Long Pulse Mode

At altitudes for which range resolution is not very important, it is sometimes possible to measure all the lags of the ACF simultaneously using a single long pulse combined with a (mismatched) filter with a short impulse response.  In the F region of the ionosphere, the scatterers are distributed over a range of more than 1000 km. As a result, the measured spectrum does not represent just a single height, but instead represents a weighted average of spectra from an altitude range equal to the width of the pulse. If the properties of the ionosphere remain constant over such a range, the technique will work well.

The frequency resolution obtained using the long pulse technique is inversely proportional to the width of the pulse. Since the incoherent scatter spectrum is relatively smooth, it might be possible to extract useful temperature estimates with a resolution of 500 Hz. The transmitter pulse would then have to be 300 km long. The sample rate must exceed the Nyquist rate in order to avoid aliasing the spectrum.

Since the JRO main antenna cannot be steered more than few degrees from vertical, this long pulse mode can only be used for measurements at high altitudes where the ionospheric parameters are constant over the 300 km extent of the pulse.

In the early days, this mode was used to get measurements as high as 10,000 km [e.g., Farley, 1991 ]. Nowadays, this mode is being used in support of most topside and protonospheric measurements including the POLITE campaign. Typical parameters are: IPP=6000 km, PW = 300 km, Receiver bandwidth = 15 km.