MUF is related to short wave (HF) propagation predictions or predicting (calculating) maximum frequency for some distance in some cases, based up on ionograms, but MUF does not fit to reporting signals received via Es or any other ionospheric propagation layer for that matter. The issue here being: MUF is used with predictions and MOF with observations.
International Telecommunications Union ITU-R (Recommendation P.373-7 10/1995, in force) recommends two definitions for MUF, which are described here in parts 1&2.
1. Operational MUF (or just MUF)
the highest frequency that would permit acceptable operation of a radio service between given terminals at a given time under specific working conditions (antennas, power, emission type, required S/N ratio, etc.)
2. Basic MUF
the highest frequency by which a radio wave can propagate between given terminals by ionospheric propagation alone, independent of power.
The difference in frequency between operational MUF and basic MUF is
in practise from 10 to 35 %.
3. Monthly median MUF and M factor
is the highest received frequency on 50% (sky wave availability is 0.50) of the days at a given hour (statistical, pre calculated). Optimum operating frequency is FOT = OWF = 0.85 * MUF. The MUFs depend heavily up on (predictions) of solar conditions which affect ionosphere's electron content.
Extensions to MUF: number of hops, distance and magnetoionic component (1F23000MUFx = one hop F2 layer propagation of 3000 km via extraordinary wave) (1E2000MUF = one hop via regular E layer at distance of 2000 km). See IPS Radio & Space Services, Introduction to HF Radio Propagation, chapter 2.3 The usable frequency range.
M3000 is a factor for obtaining the maximum frequency usable in a given oblique propagation distance from the critical frequency (foF2) at vertical incidence. The M factor for the standard distance of 3000 km is M(3000).
M(3000) = MUF(3000)/foF2
where MUF(3000) is the Maximum Usable Frequency for a 3000 km path length and foF2 is the maximum frequency of vertical incidence sounding.
Predictions of Es
Blanketing sporadic-E and foEs have prediction models than are based
on state of solar activity, time of day, time of year, geographical location,
statistically know variation of foEs. However, these serve merely is prediction
tools for HF propagation planning and do not predict occurrences of VHF
sporadic-E events with maximum observed frequencies up to 100...200 MHz
Maximum Observed Frequency (MOF) (used with oblique sounding)
is the highest frequency on which signals are observed regardless of propagation path and therefore is most proper for reporting the highest frequency where signals are being received, via Es, or whatever layer. MOF is used when studying oblique ionograms and in the description of these ionograms, we are not concerned if or not some frequency is "usable" (~MUF), but what is observed and the nomenclature inludes the following definitions MOF, LOF (lowest observed frequency) and if possible, the modes via which layer propagation has occurred, the lowest frequencies for low and high angle rays and Xo, or O raypaths. There is no room in this contex for MUF because we are talking on observed conditions and not making calculations or predictions!
Note: There may actually be some difference between oblique sounding MOF and the highest frequency on which you are able to observe signals on, since sounding equipment use no-gap scanning of radio spectrum, but there may not be stations on the air to particular direction on every frequency of interest, so sounding MOF may be actually little higher that the frequency of stations you hear on the radio.
If the propagation mode is identified, extensions similar to what is used with MUF can be used such as 2F2MOF = two F2 layer hop's maximum observed frequency or 1EsMOF = one hop sporadic-E layer reflection).
Above is an Ionogram from a vertical sounding ionosonde analyzed
automatically by software for different ionospheric layers and their properties
and producing calculated additional information. The tangential point (1)
of reflection from F2 layer is called foF2
(2). This value is used to generate
calculated (basic) MUF for distance of 3000 km (F2MUF3000)
(3) and as listed in table (4),
is calculated also for other distances down to 100 km. The sporadic E layer
is evident (5) and has foEs 5.25 MHz.
From this vertical sounding, by observing highest frequency for Es
layer (foEs) we also could calculate basic EsMUFs for different distances,
but please note, we are talking here about calculated values (which are
called MUF, or Basic MUF), not observed values. Ordinary E layer is not
observed as it has allready vanished for the night as has F1 layer.
K. Davies, Ionopheric Radio
Rec. ITU-R P.373-7 and P.434-6
IPS Radio & Space Services, Introduction to HF Radio Propagation
V.K. Lehtoranta, YLE - Finnish Broadcasting Corporation, Network Planning Services (ret.), personal communications
Common formula for MUF:
MUF = 48*SQR(N)
where N is electron density in e/m3