The saga continues ...

[MikeFromMesa]

I borrowed a working iPhone 6 to use to test some of the nav software and how it interfaces with the Sync 3 system on my vehicle using CarPlay. The good news is that CarPlay has absolutely no issue with the Sync 3 system and everything shows up properly, I have some limited control over the apps using the vehicle interface and the routing stuff works perfectly. What is odd is the displayed speed, which I assume is based on GPS data, is way off.

The software started to give me an "over speed limit" warning and when I looked at the speedometer I noticed that I was under the speed limit. In fact the displayed speed was 7-9 miles/hour over what the speedometer said it was and when I checked on my Android Phone (don't ask) I noticed that the same apps running on it showed the correct speed. This raised a question about why the iPhone would be so inaccurate when mthey Android phone was right on the money in terms of the displayed speed. What would cause a displayed speed limit to be off by so much?

We are traveling and will be returning tomorrow and I will do some testing while we are on the road. One thing I will test is whether the speed limit displayed on my wife's iPhone 8 Plus is off as the iPhone 6 is, or whether it is just the 6.

 

[pm-r]

I was wondering if your iPhone needed an iOS Update to fix a GPS bug, but it doesn't look like your model are affected:
Apple Releases iOS 9.3.6 and iOS 10.3.4 With GPS Bug Fix for Older iPhones and iPads - MacRumors

But maybe check anyway...


- Patrick
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[MikeFromMesa]

I was wondering if your iPhone needed an iOS Update to fix a GPS bug, but it doesn't look like your model are affected:
Apple Releases iOS 9.3.6 and iOS 10.3.4 With GPS Bug Fix for Older iPhones and iPads - MacRumors

Thanks. I did not know that there was a GPS bug in the old iOS versions.

The iPhone 6 in question was on an old version of iOS that did not even include support for Waze or Google Maps so I updated it before I left for the trip. The update took it from 10.? to 12.4 (I looked at what the previous iOS version was, but do not remember exactly, only that it was 10.something) and I could run Waze, Google Maps, Sygic GPS and Sygic Car through CarPlay after the update. The GPS seems to work properly as far as the navigation is concerned, but I don't know about how the speed is computed.

It was my fault that I did not check my wife's iPhone 8 Plus to see how fast it said we were traveling, but I will do that tomorrow on our trip back home.

I would check with AppleCare but the iPhone is long out of support. The incorrect speed is the only thing I noticed that was wrong as the nav stuff all seems to work properly other than that. Turns are exactly where the software says they are, it has no issue locating us properly and the No GPS signal messages that I got with the iPhone I returned are all gone.

 

[ferrarr]

Was the iPhone 6 using cell service, or Wi-Fi?

 

[chscag]

Was the iPhone 6 using cell service, or Wi-Fi?

I believe Mike purchased the iPhone for just the GPS. No SIM card, therefore no cell service.

 

[Groovetube]

Recently bought a 2019 rav4, was excited to finally use CarPlay... I have an iPhone XS, and I find the speed in Waze is often slightly out of wack. Sometimes by 3-5 or sometimes, far more than that. I honestly don’t pay attention to it. Latest iOS.

Edit: I’ll add that I am using cellular data, not WiFi.

 

[ferrarr]

If the phone is connecting to Wi-Fi and connecting to the vehicle, that could be the delay, not sure about the location issue though.

 

[MikeFromMesa]

The iPhone 6 has an inactive SIM card in it and is connected to my cell hot spot. I have been testing Sygic and it is an off-network nav system as far as routine is concerned and I assume it uses GPS for the current location. It has traffic and alerts, but those require an active network of course, and I have been assuming that the cell wifi connection is enough. Is it possible that the inaccurate speed is due to the lack of an active SIM card? I would have thought that a cell wifi connection would provide everything an active SIM card would provide. Is that wrong?

The speed displayed on the Sygic nav system is consistently 7-9 mph too high. When I am doing 30 it shows 37-39, usually 38. When I am doing 60 it shows 67-69. It is never right unless I am stopped at a stop light or traffic sign, and then it shows 0, but then I am not too worried about traffic cameras or speed radar.

Maybe what I should try is removing the SIM card from my phone and placing it in the iPhone 6 to see if that fixes the issue.

 

[MikeFromMesa]

I believe Mike purchased the iPhone for just the GPS. No SIM card, therefore no cell service.

I have been trying to avoid adding the phone to my service. It is probably another $40/month and that seems like a waste since I only use it for navigation.

 

[pm-r]

It was my fault that I did not check my wife's iPhone 8 Plus to see how fast it said we were traveling, but I will do that tomorrow on our trip back home.

Regardless of any iOS software, I cannot imagine myself relying or using any speed indication from any iDevice while I am travelling on any road, especially when I have a good reliable speedometer in front of me.

And I'm sure not going to get into any glancing at any iDevice while driving.

The rules up in this part of the country are pretty clear and strict and carry some pretty hefty fines!!!

What's wrong with just using the car's speedometer to determine your speed????



- Patrick
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[MikeFromMesa]

What's wrong with just using the car's speedometer to determine your speed????

I think you are missing the point here.

I do not rely on the speed posted on the nav device to tell me how fast I am going. I have the speedometer for that. However I do sometimes rely on the speed warning signal from the device to let me know if I have exceeded the speed limit.

There are times in cities when there is so much cross traffic, jay walkers and lane switchers that I have to keep my eyes on the road and it is good to have the nav system tell me if I have accidentally sped up too much. Sometimes on highways through built-up areas where people are merging on and off the highway, changing lanes and when the traffic is particularly heavy it is good to know that I will get a warning if my speed gets a bit too high.

Similarly when traveling in the country it is good to know that if I miss seeing a change in speed limit that the nav system will warn me so I can slow down. That is what the warning is for and if the speed limit shown by the display is off so are the warnings.

I drive an RV and on the main roads I seldom go faster than 62-64 mph and where I live, in the west, that is way below the speed limit, but often isolated intersections have lower speed limits and in heavy traffic it is often easy to miss them. If I am passing a truck I can not see any sign on the right side of the road, so the warning is very helpful.

 

[Slydude]

I think you are missing the point here.

There are times in cities when there is so much cross traffic, jay walkers and lane switchers that I have to keep my eyes on the road and it is good to have the nav system tell me if I have accidentally sped up too much. Sometimes on highways through built-up areas where people are merging on and off the highway, changing lanes and when the traffic is particularly heavy it is good to know that I will get a warning if my speed gets a bit too high.

Similarly when traveling in the country it is good to know that if I miss seeing a change in speed limit that the nav system will warn me so I can slow down. That is what the warning is for and if the speed limit shown by the display is off so are the warnings.

.

That's a great point and it's one thing we find really helpful in unfamiliar territory.

 

[MikeFromMesa]

That's a great point and it's one thing we find really helpful in unfamiliar territory.

And the point of my "story" is that when the nav system displayed speed is incorrect so are the alarms. Thus I found myself driving through small cities, keeping under the 30, 35 and 45 mph speed limits, but listening to all of the alarms going off telling me that I am speeding because the alarms are triggered by the nav system speed limits and those are completely inaccurate.

On our return trip I checked the same nav system (Sygic) on my wife's iPhone 8 Plus and compared it with the one on the iPhone 6, and they were very different. The iPhone 6 was off by its normal 8 or 9 mph while the same software on the iPhone 8 Plus was only off by about 2 mph. I also checked Waze on the iPhone 6 and it was also off, exactly the same as Sygic so it is not the software. I wonder if the difference has to do with the GPS hardware in the phone itself?

I an reluctant to turn the alarms off since they can be really helpful, but they are not of any use to me as they are. Perhaps I need something a bit newer than an iPhone 6 for the nav software.

 

[pm-r]

And the point of my "story" is that when the nav system displayed speed is incorrect so are the alarms.

I wonder what the RVers are saying or finding at their popular RV Forum websits about the accuecy of the GPS stuff out there??????



- Patrick
======

 

[ferrarr]

I wonder what the RVers are saying or finding at their popular RV Forum websits about the accuecy of the GPS stuff out there??????



- Patrick
======

I believe that devices with only one connection (to GPS), will have better accuracy than one device using a second device, to get the GPS, then using the first device for displaying on yet a third device?

 

[MikeFromMesa]

I believe that devices with only one connection (to GPS), will have better accuracy than one device using a second device, to get the GPS, then using the first device for displaying on yet a third device?

Why? This is all digital information, not analog, so there is no loss of accuracy when transferring the information. I would think it would not matter.

As it turns out there were some settings in the software for increasing accuracy in the gps information, something about including compass data and perhaps cell data as well, and changing those corrected the large speed error. When I tested with those changes made the digital speed display was much closer to the actual speed and thus the alarms were not a problem any more.

 

[MacInWin]

I think the increased accuracy actually comes from integrating the accelerometer data in the iPhone, not cell data. Compass data can show which direction the phone is facing, but that can be different from the direction it is traveling. Speed differences between the speedometer and GPS are common, with most of the error in the speedometer. Auto manufacturers have to meet certain standards for the odometer reading, so most of them choose to have the odometer be higher than the vehicle has actually travelled. There are penalties for under-reporting mileage, which they avoid by over-reporting. GPS then shows a speed generally slower than the speedometer. Also, as your tires wear out the speedometer speeds up. I'll put a paragraph on why that is after this but if you don't care, stop now.

Tires have tread that wears off as the tire is used. The overall diameter of the tire thus gets smaller as the rubber wears away. So let's say you have pretty worn tires that have lost .25 inches/6.35mm of tread from when they were new. That means the diameter of the tire is .5 inches/12.7mm smaller. So now each revolution of the wheel goes a shorter distance than on new tires. The difference is equal to pi times the difference, or 1.57 inches / 40 mm. Not very much, but it adds up. Assuming a standard 15 inch rim, with a standard diameter of 24.1 inches tire new, that tire covers 75.71 inches each revolution. But the worn tire only covers 74.14 inches each revolution. So to cover the same distance in the same time the tire has to rotate faster, which means the speedometer will show a higher number for the same speed. In our example, the difference is about 2%. At 60 mph on old tires, although the speedometer may say 60 mph, you are, in fact only traveling 58.8 mph. But GPS doesn't care about tires, it calculates speed as the distance you covered between fixes and use the time between fixes to calculate your speed between fixes. So the GPS will show 58.8 mph. And that assumes the tires are the proper size for the vehicle. Change tire size or profile and the speedometer can be off even more.

All that also assumes constant speed, which is nearly impossible to achieve in real life, so the actual speed will still vary on the GPS. The increased accuracy comes from using the accelerometers in the iPhone to calculate changes in speed during the time between fixes so that the GPS can forecast your next fix-based speed. I've seen at least one application on the iPhone that uses the accelerometer and compass to continue to provide tracking through tunnels where GPS is not available. Some cars also use the speedometer system to feed data to the navigation system inside tunnels, too.

Finally, since we are here, the accuracy of the fix can affect the speed calculation from the GPS. Each fix is calculated independently and each has a certain probability of error. If the two adjacent fixes have errors in opposite direction the speed calculated between those fixes can be much higher or lower than your actual speed. GPS applications generally apply smoothing algorithms to limit the changes between fixes for speed, heading, etc. If in previous second you travelled this far in this direction, then this second you should be traveling about that same speed/direction. Older intervals get weighted less and less as they age, but are used to keep the speed/direction from jumping all over the place. Accelerate or decelerate really hard and those smoothing algorithms can be fooled. (I'm thinking of high speed drag racers and hitting an object slamming the vehicle to a stop kind of accel/decel.)

When you think of it, SatNav/GPS is a pretty amazing technology. And we haven't even talked about how the system calculates a route from point A to point B for you!

 

[MikeFromMesa]

I think the increased accuracy actually comes from integrating the accelerometer data in the iPhone, not cell data.

Perhaps. I don't know. The nav software had some settings for increasing the GPS accuracy and I used those, but have no idea what iOS changes that made, if any. Whatever the case the mileage figure on the display was pretty much dead-on after the adjustment.

I'll put a paragraph on why that is after this but if you don't care, stop now.

Thanks. I did read what you wrote.

The math is really pretty simple. The circumference of a circle is pi x d so it is directly proportional to the diameter. As the tread shrinks so does the circumference and since the speedometer is based on turns of the wheel the actual distance covered per turn decreases as well. A bit of simple math will give a percent figure as to how much actual speed loss there is between new tires and those ready to replace. And, of course, actual fuel economy suffers as well since less distance is actually being covered.

Finally, since we are here, the accuracy of the fix can affect the speed calculation from the GPS.

As with most of the stuff that goes on under the covers in technology, it is often far more complex than appears at first glance. The decisions that the software engineers have to make when implementing something like speeds and routing in nav software often have unexpected consequences and the whole thing rests on the integrity of the data, which I suspect is not as accurate as we think.

I used to use a Rand McNally portable GPS unit. I liked it, and it was as good as any other navigation device I had used, but it often made some very strange decisions. I had it set for "fastest trip" but it always took me through the downtown of the city of Santa Fe even though there was a bypass with a higher speed limit and only 2 traffic lights. The bypass was at least 15 minutes faster, but it was 1 mile longer, and so the software always told me to go through the city. I can not tell you how many times it told me to drive through people's backyards or to turn into their driveways while navigating in rural areas, or how many times it told me to get off of the interstate only to tell me to get back on as soon as I got off.

I spoke with tech support and they blamed bad data, and that may well be right. Like most other things, the technology is a mixed blessing.