Green light for Cygnus to re-approach ISS for Sunday berthing

by Chris Bergin and Martin Payne

NASA managers have approved Orbital’s 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 morning.

Cygnus ORB-D:

Following its successful launch on Orbital’s Antares rocket on September 18, Cygnus ably pressed through its opening COTS milestones designated to its ORB-D mission.

Closing in on the ISS from behind and below, the spacecraft continued towards the orbital outpost with no technical issues until a discrepancy in the GPS readings between the Cygnus and the ISS was noted.

The problem is no “fault” of the Cygnus, but more to do with an issue that would only come to light during an actual mission – in turn showing the value of a demonstration mission such as ORB-D.

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 “week zero”.

This is similar to two-digit years in normal dates where year ’00 follows year ’99 and ’12 could mean 1912, 2012 or any century’s year 12. If the user of this two-digit date knows that the date is in the 21st century, he/she adds “2000” to the two digit number and gets the right year, 2012.

Similarly, each group of 1024 weeks forms a new GPS “century”, and these start 1980, 1999 (current “century”), 2019, etc.

Z514As long as the GPS receiver knows the approximate date, it can work out which GPS “century” it’s 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.

Cygnus uses GPS to sample its position at various times, and these samples are used to determine its trajectory. Cygnus also receives “position versus time” information from the Japanese PROX system on the ISS, providing the ISS’ location and trajectory over time. By comparing the two, Cygnus knows relative distance and speed, and can rendezvous.

Unfortunately, the time data that PROX transmits uses the raw 10 bit short format for number of weeks  – while Cygnus was expecting a 13 bit “weeks-since-start-of-the-1980 epoch” value.

Cygnus therefore misinterpreted the ISS data as a position from 1024 weeks – 19.7 years – previously. As such, the spacecraft couldn’t match this with its own navigational data and rejected it.

Z3The solution was known almost immediately, requiring one line of code to be inserted into Cygnus’ software, allowing for commonality between the GPS data sent from the ISS and its own GPS software.

As 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 – an action that had to be conducted several hundred miles away from the ISS.

All associated actions were completed successfully, with Cygnus healthy and in position to reattempt the rendezvous and berthing on Sunday morning.

“The 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 “patch” that resolved the GPS data roll-over discrepancy that was identified during the initial approach to the ISS last Saturday,” Orbital noted.

Z41“Orbital 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.”

That approval came on Friday morning, following a review by the International Space Station’s Mission Management Team.

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.

“Cygnus mission operations team has been monitoring the spacecraft 24/7 with two operational teams – the blue team and the green team – pulling alternate shifts,” Orbital added. “Program personnel are well-rested and fully prepared for Sunday’s approach and rendezvous.”

(Images: via L2’s Antares/Cygnus Section – Containing presentations, videos, images, interactive high level updates and more, with additional images via Orbital and NASA).

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