When radar data stop arriving, the problem is often because of the power or data connection, but well-maintained, modern radars are rather reliable pieces of equipment.
Weather radars constantly scan the atmosphere, providing data and colorful images of storms and their development. The radar images help the forecaster to warn about dangerous phenomena and support decision making of the end users in all aspects sensitive to rain and snow. But every now and then, the screen is black and no data are available. To keep periods of unavailability as short as possible, owners of the radars carry out preventive maintenance, constantly monitor the quality of the data, and, when something happens, they make corrections and repairs as quickly as possible.
The European radar project Operational Programme for the Exchange of Weather Radar Information (OPERA; Huuskonen et al. 2014) asked its members to share their experiences of radar maintenance and availability. Technicians from 21 countries (out of 28 member countries that have radars) responded. These are the people responsible for 148 radars throughout Europe. To our knowledge this is the first time such a large survey has been carried out. The World Meteorological Organization (WMO) conducted a survey among radar operators in 2009, when preparing the global weather radar database, but the maintenance-related questions were rather general (Sireci 2011). Radar maintenance and the causes for radar outages are sometimes mentioned in scientific literature, but most of this is anecdotal (e.g., Panziera and Germann 2010). Radar manufacturers give maintenance recommendations, but not everyone follows them verbatim. A new generation of radars provides a good opportunity to reconsider maintenance cycles and to learn from neighbors’ experiences. The aim of this paper is to share these experiences and to make them more widely available to the meteorological community.
Even when preventive maintenance is carefully carried out, experience has shown that sometimes surprising problems appear out of the blue. As an example of such a case, an amazing story came from Norway, where wintertime thunderstorms destroyed the radome (the protective dome over the antenna) of a remotely located radar. Photographs, a likely explanation of the course of events, and the solution found to protect the radar from future lightning strikes are described in detail.
RADAR NETWORKS OF THE EUROPEAN WEATHER SERVICES.
European weather services operate varying sizes of weather radar networks. Almost half of OPERA members have one to three radars. At the other end, France has 30, Germany has 20, and the United Kingdom and Serbia have 15 radars each (Italy, with its 24 radars, is not an OPERA member).
Also, the geographical extent of these radar networks varies significantly. The two radars operated in the Netherlands are 126 km from each other, while the most remote radar in the Azores, Portugal, is 1,580 km from the operational headquarters of the institute in Lisbon.
In smaller weather services, in addition to specific radar technicians, usually all technical staff know basic first-line technical support for the radar. For radars located far from the main office of the weather service in charge of them, local people living near the radar are sometimes used to help in simple tasks supervised by the professional.
Contractors are often engaged for corrective and preventive maintenance, but even in such cases many institutes follow the status of their radars by themselves [e.g., with built-in test equipment (BITE)].
MAIN FINDINGS OF THE SURVEY.
The initiative for the survey came from the OPERA Expert Team, which is an organization of radar experts of the OPERA member weather services. Many OPERA member countries have upgraded their radar networks recently. Members of the Expert Team were curious if other countries have changed the maintenance cycle at the time of the upgrade and how closely they follow the manufacturer’s instructions. It was mentioned that manufacturers make recommendations theoretically, and until now nobody gathered the maintenance technicians’ experience.
According to the survey, practically all OPERA members have their own unique maintenance procedures. Five respondents have outsourced maintenance, and two are buying maintenance case by case. OPERA radars are typically from 1990 to 2016, with a few from the 1980s still in use. The signal processor is the most common upgrade.
The target availability of radar data at the OPERA central hub is 95%. All but one mentioned they reach this figure at their national level. This target allows a radar to be unavailable 36 hours a month. Sixteen respondents replied they are over the 98% threshold, which means the radars are down less than 15 hours a month.
The most common reasons for radar being not available are external: problems with electricity supply or telecommunications. Among radar-related problems, transmitter and pedestal electronics take the lead. The longest periods of unavailability come from waiting for spare parts, followed again by external electricity and data connection problems.
At the time of the survey, none of the members had yet upgraded their entire network to dual polarization. Hence, it was not possible to extract different results for single and dual polarization in this survey.
As this was the first survey of its kind, no real trends can be extracted from the results. It may be interesting to repeat the survey after a couple of years, when all members have more experience of their new radar generation. Learning from this experience, we would probably be more careful in ensuring that everybody understands the questions the same way—now there is some uncertainty, which is the main reason the quantitative results are not discussed more in this article. A summary of the survey is given in Table 1. Readers interested in the details are encouraged to look at the full report in the electronic supplement (http://dx.doi.org/10.1175/BAMS-16-0095.2).
Summary of the survey. In this summary, only selected questions and the most popular options are shown. The full survey is available in the electronic supplement. Note that weather services that have radars from several decades filled the survey for each “generation.”
MAINTENANCE AND MONITORING OF RADAR EQUIPMENT.
Preventive maintenance.
If it is essential to keep a device running all the time (like airplanes and to some extent radars), maintenance should be carried out not only when parts are broken but also on a regular basis (i.e., preventive maintenance). An additional advantage of preventive maintenance is that it can be planned, generally with the weather in mind, in order to minimize the risk of downtime during important weather events.
As noted by Meischner et al. (1997), Serafin and Wilson (2000), and others, effective weather radar operation and exploitation begins with a well-calibrated radar. Keeping the radar calibrated is as important as keeping the radar running and ensuring that the radar produces the best-quality data. In calibration, the process of converting derived parameters (such as reflectivity) from measured quantities (power of backscattered microwaves) is adjusted to match an external reference, typically provided by a signal generator. For dual-polarization radars, the balance of the two signal channels is also monitored. Aspects of calibration of single- and dual-polarization radars are discussed, for example, in Chandrasekar et al. (2015).
Some decades ago, each radar still had its own operational staff. In 2016, most of the European weather services do not have permanent staff at the radar site, particularly in remote locations. BITE is often used to monitor a radar’s status remotely, but there are still some inspections for which a technician must visit the radar site. The maintenance schedules for OPERA members vary significantly from country to country. Some OPERA members plan their procedures so that a maintenance visit to the radar only occurs every 6 or 12 months. However, unplanned visits are needed so often that every radar is visited at least twice a year. The most common reasons for planning these visits are to carry out receiver calibration, to check the transmitter power, and to monitor the tower infrastructures (such as air conditioning and power systems). Some OPERA members check antenna pointing at the radar site at least once per month, while other members remotely check antenna pointing daily. There are those who carry out receiver calibration up to twice per week, while some do not calibrate their receiver even yearly. Some members perform monthly on-site checks of the power and quarterly checks of the pulse shape.
In addition to the radar equipment, the radome is checked for leaks, repaired when needed, and only occasionally washed and waxed. Wax protects the radome from UV radiation and keeps the surface hydrophobic so that rain falling on the radome does not form a film of water, which in turn causes attenuation (Kurri and Huuskonen 2008). It is interesting to note that only 1 of the 21 countries that replied to the survey actually waxes their radomes regularly—even though some mentioned they have been planning to do so, and some of them have waxed it in the past.
Manufacturers publish the expected lifetimes (mean time between failures) of key components. As it is very rare to change a component only because its expected lifetime is approaching its end, many radar operators keep a stock of spare parts, bearing in mind that components can fail at any time. Real-world experience of the durability of different components is exchanged between radar operators. As a radar gets older and the frequency of component failure increases, an operator may start to plan for a major upgrade or replacement. If the decision is to replace an old radar, the opportunity to incorporate new technology such as dual polarization is also considered.
Monitoring.
A radar can be running without interruption but producing bad-quality data. To prevent this, radar operators monitor the quality of the data. Monitoring can also give early indications of component degradation, allowing radar operators to replace those parts before they actually break (e.g., Beekhuis and Leijnse 2012). There are several software-based monitoring methods available that allow the radar operator to perform monitoring without interrupting radar operation, which is very important for most national weather services. These methods include, for example, using the sun for receiver-bias monitoring and antenna-pointing monitoring (Huuskonen and Holleman 2007; Holleman et al. 2010; Gabella et al. 2016), using fixed clutter targets (Silberstein et al. 2008), and using a birdbath scan (i.e., antenna pointed vertically; see, e.g., Gorgucci et al. 1999) to determine offsets in calibration of differential reflectivity (ZDR), as well as calculating daily averages of several polarimetric variables to monitor the stability of the system (Figueras i Ventura et al. 2012).
When radars are operated as a network, the reflectivity levels in the same measurement volume of the neighboring radars can be used as references for a pair comparison (Seo et al. 2013; Vaccarono et al. 2016; Vukovic et al. 2014). Vertically pointing radars, disdrometers, and rain gauges have been suggested for providing a reference that can be used for radar monitoring. While providing very valuable additional data, care should be taken in using these instruments for monitoring purposes because of differences in measurement volume sizes, vertical variation of precipitation (disdrometers and gauges), and uncertainty introduced by using radar reflectivity factor–rain rate relations (gauges). We therefore suggest applying first more direct methods (e.g., radar-to-radar or radar-to-sun comparisons) and only after that applying these indirect methods if needed.
Most radar systems include a BITE system, which is used to monitor various factors of the radar system and infrastructure. Typical features to be monitored with a BITE include, among others, the magnetron peak current, the waveguide pressurization, the system temperature, whether the transmitter is radiating, and various operating voltages and interlocks (e.g., at the doors). These BITE alarms can then lead to selected automatized actions such as a restart, security alerts, or an increase in the air-conditioning level.
Corrective maintenance.
Most modern weather radars are designed so that replacing parts is not tedious, but it may be a logistical challenge to get the spare parts to the site in reasonable time. Many radar operators have their own spare parts, and some are members of a spare part pool. This is an important decision; the value of spare parts kept in stock in a weather service running a 10-radar network can be on the order of $1 million. In the OPERA survey, “waiting for spare parts” was mentioned as the most important reason for long radar outages. The members mentioned pedestal electronics and mechanics, rotary joints, and air conditioning as the components that caused the most frequent breaks in the radar service.
IT TAKES MORE THAN JUST A RADAR TO PRODUCE RADAR DATA.
A typical C-band weather radar consumes 1–2 kW of power, and the same amount of power is consumed by auxiliary equipment such as air conditioning (heating or cooling). To get good visibility over terrain, many radars are located on hilltops and mountains, at some distance from power lines. In most European countries, dedicated powerline extensions have been built to the radar site, while some radars rely on diesel generators. Most radars are equipped with a “no-break system,” including an uninterruptible power supply (UPS) unit, which provides electricity for sufficient time to stop the radar and park the antenna safely or to keep it running until the spare power system is started (e.g., the emergency power generator). Even still, the loss of the electricity supply is reported to be among the most frequent reasons for breaks in radar operation and is also among the reasons for the longest down times.
Another important element in the radar data production chain is a stable data connection. Similar to losses in the electricity power supply, the loss of the data connection is also one of the most frequent reasons given in the survey for interruptions to radar operations and among the top reasons for the longest periods of radar unavailability. A radar can produce several gigabytes of data every day. With the introduction of dual polarization, the dataflow has increased owing to the availability of more variables from the signal processor. Furthermore, because of the ever-increasing availability of processing power and data storage space, there is a tendency to move processing from the local radar signal processor at the radar site to a centralized radar product processing system. This further adds to the increase in the data volume. Hence, the requirements on bandwidth for data transfer between the radar and the central office have increased. Because of the importance of a stable data connection (a perfectly running radar without a data connection is of very limited value), there are many precautions in place to guarantee continuous operation. For example, the Coruche and Loulé radars in Portugal have a microwave link with two channels, and both weather radars in the Czech Republic have two independent microwave links (a main and a backup link). Where links have to work over longer distance, a few interruptions per year are typical. The duration of interruptions ranges from just a few minutes to a maximum of a few tens of minutes in the case of severe convective storms that hit the links. It was found that pointing the links in different directions reduces the probability that both links are influenced at the same time by the same storm.
The quality of certain data communication methods, such as microwave links, is dependent on the weather conditions. Heavy rain can cause attenuation, and temperature and humidity changes will change the direction of the propagating signal of the microwave link; in extreme cases heavy precipitation can lead to a total signal loss (e.g., Hogg 1968), right when the radar data are most needed. Unfortunately, this is not the only effect the weather can have on the availability of radar data. Lightning (which also occurs in weather for which radar data are highly desirable) can lead to power or communication line cuts, as well as damage to the radar itself in case of a direct strike.
NORWEGIAN RADAR HIT BY LIGHTNING.
Laws and regulations demand that all radars, as well as buildings, are protected from lightning hits with a lightning conductor. Typically, the protection consists of one or more metal rods installed over the highest peak of the building. Almost every year somewhere in the world a weather radar is damaged by lightning. However, the case of the Andøya radar is quite an unusual one.
The Andøya radar (Fig. 1) in Norway is located on a mountain on an island at a latitude of 69°N, north of the polar circle. It is one of the most remote radar locations in the OPERA network.
The Andøya radar with its broken radome caused by lightning strikes during 8 Mar 2015. The radar is located in a very harsh environment.
Citation: Bulletin of the American Meteorological Society 98, 9; 10.1175/BAMS-D-16-0095.1
During the winter of 2014/15, the Andøya radar was exposed to several lightning strikes. The lightning protection had been inspected and improved upon in 2014, and the grounding was concluded to be as good as was possible given the rocky soil conditions at the radar site.
On 8 March 2015, the radar was hit by a very powerful strike. The Norwegian Meteorological Institute received a photo showing that three of the radome panels were severely damaged. The entire radome was weakened considerably. During strong winds on the following days, the majority of the radome was blown off the radar entirely.
The radar itself, or its antenna, did not experience any major damage. The orientation of the antenna was away from the hole caused by the strike in the radome. Luckily, only the counterweights were facing toward the hole because the antenna was standing still owing to ongoing repairs after earlier strikes. This probably saved the antenna from more severe damage.
Why is it that the electrical current from the lightning destroyed the radome, instead of being conducted to Earth along the lightning conductor? One plausible explanation for the extensive damage is that water was contained in the radome panel joints. In heavy weather, seawater is sprayed over the entire island. This salty seawater conducts electrical current well, probably as well as the lightning down-rod conductors, or possibly it may conduct it even better because of the large surface of the radome. Because of the transient nature of the lightning, currents tend to travel down on the conductor surfaces: the so-called skin effect (Bazelyan and Raizer 2000). Lightning currents occur in the kiloampere (kA) range and contain several hundred megajoules of energy. Such huge amounts of energy cause the water contained in the panel joints to evaporate instantaneously. This sudden evaporation creates an explosion-like effect on the radome panels, and the panels are delaminated and torn apart.
Since the case of the exploding radome, the Norwegian Weather Service has further improved its lightning protection on the Andøya radar. The solution implemented consists of four lightning rods, each protruding 1.5 m above the surface of the radome (Fig. 2). Each rod is tied to its own down conductor, mounted on the outside of the radar tower. This is to keep the energy from lightning strikes on the outside of the radar tower. Each down conductor is tied in to the common grounding system for the whole radar site.
The Andøya radar with its new lightning protection arrangements.
Citation: Bulletin of the American Meteorological Society 98, 9; 10.1175/BAMS-D-16-0095.1
This shows the importance of proper infrastructure (in this case lightning protection) that is well maintained. Furthermore, not only does regular maintenance of the radome limit the attenuation by wetting of the radome, but sealing its seams may also prevent water from going into radome panel joints, greatly reducing the probability of such devastating damage by lightning.
There had been some concerns that the new, rather massive lightning protection arrangements would cause shadowing of the radar signals in direction of the rods, causing four narrow blind sectors in the radar images. However, the rods are in the near field of the antenna, and overall the effect in the operational products seems to be very small. It is, however, visible in the long-term sums. The Andøya radar is a single-polarization radar, and therefore the impacts to dual-polarization measurements (studied, for example, in France; Friedrich et al. 2007) are avoided.
DISCUSSION.
Where radar maintenance takes place only a few times a year, experience and expertise are gained slowly. In many cases, a deeper understanding is achieved by only one person, which then forms the risk of a single point of failure. Buying and installing a new radar is a big effort in terms of budget and manpower. As a result, radars that are part of larger national networks may have been purchased in different years, and there are different models of radars in one national network, sometimes from several manufacturers. This is especially true during the recent decade, when most European countries have been upgrading or replacing their radars. As a consequence, they have often had to operate heterogeneous radar networks. This in turn leads to a need for many noninterchangeable spare parts and the expertise to maintain different radar types in one country. One possible solution to this issue (which we did not encounter in our survey) would be for different countries operating the same type of radars to join forces to maintain a common spare part pool. Exchange of knowledge, and sometimes even spare parts, is already happening within the community.
It is clear that within the European operational weather radar community each member has its own maintenance procedures, which are quite diverse. However, it is also true that many common requirements apply, providing the opportunity for closer cooperation and discussion on operational radar maintenance practices in Europe and beyond. Our analysis of the OPERA radar maintenance survey has highlighted several take-home messages for radar operators at large:
It is extremely important to take infrastructure, maintenance, and monitoring into account when purchasing a new radar. It is recommended that these aspects are explicitly budgeted. This will avoid a lot of frustration of having a radar network but not being able to use it to its full potential.
Monitor the radar constantly, invest in infrastructure, recruit wisely, and work together to exchange information, as happens in the OPERA community.
Annual operative costs of a radar network are typically on the order of 5%–10% of the radar purchase price. During the lifetime of a system (typically 10–20 years) the operator can hence pay as much for the running costs as for the hardware purchase. Despite this, the policies and practices of radar maintenance have not yet been discussed very widely in the international radar community.
ROTARY JOINTS AND BIRDBATH SCANS? SOME RADAR TECHNICAL TERMINOLOGY EXPLAINED.
When a radar antenna is pointing straight up, the antenna looks like a birdbath. Raindrops falling toward it are rather symmetrical when looked at from below. Rotating the antenna and counting an average of the signal ensures that we have a truly symmetrical input. This is used for calibration of the radar parameter related to the shape of scattering particles (ZDR). This technique is called the birdbath method.
The radar transmits strong signals and receives very weak signals. Instead of a cable, a hollow metal tube called a waveguide is used between the transmitter, receiver, and antenna. To bring the rigid waveguide through the rotating antenna dish, rotary joints are needed.
ACKNOWLEDGMENTS
The authors thank members of the OPERA Expert Team, especially Asko Huuskonen, Kieran Commins, and Sarah Gallagher, for their support in preparation of this manuscript. Originally, the idea to conduct such a survey came from Pierre Tabary and his team. We extend special thanks to the maintenance technicians who carefully answered the survey, as well as the three anonymous reviewers whose questions helped us to clarify the structure of the article.
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