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The problem is no \u201cfault\u201d of the Cygnus, but more to do with an issue that would only come to light during an actual mission \u2013 in turn showing the value of a demonstration mission such as ORB-D.<\/p>\n
The issue, which is rather complicated, relates to the facts that GPS time is specified using week numbers, and that this number is transmitted by GPS satellites as a 10 bit number that ranges from 0 to 1023. In 1999 this number reached 1023 and the next week was again \u201cweek zero\u201d.<\/p>\n
This is similar to two-digit years in normal dates where year \u201900 follows year \u201999 and \u201912 could mean 1912, 2012 or any century\u2019s year 12. If the user of this two-digit date knows that the date is in the 21st century, he\/she adds \u201c2000\u201d to the two digit number and gets the right year, 2012.<\/p>\n
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Similarly, each group of 1024 weeks forms a new GPS \u201ccentury\u201d, and these start 1980, 1999 (current \u201ccentury\u201d), 2019, etc.<\/p>\n
![\"Z514\"](\"https:\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2013\/09\/Z5141.jpg\")
As long as the GPS receiver knows the approximate date, it can work out which GPS \u201ccentury\u201d it\u2019s in, and derive an unambiguous 13 bit number of weeks since the start of the 1980 epoch. This is like converting a short date into one that includes the century.<\/p>\nCygnus uses GPS to sample its position at various times, and these samples are used to determine its trajectory. Cygnus also receives \u201cposition versus time\u201d information from the Japanese PROX system on the ISS, providing the ISS\u2019 location and trajectory over time. By comparing the two, Cygnus knows relative distance and speed, and can rendezvous.<\/p>\n
Unfortunately, the time data that PROX transmits uses the raw 10 bit short format for number of weeks \u2013 while Cygnus was expecting a 13 bit \u201cweeks-since-start-of-the-1980 epoch\u201d value.<\/p>\n
Cygnus therefore misinterpreted the ISS data as a position from 1024 weeks \u2013 19.7 years \u2013 previously. As such, the spacecraft couldn\u2019t match this with its own navigational data and rejected it.<\/p>\n
![\"Z3\"](\"https:\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2013\/09\/Z315.jpg\")
The solution was known almost immediately, requiring one line of code to be inserted into Cygnus\u2019 software, allowing for commonality between the GPS data sent from the ISS and its own GPS software.<\/p>\nAs of Sunday morning, Orbital began running regression tests to ensure no systems would be adversely affected as a result of what was a minor change. However, in order to execute the new code, Cygnus required its avionics to be reset \u2013 an action that had to be conducted several hundred miles away from the ISS.<\/p>\n
All associated actions were completed successfully, with Cygnus healthy and in position to reattempt the rendezvous and berthing on Sunday morning.<\/p>\n
\u201cThe Cygnus spacecraft remains healthy in-orbit, with all major onboard systems performing as expected. Over the past several days, the Cygnus engineering team has developed, validated and uploaded the one-line software \u201cpatch\u201d that resolved the GPS data roll-over discrepancy that was identified during the initial approach to the ISS last Saturday,\u201d Orbital noted.<\/p>\n
![\"Z41\"](\"https:\/\/www.nasaspaceflight.com\/wp-content\/uploads\/2013\/09\/Z411.jpg\")
\u201cOrbital and NASA are currently discussing the best rendezvous opportunity, with the current trajectory plan supporting Sunday morning, September 29 as the next opportunity to rendezvous and approach the ISS. This schedule is still subject to final review and approval by the NASA and Orbital teams.\u201d<\/p>\nThat approval came on Friday morning, following a review by the International Space Station\u2019s Mission Management Team.<\/p>\n
With Cygnus patiently waiting for the second attempt, 2,400 km behind the International Space Station, Orbital gave the spacecraft permission to perform the first of a series of thruster burns to begin the journey back towards the ISS to be in the right position for a rendezvous during Sunday morning.<\/p>\n
\u201cCygnus mission operations team has been monitoring the spacecraft 24\/7 with two operational teams \u2013 the blue team and the green team \u2013 pulling alternate shifts,\u201d Orbital added. \u201cProgram personnel are well-rested and fully prepared for Sunday\u2019s approach and rendezvous.\u201d<\/p>\n
(Images: via L2\u2019s Antares\/Cygnus Section \u2013 Containing presentations, videos, images, interactive high level updates and more, with additional images via Orbital and NASA).<\/p>\n
(Click here: http:\/\/www.nasaspaceflight.com\/l2\/ \u2013 to view how you can support NSF and access the best space flight content on the entire internet).<\/p>\n","protected":false},"excerpt":{"rendered":"
NASA managers have approved Orbital\u2019s Cygnus spacecraft for a second attempt to rendezvous and berth with the International Space Station. Cygnus required one line of code to be updated to its software, following a GPS discrepancy between the spacecraft and the Station. The successful action resulted in approval to press ahead with berthing on Sunday […]<\/p>\n","protected":false},"author":1,"featured_media":30002,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":"","_links_to":"","_links_to_target":""},"categories":[2],"tags":[9091,639,233,7577],"class_list":["post-39750","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-news","tag-cots","tag-cygnus","tag-iss","tag-orbital"],"acf":[],"_links":{"self":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/39750"}],"collection":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/comments?post=39750"}],"version-history":[{"count":0,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/39750\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media\/30002"}],"wp:attachment":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media?parent=39750"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/categories?post=39750"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/tags?post=39750"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}