compared to other underwater acoustic recorder designs

 The microMARS design has been guided by three fundamental objectives:

  • To enable practical and affordable large-scale passive acoustic monitoring (PAM).

  • To serve not as a single fixed-capability recorder but as a modular PAM 'tool kit' for a range of PAM projects.

  • To support quick deployments and servicing anytime, with minimal ship and support resources.

This application note explains the microMARS design features and their significance. To help you evaluate the suitability of microMARS, it provides a contrast and relative merits compared to design alternatives we found in other recorders.

Recorder modularity

Recognizing that each PAM project will have different design requirements in terms of for example battery and memory endurance, emphasis on high or low frequency signals, monitoring of loud industrial noises or faint vocalizations, microMARS is designed as a plug & play PAM toolkit. You can swap out the hydrophone end cap, adjust memory size use a short or long housing tube for smallest size or longest battery endurance etc. Keep a drawer full of microMARS hardware, and you will be set for nearly any job.

Recorders are often single fixed-capability designs with one hydrophone, battery configuration, memory size limit. Thus, the potential to adapt to future requirements can be limited.

Rapid recorder servicing at sea

Maintenance of large recorder arrays requires rapid recorder servicing. microMARS batteries and SD cards can be swapped out quickly and the recorder sealed within about a 5-minute service interval. Start a service day with a recorder at hand and swap for the deployed unit at site #1. The recovered recorder #1 is serviced in transit and deployed at site #2 etc.

Recorders often require data recovery by wired (USB etc.) transfer and charging of internal batteries before the recorder can be used again. This process may take hours. Consequently, continuous monitoring requires two sets of recorders: One deployed set, and a second set recovered for servicing.

Recorder size

On account of very low power consumption and use of SDS memory, microMARS is very small. The small size of the recorder itself means that a smaller acoustic release, a smaller anchor and less flotation suffices. A much smaller overall deployment package in turn minimizes the boat and other logistic resources needed for array deployment and servicing. Realistic microMARS arrays for long-term monitoring can and have been deployed even from RIB boats.

Most recorder designs of equivalent endurance are significantly larger than microMARS, requiring more equipment and resources for large array deployments and servicing.

Memory endurance

Required memory endurance depends on sample rate and recording period. Some tasks need only a few hundred GB while others can require several thousand GB. microMARS uses field replaceable SD cards and is shipped equipped with 32 GB cards. This 64 or 256 GB memory is enough for shorter deployments; for example to record at 100kHz for about two weeks. For longer deployments, swap with larger SD cards offering currently up to 4096 GB recorder capacity or 16x the starter value. Benefit from SD technology as card sizes increase and cost per bit declines.

Available memory is often fixed. Smaller recorders may provide little memory of a few hundred GB and thus limit recording endurance to a number of days. Other recorders offer more but can be large. Fixed memory designs do not readily benefit from advancement in memory technology.

Battery Endurance

microMARS is designed for near the theoretically lowest possible power consumption for a SD card recorder in order to keep the recorder small. About 80% of the total power consumption is by the SD cards themselves, meaning the recorder power overhead is only about 20%. Consumption various only little with sampling rate: At 250kHz sample rate, battery endurance has been field tested for at least 31 days continuous reporting with four 'D' size lithium batteries.

Depending on the design, power consumption can be much higher, in particular at high sampling frequencies. This imposes a recording endurance limit on a small recorder, or drives up the instrument package size.

Low noise performance and loud signal tolerance 

Industrial signals such as pile driving and ship traffic can overpower a recorder, while detection of distant vocalizations requires low recorder noise. microMARS thus offers a selection of field replaceable hydrophone end-caps, overall accepting signals as strong as 240 dB re. 1 µPa at 10Hz, and noise floors as low as 21 dB re. 1 µPa / √Hz above 30 kHz. In order to monitor high frequency vocalizing dolphins in the presence of industrial noise, hydrophone end caps with frequency sloped frequency responses are available. MH33-S2 accepts substantial shipping and industrial noise of 204 dB at 10Hz, and provides a noise floor of 21 dB at 30kHz. Following equalization, the dynamic range is thus 183 dB.

Some recorders are equipped with a fixed hydrophone. This by necessity limits either loud signal tolerance or low noise performance. Sloping frequency response curves that provide tolerance for loud low frequency signals while offering a low high-frequency noise floor are unique to microMARS.

Sensitivity at low and high frequencies

Larger hydrophone elements are needed to operate effectively at low frequencies, while smaller elements are required for high frequency operations. The microMARS hydrophone end caps can be swapped out based on the monitoring demands. Currently available is the MH33 end cap for operation up to 33kHz with improved low-frequency performance down to 2Hz, and MH125 for monitoring signals up to 125 kHz.

Recorders equipped with a single fixed hydrophone will be performance limited at either high or low monitoring frequencies. A small hydrophone will cover a wide frequency range, but with reduced low frequency sensitivity. A large hydrophone will limit the upper end of the operating frequency range.

Advanced and custom capabilities 

microMARS modular design promotes advanced and custom capabilities. Currently in development under a ONR contract is a sound source localization capability based on the use of a wireless network of tightly synchronized microMARS recorders. The modular hardware also supports the development of custom capabilities at a moderate cost.

Long-term investment value and the future growth path

With its fundamental structure as a modular tool kit, microMARS preserves your investment in PAM technology. Upgrades are through an on-line firmware upgrade, a swap of an electronics, housing or end cap module, or perhaps your own installation of later generation SD memory. This is in particular important for long-term monitoring needs, where your PAM capabilities can evolve with your changing requirements while providing system continuity. Some Desert Star technologies such as underwater acoustic positioning have now exceeded the 20-year development mark while maintaining continuity and investment protection for our customers. Currently on the drawing board for microMARS are upgraded sampling rates to 500 kHz, 18-bit and 20-bit A/D operation, underwater sound localization and additional hydrophone end cap designs.

Recorders often represent a fixed snapshot of technological capability. As more memory, better hydrophones, lower power consumption, advanced capabilities etc. are desired, entirely new recorder models or brands may have to be purchased. This is problematic in particular for long-term deployments and use where a switch to an entirely different new recorder can have a range of consequences from an impact on your overall deployment package, data compatibility, training investment and more.