Technology Today issue 1 2008 - Raytheon

RF SYSTEMS (continued)
Continued from page 27
SNR. With respect to a MIMO radar system’s
robustness, efficiency and ability to
mitigate multiple sidelobe effects, the performance
evaluation is usually done in
terms of the Cramer-Rao bound (CRB),
which is a lower minimum mean square
error (MSE) bound on the performance of
all unbiased MIMO estimators. However,
CRB is mostly used for angle-of-arrival estimation,
and it ignores the effects of sidelobe-induced
errors. The Weiss-Weinstein
bound (WWB) can also be used to evaluate
the lower bound MSE performance of all
MIMO unbiased estimators. The WWB
bound includes the effects of multiple sidelobes
and provides a more accurate theoretical
platform for comparing the performance
of MIMO versus phased array radars.
Future directions
A MIMO communication system is clearly a
good idea because of the inherent advantages
provided by its architecture. MIMO
radar is a derivative of MIMO communication
systems, but it is still widely considered
to be a research topic. Under the right conditions,
however, MIMO radars can offer
significant advantages, such as better performance
in a multipath environment, limited
bandwidth and power requirements,
and adaptive degrees of freedom. In particular,
robustness to multipath — especially
for shipborne radars — appears to be a
promising application of MIMO radar;
although, standard methods using phased
arrays can effectively compete.
Several other keys areas of radar performance
may also benefit from MIMO technology.
In particular, over-the-horizon radar
28 2008 ISSUE 1 RAYTHEON TECHNOLOGY TODAY
systems have inherent characteristics such
as limited bandwidth at HF, severe ionospheric
multipath, challenging clutter environment
and poor angular resolution; many
of these could be reduced or eliminated
through a MIMO approach. As noted
above, MIMO radar’s coherent transmit and
receive capability can enhance resolution;
mitigate clutter; and improve detection,
tracking and classification performance.
MIMO radars can also effectively use commercial-off–the-shelf
(COTS) MIMO technology
to reduce development time and cost.
When MIMO is compared to phased array
radars, however, other issues arise. The
complexity and feasibility of calibrating
both receive and transmit paths for tropospheric
and ionospheric errors, radar hardware
errors, and multipath require some
significant design considerations and tradeoffs
compared to phased array technology.
Moreover, standard MIMO radars can suffer
substantial SNR loss relative to phased
arrays, as discussed above. Finally, in many
cases, current phased array radars using
simpler, less expensive and less risky algorithms
can achieve some of the same
advantages as MIMO radars.
With all of its advantages and disadvantages,
however, MIMO radar — and the development
of the required technology to make it
an effective system — are worth investigating.
Future radar requirements will challenge
the capabilities of current systems and
require the investigation of other approaches.
MIMO radars may provide a platform to
address some of these challenges.
Dr. Pierre-Richard Cornely
pierre-richard_j_cornely@raytheon.com
onTechnology
Raytheon and
CALCE:
Partners in
Lead-Free Research
The aerospace–defense industry is
addressing new environmental regulations
and their effects on electronic equipment
performance. The European Union’s
Restrictions on the Use of Hazardous
Substances (RoHS) regulations have
changed supply-chain scope, causing more
suppliers to provide components/assemblies
with lead-free materials (as interconnection
or finish) rather than the traditional
tin-lead. Raytheon supports reducing hazardous
substances, but our responsibility to
our customers requires us to ask how well
these materials will perform in harsh-use
environments.
Because this issue concerns many defense
and aerospace companies (system integrators
and multi-tier suppliers) the industry
has created several working groups and
consortia to address performance issues,
risks and mitigation plans and practices.
This article discusses Raytheon’s successful
collaboration with the University of
Maryland Center for Advanced Life-Cycle
Engineering (CALCE ® ) consortium in
researching lead-free materials.
The Challenges
Differences between various lead-free solders
and the traditionally used eutectic tinlead
(63Sn-37Pb) can affect equipment
performance. As the primary interconnection
for electronic components, solder is a
critical material. However, the relative newness
of lead-free solders in
aerospace–defense applications limits the
amount of data available for evaluating
interconnection performance. Real-time
field data is always preferred to accelerated
life testing and modeling, but the relatively
swift implementation of RoHS has prevented
the timely acquisition of such data.
Although the aerospace–defense industry is
exempt from the ban on lead, the problem
still affects us; many of our commercially
obtained items will contain lead-free materials
as suppliers change their products to
YESTERDAY…TODAY…TOMORROW