In planetary exploration, environmental testing, military, emergency response, and other sensor applications, it is often desirable to acquire sensor data in areas where manual or even robotic emplacement is difficult or dangerous. For example, exploration of the permanently shaded craters on the lunar poles presents formidable challenges in providing mobility, power, and communication for robotic or human explorers. Moreover, in some applications such as planetary exploration, the mere presence of a lander or rover can disturb the local environment sufficiently to greatly reduce the quality of the data being gathered. In many cases, such sensors also have specific placement and orientation requirements, further complicating their use.

Under funding from NASA's SBIR program, Tethers Unlimited, Inc. has developed a suite of novel technologies for deploying and emplacing sensors into remote and difficult-to-access areas for planetary and terrestrial applications. This suite of technologies, called the SensorPod™ System, encompasses:

The SensorPod™ system and its derivative technologies also have numerous potential applications in the environmental, military, and consumer markets. In environmental testing and safety markets, this system will provide a low-cost means for rapidly deploying sensors into areas where it is unsafe or impossible for humans to collect measurements manually, such as in emergency response to chemical spills. In military markets, the SensorPod™ system will provide a means for troops in unsecured areas to rapidly deploy sensors to detect chemical or biological weapons and to provide a secure perimeter to detect intrusion by hostile forces. In planetary exploration applications, the SensorPod™ systems novel deployment and post-deployment supporting technologies permits sensors to be distributed over long distances in challenging terrains, enabling planetary landers to obtain in-situ thermal, chemical, seismic, and other measurements over a much wider area at a significantly lower cost, complexity, and technical risk than robotic rovers.

Advantages of Tether-Based Sensors Systems

SensorPod™ based sensor systems have several notable advantages that make SensorPod™ technologies ideal for integration into many different types of missions. Besides the unique employment capabilities (such as onto a forest canopy), tethered sensor systems have some characteristics that enable some missions, while increasing the capabilities of others.

Low mass/volume

The primary platform size is that of a 40mm grenade launcher. This size is large enough to accommodate 500 m of tether and a small sensor array, while still being a standard size launcher. Both the cold-gas and sealed pyrotechnic launch technologies allow for lengthening the package to allow increased sensor volume, or increased tether length using a breach loaded deployer with a muzzle loaded sensor package. The basic SensorPod™ technologies can also scale much smaller. The following figure shows a 10 m deployer designed to fit within a 12-gauge shotgun shell. For smaller systems where more distance is desired, it is possible to use a fiber optic tether, which typically requires lower volume per unit length due to their small diameter and high strength.


Figure shows protoypes of a 500 m deployer using a 40mm grenade housing and a 10 m deployer using a 12-guage shotgun shell housing.

Another volume savings exists because of the nested cylinder configuration of the SensorPod™ system. Essentially, the volume of the deployed sensors and the volume of the tether take up the majority of the total volume of the system. For many applications, this total volume is still much less than would be required for a comparable system utilizing batteries and wireless communication. This is particularly true if long lifetimes are desired in harsh environments (requiring substantial power just for thermal control). As radio and battery technologies continue to advance, they will likely allow for competitive wireless systems to develop (though a deployment system of some sort will still be required), but in the mean time, a tethered system allows for early implementation of these systems.

Sensor Localization

One of the added benefits of sensors attached to the tether is that they are loosely constrained in their location. The spacing along the tether not only determines the spatial resolution of the sensors, but it also provides a simple way to determine the location of one sensor relative to the others. This may not be adequate for very sensitive spatial measurements (such as interferometry), but for most basic planetary science it seems sufficient to not require additional systems to provide this information. Conversely, non-tethered “scattered” sensors have an additional localization challenge (along with a difficulty in ensuring desired distributions).

Provide High Data Bandwidth

Though proponents of wireless systems have demonstrated methods for reducing the required bandwidth (using onboard compression and variable sensing rates), most scientists we talked to were excited to have the ability to operate their sensors continuously at high sampling rates and return large amounts of data for correlation and compression at a central location. The necessary subsystems required to analyze and compress data locally have been demonstrated to some extent, but the added complexity for each node is not inconsequential. By additionally allowing the user to decide how much data they acquire for each sensor, the flexibility to investigate unexpected phenomena is greatly improved. It is still possible to take advantage of compression and autonomy technologies developed for wireless systems to even further enhance the performance of SensorPod™ if desired.

Provide Power

Most potential users we talked to were not happy with power limitations that forced them to modify their sensing plan. Also, requiring each node to provide its own power not only requires the addition of a power subsystem at every node, but it also drives the thermal design substantially, as often the batteries themselves are some of the biggest drivers of the thermal design of small systems. Even optical fibers have been demonstrated as viable power sources for limited power applications

Occasional Burst Power

Some users felt that there was a need to provide capabilities for occasional high power activities. A promising approach to this problem that allows for a smaller tether relies on a combination of parasitic power technologies developed for tether tracking beacons attached to multi-kilometer space tethers, and lessons learned while working with Maxim's 1-wire technology for use as part of an early SensorPod™ proof of concept demonstration. The approach uses low power trickle charging to provide burst power for low bandwidth activities, though some form of energy storage system is required.

Interactions with potential users of distributed sensor deployment systems revealed desires for a number of different capabilities for sensor emplacement, orientation, surface contact, above-surface positioning, and mobility. As a result, our SensorPod™ development effort has focused on implementing a range of these capabilities within a consistent sensor package form factor so as to maximize the system's utility for different applications.

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