Review of Autonomous Undersea Vehicle (AUV) Developments 
Concerning anti-submarine warfare (ASW) application of oceanography, the NSB provided the 
following [2]: 
"The real-time acquisition of environmental data is an imperative, but challenging, task for a mobile 
system that must be capable of operating anywhere in the world. The research community has gone to 
great lengths to acquire such data, but the requisite oceanographic data are both site and time specific, 
with many scales of variability, and tend to be undersampled in both space and time. The important issue 
is that up-to-date site-specific environmental data must be incorporated in the deployment of mobile sonar 
if high detection gains are to be realized. 
One development currently in its infancy is the use of networked unmanned underwater vehicles for 
synoptic (fully three-dimensional) environmental sensing. Such environmental information, obtained 
through local sampling or tomography, could be of significant operational usefulness in improving sonar 
capabilities. In addition, the imagination is stimulated by the vision that many small, dispersed platforms 
could be connected with a few manned platforms for networked warfighting. Such a vision is currently 
premature but might become practical over the time horizon of this study. The panel believes that the 
environmental sensing applications of UUV networks should be pursued first. In this way, the Navy will 
develop the enabling technologies for such networks, as well as operational experience in their 
deployment. 
The two key enabling technologies for UUV networks are their power sources and reliable underwater 
communications. There has been much activity in the area of air-independent long-duration power 
sources, but there does not seem to be any consensus regarding which of many possible avenues-- 
batteries, fuel cells, and air-independent combustion--should be pursued. This is an area in which 
considerable interest also exists in the commercial sector, and new long-duration power sources may 
arise from activities such as the Partnership for Next-Generation Vehicles. Thus, a top-down research 
approach may be effective in guiding R&D in this area into the most promising paths. 
Currently, the leading candidate for underwater communication is acoustic communications. Data rates 
of 2 to 20 kilobits per second (kbps) are currently achievable, although distances are limited to a few 
kilometers in shallow water. It is even possible to give UUVs Internet addresses and to communicate with 
them using standard Transmission Control Protocol/Internet Protocol (TCP/IP) protocols. An improved 
understanding of ocean acoustic coherence could also improve our ability to communicate underwater. 
Little work, however, has been done on the vulnerability of acoustic communications networks to acoustic 
jamming or on covert underwater communication. 
Several science and technology issues must be addressed to ensure the continuous evolution of 
improved performance for both mobile and fixed sonar systems. Although signal processing algorithms 
and computational capabilities are necessary for a high-performance sonar, the acoustic-oceanographic 
coherence is what ultimately sets the limits. The science and technology issues for coherence have 
several environmental contexts. 
Littoral waters, where these sonars must operate, can be quite shallow (10 to 200 m) or deep (kilometers) 
and include a range-dependent shelf break. In shallow water and on the shelf, strong horizontally 
anisotropic internal waves driven by tidal and topographic forcing, usually with a diurnal period, can 
modulate sound speed profiles dramatically. In upslope-downslope geometries these can precipitously 
interrupt surface duct propagation and impact coherences through mode coupling and/or ray path 
fluctuations. When bottom refraction sound speed profiles are present, bottom interaction can 
significantly impact coherence. Even well-lineated, constant-depth shallow water introduces problems 
because of differential absorption; the complexities of rapidly range-dependent slope with high geologic 
roughness are even more challenging. 
Coherence in deep water is greater than in the littoral, especially when the signals are not bottom 
interacting. This presents an opportunity to significantly increase detection ranges by pushing coherence 
to the limits. VLF deep-water experiments have demonstrated significant frequency dependence on 
coherences, with those in the lower edge of the band demonstrating remarkable ray-mode coherence, 
whereas those in the upper section have different coherences for the high-angle paths and the ducted 
paths. The cumulative effects of internal waves appear to be the problem, but there is a great deal of 
controversy about this issue. Bottom interaction has received less consideration; but it is an unavoidable 
issue, especially for active systems. 
