Fort McPherson Propagation Experiment Results

By: ksterne on: Mon., Feb. 13, 2012 04:09 PM EST (13038 Reads)

An experiment on HF propagation conducted between the Fort Hays radar site and a transponder located near Fort McPherson in the Northwest Territories of Canada occurred during the week of Feb. 13th to 17th. The transponder, setup by Dr. Paul Bernhardt of the Naval Research Lab, upon receiving a SuperDARN signal, retransmits the signal with a certain amount of delay and frequency shift. Its believed that the re-transmitted signal showed up during a grayline propagation mode during the morning of Feb. 14 as seen in the scan plot to the left.

The basic challenge for this experiment is to get radio waves transmitted from our radar to be received by Fort McPherson. However, variations in the height and density of layers in the ionosphere as well as how these factors change with the wavelength of the radio wave propagating through the ionosphere make it difficult to predict what location the transmitted pulses will come back to the ground. Some effort to predict where our signal will reach the ground has been made using the ray-tracing tool located on our website at:

http://vt.superdarn.org/tiki-index.php?page=Ray-tracing

In order to perform this experiment, our radars needed to use one single frequency as the transponder at Fort McPherson can only be set to one frequency every 30 seconds. Normally a SuperDARN radar has a small frequency hopping routine that searches for the quietest frequency and then transmits on this frequency. In order to override this frequency hopping, a couple of lines of code were added to the normalscan control program which were already present in the themisscan control program. These lines included initializing a new variable called 'fixfrq', adding a OptionProcess line to read in the variable value from the scheduler command line and lastly setting fixfrq to the transmit frequency just after the clear frequency search. For the last part, an if statement works well to gate valid values for fixfrq as well as enabling and disabling the override. For example, if fixfrq is set to zero, that's not a valid transmit frequency AND its an indication that the transmit frequency found in the clear frequency search should be used.

As seen in the scan plot above and the range-time plot below, it's believed that with both Fort Hays West and the transponder set to 14.800 MHz, a link between the two sites was established for a period of time. During the time shown here, the transponder was repeating received signals at a +50 and -50 Hz shift simultaneously. It is unsure of how a SuperDARN radar would interpret a signal with both a positive and negative shift, but this amount of frequency shift would show up with a magnitude of about 500 m/s. The scatter from the Fort McPherson area on the morning of Feb. 14 has velocities that approach the calculated value.

This is the only time period that it is believe the Fort Hays radar was able to get a signal to the transponder at Fort McPherson during this experiment. Some suggestions have been made to Paul Bernhardt on how to better setup the transponder for a SuperDARN radar. Another similar experiment is hoped to be conducted sometime in May.

Timeline of Events

Fort McPherson Propagation Experiment Results

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