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Blog Post: PerformanceBox vs AP-22 results


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Recently I began testing with the RaceLogic Performancebox gps data logger.  Previously I had been testing with the Race Technology AP-22 accelerometer-based performance meter.  This test is intended as a side-by-side comparison of the two meters on simultaneous runs.  I ran both meters at the same time, on the same ...

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Bruce

2023 Cadillac CT4-V Blackwing

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The weakness of GPS-based performance metering is that the accuracy depends on the satellite geometry at the time of testing. If one of the GPS satellites is within a 60-degree cone of the line of acceleration at the time of test, accuracy is best. Best GPS accuracy is limited by the accuracy of the time measurements of that one satellite.

Since GPS accuracy is limited to about 50 feet due to limitations on accuracy of the ephemeris data on each GPS satellite, the time measurement function needs to be accurate to about 25 ns, and it is quite likely that the PB uses existing GPS hardware and thus has a 25 ns accuracy per individual measurement. This translates into a noisiness of about 25 feet in individual position measurements. I suspect that there is an averaging and estimation process in the PB software that reduces the position errors to far less than that; how much less is best estimated from the performance of the PB. If you ever compare it to a fifth-wheel or other highly accurate performance measurement technique, that would be great. And, take the AP-22 with you...

But, back to the real world of real data as available, it looks like the PB uses the GPS trick of estimating position, velocity and acceleration in two dimensions to get the numbers it displays. I don't see how it could match the AP-22 unless it did this, and this is standard in GPS software. The best performance would be a box that has both accelerometers and GPS and uses data from both in producing its outputs.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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The Performancebox manual shows these accuracies:

Speed Accuracy 0.2 Km/h

Position Accuracy 5m 95% CEP**

Distance Accuracy 0.05% (<50 cm per Km)

Time Resolution 0.1 seconds (10 hz GPS, so 10 samples / sec)

** 95% CEP (Circle Error Probable) means that 95% of the time the position readings will fall within a circle of the stated diameter

An additional benefit is that the car magazines often use the more expensive Vbox version of the (consumer) PB, and by test have found that they give equivalent results. So at the least one might say that the PB gives times similar to the magazine results.

Here in Dallas the PB finds 6-7 satellites on the datalogs I have examined for tracking with, and manages to lock in while I am still in the driveway. I was concerned because some users have reported slow satellite lock times, but my experience has been that once I get out of the garage it finds satellites right away.

The PB datalogger also keeps up with altitude, and can be used to log height/elevation changes within 10 m at 95% CEP

I am thinking that the ABSOLUTE position of the vehicle matters less for delta measurements such as 0-60 or quartermile than the relative measurements, which actually helps in terms of accuracy?

Bruce

2023 Cadillac CT4-V Blackwing

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Relative measurements can be made using a single satellite signal, which is where I got the 60-degree cone. If they are getting 5 m CEP, then they have a 15 ns time measurement accuracy, which can be obtained using 25 ns accuracy hardware with not much averaging, and I suspect that several hardware measurements are averaged at each 100 ms position output. So, the numbers seem to hang together. Full position measurements including time and altitude four or more satellites, and GPS time is a by-product of this process. Speed and acceleration accuracy are strong functions of the averaging times and such. Evaluation of their speed and distance accuracies would require a little more analysis, but since the top-level numbers seem to be OK I think that they are likely as they state. In fact, I find that commercial products under-specify performance by a margin that guarantees that they don't get returns or bad publicity when the worst of the batch is put to the test by someone out there.

Satellite locking time usually isn't a function of where you are because the GPS satellites all have polar MEO orbits, but it can be a function of weather or the positions that the satellites are when you turn it on. Unless one is within 60 degrees of straight up when you turn it on, it will be slow in locking on to that first satellite or two. Once it has three or four, the rest come pretty quickly. Position accuracy comes from satellites near the horizon, which are locked on to last. Altitude accuracy uses satellites that are overhead, and the transit times are pretty quick, so you may have to wait longer for good altitude accuracy.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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I was interested in the altitude above sea level of my test area, and I also note relative altitude start of a run to finish to consider how flat the test area was. With the CTS I automatically get terrain-following lol. One of the great things with the PB is it is set and forget -- nothing needed during the test itself as it logs constantly and saves the entire run to an SD card. The software highlights for me where the 0-speed runs are and peals them out of the data later during post processing.

I need to work on how to cue myself better between the video and the datalogger to make sure which data run goes with which video. Also, my Flip video is the earlier 30min version, and not the newer HD and/or 60 min version. So I may want to change to a HD version just to get the 60 min test time, although normally 30 min works for video to just turn it on and forget it also.

I was close to setting a new quickest time for the CTS that session -- down to 6.03 on the AP-22, 6.10 on the PB, where my quickest ever run was 5.95 sec 0-60 on the AP-22. A little better driving at the launch and I think I could get a new time.

Bruce

2023 Cadillac CT4-V Blackwing

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For the most accurate altitudes from my Garmin Nuvi 360, I need to wait at least 15 minutes. The altitude and altitude error changes slowly, it it usually takes a couple of satellite passes to get best accuracy. One of the displays shows the positions of the satellites, relative to true North and zenith, and a set of bar charts of their signal strengths, as well as lat/long/alt and accuracies, so you can watch that too. If there is a similar display on your nav system that would help. Any given satellite will come and go in about 20 minutes, and 2/3 of them will pass within 60 degrees of zenith.

The Canon HD camcorders are here. I noticed that the estimated street prices on the Canon web page are pretty closely tracked by the prices on Amazon.com.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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The (cheap) cameras I am looking at for this type of testing are:

Aiptek HD

Flip Mino HD

Kokak Zi6 HD

Sony Webbie MHS-CM1 HD Camcorder (Silver)

The video I use in the car now is the Flip Ultra 30 min

31jhn4VDrqL._SL500_AA280_.jpg

Key needs: Acceptable video, no rattle on the car mount, low price, capture 30+ min of video with no active input, capture to SD or similar, saves to a native format Trackvision can understand

Desired: powered by the car while recording, HD resolution.

I see a lot of discussion about the Aiptek cameras, and Trackvision has specific input parameters built around them because they have been a popular choice for (cheap) in-car video.

Bruce

2023 Cadillac CT4-V Blackwing

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I'm a still photography person myself -- very much so for many years -- but I never did get a video camera myself, so I can't help you too much by commenting about experience there. When I buy something from Amazon or NewEgg, I go by the reviews that I see there, and I read a lot of them to make sure that I get true impressions from knowledgeable people that are applicable to my usage. All of the HD Video cameras that you cite have a lot of reviews and very high average ratings. Here are a couple of things that I notice:

  • The Kodak Zi6 and Flip Video Mino HD record at only 720p, which is about halfway between DVD quality and 1080p. I would get one of the others that records at 1080p.
  • The Aiptek is the only one that mentions image stabilization. That can be a very big deal with a headrest-mounted car-cam.
So, with your note that a lot of the accessories for car-cams are designated to work with Aiptek, there you go.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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