Working Group SE
of the Electronic
Communications Committee
SE 40
0.1.1.1.1.1
Paris February 2010
Source: CRAF (Axel Jessner, MPIfR Effelsberg)
Subject: Parameters of instrumental support for Leeheim measurements in the RAS
band 1610.6 - 1613.8 MHz
Date issued: 4 February 2010
Password protection required? (Y/N)
N
Previous measurements of the interference caused by the IRIDIUM MSS have been difficult to
interpret and were limited in their statistical significance. Measurements at radio astronomical
observatories (i.e. Effelsberg) were able to provide vital spectral characteristics of the interference, but
the antennas are unable to track the source of interference and the limitations on available telescope
time preclude a full assessment of the distribution of interference over the celestial hemisphere.
The Leeheim station could track and identify individual satellites, but was unable, for instrumental
reasons, to obtain full spectral and temporal coverage of the interference scenario. Future
measurements, intended to clarify remaining open questions and also to verify the compliance of
IRIDIUM with agreements concerning the suppression of interference to radio astronomy would
benefit from a combination of capabilities of radio astronomical equipment and Leeheims satellite
tracking facilities.
Based on discussions with engineers from the Leeheim monitoring station, CRAF offers instrumental
and personal support for prospective future Leeheim measurements of interference from IRIDIUM
MSS in the radio astronomical band. Details of the calculations and background are supplied in an
auxiliary document.
1. Description of the MPIfR Fast Fourier Spectrometer (FFTS)
The FFTS is a development of the MPIfR Digital Laboratory and used for radio astronomical
observations in Effelsberg and APEX (Chile). A compact, portable version of the instrument is
now available. This instrument is capable of operation in a non-astronomical environment and
could therefore be used for Leeheim satellite measurements.
Characteristics:
Spectrometer bandwidth:
 ffts
100 . MHz (base band input : 0 – 100MHz )
Number of channels:
N chan
2
hence the channel bandwidth is:

Minimum integration time
 min
14
 ffts
N chan
= 16384
3
or  = 6.104 10
Hz
0.1 . s
Minimum dead time between integrations:
blank = 50s
These times are constrained by the available bandwidth for internal data transfers and control, but continuous
recording of spectra is possible within these constraints, leading to a data loss of less than 0.05%
The signal is digitized by an eight bit ADC, therefore the maximum input amplitude of the
ADC limits the input power levels. This will result is different input limits depending on the
spread of the signal spectrum.
Optimal input level (50 Ohm):
-10 dBm (total power one single channel),
for a channel bandwidth of 38 dB(Hz) this corresponds to a spectral power density of
-48 dBm/Hz.
Maximum full bandwidth noise power density:
10
10. log
 ffts
Hz
= 90
dBm/Hz
For smaller noise bandwidths, the input level may be adjusted accordingly.
Note: The FFTS does not contain an anti-aliasing input filter, hence the input level will have to be adjusted to the
actual bandwidth of the supplied input signal.
Noise floor and IM properties:
MPIFR AFFTS 2010-02-02T16:32:30.0506PC
-50
-114
-60
Spectral Power Density dBm/Hz
Spectral Power Density dBm/Hz
MPIFR AFFTS 2010-02-02T11:12:51.0657PC
-112
-116
-118
-120
-122
-124
-126
-128
-70
-80
-90
-100
-110
-120
0
20
40
60
frequency [MHz]
80
100
-130
0
20
40
60
frequency [MHz]
80
100
The left hand panel shows the spectrum without any input signal. FFT artefacts occur at integer
fractions of the sampling frequency: 25, 50, 75 and 100 MHz. The strongest artefacts correspond
to a signal spd of -112 dBm/Hz. These artefacts are narrow band (single channel) and can be
masked out if necessary. For the remaining spectrum, the noise floor is at -125dBm/Hz.
Therefore the available dynamic range is ca. 70 dB.
The right hand panel shows the spectrum obtained after feeding two -13 dBm signals, one from a
Schomandl frequency synthesizer at 20 MHz and the other from a Wavetek signal generator at
58.51 MHz. The signals themselves showed harmonics at about -40 dBc which made
determination of higher harmonic intercept levels difficult. However from the two second order
products at f 2 f 1 = 18.51 MHz and f 2 f 1 = 98.51 MHz one could determine the
IIP2 as -2.8 dBm per channel or equivalently -40 dBm/Hz.
For a maximum input dual frequency signal level of -50 dBm/Hz we find that the second order
IM products occur at -50 dBc, or -100 dBm/Hz. An input reduction by about 10dB will reduce
the IM2 to values below the noise floor.
Third and higher order IM products were visible at about -110 dBm/Hz, but did not rise with the
input signal within the safe operating range of the FFTS.
Dataformat and Software:
The data are binary 16k 4 Byte floating point values plus a small header, in total 65kB per
recorded spectrum. For the shortest integration time of 0.1 seconds, the data rate will be 40MB
per minute.
The format is documented and MATLAB routines are available for control of the FFTS and the
data reduction.
Network operating requirements:
Two 100 MB/s internet connections for control pc/laptop and FFTS. DHCP service for the
provision of local IP addresses. The FFTS will attempt to set the internal clock for accurate time
stamping of the spectra to UTC using public NTP servers. Access to WAN would be
advantageous.
Two 240 V ac mains sockets for FFTS and laptop
2. Leeheim Characteristics and Calibration Procedures
All the information and inferences are based on information by engineers from Leeheim. No actual
measurements of the actual system parameters have so far been made by radio astronomers.
According to information from Leeheim, the 12m Antenna has a main beam gain of currently
44dBi and a beam width of 1°. The zenith system temperature at 1620 MHz (without stop band
filter) is T sys 120. K
The dependence of antenna temperature on elevation was carefully determined in 1980, when the
antenna was commissioned.
250
Antenna Temperature
200
150
100
50
0
20
0
20
40
60
80
100
Elevation Angle
Ground radiation starts to contribute more than 3dB to the antenna temperature for elevations below
6°. However, the main contribution to system temperature during the IRIDIUM measurements is the
insertion loss of the stop-band filter.
Stop Band Filter Insertion Los s
0
dB
2
4
6
8
1610
1610.5
1611
1611.5
1612
1612.5
1613
1613.5
1614
1614.5
MHz
Regridding of the insertion loss to the spectrometer channel frequencies by linear interpolation
linterp  f , L f , 
L 1(  )
10
10
and combining it with the elevation dependent antenna temperature T ant() as interpolated from the measured
values, yields the system temperature as a function of frequency and elevation:
T sys(  ,  )
T ant(  )
290 . L 1 (  )
T RX
1
The following graph illustrates the behaviour of the system temperature for different elevations within the radio
astronomical band.
System Temperature
1000
900
800
1610.6
1613.8
Kelvin
700
600
500
400
300
200
100
0
1610
1610.5
1611
1611.5
1612
1612.5
1613
1613.5
1614
MHz
The green trace shows the expected system temperature at =90° (zenit), the black trace at =5° and the red trace
at =0° (horizontal). The filter loss is responsible for the majority of the system noise, only for elevations near
the horizon (< 3°) can we expect a significant contribution of the ground radation.
One can now make a final sensitivity estimate for the envisaged Leeheim monitoring configuration:
S min(  ,  )
2. k . T s ys(  ,  )
A eff
 .  min
The result of which is shown in the graph below:
Channel Sensitivity
1000
1610.6
1613.8
Jy
800
600
400
200
1610
1610.5
1611
1611.5
1612
1612.5
1613
1613.5
1614
MHz
The configuration cannot detect 0.1 second transients on the ITU-R RA 769 level of 158.5 Jy
(= -238 dB(Wm-2Hz-1) ) , but can detect signals that are exceeding that level by about 3-6 dB.
Analysis for expected IRIDIUM signals:
The free space path loss for 1300 km tx - rx distance is given by
1.6 . GHz
1300 . km
20. log
20. log
or
GHz
km
Known values of the IRIDIUM system (SE40 (09)92 from Germany):
Lb
92.5
Iridium channel power
IM power level
L b = 158.861
dB
-13dB(W) and 24dBi gain = 11 dB(W) e.i.r.p.
-29 dB(W) e.i.r.p.
Expected signal for full power IRIDIUM carrier (main beam): P carrier
P carrier = 73.619 dBm
Stop band filter suppressed carrier:
P carrier 65 = 138.619
Expected interference signal
P IM
29
30
P IM = 113.619
30
Lb
G max
dBm
G max
dBm
k. T sys( 1613 . MHz, 30 . Grad ) . 
Noise Power level in one spectral channel at 300 K: 10 . log
Sensitivity at minimum integration time:
Lb
11
0.001 . W
10. log
k. T s ys( 1613 . MHz, 30. Grad ) . 
= 135.03
= 148.958
dBm
dBm
0.001 . W.  .  min
Optimal average FFTS input level:
10

10. log
= 47.707

dBm (per channel)
The estimates refer to the antenna output. That means that about 80 – 90 dB amplification is required to match
antenna output and FFTS input.
Interference by the IRIDIUM MSS should be clearly visible with a S/N of up to 20dB
Calibration
Strong celestial radio sources can be used to calibrate the system as had been done in 1980 when the
system was commissioned. Details and background of the calibration procedure are outlined in the
auxiliary document. Observing a transit of the moon through the main beam of the antenna will raise
the system temperature by about 50 K (Kuzmin, 1966).
The average power level on the empty sky will be
P0
10. log
T s ys. k.  RAS
P 0 = 108.496
W
dBm
T s ys T om.
Pointing to the moon will give:
Pm
10. log
P m = 107.846
30
 moon
 HPBW
W
2
. k. 
RAS
30
dBm
This is a rise of just 1dB and easily missed on a logarithmic power meter. A linear display is therefore
mandatory. A similar increase will also be seen on the transit of strong radio sources (see list in aux. document).
The FFTS records power levels of individual channels, and it can be used to calibrate the assembled receiving
system for individual channels and for the full band. The following graphs show the result of a 0.5 dB calibration
signal being switchend on for ca. 15 seconds:
MPIFR AFFTS 2010-02-02T14:17:20.4787PC
0.0315
MPIFR AFFTS 2010-02-02T14:17:20.4787PC
0
5
0.031
average power [mW]
10
time [s]
15
20
25
30
35
40
0.0305
0.03
45
10
20
30
40
50
60
frequency [MHz]
70
80
90
0.0295
-96
-95.8
-95.6
-95.4
-95.2
-95
-94.8
-94.6
Spectral Power Density dBm/Hz
0
10
20
30
40
50
time [s]
The signal is clearly visible and measurable with a good S/N ratio. Loss of sensitivity can be converted into a
loss of antenna efficiency which for the moon is calculated by
P m 30
A
10
10
P0
10
10
.
k T om. 
30
. W  HPBW
.
 moon
2
The average efficiency factor can be determined with good accuracy, enabling an absolute calibration
of the receiving station. In the same manner can individual spectral channels be calibrated. For
calibration of the main carrier band, one can use the average efficiency and correct the measured
power levels by the known filter attenuation.