The Gyfcat animation below shows a hydrogen line "drift" scan recorded by the folks over at rtl-sdr.com. The scan is performed over one day, and they let the rotation of the earth allow the Milky Way to drift over the antenna. The Stellarium software on the left shows the movement of the Milky Way/galactic plane over the course of a day for their location. The dish antenna points straight up into the sky, and they have set Stellarium to display the sky at the location of the antenna, so Stellarium sees exactly what their dish antenna is seeing.

On the right is a spectrum plot of averaged signals over time. As the Milky Way passes over the antenna a roughly triangular lump forms growing in heigth to indicate strength of the neutral hydrogen signal, which indicates the density of hydrogen present in the antenna direction.

The signal also spreads left and/or right from the exact frequency of stationary hydrogen atoms, 1420.405 751 768 Mhz,  indicates the doppler shift and therefore velocity of gas clouds relative to Earth. Left shifted (red shift)  signals are moving away and right shifted (blue shift) signals are approaching. Occasionally the signal lump can be seen to divide to reveal more than one peak. This usually indicates that different spiral arms moving at different velocities are in the beam of in the antenna. 

Spiky signals that looks like combs appearing occasionally  in the plot are believed to be terrestrial interference such as from cellular telephone signals. The tallest spike is interference from the CPU clock of the computer hosting the receiver. Avoiding these sorts of interference is becoming more difficult for Radio Astronomy over time as this technology proliferates.

In the above example, the antenna is stationary, pointing straight up at zenith, and the turning of the Earth brings different parts of the sky and the Milky Way Galaxy into view of the antenna. More about this project here: 

Cheap and Easy Hydrogen Line Radio Astronomy with an RTL-SDR, WiFi Parabolic Grid Dish, LNA and SDRSharp

However, using even a fairly small antenna that can be pointed at different parts of the sky for a period of time or rapidly scanned, it is possible to do mapping of the radio emissions, and learn things about the structure of the spiral arms.

Below left we have a photo of part the Milky Way Galaxy in all its glory using an optical camera. Below right we have an overlay of the hydrogen radio signal for same section of the galaxy. Color coding indicates the signal to noise ratio (intensity) of the Hydrogen Line signal.

This radio image map displays signal intensity .  More about this project here:

Imaging the Milky Way in Neutral Hydrogen with an RTL-SDR

Imaging the Milky Way in Neutral Hydrogen with an RTL-SDR Part 2

The radio image map of the Galaxy used above required 902 precisely pointed observations of 150 seconds each collected over the course of 8 days. 

Another way of looking at the signal data is by using doppler shift to measure velocity of independent objects in the same direction. Here is a look at detecting Galactic High Velocity Clouds (HVCs).

More about this project can be found here:

Detecting Interstellar High-Velocity Clouds with a Radio Telescope and an RTL-SDR

These images are the work of Job Geheniau, a frequent contributor to rtl-sdr.com. To read more of his his fascinating posts, go here: https://www.rtl-sdr.com/?s=Job+Geheniau

Here at VBAS, our Galactic Neutral Hydrogen Line astronomy is a work in progress. We have received the donation of a 3m parabolic dish antenna from member Jon Carleton in Georgiathat was previously used to to receive C-Band and Ku-Band TVRO television satellite broadcasts.  

We stood the dish up at our Observatory site, modified the mount so that we can elevate it easily, attached a simple "bean can" circular  wave guide feed horn in the focus. and cabled it up so we can attach an SDR receiver.

We built a simple L-Band circular wave guide feed horn out of a bean can and mounted it in the focus of the dish. L-Band encompasses the 21 cm Neutral Hydrogen Line and the 18 cm Hydroxyl Radical Line. The radiator probe is an N-Type female chassis connector with a 5.25 cm (1/4 wave) rod soldered to the internal side. This was mounted in a matching hole in the can drilled 5.25 cm from the solid end. [Provide antenna analyzer plot from 1 GHz to 2 GHz]

This L-Band subset region of radio spectrum between 1420 1nd 1666 MHz is commonly known as "The Water Hole" because it is bracketed by the frequency of hydrogen (H2) and the frequency of hydroxyl (OH), which are the basic constituents of water (H2O).  These two frequencies also are near the center of the Atmospheric Window in our that provides the least attenuation to radio signals coming from space. Search for Extra Terrestrial Life (SETI) researchers follow the somewhat poetic notion that water based life may tend to gather at the Water Hole to communicate, so there is a certain focus to searching these frequencies for possible signals indicating intelligence.