ESA-Dresden Radio Telescope Simulator
Joachim Köppen Kiel July 2019


Some brief explanations

This is a simulator of the software with which the ESA-Dresden radio telescope on 11 GHz at ISU in Illkirch is controlled and operated to make radioastronomical measurements. This instrument measures the radio flux in the continuum in a 5 MHz wide section in the TV satellite band from about 10 to 12 GHz. The simulator permits to make realistic 'observations' of sky objects in the same way and to produce data with the same quality as the real instrument. Likewise, the amount of noise is similar, as are the occurrences of radar signals, birds flying across the antenna lobe, drift of the electronic noise level, and noise from the antenna motors. It includes the quiet Sun, the Moon with its monthly variation of surface temperature and radio flux, and a number of bright radio sources.

The real instrument has these technical data:

  • Diameter of the parabolic dish: 1.2 m
  • Effective diameter of the illuminated dish: 0.8 m
  • Effective area of the illuminated dish: 0.5 m2
  • Antenna gain: 40 dBi
  • System temperature of the receiving system: 250 K
  • Sensitivity: 5300 Jy/K or 0.19 mK/Jy

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


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 sidereal 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.
  • goto Calibrator: clicking this lets the antenna go to the flux calibrator, which is a grove of trees which provide thermal radiation at ambient temperature, like the ground. Thus we look at a source of well-known temperature - about 290 K.
  • 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 Tsys = 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 grey 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.

Skyview: shows the situation in the sky above the antenna. The grey 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 ...

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 might be advisible not to make recordings too long.


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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 Tcal = 290 K), which adds to the receiver noise, represented by Tsys. The (linear!) power --- i.e. p = 10pdB/10 --- is pcal = a (Tcal + Tsys) with some instrumental factor a.
    • move the antenna to a position high in the sky, say elevation 60°, and measure
      psky = a (Tsky(e) + TCMB + Tsys)
    • As Tcal = 290 K from the calibrator, TCMB=2.7 K from the cosmic microwave background, and Tsky(e) = Tzen/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.
    • This procedure neglects the contribution from the continuum emission from the Milky Way, but if all sky positions are sufficiently far away from the Galactic Plane, this error may well be neglected...

  • ... 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
    Tsky(e) = Tzen/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 Tcal = 290 K), which adds to the receiver noise, represented by Tsys. The (linear!) power is
      pcal = a (Tcal + Tsys)
      with some instrumental factor a.
    • move the antenna to a number of positions in the empty sky at various elevations, and measure:
      psky(e) = a (Tsky(e) + TCMB + Tsys)
    • Plot the (linear!) powers psky(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 Tzen = b/m. This fit also allows us to extrapolate it to Airmass=0, i.e. if there was no atmosphere!
    • This procedure does not take into account the contribution from the continuum emission from the Milky Way. If all sky positions are sufficiently far away from the Galactic Plane, this error may well be neglected...

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Station


current Position

Offs. Hor, Vert.

Frequency 11 GHz

Ymax

Ymin

Xspan





Recorded data (UT, Az, El, pwr [dB]):