 
circuit blocks 64 and 66 include suitable signal modulators and 
demodulators, as appropriate. The OBP 68 is bidirectionally connected to 
the RF circuit blocks 64 and 66, and also to the ICR 70. 
Turning now the ground segment 12, there are provided a plurality of 
terrestrial data, or data and/or voice networks and also fixed and mobile 
user terminals. The ground segment 12 includes first gateways 72 having 
transceivers for communicating with the satellite C-band antennas 60 and 
62. These transmissions are feederlinks through which voice and data 
communications can be directed to and from a terrestrial public switched 
telephone network 74 (PSTN) and, by example, the user terminals 82 and 84. 
The various terminals and other equipment designated as 78, 80, 82 and 84 
may all be considered to be subscriber or user terminals or transceivers. 
FIG. 2 shows the gateway 72 in greater detail, it being realized that 
typically a plurality of the gateways are provided for serving different 
geographical areas. Each gateway 72 includes up to four dual polarization 
RF C-band sub-systems each comprising an antenna 90, antenna driver 92 and 
pedestal 94, low noise receivers 96, and high power amplifiers 98. All of 
these components may be located within a radome structure to provide 
environmental protection. 
The gateway 72 further includes down converters 100 and up converters 102 
for processing the received and transmitted RF carrier signals, 
respectively. The down converters 100 and the up converters 102 are 
connected to a CDMA sub-system 104 which, in turn, is coupled to the 
Public Switched Telephone Network (PSTN) though a PSTN interface 106. As 
an option, the PSTN could be bypassed by using satellite-to-satellite 
links. 
The CDMA sub-system 104 includes a signal summer/switch unit 104a, a 
Gateway Transceiver Subsystem (GTS) 104b, a GTS Controller 104c, a CDMA 
Interconnect Subsystem (CIS) 104d, and a Selector Bank Subsystem (SBS) 
104e. The CDMA sub-system 104 is controlled by a Base Station Manager 
(BSM) 104f and functions in a manner similar to a CDMA-compatible (for 
example, an IS-95 compatible) base station. The CDMA sub-system 104 also 
includes the required frequency synthesizer 104g and possibly a Global 
Positioning System (GPS) receiver 104h. 
The PSTN interface 106 includes a PSTN Service Switch Point (SSP) 106a, a 
Call Control Processor (CCP) 106b, a Visitor Location Register (VLR) 106c, 
and a protocol interface 106d to a Home Location Register (HLR). The HLR 
may be located in a cellular gateway or in the PSTN interface 106. 
The gateway 72 is connected to telecommunication networks through a 
standard interface made through the SSP 106a. The gateway 72 provides an 
interface and connects to the PSTN via a Primary Rate Interface (PRI), or 
other suitable means. The gateway 72 is further capable of providing a 
direct connection to a Mobile Switching Center (MSC). 
The gateway 72 may provide SS-7 ISDN fixed signalling to the CCP 106b. On 
the gateway-side of this interface, the CCP 106b interfaces with the CIS 
106d and hence to the CDMA sub-system 104. The CCP 106b provides protocol 
translation functions for the system Air Interface (AI), which may be 
similar to the IS-95 Interim Standard for CDMA communications. 
Blocks 106c and 106d generally provide an interface between the gateway 72 
and an external cellular telephone network that is compatible, for 
example, with the IS-41 (North American Standard, AMPS) or the GSM 
(European Standard, MAP) cellular systems and, in particular, to the 
specified methods for handling roamers, that is, users who place calls 
outside of their home system. 
Overall gateway control is provided by a gateway controller 108 which 
includes an interface 108a to a Ground Data Network (GDN) 110 which 
interconnects the various gateways one to another and to Ground Operations 
Control Center (GOCC) 112. An interface 108b to a Service Provider Control 
Center (SPCC) 114 can also be provided. The gateway controller 108 is 
generally interconnected to the gateway 72 through the BSM 104f and 
through RF controllers 116 associated with each of the antennas 90. The 
gateway controller 108 is further coupled to a database 118, such as a 
database of users, satellite ephemeris data, etc., and to an I/O unit 120 
that enables service personnel to gain access to the gateway controller 
108. 
Referring now again to FIG. 1, the ground segment 12 further includes a 
fixed terrestrial network having a second gateway 76 that is 
bidirectionally connected to the Ka-band antennas 44 and 46 of the 
satellite 10. Gateway 76 is also connected to the PSTN 74 and is also 
connected to a fiber optic network 75 through a suitable fiber optic 
interface. The gateway 76 can communicate with a number of different types 
of equipment such as data processors (e.g., multimedia PCs 78 having a 
suitable RF interface 78a connected to a suitable RF front end 78b and a 
Ka-band antenna 78c). Other devices, such as user entertainment equipment 
80 (e.g. television) can also be accommodated. The other devices can also 
be interfaces to a wireless local loop (WLL) of a type that serves an 
office building, residential area, etc. In these cases the other equipment 
80 is also provided with suitable RF circuitry and a Ka-band antenna 80a. 
The units 78 and 80 can be considered to form one or more virtual mesh, 
star or other network types having a capability to be interconnected via 
the satellites 10, gateway 76, the PSTN 74, and the fiber optic network 
75. By example only, a 400 MHz bandwidth, 1 MB/sec data link capability is 
provided between the units 78, 80 and the second gateway 76, thereby 
enabling the delivery of, by example, video, image and Internet services. 
In accordance with an aspect of this invention the system disclosed in FIG. 
1 can provide a global, wideband Internet access capability with 
negligible connectivity time. The invention also enables a direct video 
download to a TV/PC, enables the use of interactive video, and also 
enables 2-way videophone capability. Interoperability with mobile 
communication devices 82 and 84 (e.g., handheld or fixed user terminals) 
is also provided (via the first gateway 72 or the second gateway 76), as 
is interoperability with various terrestrial wireless local loop systems. 
Reference is now made to FIG. 3 for illustrating a further aspect of the 
instant invention. The sphere generally indicates the surface of the earth 
over which traverse a plurality of the LEO satellites 10. Each satellite 
10 has a beam coverage area on the surface of the earth indicated 
generally as 10a. The beam coverage areas may overlap, thus providing for 
diversity reception by user terminals and other equipment located within 
the overlap region. Also shown are a plurality of the other satellites 11 
which are in a higher orbit, such a geosynchronous orbit (GSO) or a 
medium-earth orbit (MEO). Other orbits, such as a Molniya orbit, can also 
be used. Each satellite 11 has a corresponding larger coverage region 
indicated by 11a. 
In this aspect of the invention the LEO satellites 10 are connected to, by 
example, the GSO satellites 11 via the inter-satellite links (ISL). In 
this manner transmissions from the region 10a can be relayed to the larger 
region 11a, and vice versa. The regions can be closely spaced apart, or 
can be located on opposite sides of the earth. 
Reference in this regard can also be made to FIG. 4 for illustrating 
various types of interconnectivity between and functionality of the 
various terrestrial terminals and the LEO constellation, either directly 
or via the synchronous or other constellation type. By example, the block 
122, designated First Mobile Circuit Switched, communicates using the 
L-band and S-band satellite antennas 24 and 26 of FIG. 1, and can include 
mobile voice, cellular extension, GSM compatibility, and world roaming. 
The block 124, designated Second Mobile Circuit Switched, communicates 
using the S-band satellite antennas 20 and 22 of FIG. 1, and can include 
mobile voice, PCS extension, FPLMTS compatibility, and world roaming. The 
block 126, designated Fixed Circuit Switched, communicates using the 
Ka-band satellite antennas 40 and 42 of FIG. 1, and can include fixed 
voice and data, fiber optics extension, medium speed data, private 
networks, and internet services. The block 128 (also Ka-band), designated 
International Circuit Switched, communicates via, by example, the GSO 
constellation and can provide a transport facility, an extended circuit 
switched network, international long lines, private networks, and 
international Internet. The block 130, designated International Wideband 
and Video, can include international video and wideband data distribution, 
regional video, and wideband data. The block 132, designated Domestic 
Wideband and Video, can provide domestic video and wideband data 
distribution. All of these various functions and features can be 
simultaneously active and interconnected through the constellation of LEO 
satellites 10 and the GSO (or other constellation type) satellites 11 via 
the ISL. It should be noted in FIG. 4 that inter-satellite links are also 
preferably provided between the GSO satellites 11. 
FIG. 5 illustrates an exemplary case where fiber optic cables are routed 
between major cities and metropolitan areas. Within the major cities and 
metropolitan areas an extensive fiber optic infrastructure may exist. 
Between these areas the fiber optic service is only marginally provided, 
such as in the smaller city and town regions that are tapped into the 
fiber optic trunks that interconnect the larger cities and metropolitan 
areas. Other areas have no local fiber optic service. However, and in 
accordance with an aspect of this invention, the satellite service area 
10a covers this region of little or no fiber optic service and provides an 
equivalent service via the fixed portion of the ground network 12 shown in 
FIG. 1 (i.e., the gateway 76, Ka links, satellites 10, and terminals 78 
and 80). 
FIG. 6 illustrates the connectivity between various digital TV/computers, 
the satellite 10, and local and long distance fiber optic networks. As can 
be seen, the digital TV/computer designated 140 has a direct connection 
(DC) to a local fiber-optic line and network which in turn is connected 
through a telephone or cable company 142 to a regional fiber network. The 
regional fiber network is connected via a long distance or cable company 
144 to a long distance fiber network. The long distance fiber network is 
connected to a further telephone or cable company 146 (or other entity) 
which in turn is connected to a distribution node 148. The distribution 
node 148 includes the gateway 76 and is thereby connected via one or more 
of the satellites 10 (or one of the GSO or MEO satellites 11) to the 
antenna 78c, RF section 78b and interface 78a of the PC 78 (refer also to 
FIG. 1). The connection between the antenna 78c and the RF section 78b can 
be a wired or a wireless connection. In this manner, the PC 78 is enabled 
to be coupled to the fiber network in essentially the same manner as the 
digital TV/computer 140 which has a direct connection to the fiber-optic 
network, and is thus enabled to avail itself of network and other services 
that are best served by the higher data rates made available by fiber 
optic lines. 
The PC 78 can thus be connected to others of similar type in a mesh 
network, or in a star network, to the distribution node 148 and thence to 
the serving entity such as the telephone or cable company. Further 
connections to the PSTN can also be made. The further connections can be 
to other computers of similar type, to servers and/or to larger computers 
providing network (e.g., Internet) services. 
The antenna 78c may be directional, but is preferably omni-directional with 
hemispherical or semi-hemispherical coverage. 
The use of the LEO constellation of satellites 10 provides unique 
advantages when employed with the teaching of this invention. Consider a 
mobile terminal which is moving under a tree (or other RF obstacle, such 
as a building) and is blocked to one of several satellites 10 serving the 
user (i.e., assume that the mobile terminal is located in the overlap 
region of the coverage areas shown in FIG. 3). The use of diversity 
combining from those satellites that are not blocked provides improved 
service and connectivity to the satellite constellation. It can be shown 
that this performance increase is significant and provides mitigation of 
shadowing and blocking due to movement of the user terminal. In this 
regard the disclosure of U.S. Pat. No. 5,233,626, issued Aug. 3, 1993, 
entitled "Repeater Diversity Spread Spectrum Communication System", to 
Stephen A. Ames is incorporated by reference herein in its entirety for 
illustrating suitable embodiments of a receiver employing diversity 
combining. 
When considering a LEO fixed system, the rain attenuation can be severe in 
the frequency bands above 3 GHz, and especially above 10 GHz. Rain fades 
of 20 db or more occur in the Ka band frequencies. It is widely known that 
the availability of satellite systems to deliver signals of the desired 
strength is affected by these rain fades. It is also known by 
experimentation that the duration and fade depth is affected by the 
rainfall zone that the user is in (deserts have much improved availability 
as compared to tropical forest areas). Furthermore, "rain cells", i.e., 
local rain zones around the user, have characteristics which cause rapidly 
changing conditions near user sites. In fact, for fixed locations 
operating with GSO satellites a significant amount of analysis has been 
done in predicting the availability of signals due to rain attenuation. 
Since the rain cells cannot be avoided a certain percentage of the time 
from coming between the fixed user and a GSO satellite there is not much 
the fixed user can do to compensate for the rain fade. In the past, it has 
been known to provide excessive margin to partially overcome theses 
attenuations, and in some cases to utilize another site located 35 to 50 
or more miles away, to provide a "diversity" site. Switching between these 
two sites can increase the system signal availability. However, for a home 
or office user it is not practical to provide such a diversity site. 
In accordance with an aspect of this invention, by providing more signal 
paths to the user from two or more of the satellites 10 at different and 
changing azimuth and elevation angles, the effect is to provide the 
"diversity site" at a single location. In effect it is the opposite of the 
mobile user moving under the blocking obstruction, as the rain cell moves 
with respect to the user terminal site. One suitable diversity-type of 
receiver is described in the above-mentioned U.S. Pat. No. 5,233,626, 
issued Aug. 3, 1993, entitled "Repeater Diversity Spread Spectrum 
Communication System, to Stephen A. Ames. 
Although described above in the context of specific frequency bands, 
bandwidths, data rates and the like, it should be realized that these are 
exemplary, and not limiting, embodiments of this invention. By example 
only, one or more of the Ka-band links shown in FIG. 1 could be replaced 
by a Ku-band link. Furthermore, the teaching of this invention can be 
practiced with but one LEO satellite, or with one LEO satellite and one 
GSO or MEO satellite. However, it is preferred to use larger numbers of 
satellites to provide a wide area coverage, while also enabling the use of 
the above-mentioned diversity reception techniques by the subscriber 
terminals and equipment. 
Thus, while the invention has been particularly shown and described with 
respect to preferred embodiments thereof, it will be understood by those 
skilled in the art that changes in form and details may be made therein 
without departing from the scope and spirit of the invention.