Repeated brief, broadband signals on 2242.5 MHz - what they turned out to be!

Sven Grahn

Strange traces on 2242.5 MHz

Below is a typical plot that I see almost every day just after 1500 UT while keeping an eye on the S-band frequency 2242.5 MHz (for details about my tracking setup click here ). That the horizontal trace is some kind satellite pass, I assumed when I first started seeing it in January 2008.

Brief, wideband traces on 2242.5 MHz

Deriving an orbital period

I have noted some logged passes in the table below. It seems that the signal comes about 17.5 minutes earlier each day, but always around 1500-1600 UT, i.e. a sun-synchronous satellite, if indeed a satellite.

Month Date
Day nr
Feb 14 45 15 14 0,634722
8 39 15 20 0,638889 -17
7 38 15 37 0,650694
Jan 28 28 15 11 0,632639 -18
27 27 15 29 0,645139
21 21 15 35 0,649306 -17,5
19 19 16 10 0,673611
16 16 15 22 0,640278 -18
15 15 15 40 0,652778
N 12 13 14 15 16
T 118,5417 109,4231 101,6071 94,83333 88,90625


n 12,14763 13,15993 14,17223 15,18453 16,19684


N is integer part of the mean motion. The two next rows are possible orbital period and mean motions for the various values of N. Below I have plotted the passes. They seem to form a "diamond grid"

Diamond grid

To test the hypothesis of a sun-synchronous object I tried to listen on 15 February at a time 17 minutes before the time of the pass on 14 February. And, indeed it turned out to be a low earth orbit object, but because of the reverse doppler, the frequency is not 2242.5 MHz, but some other S-band frequency. See waterfall below (the strong signal with four subcarriers is a high earth orbit object):

Doppler trace from Coriolis on 2242.5 MHz

With a little help from my friends

15 February 2008 - sorting out the alternatives

I presented these observations to the members of a list server for space trackers and on 15 February 2008 Robert Christy wrote to the list server

"... Possibilities are:
1 - a single object being being switched on for one pass per day,
2 - a single object where Sven is seeing only the day's highest elevation pass,
3 - multiple objects, only one of which transmits each day with the diamond grid being a freak effect.

My money is on number 2..."

16 February 2008 - signals source identified - Coriolis!

On 16 february 2008 Ted Molczan wrote on the same list server:

"... A winning bet! The object turns out to be Coriolis (03001A /27640), a Navy/USAF scientific satellite:
This afternoon, I recalled that such a satellite had gone up a few years ago into a sun-synchronous orbit. Since it was military, it might be on SGLS, like DMSP. But, I could not recall its name. So, I pressed on with efforts to fit a rough mean motion, and found 14.175 rev/d fit all of the observed times. This was similar to one of the values Sven had determined. So, I was convinced that it was a single object. Final step was to brute force search the file of all satellites for all instances of " 14.16", "14.17" and " 14.18", with an eye sun-synch orbits at promising RAANs [Right Ascension of the Ascending Node]. Within minutes, I found Coriolis, which was the one I had been trying to recall earlier..."
On the same day I wrote to the list server:
"... I tried it again today. I saw the reverse doppler on 2242.5 MHz, but quite  weak. There was nothing on [the frequencies listed for Criolis] 2212.5 MHz or 2222.5 MHz. BTW, Paul [Marsh] lists both 2212.5 and 2222.528 MHz..."

18 February 2008 - not on a SGLS channel

I wrote to the list server again on 18 February 2008:
"..I tuned to the SGLS channels but could not find the signal. Then tuned back to 2242.5 MHz and widened the bandwidth, first to 1 MHz and then to 4 MHz . The doppler was still inverted, but the signal structure showed up. And - it is strong as hell! I tried to tune around 2242.5 MHz but I fumbled with the tuning step. Next time I will set the tuning step to 1 MHz and quickly tune up and down flist server MHz. The signal is wide, so 0.5 MHz or 1 MHz tuning step is no big deal. The signal strength certainly peaked exactly at the Closest Approach for Coriolis, so the source is clear...

... there is a direct 'continuous tactical downlink' at 256 kb/s from Coriolis of wind data to the U.S. fleet on S-band that is picked up by the shipboard 1.2 meter terminal AN/SMQ-11. Some datalist serverthat the maximum frequency of this device is 2252 MHz..."

19 February 2008 - frequency determination and confirmation

After tuning downwards from 2242.5 MHz with a 1 MHz tuning step I finally found a signal with normal doppler at the time of a Coriolis pass I wrote at 1538 UT on 19 February 2008:
"...Please check this frequency (2221.5 MHz). Passes over the UK at about 1700 UT. Signal is broadband, about 700 kHz. Quite strong..."
After the next pass, at 1705 UT, Paul Marsh wrote:
"... a nice strong signal from Coriolis! 2221.5 Confirmed! regards, Paul...."

The signal spectrum

Three days later I recorded the spectrum below on 2221.5 MHz:

Signals spectrum of Coriolis satellite on 2221.5 MHz recorded on 22 February 2008

Why the signal on 2242.5 MHz was inverted in frequency and why a signal on 2221.5 MHz can be observed on 2242.5 MHz remains to be fully explained.
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