Found this unusual transmission around 8pm tonight, consisted of a repeating transmission of 3 different tones as following:
3 tones at 600 Hertz
3 tones at 450 Hertz
3 tones at 350 Hertz.
This pattern was transmitted over and over until about 8:15 pm when it stopped. Attached is a screenshot. Some of the tones were at different volume levels. Haven't the slightest idea what this is. If you have ever taken a hearing test, that is how it sounded.
Transmissions from the Joint Land Attack Cruise Missile Defense Elevated Netted Sensor System (JLENS) radar at Aberdeen, MD occur centered on 138.3125 and 149.4375 MHz. As this tethered aerostat (blimp) can be at altitudes like 10,000 feet, the transmissions are easily heard at distances overt 100 miles. Due to the high pulse repetition rate, these have a bandwidth over 100 kHz (about 100 kHz @ -10 dB, 150 kHz @ -20 dB, and easily seen over a 400 kHz bandwidth). They can be heard in AM, FM, or WFM modes, but are best heard in AM mode. As the pulses are very short, when received this way, the audio will contain harmonics of the pulse repetition rates.
The transmission uses intervals with pulse repetition rates of 360.23 Hz, 460.40 Hz, and 599.52 Hz that follow repeat a pattern for long periods of time.
Let the following represent the the different pulse repetition rate intervals on the two frequencies:
"a" 360.23 Hz on 138.3125 "A" 360.23 Hz on 149.4375 (780 ms)
"b" 460.40 Hz on 138.3125 "B" 460.40 Hz on 149.4375 (610 ms)
"c" 599.52 Hz on 138.3125 "C" 599.52 Hz on 149.4375 (469 ms)
[Interval time lengths are first to last pulse. In each case, the result is 282 pulses are transmitted during an interval of each pulse repetition rate. Multiplying by 282/281=1.0036 to convert to full cycle lengths obtains 783, 612, and 471 ms, respectively.]
An example transmission pattern heard is abcaBCABcabcABCAbcabCABC and then that repeats.
Another heard transmission pattern heard is aBCaBcaBcABcAbcAbCAbCabC. Note that each pattern always contains 4 intervals of each radio frequency pulse rate combination. This gives 4 x 3(PRR) x 2 (radio frequencies) = 24 intervals which takes typically about 16 or 17 seconds to transmit (the times between the frequency pulse rate intervals do not seem precisely controlled but is often roughly 57 ms).
In the above cases, the pulse repetition patterns did not change, only the radio frequency pattern changed.
There are occasional skips and rarer slight overlaps. The transmissions sometimes go off the air for a while.
The transmissions interfere with a trunked LTR system at Dahlgren, VA which uses 149.3000 and 149.4500 among its frequencies; but most users around the 138.3125 area (several DC area EDACS trunks) moved out several years ago.
*The pulse rates 360.23, 460.40, and 599.52 Hz correspond to times between pulses of 2.776, 2.172, and 1.668 ms, respectively.
David L. Wilson to SatelliteGuys! Thank you for this great information! 
