GPS/GNSS permanent stations are typically powered either through a solar (photovoltaic) system, or a connection to an AC power feed, and use a varying number of batteries. Stations relying on solar power typically contain between 4 and 30 batteries, whereas those using AC power have fewer batteries to maintain constant power during brief power outages. Both systems supply continuous 12 volt DC power to a GPS receiver, communications device, and meteorological equipment. General system loads will vary from 4 to 24 Watts. Solar powered stations with power needs greater than 9 Watts are generally split into two DC systems, so that the communications equipment power draw will not affect the GPS power system.
Size and quantity of the power system components (enclosures, batteries and solar panels) will vary based on total load and location. There are generally four types of system configurations. Configuration usage is determined by cross referencing desired load with NREL minimum solar radiation data. The system break down is as follows:
The above systems utilize a combination of solar panels, batteries, backpanel (consisting of AC/DC charge controllers, DIN rails, and circuit breakers), lightning protection, RF surge protection, and associated wiring to maintain sufficient continual voltage to the installed electronics. The enclosures that house the electronics and internal power system components are also an important part of the PBO station. The vendors and specific models of these components may vary to a slight degree throughout the duration of major projects or with what is currently available, but generally these components are interchangeable with one another.
Using solar panels to supply input voltage to DC systems has proven the most effective and reliable source of power. As opposed to wind turbines, solar panels are usually less affected by wind loads and icing, and are a better choice for remote installations. Solar panels are wired in parallel and connected to the backpanel via a solar isolation block, which incorporates a lightning protection device which is discussed below.
Lower 48 States in the US: For use within the continuous lower 48 states of the US, the initial panel of choice has been the Siemens SP-75, which output 75w and was of a standard frame size. Tamper-resistant panel frames are built to dimension for the panels. After Siemens discontinued the manufacture of these panels, other vendors were used that produced panels of this standard frame size, including BP, GE, and others. The output of the panels made by the various vendors varied from 72-80w. Availability of these panels can fluctuate, making large orders important when available.
Alaska: Originally for the PBO GPS Network in the Alaska, the panel of choice was a 60w shatter resistant panel, recommended by other network operators already working in the state. The shatterproof design was thought to be superior to conventional panels due to the extremely high wind loads, combined with heavy icing, that apply large forces on the face of the panel. Unfortunately, these panels did not perform as expected in the field. Problems arose with the surface of the panel clouding, greatly reducing the solar radiation that actually made it to the photovoltaic cells, in addition to the coating on the panel face degrading which allowed water and particles to enter the cells and damage them. In addition, the shatter resistant panels were ironically no more shatter proof than conventional panels.
Sharp 80w panels are deployed in Alaska for PBO, replacing the 60w panels. The DC stations in Alaska do not generally utilize the PBO tamper resistant panel mount, and so conforming to the standard panel frame size is not critical. The Sharp 80w panels are somewhat narrower than the SP-75 standard used in the lower 48, and are attached to huts and swingsets with a custom mounting system consisting of angle aluminum bolted to the panel and then to the hut, which securely holds them in the face of high winds.
The standard specification used by PBO for batteries at both DC and AC stations is the 12V, 110 amp-hour deep cycle gel cell battery, specifically made for slow charge/discharge situations. These batteries perform well across a broad temperature range, and do not spill corrosive contents if a cell is punctured. The batteries in a given system are wired in parallel and connected to the backpanel. The terminals of these batteries vary according to customer specifications, and come in flag, stud, and threaded stud configurations. For ease of installation in space restricted enclosures, the threaded stud terminals have become the choice of PBO. The type 1,2, and 3 systems referenced above utilize enclosures that are built to specifications that snugly house between 2-4 batteries in a lower compartment, isolated from the upper electronics compartment. Freestanding enclosures built to house solely batteries are also deployed at PBO stations, and hold either 8 or 16 additional batteries for high load applications. In Alaska, the fiberglass huts hold many more batteries, up to 30, which allows for continuous power throughout the sunlight deprived winter. This large number of batteries is often divided into separate parallel strings, both wired to the battery blocks on the DIN rail.
UNAVCO's current procedure to replace all batteries at a station every 5 years, generally before they start to seriously degrade in performance. It should be noted that some stations in existing networks have been observed to operate close to 10 years on a set of properly installed batteries.
DC and AC charge controllers regulate the power supplied to the batteries, shutting off the charging when the batteries reach full capacity, and then resuming charging when the batteries begin to discharge (if there is available power via solar panels in DC systems). The charge controllers themselves draw a small amount of power (approx. 6mA current), a factor not usually included in system selection.
The charge controller used in PBO DC powered stations is the Flexcharge NC30L12. This unit is robust and is tolerant of a wide range of atmospheric conditions. It charges batteries to a maximum voltage of 14.25v, and resumes charging when batteries drop to 13.70v. The unit also features a low voltage disconnect (LVD) that eliminates power to the system load circuit when the batteries drop below 10.92v, which reduces the possibility that batteries are drained completely dry, a scenario that generally results in severely reducing the battery lifespan. The charge controller is mounted to the backpanel, and wired to the circuit breakers.
For AC systems, a larger charge controller is installed to convert the 120v AC current into 12v DC current for battery charging. The charge controller selected by PBO for use is the Iota DLS-15, which provides a small float-charge to keep the batteries near optimum charge levels when there is constant AC power. In the event of an AC power outage that results in battery discharge, the unit will charge the batteries more rapidly when power is restored.
On the PBO backpanel, the power distribution is accomplished through a compact set of DIN rail connections that are separated into 4 blocks; solar power input, batteries, load, and ground (negative). The three positive blocks each has an associated circuit load breaker that allows the components to be turned off separately, and additionally prevents spikes from propagating through the system. The solar and battery breakers are 50A, while the load is a smaller 15A breaker.
To protect the DC power system from surges due to atmospheric static discharge via the solar panels, a lightning protection device is installed on the exterior of the enclosure. The protection device chosen by PBO is the Delta LA-302 lightning arrestor. It is wired in common with the solar panel input to the solar isolation block. It is rated to clamp a 50,000A surge in 25 milliseconds, and will clamp up to a maximum of 100,000A surge but will be destroyed in the process. It should be noted that no lightning protection can truly guard against a direct lightning strike, and in such an event, it is likely that some or all of the electronics in the enclosure will be destroyed.
To limit power surges traveling down the various coaxial antenna cables that are connected to the GPS chokering antenna and communications antennas, in-line surge protectors are installed inside the enclosure, mounted to the bottom of the top equipment shelf and grounded via the enclosure itself. PBO uses the Huber-Suhner 3402 series of RF surge protectors, which protect on frequencies up to 2.5 GHz, and can stop a surge of up to 30kA a single time, or multiple surges of up to 20kA. The mechanism at work is a small gas filled discharge tube inside the Huber Suhner device, which sparks excess current over to ground (leveling charge in both conductors of the coaxial antenna cables) when a surge travels down the cable. After the surge has discharged, the gas capsule reverts to a near infinite resistance, once again separating the two conductive elements of the coaxial cable.
The various systems described above utilize different equipment enclosures based primarily on the number and size of the electronic devices and the number of batteries to be installed. They are designed to be weatherproof, to hold one of two different backpanels, and to utilize a standard key.
Lower 48 States in the US: System types 1, 2, and 3 are generally found at most stations in the lower 48 states and hold between 1 and 20 batteries in total. These enclosures are constructed from aluminum, and then powder coated with a durable white finish. On the door of each enclosure is a decal that presents information about the GPS station, as well as contact information for those wishing to learn more about PBO. The type 1 and 2 system enclosures are divided into separate compartments, either two or three shelves. The top shelf is standard, and contains the AC or DC backpanel and holds the GPS receiver and communications equipment, and is isolated from the lower shelves via weather resistant strain relief pass-throughs. The lower shelves hold the batteries and also have knock-out ports for the entry of antenna cables and solar panel leads, the solar isolation block, and the gas capsule side of the Huber Suhner RF surge protection devices. Affixed to the exterior of the lower compartment are the lightning protection unit and the grounding lug, which is connected via copper wire to the ground rod. These enclosures are mounted to 2-4” pipe masts via tamper resistant brackets, and have a door with 2 locks, that can be lifted off its hinges to allow engineers to work on the enclosure components unhindered.
The type 3 systems can include a type 1 or 2 enclosure, as well as a separate stand alone battery chest that holds either 8 or 16 additional batteries. These battery chests are positioned on the ground near the primary enclosure, and are wired to the battery portion of the DIN rail via 10 gauge wires routed through a weather proof conduit.
Alaska: The fiberglass huts used to house the large number of batteries and Alaska style backpanel are specifically designed for use in Alaska and other low solar radiation locations, such as Mt Saint Helens. The hut is weather resistant and is more durable than the aluminum enclosures. Three of the Sharp 80w solar panels can be mounted to the back portion of the hut, which allows the engineers to fully assemble the solar power system of the hut prior to emplacement via helicopter sling line. Once in position, the hut is loaded with up to 30 batteries, and the electronics are installed. The hut is generally secured to the ground with all-thread rods driven through the corners of the bottom flange and into the ground, as well as with high tension guy wires secured to rock bolts or duck-bill anchors. Additionally, extra solar panels can be mounted to a free standing aluminum frame, known as a swingset, to give the station extra power in low sunlight conditions. However, these swingsets have repeatedly failed in high wind locations, which was one factor that precipitated the switch from 60w to 80w solar panels mounted directly to the hut.
For more information about power, please see the UNAVCO Knowledge Base on GNSS Station Power.
For more information about enclosures, please see the UNAVCO Knowledge Base on GNSS Enclosures.
Last modified: Friday, 01-Apr-2016 23:56:00 UTC