Quiet sun radio flux interpolations


See Quiet solar radio flux interpolations calculator for information about changes.


Quiet sun radio noise is often used for measurements of radio receiving systems. The radio flux from the quiet sun is a known signal in that it is measured and reported at observatories around the earth, and therefore provides a basis of assessment of radio receiving systems.

This page calculates interpolated recent solar flux data for amateur radio UHF and microwave bands from published observations.

Table 1: Observatories
Observatory Lat (°) Lon (°) Nominal observation time (UTC)
Learmonth, Australia 22.13S 114.60E 0500 
San Vito, Italy 40.40N 17.43E 1200 
Sagamore Hill, North America 42.38N 70.49W 1700 
Palehua, Hawaii 21.24N 158.06W 2300 

Table 1 shows the nominal times of observations published on the NOAA site for each of the observatories.

Enter Details:


Date: (dd/mm/yyyy) (optional, must be in the last 40 days)

Frequencies (MHz):  

Add [0,0] datapoint

Echo raw data

An alternate list of frequencies from 100MHz to 25000MHz can be entered in the above form as a comma delimited list. Frequencies outside of the range of observation frequencies (ie extrapolations) will not be reported.

How is it done?

This calculator performs a cubic spline interpolation of the observation points to estimate the solar flux at other frequencies.

Fig 1: Comparison of IPS interpolations with cubic spline interpolation

Fig 1 compares the interpolations published by Australia's IPS for its Learmonth observations on 22/08/2007 with a cubic spline interpolation. The maximum difference between the IPS interpolation and cubic spline is 0.09dB in this sample set.

IPS does not appear to state how their interpolation is performed, but examination of the above data shows that all interpolated values are within 0.5 SFU of linear interpolation of the published observations which are rounded to 1 SFU, suggesting that the interpolation is a simple linear interpolation between unrounded observations, even in regions either side of 1415MHz and 2695MHz where there is likely to be a more gradual change in slope than the linear interpolation would suggest.

Fig 2: Comparison of observations and interpolations with Tapping extrapolation

A method sometimes used for extrapolating solar flux from observations at 2800MHz uses a formula proposed by Tapping (1994).

  where S10 is the 10.7cm solar flux, f is the frequency of interest in MHz, α=0.0002 and β=-0.01.

Fig 2 compares the measurements and interpolations discussed above and that from Tapping's formula. Tapping's extrapolation does not seem a good method of estimating daily solar flux at a frequency of interest from the day's 2800MHz observations, perhaps it is not intended for this purpose.

Fig 3:

Fig 3 compares Tapping's formula for estimating 1415MHz solar flux from measured 2695MHz solar flux, with the measured 1415MHz solar flux at the Learmonth observatory in 2001. Note the grouping of observations into two clusters, the main cluster well below the regression estimate and a smaller cluster above the regression estimate.

Fig 4:

Fig 4 shows the distribution of error in decibels of the data in Fig 3. The clustering pattern noted in Fig 3 is manifest as a cluster of outliers and extreme values on this chart.

Table 2: Solar radio spectrum origin
Level of Origin Freq (MHz) Wavelength
Lower Chromosphere 15400
1.9 cm
3.4 cm
Middle Chromosphere 4995
6.0 cm
11.1 cm
Upper Chromosphere 1415
21.2 cm
49.2 cm
73.2 cm
Lower Corona  245 1.2 m
Upper Corona 75
4.0 m
12.0 m

Table 2 shows the origin of solar radio spectrum at the observation frequencies (from NOAA).

Solar activity can change the emission levels at some origin levels / frequencies / wavelengths differently to others. There is weakness is working from the WWV broadcast 'SEC Geophysical Alert' Solar Flux is that it is measured at 2800MHz, and cannot be reliably adjusted to other frequencies.

The observations are fetched from NOAA's   http://www.sec.noaa.gov/ftpdir/lists/radio/rad.txt in real time, and an interpolation performed for days where there are observations reported for more than four frequencies for the selected observatory. Calculations that are outside the frequency range of the observations used for the model are flagged as extrapolations, and are ignored.

Fig 5:

Fig 5 shows the variation of noon quiet sun radio flux from day to day as observed on 1415MHz at Learmonth in July / August 2007. The effect of the sun's 27 day rotation cycle is visible in the graph. The variation in solar flux is some tenths of a dB from day to day and needs to be considered in high accuracy measurements.

Fig 6

 Larger variations occur during the 11 year solar cycle, Fig 6 shows the variation in observed flux at 2800Mhz varies over the 11 year solar cycle, some 6dB over the cycle.


Noise Figure Meter (NFM) software


SETI: Determination of G/T

Wikipedia: spline interpolation

Other tools.


Version Date Description
1.01 07/02/2009 Initial.
1.02 04/02/2015 Added optional date.

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.

© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.