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GNSS Garmin 64 Series versus Forerunner 920XT,
Fenix 3 and epix Wrist Units for Hikers

Written by:
Professor Gérard Lachapelle
Department of Geomatics Engineering
University of Calgary

Gerard LachapelleThe past two years has seen the introduction of Garmin’s 64 series GNSS receivers and several wrist models, including the 920XT, Fenix3 and Epix discussed below.

64sThe 64 series, which consist of the 64, 64s and 64st units, come with GPS-GLONASS chips instead of GPS-only chips as is the case with the 62 series. Both the s and st models have a barometer and compass. The st model has pre-loaded maps while maps on a microSD card can be inserted in the other models. Colour displays have sufficient resolution and size to follow one’s location and trajectory; the zoom in/out option helps. The 64 series is the natural evolution of the 60 series and has the same form factor and functions of the 62 models plus more. Power consumption is roughly the same at some 1 hours, depending on batteries and signal reception conditions. Helix antennas are still used to acquire and track signals; this results in larger physical dimensions but result in slightly better performance advantages in some challenging environments than the use of embedded antennas. 64 series receivers can be paired with cadence, temperature and heart rate sensors. The outputs can be displayed on Garmin Connect and BaseCamp.

forerunner 920xtThe new 920XT and Fenix3 wrist units replace the 910XT and Fenix2. They come with GPS-GLONASS chipsets as well.  They can also be paired with heart rate, cadence, bicycle speed, bicycle power and temperature sensors.  The minimum temperature recording on the 910XT was -20˚C below which the recording jumped to 70˚C (although the dial display was still correct) due to a bit limitation issue.  I have not had the chance to test if the new units have the same issue because last winter was unusually warm but I hope it has been resolved (Nothing more disappointing than not being able to prove to anyone else that one has been hiking at -25˚C for hours and came back alive….). fenix 3The functionalities of the two units are similar, although the Fenix3 has a hiking option in addition to numerous others, including running.  The 920Xt is  targeted at triathletes who swim and bike.  I have hiked with both the running and hiking option and found no difference. I prefer the running option due to the additional information available in GarminConnect as discussed below. The two watch appearances are different, with a round versus a square bezel.  I personally prefer the 920XT as I find brightness and readability better. To use the maximum capability of either model, one should buy the new Garmin chest-mounted accelerometer equipped with a heart rate sensor. This allows the estimation of cadence, body vertical oscillation and ground contact time; these are displayed in GarminConnect and fun to analyze as a function of speed, ascent, descent and tiredness. The units now allow wireless data transfer to GarminConnect via Wi-Fi and Bluetooth, hence download via a mobile phone is possible on the way back home.

epixThe Epix is a new model available only since June, hence I have not had the chance to evaluate it thoroughly. The unit is slightly larger than the two others to display map and location.  Coordinates can also be displayed making it easier for off trail hiking and to find routes more easily than the 920XT and Fenix3 units.

All three watches are equipped with accelerometers and magnetometers. This allows for tilt compensated compass heading. The accelerometer also computes the cadence, so one can have cadence without a heart rate monitor.  Additionally, use of the units outdoors with GPS on allows their calibration and subsequent use of the units indoors with GPS off to measure distance travelled.  I have not tested this function and performance with/without cadence sensors to assess performance. Accelerometers on the heart rate monitor have more success to estimate body oscillations as this motion is more regular.

What does the addition of GLONASS bring? It is generally limited because GPS-only coverage is now sufficient for most mountain hikes.  There are still a few spots with trails beside near vertical walls and in narrow gorges where it is an advantage; these spots are few and far between. Stable position outputs do not depend only on the number of satellites in view but also on antenna type, signal tracking algorithms, etc. To increase battery life, disabling GLONASS will hep and it can save hours. Both watches have an ultratrac feature. This mode turns the GPS chipset off periodically and the accelerometer is used to calculate speed and distance between fixes.  The tradeoff is typically accuracy and position resolution.  However for those multiday backpacking trips deep in the backcountry, one can always know location.

The above describe some but not all the capabilities of these units. Since manuals are available online, potential buyers will find additional information in these to complete their selection.

One last point often the object of complaints is the inaccuracy of distances travelled. GPS does not calculate distances but provides positions with associated time tags along the user trajectory. How positions are used to calculate distances is an app designed by manufacturers or individuals (for mobile phone). Distance accuracy is not only a function of the algorithm used but of a variety of other factors, such as open sky versus forestry canopy, antenna quality, nearby mountains obscuring the antenna, etc.  On flat terrain and open sky, accuracy will generally be better than 1 to 2% of the distance travelled. Under harsher conditions, it may degrade to 5% of distance travelled.  The algorithm developer is only partly in control of the variables involved.  There are methods to obtain very high level of accuracy at a fraction of a percent of distance travelled by using more performing equipment with much higher weight, cost and deployment complexity, and therefore inappropriate for wearable use. Wearable devices with better distance calculation will come slowly and gradually.

The author has been involved with GPS signal processing development and research for 35 years. He is an avid hiker who uses GPS equipment extensively.  He has no financial or other interest in Garmin or GPS Central.


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