**JRG-87**

**ITU-R JRG 1A-1C-8B**

**5^th^ Meeting -- London -- 23^rd^ - 27^th^ April 2007**

**[Spectrum measurements of various radars]{.underline}**

**[(Switzerland)]{.underline}**

# Introduction

In order to support the work of the Joint Rapporteur Group 1A-1C-8B,
OFCOM Switzerland decided to perform some measurements of radar sideband
and spurious emissions. As the time left for this project was rather
short it was decided to measure radar equipment of well-known operators
such as the Swiss air traffic control operator Skyguide and a heritage
weather radar operated by the Swiss Federal Institute of Technology. The
aim of this report is to provide more results of radar spectra in order
to support JRG 1A-1C-8B in its efforts towards a revision of ITU-R Rec.
SM.1541-2.

# Radar Equipment measured

The following radars were investigated:

1.  Air navigation monopulse secondary surveillance radar at 1030 MHz in
    the region of Zurich airport.

2.  Air navigation primary surveillance radar at 2720 MHz in the region
    of Zurich airport.

3.  Airport surface movement radars at 9175 MHz and 9420 MHz in the
    region of Zurich airport.

4.  Weather radar at 5628 MHz of the Swiss Federal Institute of
    Technology in Zurich.

The specifications of the different radars investigated are detailed in
Table 1. It should be noted that the weather radar at 5628 MHz was
tested with two different pulse lengths, thus corresponding to two
different spectrum masks. As it is operated with a BP filter added to
the original design, the spectrum both prior to and after the filter was
measured.

# Measurement setup and procedure

The block diagram of the measurement setup is depicted in Fig. 1.

It should be noted that:

-   All measurements were done at monitor points. This can clearly be a
    disadvantage if this point is prior to a combining filter such as in
    the 9 GHz radars. On the other hand it is the only way of measuring
    in a densely loaded area such as an airport. A proper way would be
    to take into account the filter specs, but they were not available
    and measurement was not possible.

-   A single notch filter operating in the bands 2.7 GHz and 5.5 GHz was
    available. It was used wherever possible. As a result of the
    insertion of this notch a dynamic range of greater than 90 dB was
    achieved.

-   Care was taken to use the spectrum analyzer under optimum
    conditions; therefore an external attenuator with 1 dB steps was
    inserted at the input.

-   The spectrum was measured in at least 3 overlapping subranges (lower
    part, centre part, upper part) in order to have maximum dynamic
    range without overloading the analyzer. The three subranges were
    recombined in Excel to obtain the full spectrum. In this way it was
    possible to improve the dynamic range by approx. 10 dB without
    creating an overload situation to the analyzer. It was found that an
    overload situation can be safely avoided if the analyzer is used
    with 3 dB headroom to the point where overload is indicated. Even if
    there is only a slight overload to the instrument, the spectrum
    cannot be recombined in Excel without creating a sharp step at the
    combining frequency. In this way we have a sensitive indicator to
    ensure proper measurements.

-   The tolerance curves were calculated strictly according to Rec.
    SM.1541-2 based on pulse length and rise- and fall-time, i.e. the
    assigned frequency was calculated using the pulse length only as
    well as using rise- respectively fall-time and pulse length. From
    the two resulting bandwidths the smaller was chosen.

# Results

The measured spectra of the radar systems are displayed in Fig. 2 to 8.

## MSSR 1030 MHz (Fig. 2)

This radar has a spectrum that easily meets the spec given by the 20
dB/decade mask and also the planned future spec based on the 40
dB/decade mask from the design objective. From all systems tested this
is the only one easily meeting current and future specs.

## Primary surveillance radar 2720 MHz (Fig. 3)

This radar system represents the biggest surprise of all system
measured. Despite of the fact that it is a solid state design it fails
to meet even current specs. The reason for this fact is unknown, but it
clearly shows that every radar system can be an interference source.

## Surface movement radars 9175 and 9420 MHz (Fig. 4 & 5)

These radar systems operate with a common antenna so they need a
combining filter that, if implemented as band pass design, will further
improve the spectrum shaping. However the roll-off is rather smooth,
specially in the light of the fact that this radar is a magnetron-based
design.

## Weather radar 5628 MHz (Fig. 6 to 8)

This weather radar is a heritage system which is the only remaining in
Switzerland with this design. The spectrum generated by the radar itself
is rather wide (Fig. 6) and the limits of SM.1541-2 could not be met.
Therefore a band pass filter was added which efficiently limits the
spurious response when the system was commissioned. The filter response
can be clearly seen in the output spectrum measured (Fig. 7 & 8). With
the filter inserted even a 40 dB/decade slope can almost be met. This
clearly shows the potential of filters.

# Conclusions

## Conformity with current Rec. ITU-R SM.1541-2

-   As can be seen in the different figures, conformity with the current
    Rec. SM.1541-2 cannot be guaranteed under all circumstances. The
    radar system exceeding the most the current spec is the primary
    surveillance radar at 2720 MHz with the solid state design and not
    as could be expected a magnetron based system. This shows that every
    radar system has the potential to violate the limits.

-   The reason why the specs are so much violated can only be guessed.
    The system was originally combined with a second radar to the same
    antenna. Of course a combining filter was inserted in order to
    separate the 2 systems. This filter was removed with the second
    radar "because it was not needed any more". We assume that with this
    filter inserted the specs would be met.

-   As a consequence of the measured results the question arises of what
    should be done from the regulator side to ensure conformity to the
    limits, whether based on current or revised specs. It is obvious
    that the radar operator is mainly interested in proper system
    operation and far less in respecting specs. So the regulator should
    clearly become more active to ensure and enforce internationally
    agreed specs.

-   Since the system that is clearly out of specs is used all over
    Europe the question arises if this is the only one that fails to
    meet the limits or if all similar systems show the same spectrum
    characteristics.

## Use of filters in radar systems

-   Use of filters in radar systems is standard since a long time. Often
    2 systems with adjacent frequencies are combined to the same
    antenna, requiring combining filters. So from a technical point of
    view, there will be no problems inserting filters into the antenna
    path in most cases.

-   Radar systems measured here are high quality systems that require
    quite some investment. Adding a filter for improving specs does not
    pose an undue load on the radar industry or on the standard users.

## Defence environment

-   In a military environment, filters are in use already for radar
    systems. A system recently measured in Switzerland easily meets the
    current Rec. SM.1541-2 with 20 dB/decade slope as well as a planned
    slope of 40 dB/decade.

-   It can be assumed that in a modern battlefield area many different
    radar systems will have to operate in parallel in ever changing
    configurations. In such environment it is essential that every
    system has a strictly limited bandwidth in order not to interfere
    with others. Moreover the defence part of the radar operators can
    only profit from more stringent specs as it will experience
    generally less interference from civilian sources as well.

**Table 1: Radar Systems Measured**

  --------------- -------------- -------------- ----------- ----------- ------------- ---------
  **Parameter**   **Radar 1**    **Radar 2**    **Radar 3** **Radar 4** **Radar 5a**  **Radar
                                                                                      5b**

  Purpose         Air navigation Air navigation Surface                 Weather radar 
                                                movement                              
                                                radar                                 

                  Monopulse      Primary        Surface                               
                  secondary      surveillance   movement                              
                  surveillance   radar          radar                                 
                  radar                                                               

  Radar           Raytheon       Raytheon       Terma                   Enterprise    
  manufacturer                                                          Electronics   
                                                                        Corp. USA     

  Reference       MSSR           ASR-10SS Mk2   Scanter                 WR100-5       
                                                2001                                  

  Frequency       1030 MHz       2720 MHz       9175 MHz    9420 MHz    5628 MHz      

  Output device   Solid state    Solid state    Magnetron   Magnetron   Magnetron     

  Peak Power      1 kW           16 kW          20 kW       20 kW       250 kW        

  Pulse length    750 ns         1.6 µs         60 ns       54 ns       580 ns        3.0 µs

  Rise time\      88 ns\         200 ns\        4.3 ns\     5.7 ns\     36 ns\        29 ns\
  Fall time       60 ns          2.6 ns         16.8 ns     15 ns       457 ns        567 ns

  PRF             150 Hz         770 Hz         8000 Hz     8000 Hz     1180 Hz       250 Hz

  40 dB Bandwidth 29.8 MHz       40 MHz         474 MHz     432 MHz     42.7 MHz      19.4 MHz

  Compliant with  yes            no             yes         yes         yes           yes
  SM.1541-2?                                                                          

  Compliant with\ yes            no             yes         no          no            no
  40 dB/decade                                                                        
  spec.?                                                                              

  Spectrum        Fig. 2         Fig. 3         Fig. 4      Fig. 5      Fig. 6; Fig.  Fig. 8
                                                                        7             
  --------------- -------------- -------------- ----------- ----------- ------------- ---------

**Fig. 1: Block Diagram Test Setup**

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