Food for thought when precise timing matters


#1

The issue got my attention recently when I was accused of making an error when calling a boat back for crossing the line early at the start of a local sailboat race. I was using GPS time and the racer in question was using a cell phone without GPS connectivity.
Dedicated GPS receivers and cell phones directly linked to GPS signals will display time with accuracy within fractions of seconds. Cell phones without GPS connectivity receive time information from cell towers and may be off by several seconds. Readouts on two cell phones receiving their signal from a common cell tower may be in sync but still seconds off GPS time. Two cell phones although close to each other may be receiving time information from different towers. In that case, they will be out of sync in addition to displaying inexact times.
Android based phones reportedly have been ahead of GPS time by as much as 15 seconds because of the administrators’ failure to coordinate with the changes applied to official UTC time that compensate for slight variations in the speed of the earth’s rotation.
We’ve come a long way since the railroad companies established time zones so they could herd their customers more or less on time to meet the trains but there’s still a little room for improvement.


#2

Was this a standard 5 minute start sequence with either horns or a gun shot for the start time? I’ve never used a GPS or a cell phone to time a start sequence. In my experience the race committee starts the sequence and you start a 5 minute count down. If your bow is over the line at the gunshot, you get called back. It’s been a few years, so maybe things have changed.


#3

I’m using the pursuit format for local races so the participants are assigned a start time based on their PHRF rating. It’s up to them to cross the starting line at their assigned time. It’s designed so the finish is the exciting part and they all make it to happy hour at about the same time.


#4

Whatever WWV says is the legally correct time in the US. “Phone time” loses bigly.


#5

My kind of racing


#6

Yeah, WWV gets it straight from the atomic clock in Fort Collins so the same as displayed on a GPS receiver.
The hotshots with the pure racing hulls consider it sacrilegious to attach any hardware to the boat. They rely on hand held VHFs and they get out of range pretty quickly. After that first false start at the start of the season, I explained to them what had happened and they piped down.
Even though the cruising class boats have VHFs, most of them are mounted inside the cabin which defeats the purpose but they don’t get that if you have a radio, you’re supposed to be able to use it while underway.


#7

Yah boy, they don’t call it beer can racing for nothing…


#8

If that’s the case, why are they taking it so seriously that a few seconds make a difference?


#9

“GPS time” and the time displayed on a GPS is being conflated. Technically “GPS time” is UTC time without the leap seconds. The current difference between UTC and GPS time is about 19 seconds.

The time displayed on a GPS units is GPS time which has been converted by the receiver to UTC or some LT. It could differ from UTC by a couple of seconds.

The time from WWV is UTC.


#10

Most of the guys on the cruising boats are laid back. They’re out to have fun after coming home from work on Wednesday nights and they’re focused on not spilling their brewskis and looking suave in front of the ladies so they couldn’t care less if they start half a minute late. It’s the diehards in the little J 24s that are out for blood.


#11

This is from the intertubes, it explains the difference between “GPS Time” and the time displayed on the GPS.

Joe Mehaffey comments:
This means that IF your GPS does not have (or does not save) the leap second offset from last time it was operated, your time may be off by perhaps 12 seconds until the complete NAV MESSAGE is received by the GPS. Jack and I have observed that “typically” Garmin GPS receivers display time which is delayed from about 1/2 to 1 second behind UTC. Lowrance GPS receivers are usually between 1 and 2 seconds delayed behind UTC. In both cases, this is a result of the display driver subroutine having low priority as the “GPS internal clock” is within a few nanoseconds of correct


#12

According to the QPS website the most recent leap second was inserted on December 31, 2016 at 23:59:60 UTC but there is a limit to the number that can be added. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits).
I think that so far something like 47 leap seconds have been added since the practice started. According to the same report, “The ITU World Radiocommunication Conference (WRC-15), in session in Geneva in November 2015, has decided that further studies are required on the impact and application of a future reference time-scale, including the modification of UTC and suppressing the so-called ‘leap second’. A report will be considered by the World Radiocommunication Conference in 2023.”

The report also states that GPS time is theoretically accurate to about 14 nanoseconds. However, most receivers lose accuracy in the interpretation of the signals and are only accurate to 100 nanoseconds.

Close enough for beer can races.


#13

That’s the error in the “GPS internal clock”. What is seen by the user is the " display time" Typically there is a delay between internal clock and display of 1/2 to 2 seconds.

EDIT: I should say there could be a delay of 1/2 to 2 seconds, I don’t know if that’s typical.


#14

That’s in line with what I’ve read. Something to do with the display unit’s filtering and verification process.


#15

Its very basic, but I love the clarity of the explanation here:


#16

By eye I can’t see the difference between Time is and a radio controlled digital clock. I understand that differences less than 0.2 seconds can not be seen by eye. I can easily see there is difference between two separate GPS receivers which sit side by side in the wheelhouse.

Atomic clocks are DIY now,here are directions, the oscillators can be purchased used from eBay here.


#17

The largest spread I’m seeing when comparing different displays is 0.9 s. Well within published parameters.
A vessel approaching the start line travelling at a ground speed of 6 knots would cover a distance of 0.911’ in 0.9 s.
From a committee boat riding at anchor at one end of the 200’ start line, I’ll compare that with the difficulty of detecting a 0.2 s difference between 2 display readouts side by side.


#18

0.9 sec at 6 knots VMG >>>

6 NM / 3600 sec * 0.9 = 0.0015 NM in 0.9 sec >>>
0.0015 NM * 2025 yd ≈ 3 yards in 0.9 sec

…that should well be visible along the start line.


#19

Yeah, 6 kts is ~10 ft per second.


#20

Yes, the correct distance is 9.11’, not .911’. That darn calculator has let me down again.