EUMETSAT
COMPATIBILITY ANALYSIS BETWEEN THE METEOROLOGICAL SATELLITE AND
THE METEOROLOGICAL AIDS SERVICES IN THE BAND 401-403 MHz
1 Introduction
The frequency band 400.15-406 MHz is currently allocated to the Meteorological Aids Service with a primary status. Since WRC-97, the sub-band 401 - 403 MHz is also allocated on a primary basis to the Meteorological Satellite Service (Earth-space). Parts of the frequency band 401 - 403 MHz are used for the operation
of Data Collection Platforms (DCPs) transmitting to meteorological satellites. Previous studies have already
indicated that compatibility of the 2 services can be achieved by several means, such as frequency separation, time coordination, or geographical separation.
In view of several updates to relevant system characteristics and permissible interference criteria contained
in ITU-R recommendations, this study has been conducted with the objective to review any potential requirements for geographical separation in case of co-frequency operations. Figure 1 shows an overview of
the 2 cases which need to be considered. One is potential interference from the radiosonde to the meteorological satellite and the other is potential interference from the DCP transmitter to radiosonde receivers via lineof-sight propagation, propagation by diffraction, tropospheric scattering or layer ducting.
METEOROLOGICAL
SATELLITE
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RADIO-SONDE
DCP EARTH
STA TION
interference via
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diffraction, scatteri
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cti
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RADIOSONDE
RECEIVE R
FIGURE 1
POTENTIAL INTERFERENCE CASES BETWEEN RADIOSONDES AND DCPs
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2 Characteristics of Data Collection Platform Systems
Data collection platform systems are mainly operated in remote and sparsely populated locations. Their objective is to make observations and measurements of a wide range of environmental data such as physical,
chemical or biological properties of oceans, rivers, lakes, solid earth and atmosphere. These data are encoded
and transmitted by the DCP via whichever satellite is within the field of view to the central ground station for
further processing. Besides the majority of self-timed DCPs, numerous so-called alert DCPs are installed to
report in emergency situations which include distress signals and data for hazard/disaster recognition. There
are two components to such a system, one is the ground terminal or transmitting platform, the other is the
satellite receiving these data, converting them to a different frequency and transmitting them to a central meteorological station.
2.1
Satellite Characteristics
Every satellite which is part of a Data Collection Systems (DCS) can receive signals from up to several thousand DCPs. The DCS may contain several satellites of the same type. Table 1 contains a summary of DCS
currently operated by a number of administrations and organisations worldwide:
TABLE 1
Characteristics of currently operated data collection systems
Data Collection System
Operations in Band
Bandwidth
Meteosat (Europe)
402.0 – 402.2 MHz
200 kHz
GOES (USA)
401.7 – 402.1 MHz
400 kHz
GMS (Japan)
402.0 – 402.4 MHz
400 kHz
Elektro (Russia)
401.7 – 402.1 MHz
400 kHz
ARGOS (France)
401.6 – 401.7 MHz
100 kHz
The DCS employs a hybrid of frequency division multiple access (FDMA) and time division multiple access
(TDMA). The DCP receive bands of the meteorological satellites have been coordinated within the CGMS
(Coordination Group for Meteorological Satellites) to allow for an International Data Collection System
(IDCS) which is designed to support mobile DCPs, i.e. those DCPs on ships, ocean buoys, aircraft or balloons which move from the telecommunication field of view of one geostationary spacecraft to another. The
IDCS consists of 33 channels of 3 kHz bandwidth (402.0 - 402.1 MHz).
The remaining transponder bandwidth of the various Meteorological Satellites (MetSat) is used for nonmobile regional DCPs. These regional bands are arranged in channel widths of either 1.5 or 3 kHz and in
addition specific times are allocated in a TDMA fashion to self-timed DCPs for transmission in their assigned slots. A few of these channels are purely dedicated to alert DCPs for emergency communications including safety of life aspects.
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2.2
DCP characteristics
The European Meteosat DCS has 66 channels with a bandwidth of 3 kHz each and receives transmissions
from 2000 to 3000 DCPs. The individual DCPs transmit up to 1 minute every hour. Further technical characteristics are:
Bit rate
Modulation
Coding
Transmission bandwidth:
Uplink EIRP
Antenna type
100 bps
±60˚ PCM/PM/Bi-Φ (unfiltered)
BI-Φ-L (Manchester coding)
1.5 kHz
40 - 52 dBm
low gain helix or hemispherical circular antenna (RHCP)
For the helix antenna, a maximum gain of 11 dB and a half power beamwidth of 38 has been assumed. The
antenna gain distribution is given in figure 2.
12.0
sensor antenna gain (dBi)
10.0
8.0
6.0
4.0
2.0
0.0
-2.0
-4.0
-6.0
-8.0
-50
-30
-10
10
30
50
off-axis angle (deg.)
FIGURE 2
GAIN OF HELIX DCP ANTENNA
Operation of this type of DCP will be maintained for a number of years in the international and regional
DCS. Depending on the application, 'high rate' DCPs will eventually replace the current DCPs. The high rate
DCPs will have data rates around 1200 bps. Further study may be required for high rate DCPs.
2.3
Relevant Sharing and Coordination Criteria
Recommendation ITU-R SA.1164-2 contains the relevant interference criteria for service links of DCS. The
single entry interference levels presented in Table 2 are recommended to be used as sharing criteria. It shall
be noted that the sharing criteria of Table 2 are intended to be applied in frequency sharing analyses and the
coordination of frequency assignments (i.e., as the minimum levels of accepted interference for applicable
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pared with that assumed in Annex 1 of SA.1164 in order to help determine whether an interfering signal
power greater than the permissible single entry level can be accepted. Generally, this consideration may enable acceptance of interference levels that may be as high as those specified in the applicable interference criteria (Recommendation ITU-R SA.1163). For GSO-DCS, this would be –187.4 dBW per 100 Hz not to be
exceeded for more than 20% of time or –173.4 dBW per 100 Hz not to be exceed for more than 0.1% of
time.
TABLE 2
Sharing criteria for stations in the EESS and MetSat service
Frequency
band
(MHz)
Function and type
of earth station
Station
subject to
interference
Interfering signal power
(dBW) in the reference
bandwidth to be exceeded no
more than
20% of the time
Interfering signal power
(dBW) in the reference
bandwidth to be exceeded no
more than
p% of the time
Space-toEarth
Terrestrial
Space-toEarth
Terrestrial
401-403
Earth-tospace
Non-GSO data
collection,
low-gain antenna
Space
station
–183.1 dBW
per
1 600 Hz(1)
–184.8 dBW
per
1 600 Hz(1)
–175.9 dBW
per
1 600 Hz(1)
p  0.05
–176.2 dBW
per
1 600 Hz(1)
p  0.05
401-403
Earth-tospace
GSO data
collection,
low-gain antenna
Space
station
–190.9 dBW
per
100 Hz(2)
–197.4 dBW
per
100 Hz(2)
–173.4 dBW
per
100 Hz(1)
p  0.075
–173.6 dBW
per
100 Hz(2)
p  0.025
(1)
The interfering signal powers (dBW) in the reference bandwidths are specified for reception at elevation
angles  5°.
(2)
The interfering signal powers (dBW) in the reference bandwidths are specified for reception at elevation
angles  3.
NOTE 1 – The single-entry interfering signal power thresholds in Table 1 are the permissible levels of interfering signal power that fall within the specified reference bandwidth. Accordingly, the total power in interfering signals that are narrower than the reference bandwidth should be considered in frequency sharing
analyses. In cases where the interfering bandwidth exceeds the reference bandwidth or does not fully overlap
the passband of a specific receiver under study, the available frequency dependent rejection should be applied in conjunction with the specified permissible interference levels. The pertinent ITU-R SM Recommendations should be consulted for guidance on this matter.
NOTE 2 – The sharing criteria can be expressed as permissible power flux-density into the main beam of the
receive antenna by subtracting 10 log (G 2/4) from the values given in Table 1, where G is the antenna
gain and  is the wavelength.
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3 Characteristics of Radiosonde Systems
Radiosondes are operated to obtain measurements of meteorological parameters such as upper air pressure,
temperature, relative humidity, windspeed and direction in the atmosphere up to an altitude of 36 km. Radiosondes often drift away more than 100 km from the launch site with a maximum of around 300 km with extremely strong winds. The radiosonde measurements are vital to national weather forecasting capability and
hence severe weather warning services for the public involving protection of life and property.
Recommendation ITU-R SA.1165 contains a detailed description of a number of radiosonde systems. The
radiosondes and associated tracking systems provide simultaneous measurements of the vertical structure of
temperature, relative humidity and wind speed and direction over the full height range required. The variation of these meteorological variables in the vertical contains the majority of the critical information for
weather forecasting.
The observations are produced by radiosondes carried by ascending balloons launched from land stations or
ships or dropsonde deployed from aircraft and carried by a parachute. Radiosonde observations are carried
out routinely by almost all countries, two to four times a day with transmission durations up to 3 hours. The
observations are then circulated immediately to all other countries within a few hours. The observing systems and data dissemination are all organized under the framework of the World Weather Watch Programme
of WMO. The standard observations for civil radiosondes are nominally performed at 0000 and 1200 UTC,
but the actual launch times vary according to national practice and in some cases will be at least threequarters of an hour earlier than the nominal time. The launch may also be up to two hours later than nominal
if there are problems with preparation of the radiosonde prior to flight, if local air traffic regulations limit
launch times or if there is a malfunction during the initial flight. Intermediate observations at 0600 and 1800
UTC are also performed routinely in several countries.
The current number of radiosonde stations reporting regularly is about 900. About 800 000 radiosondes are
launched in a year in association with the WMO network and it is estimated that about another 400 000 radiosondes are used for defence use and specialized applications. Additional radiosondes and dropsondes are
launched periodically, often from temporary sites using mobile systems in response to abnormal weather or
requirements for testing. It shall be noted, however, that the above number includes also radiosondes operated around 1670 MHz.
3.1
Radiosondes Receiver Characteristics
Typical characteristics of currently used receivers taken from SA.1165 are given in table 3.
TABLE 3
Characteristics of 403 MHz antennas
Omnidirectional
(dipole, ground plane)
Directional corner
reflector, six corners
397-409
400-406
Horizontal gain (dB)
Omnidirectional
8
Vertical gain (dB)
Omnidirectional
–3
Amplifier NF (dB)
 3.5
 2.5
Amplifier gain (dB)
13
20
Type
Frequency range (MHz)
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3.2
Radiosondes Transmitter Characteristics
Typical characteristics of currently used transmitters taken from SA.1165 are given in table 4.
TABLE 4
Radiocommunication characteristics of 400 MHz radiosonde transmitters
Tuning range (MHz)
400.15-406
Maximum drift in flight (kHz)
 800
Nominal output power (dBW)
– 6.0
Maximum antenna gain (dBi)
2
Modulation
FM
Modulating PTU signal (kHz)
7-10
Deviation of the PTU signal (kHz)
45  15
Deviation caused by the VLF/Loran-C signal relay link (kHz)
100/300
Occupied bandwidth with Omega VLF (kHz) (– 40 dBc level)
280
Occupied bandwidth with Loran-C (kHz) (– 40 dBc level)
480
Occupied bandwidth with GPS (kHz) (– 40 dBc level)
200
Equivalent information rate of the PTU signal (bit/s)
1 200 (1)
Equivalent information rate of the PTU and GPS signal (bit/s)
2 400
Out-of-band emission (dBc)
 – 65
(1)
The information transmission rate is intended to indicate the actual data rate transferred from the
radiosonde to the ground receiver. Because of the current modulation techniques used by radiosonde systems, further study is needed to estimate these values.
The radiosonde transmitting antennas are usually quarter wave length monopoles.
3.3
Relevant Sharing and Coordination Criteria
Recommendation ITU-R SA.1262 contains sharing and coordination criteria for radiosonde systems. A revision has been drafted at the Working Party 7C meeting in August 2000 and the following tables contain already the updated information. In view of the relatively short duration of a DCP transmission, the short term
interference criteria may be the most appropriate one. The relevant data are contained in table 5:
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TABLE 5
Interference criteria for radiosonde systems in the MetAids
NAVAID radiosonde
system with
directional antenna
400.15-406 MHz
NAVAID radiosonde
system with
omnidirectional
antenna
400.15-406 MHz
300 kHz
300 kHz
Interference signal power (dBW) in the reference
bandwidth to be exceeded no more than 0.02% of
the time  I(0.02)
– 139.5
– 140.1
Interference signal power (dBW) in the reference
bandwidth to be exceeded no more than 1.25% of
the time  I(1.25)
– 150.9
– 143.5
Interference signal power (dBW) in the reference
bandwidth to be exceeded no more than 20% of the
time  I(20)
– 154.0
– 153.5
Parameter
System reference bandwidth
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4 Interference analyses
4.1
Interference assessment from Radiosonde Transmitters to DCP Receivers
For this link, the calculations are relatively straightforward. What is not exactly known is the actual bandwidth occupied by the radiosonde signal and hence the maximum interference power spectral density. Assuming that a very strong carrier component is not dominating the spectrum, one can use the equivalent information rate to estimate the mean power spectral density of the radiosonde. The geometrical constellation
is shown in Figure 3:
METEOROLOGICAL
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RADIO-SONDE
RADIOSONDE
RECEIVER
DCP EARTH
STATION
FIGURE 3
GEOMETRICAL CONSTELLATION FOR POTENTIAL UPLINK INTERFERENCE
The basic equation for interference received by the meteorological satellite
 isgivenby:
Pir Pd  GMET  ls  le  lp  lso  GRS
where: Pir
Pd
GMET
ls
GRS
:
:
:
:
:
Interference power density received
by satellite
Power density emitted by radiosonde transmitter
Antenna gain of satellite towards radiosonde
Space loss between satellite and radiosonde
Antenna gain of radiosonde transmitter towards satellite
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In order to obtain a feeling for the range of typical interference scenarios, 3 different cases have been considered. A worst case with maximum radiosonde power and high antenna gains for both the radiosonde backlobe and the MetSat DCP receiver, a best case with favourable assumptions and a mean case with intermediate values. Table 6 shows typical link budgets for the received interference levels at the meteorological satellite.
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TABLE 6
INTERFERENCE LEVELS RECEIVED AT METSAT
System Characteristics
adverse
case
mean case
favourable
case
METSAT orbit height
36000
36000
36000 km
Carrier frequency
0.403
0.403
0.403 GHz
-6.0
-7.0
-8.0 dBW
0.0
-2.0
-4.0 dBi
1200
1200
1200 bps
Radiosonde power amplifier
Radiosonde antenna gain towards MetSat
Equivalent information rate
Equivalent bandwidth based on equ. information rate
30.8
30.8
-16.8
-19.8
Distance between radiosonde and MetSat
36000
38000
40000 km
Propagation loss
175.7
176.2
176.6 dB
6.5
6.0
Received interference power at MetSat
-186.0
-190.0
-193.4 dBW
MetSat long term interference protection criterion
-190.9
-190.9
-190.9 dBW
4.9
0.9
-173.4
-173.4
-173.4 dBW
-12.6
-16.6
-20.0 dB
Equivalent interference power density
MetSat antenna gain
Interference excess for long term criterium
MetSat short term interference protection criterion
Interference excess for short term criterium
30.8 dBHz
-22.8 dBW/100Hz
6.0 dBi
-2.5 dB
It can be seen that the long term criterion is not met at all times. A worst case interference excess of around
4.9 dB could occur. The short term criteria are met in all cases.
As pointed out in section 2.3, acceptance of interference levels may be considered which are as high as those
specified in the applicable interference criteria (Recommendation ITU-R SA.1163). For GSO-DCS, a long
term criterion of –187.4 dBW per 100 Hz not to be exceeded for more than 20% of time may be considered
appropriate. In this case, the remaining interference excess would be reduced to around 1.4 dB.
Cumulative interference from several radiosondes operating on the same nominal frequency could also occur
in view of the earth covering MetSat antenna. Assuming 900 simultaneous radiosonde launches worldwide
and a visibility of a 42% of the earth’s surface by a geostationary meteorological satellite, simultaneous signals 378 radiosondes could be received by the satellite. However, in view of the large drifts of the radiosonde
carrier frequency, it is very likely that the signals will be spread out and that only a few radiosonde signals
will be found within any individual DCP bandwidth.
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4.2
Interference assessment from DCP Transmitters to Radiosonde Receivers
The separation distance between DCP transmitters and radiosonde receivers can be calculated based on Recommendation ITU-R P.452. This recommendation addresses long term effects such as propagation by diffraction (Rec. ITU-R P.526) or troposcattering as well as short-term propagation effects such as reflection,
refraction, ducting and hydrometeor scattering. Figure 4 gives an overview of the geometrical constellation
and the associated propagation mechanisms.
METSAT
TROPOSPHERE
SCATTER / REFLECTION
REFRACTION / DUCTING
RADIOSONDE
DIFFRACTION
RADIOSONDE
RECEIVER
DCP TERMINAL
FIGURE 4
POTENTIAL GEOMETRICAL CONSTELLATIONS AND PROPAGATION MECHANISMS
A number of different losses contribute to the total required signal attenuation of the long-term propagation
mechanism. The free space loss is given by Ls = 20 log (42 d f) and is the basic transmission loss by
spreading of the signal in space. Further attenuation will occur due to atmospheric effects, diffraction due to
the Earth's curvature, path obstacles and vegetation. For frequencies below a few GHz, the vegetation loss
may be neglected. Atmospheric attenuation can be calculated from Rec. ITU-R P.676 but is also insignificant
around 403 MHz.
Propagation by diffraction determines often the dominating signal component if the permissible interference
probability is not very low. However, for rather small percentages of time, during which interference is permissible, other propagation mechanisms are likely to result in stronger interfering signal components than the
diffraction path. It is therefore necessary to investigate several propagation modes of potential significance.
For the DCP, an antenna centre point at a height of 2 m has been assumed. The off-axis antenna gain is 0 dBi
at an elevation angle of 37 degrees. An average antenna height of 2 meters above ground has also been assumed for the radiosonde receiver. The following basic equation applies to the calculation of the separation
distance:
Lt = Ptx + Gtx + Grx - Prx
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where:
Lt:
Ptx:
Gtx:
Grx:
Prx:
Total required signal attenuation or permissible transmission loss (dB)
Power density of transmitting station (dBW/Hz)
Gain of transmitting station towards receiving station (dBi)
Gain of receiving station towards transmitting station (dBi)
Permissible interference power density at receiving station (dBW/Hz)
A mathematical model based on Recommendation P.452 has been used to derive an estimation for the required separation distances. The matter is very complex and only a few cases could be investigated. Main
parameters were antenna centre point altitudes and shielding by hills or mountains.
The results for the required separation distances are contained in Table 7.
TABLE 7
REQUIRED SEPARATION DISTANCES FOR VARIOUS PROPAGATION MECHANISMS
Radiosonde characteristics
NAVAID - directional antenna
NAVAID - directional antenna
NAVAID - directional antenna
NAVAID - omnidirectional antenna
NAVAID - omnidirectional antenna
NAVAID - omnidirectional antenna
interference interference
probability
criterion
0.02%
-139.5
1.25%
-150.9
20%
-154.0
0.02%
-140.1
1.25%
-143.5
20%
-153.5
mean
distance
40
60
35
25
40
55
maximum
distance
245
180
125
120
65
135
For the above results, it was assumed that very little site shielding would be available, hence, the dominating
effect of signal refraction and tropospheric scatter in all cases. Only obstacles between 100 and 200 meters at
a distance of at least 10 km were assumed. Site shielding would significantly reduce the required worst case
separation distances to less than 100 km.
Detailed results for the various components as well as the impact of site shielding are shown in figures 5 and
6. The specific cases selected were a radiosonde receiver with a directional antenna, an interference excess
probability of 1.25%, and shielding with 300m and 100m obstacles at a distance of 10km, respectively.
It shall be noted that the probability of main beam coupling between a DCP transmitting antenna and a radiosonde receiver antenna is itself already very low. The probability that the 3 dB mainbeams point at each other is in the order of 0.1-0.2% for the 11 dB helix DCP and the 8 dB sector antenna. This suggests that the
case of 0.02% interference excess is not representative and should be disregarded as the combined probabilities of main beam coupling and interference excess of 1.25% result already in far less than 0.02%.
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REQUIRED SEPARATION DISTANCES FOR VARIOUS PROPAGATION MODES
Transmission loss (dB) -
200
190
180
170
diffraction loss
ducting / layer reflection loss
troposcatter loss
combined trans-horizon loss
required transmission loss
adverse case
favourable case
160
150
140
20
40
60
80
100
120
140
160
180
200
Separation distance (km)
FIGURE 5
DIRECTIONAL ANTENNAS, SOME SHIELDING AND 1.25% INTERFERENCE EXCESS TIME
REQUIRED SEPARATION DISTANCES FOR VARIOUS PROPAGATION MODES
Transmission loss (dB) -
200
190
180
170
diffraction loss
ducting / layer reflection loss
troposcatter loss
combined trans-horizon loss
required transmission loss
adverse case
favourable case
160
150
140
20
40
60
80
100
120
140
160
180
200
Separation distance (km)
FIGURE 6
DIRECTIONAL ANTENNAS, LITTLE SHIELDING AND 1.25% INTERFERENCE EXCESS TIME
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5 Conclusions
Regarding the interference from radiosondes to meteorological satellites, the short term criteria as specified
in Recommendation ITU-R SA.1164 are met in all cases with a significant margin.
The long term interference protection criterion is not met for the worst case assumptions. An interference
excess of around 4.9 dB could occur. The mean case and best case assumptions meet the required criteria for
a single radiosonde emission.
Based on Recommendation ITU-R SA.1163, a long term criterion of –187.4 dBW per 100 Hz not to be exceeded for more than 20% of time may be considered appropriate in special circumstances. In this case, the
remaining long term interference excess would be around 1.4 dB.
Cumulative interference from several radiosondes operating on the same nominal frequency could also occur
in view of the earth covering MetSat antenna and the operational procedure to launch the radiosondes at approximately the same times.
Regarding terrestrial interference from DCP transmitters into a radiosonde receiver, a number of different
propagation mechanisms and system assumptions have to be considered. The required separation distances
range between 25 and 245 km, assuming very little site shielding, i.e. basically flat terrain.
Key factors for short separation distances are low antenna gains due to off-pointing and site shielding. Antenna off-pointing by more than 20 degrees and site shielding to elevation angles above 1.5 degrees is likely
to reduce the required separation distances to less than 20km.
The above results suggest, that co-frequency sharing should only be considered on a case-by-case basis
where the DCP locations and characteristics are known and where site shielding is available. Coordination
will be required.
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