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Stereo CUTLASS installed at the Iceland radar site
(Click on the picture to see a larger version.) More detailed pictures are available here.
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Stereo CUTLASS installed at the Iceland radar site
(Click on the picture to see a larger version.) More detailed pictures are available here.
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STEREO was developed to improve the temporal resolution of the CUTLASS SuperDARN radars by adding a second beam direction to the conventional single beam configuration. STEREO relies on the capability of the transmitters in Type 3 SuperDARN radars to operate at duty cycles of at least 7% which is twice that required for normal SuperDARN operation. (A Type 3 radars is a specific design configuration of a SuperDARN radar developed at Leicester and which has been used as a basis for the implementation of several recent radars.)
STEREO consists of two radars each of which is able to transmit and receive independent signals but which share common transmitters, antennas and control system. Radar pulses from each channel are differentiated by their frequency and it is necessary to use separate signal generation, beam forming and receivers in each of the two channels. The common components, however, constitute the major capital cost within a SuperDARN radar so Stereo is significantly cheaper than two radars.
A SuperDARN radar is a phased array radar operating in the frequency range 8 to 20 MHz. It employs 16 horizontally polarised log periodic antennas each driven by a 600W pulse transmitter which includes a Tx/Rx switch to allow the antenna array to be used for both receive and transmit. A bi-directional beam forming network consisting of switched fixed time delays allows any one of 16 possible pointing directions to be selected and allows a single receiver and signal source to be connected via a further Tx/Rx switch. A second receive only antenna array consisting of 4 antennas similar to those in the main array is normally deployed to allow the measurement of the angle of arrival of backscattered radar signals.
SuperDARN radars employ a seven pulse sequence with an individual pulse length of 300 microsecond corresponding to a range resolution of 45 km from which the autocorrelation function is determined for ranges from 180 to 2500 km. The Doppler shift is determined by a phase fitting procedure to provide the line of sight velocity of irregularities within the range of +/- 2000 m/s with a resolution of some tens of m/s. Backscatter power and spectral width are determined directly from the autocorrelation function.
The STEREO implementation is illustrated in figure below (pdf version available here) which is a functional representation of the complete system. The original configuration is extended by the duplication of the phasing matrix, receiver and transmit signal generation. A new version of the BAS Box, originally a hardwired patch panel providing convenient interconnection of control signals throughout the system, has been developed. The Stereo BAS Box adds arbitration logic to protect the system from requests to transmit on both channels simultaneously and to ensure that the Rx/Tx switches in both Stereo channels are set with regard to transmit requests in both channels. By default both channels are able to receive simultaneously but neither channel is able to receive whilst either channel is transmitting. The new Stereo BAS Box also incorporates remote monitoring of potential conflicts detected by the arbitration logic as well as the aggregated duty cycle and other performance indicators.
A second A/D interface is provided within the main computer to allow the additional four channels of analogue I/O (In-phase and Quadrature signals for both the main and interferometer receivers) to be simultaneously sampled. The original timing computer which acts as a programmable timing signal generator, includes the ability to generate the required signals for both Stereo channels. The most recent version of the radar operating system, RST, developed by the Applied Physics Laboratory (APL) at Johns Hopkins University includes the ability to operate Stereo CUTLASS as well as normal SuperDARN radars.
In operation Stereo sets the beam direction for each of the two channels which may be the same or different by sending the appropriate control signals to the two phasing matrixes. The timing computer generates individual, non-overlapping pulse sequences for each of the two channels. The conceptually simplest arrangement is to use the same pulse pattern on both channels with one delayed with respect to the other by a single pulse period. These pulse sequences generate equivalent RF drive signals at different transmit frequencies and these are routed through the phasing matrixes and Tx/Rx switches to the transmitters which generate the appropriately phased signals in the antenna array. When not transmitting the received signals are routed back through the phasing matrixes to the receivers and the down converted signals sampled by the main computer. The autocorrelation function for each pulse sequence is averaged over an integration period, normally three seconds, is fitted to determine power, velocity and spectral width parameters for each of the Stereo channels. The process continues for as many pointing directions as required in an individual scan and the interleaved data from both Stereo channels recorded on disk.
Stereo CUTLASS provides is effectively equivalent to two independent radars does have several limitations caused by hardware constraints. The two channels must operate at different frequencies to ensure that backscattered signals from one channel are not confused with those from the other channel. The separation in frequency must be greater than the receiver bandwidth of 20 kHz but this allows the two channels to operate within the same frequency band which is for CUTLASS normally in the region of 100kHz.
In normal SuperDARN operation, the inability to receive and transmit simultaneously results in so called bad lags where, at certain ranges, data from some autocorrelation lags is missing. These missing points do not normally prevent data fitting as long as the fitting process takes account of them. In Stereo operation the number of bad lags is increased since there are more times when the received signal is not available to the fitting process. By suitable arrangement of the offset between the two transmitted pulse sequence the impact of the additional bad lags is not significant.
The current Stereo software implementation within RST requires the use of the same integration time for each channel. Whilst there is no fundamental reason for this limitation it would require significant programming effort to overcome. In addition in the Stereo control programs developed to date have used the same pulse sequence in both channels although there is no reason why different pulse sequences could not be used as long as they are designed to avoid overlapping transmit requests.
SuperDARN radars have demonstrated the unique ability to generate maps of irregularities with high time resolution over extended regions during normal operation. This ability is significantly enhanced by the second Stereo channel which may be programmed to provide additional information without impacting on normal scanning. Some of the possible applications are illustrated below:
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