Extraterrestrial Interference to NOAA APT Images

ET interference to NOAA-15ET interference to NOAA-12There is one hell of a lot of space junk in low earth orbit, and some of it is transmitting radio waves where they are not wanted, which can occasionally interfere with transmissions of NOAA APT images.  Such interference may show itself as periodic black bands across the image, as shown on the thumbnail images to the left and right. (click on the thumbnails for full sized images).  In this example, interference was first received by NOAA-12 (left), and then by NOAA-15 (right) 53 minutes later.  What these two satellites have in common is that they both transmit their images at 137.5 MHz, so something else is also transmitting on or very near this frequency.  By analysing the spacing between the stripes, one may determine that the interference repeats every 29.9±0.7 seconds.  If the interfering signal is from another satellite (space-junk or active) this suggests that this satellite may be tumbling with an ≈30 second period.

ET interference to NOAA-15Folk who routinely monitor APT images sometimes nickname this banding pattern 'piano keys'.  To prove that the noise really is extraterrestrial, and not something weird being picked up from local electrical mains (for example), the thumbnail image to the right was received at station located 315km east of us, at Swansea, Central Coast, NSW.  It was also received simultaneously with the image displayed directly above it, and both images show a similar banding pattern over southern Australia.  Moreover, if one analyses the time spacing between the stripes in the righthand image, one again measures a period between interference peaks of 29.9±0.7 seconds.

So who's doing the dirty deed!  The prime suspects are the junked satellites NOAA-6, NOAA-9 and Tiros-N.  All are known to be transmitting a radio wave carrier on or about 137.500MHz.  At the time that the right hand images were acquired (March 14th 2005, ≈20:25 hours UTC), the only nearby suspect satellite was NOAA-6, cruising at an altitude of 780km, southbound over the western Pacific Ocean.  A satellite's eye view of the Earth is shown below, as seen aboard NOAA-6 at 20:25 UTC, and Eastern Australia is plainly visible.  This colourful image comes courtesy of Formilab Earth  Viewer.

NOAA-6 ViewOther possible sources of interference from active satellites have been identified, notably from Orbcomm satellites.  These satellites transmit digital data in the 137-138 MHz Space Band, and there is some evidence that they can also interfere with NOAA APT Signals.