- Calibration to enable measurements to be displayed as noise temperatures (K), power (dBm/Watts) and spectral flux density (Jy).
- Utilities are included for estimation and calculation of noise temperature components (Tsys, Trx, Tgal, Tatm, Tsky, Tsp) and sensitivity (sigma Tsys and sigma Sv).
- Spectra can be displayed against frequency and velocity (with a configurable reference spectral line), with the velocity adjusted to topocentric, Solar System barycentric or the Local Standard of Rest (LSR) reference frames.
- Calculation and plotting of radial and Galactocentric distance to HI clouds, based on spectral peaks.
- Position of HI clouds can be sent to Star Tracker plugin for visualisation on the Galactic line-of-sight image and created in to an animation mapping out the Milky Way's spiral arms.
- A Gaussian fitting tool in the spectrometer for HI cloud kinetic temperature and column density estimation.
- A Gaussian fitting tool in the radiometer to enable antenna HPBW measurement from Solar drift-scans.
- Ability to record and plot real-time surface air temperature and other sensor measurements (component voltages / temperatures) alongside radiometer measurements.
- Ability to export charts to animated .png files and static image files.
- Reference spectra from the LAB (Leiden/Argentine/Bonn) Galactic HI survey can be automatically downloaded and plotted for comparison against user measurements.
- 2D sweeps can be made and plotted in different coordinate systems (Az/El, Galactic, offsets around a target and drift scans).
- All spectra are held in memory and can be scrolled through.
- Data can be saved and loaded from .csv files.
- Hardware for calibration (E.g. RF switches) can be automatically controlled.
Several of the features in this plugin are tailored towards measurements of neutral hydrogen (HI) that is dispersed throughout the Milky Way's interstellar medium (ISM).
The HI in the ISM is a particularly interesting astronomical radiation source for SDR users, as the ground-state hyperfine transition of HI has a rest frequency of 1420.405MHz,
which is both quite powerful (due to the vast amount of HI spread throughout the Milky Way) and at a frequency that is easy to detect with relatively small dishes and low-cost LNAs and SDRs.
The HI spectrum can be used to determine some of the Milky Way's spiral structure and calculate rotation curves for the inner Milky Way, which suggest the presence of dark matter.
In radio astronomy it is common to use noise temperatures rather than power, via the relation:
Use the wheels to adjust the frequency shift in Hz from the center frequency of reception. Left click on a digit sets the cursor position at this digit.
Right click on a digit sets all digits on the right to zero. This effectively floors value at the digit position.
Wheels are moved with the mousewheel while pointing at the wheel or by selecting the wheel with the left mouse click and using the keyboard arrows.
Pressing shift simultaneously moves digit by 5 and pressing control moves it by 2.
Sample rate in millions of samples per second. This determines the maximum bandwidth for the radiometer and spectrometer and thus the maximum Doppler shift range.
Typically this should be set to match SDRangel's baseband sample rate.
Specifies the number of FFTs that are summed in each average. Higher integration counts increase measurement time, but also increase sensitivity (up to the limit imposed by receiver gain variation).
Displays in seconds or minutes how long it will take to make a single measurement. This is dependent on the sample rate (2), Integration Count (5) and number of channels (6).
Specifies the Star Tracker feature that determines the observation target.
A corresponding Star Tracker feature is required to calculate and display numerous values within the Radio Astronomy plugin and also for performing sweeps.
Sets the receiver noise temperature in Kelvin or noise figure (NF) in dB. This is the noise temperature / figure for the combination of the LNA, feed line and SDR (at a specific gain setting).
The value set here can be measured using SDRangel's [Noise Figure plugin](https://github.com/f4exb/sdrangel/blob/master/plugins/channelrx/noisefigure/readme.md)
or estimated from datasheet values for the individual components using the [Friis formula](https://en.wikipedia.org/wiki/Friis_formulas_for_noise).
Sets the contribution to noise temperature from the Cosmic Microwave Background (CMB). This is 2.73K when an antenna is pointed at the Sky, but may be set to 0K if measurements are made
where there is no CMB contribution (E.g. if the feedhorn is covered with absorbing foam or a 50Ohm terminator or other noise source is used for calibration).
Sets the contribution to the noise temperature from the Galactic background. This is frequency dependent and varies with direction. It does not include
the Galactic foreground (i.e. the increased noise temperature when looking in the Galactic plane).
If the link button to the right is unchecked, a value can be entered manually.
If the link button is checked, Tgal is calculated using:
Sets spillover noise temperature. This is unwanted noise due to thermal ground radiation or other thermal radiation sources such as buildings and trees that can be picked
up via an antenna's side and back lobes.
It can be very dependent on azimuth and elevation in urban environments.
An estimate for Tsp can be made via the hot/cold calibration process.
Contribution to noise temperature due to atmospheric emission. Atmospheric emission is dependent upon frequency, opacity (which is dependent on air temperature, pressure and water vapour) and antenna elevation.
If the link button to the right is unchecked, a value can be entered manually.
If the link button is checked, Tatm is calculated using:
Tair specifies the surface air temperature at the antenna location in degrees Celsius.
If the link button to the right is unchecked, a value can be entered manually.
If the link button is checked, Tair is set to the air temperature value received from the Star Tracker plugin, which itself is periodically downloaded from openweathermap.org for the antenna's location.
tau_z specifies the Zenith opacity. This value determines atmospheric absorption and emission. It is dependent upon air temperature, pressure and water vapour.
The default value of 0.0055 roughly corresponds to clear air as per ITU-R P.372-14 figure 5 at 1.4GHz.
This displays the value of sigma Tsys0, which is the standard deviation / RMS of Tsys0, and gives an indication of the sensitivity. It is calculated as:
delta G / G specifies the gain variation of the LNA / receiver. Gain variation places a limit on the sensitivity improvement available by increased integration counts.
This displays the antenna half-power (-3dB) beamwidth (HPBW) in degrees or beam solid angle in steradians, as set in the Star Tracker plugins set by (10).
In order to reduce bandwidth to the server supplying this data, it is recommended to use this option sparingly.
If the series does not appear on Windows (and you see "SSL handshake failed" in the log file), you may need to open https://www.astro.uni-bonn.de/hisurvey/euhou/index.php in your Web browser first, so that the certificate for the website is downloaded.
When checked, the marker table will have six additional columns that display estimates of the distance to a HI cloud corresponding to the marker and the tangent point
along the line of sight.
- Vr is radial velocity of the cloud in km/s, relative to the selected reference frame.
- R is the distance from the cloud to the Galactic centre in kiloparsecs (kpc).
- d is the line-of-sight distance to the cloud in kpc. In some instances there can be two possible solutions.
- Plot max determines whether the smaller or larger solution to d is sent to the Star Tracker plugin for display.
- Rmin is the minimum distance to the Galactic centre in kiloparsecs (kpc) along the line of sight (i.e. at the tangent point).
- Vmax is the orbital velocity at the tangent point.
Vmax can be plotted against Rmin to determine the rotation curve of the Milky Way.
d can be plotted against Galactic longitude in Star Tracker to map out the Milky Way's spiral arms.
The spectrometer GUI will also display two additional fields, R0 and V0,
which allow you to enter the distance from the Sun to the Galactic centre and the Sun's orbital velocity
around the Galactic centre, which are used in the above calculations.
When checked, a horizontal axis showing Doppler shift in km/s is added to the top of the spectrometer chart and a vertical reference spectral line is plotted at 0km/s.
The rest frequency of the spectral line can be set via the reference spectral line field or manually entered.
The relationship between the frequency and velocity axes is determined by the selected reference frame.
When checked, the Gaussian fitting tools are displayed. These allow a Gaussian to be fitted to a spectral peak for kinetic temperature and column density estimation.
When checked, the marker table is displayed and the user may place two markers (M1 and M2) on the chart for accurate display of the corresponding values.
When checked, the X axis is reversed. This allows switching between an axis that increases with frequency (which is most common in engineering) or increases with velocity (which is most common in radio astronomy).
Determines the reference frame used for calculating velocities from frequency.
- Topo is a topocentric reference frame (i.e. relative to the observation location).
- BCRS is the barycentric celestial reference system (i.e. relative to the Solar System's barycenter (centre of mass)).
- LSR is the local standard of rest (i.e. relative to the local standard of rest, which accounts for the Sun's movements relative to other nearby stars).
Professional astronomers tend to plot spectra using the LSR, so any observed Doppler shift can assumed to be due to the source moving.
When checked, the Gaussian fitting tools are displayed. These allow a Gaussian to be fitted to the data, allowing measurement of the HPBW of the antenna.
When checked, the marker table is displayed and the user may place two markers (M1 and M2) on the chart for accurate display of the corresponding values from the measurement series.
When checked, the marker table is displayed and the peak Max and Min markers are displayed at the maximum and minimum values on the measurement series.
An estimate of the HPBW in degrees of an antenna whose main lobe corresponds to the Gaussian profile of a drift scan of the Sun, using a linear scale (E.g. Y axis must not be in not dB).
Specifies the palette / gradient used to plot the 2D map. This can either be colour or greyscale. The gradient is applied linearly between the Min and Max values.
Power measurements in SDRs are typically relative (E.g. dBFS) rather than absolute (E.g. dBm). In order to produce absolute power measurements,
and thus noise temperature measurements, we need to perform a calibration process that calculates a mapping from the relative power value to an absolute value.
Also, there are multiple unwanted noise sources that contribute to the measured power (LNA and receiver noise, for example),
that we wish to subtract from our power measurement, to get a measurement of the power of the radiation received from the astronomical object we are observing.
The first step is to measure the noise of the receiver, Trx. This is the combined noise of the LNA, feed line and SDR, for a particular gain setting.
This can be measured with a calibrated noise source connected to the LNA input using SDRangel's [Noise Figure plugin](https://github.com/f4exb/sdrangel/blob/master/plugins/channelrx/noisefigure/readme.md),
or estimated from datasheet values for the individual components using the [Friis formula](https://en.wikipedia.org/wiki/Friis_formulas_for_noise).
It is also possible to calculate this within the Radio Astronomy plugin by running a hot and cold calibration. The plugin will then use the Y factor method
to estimate Trx, and this will be displayed in the Trx field, below the chart. Whatever method is used, the value should be entered in to the Trx field in the Settings area.
In order to map relative powers to absolute powers (and temperatures), a hot calibration should be run. To run a hot calibration, the noise
temperature of the calibration source is entered in to the Thot field (or power into Phot) and then press the "Start hot calibration" button. (The process
is likewise to run a cold calibration). The main consideration for a user, is what can be used as a calibration source and how is it connected to the antenna/receiver.
There are two ways, with and without an antenna:
For parabolic dishes or horn antennas, an object at a known temperature can be used, so long as it completely covers the feed horn aperture.
The object needs to be as close to an ideal blackbody as possible, with high emissivity at the frequencies of interest, so that the temperature
of the object results in an identical increase in noise temperature in the antenna. If the dish is steerable to point towards the ground, the temperature
of the ground may be used.
It is also possible to calibrate by directly connecting a noise source to the LNA input. This could be as simple as 50Ohm termination resistor,
which should result in a noise temperature corresponding to the physical temperature of the resistor, assuming good impedance matching and very low insertion loss.
One large unknown can be the spillover temperature, Tsp. This is the noise contribution due to ground or building thermal radiation leaking in
to the feed horn from the back or side lobes. Once Trx is known, is possible to estimate Tsp by performing a hot and cold calibration,
where the hot calibration uses an object blocking the feed, but the cold calibration has the feed unblocked pointing to a cold part of the sky.
The temperature of the cold sky can be estimated from an all-sky survey in Star Tracker, and this is displayed under the calibration chart as Tsky.
If Thot is measured with Tsp=0, Tcold is Tsky, and Trx is known, then the plugin can estimate Tsp for the cold measurement. Note that Tsp is typically strongly
dependent on the antenna's elevation and azimuth, as this changes the amount of ground thermal radiation that gets in to the antenna.
When clicked, shows the Calibration Settings dialog.
The Calibration Settings dialog allows a user to control hardware used for calibration. It supports two methods: GPIO pins in a SDR can be toggled during calibration and/or
commands/scripts can be run before and after calibration. The pre-calibration delay setting specifies a delay in seconds between the GPIO being toggled or start command
being executed, before the calibration routine in the plugin starts.
An example of its use would be to electronically switch in a 50Ohm resistor to the LNA input when calibration is run, using one of the SDR's GPIO pins to control the RF switch.
When checked, results of a new calibration will be applied to all existing measurements. When unchecked, the calibration will only apply to new measurements.
Many equations are from Essential Radio Astronomy by James Condon and Scott Ransom: https://www.cv.nrao.edu/~sransom/web/xxx.html
The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: https://arxiv.org/abs/astro-ph/0504140 and EU-HOU project: https://www.astro.uni-bonn.de/hisurvey/euhou/index.php
Thermometer icons are by Freepik from https://www.flaticon.com/
Reverse icon by Creaticca Creative Agency from https://www.flaticon.com/
