GPS World, March 2015

EXPERT ADVICE A Leap into the Unknown Mark Sampson A Leap Second will be introduced this year at 23 59 on June 30 This phenomenon comes around periodically and is necessary for keeping Coordinated Universal Time UTC in line with the small vagaries of the Earths slowing rotation Although it is an event that will pass unnoticed by the majority of people it has implications for anyone involved in the development of GNSS enabled devices For some it can be the cause of a major headache The behavior of a new receiver when subjected to a leap second may prove critical in certain instances and without robust characterization it can lead to inconsistent performance Part of the problem with the leap second is its irregularity Occurring every two or three years it means that receiver technology moves on in between and because the Earths slowing rotation is not at a constant rate of change it cannot be predicted when the next one will be announced A rapidly developing market of GNSS products having to deal with random alterations to its time framework is not an ideal situation Suitable preparations clearly should be employed The behavior of a new receiver when subjected to a leap second may prove critical in certain instances and without robust characterization it can lead to inconsistent performance It has already happened this year on January 21 GPS signals started to include information which effectively announced this years leap second event with the relevant data for future delta time and week and day numbers This caused issues with some receivers that werent expecting it some units applied the additional second immediately It would be interesting to see how these systems might have reacted during an actual leap second transition Receiver logic flow requires testing so that any GPS receiver can remain compliant with the ISGPS 200 standard and potential problems must be mitigated and controlled The use of a GNSS simulator which outputs a scenario containing the leap second event allows for the receiver and any systems around it to be exercised over and over again ironing out any anomalies to ensure total reliability The recent issues with those noncompliant GPS engines highlights the advantage that simulation provides The consistency it delivers enables a very thorough testing schedule which will in turn lead to a straightforward application of the time change One school of thought holds that leap seconds should be abandoned and that we should stick to atomic time from now on Their removal would mean that by 2100 the Earths rotation would be some two to three minutes behind humanitys precise atomic powered 24 hour clock and half an hour or so by 2700 The World Radiocommunication Assembly which has control over such matters had been postponing a decision on whether to abolish the leap second for over a decade another vote is due this year It wouldnt be any great wonder if this prevarication continues so whilst it still exists it is best to concentrate on what this Junes extra second might have in store for anyone currently developing a GNSS product Armed with a simulator the unpredictability of leap second scheduling should no longer be a major concern Should this years vote be again inconclusive those who have taken the positive step of acquiring a GNSS simulator will be in good shape to deal with the next time the clocks show 23 59 60 MARK SAMPSON is LabSat product manager for RaceLogic GPS World March 2015 www gpsworld com 12