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{{DISPLAYTITLE:GPS Accuracy of Garmin, Polar, and other Running WatchesHeart Rate Variability (HRV)}}<div style="float:right;">__TOC__</div>I evaluated the real-world accuracy Heart Rate Variability (HRV) can be used to measure stress, either to evaluate recovery status or exercise intensity.=What is HRV?=Heart Rate Variability (HRV) is a measure of GPS watches while running over 12,000 miles/19,000Km and recording over 50,000 data points as part of my evaluation the irregularity of the [[Best Running WatchHeart Rate]]es. Under good conditions most of the watches are remarkably goodThe time between heartbeats varies slightly, but even when things get a little tough the differences become more apparentaverage [[Heart Rate]] is steady. HoweverFor example, '''none a [[Heart Rate]] of the watches have GPS accuracy that 60 BPM is good enough to be used for displaying your current pace'''an average of one beat per second. As a resultHowever, I've added the test results for various [[Footpod]]s as they can be far more accurate than GPS, but more importantly they tend to have far less moment-to-moment variation actual time between heartbeats could vary so they can give a far better display of your current pacethat some beats occur after 0. (Note that my accuracy tests focus on the ability to measure distance8 seconds, not and some after 1.2 seconds. In the moment in time positioncontext of HRV, this irregularity is a good thing, though the two are obviously relatedand lower HRV indicates an increased level of stress.) [[File:GPS Accuracy=HRV to Measure Recovery Status=* HRV can be measured during exercise or at rest.png|none|thumb|800px|An infographic * There are various ways of the accuracy of the GPS running watchesanalyzing HRV that provide different values, and these methods have different benefits. The top right corner represents the most accurate watches. (This graphic uses ISO 5725 terminology.)]]The table below is a simplified summary of the results* Resting HRV tends to decline with training stress, where a '10' would be but there are wide variations between individuals and there are other factors that can influence HRV on a perfect device. (For an explanation of the ISO 5725 terms 'trueness', 'precision' and 'accuracy', see belowdaily basis.){{:GPS Accuracy-summary}}* There is evidence that HRV can be used to detect [[Overtraining Syndrome]], but only by comparison with prior HRV data.The values used are simply 10 minus * Generally, HRV is greatest at rest and the value for trueness (average) and precision (standard deviation from true)variability declines as the heart rate rises. The overall Therefore, looking at HRV to Heart Rate ratios is the combination of trueness and precisionimportant rather than looking at raw HRV values. Repeatability * HRV is how consistent a watch is in providing linked to aerobic fitness, with the same value for the same course segment. '''Important''': Manufacturers do not typically release fittest individuals having the type of GPS chipset usedgreatest variability, so the information in and this table is based the best available data, but it should can be treated with caution. =Methodology=''Main article: used to predict [[GPS Testing MethodologyVO2max|V̇O₂max]]''<ref name="Hottenrott-2006"/>. Simply taking a GPS watch on a single run does not provide sufficient data to reasonably evaluate its accuracy* Lower HRV is associated with greater risk of death after heart attacks<ref name="Lombardi2000"/>. To gather the data for this test I ran the same route repeatedly, recording laps every quarter mile. The course is challenging for GPS, with lots of twists, tree cover, power lines* Some [[Best Running Watch| Running Watches]] can record or display HRV, turn arounds and goes under a bridge. However, I believe that it's reasonably representative of real-world conditions, and probably less challenging than running in the city with skyscraperssome have software to use HRV to predict recovery or [[VO2max|V̇O₂max]]. =Accuracy, Trueness HRV and Precision (plus Repeatability)Overtraining Syndrome=For this evaluation I'll use the ISO 5725 definition of[http://en[Overtraining Syndrome]] is a serious long term problem for athletes.wikipedia.org/wiki/Accuracy_and_precision Accuracy as the combination of trueness The science around HRV and precision]. Overtraining Syndrome is tricky to interpret for several reasons:{| class="wikitable" |- valign="top"|[[File:High precision Low accuracy* Many of the studies evaluate the change in HRV with increasing training load (overload).svg|none|thumb|x200px|This overload is an example of high precision, as all the hits are tightly clusteredquite different from Overtraining Syndrome and the results do not necessarily transfer. HoweverBy comparison, the trueness is poor few studies look at large groups of athletes to see what happens as all the hits are off center, so accuracy is lowsome of them suffer Overtraining Syndrome.]]|[[File:High accuracy Low precision.svg|none|thumb|x200px|This shows good trueness* Differing HRV metrics (see below) are used in different studies, as all the hits are around the centermaking comparison difficult. On average they are on target, * The HRV is often measured while resting but there is poor precisionawake, as the hits and HRV can be sensitive to changes in mood or stress which are scatteredmore variable while conscious.]]|}We can look at trueness by measuring the average lap length * Relatively short time periods are used, and precision by measuring the standard deviationOvertraining Syndrome typically requires a longer study period. I use the traditional approach =HRV Metrics=There are a number of mathematical approaches to standard deviation (variation from mean) as well as a modified approach that uses variation from the true valueevaluating HRV. (It is more common in many fields to use "accuracy" to mean closeness to true value and "validity" to mean the combination Most of accuracy and precision. Howeverthese metrics do not adjust for heart rate, I feel that the meanings used by ISO 5725 are closer to the common usageso HRV appears disproportionately higher at lower heart rates, confounding analysis. If a company sold These include:* '''rMSSD''accurate' 12 inch pipes and shipped half . This is the square root of the mean sum of them as 6 inches the squared differences between R–R intervals. Using rMSSD typically has less measurement error and half is less influenced by breathing rate than other metrics. It is also used as 18 inches, they would meet the traditional definition basis of accuracy, but few people would be happy with the productnext two metrics. ) In addition* '''Ln rMSSD'''. This is the natural logarithm rMSSD, I calculate and this produces a value for "repeatability", smaller number which is a measure of how likely a watch is tends to give be in the same distance measurement for a specific courserange 3.0-8.0. * '''Ln rMSSD to R-R Interval Ratio'''. I calculate Using the standard deviation for each segment ratio of Ln rMSSD to the course, and then take the averageheart rate (interval between beats or R-R Interval) adjusts for changes in [[Resting Heart Rate]] (RHH). A high repeatability score can mask poor accuracy and can convince users they An athlete could have a good devicereduced HRV purely due to a slightly elevated RHH. =Accuracy=* '''SDNN'''. The table below shows summary data for each devicestandard deviation of R-R intervals. The count field problem with SDNN is how many measurements I have for that combination of condition and deviceif the heart rate is changing, with each measurement being a quarter mile distance. I generally aim for over 1(going up or down steadily),000 data points to even out then the effects of weather, satellite position and other factorsSDNN will be inappropriately high. The Trueness is the absolute of the mean* '''High Frequency Power (HF)'''. Spectral analysis can provide the power in the high frequencies, though nearly all watches tend typically 0.15 to read short0. The standard deviation 4 Hz (high frequency here is provided based on relative.)* '''Low Frequency Power (LF)'''. Like HF but for the variance from the mean and the variance from the known true valuelow frequencies, typically 0. The average pace error is shown 04 to give a sense of how much error you0.15 Hz,* '''Normalized LF power (LFn)'''re likely to see in the display of current pace. This is an average error not a worst caseLF/(LF+HF). The data shown below is a summary the accuracy based on all the sections* '''pNN50'''. If you'd like The percentage of R-R intervals that differ by more detailed information, than 50ms. I've split off the [[Detailed Statistics for GPS Running Watches]] for the results under different conditionsfind this is far too sensitive to heart rate to be of much use. {{:GPS Accuracy-statistics}}=Watches with HRV Recording=The "Accuracy (Combined)" column has an indication There are a number of statistical significance compared with watches that will record HRV, or more accurately, will record the most accurate entry. The key beat-to this indication is: † p<0-beat time for later HRV analysis.05, * p< 0'''Recent Garmin Watches'''.01, ** p< 0require you to download [https://fellrnr.001, *** p< 0com/enable_hrv_settings_file.0001, **** p< 0fit enable_hrv_settings_file.00001, ***** p< 0fit] that you copy onto the watch.000001==Progress of newer watches==I expected GPS watches You must connect the watch to improve with time, but a computer and copy the opposite appears file to be happening. With the Garmin devices especiallyfolder "GARMIN\NEWFILES", which on Windows may require you can see that to show hidden folders. Simply disconnect and the older watches generally do far better than watch will restart, processing the newer onesFIT file. I suspect You can disable HRV with this is due to compromises to get better battery life and smaller packaging and the cost of GPS accuracyfile [https://fellrnr.com/disable_hrv_settings_file. ==Smartphone Accuracy==There are various things you will need to do in order to get the level of accuracy I found with Smartphonesfit disable_hrv_settings_file. See [[Running With A Smartphone#Optimizing GPS Accuracy| Optimizing Smartphone GPS Accuracyfit]]. The watches include [[Garmin Epix]], [[Garmin 920XT]] for details. ==Interpretation and Conclusions==What do these statistics mean? This is my interpretation:* Under normal conditions the GPS accuracy is quite good for most devices.* The accuracy of a calibrated , [[Garmin 620]], [[FootpodGarmin 235]] is far better than any GPS device. Without calibration the Footpod is more accurate than any watch currently on the market with the exception of the 310XT/910XT with a Footpod backing up the GPS. * The [[Polar M400, [[Garmin Fenix 3]], [[Garmin Fenix 2920XT]], and [.* '''[[Garmin 10910XT]] are noticeably poorer than the other devices'''. I found This requires you to cycle power off and then on again, then hit the accuracy of up button, then the M400/Fenix2/down button, repeating 10 in general usage to be rather grim, and I did some testing pairing them up with the 610 or the 310XT. In all cases times until you get the Fenix2/10 would have poor accuracy compared with the 610 or 310XT on the same rundiagnostic menu. * The Fenix2 would repeated loose satellite reception, something I've not seen (the M400 ''Fenix 5X'''. [[Garmin Fenix 5X]] has done this once). The statistics do not reflect just how bad the Fenix2 is, as some of the data is too bad a menu option to analyzeenable and disable HRV. * The results of the Garmin 610 & 620 indicate the problems with the 10 are not inherent in a smaller device'''Suunto Watches'''. These simply record HRV data automatically. * The improvement in GPS accuracy of the 620 with updated firmware shows just how important the software can be. With the earlier firmware the 620 lost over a mile over a 20 mile run! * '''Polar V800'''. The accuracy [[Polar V800]] will display HRV, though the details of all devices is better in a straight line than on curves or twisty routes'''the calculation are not provided. My course is You can use the V800 to record HRV data, but not as part of a tough test for GPS devices with many curves and only a few relatively straight sectionsnormal workout which limits the value. * Not surprisingly, for many devices accuracy drops going under the bridge. However, some devices do great in this section, probably because it's fairly straight=Software to Analyze HRV=There are a number of ways you can use HRV as an athlete. * More interestingly the trueness just after the bridge is even lower, suggesting There are a number of [[HRV Apps]] for smartphones that the GPS watches are struggling cheap and easy to reacquire the satellitesuse. * The turnarounds are even less accurate than going under Firstbeat has a bridge, but Power Lines do not seem to impact accuracy noticeablysystem that measures HRV overnight and includes analysis software. This is probably the best solution, but it's also rather expensive for the recreational athlete, costing over $1,000. * The Some [[FootpodBest Running Watch| Running Watches]] improves the accuracy can record HRV for use in Firstbeat algorithms or other analysis.* A number of running watches have the 310XTFirstbeat software built in for calculating aerobic training load and recovery time. ** Note that I'm intentionally using an uncalibrated Footpod (factor = 1.000) to gather data for a comparison of Foodpod and GPSRunning watches also include algorithms for estimating aerobic fitness or training intensities based on HRV. * The older Garmin 205 does remarkably well [[File:Fenix 5X HRV Runalyze. =Footpod Accuracy=The accuracy of a Footpod is far higher than GPS, as well as more consistent and quicker to react to changes in pace. For any given run, the average pace error jpg|center|thumb|700px|HRV from the Footpod is only 7 seconds/mile (at a 9:00 min/mile pace) or 5 seconds/Km (at a 5:30 min/Km pace)Fenix 5X in RUNALYZE]]=References=<references><ref name="Hottenrott-2006">K. Hottenrott, O. In practical termsHoos, I've found that I always have to use a Footpod to pace a marathon or for critical speedwork. For details of how the Footpod calibration was doneHD. Esperer, see [[GPS Testing Methodology]Heart rate variability and physical exercise. Current status]. =Which Chipset? =While the specific chipset used in a GPS watch will impact its accuracy, there are many other factors that come into play. The physical packaging of the chipsetHerz, volume 31, issue 6, the antenna usedpages 544-52, the particular features that are implementedSep 2006, and the software that interprets the raw data will influence the overall accuracydoi [http://dx.doi. It's important to note that the SiRF chipsets such as "SIRFstarIV" are not a single chipset, but rather an overall architecture with several specific chipsets bearing the same nameorg/10.1007/s00059-006-2855-1 10. =Even GPS Watches have Bad Days=While it's tempting to take the various GPS watches on a single run and simply compare the totals1007/s00059-006-2855-1], this is a flawed approachPMID [http://www.ncbi.nlm.nih. Evaluating the devices GPS accuracy on the basis of a single sample does not tell you muchgov/pubmed/17036185 17036185]</ref><ref name="Lombardi2000">F. It's a bit like evaluating an athlete's ability on the basis of one event; everyone has good days and bad daysLombardi, Chaos Theory, Heart Rate Variability, and Arrhythmic Mortality, Circulation, volume 101, issue 1, and that applies to GPS watches as well. To illustrate this2000, the images below are from two runspages 8–10, recorded on 9ISSN [http:/20 and 9/22www.worldcat. In each run I recorded data on both the 310 and 910 watchesorg/issn/0009-7322 0009-7322], hitting the lap button on both at as close to the same time as is humanly possible. On 9doi [http:/20 the 910XT was far more accurate than the 310XT, but on 9/22 the situation is reverseddx. If you were to have evaluated the two watches on the basis of a single run, you would conclude that one is much better than the otherdoi.org/10.1161/01. But which device would win would depend on the particular dayCIR. This is why I've accumulated a lot of data to do a statistical analysis to work out which is really better101.1. {| class="wikitable" |- valign="top"|[[File:310XT Bad8 10.jpg|none|thumb|x500px| The {{Garmin 310XT}} having a bad day1161/01.CIR.101. You can see on the upper half of the course where it got a little confused and off track1. 8]]</ref> |[[File:910XT Good.jpg|none|thumb|x500px|The {{Garmin 910XT}} on the same run having no problems, and only the standard, expected level of inaccuracy.]]|- valign="top"|[[File:310XT Good.jpg|none|thumb|x500px|Two days later and it's the turn of the {{Garmin 310XT}} to have a good day.]]|[[File:910XT Bad.jpg|none|thumb|x500px|Again, this track is recorded on the same run as the image to the left. The {{Garmin 910XT}} gets a little confused at the start, and then again around lap 27.]]|}=Some Devices Are Better Than Others=Below is a section of two runs showing the same section of the course, both taken at the same time, one from the Garmin 310XT and the other from the Garmin 620 with the early firmware. (With the later firmware the tracks from the 620 look like the 310XT.) {| class="wikitable" |- valign="top"|[[File:ExampleGarmin310.jpg|none|thumb|x500px| You can see the GPS tracks (thin red line) are close together and the lap markers (yellow diamonds) are clustered nicely. The blue dots on the GPS tracks are the actual GPS recordings.]]|[[File:ExampleGarmin620.jpg|none|thumb|x500px|By contrast, the 620 has much wider GPS tracks and dispersed lap markers. ]]|}=GPS Short and long measurements=As you can see from the images below, the GPS track tends to take shortcuts around bends, reducing the length of the measured track. This cutting of the corners indicates the devices are doing some post-hoc smoothing to try to overcome the GPS errors. The more smoothing they do, the better the accuracy is likely to be in a straight line and the worse it is around corners or twisty courses. In my discussions with engineers working on GPS systems, this type of smoothing is often performed with a[http://en.wikipedia.org/wiki/Kalman_filter Kalman filter]. (When I tested using software without smoothing I found the measurements were long on my course rather than short, which is almost always the case.)[[File:GPS Shortcuts.jpg|none|thumb|500px|The GPS tracks in red showing the tendency to cut the corners on the curves.]]Often GPS measurements of races, especially marathons record a longer distance than the race. This is partly because the USATF technique for measuring the distance takes a path that is no more than 12 inches away from the tangent (corner), and few runners are able to run that close. In a large marathon you can be forced to take a line that is a long way from the tangent. The other factor is that on a straight line, the GPS error tends to give a slightly longer measurement. {| class="wikitable" |- valign="top"[[File:GPS Marathon.jpg|none|thumb|500px|Here you can see the GPS line is not following the straight road, giving a longer reading on the Thunder Road Marathon. Notice that the GPS is also cutting the corner at the top (we didn't run through the building).]]|[[File:GPS MarketSt.jpg|none|thumb|x300px|Here's another example of running down Market Street in San Francisco, where you can see the errors that would add to the distance. ]]|}=GPS Accuracy and Weather=GPS Accuracy is slightly better with clear skies than with cloud cover. The difference between completely clear and fully overcast is generally less than 0.1% and my testing includes a similar mix of cloud cover for each watch, so I ignore this difference. However, rain can degrade accuracy by 0.3-3.1%, with the better watches being impacted the least. Because it does not rain that frequently where I test, this has created some potential bias in my testing so I now ignore measurements taken during the rain. This has only made a slight difference to the results, but it ensures consistence. =GPS Accuracy and Seasons=I run in a wooded area with mostly deciduous trees, so the foliage varies by season. This foliage can have a noticeable impact on GPS accuracy, with better accuracy during the bare winter months than the rest of the year. This difference is mostly 0.1-1.5%, but in some cases can be as large as 2.5%. Because of this, my testing now ignores data from the winter months when the trees are bare. The short winter here in the south of the US means that the impact on the overall results are small, but like the weather impacts noted above, this does ensure greater consistency. =GPS Accuracy and Pace=[[File:AccuracyAndPace.jpg|none|thumb|500px| A plot of GPS precision against pace. The red line is the correlation.]]There have been reports of GPS accuracy changing with pace, but as you can see from the graph above, my testing does not show this. =GPS and GLONASS=I have found that GPS plus GLONASS produces less accuracy than GPS alone, something that is a little counterintuitive. I have no definitive explanation for this, and I do have a working hypothesis. My thought is that enabling both GPS and GLONASS will increase the number of satellites above the horizon, and a modern chipset can have over 50 channels. This means the chipset will have access to far more satellites with both systems enabled. However, I don't believe that the chipset will use all the available satellites when calculating its position. In an urban, or wooded environment, the satellites nearest the horizon will have the weakest signal, and the satellites closest to directly overhead will have the strongest signal. If the chipset were to use only the strongest 5-6 signals, then it's likely to choose the satellites that are closest to being directly overhead. That means the satellites chosen are relatively close together, which is a poor geometry that reduces accuracy. (In GPS terms this is called Dilution of Precision, or DoP.) I've talked to a GPS specialist who tells me that they have seen this in GPS systems they've tested (though not necessarily consumer grade systems.) What this means in the real world is that if you're in an environment with a partial view of the sky due to tree cover for low buildings then GPS on its own is likely to provide better accuracy. If you're in an environment with a clear view of the sky from horizon to horizon, then it's less clear to me which system is likely to provide better accuracy, and I've not tested this in practice. Given that the theoretical accuracy of GLONASS is not quite as good as GPS I'm not sure that enabling both systems will improve matters. It's possible that GLONASS will do relatively better at extreme polar latitudes due to its different orbital patterns.==Garmin 920XT and GLONASS==The [[Garmin 920XT]] is significantly worse with GLONASS enabled. {| class="wikitable"!Device!Accuracy!Trueness!Precision!Repeatability|-|Garmin 920XT|style="background-color: #FAE983;"|6.6|style="background-color: #D2DE81;"|7.5|style="background-color: #FED680;"|5.9|style="background-color: #D1DD81;"|7.5|-|Garmin 920XT (GLONASS)|style="background-color: #FEC77D;"|5.5|style="background-color: #FAE983;"|6.6|style="background-color: #FCA777;"|4.6|style="background-color: #E1E282;"|7.2|}==Suunto Spartan Ultra and GLONASS==The [[Suunto Spartan Ultra]] seems to do particularly poorly with GLONASS enabled. {| class="wikitable"!Device!Accuracy!Trueness!Precision!Repeatability|-|Suunto Spartan Ultra 1.6.14|style="background-color: #E2E282;"|7.1|style="background-color: #79C47C;"|9.5|style="background-color: #FED881;"|6.0|style="background-color: #D8DF81;"|7.4|-|Suunto Spartan Ultra 1.6.14.GLONASS|style="background-color: #FCB179;"|4.9|style="background-color: #D4DE81;"|7.5|style="background-color: #F9756E;"|3.3|style="background-color: #FDB57A;"|5.0|}==Garmin Epix and GLONASS==The [[Garmin Epix]] has slightly better accuracy with WAAS than without it, and GLONASS didn't degrade the accuracy the way it does with other devices. My belief is that enabling WAAS effectively disables GLONASS, as WAAS is GPS specific (EGNOS Ground Segment is the equivalent of WAAS for GLONASS.){| class="wikitable" |- valign="top"!Device!Accuracy!Trueness!Precision!Repeatability|-|Garmin Epix with GLONASS+WAAS|style="background-color: #FFE082;"|6.2|style="background-color: #D6DF81;"|7.4|style="background-color: #FDBE7C;"|5.3|style="background-color: #D6DF81;"|7.4|-|Garmin Epix with WAAS|style="background-color: #FFDF82;"|6.2|style="background-color: #C8DB80;"|7.7|style="background-color: #FDB77A;"|5.1|style="background-color: #F7E883;"|6.7|-|Garmin Epix|style="background-color: #FDC37D;"|5.4|style="background-color: #F3E783;"|6.8|style="background-color: #FB9C75;"|4.4|style="background-color: #F2E783;"|6.8|}=GPS Accuracy and Sampling Rate=GPS watches default to recording a sample frequently enough that accuracy is not compromised. However, several devices offer the option of recording less frequently to improve battery life at the cost of accuracy. These devices actually turn off the GPS receiver, turning it on periodically for just long enough to get a fix. The images below are from the [[2014 Badwater 135]] using the [[Suunto Ambit2| Suunto Ambit2 R]] with recording set to one minute intervals. As you can see, accuracy suffers on curves, but is fine on the straights. For a course like Badwater, the one minute recording interval was fine as the course has few turns. {| class="wikitable" |- valign="top"|[[File:GPS Sampling Curve.jpg|none|thumb|x300px|On a curve, the infrequent samples tend to 'cut the corners' and are quite inaccurate.]]|[[File:GPS Sampling Straight.jpg|none|thumb|x300px|On the straight sections, the one minute sampling does not lose any accuracy.]]|[[File:GPS Sampling Comparison.jpg|none|thumb|x300px|Here's a comparison of 1 minute sampling (red) with 1 second sampling (blue). On my GPS testing course the 1 minute sampling lost nearly 2 miles over a 16 mile run.]]|}=GPS Accuracy and Recording Rate (Smart/1-Second)=While the GPS sampling rate mentioned above has a huge impact on GPS accuracy, the same isn't true for recording rate. These two ideas seem to get confused. GPS sampling rate allows a watch to turn off the GPS receiver for short periods to conserve battery life while sacrificing GPS accuracy. Some Garmin watches can be configured to either record every second, or only record when something happens, such a change in heart rate or change in direction, something they call "smart recording." With a smart recording in normal GPS mode, the GPS system is continually active, so there's no loss in accuracy. To verify this, I tested the [[Garmin Fenix 5X]] in both the smart recording mode I normally use, and one second recording mode for comparison. As you can see, the two modes are virtually identical, and the differences are most likely due to chance (p=0.72). {| class="wikitable sortable"!Device!Accuracy!Trueness!Precision!Repeatability|-|Fenix 5X 4.30 Smart Recording|style="background-color: #FEC97E;"|5.6|style="background-color: #EDE683;"|6.9|style="background-color: #FCA377;"|4.5|style="background-color: #F3E783;"|6.8|-|Fenix 5X 4.30 One Second Recording|style="background-color: #FDBF7C;"|5.3|style="background-color: #E5E382;"|7.1|style="background-color: #FB9073;"|4.0|style="background-color: #FBE983;"|6.6|}=Device Specific Notes=For those interested in some of the details of how devices are configured for testing, here are some additional notes. * Garmin devices are set to 'smart recording'. I did try an informal test with the 620 using 1-second recording, but it appeared to make no difference. * For details of the calibration of the [[Footpod]] see [[GPS Testing Methodology]].* The Fenix 2 was tested with and without WAAS support activated; WAAS helped slightly. * The [[Garmin 920XT]] was tested with Watch Firmware 2.50, GPS Firmware 2.70 using smart recording. =Garmin 620 Issues=The Garmin 620 had some notorious problems with its GPS accuracy. The table below shows the changes with various firmware versions, culminating in the GPS-3.30 firmware that resolved the issues. I've including some testing I did without EPO data (NoEPO row below) and with a Footpod (+FP row below). {{:GPS Accuracy-g620}}{| class="wikitable" |- valign="top"|[[File:Garmin620 Offset1.jpg|none|thumb|x500px|Here you can see the last repeat is offset. Starting at lap marker 49, the track follows the same outline as the more accurate tracks, but is offset. So marker 50 should be near 4, 51 near 37, 52 near 2, 53 near 1, and the finish near the start.]]|[[File:Garmin620 Offset2.jpg|none|thumb|x500px|This is a simple out and back run of ~3 miles/5 Km, but you can see after the turn around the Garmin 620 records a gradually widening gap, even though it follows the right overall shape. (The outbound track is fairly accurate, the return is messed up.)]]|}=Garmin Fenix 2 Issues=Like the Garmin 620, I've had similar GPS accuracy issues with the Fenix 2. In fact, the Fenix 2 is the only device I've ever had that has given the "lost satellite reception" message on my usual running route. Because of these issues Garmin replaced my Fenix 2 under warranty, and below are the results for the original and new watches. The replacement watch also gave "lost satellite reception" repeatedly and the error values for the Fenix 2 do not reflect these problems as the data from those runs was useless for analysis. I suspect there are three (possibly related) problems with the Fenix 2:# The MediaTek GPS chipset is not as accurate as the SiRF chipset. The best results from the Fenix 2 are generally mediocre. # The Fenix 2 records the right shape track, but offset by some distance. This does not look like a typical accuracy problem that would manifest itself randomly. # Occasionally the Fenix 2 will report "lost satellite reception", and I have several instances of this where the date and time were wrong after reception was lost. If a GPS device has the wrong time, then it will expect the satellites to be in different positions and will be unable to acquire a position fix. I have four instances where the workout file was stored with a date in April 2019, indicating that was the date when I terminated the workout and attempted to reacquire satellite lock. In one case I noticed the date and time was set incorrectly on the watch display after the satellite lost message. There are also reports from various users about lost satellite reception and the 2019 date. This problem might also explain the offset track above, but only if the clock was out by a very small amount.{{:GPS Accuracy-Fenix2}}{| class="wikitable" |- valign="top"|[[File:Fenix2 Getting Lost.jpg|none|thumb|x400px|This is an example of just how bad the Fenix 2 can be. This is a short run, with the start and finish in the same place. The track up to marker 18 is not bad, but then the Fenix 2 loses reception for a couple of miles. When it gets reception back, it tracks wildly off course, ending up with a position that's out by around a mile.]]|[[File:Fenix2 Getting Lost3.jpg|none|thumb|x400px|Another example of the Fenix 2 getting lost. You can see marker 41 is a long way off the route, probably about half a mile off. Notice how messy the rest of the track is as well.]]|[[File:Fenix2 Getting Lost4.jpg|none|thumb|x400px|Here you can see the Fenix 2 track is a confused mess.]]|- valign="top"|[[File:Fenix2 Getting Lost5.jpg|none|thumb|x400px| The first part of this run goes okay, but at marker 61 things to go a little astray, and at marker 65 the GPS lock is lost, then briefly regained until marker 70. Not unreasonably, the Fenix 2 assumes straight-line movement until GPS lock is reacquired, but then rather bizarrely seems to assume that the straight-line movement is correct and records a track that is about half a mile/1 Km off.]]|[[File:Fenix2 Short1.jpg|none|thumb|x400px| This is more how the GPS track should look, but even on this run the Fenix 2 lost nearly a mile in a 20 mile run.]]|[[File:Fenix2 Getting Lost6.jpg|none|thumb|x400px|This GPS track looks reasonable until marker #54, and then the track gets offset, but strangely it stays offset until the last marker.]]|}=Next Steps=This is an initial analysis of the data I have, and there are a number of further evaluations to do.* Check how GPS accuracy changes over the course of a run, as I've seen a distinct tendency for the watches to say they are good to go when they don't really have an optimal lock on the satellites. I wait for 5+ minutes between the watches saying they have sufficient satellites locked in, so this should not be a problem with the data shown here, but I could do some tests where I turn on the watch from a cold state, then start running as soon as they claim they have a lock. * Look at how accurate the GPS watches are for measuring elevation, and compare with barometric data.* Write up general GPS accuracy.</references>