Observing the "Warbler" - Little Leo

First Sounds

After several years of relative retirement from the world of satellite tracking, I switched my receivers on again during the late summer of 2000. Hardly had I got everything up and running before I was pitched headfirst into a satellite detective story. Still finding my feet among the frequencies, I found a listing that included a suggestion as to where signals from satellites in the US Defense Meteorological Satellite Program (DMSP) might be located. Out of curiosity I left a receiver tuned-in in the hope that something might come by.

An hour or so of silence then suddenly - something "warbling" against the background hiss. Not only that but Doppler too!!

Which Satellite?

Now to the detective work. First - ask the tracking community if anyone had picked up signals around 400.3 MHz before. The response was pretty well zero - apart from other people who had the DMSP frequency listed. No-one appeared to have picked up any signals around there.

More observations needed - so more listening and eventually picking it up again. Simple arithmetic offered 94.7, 101.4 or 109.2 minutes as possible orbital periods. Intuition suggested 101 minutes as the best bet. Further observations refined the figure to 101.24 minutes, ruling out it being a DMSP satellite as they use orbits with periods around 101.8 minutes.

Now to find which satellite was actually emitting the signals. There was nothing for it but to trawl the catalogue for payloads with the right orbital period. A component of the 1998-43 launch came out as the best bet - but which one? The obvious next step was to take each object in turn and find its location at the times I observed the transmissions.

Resurs O1-4 (1998-43A) fitted them all. The clincher came when I used the Doppler to measure the time of closest approach on rev 12122 and compare it with the Resurs O1-4 prediction - a perfect match!

What Is It Doing?

Finally - what do the transmissions represent? Sven Grahn came up with the best suggestion. Resurs O1-4 carried a secondary payload into orbit, in addition to its remote sensing equipment - LLMS (Little Leo Messaging System), a European Space Agency Store/Dump data communications package. Computer data can be uplinked from remote locations around the world and then, once per orbit, the data is downloaded to a ground station in Spitzbergen, Norway for onward transmission. My original thought was that the signal was the trigger from the satellite to let ground terminals know that it is within range and available to receive their transmissions. Now that the transmission has been found on such a wide range of frequencies (see below), it seems more likely that we are looking at a spread spectrum transmission.

Observations

The notes below summarise my observations at 400 MHz between 2000 October 30 and November 12. The presence and nature of the signals around 400.33 MHz have been confirmed by Sven Grahn and Chris Wood, both fellow members of the Kettering Group.

2000 Oct 30

12056

2216.15

2219

Weak, clear, warbling signal with Doppler noted on 400.329 MHz

2000 Oct 31

12070

2153.30

2159

Initially 400.300 MHz, at least two components, one of which was 400.327 MHz at LOS - Time difference between days suggests period of 94.7, 101.4 or 109.2 minutes

2000 Nov 5

12079

2142.20

2148.40

Signal on 400.326 MHz, followed Doppler down to 400.315 MHz - estimated period using earlier observations is 101.24 minutes. At least four distinct frequencies involved - pulse rate is 7.7 Hz

2000 Nov 5

12080

2300

2333

Nothing heard

2000 Nov 6

12093

2122.20

2125.30

400.326 MHz at acquisition - confirmed as multiple components

2000 Nov 6

12094

2258.30

2308

400.331 MHz at acquisition - two components approx 4 kHz apart tracked

2000 Nov 7

12107

2057

2106

Components noted initially near 400.338 MHz and 400.341 MHz - scan of adjacent frequencies shows five components at 4 kHz intervals, centred near 400.330 MHz, sounds like simple pulses but Spectrogram analysis suggests they could be PDM, signal drops out briefly every seven seconds

2000 Nov 8

12122

2211.30

2224

Located on 400.341 MHz - five components confirmed - centre frequency is 400.326 MHz. Doppler shows closest approach at 2219 - which fits Resurs O1-4. Checks of earlier passes also show a fit with the same satellite

2000 Nov 9

12128

-

-

APT noted on 137.850 MHz on tuning in at 0845, lost 0850 - low elevation and NH on 400 MHz

2000 Nov 9

12137

2338

2339

Signal located on 400.337 MHz, reducing to 400.335 MHz - elevation was less than ten degrees for this pass. Resurs O1-4 APT switched on 137.85 MHz at 2343.20 with LOS 2344

2000 Nov 12

12177

1100

1103

Observation from Jarrow, UK - closest approach approx 1102 - working with indoor dipole (hence short duration of observable signal). APT present on 137.95 MHz when on watch at 1058. APT still present during check at 1105 but no longer present on re-tuning from 400 MHz at 1106

2000 Nov 12

12178

1239

1242.45

Observation from Jarrow, UK - working with indoor dipole. APT present from 1235.50 to 1248.30.

Since the initial observations were made, more components of the Little Leo signal have been detected on a range of frequencies around 400.6 MHz. Discovery of some of the other components came about as a result of the launch of Munin (2000-75C/26621). Munin is a Swedish nanosatellite (20 cm cube) which went into orbit on 2000 November 21 as a small additional payload aboard the Delta 7320rocket which orbited NASA's Earth Observing 1 (EO-1) satellite. Munin's transmitter operates at 400.55 MHz and some of the additional Little Leo signal frequencies were detected as a result of listening for Munin.

The Signal

To the ear, Little Leo's transmitter sounds like a small bird " warbling" away to itself. The spectrogram below shows it in detail - there are thirteen pulses per second, and the gaps are where the signal drops-out periodically. The two dropouts recur every seven seconds. The slope of the line is caused by the drop in frequency due to the Doppler effect.

The signal that was tracked early November consists of five distinct, but similar, components - the central one is at 400.326 MHz, and the others are arranged symmetrically either side at +/- 4 kHz and +/- 8 kHz. All the other signal components are similar in sound with the seven-second periodicity, and they appear in groups all the way up to at least 400.79 MHz.

Page updated: 2000 December 10