The 'RPI' instrument is based on a principle that has been used in other instruments called 'ionosonds' for the last 60 years. Modern versions are called Digisondes which use digital technology to scan and record the data.
The ionosphere is a region surrounding the Earth which allows radio waves to be reflected if they are below a 'cut off' frequency which is set by the density of electrons in the ionosphere. The formula is a simple one. The cutoff-frequency, in megacycles, equals 9 x ( electron density)^1/2 where you provide the electron density in units of electrons per cubic centimeter.
For radio waves well below this cutoff, the ionosphere acts like a very good mirror, and allows the waves to 'bounce' over the horizon in what is called the 'sky wave' mode. For radio waves well above this cutoff frequency, the ionosphere is virtually transparent and the signals continue out into space. This is why television or 'FM' signals are hard to receive unless you are within the same horizon patch as the transmitter. For frequencies between 1 megacycle and 90 megacycles, the behavior of the ionosphere as a mirror is more complex, with both reflection and signal attenuation being important.
Ionosonds are ground-based radio transmitters that send a directional beam of radio waves into the ionosphere, and measure the strength and time delay in their return. By scanning the transmitter and receiver in frequency from 1 to 30 megacycles, the different reflection properties of various layers of the ionosphere can be read-out. The delay time tells you the distance above the ground of the reflecting layer, and the frequency that is reflected tells you the density of the electrons at that layer. there are hundreds of ionosonde stations on the earth, monitoring the zenithal changes in the local ionosphere density and vertical structure.
The problem for space physicists is that the very low density plasmas in the magnetosphere would require very low frequency pulses of radio energy to detect from the ground, but at the same time, the ionosphere reflects these radio signals. This means that ground directed 'kilohertz' pulses never make it into space to be reflected, and any such incoming radio waves would be scattered by the ionosphere and never reach the ground.
The RPI instrument is simply a 'top-side ionosonde' which will operate in space the same way that a ground-based 'bottom-side ionosonde' has been used to study the ionosphere.
Ten Watt pulses of radio waves scanned from 3,000 cycles to 3 megacycles, will be transmitted from the spacecraft, and then a few milliseconds later, the RPI will be converted into a receiver for the reflected radio signals. By measuring the intensity of the received signal in each frequency channel, the density of the reflecting plasma region can be determined. The delay between the transmitted pulse and its reception will give the distance to the plasma region. These pulse cycles will be repeated every 2-4 seconds.
The instrument will measure plasma densities from 0.1 to 100,000 electrons/cc, and determine the relative speed of the plasma with respect to the satellites rest frame to a resolution of 400 meters/sec. Changes in the entire plasma system will be detected over time intervals as short as 4 seconds.
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