By David L. Wilson
Monitoring Times - Summer 1985It should be readily realized in the radio hobby that what is one man's interference is another man's interest. Such is the case with the USSR OTH-B (over-the-horizon backscatter) radar.
Also known as the "woodpecker" or "pulsar," this device has drawn much interest by the interference that it generates. In fact, the WORLD RADIO TELEVISION HANDBOOK'S 1984 edition reviewed two products designed to counter this interference.
On the other hand, with proper equipment, the woodpecker can be an object of study -- as it is to this author.
The USSR OTH-B radar is believed to use two locations in the western USSR, Minsk amd Nicolayev, one location is used for transmitting and the other for receiving. This prevents the receiver site from being "overwhelmed" by thetransmitted signal. (This is a common practice for many operations at HF frequencies.)
An analysis of the woodpecker has been made using an externally triggered scope.The remainder of this article discusses the results of that analysis.
The USSR woodpecker has been observed using three repetition rates: 10 Hz, 16 Hz and 20 Hz. By far the most common rate is 10 Hz. In fact, the 16 Hz and 20 Hz modes are so rare that an analysis of those modes has not been made; however, it is probable that their operations are the same as those of the 10 Hz mode
The woodpecker generally is found to operate in either of two modes which will here be called STATIC MODE and DYNAMIC MODE.
In the STATIC MODE, four frequencies are used, each of which is associated with one of the time windows during which a pulse is transmitted.
For example, the woodpecker was observed using 16450, 16490, 16570 and 16390kHz.
During time window 1, 16450 transmitted a pulse;
during window 2, 16490 transmitted a pulse,
during window 3, 16570 transmitted a pulse;
during window 4, 16390 transmitted a pulse;
then the 72 ms silent period; and then the pattern repeats.
Before going any further, let us note an important fact. The pulses transmitted by the woodpecker have a wide bandwidth typically 40 kHz; thus, all listed frequencies are approximate. The shape and length of the pulse is not known.
The pulse does not occupy all of the 7 ms transmission window; measurements indicate the pulse width to be between 3 and 6 ms; possibly it can be varied.
It would not be surprising if the pulse actually consists of several "sub-pulses" due to reciever reaction time and multipath, it is very difficult to analyse the exact structure of the pulse.
in the DYNAMIC MODE each of the four frequencies uses all four transmitting windows, stepping through them in 6 ms intervals.
In the 10 Hz mode, a pulse is transmitted every 0.1 sec. (100 ms). Using the triggered oscilloscope, it is found that there are
actually four adjacent 7 ms transmission windows in the 100 ms period. In time order, let us call these 7 ms transmission windows
1, 2, 3 and 4. In other words, we may view the 100 ms period as consisting of
window 1 (7 ms),
window 2 (7 ms),
window 3 (7 ms),
window 4 (7 ms),
and the remaining 100-28=72 ms in that order.
The stepping order on each frequency may be 1 - 2 - 3 - 4 or 4 - 3 - 2 - 1 so that each frequency occupies a different transmitting window during a typical six second period.
As an example, the dynamic mode was noted using 8070, 8230, 8310 and 8260 kHz.. In the first six seconds, the transmission-to-
transmitting window assignment was
8070/1 8230/2 8310/3 8260/4;
for the next six seconds it was
8070/2 8230/3 8310/4 8260/1;
then for six seconds it was
8070/3 8230/4 8310/1 8260/2;
finally, for six seconds it was
8070/4 8230/1 8310/2 8260/3.
The pattern then repeats after 24 seconds (4 steps x 6 seconds).
The result is that the dynamic mode looks like the static mode for six second intervals. Only to a very trained ear can the difference between dynamic and static modes be detected.
It is suspected that the loss of synchronization that sometimes occurs with woodpecker blanking devices is caused by the transmitting window shifts in the dynamic mode, not by varying rate as stated in the WORLD RADIO TV HANDBOOK.
Why does the dynamic mode use both the 1 - 2 - 3 - 4 and 4 - 3 -2 - 1 transmitting window cyclings? The answer came one day when, 15960, 16370, 17480 and 16820 kHz were observed using the 1 - 2 - 3 - 4 cycling, while 16230, 15730, 16020 and 16130 kHz were using the 4 - 3 - 2 - 1 pattern.
Keeping in mind the wide bandwidth of these signals, it is likely that the two different cyclings allow simultaneous use of two separate four-frequency, dynamic-mode systems in the same frequency band without mutual interference. However, usually only one set of four frequencies has been noted at a time.COMPLICATIONS
In either the static or dynamic mode, one or all frequencies may suddenly change, often so rapidly as to frequently make it difficult to establish which four frequencies are in use at a given time. Sometimes the woodpecker returns to a frequency abandoned earlier.
No pattern has been found for these frequency changes; they appear to be random. At times the frequencies stay constant for long periods; at other times the frequencies change at such a rapid rate as to make analysis difficult.
While a listener may encounter difficulty trying to separate two overlapping woodpecker frequencies, a triggered scope will easily identify the two signals.
On rare occasions the woodpecker has been noted with other 10 Hz modes, often consisting of more than one pulse. In one instance, the woodpecker was observed transmitting pulses in all four 7 ms transmitting windows on a single frequency. This could be described as static or dynamic mode with all four frequencies the same.
In another case, the woodpecker was found to be transmitting a second pulse 23 ms after the first; in effect, this made eight transmitting windows. This operation was of the dynamic type.
Whether these rarely seen modes are actual operating modes, test or tuning modes, or accidents, is unknown. Perhaps other modes or some of the questions that arise from the above analysis may be answered by further observations.
I am most grateful to David L. Wilson who kindly sent me a copy of his article and in addition allowed it to be published here, and also to Rachel Baughn Editor of Monitoring Times for permission to use the copyright.