[amsat-bb] Re International Space Station-Bounce on 1296 MHz (simpler calculation using RADAR equation )

i8cvs domenico.i8cvs at tin.it
Fri May 31 23:40:33 PDT 2013

Hi Domenico,

Thanks for your interest in the ISS reflections

Now about your questions.

1. I started calling Andreas as soon as my dish could see ISS.
    That was on May 23 at 8 degrees elevation.
    So the distance would have been close to 2000 km at that time.
    Andreas reports that he heard me right from the beginning!
    Own echo's is not possible because the time is too short

2. We heard the strongest signals when ISS was right overhead. In my case
     that was at about 60 degrees elevation.
     Distance probably somewhere around 500 km?
     On the SDR recording from Andreas we can see the signal peaking 20 dB
     over the noise and sometimes even a bit more.

3. We both used analog CW but we both listened in the SSB passband (approx.
     2.5 kHz) to have some margin of error is case our Doppler compensation
     was not 100% OK.
     But once I found Andreas I did not touch the RIT of my rig anymore
     during the QSO. Doppler compensation worked fantastic!

We hope to try other modes in future. FSK441 but also JT65 in 30 sec period
mode, and when ISS is overhead probably SSB is possible

I am also involved in the restoration of the 25 m Dwingeloo dish PI9CAM (see
And the dish will be on air in a few months from now (I hope)
We hope to experiment with passive satellite reflection there.
ISS will be the start and with the dish having 48 dB gain it will be
possible with QRP
But maybe it's possible with other low orbiting satellites too....

To be continued!


Hello Jan, PA3FXB

It is very interesting that you heard the strongest signals
when the ISS was right overhead and that in your case that
was at about 60 degrees elevation with distance from the
ISS somewhere around 500 km and that in the SDR recording
from Andreas you can see the signal peaking +20 dB over the
noise and sometimes even a bit more.

It is also interesting that both of you were using analog
CW and both listened in the SSB passband (approx. 2.5 kHz)

As you can see looking at my previous budged calculations
the expected S/N ratio on CW was +6.9 dB using a receiving
bandwidth of 500 Hz on CW

It is possible that my calculations showed a less level in
ratio S/N = +6.9 dB in comparison to your received +20 dB
because for calculation with the RADAR equation I have
used a reflection factor for the ISS of only 10% wich in
reality is very low because for the Moon  we use a
reflection factor of 7% but the Moon is made of stone and
not a metallic reflecting object like the ISS.

Considering that the ISS is made almost of metallic structural
material as well for supporting the solar panels and considering
that the efficiency of a metallic parabolic dish is never better
than 50% I have uptodate my previous link budged calculation
using a reflection factor of 50% for the ISS and as you can read
belove the S/N ratio at a distance of 700 km jumped up to +17dB
over the noise !

This means that your experimental investigation receiving +20 dB
match well with my calculation showing +17 dB and for the future
we can consider that a reflection factor for the ISS of 50% is a real
figure to be used.

Since you live in a quite location the antenna temperature at
1296 MHz when aimed toward the Cold Sky can be only 5 degrees
kelvin instead of the previously estimated 50 degrees kelvin
so that the overall Noise Floor of your receiving system
decreases by 3.29 dB

Read please the following revised calculations using a ISS
reflection factor of 50% instead of 10% at a range of 700 km
and 5 degrees kelvin for the equivalent antenna temperature Ta
instead of 50 kelvin

The following calculation is easer than the previous one because
uses directly the RADAR equation.


 We consider the ISS like a passive reflector with reflectivity
 factor of 50 % to try a QSO by reflection Earth-ISS-Earth


 1) The solar panels of the ISS plus the central body large like a
      Boeing 747 have a metallic reflecting surface of about
      2000 square meters and we consider the ISS like a circular
      RADAR target having being a metallic plate an estimated
      reflectivity factor S of 50 % at SHF

 2) The range EARTH-ISS at elevation of 35 degrees is about
      700 km

 3) Our EME station at 1296 MHz uses a 3 meters dish in diameter
      with gain of 29 dB and 200 W at the feed

 4) The overall noise figure of our receive system is NF = 0.5 dB
      while the antenna temperature is 5 kelvin when pointed at the
      Cold Sky and we receive on CW using a filter with a BW large
      500 Hz

 5) We use only analogic reception without digital software like
      WSJT or similar tecniques.


 Aiming the dish towards the ISS when distant 700 km and
 transmitting on CW and using the RADAR equation we
 calculate the Signal to Noise ratio S/N to see if on CW the
 echoes reflected by the ISS are above or belove the Noise
 Floor of receiver.

                                  RADAR EQUATION

                                      Pt x Gt x Ar x S
                  Pr =      -----------------------------
                                  ( 4 x 3.14 x R^2 ) ^2
  Where :

  Pr  = power received in watt

  Pt  = power transmitted = 200 watt

  Gt = isotropic gain of a ground antenna at 1296 MHz  = 29 dB
           or 794.3 time in power

  Ar = aperture area of isotropic antenna at 1296 MHz = 0,0043
           square meters

  S = Sigma or Radar Cross Section i.e. the surface of the ISS in
         square meters with reflecting coefficient of 0.50 = 50 %

  R = distance or range EARTH-ISS = 700 km = 700000 meters

  (4 x 3.14 x R^2)^2  calculates the surface of a sphere having a
  radius R=700000 meters squared 2 to take account of the
  round trip  "EARTH-ISS-EARTH"


 S = Sigma of the ISS with reflecting surface of 2000 square meters
 and reflection coefficient of 50 % = 2000 x 0.50 = 1000 square meters

Calculation of the aperture area  Ar of isotropic antenna at 1296 MHz

                 / 2                      2
               /\              0,2314
 Ar = ---------- =  ----------- = 0,0043  square meters
          4 x 3,14       4 x 3,14

 Calculation of the received power Pr on the EARTH
 collected by the antenna with gain of 29 dB or 794.3
 time in power at 1296 MHz

          200 x 794.3 x 0.0043 x 1000                     -20
 Pr = ------------------------------------- = 1.80 x 10      watt
              (4 x 3.14 x 700000^2 )^2

Pr = 10 log     1.80 x 10        =  -197.4 dBW


 Data of the 1296 MHz receiving system :

 Overall Noise Figure of receiving system NF= 0.5 dB = 35 kelvin
 Bandwidth BW of receiver on CW  = 500 Hz
 Equivalent Noise Temperature Ta of the antenna when aimed toward
 the Cold Sky = 5 kelvin

 Calculation to get the Noise Floor KTB of receiver

 Where :
 K = Boltzmann constant = 1,38 x 10      joule/kelvin

 T = Equivalent noise temperature Ta of the antenna  plus the
 equivalent Noise Temperature Te of receiver i.e. T= (Ta + Te)

 Computation of the overall Noise Factor F for the receiver
 F = 10 ^ (NF/10) and so F = 10 ^(0,5 / 10) = 10 ^0,05 = 1,12
 in factor

 The equivalent overall Noise Temperature Te of the receiver is
 Te = ( F-1) x 290 = (1,12 -1) x 290 = 35 kelvin

 The overall Noise Floor KTB of the receiving system with the
 antenna connected is :

 KTB = K  x  ( Te + Ta ) x BW  and in numbars :

  Noise Floor KTB = 1,38 x 10    x  ( 35 + 5 ) x 500 = -185.5 dBW

 LINK CALCULATION  "round trip" EARTH -ISS-EARTH at 1296 MHz
 on CW

 Power Pr received on EARTH  over
 1296 MHz isotropic antenna ........... - 197.5 dBW
 Antenna Gain in RX.........................  +  29 dB
 Power available at receiver input .... - 168.5 dBW
 RX Noise Floor......................... ........- 185.5 dBW
 S/N ratio CW at RX audio output...  +  17.0  dB


 At a range of 700 km from the ISS and using a 3 meters dish with
 200 W at the feed and a receiving system with a Noise Floor of
 -185.5 dBW = - 155.5 dBm it is possible to hear your hown echo
 signals in plain analogic CW with a Signal to Noise ratio S/N of
 +17.0 dB wich is very strong.

 If two stations are equipped with the same equipments it is
 possible to make good QSO for a short time when the ISS
 is close range around 700 km at elevation of about 35

 Increasing the BW of receiver at 2700 Hz to try receive SSB
 it is possible since 10 time log (2700/500) = 7.3 dB and
 +17 dB - (+7.3 dB ) = +9.7 dB and so the signal scattered back
  by the ISS in SSB will have a S/N ratio of about + 9.7 dB
  i.e. well over the Noise Floor of your receiver.

 It is necessary to use a precise traking system particularly fast
 to move the dish as soon the ISS approach the TCA because
 the beam wide of a 3 meter dish with gain of 29 dB is large
 only about 5 degrees at the -3 dB points so that traking the
 ISS at 1296 MHz with a 3 meter dish or even larger for EME
 it seems to be the more critical point of the experiment.

 73" de

 i8CVS Domenico

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