::Aero-Experiments::

Notes on the glide ratio functions of some Garmin GPS receivers with pressure sensors, including the GPSmap 76S/CS/CSx, GPSmap 60CS/CSx, and Etrex Vista/Vista C/Cx

July 24 2006 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

Part One—General notes on the altitude-related functions of some Garmin GPS receivers with pressure sensors, including the GPSMap 76S/CS/CSx, GPSMap 60CS/CSx, and Etrex Vista/Vista C/Cx.

Part Two—The “missing” tenths digit-- notes on the “glide ratio to destination” function of some Garmin GPS receivers with pressure sensors, including the GPSMap 76S/CS/CSx, GPSMap 60CS/CSx, and Etrex Vista/Vista C/Cx.

Part Three—Notes on the “vertical speed” and “current glide ratio” functions of some Garmin GPS receivers with pressure sensors, including the GPSMap 76S/CS/CSx, GPSMap 60CS/CSx, and Etrex Vista/Vista C/Cx.

Part Four—Brief notes on the notes on the availability of glide ratio functions on some Garmin GPS receivers without pressure sensors

Part One—General notes on the altitude-related functions of some Garmin GPS receivers with pressure sensors, including the GPSMap 76S/CS/CSx, GPSMap 60CS/CSx, and Etrex Vista/Vista C/Cx.

The comments in this section are based on my experiences with the Etrex Vista and GPSMap 76S, but they also apply to many other Garmin GPS receivers with pressure sensors, including the Etrex Vista C/Cx, the GPSMap 76CS/CSx, and the GPSMap 60CS/CSx. To the best of my knowledge all these units use the same algorithms for their altitude-related functions.

With any barometric altimeter, fluctuations in the barometric pressure can change the elevation reading at a given point on the earth’s surface by many hundreds of feet over the course of several days, unless the user corrects the altimeter by setting it to reflect the known elevation, or to match a GPS-derived estimate of the elevation.

I usually use my Etrex Vista and GPSMap 76S in the “autocalibrate off” mode. I manually calibrate the barometric altimeter before each flight to match the known elevation of my starting point, or to match a GPS-derived approximation of the elevation. When the “autocalibrate” function is enabled, the unit uses GPS-derived altitude data to slowly adjust the calibration of the barometric altimeter. This adjustment is always very gradual—for example a 50-foot change in the calibration of the barometric altimeter can take as long as 15 minutes. The slow action of the “autocalibrate” function means that random fluctuations in the GPS-derived altitude data won’t create abrupt, random changes in the pressure altimeter reading. However, the slow action of the “autocalibrate” function also means that the user can’t rely on this function to yield accurate barometric altimeter readings immediately after the GPS unit is switched on. Therefore, even when the “autocalibrate” function is on, pilots will often find it desirable to manually recalibrate the pressure altimeter before the start of a flight.

Even when the autocalibrate function is allowed to run for an hour or more, the barometric altimeter reading may still have a large error in it, if the quality of the GPS-derived elevation data is poor. A GPS unit generally needs a good lock on at least four or five different satellites that are well distributed around the sky in order to produce good GPS-derived altitude or elevation information, and even in this case the altitude or elevation reading will sometimes be off by off a hundred feet or more. If many more satellites are in view, the satellite-derived altitude or elevation data should be quite accurate. On the other hand, if only 3 satellites are in view and they are all located near the zenith, the satellite-derived altitude or elevation data will be extremely inaccurate.

Since satellite-derived altitude or elevation data is not always very accurate, on the Garmin Etrex Vista and GPSMap 76S the “elevation” display is driven directly from the internal pressure sensor data, averaged over about 5 seconds. If the user switches off the "autocalibrate" function, then it appears that the "elevation" display on these units is not affected in any way by the satellite-derived elevation data or by the quality of satellite reception. The elevation data used for the "glide ratio to destination" function appears to be the same as the data that appears in the “elevation” display window, so again, if the unit is being used in the “autocalibrate off” mode, the “glide ratio to destination” function will not be affected in any way by the satellite-derived elevation data.

However, unlike the “elevation” and “glide ratio to destination” displays, the “vertical speed” display and the “current glide ratio” display are both adversely affected when the satellite reception is degraded to the point that satellite-derived elevation data becomes unreliable. This is true even when the GPS unit is being used in the “autocalibrate off” mode. This is very counterintuitive. We’ll explore this in more detail in Part Three of this article.

When the Etrex Vista or GPSMap 76S are used in the “GPS off” mode, the “vertical speed” display seems to be based on the “elevation” display, but time-averaged over about 15 to 20 seconds. When the Etrex Vista or GPSMap 76S are being used in the “GPS on” mode, the “vertical speed” display seems to be based mainly on the “elevation” display, with no additional time-averaging, but with some consideration given to satellite-derived elevation data, as noted above. Again, we’ll explore this in more detail in Part Three of this article. The “current glide ratio” display (which naturally is only available in the “GPS on” mode) seems to be based directly on the “vertical speed” display and the “speed” (groundspeed) display, with no additional filtering or processing. Since satellite-derived elevation data plays some role in the “vertical speed” display, the satellite-derived elevation data also plays some role in the “current glide ratio” display. Again, we’ll explore this in more detail in Part Three.

As an interesting aside, we’ll note that on the Garmin Etrex Legend, which has no pressure sensor, the "vertical speed" display (which must be based purely on satellite-derived data) is smoothed or averaged over a time period that also appears to be about 15 to 20 seconds. I found it interesting that the purely satellite-derived "vertical speed" display on my Etrex Legend, and the purely pressure-sensor derived "vertical speed" display on my Etrex Vista or GPSMap 76S in "GPS off" mode, both behaved in very similar ways, with similar readings and similar lag times, during tests in a vehicle moving at a constant speed on a hilly road in a landscape where satellite reception was not compromised by trees or terrain. Both the Vista or GPSMap 76S in "GPS off" mode, and the Legend which lacks a pressure sensor, were much slower to show that the vehicle had changed from a climb to a descent, or vice versa, than were the Etrex Vista or GPSMap 76S in the normal, "GPS on" mode.

A "glide ratio to destination" display is always intrinsically much more stable than a "current glide ratio" display, because the "glide ratio to destination" function depends only on the glider's position in space relative to the target, not on the glider's horizontal and vertical velocities. The “glide ratio to destination” display is not dependent upon an accurate measurement of the glider’s vertical speed at any given moment. An updraft or downdraft can produce a very large, immediate change in the “current glide ratio”, but will only produce a gradual change in the “glide ratio to destination.”

I’ve found that the “glide ratio to destination” function on the Etrex Vista and GPSMap 76S works quite well.

However, the “glide ratio to destination” display would be significantly more useful for hang gliding and paragliding applications if it had a "tenths" digit, as is the case with some GPS-compatible variometers such as the Brauniger IQ Comp GPS, and also with some GPS units such as the MLR SP24 XC VL (which is specifically targeted toward paragliding and hang gliding.) A "tenths" digit allows the user to better detect slow, long-term trends in the "glide ratio to destination " value, particularly with hang glider and paraglider glide ratios, which are often fairly low (e.g. less than 10 to 1).

Why would a pilot be interested in detecting slow, long-term trends in the “glide ratio to destination function? Let’s look at three examples. If over a period of several minutes, the “glide ratio to destination” figure slowly scrolls from "5.8" to "5.7" to "5.6", or from “20” to “19” to “18”, this lets the user know that he is currently on a glide path that will overfly the target with altitude to spare, assuming that the current atmospheric conditions continue all the way to the target. Likewise, if the “glide ratio to destination” display is slowly scrolling from "5.7" to "5.8" to "5.9", or from “20” to “21” to “22”, this alerts the user that he is on currently on a glide path that will fall short of the target.

Since the “glide ratio to destination” display on the Etrex Vista and GPSMap76S is confined to whole numbers with no “tenths” digit, is difficult to detect slow trends in the “glide ratio to destination” display whenever the “glide ratio to destination” is below 10:1. For example, by the time the “glide ratio to destination” figure has scrolled from “4:1” to “5:1”, the glide slope to the destination has degraded by a full 2.7 degrees, or 19%. Note that while sailplane pilots very rarely see glide ratios below 10 to 1, it’s very common for hang glider pilots and paraglider pilots operating in strong headwinds to see glide ratios of 4:1 or less. My number one suggestion to Garmin for improving the functioning off all their GPS units – and especially their pressure-sensor-equipped GPS units—for hang gliding and paragliding applications would be to modify the algorithm for the “glide ratio to destination” display so that it includes a “tenths” digit, at least in cases where the “glide ratio to destination” value is below 10 to 1.

In theory, many of the above comments also apply to the “current glide ratio” display as well as to the “glide ratio to destination” display. However, in actual practice, in all but the smoothest conditions, the “current glide ratio” number fluctuates too much for a “tenths” digit to be of very much value on this display, except when the glide ratio is extremely poor (perhaps 5 to 1 or less), in which case a “tenths” digit would be of some value on this display as well.

The “glide ratio to destination” display on the Etrex Vista and Garmin GPSmap 76S is based entirely on the current pressure-sensor-derived elevation, and the elevation of the destination waypoint, and the distance to the destination waypoint. Satellite-derived elevation data plays no role at all in the “glide ratio to destination” display, except for the very slow changes in the calibration of the pressure sensor that are applied by the “autocalibrate” function, if it is enabled. Don’t be confused by the comments in “Part 3”—apart from the very gradual effects of the “autocalibrate” function, satellite-derived elevation data only affects the “vertical speed” function and the “current glide ratio” function, not the “elevation” function or the “glide ratio to destination” function.

Part Three—notes on the “vertical speed” and “current glide ratio” functions of some Garmin GPS receivers with pressure sensors, including the GPSMap 76S/CS/CSx, GPSMap 60CS/CSx, and Etrex Vista/Vista C/Cx.

While flying with the Etrex Vista/Vista C/Cx or GPSMap 76S/CS/CSx or GPSMap 60CS/CSx, in a steady descending glide, have you ever seen the “current glide ratio” display slowly scroll up to infinity, and then abruptly “snap” back to a realistic number? If so, read on: we’ll explain the cause of this curious fault, and give some hints for avoiding it.

I keep my Etrex Vista and Mapsource76S GPS's automatic altimeter calibration function switched off at all times, so I very was surprised to discover that the peculiar behavior that I was occasionally observing in the "vertical speed" and "current glide ratio" displays was related to the quality of the GPS satellite reception. After further experiments with the Etrex Vista and the GPSMap76S, here's what I've found:

In a nutshell, on the Etrex Vista and GPSMap 76S and other similar Garmin GPS receivers with pressure sensors, the “vertical speed” and “current glide ratio” functions are subject to a rather peculiar failure mode whenever the satellite reception is alternating between good and marginal.

As we’ve noted above, a GPS unit generally needs a good lock on at least four or five different satellites that are well distributed around the sky in order to produce good GPS-derived altitude or elevation information, and even in this case the altitude or elevation reading will sometimes be off by off a hundred feet or more. And as we’ve noted above, the Etrex Vista and GPSMap 76S use their internal pressure sensor (barometric altimeter) to drive their "elevation" and “glide ratio to destination” displays, rather than using satellite-derived altitude data to drive these display, so these displays can be thought of as being completely independent of the satellite-derived elevation data, though the “autocalibrate” function, if switched on, does use GPS-derived altitude data to make a very gradual adjustment to the calibration of the barometric altimeter.

However, the Garmin Etrex Vista and GPSMap 76S normally use information from the GPS satellites as well as information from the pressure sensor to create the "vertical speed" display. Evidently, by comparing the data from the satellites with the data from the pressure sensor, spurious signals (such as would be caused by gusts of wind) can be eliminated. This allows for the creation of an accurate "vertical speed" display without averaging the data over a long time period. Since the "vertical speed" display on the Etrex Vista and GPSMap76S is not a time-averaged value, or is averaged over a very short time interval, it is very responsive. Yet, unlike the "elevation" display, it is usually not affected by gusts of wind or other temporary pressure fluctuations that are not associated with a change in the unit's actual altitude.

As noted above, the Etrex Vista and GPSMap76S appear to calculate the "current glide ratio" value by dividing the number displayed in the "speed" window by the number in the "vertical speed" window. Little or no additional smoothing or time-averaging appears to be involved. Therefore the accurate, responsive behavior of the "vertical speed" display, as described immediately above, is also reflected by the accurate, responsive behavior of the "current glide ratio" display.

However, there is a large "Achilles' heel" to this design. At any instance when there are not enough visible satellites to yield good satellite-based vertical speed data, the "vertical speed" display becomes faulty: it slowly scrolls down to "zero" and then remains there until the satellite reception improves again. When the "vertical speed" display fails in this manner, the "current glide ratio" display also fails: as the "vertical speed" display scrolls to zero due to poor satellite reception, the "current glide ratio" display scrolls toward infinity and then goes blank.

When I first saw the “current glide ratio” display of my Etrex Vista fail in this manner, I was extremely puzzled. Only after some experimentation did I recognize that the problem was actually a problem with the “vertical speed” figure, and was directly correlated to a degradation of the satellite-derived elevation data, due to a non-optimal satellite arrangement and/or a poor view of the sky.

Since the "vertical speed" display appears to be not time-averaged or to be averaged over a very short time interval, I don't know why the vertical speed display slowly scrolls to "zero" when satellite reception becomes compromised, instead of simply switching abruptly to a reading of "zero". When satellite reception improves again, the "vertical speed" display switches abruptly from a reading of "zero" to a realistic value, and the "current glide ratio" window abruptly begins displaying a realistic value.

As a very rough rule of thumb, the "vertical speed" and "current glide ratio" displays are likely to fail as described above whenever the GPS unit is not locked on to at least 4 or 5 satellites that are well distributed around the sky. (Satellites that are located low on the horizon are much more useful for purposes of computing the elevation or the "vertical speed" value than are satellites located high overhead.) As another very rough rule of thumb, the "vertical speed" and “current glide ratio” displays are likely to fail as described above whenever the "GPS accuracy" value is greater than 30 feet.

All this can easily be confirmed by noting the behavior of the "vertical speed" and "current glide ratio" displays while driving in a car on a mountain road. The areas where the "vertical speed" and "current glide ratio" displays fail will correspond completely to the areas where satellite reception becomes somewhat limited due to terrain or trees. Other functions such as "groundspeed", "heading", and "glide ratio to destination" will often continue to function quite well in these areas. This is easiest to see when 2 GPS units are available, so one can be set to show the "satellite reception" screen and the other can be set to show the "vertical speed" and "current glide ratio" displays.

The "vertical speed" and "current glide ratio" functions are the only GPS functions on the Garmin Etrex Vista and GPSMap 76S that involve satellite-based vertical speed calculations. That is why these are the only two functions that are hyper-sensitive to the quality of the satellite reception. After the "vertical speed" and "current glide ratio" displays fail as described above, because too few satellites are in view to provide good satellite-based vertical speed data, the GPS unit will typically tolerate much more degradation in the quality of satellite reception before the "groundspeed" and other GPS functions fail. When satellite reception becomes extremely poor (typically, when the unit is locked on to fewer than 3 satellites) the "groundspeed" and other GPS functions will fail. When this happens, the various display windows will freeze for about 30 seconds, after which these display windows will blank out and the "lost satellite reception" warning will appear.

At this point the "vertical speed" display will switch to a mode where it is driven purely by data from the pressure sensor, just as it is when the user selects the “GPS off” mode of operation.

If the user appreciates the very responsive nature of the "vertical speed" and "current glide ratio" displays, and is using the GPS unit in a situation where satellite reception will not be compromised by terrain or by the structure of the vehicle, then none of this is a problem.

However, the way that the "vertical speed" and "current glide ratio" displays demand very good satellite reception is often an issue when using the Etrex Vista or GPSMap76S in an airplane, since satellite reception is often somewhat limited by the aircraft structure. (The "current glide ratio" display can be quite useful in an airplane for planning long descents or for emergency situations.) Pilots who want the "vertical speed" or "current glide ratio" display to function well in flight will need to position the GPS for the best possible view of the sky, even if this makes the GPS unit harder to see. The GPSMap76CSx and GPSMap60CSx, both of which have particularly sensitive antennae design, would undoubtedly be much more tolerant of a less-than-optimal view of the sky than are the Etrex Vista/Vista C/Cx and GPSMap 76S/CS and GPSMap 60CS. Therefore the GPSMap 76CSx or GPSMap 60CSx might be particularly good choices for use in an aircraft with some overhead metal structure, in cases where the user places a high priority on the optimal functioning of the “current glide ratio” display. (Despite this, I prefer the GPSMap 76S over its newer counterparts for all flying applications—see the related article on Aeroexperiments website entitled “Additional notes on the Garmin GPSMap 76S” to learn why.) Also, the problem of inadequate satellite reception could undoubtedly be avoided entirely through the use of an external antenna, positioned for optimal reception. This is apparently available for all the GPSMap products, but not for the Etrex products.

Fortunately, in sailplanes the satellite reception will usually be quite good, but even here, in some cases the GPS unit may need to be mounted in a position that compromises ease of viewing if the "current glide ratio" display is to function optimally.

When hang gliding with an Etrex Vista GPS, I’ve definitely found that I need to take some care in positioning the GPS unit if I want the “current glide ratio” and “vertical speed” displays to work optimally. These functions should work fine if the GPS is oriented so that the display faces straight up, which gives the best reception for the patch antenna used in the Etrex series. However, to avoid damage during hard landings and ground handling, I prefer to mount my Etrex Vista and variometer on a down tube rather than on the base tube. If I position the GPS unit on its side so that the display faces toward me, this gives me the best view of the screen, but the satellite reception is inadequate for good functioning of the “current glide ratio” and “vertical speed” displays. Therefore I position the GPS unit so that the display faces toward a point about 45 degrees between the horizon and the zenith. This provides a compromise between ease of viewing and good satellite reception. I find that this yields very satisfactory "groundspeed", "heading", and "glide ratio to destination" displays, but the "current glide ratio" display occasionally fails in the manner described above. This problem is avoided with the GPSMap76S, which has a quad-helix antenna rather than the Etrex Vista’s patch antenna. The GPSMap76S gets nearly the same quality of reception regardless of whether the unit is mounted with the display facing upwards, or with the display facing sideways. For this reason the GPSMap76S is a bit better suited than the Etrex Vista for mounting on the down tube of a hang glider, at least in cases where the user places a high priority on the optimal functioning of the “current glide ratio” display.

I find myself questioning whether it is really such a good idea for Garmin to smooth the pressure-sensor derived vertical speed data by comparing it to the satellite-derived elevation or vertical speed data, with little or no time-averaging. The main problems with this arrangement are twofold. We’ve already explored the way that these displays fail when the satellite reception is inadequate to yield good satellite-derived vertical speed data. The other problem is that since little or no time-averaging is involved, the “vertical speed” and “current glide ratio” displays are difficult to use if any turbulence is present. There’s no doubt that the very fast, accurate functioning of the “vertical speed” and “current glide ratio” displays on the Etrex Vista and GPSMap 76S is quite nice in smooth air. However, in turbulent air the “current glide ratio” display would be a bit less “twitchy”, and therefore a bit more useable for fine-tuning the pilot’s choice of speed-to-fly, if it were time-averaged over 5 seconds or so, as is the “current glide ratio” display on my Brauniger “IQ Comp GPS” variometer. With a bit of time-averaging, there is no real need to use satellite-derived elevation or vertical speed data to eliminate spurious pressure signals: the “current glide ratio” display on my Brauniger IQ Comp GPS variometer works quite well, and doesn’t depend on GPS-derived elevation or vertical speed data in any way. I find this display to be “steady” enough that it is sometimes useful for fine-tuning my choice of speed-to-fly even in moderately turbulent air, which is something that I can’t do very well with the “twitchier” current glide ratio display on the Etrex Vista or GPSMap 76S. A third problem with the “vertical speed” and “current glide ratio” displays on the Etrex Vista and GPSMap 76S is that the “vertical speed” value sometimes tends to “stick” on a “zero” reading. This is undoubtedly another byproduct of the way that the pressure-sensor-derived “vertical speed” data is filtered by comparing it to satellite-derived elevation or vertical speed data to eliminate spurious signals from wind gusts. All things considered, I feel that even for hang gliding and sailplane applications where the satellite reception is usually unimpaired, the “vertical speed” and “current glide” ratio displays on the Etrex Vista and GPSMap 76S would both function slightly better in flight if they were averaged over a few more seconds, rather than the current situation where there is little or no time-averaging of these displays. Ideally, the user could even select the time period over which the “vertical speed” and “current glide ratio” displays were averaged, so that these displays could be as responsive or as smoothed as the user wished.

At this point it’s worth noting that the sensitive variometers that have been used for many decades to measure vertical speed in sailplanes and hang gliders have always been driven purely from pressure sensors and have always worked very well, even when they are set up to be averaged over extremely short time periods. Since the Etrex Vista and GPSMap 76S have pressure sensors, it seems to me to be an unnecessary aggravation that their vertical-speed-related functions should be the first functions to fail when the satellite reception becomes somewhat marginal. Pilots who always fly with an external antenna (or who use the “CSx” version of the GPSMap 76 or GPSMap 60), usually fly in smooth air, and value the extremely fast response of the “vertical speed” and “current glide ratio” functions in their present form may disagree.

At the very least, even if GPS-derived vertical speed data is incorporated into the “vertical speed” and “current glide ratio” functions, these functions should be designed to fail in a manner that is obvious to the pilot rather than in a manner that is cryptic to the pilot. Whenever the satellite-derived vertical speed data becomes faulty, it would be much better if the “vertical speed” and “current glide ratio” displays both went blank, rather than the current state of affairs where the “vertical speed” display slowly scrolls to zero before going blank and the “current glide ratio” display slowly scrolls to infinity before going blank.

Better yet, even if GPS-derived vertical speed data is normally incorporated into the “vertical speed” and “current glide ratio” functions, the unit could be designed to immediately switch to vertical speed computations that are driven solely from the pressure sensor whenever satellite reception is less than excellent, rather than the current situation where these displays fail as described above.

We'll close this section with a few notes on how the "vertical speed" display on the Etrex Vista and GPSMap76S functions when it is driven purely from the pressure sensor either because the user has switched off the GPS satellite reception entirely, or because the unit is displaying the "lost satellite reception" warning. We'll also give some notes on how the "vertical speed" display functions on other Garmin units such as the Etrex Legend, which has no pressure sensor.

On the Etrex Vista and GPSMap76S, when the user selects "GPS off", halting all GPS satellite reception, then the "vertical speed" display is driven entirely from the pressure sensor. The "vertical speed" display is also driven entirely from the pressure sensor whenever the "lost satellite reception" warning appears. Of course, in both of these cases, the "current glide ratio" function is no longer available, because no groundspeed information is available. When the "vertical speed" display is driven entirely from the pressure sensor, it is averaged over a time period of about 15 to 20 seconds. On the Garmin Etrex Legend, which has no pressure sensor, the "vertical speed" display (which must be based purely on satellite-derived data) is smoothed or averaged over a time period that also appears to be about 15 to 20 seconds. I found it interesting that the purely satellite-derived "vertical speed" display on my Etrex Legend, and the purely pressure-sensor derived "vertical speed" display on my Etrex Vista in "GPS off" mode, both behaved in very similar ways, with similar readings and similar lag times, during tests in a vehicle moving at a constant speed on a hilly road in a landscape where satellite reception was not compromised by trees or terrain. Both the Vista in "GPS off" mode, and the Legend which lacks a pressure sensor, were much slower to show that the vehicle had changed from a climb to a descent, or vice versa, than was the Etrex Vista in the normal, "GPS on" mode. This shows that there are definitely some advantages to driving the "vertical speed" and "current glide ratio" functions from a combination of data from the pressure sensor and data from the GPS satellites, with little or no time-averaging, rather than driving the unit solely from data from one source with some averaging over time to filter out spurious signals. However, once again, one should keep in mind that the pressure-sensor-derived vertical speed data could be averaged over a much shorter time period, on the order of 5 seconds, as is the practice with instruments such as the Brauniger IQ Comp GPS which combines GPS-derived groundspeed data with pressure-sensor-derived vertical speed data to create a highly functional “current glide ratio” display.

Part Four—brief notes on the notes on the availability of glide ratio functions in some Garmin GPS receivers without pressure sensors

Immediately above, we described how some Garmin GPS units with no pressure sensors do have a vertical speed display that works quite well, and appears to be averaged over about 15 to 20 seconds worth of satellite-derived elevation data.

I’ve noticed that Garmin now includes “glide ratio to destination” and “current glide ratio” functions on some of their GPS units that lack pressure sensors, such as the Etrex Legend C, and all of the portable aviation units (96, 196, etc). I haven’t flown with any of these units, and I don’t know how well these features would function for soaring applications. It seems likely that they work quite well for high-speed, high-flying aircraft. Note that a pressure-sensor-derived glide ratio display would be useless in an airplane with a pressurized cockpit. Note also that when an airplane is traveling at high altitude, even several hundred feet of error in the GPS-derived elevation data wouldn’t have a large effect on the accuracy of the “glide ratio to destination” function. Likewise, when an airplane is traveling at a high velocity, relatively large changes in vertical speed will be associated with relatively small changes in the current glide ratio, and relatively small changes in the current glide ratio will create large changes in vertical speed, so the accuracy of the “current glide ratio” function should be fairly good during high-speed flight even though the vertical speed data is purely satellite-derived. Also, it’s conceivable that the nature of errors in GPS-derived altitude data may be such that even if the altitude is several hundred feet off in absolute terms, the vertical speed computation, and thus the “current glide ratio” computation, may still be relatively accurate. It’s also possible that if a purely-satellite-driven “current glide ratio” display is averaged over 15 to 20 seconds, it might even be more useable during some soaring situations in turbulent air than is the overly “twitchy” current glide ratio display of the GPSMap 76S or Etrex Vista, as described above.

Of course, unless the satellite reception is quite good, a purely-GPS-driven “glide ratio to destination” function would be unreliable within a few thousand feet of the ground, when an altitude error on the order of one or two hundred feet would be quite significant. For purely-satellite-driven vertical speed and glide ratio functions, the quality of satellite reception will be much more important than it is for all other GPS functions (groundspeed, heading, etc). Here it is worth pointing out again that the patch antennae used on the Etrex series must point upward for the best view of the sky, which means that the display screen must face upwards, while the quad-helix antennae used in the GPSMap 60/76/96 series doesn’t experience very much change in quality of satellite reception as the GPS unit is rotated around it’s long axis. This makes the GPSMap 60/76/96 units a bit better suited than the Etrex units for mounting on the down tube of a hang glider, especially when the user places a high priority on the functions that depend on GPS-derived elevation and vertical speed data. On a similar vein, we’ll note again that the GPSMap76Cx and GPSMap60Cx have more sensitive antennae than the GPSMap76C and GPSMap60/60C, so in terms of ensuring adequate satellite reception for good functioning of purely-satellite-driven “vertical speed”, “current glide ratio”, and “glide ratio to destination” functions, these newer “Cx” units would be better choices for use in an aircraft with some overhead metal structure, unless an external antenna is employed.