How does hf radar work

One of those antennas is a transmit antenna, and one is a receive antenna. On the left is the transmit antenna, and on the right is the receive antenna. The transmit antenna sends out a 5 MHz signal out over the surface of the ocean, and the receive antenna collects the signal that is bounced back by the ocean waves.

In case of long paths e. A continuity test should be always performed after connector installation to ensure proper insulation between shield and central pin. The cable and connector role is often underestimated, while they represent a crucial component of the system and may compromise the quality of the signal. Since they have a low impact on the total cost of HF radar systems, they should match with high quality standards.

Moisture penetration inside the cables is the main reason for efficiency loss in time, and should be prevented under all circumstances by using specialized cables some plastic sheaths are more effective than others. Furthermore, great care should be taken by protecting the connectors from direct exposure, using specific greases and self-fusing insulation tape when connecting them.

In general, all cables should be handled with care, not being stressed, twisted or narrow bent. Cables already used in the field should be avoided for new installations.

If they are the only option, they must be carefully checked with visual inspection and specific instruments as multimeters and cable fault finders for first insight, and with more advanced instruments for complete testing of electrical characteristics. Within the theoretical limits given by the chosen operating frequency, the HF radar performances can be significantly reduced in case of a bad tuning of the transmit antenna, i. For this reason, it is always recommended to check each antenna after installation in the field.

A VSWR value below 2 is recommended, while a greater value means that reflected power is coming back to the electronic devices which, besides resulting in lower efficiency, can lead to damage of the electronic equipment. If no tuning possibilities are available, an antenna tuner can be used to minimize the reflected power.

Note that the SeaSonde transmit antenna is tuned by the manufacturer prior to delivery and no tuning by operators is foreseen. However, if required, an approximate and limited tuning can be performed by changing the length of the antenna whips by trimming, and by adding or removing the capacity top-hat in the low-frequency band, or optimally matching the length of the transmission line.

WERA systems are pre-tuned by the manufacturer but can be further fine-tuned in the field. After the tuning of each antenna element of the transmit and receive array, an internal calibration of the entire systems is needed, which takes into account the antenna tuning and cable length for each element. For adjusting the level of the power amplifier, the output power of the transmit antenna should fulfill the following requirements:.

For FMiCW systems, the blank delay should be used to adjust the saturation in the receiver. The analysis of the radar signals to resolve the azimuth needs a good knowledge of the receiving antenna specificities.

Any HFR system is subject to the influence of the nearby environment, and should be always calibrated once in place Kohut and Glenn, ; Yang et al. Calibration can be performed with different techniques, among which two are the most common: internal calibration method, commonly applied to BF systems, and far-field calibration method, also known within the HFR community as antenna pattern measurement APM , usually applied to DF configurations.

This is particularly important for compact DF antenna like SeaSonde, where the electromagnetic environment can affect phase and gain more significantly than in case of large antenna array.

For BF configurations, the analytical antenna array response function or antenna manifold is used. It is computed by solving the electromagnetic equations for the waves propagating to the array using the precise positions less than 0. Phase and gain calibration is required and is performed internally.

Furthermore, dedicated software for the BF systems can compensate variations of the antenna characteristic caused by environmental or technical conditions Helzel and Kniephoff, For WERA systems, all antenna parameters are automatically monitored, at least once per hour and a warning is automatically generated if any parameter reaches a critical value, which might cause the automatic calibration procedure to fail. In such a case a preventive maintenance should be carried out. In some extreme conditions an APM can be helpful to improve the quality of the BF, in particular when being utilized for wave measurement applications.

For DF systems APM is mandatory and can be performed, following a well established procedure, using a boat equipped with a transponder provided by HFR system manufacturer.

The former uses Doppler echoes from vessels cruising in the area, which in many cases lead to a return signal similar to that of a transponder Emery et al. The latter employs small aerial drones Washburn et al. Flying drones speed up the APM procedure but in other hands are subject to strict regulations in order to be operated. Correct network setup is crucial for data retrieval and remote management, which in turn make the HF radar operations more sustainable as the operators do not need to travel often to the sites for data backup and system monitoring.

With any kind of network connection at the HF radar site Ethernet, mobile, satellite etc. When setting up a new HF radar system, operators are requested to insert specific configuration parameters such as the identification of the system, the geographical information, the operating frequency and bandwidth, the processing options. One important step in HF radars signal processing chain is the detection, inside the power spectra maps, of the regions containing the first order bragg peaks, i.

Such detection, better known as first order limits FOLs determination, is usually performed by the HF radar acquisition and processing software with automatic algorithms that look for the Doppler peaks at one channel usually the omnidirectional vertical loop for SeaSonde systems and must be tuned by the operator during the initial period of acquisition through the selection of some parameters and visual inspection of the results.

Improper FOLs identification may introduce large errors or missing values of sea current velocity. Depending on sea state, interferences, presence of other targets e. This is particularly true in those areas where a high variability in currents and waves regimes leads to an overlap of first and second order Bragg peaks with high variability in time and along the HF radar range.

In these circumstances, operators should carefully monitor the performance of their initial setup and optimize it for different sea conditions. When this is not possible, different subset of data should be reprocessed each one with a specific set of processing parameters.

These considerations apply especially for DF systems, where the power spectra contain the contributions from all the azimuthal directions. In some cases the application of advanced methods for FOLs detection can improve the quality of the data Kirincich, As all in situ instrumentation, also HF radar suffers from the normal deterioration, in particular - but not only — of the outdoor components.

Besides that, major damages have been experienced due to severe weather events inducing storm surge or lightning, which often lead to electrical damages or antennae breakage. Another not to be underestimated source of damage results from animals in particular to cables or human vandalism.

On the other hand, issues may arise from specific hardware or software failures without any external cause, such as data corruption due to storage failure, or software modules and applications unexpectedly freezing.

For all these reasons, even if HF radar systems can be considered fully automated acquisition platforms, regular monitoring is necessary to ensure continuous and correct operation. The first, and more frequent, kind of maintenance can be performed remotely.

With an internet connection enabled, the system status as well as its data acquisition process and data themselves can be easily diagnosed and verified by means of web tools, email alerts, screen sharing applications, even on a daily basis.

As a complement, on-site inspection is unavoidable and is recommended on a regular biannual basis, in order to confirm the good status of the equipment and prevent issues, but also to perform scheduled actions such as data backup on external disk, UPS battery replacement.

Additional specific on-site inspections are recommended after a severe weather event e. According to Cook et al. Results of remote and on-site checkings should be included in periodic reports, to help the operator to keep track of maintenance history. Once the HF radar station is deployed, its proper functioning can be assessed by the diagnostic reporting on its performance. HF radar systems, as discussed in the previous sections in this paper, can be implemented with different hardware and software design, and different signal processing techniques.

For this reason, they can provide different performance indicators. However, all HF radar systems allow recording common operating parameters like temperature and voltage of subcomponents e. As an example, any significant permanent variation of the reflected power is most likely related to a hardware failure e.

Signal to Noise Ratio is usually not stable because of environmental conditions and periodically fluctuates with repeated daily pattern, but any significant permanent change in this pattern or in its variation range should be investigated. Much of this information is reported within the radial file itself, but diagnostic files help to aggregate and show them, through DiagDisplay application, in a bigger picture.

Plots of diagnostic parameters over custom time windows can be also exposed in a web page served by the RadialWebServer. Another application, the RadialSiteReporter, is able to perform a scheduled detailed check of all the software and hardware components and to produce and send to the operator detailed email alerts with highly customizable rules. These alerts are also shown by the RadialWebServer, plus are added to an alert log. The WERA software provides hardware and software diagnostics on all levels of data acquisition or data processing.

High priority messages will be sent out immediately, lower priority messages only once per day or per week. An anti-spam filter makes sure that no messages within some hours are send out more than once. In case of a detected problem, the message will contain possible solutions for the problem. Plots of time series files for different kinds of parameters, like internal voltages, temperatures, forward and reverse transmit power, hard disk usage, antenna performance, can be accessed by the operator for further analysis and troubleshooting.

Alerts should be thrown when these ranges are exceeded because of risk of hardware or operation failures. Table 5. Table 6. The main objective of this section is to provide general concepts and definitions in order to formalize the data processing and management steps regardless of the HF Radar system adopted, with focus on Near Real Time data flow suitable for operational services.

HF radar data is in situ gridded data sampled at high-frequency that has to be managed according to its peculiarity and complexity, which derives from the fact that it includes diverse data types radials and totals and with different kinds of native formats from different HF radar systems. Different steps in the data life cycle as schematized in Figure 3 have been defined following the processing levels specified in Table 7.

The first step consists of raw electromagnetic data acquisition and extraction of radial components of the ocean currents performed by each HFR system. The output, the so-called radial files, must be transferred to a processing station responsible for combination of overlapping radial vectors measured by two or more HF radar systems.

The processing station is a generic name for the IT infrastructure that will perform the combination, either at the data-provider level or as third-party competence center e. According to the definition of data levels, this flow is summarized in the following scheme:. HF radar manufacturers provide software suites for data acquisition and radial velocities production. The two most common commercial software packages for HF Radar acquisition and pre-processing are:. Third party software for data acquisition and preprocessing is available under commercial license for beam forming HF radar systems, see Table 8 for more details.

Operators are invited to investigate the license conditions, especially the policy for software updates, which may differ significantly, in order to allocate the correct budget for maintaining the systems. The combination of radial velocities from two or more remote stations for obtaining total surface current velocity files is covered in this paragraph.

The most commonly adopted combination algorithm of radial vectors into total vectors is the unweighted least squares fitting UWLS algorithm. HF radar manufacturers provide combining suite software under commercial license Table 9 often as separate product with respect to the acquisition and pre-processing software described in the previous section.

Best practices suggest that the NetCDF should be adopted as file format either for radial and total vectors files. Mandatory, suggested or ancillary variables and attributes and their naming conventions are a matter of discussion. As a data post-processing example, the authors want to report here the case of the application of QC tests and their description, according to the definition of data levels and to the European common data and metadata model for surface currents Corgnati et al.

The flow is summarized in the following scheme:. As described by Chapman et al. It is thus highly recommended to provide information about the validity and correctness of HFR measurements via Quality Control QC procedures. These tests, listed as mandatory in the European common QC model for near real-time HF radar current data and textually reported here from Corgnati et al.

The Codar Current Newsletter suggests not to use variance data for real-time QC, due to the fact that the CODAR parameter defining the variance is computed at each time step, and therefore considered not statistically solid. Therefore, this test is applicable only to Beam Forming BF systems. Since this method implies a one-hour delay in the data provision, the current hour file should have the related QC flag set to 0 no QC performed until it is updated to the proper values when the next hour file is generated.

This test is applicable only to DF systems. As stated for radial data, this test is applicable only to Beam Forming BF systems. The Temporal Derivative test is applied. These mandatory QC tests are manufacturer-independent, i. Each QC test will result in a flag related to each data vector which will be inserted in the specific test variable.

These variables can be matrices with the same dimensions of the data variable, containing, for each cell, the flag related to the vector lying in that cell, in case the QC test evaluates each cell of the gridded data, or a scalar, in case the QC test assesses an overall property of the data. For some of these tests, HF radar operators will need to select the best thresholds.

Since a successful QC effort is highly dependent upon selection of the proper thresholds, this choice is not straightforward, and may require trial and error before final selections are made. Analytics analytics. Analytical cookies are used to understand how visitors interact with the website. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc.

Uncategorized uncategorized. Undefined cookies are those that are being analyzed and have not been classified into a category as yet. Performance performance.

Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors. In one study, researchers deployed drifters within a 2 km by 2 km square, returning drifters that floated out of the square, then compared the data from this drifter with data from HF radar.

The two datasets closely agreed. Still there are minor bugs that need to be worked out, and researchers are still determining the range of errors that can occur. Currently NOAA and the Scripps Institution of Oceanography are developing a system that links all of the HF radar installations to a central database that is available to the general public in formats that are easy to understand.

Click image to enlarge. Data on coastal currents will be useful to many people. They will help physical oceanographers model circulation along the coast and in bays and estuaries and observe how currents change during storms or when eddies intrude.

Physical oceanographers will also be able to incorporate these data into their larger ocean circulation models, filling in important gaps. Scientists studying coastal ecosystems will be able to use the data to study the movement of juvenile fish and invertebrates, the influx of nutrients from the deep ocean, and the outflow of pollutants from land.

The data will help the Coast Guard improve its search-and-rescue operations and track oil spills. The data will also help ships navigating into ports and commercial and recreational fishermen planning their expeditions.