In this chapter we will be discussing information on a number of oceanography products and environmental factors of utmost importance to the Aerographers Mate.

By being familiar with these products, parameters, limitations, and request procedures the Aerographer can provide the on-scene commander with a detailed accounting of environmental conditions above, as well as below, the oceans surface.

We will first discuss the products available from the Navy Oceanographic Data Distribution System (NODDS). NODDS was developed in 1982 as a means to make FLENUMETOCCEN (FNMOC) environmental products available to METOCFACS and METOCDETS who had no direct access to this data. Through the years, the system has grown in use as product support has expanded. NODDS 3.0 was distributed in December 1991, and it was unique in its approach to environmental data communications. Once a user has defined the products desired for a specific area, an automatic process of acquiring data is initiated.

Using a commercial "off the shelf" licensed communications software package, the system dials FLENUMMETOCCEN and requests the data fields from a security shell in a host mainframe computer. The required data is extracted from one of the global data bases as a compacted ASCII transmission which is generated for each field/product. By transmitting field data and limiting the area of extraction, the transmissions are small and communications are efficient. Once the raw data is received by the users NODDS, the required contouring, streamlining, shading, and so forth, is performed automatically until all products are in a ready-to-display format.

The NODDS Users Manual contains explanations of system functions and step-by-step procedures for using the NODDS terminal, By selecting the "Convert Data" option of the "Data Manager" file from the main menu the user can convert the NODDS geographic displays to alphanumeric displays. Underway units may also access NODDS data using a VHF Stel Modem along with a STU-III Secure phone. There are limitations associated with all of the NODDS acoustic products listed in this section, such as low grid resolutions and graphic depiction errors. A general description of each product will be covered along with example outputs. Further discussion on parameter derivation and user provided inputs may be found in the NODDS Products Manual, FLENUMMETOCCENINST 3147.1. Now lets look at some of the products available from NODDS.

CONVERGENCE ZONE RANGE (CZR)

LEARNING OBJECTIVES Recognize characteristics of a convergence zone. Evaluate CZR products. Identify the two graphic outputs of the product.

The CZR product predicts the expected ranges to the first convergence zone for a sonar. Convergence zones are regions in the deep ocean where sound rays, refracted from the depths, are focused at or near the surface. Convergence zones are repeated at regular range intervals and have been observed out to 500 nmi or more. Convergence zone ranges are those ranges capable of being achieved when operating sonar in the path of a convergence zone.

The distribution of sound throughout the deep ocean is characterized by a complex series of shadow zones and convergence zones. The presence and extent of these zones are determined by the sound speed profile, the location of the surface, bottom, and source relative to the profile, and the existence of caustics.

A caustic is the envelope formed by the intersection of adjacent rays. When a caustic intersects the sea surface or a region at or near the surface, a convergence zone is created. Convergence zones are regions of high sound intensity. Thus, a receiver may be expected to pick up high sound intensity gain within a convergence zone versus outside of it, where only a single strong propagation path occurs.

The existence of a convergence zone requires a negative sound-speed gradient at or near the surface and a positive gradient below. In addition, there must be sufficient depth for usable convergence zone to occur, that is, the water column must be deeper than the limiting depth by at least 200 fathoms.

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