Background
In 1994, C-MAX introduced the CM800 system and now more than 100 of these systems are in use around the world. Some key features of the CM800 were the use of an industrial PC for the top end display and recording, the use of 3.5" magneto-optical (MO) disks as the recording medium, the use of a custom splashproof keypad for control and annotation, and the option of a fibre optic (FO) tow cable for intermediate depths. At the time these features were unique in a sidescan sonar. In the meantime the PC has become common-place for sonar processing and MO disks have progressed from 128MB capacity to 1.3GB.
The challenge for C-MAX was to improve on the CM800 and at the same time to address the growing market for systems with a deep water capability.
The key to achieving these aims was the redesign of the tow cable telemetry, the subsystem that brings the sonar data up the tow cable and sends commands down to the towfish. In the CM800 the telemetry system was restricted to FO cables up to 3km long or electrical tow cables up to 500m. A disadvantage of the FO tow cables, apart from their cost, was the fact that they did not carry power and therefore batteries were needed in the towfish.
The new CM2 digital telemetry changed all that, doing away with the need for batteries in the towfish. The telemetry is so versatile that it can handle coaxial or twisted pair cables from 50m up to 6km long.
At the shallow end of the market, "soft" hand-hauled tow cables can be used. For slightly greater depths there is the option of using up to 300m of 4.7mm armoured cable on a power winch that is light enough to be hand carried.
For the heavy (deep) end of the market the versatility of the telemetry means that virtually any 2-wire cable can be used, including most types that may have been inherited from older equipments. A key point to note is that the CM2 is a true digital system, with the images being digitized in the towfish. Unlike sonars that rely on analogue towfish and analogue telemetry there is no loss of image quality in the tow cable.
Aside from the new telemetry the CM2 was given the benefit of the years of experience with the CM800. New options for data acquisition and display were introduced, including an all-in-one clamshell unit (the C-Case) and, most recently, the compact Sonar Transceiver Unit (STR) that links to a laptop via a USB cable.
In the towfish, the basic configuration was carried over from the CM800 but improved in nearly every respect. Now that batteries were not required the towfish was shortened slightly. The transducers are now protected by wider "wings". As well as providing protection these "wings" serve two other functions. The first is that they maintain the pitch stability of the towfish. The second is that they act as acoustic reflectors, primarily for the high frequency beams, cutting down the signal in the upper sidelobes by an average of 20dB. This significantly reduces any interference by surface echoes when working in shallow water.
The transducers retain C-MAX's unique configuration that includes an additional pair of depressed short arrays that ensure good coverage directly below the towfish, and also act as an echo sounder to allow the towfish to measure its altitude automatically.
The CM2 towfish keeps the stainless steel construction and the double O-ring sealing design that has ensured that no CM2 or CM800 towfish has ever leaked. The CM2 towfish also keeps the breakaway mechanism that, if an obstruction is hit, causes the towfish to flip and tow from the rear via a safety lanyard. This mechanism has proved so effective that very few towfish have been lost. A handle was added to the CM2 towfish to make one-man launch and retrieve even easier.
The two areas of weakness in the CM800 were related to the towfish external connectors and to the chip sockets on its internal electronics board. In the CM2 the external connectors were replaced with a larger, much more robust type. Internally the new electronics board has no sockets and towfish software updates are downloaded electronically instead of by exchanging chips. Our experience of the CM2 reliability and our confidence in its design led us to increase the warranty period for the towfish, the data acquisition unit and the keypad from 12 months to 36 months!
In summary, the CM2 can use a much wider variety of cables than its predecessor, is even more robust, and has been improved in many other respects.