Advancing Water Resources Research and Management
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Advancing Water Resources Research and Management |
| AWRA 2000 Spring Specialty Conference |
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| Anchorage, Alaska, April 30 - May 4, 2000 |
ABSTRACT: Continuous data sets are the goal of all hydrologists and meteorologists. As we attempt to expand our data collection effort in extreme climatological environments this challenge is increased. On the North Slope of Alaska we operate 18 remote sites that collect hydrologic and metrological data. The most remote sites have scheduled visits only twice per year. If equipment problems or malfunctions developed, considerable time (and therefore considerable data) could be lost before the problem was discovered. Therefore, a system needed to be developed with the following capabilities: daily data communication access to each site, redundant data communication paths, two way data communication for error checking and problem determination, low power consumption with 12 volt battery source, reasonable initial and operating costs, operate at extreme temperatures and unattended data collection.
The installed system consists of a communication network of computer to modem to telephone to cellular to VHF radio modems to data logger. Computers via telephone modems are able to access base stations (two bases for path redundancy) in Prudhoe Bay that relay data requests to the appropriate site via VHF radio (see Figure 1). Because of distances involved (approximately 200 km [120 miles] from Prudhoe Bay to the limits of the upper Kuparuk River basin), two repeaters were installed on elevated points within the basin.
While this communication system does provide all the capabilities listed above, there are several problems that still cause considerable concern. During the summer our largest problem has been bears. They have destroyed antennas, radios, co-axial cables and equipment enclosures. During the winter our problems are the extreme cold and rhime. The extreme cold reduces the capacity of the batteries and has frozen them. The rhime detunes antennas and reduces the range of the radio communications.
KEY TERMS: remote data communications, VHF radio network, extreme environments, data collection, remote power supply.
Data retrieval from a remote site can be difficult and/or expensive. To accomplish data collection from isolated sites we developed a radiotelemetry network using computer control. Before we installed the system, we completed a comprehensive plan starting with the calculations necessary for determining the locations of repeaters, power of the transmitters, antenna selection, and coax selection. Another important consideration of repeater location is that VHF (Very High Frequency generally ranging from 130 to 174 MHz) communication is mainly line-of-sight so there should not be obstacles between any two sites that are trying to communicate.
Transmit Power
There must be enough transmission power in any radio frequency (RF) link to complete communication.The possible sources of power are the radio and the antennas.Conversely, power is lost both through the cables (coax loss) and over the communication distance (path loss).
The unit of power that is commonly used for RF power calculations is the logarithmic decibel milliwatts (dBm) where 0 dBm represents one milliwatt of power. The signal power must be greater than –95 dBm (approximately 3x10-13 Watts) to have a good radiotelemetry link.Conversion of Watts to dBm can be done with the following formula:
Therefore a 5 Watt radio is producing 36.99 dBm while a 50 Watt radio is outputting 46.99 dBm.
Antenna Gain
The easiest method for improving antenna gain is to make the antenna directional.However, all of our sites use omni-directional antennas for redundant network paths (any site can be used as a repeater for another site if one site malfunctions).Therefore a stacked 5/8 wavelength vertical polarized antenna with a gain of 3 dBd is used.The stated gain of an antenna must use a reference to have any meaning.Most commercial antenna manufactures use a free space half-wave dipole as the gain reference and the gain will be shown with the subscript “d” (for dipole).So the number 3 dBd indicates 3 dB more power in the direction of maximum radiation than a dipole.It is relatively easy to obtain 3dB of omni-directional gain with respect to a vertical half-wave dipole but higher gains require much more complex structures, multiple feedpoints, power dividers, phasing sections, and impedance matching networks.These higher gain antennas are generally much more expensive and also have greater problems with decoupling.
Decoupling prevents the coaxial cable and mounting structure from becoming inadvertent parts of the antenna.A properly designed vertical antenna is said to be “decoupled” from the feed-line and supporting mast. Failure to achieve decoupling can have a huge negative impact on the radiation pattern of the antenna, drastically lower the gain, cause the direction of the maximum radiation to be either raised or lowered from the horizon, and allow RF energy to be guided down the outside of the feedline to the transmitter, where it can be coupled into the electric wiring, telephone wiring, and other electronic equipment. The antenna that has been found to offer the best decoupling in combination with good mechanical design (withstanding severe environmental stresses of high winds, low temperatures, icing, etc.), good radiation pattern and gain, and reasonable cost is the IsoPole 144 (See sketch of VHF Antenna in Figure 1).
Path Loss
Path loss can be defined by the equation:
where, PL = Path Loss (dB),
F = Frequency (MHz),
D = Distance (miles).
In our case we use 152.0125 MHz for the radio transceivers and the greatest distance is 60 miles (100 kilometers). So the worst path loss in the radiotelemetry network is 115.80 dB.
Coaxial Cable Loss
| RG-58A/U | RG-8A/U | LMR-400-UF | |
| 50MHZ | 3.5 | 1.9 | 1.0 |
| 150MHZ | 6.2 | 3.1 | 1.7 |
| 220MHZ | 7.9 | 3.9 | 2.1 |
| 450MHZ | 9.5 | 5.0 | 3.1 |
The coaxial cable chosen was LMR-400-UF with a total length of approximately 30m (transmitter and receiver) so the attenuation is approximately 1.7 dB. While some cables have even lower attenuation, the LMR-400-UF provides the best combination of extreme temperature flexibility, attenuation and price.
Signal Power
Signal power can be defined by the equation:
where, SP = Signal Power (dBm) Power of signal received,
TP = Transmit Power (dBm) Rated output power of transmitting radio,
PL = Path Loss (dB) Power lost over the distance of communication,
AG = Antenna Gain (dB) Total power gained by both the transmit and receive antennas,
CL = Coax Loss (dB) Total power lost through both lengths of cable connecting the transmit and receive radios to the antennas.
Using the values for “worst case” path between sites in our network results:
SP = 46.99 + 6 – 115.8 – 1.7 = -64.51 dBm
So the “worst case” path exceeds the minimum of –95 dBm and thus the network is viable.
Selected Components
In addition to components stated above (antenna and coax), there are several other components to a radiotelemetry network (Figure 1). Each component was chosen considering the extreme environmental conditions in which it operates.
Figure 1. Pictorial representation of the radiotelemetry network installed by
Water and Environmental Research Center staff on the North Slope of Alaska.
The RF Modem is the main communication control device in a radiotelemetry network. The RF Modem enables a base site to communicate with up to 254 different RF stations. The purpose of the RF Modem is to control operation of the radio and provide protection for data integrity. The RF Modem controls the communication sequences, sets data to be transferred into data blocks, creates signatures of data blocks, modulates the radio’s carrier wave, and stores information on communication quality. The Campbell Scientific RF95-XT is a microprocessor controlled device which codes all transmissions for a specific communication path. The device specified is tested for extended temperatures (-55o C).
The purpose of a radio is to transmit and receive the modulated carrier wave. The Alinco Electronics Inc. DR-140PKT provides 50 watts output power using variable reactance frequency modulation. Spurious emissions are -60 dB with a maximum frequency deviation of 5 kHz. The receiver uses superhetrodyne dual conversion providing excellent sensitivity and selectivity. The radio is designed for amateur radio operation (144-148 MHz) so it was modified for the 152.0125 MHz frequency.
The power system (consisting of the battery, solar panel, and solar panel regulator) is being used at the majority of the field sites with excellent results. The power system for the radio telemetry network components is kept separate from the data logger power supply for data integrity reasons (see Figure 2).
Figure 2. Pictorial of the power system used at remote sites
(data logging on right and data transmission on left).
The control module monitors the power system condition, records equipment temperatures and controls the control module remote switch. The unit provides maximum flexibility because all changes are accomplished by software rather than hardware.
The control module remote switch manages the power to the radio thus reducing the total power consumed by the system so that the ten watt solar panel provides ample recharge capabilities. The system draws approximately 0.002 amps in the sleep mode; 0.350 amps in the receive mode; and 10.5 amps in the transmit mode. It is anticipated that a site has 23.75 hours of sleep mode, 14 minutes of receive mode, and 1 minute of transmit mode per day. The contacts of the switches must be able to handle the power of the entire system. The switch has been designed and fabricated by staff at the Water and Environmental Research Center of the University of Alaska Fairbanks.
The radiotelemetry network was installed on the North Slope of Alaska (see Figure 3) during the summer 1997. While there have several months of the entire radiotelemetry network operating successfully, there have been several instances of outages. During the winter, the major causes for outages have been rhime, wind and frozen batteries. Our original VHF antenna detuned as the rhime accumulated. The original antennas had wire loop used for tuning the antenna and adjusting the SWR (standing wave ratio – ratio of what is transmitted to what is reflected back to the transmitter) that would become encrusted with ice and thus the antenna was severely detuned. As the antenna detuned, the transmission and receiving ranges decreased (sometimes to ineffectual ranges). Replacing the original antennas with IsoPole 144 type antennas is solving this problem. Also the wind would adversely affect the high gain directional yaggi antennas used on the cell telephones. The wind varies the signal and introduces noise so that the modems drop the connection. On some windy days it is impossible to transmit data. Most of the frozen batteries were the result of solar panel wires being damaged by animals.
The major causes of summer outages were due to bears destroying sites. The bears seemed to be most interested in the equipment enclosures (tarp covered ice chests that had never been used for food storage). A total of five enclosures were completely destroyed during the summer of 1998 (lids ripped off, sides chewed through, etc.). Bears also chewed through coaxial cables, tipped over antennas, and pulled wires out of the instruments. Smaller animals enjoyed chewing on wires at selected sites (wire insulation was found in ground squirrel nests). Birds also damaged the decoupling radials on several of the original antennas that were installed.
The equipment enclosures have been exchanged with stronger boxes that are attached the ground with four 1.3m metal stakes holding it down, wires have been shortened and raised, heavier antenna tripods have been installed, the IsoPole antennas have been installed, and the scheduled “on time” of the communication system has been varied to allow for more “wind” days. However, there are still equipment outages that will not be explained until the next trip to the sites.
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