Wireless Finish Timing Accuracy

I thought that folks may be interested in an experiment that we performed which compares wireless finish timing with hard-wired finish timing. Over the course of more than 200 runs, the difference was always less than 0.0007s, and usually less than 0.0005s. This is well within the requirements for ski races at all levels. See below for details.

For non-scored races, the USSA currently allows a wireless connection between the start-gate and timer, but not between the finish eyes and timer. Allowing wireless finish connections seems like a natural next step, especially since the distance is so much shorter. It is my hope that this will be allowed in the near future.


Experiment Details

Purpose
The purpose of this experiment is to determine the accuracy of a wireless finish compared with that of a wired finish for an alpine ski race.

Setup
A single Alge PR1aW photocell was used at the finish. It was connected to two timers. It was (a) hard-wired to an Alge Timy-PXE; and (b) connected wirelessly over the Alge WTN to a Timy3-WP. Channel 0 of both timers were shorted together and hard-wired to the start gate.

This experiment took place at the U8/U10 Town Meeting Day Slalom at Cochran's Ski Area, VT on March 1, 2016.

Results
In this section, SysA is the hard-wired system, and SysB is the wireless system.

The plots below shows two curves: the difference between start times recorded by each timer and the difference between the net run run times for each system. We note a few things.
  • Over the 65 minutes that it took to complete run 1, the timers drifted by almost 0.002s. This is evident from the start time difference. Note that this drift will not have any significant impact on the net run times.

  • The wireless net run times are usually within 0.0005s of the wired times, and never more than 0.0007s. This is well within the requirements for alpine ski races at all levels. Note that, by comparing net run times, we have removed the effect of timer drift.

  • Run 2 results are similar, if somewhat better. Over a similar amount of time, the timers drifted only about 0.001s, and the net run times were always within 0.0005s.


As an aside, it's interesting to note that timer drift seems to be affected by actually receiving timing impulses. I note this since simply leaving the timers running overnight (without any impulses) results in less than 0.0002s of drift.

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Original Post
Great experiment.

I know from personal experience at our dual NASTAR finish at Bear Creek that the PR1aW works well even in close finish situations. Mind the battery use and it is a super addition to any racing scenario that needs a flexible finish option.

Thanks for investing the time in setting this up.

You have a perfect "lab" there. Great maple syrup too!

We will encourage some of the other timingguys to try this before the end of the season.

Certainly could be a topic of discussion for upcoming USSA Timing Group meetings.

I think that no one anticipated the advent of the inexpensive WTN option. Within reason the ability to use wireless finish already exists by inference.
You mentioned minding the battery ...

I asked this in a previous post but received no responses: Do you have a feeling for the PR1aW battery life when the radio is at full power (100mW)?

The manual has a chart which states ~28h operation for NiMH 2700 mAh batteries, but I suspect that it's for a radio power of 10mW.

Thanks.
Actually, I realized that by doing a little algebra with the stated battery times with and without the radio, I can arrive at the answer.

Battery life for 2700 mAh NiMH batteries with 1 pulse/minute at -20C (-4F):

No Radio: 57h (from manual)
Radio at 10 mW: 28h (from manual)
Radio at 25 mW: 16h
Radio at 50 mW: 9h
Radio at 100 mW: 5h

Does that sound about right?

Thanks.
quote:
PR1aW

I ran a couple of winter events and used the PR1aW in the high power output mode. It died every time just when having the prize giving ceremony (5-6 hours after I set-up the system) so your calculation seems right to me. Have to admit that weather conditions at all events have been rather mild at -5°C (23F). As I have line of sight between S and F, I will decrease to 50 for the next events.

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