Radar Correspondence Group

**Notes on Informal RCG Discussions from May 26, 2005 in Southampton,
UK**

Frank Sanders, Chairman

**Introduction:** At the end of the meeting of Joint Rapporteurs Group
1A-1C-8B in Southampton, UK, informal discussions were held amongst the
attendees on topics of interest to the Radar Correspondence Group (RCG).
(Most of the JRG 1A-1C-8B participants are members of the RCG, and
vice-versa.)

At the outset of the discussions, it was agreed by the participants
that:

1)  No decisions regarding RCG business would be made, as this was not
    an RCG meeting;

2)  That notes regarding the content of the discussions would be made
    available on the RCG web site afterward;

3)  That all official business regarding ITU-R WP 8B input documents
    always occurs at WP-8B meetings.

**First topic:** There was discussion about the feasibility of doing all
work of the RCG via correspondence, with no possibility of actually
holding meetings. Some participants suggested that it would be highly
beneficial for the RCG to meet occasionally. The possibility of removing
the word "correspondence" from the name of the group was suggested. In
any event, there seemed to be consensus that, if the members of the RCG
could meet occasionally (say, once per year), and if no cost for such
meetings were incurred by the Secretariat of the ITU, it would be of
substantial benefit to the furtherance of the business of the RCG to do
so.

**Second topic:** The question was raised as to how inputs from the RCG
could be submitted to WP-8B. It was suggested, as an answer, that the
RCG Chairman can provide information to the Chairman of WP 8B in a
report from the RCG, and that of course every Administration can always
provide inputs to WP 8B on topics related to RCG inputs via the ordinary
channels and such documents will then be handled in WP 8B drafting
groups in the ordinary way.

Following this discussion, it was decided that it would be beneficial to
discuss each of the RCG inputs that had been provided to date in 2005.
These were documents RCG-25 through RCG-42. For each document, the
author (or some representative from the author's Administration)
provided a brief set of introductory remarks, followed by a general
question-and-answer style of discussion amongst the participants.

The substance of these discussions is summarized below for each of the
RCG documents between 25 and 42.

**RCG-25:** This document compares radiated radar emission spectra
measured at various distances. There is not much difference between
spectra measured at (D^2^/lambda) versus 2(D^2^/lambda), where lambda is
the wavelength of the center frequency. In the best of all possible
worlds, the value of lambda would be the *highest* frequency measured.
But as distance from the radar increases, the dynamic range available
for the measured spectrum tends to decrease. So measurement personnel
need to strike a balance between theses factors when performing radiated
radar emission spectrum measurements.

**RCG-26:** This document examines the issue of whether a maritime
radar's antenna beam scanning in space can be stopped during a radiated
measurement of the spectrum. The conclusion is that the radar beam
direction in space relative to the axis of the antenna changes as a
function of frequency, and therefore the radar *must* continue to rotate
during a measurement. This will be true for all radar types that exhibit
this sort of antenna beam behavior (that is, change in the direction of
the beam relative to the physical antenna axis as a function of
frequency).

**RCG-27:** This paper compares radiated radar emission spectra in
several measurement bandwidths. It is concluded that bandwidths wider
than 1 MHz reference bandwidth can be successfully used, assuming that
the bandwidth does not exceed the limits set forth in M.1177. However,
if the radar emission spectrum fails to meet a mask limit when measured
in a bandwidth that exceeds 1 MHz, then it would be advisable for
measurement personnel to re-measure the part of the spectrum that is
failing the mask in a 1 MHz reference bandwidth, and see whether the
radar meets the mask in the narrower bandwidth. If it does, then the
radar passes the mask. This approach (measuring in a wider bandwidth
than 1 MHz, but within the limits set forth in M.1177, and reducing the
measurement bandwidth to 1 MHz only for parts of the spectrum that fail
the mask limit in the wider bandwidth), will save time and effort in
radiated spectrum measurements without losing accuracy in measurement
results.

**RCG-28:** This paper analyzes earlier work in 8B/84, in which the
frequency response of a maritime radar antenna was measured as part of a
project that sought to construct the emission spectrum of a radar using
a variation on the Indirect Method of M.1177. No one was present who
could speak for the authors of 8B/84, but the possibility was raised
that uncertainties in the measurement of antenna frequency response
might account for some features of the results presented in 8B/84.

**RCG-29:** This paper suggests that there are a number of inherently
substantial sources of uncertainty in the Indirect Method of M.1177, and
calls for more study of these error sources.

**RCG-30:** This paper discusses several technical topics related to
M.1177 that the author argues should be further clarified. Especially,
it was commented that ITU-R documents that provide the specification of
emission limits and that form the basis of testing using ITU-R
Recommendation M.1177 are inconsistent. Some confusion arises from the
fact that for some services the limits are specified at the transmitter
output but for radar they are specified as off-air measurements in line
with ITU-R Recommendation M.1177. In particular it was remarked that
there is confusion in the specifications between power, spectral power
density and EIRP. This paper may form the basis for a proposed change in
M.1177 at some later date.

**RCG-31:** This paper comments on the uncertainties in the Indirect
Method in M.1177. It was commented that perhaps there would be some
interest at some future date in removing the Indirect Method from
M.1177, but at least one participant commented that the Indirect Method
has proven useful in certain circumstances, and should therefore be
retained, but perhaps with some additional cautionary wording

**RCG-32:** This paper contains comments from the UK on 8B/129, the
liaison statement from 7C to 8B on WRC-07 Agenda item 1.3, Resolution
747 (WRC-03): Possible extension of the existing primary allocations to
the earth exploration-satellite service (active) and the space research
service (active) in the band 9 500-9 800 MHz. A number of useful
comments were included in this paper, which should prove to be very
helpful to 8B when it meets in Geneva in September to actually draft a
response to 7C, and the participants thanked the UK for providing this
input.

**RCG-33:** This paper addresses rotary joint issues in the use of the
Direct Method of (radiated) radar emission measurements. It is possible
that variation of a few dB in output levels could occur as a maritime
radar antenna rotates. Discussion ensued about the advisability of
verifying VSWR characteristics of rotary joints as part of measurement
procedures.

**RCG-34:** This paper expands on the theme of RCG-26, but with much
more sophisticated measurement methods that clearly demonstrate the
variation in the direction of the main beam of radiation from a
slotted-array maritime radar antenna as a function of frequency. The
implication is that radiated measurements on such radar systems should
be performed with the antenna rotating, not held stationary.

**RCG-35:** This paper expresses concerns about the difficulty of
accounting for the effects of the rotary joint in the Indirect Method.
It was commented that it may be difficult in general to obtain agreement
between the results of the Direct Method and the Indirect Method when
they are both used to measure spectrum emissions of the same radar. Much
discussion ensued about difficulties in accounting for joint behavior in
various types of radar systems.

**RCG-36:** This very thorough paper describes the results of extensive
measurements and simulations of radar emission spectrum measurements
performed with a variety of measurement bandwidths. It was shown that
the best resolution could be achieved if the measurement bandwidth were
about (1/4) (1/t), although it was acknowledged that the use of such a
narrow bandwidth makes the time required for the measurement become
relatively long.

**Most significant results of the RCG informal discussions of May 26:**

1)  There seemed to be a consensus amongst the participants that radar
    emission spectra can be measured in a variety of bandwidths for
    compliance with emission mask criteria, provided that the
    measurement bandwidth is less than or equal to the limits stated in
    the existing version of M.1177. (For example, a maritime radar with
    a 70 ns pulse width might be measured in an 8 MHz bandwidth.) The
    best resolution can be achieved if the measurement bandwidth does
    not exceed (1/4) of (1/t), although measurements can become lengthy;
    the wider the measurement bandwidth, the faster the measurement can
    proceed. This raises the question of how to balance these effects in
    measurements.

    One way to proceed for purposes of verification of emission mask
    compliance would be to measure a radar emission in a bandwidth that
    is as close as possible to the upper limit of the range stated in
    M.1177, but if part of the radar emission spectrum does not meet the
    limits of an emission mask as measured in that bandwidth, then
    measurement personnel may find it desirable to re-measure the
    failing part of the emission spectrum in the reference bandwidth of
    1 MHz. If the radar were to pass the mask in the narrower-bandwidth
    measurement, then it would be appropriate to report that result.

On the other hand, if the purpose of a measurement is to resolve fine
details in emission spectrum features, then it would be better to
perform the measurement in a narrow bandwidth, on the order of (1/4)
(1/t), as noted in RCG-36.

2)  Although not very much support was voiced for the technique of
    measuring radar emissions through hardline couplers with the antenna
    detached at the rotary joint (the Indirect Method), it was also
    commented that there are specific situations in which such
    measurements are desirable. Currently the UK and Germany have the
    most experience with this technique, and other Administrations have
    very little experience with it. Several Administrations have
    implemented variations of the Direct Method at their radio
    measurement facilities. Most Administrations seemed most comfortable
    with measurements of radiated radar emissions using the Direct
    Method of M.1177 for purposes of determining emission mask
    compliance. But again, it was commented that there are some
    instances in which there is still a need to utilize the in-guide
    measurement technique.

3)  We are still experiencing difficulties in the disparity between the
    specifications for radar spectrum emission limits as described in
    various ITU-R documents versus the realities of the types of data
    that can actually be obtained (that is, measured) on such emissions.
    Recommendation M.11177 describes the *techniques* and *parameters*
    that should be used to measure radar emissions, but the data so
    obtained will inevitably be power levels that have been measured in
    certain filters that have particular characteristics of bandwidth
    and shape. An ongoing challenge for measurement personnel is to
    convert measured data into the types of information that are called
    out in ITU-R emission mask criteria.

4)  We anticipate that some Administrations may provide Contributions to
    WP 8B at the September meeting, calling for some modifications to
    the current version of Recommendation M.1177-3. Some of those
    expected Contributions will likely follow the content of some of the
    RCG inputs that have already been provided. The discussions that
    occurred at Southampton should help to move forward the work of WP
    8B on this Recommendation when 8B meets in September.

5)  It was the consensus of the participants in the discussion in
    Southampton that it would be highly beneficial for the members of
    the RCG to be able to meet officially at no cost to the ITU-R
    Secretariat, perhaps about once a year, and that the Chairman of the
    RCG should take up this matter with the Chairman of WP 8B as part of
    the RCG Chairman's Report to 8B at the September meeting.