It’s a wonderful introduction about the experiment mode and data processing for the coming advanced tristatic radar system. Much information included. And I have some questions:

1. As mentioned in paper, the antenna arrays are horizontal for the remoting receivers. So what is the lowest altitude of the common volume? And because the antenna gain would decrease when the elevation becomes small, what is the SNR level at the lowest altitude?
2. What is the synchronous mode for the tristatic radar system? Does the remote site start to measure when the core site still transmits? What cause the blank in the Figure 3 (e)?
3. Do you combine the ACFs from the core and remoting sites, when you invert the plasma parameters? How do you consider the effect of drift velocity from different sites in the spectrum if you put the ACFs together in the inversion?
4. In the results of Figure 9, the decrease of Te appears with the increase of Ne. They are usually related in the inversion. Does the fluctuate at the altitude about 110-120km come from the coupling between the parameters or the true state of ionosphere?  

By a way, there is an error for the expression at line 648.

This interesting manuscript explores transmission codes and analysis techniques for the nearly-completed EISCAT3D incoherent scatter radar.  This new tri-static radar will have impressive flexibility and sensitivity and will require innovative modulation schemes to fully realize its potential.  The manuscript describes such a scheme along with the processing required to implement it.  It also includes the results of simulations of the mode as well as measurements from the KAIRA receive array used in conjunction with the EISCAT VHF transmitter to demonstrate some of the basic principles.

The modulation described in the manuscript is a multipurpose mode which combines various techniques to measure ACFs at a wide range of lags and range resolutions.  This supports measurements from the D region through the E and F regions while maintaining a relatively high RF duty cycle.

I have the following questions for the authors which would help in the interpretation of the results.

1. The manuscript describes the impact of self-clutter on the measurements and the reduction of this clutter in the data from the remote sites. One thing I did not see was whether the analyses and simulations covered the impact of range aliasing in the measurements from Skibotn. Looking at Figure 2, the pulse starting at 0 ms will also be seen in the returns after the pulse at 1.2 ms, but from ranges 180 km further away.  This means that those samples will have contributions, for example, from both the D- and F-region altitudes.  Has this been accounted for in the simulations?  Is the added self-clutter included?
2. Section 3.3 briefly mentions the problem of ground clutter in the Skibotn measurements. Given that the coded pulse itself is 600 microsec in duration, this means that any ground clutter will add to the 90 km of equivalent range that must be blanked from the first bauds of the pulse. Are there any estimates of the maximum range of the ground clutter returns around Skibotn?  How severe are the impacts of the partial decoding of the pulse around this altitude?  Might a shorter modulation be needed to make D-region measurements?
3. Have the authors given any thought to the removal of returns from low earth orbit (LEO) satellites in the lag profile estimates? The region around 800 km altitude is becoming more and more problematic in this regard and it would be interesting to know how this modulation and processing might be impacted by such signals, at least in a general sense.
4. It would be helpful to have a discussion about the different spatial resolutions of the measurements from Skibotn and those from the remote sites. The mono-static case at Skibotn is fairly straightforward as the 2.1-degree beam width implies a 3.7 km horizontal extent of the scattering volume at 100 km and 11 km horizontal extent at 300 km altitude. The basic range resolution of the measurements is 0.75 km from the phase coding, so each lag estimate comes from a roughly pancake-shaped region of space perpendicular to the beam steering direction.  The remote site measurements are more complicated, however, because the impact of the phase coding does not reflect the altitude, even for a vertically oriented transmit beam.  A signal scattered from one edge of the transmit beam at 100 km takes 25 microsec to reach the other edge of that beam (traveling perpendicularly to the transmit direction).  It would be helpful if the authors could discuss how this might impact combining high range-resolution measurements from the three stations (Skibotn, Karesuvanto, and Kaiseniemi).