The System: Leap-Second Confusion

March 1, 2015 - By GPS World Staff

The United States Civil GPS Service Interface Committee (CGSIC) has issued a notice about a problem some receivers are having implementing the correct time. The U.S. Coast Guard Navigation Center has received reports of synchronization issues since the implementation of a leap second on Jan. 21. Users experiencing this problem should contact the receiver manufacturer for a firmware or software update. Here is the text of the CGSIC notice:

All CGSIC: 2015 GPS Future Leap Second Implementation

The GPS 50 bit-per-second navigation message transmitted by each GPS satellite (specifically Page 18, subframe 4) includes the parameters needed to relate GPS time to UTC (Coordinated Universal Time). That relationship is maintained through leap second implementation transitions by IS-GPS-200 compliant user equipment. For leap second transition, user equipment must utilize the notice regarding a scheduled future delta time due to leap seconds (ÄtLSF), together with the week number (WNLSF) and the day number (DN), at the end of which the leap second becomes effective.

On or about Jan. 21, 2015, those GPS navigation messages began to include futurevleap second data which indicates an increase in the leap second to become effective at the end of June 2015. IS-GPS-200 revision H, dated 24 Sep 2013 paragraph 20.3.3.5.2.4 Coordinated Universal Time (UTC), documents the appropriate algorithm details to ensure correct utilization of the parameters above (including all potential truncated week number transitions and variations in time of processing relative to satellite upload timing near the future leap second effectivity).

The data upload for the June 30 leap second, initiated with SVN48/PRN07 at 18:33:56z on Jan. 21, was correctly executed. However, there are several receivers brands/models that seem to be mishandling this information and applying the leap second now. This is creating a negative one-second offset in faulty receivers. The U.S. Coast Guard Navigation Center has reports of these receivers causing synchronization issues with radios, computer systems, and data logging equipment.

Users experiencing issues with GPS receivers that began on Jan. 21 should contact the receiver manufacturer to determine if the latest firmware or software patch can correct the issue.

Galileo FOC Three and Four Fit to Fly

The third and fourth Galileo Full Operational Capability (FOC) satellites are a confirmed “fit” for their Arianespace Soyuz launch March 27, having made initial contact with the mission’s dual-payload dispenser in French Guiana, according to Arianespace.

The fit check was completed over a two-day period inside the Spaceport’s S1A payload preparation building. The two satellites were installed separately, with the Flight Model #3 (FM3) spacecraft integrated on — and subsequently removed from — the dispenser on Feb. 9. Flight Model #4 (FM4) underwent the same process the following day.

The payload dispenser for Galileo was developed by RUAG Space Sweden for Arianespace, and carries one satellite on each side. It will deploy the spacecraft during the Soyuz launch by firing a pyrotechnic separation system to release them in opposite directions at the orbital insertion point.

Final integration on the dispenser will be performed during upcoming processing at the spaceport, and will be followed by the completed unit’s installation on Soyuz.

The March 27 mission — designated Flight VS11 in Arianespace’s numbering system — will be the company’s fourth launch carrying spacecraft for the Galileo constellation.

Air Force Orders Two More GPS III Satellites

The United States Air Force plans to order two more GPS III satellites from contractor Lockheed Martin. Lockheed Martin is under contract to build eight GPS III satellites, with the first planned to be launched in 2016. The contract includes options for up to four more satellites.

However, the Air Force plans to open up construction of subsequent GPS satellites for competitive bidding with GPS III space vehicle 11. The satellites are part of the Air Force’s $167.3 billion budget request for fiscal 2016, up from $152.8 billion provided by Congress for fiscal 2015.

The Air Force also intends to buy only one GPS satellite — from Lockheed Martin or a different contractor — in 2017 rather than the three included in the current budget blueprint.