Some brief explanations

This is a simulator of the software with which the 1.4 GHz antenna at DL0SHF in the village of Rönne is controlled and operated to make radioastronomical measurements. It permits to make 'observations' of sky objects in the same way and to produce data in the same quality as the real instrument. The Radiometer version measures the radio flux in the continuum from about 1420 to 1428 MHz.

This version can also simulate the direct flux calibration if one were to hold in front of the feed antenna an absorbing body of known temperature and large enough to cover it completely. (unfortunately, this is not feasible with the real antenna!) With a calibration at two known temperatures it is possible to measure the temperature of the Cosmic Microwave Background

The real instrument has these technical data:

• Diameter of the parabolic dish: 9 m
• Effective diameter of the illuminated dish: 7 m
• Effective area of the illuminated dish: 40 m²
• Antenna gain: 40 dBi
• System temperature of the receiving system: 50 K
• Sensitivity: 69 Jy/K

Access to the simulator is organized in three pages: Operate, Skyview, and Output. Here is a description of the controls:

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Operate: this is the main page, from which all operations are effected, and where all information are displayed.

• Startup: click this to start the operations. If everything starts up properly, the clock will show the current time in UT, and the local sideral time LST. Also, now all controls can be operated.
  A second click will stop the operations -- however, this would make real sense only with the real instrument, of course.
• Direct calibration ...: clicking one of these three buttons simulates holding an absorbing body at a chosen known temperature in front of the feed antenna. The temperatures are 290 K (usual conditions), 195 K (sublimation temperature of solid CO₂ (dry ice)) and 77 K (boiling temperature of liquid N₂).
• goto Park: this lets the antenna go to its parking position.
• Sun now: a click will display the current position of the Sun in the sky in the fields to the left of the Goto button. Note that this will not move the antenna. If the Sun is below the horizon, the text fields will show a yellow background.
• Moon now: displays the current position of the Moon. Again, this does not move the antenna.
• Sun ...: from this list of sky objects one may choose an object, whose present position in the sky is displayed.
• now: one may choose to pick the object's position at the current moment ("now") or at some later time (e.g. " in 10 min").
• Go & Track: clicking this commands the antenna to go to the position displayed (left of Goto button), and to track it on its way across the sky. When tracking, this button's text is blue.
  Clicking on it again stops the tracking, and the antenna halts at the last position.
• current Position: displays the sky position (azimuth, elevation) where the antenna is now pointing at.
• AzEl: (this button will have additional features ...)
  The two text fields to its right are used to display the present position of the sky object selected as above, but the user can also enter the position (azimuth, elevation) to which the antenna should go.
• Goto: clicking this button lets the antenna move to the position given in the text fields to the left.
• Offs Hor. Vert.: the user may enter any offsets to the current position. Enter your values and hit the 'Enter' key of your keyboard.
  Note that the offsets are in real angles, so that the horizontal offset is Δh = Δa * cos(e) differs from the offset in azimuth!
• Record: click on this button starts recording all measurements as text on the Output page. Another click will stop recording. Recording data is only possible when the program works in the current time (Now). Note that the Output page starts afresh when the next recording is started.
• the Plot: shows the received power in terms of decibels (dB) relative to the level of the receiver noise (represented by the system temperature T_sys = 50 K for this instrument) as a function of time.
  Passing the mouse over the plot area will display the time and power at the mouse position.
  The curve is gray when one is not recording, but red when data are recorded
• Ymax, Ymin: enter the desired value(s) and hit the 'Enter' key. The plot is refreshed to show it with the new plot limits.
• Xspan: choose the desired time span to be shown. The plot will always start with the time when the simulator was started or - when small time spans are chosen - at an even number of span intervals later. The simulator has a memory for about one hour.
• Set: applies the above changes in the plot limits to the plot.

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Skyview: shows the situation in the sky above the antenna. The gray area near the horizon shows the ground, two antenna masts, and the small grove of trees in the south-west which serves as the flux calibrator.
Sun and Moon are indicated as yellow and cyan disks, other celestial sources are shown as red dots with their name. The blue curve across the sky is the mid-plane of the Milky Way, with galactic longitudes of 90 and 180° marked as blue circles, and the Galactic centre as a blue dot.
When the simulator has been started up, this plot is updated in real time.

• Now: click this to show current situation.
• time - 1 hr: click this to show the situation one hour earlier ...
• time + 1 hr: ... or later
• Position: a mouse click on the plot then shows the position (azimuth, elevation) -- this works OK with Chrome, Firefox, and Edge, but on other browsers one may find that it works only if the window is not scrolled ...

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Output: when data are recorded, they are displayed on this page. From here they may be copied and pasted into a text file for further interpretation. The format is identical to that used by the real software: each datum is composed of time [UTC], azimuth, elevation, power [dB]. When a large number of data are recorded - for a couple of hours, say - the browser may have problems showing all the data easily. Therefore, it is better to avoid recordings longer than about one hour.

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How to ...

• ... make a drift scan of the Sun: This is to point the antenna at a position where the Sun will be some time later. One waits for the Sun to pass through the centre of the antenna beam, thereby allowing to measure the peak power (i.e. the power from the solar radio emission) but also the antenna pattern, i.e. the shape of the antenna beam and measures the angular resolution of the antenna (or HPBW = Half Power Beam Width) which is an important value for the interpretation of the measured solar data.
• choose "Sun" as object
• choose for example "in 10 min" as the wait time
• click Goto
• record the data
• wait for the Sun's passage ...
• ... monitor the solar radiation: This would be needed when you are interested to search for solar radio bursts due to activity. One simply tracks the Sun and records the data.
• click "Sun now" or choose "Sun" as object and "now" as wait time
• click Go & Track
• record the data as long as you need or want (I've no idea yet what happens when the Output page gets filled with lots of data ... hence no guarantee ...
• ... measure the system temperature of the instrument: Because the noise produced by the receiving electronics itself is one important (if not dominant) part of the measured power, it determines the sensitivity of the apparatus and its knowledge is necessary when one wants to interpret the data quantitatively.
• go to the Calibrator to measure the radio noise produced by the trees or the ground T_cal = 290 K), which adds to the receiver noise, represented by T_sys. The (linear!) power --- i.e. p = 10^p_dB/10 --- is p_cal = a (T_cal + T_sys) with some instrumental factor a.
• move the antenna to a position high in the sky, say elevation 60°, and measure
  p_sky = a (T_sky(e) + T_CMB + T_sys)
  
• As T_cal = 290 K from the calibrator, T_CMB=2.7 K from the cosmic microwave background, and T_sky(e) = T_zen/sin(e) are known or can be measured, the two above equations can be resolved to yield the system temperature. Then one may also determine the instrumental factor a.
• ... measure the sky's thermal radiation: Although the sky does not absorb much radiation at 1420 MHz, it does contribute to the noise measured by the antenna, because it emits thermal radiation. This foreground noise is present in EVERY observation of a celestial object, and hence we need to know it well, if we want to interpret our data.
  Fortunately, this atmospheric emission can well be modelled by a simple expression: The sky temperature depends on the elevation e in this manner
  T_sky(e) = T_zen/sin(e)
  The zenith temperature is determined in this way.
• go to the Calibrator to measure the radio noise produced by the trees or the ground T_cal = 290 K), which adds to the receiver noise, represented by T_sys. The (linear!) power is
  p_cal = a (T_cal + T_sys)
  with some instrumental factor a.
• move the antenna to a number of positions in the empty sky at various elevations, and measure:
  p_sky(e) = a (T_sky(e) + T_CMB + T_sys)
  
• Plot the (linear!) powers p_sky(e) as a function of the so-called 'Airmass' = 1/sin(e), and fit a straight line through the data points, such as p = b + m/sin(e).
  From this fit one gets the zenith temperature T_zen = b/m.
• ... measure the CMB temperature: This is done in a manner very similar to the measurement of the sky's thermal radiation and the determination of system temperature, but with the important difference that the 'direct flux calibration' is done at two different calibrator temperatures:
  • go to any position with the empty sky
  • perform direct calibrations with both the normal 'hot' temperature T_H = 290 K and a lower 'cold' temperature T_C.
  • move the antenna to a number of positions in the empty sky at various elevations (between 10° and 60°), and measure.
  • finish the observations by another calibration with both temperatures.
  The analysis goes like this:
  • Since during each calibration the feed antenna gets only the thermal radiation from the calibrator, the (linear) power received is given by: p(T_cal) = a(T_sys + T_cal). From the two calibration measurements one gets the instrumental factor a = (p(T_H) - p(T_C))/(T_H - T_C). The power value at T_cal=0: p₀ = p(T_H) - a*T_C = a*T_sys then gives the system temperature T_sys = p₀/a
  • As the (linear) power of the sky noise at elevation e is: p(e) = a (T_zen/sin(e) + T_CMB + T_sys), one plots the measured power as a function of the 'Airmass' = 1/sin(e), and fits a straight line through the data points, such as p(e) = b + m/sin(e). This yields both the zenith temperature T_zen = b/m and the CMB temperature T_CMB = (b-p₀)/a

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