Riometer — Background

A RIOMETER (Relative Ionospheric Opacity METER) is an instrument used to quantify the amount of electromagnetic wave ionospheric absorption in the atmosphere. As the name implies, a riometer measures the "opacity" of the ionosphere to radio noise emanating from distant stars and galaxies. In the absence of any ionospheric absorption, this radio noise, averaged over a sufficiently long periods of time, forms a quiet-day curve (QDC). Increased ionization in the Earth's ionosphere will cause absorption of radio signals, and a departure from the QDC. The difference between the QDC and the riometer signal is an indicator of the amount of absorption. The Riometer was developed in the mid 1950s by scientists at the University of Alaska who were researching the radio propagation effects of aurora.

Commercially-produced riometers in the last three decades have largely been of the analog-servo variety. While this approach was in the past state-of-the-art, the techniques embodied in the design come from a pre-ubiquitous-computing era. In effect, the servo-mechanism may be considered a crude "analog computer" implementing a very simple, narrow-focused algorithm.

The emergence of relatively inexpensive DSP techniques in the 1980s suggests that the 1950s-era servo design could reasonably have been re-visited at that time, but never was.

Five years in the making, Keo Scientific's riometer may be considered a 'next-generation' riometer. In small-instrument riometry, continuum radio astronomy and solar observations, the most common instrument configurations are the Dicke radiometer and the phase switched interferometer. For our riometer, we are employing SDR-based alternatives to these systems. Our RF front-end is simple, and small, and has only enough gain to drive our RF sampler, thus reducing inherent gain fluctuations. Furthermore, our front-end is temperature stabilized, thus reducing further instability to negligible levels, even for fine-structure absorption studies.

A purely-analog design makes many things extremely difficult and often expensive to implement, including frequency and bandwidth agility. The modern RF environment, however, is utterly-hostile to the type of science that riometers are trying to address. Even in the so-called protected sections of the RF spectrum, narrow-band RFI is an ongoing problem that can render-useless a large numbers of scientific observations.

By leveraging recent innovations in RF technology, we have produced a riometer that is agile and robust, meeting all of the requirements of radio-based ionospheric science in the noisy modern world.

Our unique approach means that new features can be implemented entirely as software updates that can be installed remotely, a benefit not enjoyed by traditional analog riometers.

The KeoRio: Key Features

As explained above, the use of modern digital techniques lends itself to a variety of features that cannot be reasonably realized in a "traditional" analog riometer:

  • Active narrow-band RFI mitigation based on proprietary dynamically-adaptive filtering with persistent interferer memory
  • Dynamic observing bandwidth user-selectable from 50KHz to 5MHz
  • Dynamic observing frequency user-selectable from 18MHz to 45MHz
  • Real-time computer display of RMS detected power
  • Real-time computer display of RFI-excised spectrum
  • High temporal-resolution logging including both UTC and sidereal time-stamps
  • Maximum flexibility in post-processing tools
  • Optimized RF gain to reduce gain fluctuations (0.012 dB Gain Stability)
  • Automatic reference switching without the need for complex servo circuitry
  • Many fewer components in the RF chain-stability, reliability, repeatability
  • So-called data-slicing instead of an analog servo
  • GigE interface

A brochure with technical specifications on our riometer follows. However, if you have time on your hands, we encourage you to design and build your own riometer (it's fun!) — we provide links to a couple of helpful papers below (email us if you have papers to add to the list).

Photos: Prototype testing in Northern Canada (Summer 2014).

Keo Riometer Prototype Testing Keo Riometer Data Output