RUT: RST For Signals "In the Noise"
n5bf/6, posted 2008 January 9, modified 2008
January 18
Heritage
From the beginning radio operators have used
standardized
systems of signal reporting to quantify station performance and
propagation. When used correctly,
this courtesy of the receiving operator can be greatly helpful to the
transmitting operator.
The system I first used is described in The
Radio
Amateur's Operating Manual by
George
Thurston, W4MLE for 1969, p. 22:
The R-S-T System
Readability
1 – Unreadable.
2 – Barely readable, occasional words
distinguishable.
3 – Readable with considerable difficulty.
4 – Readable with practically no difficulty.
5 – Perfectly readable.
Signal Strength
1 – Faint signals, barely perceptible.
2 – Very weak signals.
3 – Weak signals.
4 – Fair signals.
5 – Fairly good signals.
6 – Good signals.
7 – Moderately strong signals.
8 – Strong signals.
9 – Extremely strong signals.
Tone
1 – Extremely rough hissing note.
2 – Very rough a.c. note, no trace of musicality.
3 – Rough low-pitched a.c. note, slightly musical.
4 – Rather rough a.c. note; moderately musical.
5 – Musically modulated note.
6 – Modulated note, slight trace of whistle.
7 – Near d.c. note, smooth ripple.
8 – Good d.c. note, just a trace of ripple.
9 – purest d.c. note.
If the signal has the characteristic steadiness of
crystal
control, add the letter X to the RST report. If
there is a chirp, the letter C may be added to so
indicate. Similarly for a click,
add K.
Discussion
My first transmitter was homebrew, cathode keyed,
and crystal
controlled. Some of the crystals
used chirped badly, several hundred Hz with a time constant of perhaps
half a
second. I received many honest
signal reports with "C" appended.
My first receiver had no meters of any kind. Indeed, the main tuning dial did not
even read frequency. I had to find
WWV by just tuning across the various bands, and plot graphs,
interpolating
where the amateur bands of interest would be.
All of my signal reports at the beginning were,
therefore, "by ear." Readability is
subjective
anyway but, by the guidelines given above, so is strength.
More recently, many amateur radios sport a signal
strength
meter in some form and although it is difficult to calibrate such a
meter for
various modulations, noise characteristics, and frequencies, there
seems to be
some at least informal agreement on a standard S-9 at 5 microvolts
received
signal with each S-unit representing 6 dB (that is, a doubling in
voltage on
the meter). So, the current S-unit
system is logarithmic, like the response of the ear.
From this information, one can construct the
following
table:
S dBm
uV femtowatts @ 50 ohms
9 -93
5.0 500.
8 -99
2.5 130.
7 -105
1.3 32.
6 -111 0.63 7.9
5 -117
0.32 2.0
4 -123
0.16
0.50
3 -129
0.079 0.12
2 -135
0.040 0.032
1 -141
0.020 0.0079
0 -147
0.010 0.0020
(This is according to the "above 30 MHz convention
used by DSP-10. Below 30 MHz, S-9 is customarily taken 20
dB higher at -73 dBm or 50 microvolts.1)
There is, however, now considerable unfortunate
ignorance or
abuse of the system leading to contesting jokes like:
"You're 599 OM, please repeat all."
and
"599 is just the contesting synchronization word."
This is unfortunate inasmuch as there would be
value in
these reports in, for example, Top band tests where they are still
required but
not honored, in evaluating the performance of your power level and
antenna over
a wide range of contacts.
Those doing radio design and construction and
serious weak
signal work these days tend to think of signal strengths in terms of
dBm. Still, the RST concept is, or at
least
was, well ingrained in the radio operator's skill set.
R-U-T
for "In the Noise" Weak Signal Research
It occurred to me that the RST system might be
extended to
the sub-audible-noise-floor research now being done in amateur radio
circles,
such as the DSP-10 group.
Before going on, let me clarify that the work I am
talking
about here is not really "below the noise" which is, by definition,
undetectable, it is merely below the noise in a standard audio
bandwidth,
human-audible receiver, the situation with which we are all so familiar
that we
think of it as ground truth. The
techniques we are now beginning to investigate are ways of looking for
or at
signals which are much weaker than those that can be detected in that
way, "by
ear." Even the simplest,
non-coherent averaging, promises 20-30 dB more depth for the patient
(20 dB) to
truly patient (30 dB) operator.
Here I propose an extrapolation of the RST system
at and
below S-0 and redefine the "readability" and "tone" criteria to
quantities
appropriate to this sort of work.
This extrapolation will be intuitive to the experienced radio
amateur
and will give us means to gauge the difficulty of contacts achieved or
contemplated. I call it R-U-T for
Readability, Under-strength, and Tone.
In this system, readability is a statistical
determination
of the probability of detection compared to the noise statistics. Averaging techniques for determining
absolute signal level from the (S+N)/N equation, for example, will
produce
statistical information about the noise "N". The
detected signal, "S" will have some strength compared to
the noise statistics. I propose
that the readability metric be estimated signal strength expressed in
standard
deviations of noise.
Readability
1 – one standard deviation, 68% chance of
detection,
not significant.
2 – two standard deviations, 95% chance of
detection, "minimums"
3 – three standard deviations, chance of false
positive, 370:1
4 – four standard deviations, chance of false
positive, 15,800:1
5 – five standard deviations, chance of false
positive, > 1,700,000:1
This could be extended beyond "5" but it appears
that this
would be pointless.
Strength Under S-0
U is "S" from the (S+N)/N equation.
Beginning from the bottom of the table
above, reading "floored" measure (i.e. -173 dBm is U-5, not U-4):
0 – -147 dBm
1 – -153 dBm
2 – -159 dBm
3 – -165 dBm
4 – -171 dBm
5 – -177 dBm
6 – -183 dBm
7 – -189 dBm
8 – -195 dBm
9 – -201 dBm
It appears that the original RST system was
designed to
limit the report to one digit in each of the places to minimize
confusion,
thus, convention and meter calibration have, for values beyond S-9, "dB
over
S-9." Here, similarly, we can say "dB
below U-9."
U-9 – 10 dB – -211 dBm
U-9 – 20 dB – -221 dBm
This gives us a reasonable range within which to
work.
Tone
This refers to the stability of the master
oscillator used
within the time periods of interest. Symbol times in this kind of work
can vary
from seconds to tens, hundreds, or thousands of seconds.
1 – one part in ten (human speaking, "one thousand
one, one thousand two,..."
2 – one part in a hundred
3 – one part per thousand, 10-3 (around
a
minute per day)
...
6 – one part per million, 10-6 (half
minute
per year)
...
9 – one part per billion, 10-9 or
better. (Obtainable with GPS or
equivalent disciplining.)
Stations attempting PUA43 tests at 10 GHz would
need T-8 to
T-9 minimum over the symbol transmission period of about a second.
Example:
At
HamRadio/Dsp10/PhaseFive/Eme2/Eme2.html
I claimed to have achieved QRPpp (5 watts to a
single yagi)
EME using the DSP-10 EME2 mode and integrating for over 10,000 seconds. The resulting signal strength estimate
was -187 dBm with 4.2 sigma confidence.
The oscillator used has been measured to have an offset of about one part per million but is
thought to be stable
to parts in 10-7 or better. Since
this is a self-echo detection
mode, the "relative" stability of the oscillator is only important over
the
five seconds of one transmit-receive cycle. We
would say RUT 477.
Example:
I can usually see the N6NB/B DM05 in LTI (beam
heading 347)
but only on a rare good day can I hear it audibly.
On such good days, I would give it an RST of 319.
The DSP-10 S-meter would say S-1 or
S-2.
The first figure in the link above shows a day
where ÒSÓ was
calculated at -158.7 dBm after 195 sample points in Long Term
Integration
(LTI). Looking at the Òyellow
lineÓ display, it looks like this is better than 5-sigma, so we
would say RUT
529. On this day, the signal was
barely audible (RST 119 - 219), so R-5 for a long integration is
reasonable.
Example:
The RUT convention can also be used to predict
signal levels
needed for certain experiments.
For example, if I added a 100 W. amplifier to the QRPpp EME2
example, I
might promote it to QRPp EME and say that RUT of 356 was the minimum
required
to claim detection. This might
take only a few thousand seconds of integration. By similar
arguments (see the link above and my paper on EVE/EME) EVE (Venus)
might require something like 20 dB below 299.
1 IARU Region 1 Technical
Recommendation R.1, Brighon 1981, Torremolinos 1990, STANDARDISATION OF
S-METER READINGS:
1. One S-unit corresponds to a signal level
difference of 6 dB,
2. On the bands below 30 MHz a meter deviation of S-9 corresponds
to an available power of -73 dBm from a continuous wave signal
generator connected to the receiver input terminals,
3. On the bands above 144 MHz this available power shall be -93
dBm,
4. The metering system shall be based on quasi-peak detection
with an attack time of 10 ms +/- 2 ms and a decay time constant of at
least 500 ms.
Cited in An Accurate S-Meter for
Direct Conversion Receivers, Gary W. Johnson, WB9JPS, QST 2008
February, p. 33.