On Jan 22, 2012 the CODAR signal on 4543 KHz was observed during the CME  impact on Earth.  The CODAR chirp was compressed and plotted with a 5000 km bi-static range. The location of the transmitter is 30.3830 N, 86.4327 W. The receiver is at 33.86N and 84.33W .

The processed CODAR plot was superimposed on the ACE plot to show the moment of impact and any possible effects on the time of arrival (or doppler) of the received CODAR chirp. [Click on image to enlarge]

There appears to be some ripples around 0504 UTC after a period of slow rise of the height of the ionospheric layers. The 0504 UTC ripple coincides with the arrival of the CME at the ACE spacecraft that is located on the L1 Lagrangian point. It is hard to believe that the effects of the CME would be already visible on earth. In order to see if the moment of impact correlates, the CODAR plot would have to be aligned with the actual moment of impact in earth. Some of the ripples that occured about 1 hour later could still be caused by the CME.

Some Background

CODAR is an example of an FMCW radar. (FMCW) is a radar system where a known stable frequency continuous wave radio energy is modulated by a triangular modulation signal so that it varies gradually and then mixes with the signal reflected from a target object with this transmit signal to produce a beat signal.

Most of the CODAR signals are around the US coast. These signals propagate well through the ionosphere and can be used to map the different layers of the ionosphere. The most common frequencies are:

4.513 MHz - 4.900 MHz

8.380 MHz - 8.390 MHz

12.020 MHz - 13.555 MHz

16.050 MHz - 16.315 MHz

24.100 MHz - 26.190 MHz

40.750 MHz - 44.210 MHz

All of the CODAR stations observed so far use down chirping. This means that the frequency moves down as time progresses. The slope of the chirp seems to vary depending on the geographical area and frequency. Most of the chirp rates in MHz/s appear to be odd multiples of 0.0036756 MHz/s . This is probably a rate based on the transmitter DDS frequency and number of points. Most of the chirps have repetition rates of 0.5 or 1.0 seconds and appear to be locked to GPS. There are a few “drifters” that do not appear to be locked.

The chirps are not continuous and appear to be gated to allow the transmission and reception at the same site. This gating adds AM sidebands to the chirp signal. These sidebands appear as lower amplitude signals to the left and right of the main pulse once the signal is de-chirped.

Empirical Data - Chirp Rates (see pdf below)

25.600 MHz Approximate Chirp Rate = -0.2022 MHz/s

4.543 MHz Approximate Chirp Rate = -0.018378 MHz/s

4.660 MHz Approximate Chirp Rate = -0.018378 MHz/s

13.450 MHz Approximate Chirp Rate = -0.09926 MHz/s

4.526 MHz Approximate Chirp Rate = -0.025733 MHz/s

Once we know the chirp rates, it is possible to use a matched filter to de-chirp the signal. This processing has been built into the SpectraVue application. It is processed in the frequency domain by taking an FFT of the IQ received data and multiplying it by a copy of the transmitted pulse. Once the result is converted back to the time-domain, the range information can be extracted.

PI 2012