Note for Solarwatchers: SOHO EIT is in “the keyhole”

We recieve lots of questions from our readers considering SOHO EIT data. SOHO’s near-real time images from EIT instruments stop uploading in January 6, however there is nothing wrong with SOHO instruments  because EIT is in a telemetry keyhole. The lack of fresh images came as no surprise to the SOHO science community. And they may have stumbled across the rather cryptic terms “keyhole”, as in “keyhole period” or “keyhole operations”. (You can observe similar images via SDO AIA)

The term “keyhole”, in antenna terminology, stems from a characteristic feature of aerial navigation maps showing the coverage of e.g. a radar antenna: A circular area near the antenna is shaded due to too-short range. In addition, any obstructions (hilltops, buildings) near the antenna will cause wedges to be shaded due to lack of coverage. Taken together, the appearance of the shaded area is often that of a keyhole. The term has since been generalised to refer to any area not covered by an antenna.

In order to get a good downlink signal, SOHO’s High Gain Antenna (HGA) needs to be pointed correctly as the spacecraft traverses its highly elliptical orbit around the L1 Lagrangian point, some 1.5 million kilometres sunward of the Earth. SOHO has “keyhole periods”, occurring twice per orbit. Each orbit around L1 takes about half a year, so keyholes affect SOHO operations roughly every 3 months.

ESTIMATED DATES OF FUTURE SOHO KEYHOLES



2011 	11-Jan-11	  15-Jan-11	    26-Jan-11	      30-Jan-11

     	12-Apr-11	  15-Apr-11	    23-Apr-11	      25-Apr-11

     	08-Jul-11	  12-Jul-11	    24-Jul-11	      28-Jul-11

	10-Oct-11	  13-Oct-11	    20-Oct-11	      23-Oct-11

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2012 	02-Jan-12	  06-Jan-12	    17-Jan-12	      21-Jan-12

	02-Apr-12	  04-Apr-12	    12-Apr-12	      15-Apr-12

	27-Jun-12	  01-Jul-12	    13-Jul-12	      18-Jul-12

	30-Sep-12	  02-Oct-12	    10-Oct-12	      13-Oct-12

	23-Dec-12	  27-Dec-12	    07-Jan-13	      11-Jan-13

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2013    23-Mar-13	  26-Mar-13	    04-Apr-13	      06-Apr-13

	18-Jun-13	  22-Jun-13	    04-Jul-13	      08-Jul-13

	20-Sep-13	  23-Sep-13	    01-Oct-13	      04-Oct-13

	13-Dec-13	  17-Dec-13	    29-Dec-13	      02-Jan-14

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2014    11-Mar-14	  15-Mar-14	    26-Mar-14	      30-Mar-14

	02-Jun-14	  09-Jun-14	    26-Jun-14	      04-Jul-14

	08-Sep-14	  12-Sep-14	    23-Sep-14	      27-Sep-14

	30-Nov-14	  07-Dec-14	    23-Dec-14	      30-Dec-14

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2015    02-Mar-15	  06-Mar-15	    17-Mar-15 	      21-Mar-15

	24-May-15	  31-May-15	    16-Jun-15	      24-Jun-15

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SOHO website

All images credits:  “Courtesy of SOHO/ consortium. SOHO is a project of international cooperation between ESA and NASA.”