Small form-factor data acquisition devices can be thought of as the sharp end of the stick in a system of remotely deployed sensors. Often the reason they are small form-factors (SFF) is because the details of the deployment (size, weight, cost, etc.) prevent the use of COTS form-factors such as PMC, AMC, etc. Regardless, remotely deployed sensors of any form-factor are in fact data producers for a centralized data consumer process. The fact that they are not connected to their hosts by a data bus requires them to use unconventional I/O methods.
There are various objectives of remote sensor deployment. Among them are the need to make measurements at remote locations, to simultaneously measure a variable in multiple widespread locations, and the need to measure a variable where the sensor will be lost or destroyed in a relatively short time. Additionally, remote deployments often carry with them restrictive space, weight and power requirements. Air-launched sonobuoys are possibly the ultimate example of remote deployment. Because they operate on batteries, power consumption must be frugal. They are deployed once and are likely to never be recovered, so they must be cheap. Additionally, sonobuoys are usually deployed in significant numbers (99 channels are theoretically available) from patrol aircraft or helicopters, so size and weight must be kept low.
With the special requirements of remote sensing in mind, SFF DAQ modules are deployed in one of three ways: fixed sensors with a hardwired connection, fixed sensors with a wireless connection and mobile sensors with a wireless connection.
Fixed sensor arrays are often encountered in industrial process control applications, where a number of sensors transmit data back to a central control system. Early systems used analog parameters, such as electric current in a control loop, to represent physical conditions such as flow rates, pressures, or temperatures. Modern systems represent the process variables as digital data and communicate with the controller using commodity communication links such as USB. The rapid acceptance of USB 2.0 in industrial control provides an example of fixed sensors and hardwired connections.
Over the last few years, USB has become a ubiquitous mechanism for connecting computers and various I/O devices. It is readily available on virtually all styles of PCs, and also on most single board computers and stand-alone instrumentation, regardless of the host OS. USB also offers the user a combination of low price, good performance, reliability and “plug and play” installation. Because differential signaling is used, USB has strong resistance to noise typically encountered in factory environments. The easy installation capabilities of USB also allow industrial sensor systems to be quickly modified. Hot swap/hot plug capability means that in many cases USB modules can be unplugged and reinstalled in new locations without interrupting system operations.
USB 2.0 offers this easy connectivity for up to 127 devices, enough to monitor several complex industrial processes from a single PC. And with commercially available USB hubs and extenders, the reach of USB connections can be extended to well over 100 meters. One of the biggest benefits of USB for remote deployments is the fact that the same cable that provides communication also provides the sensor power. Although the total power available from a USB host is low (mandated at 0.5A @ 5V), many commercially available hubs provide additional power to USB devices.
Fixed wireless connections are used in applications where physically connecting the sensor nodes is inconvenient, undesirable, or impossible. In factory applications similar to the fixed and hardwired example, a wireless architecture will enable rapid deployment of a new sensor network with little or no additional infrastructure.
An excellent example of a legacy “fixed” (to use a term loosely) wireless SFF DAQ system is the sonobuoy. These are typically either active or passive air-launched sensors that are used in various marine applications from submarine detection to sea state monitoring, to animal tracking. Each sonobuoy contains its own radio transmitter, which relays data back to a receiver located in a patrol aircraft or helicopter. The buoys themselves do little or no processing, merely relaying data back to a host system. Each sonobuoy typically transmits on a preset carrier frequency, and the host is responsible for receiving all the deployed channels, or at least those of interest.
Figure 1 shows a typical airborne acoustic processing system. The basic signal acquisition system architecture has changed little since the Second World War. This type of signal distribution was originally based on analog FM radio technology, but advances in digital signal processing and software defined radio technology now allow use of modern waveforms. Typical legacy airborne anti-submarine warfare (ASW) systems support 8 to 32 concurrent sonobuoy receiver channels selected from any of 99 channels in the VHF band 136 to 174 MHz.
Mobile Ad-hoc Networks (MANETs) represent one of the most challenging applications of small form-factor data acquisition modules. However, creative implementation of MANETs is yielding some very efficient ways to collect data from remote, sometimes dangerous, environments. Ad-hoc wireless sensor networks consist of large numbers of small, cheap and often single-use devices, each capable of limited computation, wireless communication and sensing. The actual sensor nodes are deployed as close to the signals of interest as possible, and ideally the sensors are actually mobile. Rather than routing signals of interest directly to a centralized location for sampling and processing, information moves among the nodes en route to its destination.
Wireless sensor networking technologies are heavily employed in the U.S. DoD JTRS HMS specification, which defines radio equipment characteristics for a wide range of applications, and re-defines the problem as wireless tactical networking.
The JTRS HMS Requirement
The U.S. Department of Defense Joint Tactical Radio System (JTRS) has placed new space, weight and power challenges on the handheld, man pack and small form fit (HMS) radios. These radios are intended for deployment in a wide variety of systems and platforms such as unattended ground sensors, UAVs, robotic vehicles, and on the soldiers themselves (Figure 2).
The objectives of the JTRS HMS include networked communications among various levels of command and the ability to support technology insertion upgrades while maintaining interoperability with existing equipment. With this in mind, wireless tactical networking is one of the most critical capabilities the JTRS program will deliver. Like commercial wireless sensor networks, wireless tactical network nodes (soldiers) often move and the bandwidth of the links can be very limited. MANET protocols mentioned above are designed to handle these wireless environments. The MANET protocols will be used in conjunction with new JTRS networking waveforms, to permit soldiers in the field to connect to the DoD’s Global Information Grid. Software Defined Radio technologies are being employed to meet the performance objectives within the aggressive new space, weight and power limits.
WSNs (and WTNs) have their roots in packet radio systems. Military packet radio networks from their very beginning developed hardware, software and protocols that could adapt to the changing topologies and environments that were expected on a battlefield. Growing out of the University of Hawaii’s ALOHANET, the DARPA-sponsored Packet Radio Network (PRNET) project extended the single-hop packet radio into a multi-hop packet radio network. Unlike most amateur packet radio networks, the PRNET project designed and tested protocols in environments where the nodes were expected to be mounted on mobile platforms, such as trucks. As a result, the protocols had to adapt automatically to changes in topology, although routes were expected to remain stable for at least a few minutes.
Mobility of the sensor nodes increases the complexity of the networking problem considerably. In contrast to traditional wired networks, routing in a WSN is both highly dynamic and ad hoc. The idea of ad hoc networking is sometimes also called “infrastructureless” networking, since the mobile nodes in the network dynamically establish routing among themselves to form their own network on the fly. For example, after the initial deployment, sensors may fail due to damage or battery depletion. Also, mobile radios or sensors are likely to experience changes in their position, available energy and task details. Changes in the environment can dramatically affect radio propagation, causing frequent network topology changes and network partitions.
Wireless sensor networks often deploy large numbers of sensors. Although they don’t require high bandwidth, they usually require low latency. And because the sensors themselves are usually battery powered, minimal power consumption is a must. These requirements are similar to those imposed by the JTRS HMS radio specifications, which are heavily based on networked communications. Because technologies such as Bluetooth and ZigBee are aimed at providing short range wireless ad-hoc networking capability to low-cost, battery-powered devices, techniques and algorithms developed for commercial markets could well be adapted for military use. For commercial applications, ZigBee (IEEE 802.15.4) in particular has great potential in the area of wireless sensor networks (Figure 3).
ZigBee Wireless Networks
ZigBee’s general characteristics include use of dual PHY (2.4 GHz and 868/915 MHz) with Data rates of 250 Kbits/s (@ 2.4 GHz), 40 Kbits/s (@ 915 MHz) and 20 Kbits/s (@ 868 MHz). The protocol is optimized for low duty cycle applications (<0.1%). CSMA-CA channel access yields high throughput and low latency for low duty cycle devices like sensors and controls. It also provides for an optional guaranteed time slot for applications requiring low latency as well as for low power usage with battery life ranging from multi-month to years.
ZigBee allows for multiple topologies including star, peer-to-peer and mesh, and is a full handshake protocol for transfer reliability with a typical range of 50m (5-500m based on environment). ZigBee uses spread-spectrum technologies to avoid multi-path fading and increase robustness. This approach allows for improved signal immunity in the presence of radio interference.
The IEEE 802.15.4 standard defines two PHYs representing three license-free frequency bands that include sixteen channels at 2.4 GHz, ten channels at 902 to 928 MHz, and one channel at 868 to 870 MHz. The maximum data rates for each band are 250 Kbits/s, 40 Kbits/s and 20 Kbits/s, respectively. The 2.4 GHz band operates worldwide while the sub-1 GHz band operates in North America, Europe and Australia/New Zealand. The IEEE standard is intended to conform to established regulations in Europe, Japan, Canada and the United States.