Calculating mission phases with OMEGA's X-33 Skywalker

18 October 2020

Calculating mission phases with OMEGA's X-33 Skywalker

The new X-33 Skywalker is a return to re-claiming the mantle of an astronaut’s watch. I know the original X-33 “Mars watch” was meant to be that anointed successor, but the impression I got was that they were frankly interchangeable with the Timex Ironman in the crew preference items for Shuttle and ISS missions. Certainly a fine pilot’s watch, but not the must-have watch that the Speedmaster was for EVA.

With the new X-33 Skywalker, you will find many of the functions you’d expect from the successor of the original X-33, such as multiple time zones, alarms, mission elapsed time (MET), date and a chronograph. It also has something invented by ESA astronaut Jean-François Clervoy, called phase elapsed time (PET) and it this that makes the watch unique.

Timing has always been critical for successful navigation. When navigating the high seas, working out a ship's latitude and longitude was essential to determine location. Latitude could be measured by the angle to a celestial body (the sun in daytime, the pole star at night). The problem of longitude was solved with a marine chronometer, a timing device able to maintain accuracy at sea so the time difference from GMT could be calculated. In the space age when it came to celestial navigation, a time measuring device was needed to meet the challenges of making course corrections in space.

For spaceflight navigation, ignore if you will the image of Luke Skywalker in an X-fighter, and think more of Jim Lovell in Apollo 13. The reality of navigating a spaceship entails a series of minutely timed and controlled events, whether they are orbital corrections, interplanetary insertions or vehicle rendezvous and un-docking, to name a few characteristic mission phases.

Astronaut Takuya Onishi maneuvering SSRMS to release Dragon spacecraft
(August 26. Credit: JAXA/NASA/Takuya Onishi)

Phased Elapsed Time (PET)

The unique Phased Elapsed Time function (PET) that allows the wearer to set a date in the past or future down to the second and have the watch calculate how much time has elapsed or is left. PET combines the traditional function of the timer and alarms, but in such a way that both are much easier to set without recourse to troublesome calculations. It is an incorrect assumption that this watch adds nothing that a smartphone (or smart watch for that matter) does not provide. PET is a patented function (Patent # US20080130418), and is only to be found in the X-33 Skywalker.

To fully understand the power of PET, one has to take a step back and perform some category definitions. Specifically, the new X-33 provides a selection of timescales and a selection of functions, and it is important in understanding PET to see how these work together.

The timescales include a choice of time zones, a reference time frame (called in the X-33 GMT but also known as UTC or an arbitrary ZULU time), and a mission elapsed time (MET) that can be initiated independently to the other timescales.

The functions include three alarms, distinguished with different beeps; a chronograph indicated in the watch as “CHR” which records elapsed times as a stopwatch would do; and a timer “TMR” that counts down towards zero a pre-defined and relatively short period of time. PET, although sounding similar to MET, is not a timescale but a function that allows marking an event in one of the timescales such as MET, GMT or a time zone.

PET provides the function to set a precise time/date with reference to one of the aforementioned timescales, and counting down the remaining time to – or recording the elapsed time from – that point. An example of the distinction between forward and backward timing in practice could be an EVA phase, where the passage through an automated airlock is coordinated from a set start point, but the duration of EVA can be more elastic so long as a safety margin is maintained. Crucially, the PET can be set for forward (elapsed) counting from a point in the past! In other words, it would not be necessary to set PET at the point of lift-off but entirely possible to do it beforehand or even afterwards when there is time to do so.

In the X-33 Skywalker, elapsed time is considered positive (as in D-Day plus the number of days after a famous amphibious assault landing of the same name) and remaining time as negative (as in T minus however many H:M:S until a rocket launch from Cape Canaveral). The PET range catered for is from -99 to +999 days. If 999 days was insufficient time between phases, an intermediary phase could be created.

Using PET to automatically start a timer/chronograph.

The first innovated timing function made possible by PET is the ability to automatically initiate the start of a timer (countdown function) and chronograph (elapsed time function). Without PET, all timers and chronographs are manually triggered and this functionality is still included as functions in the Speedmaster X-33 Skywalker. But manual triggering of the timer creates issues when a countdown is required to end at a specified future point in time. The only way to accomplish this without PET is to calculate duration from an imminent future point and then manually trigger the timer function. With PET the alarm has an in-built count down function (with elapsed time going forward, too). It really is that simple, but elegantly addresses a real gap in time keeping functionality.

Using PET and MET to calculate the setting of an alarm.

Another use of PET is combining it with Mission Elapsed Tie (MET). Imagine if you will that there is a moment either just past or in the near future that marks the beginning of a chain of events culminating in an end point a precise period of time into the future. Now, you would like to set an alarm for this future point in time, but you only know the start time and duration, not the end point. For example, the start point is or was at 11:43, and the duration is 1 day, 5 hours and 17 minutes. I hope you can immediately appreciate that it’s not a trivial task setting this alarm. Of course it can be readily done, but imagine if it has to be calculated in circumstances that are quite stressful, when you are tired, in a hurry, overloaded with other tasks and unfortunately aware of severe consequences if you get it wrong. Then you might prefer PET, with the unique advantage that is there are only two settings that need to be made for the alarm to be set at precisely the right time:

  1. Set the Mission Elapsed Time (M.E.T.) to the start point.
  2. Set PET as duration required.

Done, and brilliantly so.


Unfortunately, it’s an inescapable fact that many future X-33 Skywalker owners will never make it into space. That doesn’t mean the functionality is without earthbound uses. In the age of 24 hour global business activity, with every hour filled somewhere and needed to be accounted for, I can see the utility of being able to monitor the time remaining in which one has to prepare for a remote meeting set up in another time zone, and then being able to automatically record the time actually spent on this activity. Admittedly though, getting these timings wrong would be unlikely to caste the hapless space cadet adrift orbiting the sun for eternity.

I think the X-33 Skywalker nods in the direction of my original Speedmaster, updated for half a century later yet still faintly similar with its black face and white baton hands. Even the name “Skywalker” reminds me of the hubris of naming a watch ”Speedmaster” on what I always considered was a tenuous link to the functionality of a tachymeter bezel. Perhaps like the Speedmaster’s qualification for lunar EVA, the new X-33’s PET functionality will not be used as much as mundane time telling either, but that doesn’t mean I don’t want the adventure it conjures up all the same.

The OMEGA Speedmaster Skywalker X-33