A HYBRID SENSOR NETWORK FOR CANE-TOAD MONITORING


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Introduction

The project investigates a wireless, acoustic sensor network application—monitoring cane toads in Kakadu National Park, Northern Territory, Australia. Our goal is to use automatic recognition of animal vocalizations to detect the existence of cane toads. This is a challenging application as it requires high frequency acoustic sampling, complex signal processing and wide area sensing coverage.

We set up two prototypes of wireless sensor networks that recognize vocalizations of up to 9 frog species in northern Australia. The first prototype is simple and consists of only resource-rich Stargate devices. The second prototype is more complex and consists of a hybrid mixture of Stargates and inexpensive, resource-poor Mica2 devices operating in concert. In the hybrid system, the MICA2s are used to collect acoustic samples, and expand the sensor network coverage. The Stargates are used for resource-intensive tasks such as Fast Fourier Transform (FFT) and machine learning. To enable the hybrid system, we design and incorporate three algorithms to account for the sampling, processing and communication bottlenecks of the MICA2s:

·        high frequency sampling

·        compression and noise reduction, to reduce data transmission by up to 90%

·        sampling scheduling, which exploits the sensor network redundancy to increase effective sample processing rate

We evaluate the performance of both systems over a range of scenarios, and demonstrate that the feasibility and benefits of a hybrid systems approach justify the additional systems complexity.


Cane-toad Monitoring Issue

Cane toads (Bufo marinus) were introduced to control sugar pests in Australia about seventy years ago. Their continuous spreading, due to lack of natural predators through-out northern Australia have caused major concerns by biologists and ecologists. The dark region in following figure shows the current distribution of cane toads in northern Australia.

Kakadu National Park is a World Heritage area, and its status is threatened by the possibility of colonization by cane toads. Cane toads have recently been spotted at approximately 15 to 30 km to the east of the park, which makes it important to monitor them in this area.


Devices

MICA mote family: MICA2 is the third generation of Berkeley mote manufactured commercially by Crossbow. It has a 7.7MHz Atmega processor and 512KB on-board flash memory. It can transmit at a maximum data rate of about 19 kbps and is powered by two AA size batteries. Its recent cousin MICAz has a ZigBee compliant RF transceiver and can support up to 250 kbps transmission rate. We use the MICA2 sensors as our resource-poor sensors.

X-Scale Single Board Computer - Stargate, manufactured by Crossbow, is a high performance processing platform that offers much more resources than MICA motes in terms of computation power, memory, energy, and transmission capability. It operates on a 400MHz Intel PXA 255 processor and has 96MB memory in total (64MB SDRAM and 32MB flash memory). It can be powered by a li-Ion battery and can support Wi-Fi (11mbps when using IEEE 802.11b) transmission. We use Stargates as our resource-rich sensors.


High-level System Structures

 Stargate-only System Structure

Figure 1: Stargate-only System Structure

 

The system samples acoustic data using a desktop microphone. The sound spectrogram is then generated to get signals in frequency domain from input in time domain. The sound attributes, including local peaks and other necessary variables, are extracted from the spectrogram and used as the input of machine learning classifiers, one for each frog species. To increase correctness and reliability of the recognition, a hierarchical recognition structure is employed, termed as voting process in above figure.

 Stargate-MICA2 Hybrid System Structure

Figure 2: Hybrid System Structure

In this system, instead of sampling acoustic data using desktop microphone, we use 2 MICA2s to sample acoustic data and compress it before sending to the Stargate via radio channel. Upon receiving data from the satellite motes, the Stargate will decompress received data before processing it. As more than 1 MICA2 is used, a scheduling algorithm is designed and employed to have them sense acoustic data in alternate periods.


Result Screen Shots

In this experiment, two frogs (Bufo marinus/cane toad, Cyclorana cryptotis) are detected successfully by our system. More screen shots:



Papers


Presentation and News


Future Development

We are porting the system from Mica2 to MicaZ to get higher bandwidth between Mica and Stargate. (Jan. 2005)