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Mach Z not stock king of the lake.

Nav systems and simply recording speed are two different things. Not all Nav systems are equal. Sort of like iphone users using Google Maps vs Apple's Maps. Same device but varying results.

Oh I understand the difference and your example makes my point. My comment was more about how GPS is seen as 'beyond reproach' in some cases and almost untrustworthy in others despite the fact that the actual GPS data is the same no matter what the application. In your example, the difference between Apple Maps and Google Maps has zero to do with the actual GPS trilateration, it has everything to do with the programmers trying to make decisions on top of that data.
 

GPS is accurate.
If you think your sled is too fast for gps i laugh. I fly a Lear 45 jet for a living. We use ipads for all our charts and approach plates. It shows the same numbers as our $500,000 dual FMS set up in the jet.
You might need to add a few turbos to hit 500 mph....
GPS is accurate. The ipad has no trouble keeping up.

But you think a 100 buck Bushnell made in China radar gun is accurate........ok
 
GPS is accurate.
If you think your sled is too fast for gps i laugh. I fly a Lear 45 jet for a living. We use ipads for all our charts and approach plates. It shows the same numbers as our $500,000 dual FMS set up in the jet.
You might need to add a few turbos to hit 500 mph....
GPS is accurate. The ipad has no trouble keeping up.

But you think a 100 buck Bushnell made in China radar gun is accurate........ok
Whats takeoff speed? What distance does it take to reach it, and time to reach that speed. Compare that to what sleds are doing. There’s the difference.
 
GPS is accurate.
If you think your sled is too fast for gps i laugh. I fly a Lear 45 jet for a living. We use ipads for all our charts and approach plates. It shows the same numbers as our $500,000 dual FMS set up in the jet.
You might need to add a few turbos to hit 500 mph....
GPS is accurate. The ipad has no trouble keeping up.

But you think a 100 buck Bushnell made in China radar gun is accurate........ok
Cool I’ve reached 300 mph on my 998 And 146 mph in my bass boat according to GPS.

All GPS are not created equal. For the most part they are very accurate but can be out to lunch.
 
Turbo Dynamics GPS, speed ,distance times 60 ft ,660, 1000 etc works great.
 
Whats takeoff speed? What distance does it take to reach it, and time to reach that speed. Compare that to what sleds are doing. There’s the difference.
Whats takeoff speed? What distance does it take to reach it, and time to reach that speed. Compare that to what sleds are doing. There’s the difference.
V1 117 KIAS
Vr 118
V2 128

TO Distance (0 to V1 abort - 0) 5218 feet
 
Cool I’ve reached 300 mph on my 998 And 146 mph in my bass boat according to GPS.

All GPS are not created equal. For the most part they are very accurate but can be out to lunch.
Just like junky radar guns. When was the radar gun last calibrated and to what standard? Police radar guns have to be.
 
What radar gun are u referring to ?
Any radar gun really. If people are debating how to accurately measure speed, everything has error to some extend to it. We tried radar runs this year with a friends Bushnell radar gun. It was a sunny day and we had lots of snow this year. It would once in a while toss out some stupid speed. Not sure if the radar was reflecting off something. It appears to work great in the summer. He uses it trying to measure speed of RC planes.
 
GPS is accurate.
If you think your sled is too fast for gps i laugh. I fly a Lear 45 jet for a living. We use ipads for all our charts and approach plates. It shows the same numbers as our $500,000 dual FMS set up in the jet.
You might need to add a few turbos to hit 500 mph....
GPS is accurate. The ipad has no trouble keeping up.

But you think a 100 buck Bushnell made in China radar gun is accurate........ok

To your point, GPS is not foolproof nor is any other measurement device. They are only as good as their operators.

But.. for GPS the entire issue is about what it was designed to do and its inherent accuracy, and most importantly the minimum level of accuracy you can have. GPS takes time to acquire and interface with satellites and once it does, it only operates at a very low sampling rate. This alone means that there is a floor of accuracy (minimum possible accuracy) associated with any GPS system.

When you are talking about long distances (like flying), GPS is fine but then you do not need for GPS accuracy much beyond 90' (un-assisted standard GPS accuracy) for flying to begin with. However, using GPS for measuring speed over very short distances is not something that GPS as a system is designed to do. For measuring outright speed from a standstill, GPS doesn't even have the time to adjust when you are talking about small amounts of movement, in fact, it cannot even detect small amounts of movement so any system that relies on GPS has to take an educated guess by using other sensors in this case MEMs sensor of 3 or more axis sampling so that it can 'guess' what direction you are moving in and it can only 'infer' rate of change (speed). What you need to keep in mind about GPS is that it is a very low power signal traveling 12,000 miles to reach your receiver. Your receiver is only getting updates (sampling) at 10hz. All GPS systems operate in a constant state of error which is why GPS systems do not give you absolute measurements, they give you a measurement with a probability. The challenge here is that small vehicle movements, or movements over a very short amount of time do not even register with a GPS. GPS relies on constant motion over reasonable distances to build up a confidence level by analyzing not just the GPS data (which has inherent errors) but by synthesizing other sensors to make up for relatively sparse data that comes from the GPS low sampling rate of 10hz.

This is without getting into outright GPS errors where you can get readings that simply do not make sense. This can happen due to bugs in implementation but the primary factors are going to be environmental in nature (signal reflection, degradation). This is why when you drive through a city or in a tunnel or even in inclement weather, GPS navigation systems will sometimes make huge corrections in the data. I have seen speeds as high as 450MPH while sitting stuck in city traffic, I have also seen speeds of over 1200MPH on my motorcycle. Obviously errors but it hints at broader limits of the system since GPS was never designed for accurately measuring speed over short distances.

Contrast this to radar. Since you picked a $100 radar gun as an example (I would not have), lets start there.

Your $100 radar gun works on a very simple principle, that's why it is cheap to make. Not sure why you picked on a $100 gun as if the $100 is supposed to illustrate how cheap they are. GPS receivers only cost a few dollars to build, I can build a GPS system that is as accurate for your use (flying) as the most expensive systems available and it will cost about $100.00, to your own point the GPS in your iPad costs maybe 10$ max. What you are paying for with a GPS is the screen, build quality, software, mapping, and certifications.

Back to radar guns. A radar gun sends an electromagnetic pulse at very high frequency. That outgoing pulse in the case of Ka band is modulating at some 40Ghz. That is 40 billion times per second or fundamentally and more importantly, 4 billion times faster than your GPS sampling. To figure out speed, the system only has to measure time of flight, how long it takes the signal to travel from the emitter to the object and back again. Once you acquire the signal framing you are simply using 3rd grade math (albeit with very large numbers) to subtract one timeframe from another. Yes it gets more complex but in principle it is a much higher accuracy system that by design has the inherent accuracy required to measure very small movements at any speed. IOW radar guns were designed to measure speed, GPS systems were not, they were designed to give you location within a probability level.

As an illustration, move an object 1 foot in 1 second. A GPS will not be able to define/detect the movement ***at all*** it is far below the threshold of accuracy that is built in to the GPS system. With a low quality design you could potentially move the GPS 90' in 1 second and not get an accurate answer. If you drew a circle with a 30 foot radius and moved randomly inside of it you may get nearly the same location no matter what your actual location is. That represents a 10% error in a 300 ft run. Use a radar gun and it will tell you not only how far you moved within several mm of accuracy, but how much closer you are now and how long it took you to get there. It does lack triangulation capabilities but none are needed to measure distance in a well set up system.

My original point however is was not about GPS accuracy, my point is that 'GPS' is held up as a gold standard for measuring speed by some folks when in reality no engineer would ever have designed GPS the way it is if the point was to measure speed. It lacks both the proper signal accuracy and the proper sampling rate to ever be used for absolute speed measurements over short distances.

Not saying that GPS can't be accurate enough for individual data logging, its accurate enough as long as you are aware of the problems it has. It should never however be used in competition.

re: Police radar guns and calibration

Police radar guns have to be calibrated just like ANY test equipment. All of the equipment in our labs from multimeters to high end signal analyzers are calibrated on a schedule. But... most of what is being calibrated is small things like oscillator accuracy and errors are measured in parts per million. I have never had a piece of test equipment give me a seemingly accurate result that was somehow grossly erroneous. What is more likely to happen is that it is off by several single digits of accuracy to the right of the decimal point which will still make it grossly 100s of times more accurate than a GPS system when used in the above use cases. The only reason that radar calibration gets brought up in court cases is to create a technicality by which the judge cannot with perfect certainty prosecute a case that was brought about with a gun out of certification. The gun will still have an extremely high likelihood of being accurate enough.
 
I would argue your required gps accuracy comment. We fly LPV RNAV approaches to 200 feet above the ground... If we had 90 feet in error this would be an issue. Up or down or side to side.


You guys are picking and choosing your examples which is why I picked the Bushnell example. The Bushnell may have been operator error. Maybe a limitation? If your using some old car gps to compare every gps against thats not fair either. We use the new ipad mini in the jet and its times and speeds are spot on to the FMS calculates. Thats even getting its signal through a thick heated windshield. A lot of current goes through them. The old ipads had issues but these new ones are awesome.

Radar has its limits too. K band has lots of noise these days. Ka too. One of the many reasons why police switched to laser. Way quicker too to get a speed too.

I see what you are saying but making a blanket statement that GPS is horrible for measuring speed is silly. Or all GPS give out nonsense numbers. Or all GPS hunt for a location.

But not to say I completely disagree with you I do have a Humminbird fish finder. Its GPS sucks. Takes 15 minutes to figure out where it is. Just sitting still it shows points all over where it thinks it is. Its a piss poor gps. Its refresh rate is pathetic.

GPS has come a long ways.
 
I would argue your required gps accuracy comment. We fly LPV RNAV approaches to 200 feet above the ground... If we had 90 feet in error this would be an issue. Up or down or side to side.


You guys are picking and choosing your examples which is why I picked the Bushnell example. The Bushnell may have been operator error. Maybe a limitation? If your using some old car gps to compare every gps against thats not fair either. We use the new ipad mini in the jet and its times and speeds are spot on to the FMS calculates. Thats even getting its signal through a thick heated windshield. A lot of current goes through them. The old ipads had issues but these new ones are awesome.

Radar has its limits too. K band has lots of noise these days. Ka too. One of the many reasons why police switched to laser. Way quicker too to get a speed too.

I see what you are saying but making a blanket statement that GPS is horrible for measuring speed is silly. Or all GPS give out nonsense numbers. Or all GPS hunt for a location.

But not to say I completely disagree with you I do have a Humminbird fish finder. Its GPS sucks. Takes 15 minutes to figure out where it is. Just sitting still it shows points all over where it thinks it is. Its a piss poor gps. Its refresh rate is pathetic.

GPS has come a long ways.

The 90' is horizontal error rate in unassisted GPS, todays GPS systems are much better but vertically they are off about 1.5-2x their horizontal accuracy rate. If your GPS is currently operating at 1M of accuracy horizontally, it is only 1.5-2M accurate vertically. Something to keep in mind. An ipad for example is only accurate to 16' with a 95% confidence level but even if that accuracy is 4' it still could be off as much as 8' in the vertical dimension.

My point is this. For short distance timing measurement, a GPS system alone just isn't the right tool. You need to improve on the data that GPS provides by using external data. (hence dragy), and again, my point wasn't about the accuracy/inaccuracy of GPS so much as it was about people putting it on a pedestal.

RE: Picking and Choosing examples. I am offering simple examples that highlight the limitations of GPS. While GPS devices themselves have gotten better, they have done so primarily because there are now more satellites/systems to choose from (e.g. GLONASS and WAAS) these systems do nothing to address the core issue for accurately measuring speed over short distances which is that there isn't enough signal or sample rate for accurate measurements over short distances. GPS just wasn't designed to solve that problem, it is a location based, navigational assistance system. It was never designed to be a high accuracy speed measurement system. While you can do some simple math to extrapolate how long it took you to get from point A to point B, the answer you get will have many contributing factors which could make the result inaccurate.

Here is a very simple example.

You are traveling at 100mph, this means you are covering 146' per second or 14.6' per GPS sample. A newer GPS system has location accuracy of 3-12 meters or ~9-36' per sample, a smartphone or tablet (as an example) is accurate to about 16' per sample with a 95% confidence level.
So, you are in the real world moving 14.6' per second, but from the perspective of the GPS the 95% confidence result could be somewhere between not moving at all (no change in location) or you moving up to 32 feet between subsequent samples (almost twice the actual real world difference). Over time, all of the noise (erroneous samples) are filtered out by cumulative accuracy, meaning the more samples you have to work with over a longer period of time the higher the degree of confidence and the higher the real world accuracy, that's why it works for your plane. If GPS operated at higher sample rates this would not be an issue, but it doesn't.

Dragy and other race logging systems get around these limitations by integrating external sensors, typically a 6 or 9 DOF accelerometer. Ever wonder why we need a dragy device if we already have a cell phone? Its because the cell phone alone doesn't have the required accuracy on top of which GPS alone also cannot be relied on. Therefore you need the external devices to create the additional accuracy, but even when it is working at peak accuracy dragy will not be as accurate as a set of timing lights or lasers.

As another example, here are some shootout results.

Focusing on the thundercat.

In the first 1.5 seconds it traveled 60 feet (40 feet/s), in the following 2.68 seconds it traveled an additional 270 feet (@ 79 feet/s). That means that you have 15 GPS samples in the first 60 feet and only 42 samples across the entire 330 ft, at 660ft you only have 66 samples total and you are moving at 99 feet/second. The sample rate simply isn't there. A radar gun would have millions of samples (minimum) over the same length of time. A timing trap (lights) would have exactly one sample, the only one that mattered. The irony is that for GPS, the faster you go over a shorter window of time (drag racing) the less reliable the GPS will be.

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It's alive...but why am I not impressed?!?! Maybe cuz it's a 2s. While I've never had any major catastrophes with them, I just don't find them appealing anymore.

 
It’s almost time to show the Polaris faithful how a real turbo motor works . lol
My buddy has high hopes for it .
 
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