Passive radar observations using CODAR and WERA signals.

Coastal ocean dynamics applications radar (CODAR) describes a type of compact High Frequency (HF) RADAR developed between 1973 and 1983 at NOAA's Wave Propagation Laboratory in Boulder, Colorado. CODAR uses HF surface wave propagation to remotely measure ocean surface waves and currents.

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

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 I/Q 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.


This image shows the heigh of the ionospheric layers over a period of many hours. Click image to zoom.


Spreadsheet with the different CODAR and WERA sweep parameters. Not all of them have been verified.


The hardware used to capture these radar plots is the RFSPACE SDR-IP or NetSDR-01-02 receivers locked to a Stanford Research FS725 Rubidium Standard. The Rubidium is disciplined with a Trimble Thunderbolt. The 10 MHz reference locks the receiver internal 80 MHz oscillator and the 1 PPS is used for accurate timing. The antenna is a homebrew amplified loop similar to the Wellbrook. The Cloud-IQ now includes an internal trigger for CODAR reception with no PPS signal requirements.



4543 CODAR signal start at 0216 UTC JAN 27, 2013. 5000 Km bistatic range.


4543 CODAR signal start at 2315 UTC JAN 21 2013. 5000 Km bistatic range.


4543 CODAR signal start at 0121 UTC JAN 23, 2013. 5000 Km bistatic range.


4543 CODAR signal start at 0018 UTC JAN 24, 2013. 2500 Km bistatic range.


4543 CODAR signal start at 0400 UTC JAN 25, 2013. 5000 Km bistatic range.


4543 CODAR signal start at 0118 UTC JAN 26, 2013. 5000 Km bistatic range.


This image shows bi-static range on the X axis and elapsed time in the Y axis. There are several interesting loops present. This same data was processed from left to right using a Matlab script to estimate the doppler  shift for every layer. The Youtube movie is shown below: