Sunday, 2 March 2014

Software defined radio wide band wide beam riometer

During my visit to Finland a few weeks ago, I setup two computers with two different prototype radio frontends to test my idea of using a USRP N200 and a single LOFAR LBA antenna as a low cost riometer with next generation capabilities.

First of all, a riometer (Relative Ionospheric Opacity METER) is an instrument that measures the absorption of cosmic radio noise in the D-region of the Earth's ionosphere. We assume that the cosmic radio noise statistics are unchanged. Thus, we assume that at a certain time of celestial day, we should pick up the same cosmic radio noise power with our antenna beam, as it sees the same swath of sky. The only variations to this power is attributed to changes in ionospheric absorption, which is significant below 50-100 MHz.

In reality the radio sky isn't a completely stationary noise source. Occasionally there are radio emissions from the Sun, which are sporadically seen as spikes in power during the daytime in riometer data. Jupiter is also a strong and variable radio source, and its emissions might cause confusion when interpreting riometer measurements. These type of sporadic emissions need to be excluded from any ionospheric data products derived from riometer measurements.

The advantage of using a wide band for riometry is at least four fold:
  1. More bandwidth = more statistics
  2. We get a measure of the frequency dependent absorption, which contains information about the electron density height profile
  3. More tolerance to interference, as we can find the noise floor around the interference, as long as it doesn't completely saturate the amplifiers. 
  4. The wide band nature of the instrument allows us to more easily identify transient radio emission events, such as Solar or Jovian radio emissions.
Anyway, here are some of the initial results. This is simply a dynamic spectrum and a raw power plot derived from the dynamic spectrum. I have not attempted to yet fit a quiet day surface to extract the frequency dependent absorption, although there is no reason why this wouldn't be possible as we have already done it successfully with the LOFAR digital backend. 

The first ~five days of raw riometer power. Several auroral precipitation absorption events happened during this interval, which was a lucky coincidence, allowing us to not only verify that the daily cosmic radio noise variation is periodic, but that we can see D-region absorption. The blue curve is labeled "Army", because this RF frontend utilizes some pretty heavy duty US army surplus tuneable helical filters. The "Pelti" frontend is made from standard minicircuits boxed components. 

The dynamic spectrum between 25 and 50 MHz. Again, nearly five days of data and many absorption events. The strong interference at 36.9 MHz is the Sodankyla Geophysical Observatory meteor radar, which is located about 10 km from the riometer site. 
I'd like to acknowledge Tomi Teppo and Toivo Iinatti for their assistance in setting up these wide band wide beam riometers.