Telemetry Takes Over
New wireless communications technology is increasingly being used for water data reporting
- By Scott South
- May 01, 2005
Unlike government bodies in the recent past, which specialized in collecting and storing data, government agencies today seek to actively use water data for improved understanding and management of environmental conditions -- for example, through modeling and projection work. For this reason, it is imperative that monitoring agencies, scientists, and researchers receive water data in a reliable, timely, and easy-to-understand manner.
The roll-out of wireless technology for water data reporting is already underway in many areas. The need for more information on a timely basis has spurred counties and cities to establish wireless networks that enable real-time assessment capability. These easy-to-deploy networks also provide an infrastructure for real-time water data monitoring.
Intelligent environmental sensors and data collection platforms, several of which are already proven and on the market, offer compatibility with numerous wireless network protocols, including satellite and Bluetooth® communications. With many options available, it is important to understand the features, advantages, and limitations of each in order to select the optimal solution for a given data collection system.
Essentially, wireless communication options are simple. They include a modem that provides a defined radio frequency (RF) and an antenna. However, radio waves or signals exhibit very different propagation characteristics depending on their frequency band, so engineers must take into consideration these characteristics when designing a wireless system.
The complication comes with the numerous types of communication modems built to capture these signals. These modems and their respective frequency bands include:
- Very High Frequency (VHF): 30 megahertz (MHz) to 300 MHz
- Ultra High Frequency (UHF): 300 MHz to 1,000 MHz
- Cellular - handset / modems: 824.01 MHz to 848.97 MHz; cell towers: 869.01 MHz to 893.97 MHz
- Spread Spectrum: 902 MHz to 928 MHz and 2,400 MHz to 2,483.5
Bluetooth: 2400 MHz to 2483.5 MHz
- LEO (ORBCOMM) satellite uplink: 148 MHz to 150.05 MHz; downlink: 137 MHz to 138 MHz
- GOES Geostationary satellite uplink: 401.7010 MHz to 402.0985 MHz; downlink: 1600 MHz
Factors to Consider
In addition to frequency band and modem output power, there are other factors to consider, all of which influence the optimal range of communication. One is "Line of Sight," which means one antenna must "see" the other, without obstructions. Line of sight is not mandatory, but when available, greatly improves performance and range. This is especially true with higher radio frequencies where signal strength is reduced from obstructions such as walls, trees, foliage, and concrete and eliminated with metal objects or structures.
Positioning, gain, antenna tuning, atmospheric conditions, time of day, ambient frequency, noise, and terrain are also all important variables affecting RF signal and communication range.
Because of the flexibility and range (typically good for up to 30 miles) VHF/UHF systems are often used in environmental monitoring applications involving gate or pump control, and with SCADA (supervisory control and data acquisition) systems. Frequency bands are typically 66 to 79 MHz; 132 to174 MHz; 216 to 266 MHz; 380 to 512 MHz, and 928 to 960 MHz.
It is important to carefully evaluate the condition of the remote site when configuring a VHF/UHF system. Typically an engineering firm performs a radio propagation study to determine the best configuration and whether additional repeater sites are required. Such additions could greatly increase the costs.
VHF/UHF telemetry systems also incur licensing costs from regulatory authorities such as the Federal Communications Commission (FCC). However, since the user is assigned a specific frequency, interference from other radios is low. In addition, users have control over their data and can define the frequency of interrogation and transmissions. Data packets are typically sent with transmission confirmation and are error-check and corrected. The configuration of the data packets is either done by the datalogger or by a smart packet radio.
Telemetry is the science and technology of automatic measurement and transmission of data by wire, radio, or other means from remote sources, as from space vehicles, to receiving stations for recording and analysis. Originally designed for voice communication, the cellular infrastructure uses a network of base stations and antennas, called "cells," to cover a large area. Cell sizes range from sixth tenths of a mile to 30 miles in radius. The Global System for Mobile Communications (GSM) is growing in the United States and uses much smaller cells, fewer than six miles across.
Cellular telemetry in environmental monitoring applications works well in areas with strong and reliable cell coverage. Although cellular coverage is increasing, there are still many remote areas where it is not available or the signal is weak. Also, because the cellular infrastructure uses a control channel to send packets of data, cellular voice coverage does not always equate to quality data coverage. Therefore, it is critical to ensure quality data service is available before choosing this option.
Circuit switched data services are billed by time and are suitable where infrequent connection is needed and large data files are transferred. Packet data service is suitable for data collection or alarming applications because small amounts of data are involved and remote systems often monitor continuously.
The FCC has allocated some specific frequency bands for flexibility. Called Spread Spectrum, these bands have two primary advantages: they are unlicensed and free.
Spread Spectrum involves two coding techniques: Direct Sequence or Frequency Hopping. Direct Sequence enables digital radio transmission on multiple radio channels. Frequency Hopping concentrates the radio power on one narrow channel at a time for a very short duration and typically are better in environmental monitoring applications where a strong penetrating radio signal is more important than high data rates.
The 900 MHz band allows for a better radio propagation and penetration properties than 2.4 GHz, however the 2.4 GHz band is designed for much higher data rates. Bluetooth and Wi-Fi communications use the 2.4 GHz band. For environmental monitoring applications, where the amount of data transmitted is small and the radio paths are not congested, the 900MHz band gives better performance.
A Spread Spectrum system is more easily set up than a VHF/UHF system and a site radio propagation study is usually not required. However, because these bands are free, signals may be heavily polluted by other unlicensed systems and may degrade signal integrity and range.
In a new type of water operation, two large golf courses in Augusta, Ga., use spread spectrum transmitters connected to dataloggers to send data on soil moisture, salinity and temperature from buried probes on the greens to a PC in the maintenance building. The wireless data feed goes directly into a SCADA system, allowing the superintendents to view soil trends and trouble spots, to help them optimize watering and fertilization.
A relatively new radio protocol increasingly used in environmental monitoring applications, Bluetooth is a low-cost, low-power, short-range radio link between mobile PCs, PDAs, mobile phones, and electronic instruments. This simple two-way radio solution allows different electronic instruments to "talk" to each other without cables or infrared.
When combined with dataloggers through an RS232/RS485 Bluetooth serial port adapter with an external antenna, Bluetooth technology offers many other advantages. It eliminates the need to open enclosures that house the datalogger in order to establish communications. It can be used to collect data from a remote site or it can be employed as the link from the data collection platform to a nearby telemetry modem whose location is better positioned for RF performance.
In monitoring applications, datalogger connections can be established without users leaving their vehicles, eliminating the need for field staff to hike up and down to rugged sites. For industrial water sites, strategically placed Bluetooth-enabled dataloggers can replace cabled solutions for data transmission to a central computer or laptop.
One example is the Stevens Water Monitoring System's Class 1 RS232 Bluetooth adapter that enables a wireless link between any datalogger with a serial port and another Bluetooth enabled computer, PDA, telemetry modem or device.
A U.S. Geological Survey (USGS) office uses a Bluetooth Adapter connected to a datalogger located in a stream gage house, which is housed on the opposite side of the river from the access road. An external flush mount antenna connected to the adapter is mounted on the outside of the gage house. As a result of this installation, the USGS is able to communicate wirelessly with the datalogger from the access road.
Eliminating the cable connection between a datalogger and other equipment connected to the datalogger's serial port makes for less cumbersome and more productive fieldwork. In another example, various governmental agencies have used an RS232 Bluetooth Adapter connected to an instrument on a mini-catamaran platform for river discharge measurement. The Bluetooth wireless data link between the instrument and the user enables easy deployment and quick data collection as the mini-catamaran is pulled across the river with a tagline.
In areas where an RF signal is impeded, such as a sewer manhole, valley, or structural interference, a Bluetooth-enabled datalogger can wirelessly relay data data to a nearby radio, satellite, or cell modem for long-range communication to or from a remote central office monitoring station.
One additional Bluetooth benefit minimizes costs even more. There is no need to purchase expensive environmental rugged computers / PDA enclosures for downloading data.
Bouncing an RF signal off a satellite is one of the best options for remote installations, especially in locations where no other reliable RF or telephone coverage is available, or where the infrastructure cost, including using repeaters, is not economically feasible. There are two primary satellite systems that offer remote environmental monitoring applications: Geosynchronous Earth Orbit (GEO) and Low-Earth Orbit (LEO).
GEO satellites are positioned at a fixed point approximately 22,000 miles above the earth's equator. This height matches the earth's rotation speed and allows the satellites a full hemispheric view from a stationary position. They are primarily used for weather imagery to enhance forecasting.
The United States normally operates two GEO meteorological satellites, named GOES (Geostationary Operational Environmental Satellite) West and GOES East. What is not commonly known is that in addition to weather imagery, these satellites allocate frequency for communication of environmental monitoring data using a relay system called the GOES Data Collection System (DCS).
A primary advantage of GOES is financial: any federal, state or local agency -- or any government-sponsored environmental monitoring group -- can apply to the National Oceanic and Atmospheric Administration's National Environmental Satellite, Data, and Information Service (NOAA/NESDIS) for permission to use the GOES DCS at no charge. The other main benefit to GOES DCS is that, while it is a one-way transmission and offers no transmission verification and no retransmission of missed data, the data transmissions are very reliable and data is easily shared among government users.
LEO satellites typically orbit about 400 to 800 miles above the Earth's surface and pass over a local area during a 20-minute span. A complete orbit ranges from 90 minutes to two hours, at approximately 17,000 miles per hour. LEO systems use a satellite-to-satellite hand-off to maintain communications and are best for short, narrowband communications.
ORBCOMM, a leading LEO system used in environmental monitoring applications, provides global coverage with 30 satellites, and is capable of sending and receiving two-way data packets anywhere in the world. It verifies data transmission to minimize the risk of missing data. Unlike GEO satellites, most LEO satellite systems are commercial ventures; therefore, use of the satellite system requires a fee.
Subscriber communicators pass data messages to and from Gateway Control Centers (GCCs) over ORBCOMM satellites. GCCs then route messages to third party services such as one operated by Stevens, which then delivers the data to users via Internet, e-mail or dedicated delivery lines. Since the lifespan of a LEO satellite is five to eight years, another important consideration for long-term users is the satellite maintenance and replacement plan the LEO provider offers.
TECQ Turns to Satellite Systems for Tracking Nutrient Loading Reduction
The Texas Commission on Environmental Quality (TCEQ) has a goal of identifying trends and tracking loading reduction into the Bosque and Leon River watersheds, two central Texas watersheds that are affected by point and non-point sources. It uses the data to assist in implementation of environmental management plans.
To address this challenge TCEQ has deployed four continuous water quality-monitoring stations in the watersheds for the two rivers. It uses Stevens - Greenspan AquaLab Analyzers to monitor physical and nutrient water quality parameters. AquaLab's ability to provide frequent measurements is important to the program's emphasis on continuous monitoring.
TCEQ relies on measurement accuracy and real-time reporting of the parameters. They make full use of wireless capability by sending data over an GOES high data rate satellite transmitter hooked up to the analyzer's datalogger unit. TCEQ is able to select how frequently they want the data transmitted. It was important that the water monitoring and data collection platforms they chose to deploy offer easy compatibility with wireless systems.
On the Horizon
The first version of a new short-range wireless technology standard called Zigbee was ratified in December 2004. Zigbee is not meant to replace Bluetooth but rather to offer a wireless option for organizations who might want to set up entire systems of low data rate networked sensors that can communicate with each other as well as with a central computer or data collection platform.
Zigbee may be just the thing for industrial water and watershed monitoring alike -- imagine a network of instruments and sensors that can "talk" with each other to automatically assess and respond to water related events. The Zigbee chips are projected to be low-cost and offer major power savings for a lifetime of 5 years. The range of the radios is 50 meters.
Zigbee devices can form star topology or even peer-to-peer local area networks, offering data rates of up to 250 kilobytes per second (kbps). The protocol supports up to 255 devices per network and operates using CSMA-CA (Carrier Sense Multiple Access/Collision Avoidance) channel access across 2.4 GHz and 868/915 MHz frequency bands. CSMA-CA requires a device to broadcast a signal onto the network in order to listen for collision scenarios and to tell other devices not to broadcast, keeping data transmission reliable.
New Dataloggers Simplify Wireless Telemetry Applications
While most of today's dataloggers can be configured for wireless communications, some are already optimized for wireless data communications. These loggers are smaller, cheaper, rugged, and can be easily programmed using Windows-based software for telemetry applications including satellite, radio, telephone, and Ethernet links. Data can even be accessed via a wireless internet connection. The new loggers can upload data to a PC for easy viewing in tabular or graphical format, or for importation into popular spreadsheets such as Microsoft's Excel.
One last factor to consider is ongoing cost. Of the various telemetry options, only cellular and LEO satellite systems involve a communication fee based on time or data transmission or both. Fees range from $30 to $70 per month. GOES Satellite, VHF, UHF, Spread Spectrum, and Bluetooth communications do not incur monthly fees.
With information about the options at hand, only a few steps need to be taken to choose a system: evaluate the water monitoring application; outline data collection needs, making sure to include expansion expectations; and consider costs.
This article originally appeared in the 05/01/2005 issue of Environmental Protection.