How to analyse your measurements of the signal strengths

Joachim Köppen Strasbourg 2013

- The data file
- First analysis
- The station gain
- Analysis of the complete pass
- download instructions to predict satellite positions during a pass
- Alternative Approach: using the Worksheet
- Analysis with enhanced appearance
- Results and Conclusions

In this example of XI-IV data we see

- the maximum signal strength at closest approach was a bit more than -100 dBm
- however, shortly after AOS, the signal was even stronger for about a minute or two
- during the pass, the signal showed almost periodic changes, with a period of more than one minute. It is likely that these fluctuations are caused by the tumbling motion of the satellite, during which the transmitting antenna is directed towards us or is obscured by the satellite's body.

From the maximum signal, which would normally occur at minimum range, we can
now use the **link budget** to work out the power of
the satellite transmitter.

- our measurements refer to the power at the receiver's antenna socket. We have to correct for the losses in the coaxial cable (-2.5 dB) to the roof, and for the gain of the mast-head pre-amplifier (+20 dB)
- our antenna gain: +19.3 dBi
- there are about 3 dB losses from the long coaxial cable from the antenna to the receiver
- free space loss: needs the range
- neglect other factors, like ionospheric absorption ...
- assume a reasonable value for the transmitting antenna gain: 0 dB

- If Cute-1 had its nominal power of 100 mW (+20 dBm), the station gain would be +33.0 +/- 0.8 dB
- If XI-IV was transmitting with 100 mW, the station gain would be +30.0 +/- 0.6 dB.

As it is unlikely that the transmitter power of a satellite becomes higher with
time, and as the value obtained with Cute-1 is closer to the theoretical value,
we believe that **our ground station has a measured gain of +33.0 dB**.
We recommend to use this value in your analyses and enter it in the PassFinder
applet to get the predicted signal powers. The remaining difference of 3 dB could
well be the sum of all additional losses from the connectors and the uncertainties
in the calibration of the measuring software.

Now that we have established a good value for the station gain, we can try to measure and monitor the powers of satellites: The above values also show that signal strengths of the two satellites differ significantly, by 3 dB. It appears very likely that the transmitter of XI-IV is really lower. It nominal power is indeed 80 mW (+19 dBm). May be this has come down by another 2 dB since its launch in 2003 ... From a single observation of XI-V, Feng-Lei estimates a transmitter power of +15 dBm (32 mW), whereas its nominal power should be the same as that of XI-IV.

- use the PassFinder applet to produce a detailed output of the pass
- use a ready-made Excel workbook (see instructions below)
- build your own worksheet using the detailed description of the method and the formulae.

Let us use the first option: compute with the PassFinder applet
the passes predicted for the satellite on the day of observation.
Also enter the correct value for the station gain. Then click
**Pass: Textoutput** and use the buttons **next pass** and
**previous pass** to choose the proper pass. Please note that the times are
given in local time (CEST)!! Then you will get this table in simple ascii text:

It is a good idea not to plot the predicted data straight away, but to allow that the predicted curve can be shifted in time as well as in the power level. In this way you can correct for some delay of the observed data in case the recording was did not start right at AOS. Similarly, you can modify the predicted signal strength in case the real transmitter power might differ from the one assumed in the predictions. Therefore create two cells which contain the values for the time and power shifts, and then create two new columns which contain the predicted times plus the time shift, and the predicted powers plus the shift value. These two columns you then add to the plot of the measured signal strengths, to get a plot like this:

Now we have to see what can be deduced from this comparison: see the results for our example pass

**Alternative Approach: using the Worksheet**

- enter the general data: altitude, inclination, inclination and frequency of the satellite
- put in the Azimuths at AOS and LOS and the maximum elevation. Also,
put in a guess value for the
**asc.node**which is the longitude where the satellite crosses the equator from the South to the North. From the map shown by NOVA you will already have an estimate for this longitude. Note that the worksheet takes positive values for Eastern longitudes! - have a look at the plot
: **asc.node**value until the curve fits snugly in the box, as shown above. - now you can make predictions for any parameter! Plots exist already for
longitude/latitude, the Doppler shift ...
- ... and the Signal(time) curve:
**EIRP**and the**station gain**which summarizes the receiver antenna gain, the preamplifier gain, and the cable losses, since our measurements refer to the receiver's antenna socket!

**Analysis with enhanced appearance**

In this example of XI-IV data we see

- the maximum signal strength at closest approach was a bit more than -100 dBm
- however, shortly after AOS, the signal was even stronger for about a minute or two
- during the pass, the signal showed almost periodic changes, with a period of more than one minute. It is likely that these fluctuations are caused by the tumbling motion of the satellite, during which the transmitting antenna is directed towards us or is obscured by the satellite's body.

- while the duration of the pass agrees very well with the prediction, the maximum signal does not seem to happen at the predicted time
- the signal just after AOS - when the satellite still is close to the horizon - is larger than at any moment thereafter. Could this be an unusual mode of propagation?
- if one wants to match the peak value of -100 dBm with a satellite's EIRP of 100 mW (from the published satellite specifications) one would need a value of only 28 dBi for our antenna gain. However, the GENSO specifications are 19 dBi antenna gain plus 20 dB preamplifier gain ... thus there seem to be unaccounted losses somewhere in the link budget ...
- ... but if we put in the 36 dBi to account for our antenna, preamplifier, and
cable losses, the predicted signal strength shortly after AOS would be
about -110 dBm, which is close to the measured signal! We must
**conclude that this "extraordinarily high" signal at AOS represents the full power received, but that at later times the satellite's antenna was simply not oriented for optimal reception by our station**!

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last update: Apr. 2013 J.Köppen