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PostPosted: Tue Nov 22, 2011 4:32 am 
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Hi,
I am a new user to the LapTimer App (last week) and have to say it has an amazing set of features packed into a small package. I was interested in 5 Hz acquisition, so I also picked up the Emprum device. Just having got an iPhone 4S, I hadn't really tested its GPS ability with any other application, so I didn't know what to expect. From the forum posts, the consensus seemed that at least for the iPhone 3 series, an external GPS was preferred. But I haven't seen a direct comparison posted for the iPhone 4S. So I conducted a very informal comparison using LapTimer as a readout and recording data to/from my daily commute to work, which is nearly all highway driving with unobstructed views of the sky. Just to note, the iPhone was in 3G only mode (WiFi disabled) using iOS 5.0.1 and for the Ultimate I am using firmware v1.0.6.

As I expected, the average acquisition rates of the iPhone GPS was around 1.0 Hz, while I achieved 4.6 - 4.8 Hz with the Emprum. Since the Emprum uses NMEA, I monitored the fixed satellites. In my travels, I normally had 9 to 11 sats in view, with fixes on 7 - 9 of them. Although their signal strength were all average, all signal strength bars gave a "green" indication for most of the open areas. On occasion, the signal did slip to yellow briefly. So far this is pretty much what I had expected and pretty good given I hadn't done any tweaking with in-car position. So although precision appears to be much better when using the Emprum, I was surprised to find out that iPhone 4S GPS was reading out 2-3X better accuracy than the Ultimate. According to the LapTimer in GPS view, the iPhone managed between 16 to 32 ft (~5 to 10m) positional accuracy, while the Ultimate was about 32 to 64 ft (~10 to 20 m). I've attached a screenshot comparison of GoogleEarth street view of my recording that illustrates these differences, showing the same road in both cases (just traveling in two different directions). I chose a section of my trip that wasn't on the highway, but around a couple of nice corners to illustrate the effect on accuracy. The differences of accuracy between these devices was still 16/32 ft vs 32/64ft on open highways.

I was wondering how others' experiences were with these devices. Honestly I'm shocked that the iPhone achieved 16ft accuracy, which was fairly consistent. How does it achieve better accuracy than an external device, perhaps I have not optimized the Emprum devices enough, though why would it be more particular about position than the iPhone, both of which were in the same location for the tests. Has anyone achieved 16 ft accuracy with their external devices? Overall, I'm not sure the increased acquisition rate outweighs the loss in precision? Any other insights into my informal Day 1 comparison are welcome!

PS As a side question, in the Satellite view, what is the difference between "3D" and "3D+" accuracy?
-Todd

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PostPosted: Tue Nov 22, 2011 8:27 am 
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Hi,

Two answers on this, the short one first: accuracy for the internal and external GPSes connected via NMEA are calculated differently and cannot be compared. Accuracy delivered by the iPhone4+4Ss internal GPSes is approximately the same as the one delivered by the Emprum Ultimate GPS.

To understand the situation, we need to start with some GPS foundation: all accuracies shown are derived from the so called HDOP = horizontal dilution of precision (cf. http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS)). You may understand HDOP as indicator for errors in the measurement that will affect the accuracy delivered. You calculate the accuracy by multiplying this HDOP value by a device specific factor. The better the GPS sensor is, the smaller is the factor. Sensor producers will typically use an optimistic factor here to show how good their device is. As HDOP is a statistic value, you cannot take it as 100% reliable anyway.

Coming back to the iPhone internal GPS, iOS does not deliver the HDOP value at all but instead passes a calculated accuracy to apps like LapTimer. LapTimer displays this value 1:1. The little colored bars showing the GPS status in many of LapTimer's views derives color from accuracy too - because it is the only measure delivered.

For external GPSes connected using LapTimer's NMEA integration works different. Here, LapTimer receives the HDOP value and multiplies it with a factor originally found for Sirf / Sirf II chips. This chips where weaker than today's chips (like the SirfStar III or the chips used by the Emprum or the Dual devices). The factor applied by LapTimer is too big for these new chips and generates - well - very conservative accuracy values. Furthermore, in this scenario, the colored bars are not derived from accuracy but the number of satellites currently locked.

I plan to have all of this adjustable in LapTimer v15. Until then, I recommend to not compare both scenarios directly. If you want to see what Emprum derives as accuracy, you may temporarily turn off NMEA integration in LapTimer: remove the Emprum from your device, goto Settings->Expert Settings and turn off 'Use GPS Accessories' in the 'GPS Tweaks' section. Plug in the dongle again and watch accuracy.

The difference between 3D and 3D+ is that 3D+ denotes a differential GPS lock. For differential locks, accuracy increases by another factor of approximately 2 - which needs to be reflected in the device specific factor again (but currently isn't). The 3D+ lock is generated from an additional satellite dedicated to error correction only. The 3D+ coverage is by far lower than that for 3D. You are lucky if you get it :-)

- Harry

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PostPosted: Tue Nov 22, 2011 2:35 pm 
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Hi Harry,
Good to know comparison is misleading at the moment, that was my initial intuition as well. Thanks for the detailed explanation, this makes perfect sense.
-Todd


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PostPosted: Tue Dec 13, 2011 4:27 pm 
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How is speed and time effected?

I'm sure those become more accurate as well. Time being harder to test...


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PostPosted: Tue Dec 13, 2011 6:19 pm 
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I do not believe that 3G+ will effect time and speed to an extend it would matter. You may contact Emprum for this.

- Harry

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