Harry,
Tomorrow is the Silver State Challenge Classic (sscc.us), a 90 mile open road time based rally. Many of the drivers and navigators are ultra competitive and I'm sort of catching the bug. In my 100mph class the goal is to reach the finish line EXACTLY 56:00.00 minutes after the start. You are started at the top of each minutes based on a system using a GPS clock. The top place finishers come in within a few tenths of a second from their target time.
I am trying to figure out all the things that could introduce even the slightest error into the process.
I noticed that there appears to be a 1 second or so delta between the iPhone's clock and the GPS Fix Time. Which clock does HLT use?
My main concern is how best to start HLT? Ideally there would be "Start at Set Time" trigger which I suggested in another thread. Right now I am planning to use a Standing Go trigger. Using the iPhone 4s internal GPS, how far do you think I'll have to move before the event is triggered? I want to try to build this lag into my timing.
Or do you thing I should manually start HLT? Even this has an error, estimated by some here as 0.2 seconds (which they try to compensate for).
Thanks,
Deven
iPhone time vs GPS time
Re: iPhone time vs GPS time
Hi Deven,
That's a complicated topic as LapTimer works in different time systems... As long as a NMEA GPS is connected, LapTimer uses UTC time delivered by GPS. This time is highly accurate in terms of timestamp / position relation and used to make LapTimer high precision lap timing - completely independent of sensor delays, wrong system time etc.
As strong and accurate this is, as difficult is mapping this time to real time. The reason is due to the aged satellite signals when arriving on earth, plus processing time in the GPS chip, plus transfer time between the chip (consider the GPS update rate) and actual processing in LapTimer. Adding this up you will be typically somewhere in the range between a 0.5 and a 2 seconds delay. GPS update rates make a big difference here!
On a side note, taking this delays into account is crucial for video overlaying too, as the (old) GPS time needs to be mapped to the near real time video... You can see the delay LapTimer uses for sensor synchronization and video overlaying in LapTimer -> Settings -> Expert Setting -> Sensor delays.
Back to our topic you may have seen LapTimer always starts laps late and stops late too - the delay is the reason for this. As many people get nervous with this item, I want to repeat LapTimer will calculate lap time based on the highly accurate GPS time stamp and not any system clock. Most smartphone lap timer solutions work with system time, which is inferior (see http://www.gps-laptimer.de/GPS_and_Accuracy.html, "Time of position accuracy"). It is the only option you have when the sensor is not integrated with the NMEA stream.
What does this mean for you? As long as the start time is given from "outside" (i.e. the atom clock time we were talking about), you need to add the sensor delay topic. So under the assumption you start based on an external signal, LapTimer will start approximately the named delay late and keep that delay until crossing the finish. So you actually need to arrive with a lap time display of 55:58.50 when crossing finish (LapTimer will report 56:00.00 as the actual lap time - one and a half second later).
Under the assumption you can manually time +-0.2s, this means you are probably better off using a manual stop watch (or operate LapTimer manually - it will use the system clock then) in your special scenario.
- Harry
That's a complicated topic as LapTimer works in different time systems... As long as a NMEA GPS is connected, LapTimer uses UTC time delivered by GPS. This time is highly accurate in terms of timestamp / position relation and used to make LapTimer high precision lap timing - completely independent of sensor delays, wrong system time etc.
As strong and accurate this is, as difficult is mapping this time to real time. The reason is due to the aged satellite signals when arriving on earth, plus processing time in the GPS chip, plus transfer time between the chip (consider the GPS update rate) and actual processing in LapTimer. Adding this up you will be typically somewhere in the range between a 0.5 and a 2 seconds delay. GPS update rates make a big difference here!
On a side note, taking this delays into account is crucial for video overlaying too, as the (old) GPS time needs to be mapped to the near real time video... You can see the delay LapTimer uses for sensor synchronization and video overlaying in LapTimer -> Settings -> Expert Setting -> Sensor delays.
Back to our topic you may have seen LapTimer always starts laps late and stops late too - the delay is the reason for this. As many people get nervous with this item, I want to repeat LapTimer will calculate lap time based on the highly accurate GPS time stamp and not any system clock. Most smartphone lap timer solutions work with system time, which is inferior (see http://www.gps-laptimer.de/GPS_and_Accuracy.html, "Time of position accuracy"). It is the only option you have when the sensor is not integrated with the NMEA stream.
What does this mean for you? As long as the start time is given from "outside" (i.e. the atom clock time we were talking about), you need to add the sensor delay topic. So under the assumption you start based on an external signal, LapTimer will start approximately the named delay late and keep that delay until crossing the finish. So you actually need to arrive with a lap time display of 55:58.50 when crossing finish (LapTimer will report 56:00.00 as the actual lap time - one and a half second later).
Under the assumption you can manually time +-0.2s, this means you are probably better off using a manual stop watch (or operate LapTimer manually - it will use the system clock then) in your special scenario.
- Harry
Re: iPhone time vs GPS time
Harry,
Thanks for the quick and thorough reply!
I will use a stopwatch, but I also need to use HLT as I've given it a virtual reference lap with my target speeds for each section of the course. This way the Timer view can give me a running update of how I am doing against my target times.
Assuming my reference lap is exactly 56 minutes, I start at exactly 8:04, the Internal GPS sensor delay is zero, and finally the actual sensor delay is 1.5, how will things be recorded and displayed?
Will the HLT start time be 8:04:01.5? If so, then I could change my virtual reference lap time to 55:59:58.5 and I would be good, right?
How does setting the Internal GPS sensor delay to 1.5 affect this? Would the start time be recorded at 8:04:00? And in the Timer view the first lap time displayed would be 1.5? And should my virtual reference lap now be 56:00:00?
Thanks so much,
Deven
Thanks for the quick and thorough reply!
I will use a stopwatch, but I also need to use HLT as I've given it a virtual reference lap with my target speeds for each section of the course. This way the Timer view can give me a running update of how I am doing against my target times.
Assuming my reference lap is exactly 56 minutes, I start at exactly 8:04, the Internal GPS sensor delay is zero, and finally the actual sensor delay is 1.5, how will things be recorded and displayed?
Will the HLT start time be 8:04:01.5? If so, then I could change my virtual reference lap time to 55:59:58.5 and I would be good, right?
How does setting the Internal GPS sensor delay to 1.5 affect this? Would the start time be recorded at 8:04:00? And in the Timer view the first lap time displayed would be 1.5? And should my virtual reference lap now be 56:00:00?
Thanks so much,
Deven
Re: iPhone time vs GPS time
All the delay topics where just to explain the different time system. Please do not change them to get different real time displays as it will result in wrong data recordings. The delays should always be set to match reality (which is +-1.5 seconds for the internal GPS). As LapTimer will eliminate the delays in recordings (they will actually come with the original UTC timestamps - which means 0 delay), just do not take delays into account for the reference lap. The start time for the reference lap is actually irrelevant anyway as only the relative times are used for predictive lap timing.
- Harry
- Harry
Re: iPhone time vs GPS time
I have a GPS time received from these guys http://www.timetoolsglobal.com/informat ... me-server/ that has a one pulse per second output on a RS232 control line. It also provides timing and positioning information in an NMEA format. The RS232 level 1PPS output appears to very accurately mark the start of each second, which could be used to provide a very accurate GPS time reference. Does anyone know how it could be used ?
Everett.
Everett.