Asko Huuskonen

General incoherent scatter analysis and GUISDAP

Abstract In modern incoherent scatter measurements very complicated coding schemes are used to achieve the necessary statistical accuracy in the measurements. The ambiguity functions specify how the measurement weights the plasma autocorrelation function in the two-dimensional space of range and lag. Alternatively one can work in the space of range and frequency, and it is necessary to derive the ambiguity (instrument, weighting) functions also in terms of these variables. In this paper we summarize the theory of ambiguity functions and discuss the use of lag profiles as tools of describing arbitrary kinds of incoherent scatter measurements. We also develope a general way to calculate error estimates of estimated autocorrelation function values values. Finally, we shortly discuss how the formalism is used in the GUISDAP (Grand Unified Incoherent Scatter Design and Analysis Package) software.

The EISCAT Svalbard radar: A case study in modern incoherent scatter radar system design

The EISCAT (European incoherent scatter) Svalbard radar (ESR) was officially inaugurated on August 22, 1996. This event marked the successful completion on schedule of the first phase of the EISCAT Svalbard radar project. In contrast to previous incoherent scatter radars, the ESR system design was adapted to make use of commercial off-the-shelf TV transmitter hardware, thereby reducing design risk, lead times, and cost to a minimum. Commercial hardware is also used in the digital signal processing system. Control and monitoring are performed by distributed, networked VME systems. Thanks to modern reflector antenna design methods and extreme efforts to reduce the receiver noise contribution, the system noise temperature is only 70 K, thus making the ESR about 30% faster than the much more powerful EISCAT UHF radar in F region experiments! Once the transmitter power is increased to 1 MW, it will become about 2–3 times faster than the UHF radar. State-of-the-art exciter and receiver hardware has been developed in-house to accommodate the special requirements introduced by operating the radar at the exceptionally high duty cycle of 25%. The RF waveform is generated by a system based on four switchable direct digital synthesizers. Continuous monitoring of the transmitted RF waveform by the receiver system allows removal of klystron-induced spurious Doppler effects from the data. Intermediate-frequency sampling at 7.5 MHz is employed, followed by fully digital channel separation, signal detection, and postdetection filtering in six parallel receiver channels. Radar codes for both E and F layer observation have been designed and perfected. So far, more than 40 hours of good quality ionospheric data have been collected and analyzed in terms of plasma parameters. While the tragic loss of the Cluster mission suddenly changed the plans and dispositions of a majority of the ESR user community, the radar has still been in high demand since its inauguration. It is now being operated by EISCAT staff on a campaign basis, to provide ground-based support data for a number of other magnetospheric satellites, notably Polar and FAST, and will be opened to the EISCAT user community for special program operations later in 1997.

Determining Weather Radar Antenna Pointing Using Signals Detected from the Sun at Low Antenna Elevations

Abstract A method to determine the elevation and azimuth biases of the radar antenna using solar signals observed by a scanning radar is presented. Data recorded at low elevation angles where the atmospheric refraction has a significant effect on the propagation of the radio wave are used, and a method to take the effect of the refraction into account in the analysis is presented. A set of equations is given by which the refraction of the radio waves as a function of the relative humidity can easily be calculated. Also, a simplified model for the calculation of the atmospheric attenuation is presented. The consistency of the adopted models for the atmospheric refraction and atmospheric attenuation is confirmed by data collected at a single elevation pointing, but over a long observing time. Finally, the method is applied to datasets based on operational measurements at the Finnish Meteorological Institute (FMI) and Royal Netherlands Meteorological Institute (KNMI), and elevation and azimuth biases of the ...

High resolution EISCAT observations of the ion-neutral collision frequency in the lower E-region

Abstract A multichannel Barker coded EISCAT experiment with basic spatial and temporal resolutions of 600 m and 10 s, respectively, is described. The experiment was run on 13–17 February 1984 and data obtained in a diffuse aurora as well as in an auroral arc are used for determining the ion-neutral collision frequency in the lower E -region. Post-integration over 20 and 3 min, respectively, is carried out in the two cases and the plasma parameters are fitted with the assumption of equal ion and electron temperatures. The effect of varying the temperature ratio is discussed. The obtained collision frequencies are generally found to be in accordance with profiles based on the CIRA (1972) model atmosphere. In addition to collision frequencies, vertical ion velocities and electron temperature profiles are presented. It is found that the velocities are consistent with the observed electric fields.

Operational Monitoring of Weather Radar Receiving Chain Using the Sun

Abstract A method for operational monitoring of a weather radar receiving chain, including the antenna gain and the receiver, is presented. The “online” method is entirely based on the analysis of sun signals in the polar volume data produced during operational scanning of weather radars. The method is an extension of that for determining the weather radar antenna pointing at low elevations using sun signals, and it is suited for routine application. The solar flux from the online method agrees very well with that obtained from “offline” sun tracking experiments at two weather radar sites. Furthermore, the retrieved sun flux is compared with data from the Dominion Radio Astrophysical Observatory (DRAO) in Canada. Small biases in the sun flux data from the Dutch and Finnish radars (between −0.93 and +0.47 dB) are found. The low standard deviations of these sun flux data against those from DRAO (0.14–0.20 dB) demonstrate the stability of the weather radar receiving chains and of the sun-based online monitor...

The accuracy of incoherent scatter measurements: error estimates valid for high signal levels

Abstract We study incoherent scatter data errors with special emphasis on the situation with good signal-to-noise ratio. Because part of the statistical fluctuations in the autocorrelation function estimates arise from the signal itself, there is good reason to believe that these fluctuations may show significant correlations between different lags and/or between different ranges. We derive formulae suitable for the estimation of errors and their covariances. We also study the covariances numerically and show that, while in the case of multi-pulses the covariances are negligible, they can be significant for alternating codes and that they most certainly are significant for long pulse measurements. We also study the effect of taking the covariances into account in the analysis of basic incoherent scatter parameters. Here we show that the fluctuations in the analysis results will be smaller when optimal weights are used, and that ignoring covariances for long pulse measurements will lead to underestimation of the error bars by up to a factor of two in certain cases. The effect in multi-pulse measurements is indeed negligible, while the effect in alternating code measurements stays rather small, at most a factor of 1.5 error in error estimates. Finally, we discuss the consequences of these calculations for experiment design.

The Operational Weather Radar Network in Europe

The operational weather radar network in Europe covers more than 30 countries and contains more than 200 weather radars. The radar network is heterogeneous in hardware, signal processing, transmit/receive frequency, and scanning strategy, thus making it fundamentally different than the Next Generation Weather Radar (NEXRAD) network. Another difference is that the density of the European weather radar network is roughly twice that of the NEXRAD network. Within the European National Meteorological Services (EUMETNET), a grouping of services, the Operational Program for Exchange of Weather Radar Information (OPERA) has been working since 1999 on improving the harmonization of radars and their measurements. In addition, OPERA has facilitated and stimulated the exchange of radar data between its members, among others, by the development of a radar data information model and jointly agreed data formats. Since 2011, a radar data center (“Odyssey”) has been in operation, producing network-wide radar mosaics from ...

Fractional lags in alternating codes: Improving incoherent scatter measurements by using lag estimates at noninteger multiples of baud length

Alternating codes are nowadays a standard measurement technique in the incoherent scatter observations. In most conditions they give the best accuracy for the plasma autocorrelation function estimates in the E and F region measurements. We explain here how oversampling can be used to further increase the speed of the experiments. In an oversampled alternating code the sampling interval and the length of the postdetection filter are a fraction of the baud length, and autocorrelation function estimates are calculated both for integer lag values, which are multiples of the baud length, and for the intermediate values, which are called fractional lags. We show that alternating codes with fractional lags give data for multiple range resolutions and that the experiments are always more efficient than the standard alternating code experiments. Finally, we study how the oversampling affects the signal processing.

Measurements of the Transmission Loss of a Radome at Different Rain Intensities

Abstract Results on the transmission loss of a dry and a wet C-band weather radar radome at different rain intensities are presented. Two methods were used in the study, both carried out under laboratory conditions. In the first method, the complex permittivity of a dry radome is measured and the transmission loss calculated. To analyze the transmission loss of a wet radome, the thickness of a continuous water layer on the surface of a radome at different rain intensities and the complex permittivity of water are calculated. In the second method, the transmission loss is measured as a free space transmission measurement with a 1.3-m2 piece of a radome panel. The piece is measured as dry and as doused by a rain system designed for the measurements. The measurements are performed with a dirty, cleaned, and waxed radome to examine the effects of maintenance measures with an old radome on the transmission loss. Because the transmission loss as a function of rain intensity is measured with a small piece of rad...

Operational Monitoring of Radar Differential Reflectivity Using the Sun

Abstract A method for the daily monitoring of the differential reflectivity bias for polarimetric weather radars is presented. Sun signals detected in polar volume data produced during operational scanning of the radar are used. This method is an extension of that for monitoring the weather radar antenna pointing at low elevations and the radar receiving chain using the sun. This “online” method is ideally suited for routine application in networks of operational radars. The online sun monitoring can be used to check the agreement between horizontal and vertical polarization lobes of the radar antenna, which is a prerequisite for high-quality polarimetric measurements. By performing both online sun monitoring and rain calibration at vertical incidence, the differential receiver bias and differential transmitter bias can be disentangled. Results from the polarimetric radars in Trappes (France) and Bornholm (Denmark), demonstrating the importance of regular monitoring of the differential reflectivity bias, ...