Using a GPS in soaring flight
November 19 2008 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org
1) Introduction
2) Using the map
screen for cross-country flying, thermalling, wave flying, and more
3) Notes on
“heading”, “track”, “bearing”, and “course”
4) Notes on the
“current glide ratio” and “glide ratio to destination”
5) Mounting a GPS for hang gliding, polarization issues,
protecting the display face
6) A simple leg-strap mount for use in sailplanes
7) Naming waypoints
8) Giving a location in an emergency
9) The GPS as an emergency cloud-flying aid
10) Additional links
1) Introduction
This article is
primarily aimed at conveying some useful techniques I’ve developed while using
a handheld-style GPS unit during flight in hang gliders, sailplanes, and
airplanes. The focus will mainly be on
soaring flight, but some items will also be relevant to powered flight. At times I’ll use some particular Garmin
GPS’s as examples, solely because these happen to be the GPS units that I’m
most familiar with.
For brevity we’ll
use the term “GPS” in this article to refer to the entire GPS receiver unit
with the associated display screen, etc.
2) Using the map
screen for cross-country flying, thermalling, wave flying, and more
In thermalling flight, it is valuable for a pilot to know the precise wind direction. For thermalling flight, it is useful to zoom the GPS’s map
screen is zoomed in far enough that individual thermalling circles are visible-- I find that the 300-foot scale works well for hang gliding; a larger scale will serve better for flight in sailplanes. When the individual thermal circles are visible, after several consecutive circles the wind direction becomes apparent from the drift in the circles, and the wind speed can be estimated from the degree to which the circles overlap. I find that by glancing at my
GPS’s map screen while thermalling, I can estimate the wind direction much
faster and much more accurately than can my GPS-interfaced variometer, which
only estimates the wind direction to the nearest 30 degrees, and sometimes
suffers from errors of greater than 30 degrees, especially when I choose to fly
without an airspeed probe. Especially in light winds, it’s quite interesting
how strongly and suddenly the wind direction can change with altitude during
the course of a thermal climb. This
will often be difficulty to detect purely from visual references, especially at
high altitude.
When a thermal is lost, it may often be found by searching in the upwind direction. This makes sense: the thermal
column is tilted, and the glider has a downward velocity--the glider's inherent
sink rate--in relation to the rising air in the thermal.If the glider flies uniform 360-degree turns
with no wind correction, the glider often will eventually sink down through the
slanted, downwind wall of the thermal. If the thermal is not found upwind, then the pilot may find it
productive to keep an eye on the GPS map screen while setting up a brief search
pattern (which may be as simple as a single wider circle, or may be a more elaborate rectangular box pattern), keeping in mind that
the thermal will be drifting with the wind. Also, a new thermal can often be found by flying upwind along a line
that exactly follows the drifting track of the earlier thermal, especially in
situations where thermals are originating from well-defined sources or trigger
areas.
I prefer to use
the moving map screen in the "north up" mode rather than the
"track up" mode. In the
“track up” mode it is difficult to quickly judge which direction is north,
which also makes it difficult to quickly estimate the direction of thermal
drift. In “track up” mode there are
also delays associated with the need to frequently redraw the map while
circling. However, in the “north up”
mode one sometimes does find oneself accidentally making course corrections in
the wrong direction when flying southwards.
This tendency will decrease with practice.
A GPS is an extremely valuable aid to wave soaring. In wave, the lift is
stationary and the ground is distant. If there are no nearby lenticulars to aid the pilot in positioning himself in the wave, then it is extremely helpful for a pilot to
mark waypoints wherever the lift is good, so that these places may be returned
to easily even though there are no good nearby visual references. The fewer the number of button-presses are required to mark a way point, the better suited a particular GPS receiver will be to wave soaring and other similar applications. For example, on my Garmin GPSmap76S, I only have to push one single button twice to mark a waypoint; marking a waypoint with my Etrex Vista is significantly more complex. In the future, this real-time lift-mapping
function will undoubtedly be automated on some of the more exotic
variometer-GPS combinations, and perhaps this is already available. Of course, during a prolonged flight in a
small area of ridge lift or wave, the “breadcrumb trail” or track line tends to
build up quite heavily in the areas where the lift is good, which provides sort
of a primitive automatic lift-mapping function, even if the user doesn’t go to
the trouble of actually marking any waypoints in these areas.
For wave flying, it would be ideal for a pilot to have one button available for marking lift and a second button available for marking sink. I accomplish this in a more clumsy way modifying sink waypoints names, so that the sink waypoints have names like "A32" or "033BBB" while the lift waypoints have simple names like "034" and "035". This takes about 5 or 6 extra keystrokes but can be worthwhile-- it is interesting and useful be able to map out the alternating bands of lift and sink representing the primary, secondary, and tertiary wave systems, etc. In addition to dedicated "mark lift" and "mark sink" buttons, a GPS system purpose-designed for wave flying might have a "hide recent" button-- this feature would hide all newly-created waypoints so that the map could be "wiped clean" after a significant change in wind flow and wave lift distribution.
When a pilot is initially searching for wave, it is useful to know the exact wind direction. The most accurate way to judge the wind
direction is to let the aircraft fly at a precisely constant airspeed in a very shallow bank,
starting on a heading that will allow the nose to slowly pass through the
estimated upwind direction. As the
aircraft’s heading slowly changes, note the precise direction of travel that
yields the lowest ground speed. At this
point the aircraft is travelling directly upwind.
At this point the pilot can estimate the wind velocity by subtracting the groundspeed from the estimated airspeed, bearing in mind that a pitot-type airspeed indicator has a substantial error at altitude. For a more precise check, the pilot can observe his groundspeed while flying in the reciprocal, downwind, direction. By splitting the difference between the two readings, the wind speed can be accurately determined independent of airspeed indicator errors. If the pilot doesn't want to give up too much distance downwind, he should turn directly to the reciprocal of the upwind direction, rather than search to find the heading that yields the greatest groundspeed. Of course, in a soaring craft it will be rarely worthwhile to fly downwind just to calculate the windspeed. The situation where I most frequently go through this particular
procedure is when scouting for wave lift in a light airplane, where the engine can be used to help re-position the aircraft in the lift after making the downwind run. This upwind-versus-downwind comparison
technique can also be carried out during a series of steady 360-degree circles
(e.g. when a glider is circling in a thermal), but with less accuracy, because
the GPS might not happen to update the groundspeed display at the precise
moments that the aircraft is pointing directly upwind or directly
downwind.
A tip for quickly calculating reciprocal azimuths in flight: add “2” to the “hundreds”
digit of the original figure and then subtract “2” from the “tens” digit of the
original figure. If that will yield a result
greater than 360, then go back to the original figure and add “2” to the “tens”
digit, and then subtract “2” from the “hundreds” digit. Example: if the original figure is 191,
adding “2” to the tens digit yields 211, and then subtracting “2” from the
hundreds digit yields 011. Pilots who
have trouble with this at altitude are probably hypoxic, and should descend immediately!
Using the GPS to find the precise wind direction is especially useful when searching for wave downwind of a single mountain peak on a cloudless day. Since the wave will be confined to a narrow
band extending directly downwind from the peak, and since the wave will not be
present everywhere within this narrow band, it’s very important to keep the search focused in the right area. To accomplish this, the pilot should set his GPS to display the bearing to the peak which is generating the wave. After determining the precise wind direction by noting the precise heading (course over ground) that yields the lowest groundspeed, the pilot can then fly at an angle to the wind until the bearing to the peak matches the wind direction. The pilot is now directly downwind of the peak, and the wave should be somewhere directly upwind or downwind of this point.
For a visual aid to the above, here's a trick for drawing a line on the GPS screen that extends downwind from the peak: create a distant waypoint in the downwind direction and create a
“route” that goes from this distant waypoint to the mountain peak. Alternatively, set the
GPS map screen to show a “course” line , and
activate a “goto” function at some moment when the aircraft lies directly
downwind of, and preferably is rather distant from, the mountain peak. Either of these procedures will draw a
fixed reference line on the map screen of the GPS.
For all of the above techniques, it will be very helpful if several numerical data fields can be displayed on the map screen of GPS. For soaring flight, I set up my Garmin GPSMap76S to display 6 numerical data fields on the map screen, which I use to display "speed", "heading" (i.e. course over ground), "bearing", "distance", "current glide ratio", and "glide ratio to target". The Garmin GPSmap 76S is a bit unusual in
this respect—its large display screen and small numerical data fields allow the
user to display a large number of numerical data fields on the map screen,
while still leaving plenty of screen area for the map display itself. In fact, as many as 9 numerical data fields
can be displayed on the map screen of the GPSmap 76S, while still leaving a
reasonable amount of screen area for the actual map display. Most other GPS’s, including the newer GPSmap
76C/Cx/CS/CSx, are less capable in this regard. For more on this, see the related articles on this website
entitled “More on the Garmin GPSmap 76S” and “Map screen size
comparison of some handheld Garmin GPS units with numerical data fields
enabled.”
I’ve noticed that
many hang glider and paraglider pilots prefer to set their GPS’s to the
compass-like display screen for cross-country flight. Perhaps they feel that this presentation is simpler to interpret
than the map display screen. I find the
map screen to be much more useful than any other screen. When optimally configured, the map screen
contains all the information that is present on the compass-like display
screen, and more.
I always set up
the map screen of my GPS to include a “bearing” line which represents the
direction to the target waypoint. The
orientation of the little triangle-shaped “current position” icon represents
the current ‘heading”, or when the magnetic compass function is disabled (as it
always should be during flight), the current direction of travel over the
ground. By aligning the tip of the
“current position” icon with the “bearing” line, I can ensure that I am
travelling directly toward the target waypoint, with the nose pointing at the
correct “crab” angle, so that I maximize my velocity-made-good in the intended
direction. This technique of aligning
the map screen’s “heading” line (or the tip of the triangle-shaped “current
position” icon) with the “bearing” line to the target waypoint is especially
useful during flight at high altitudes, where there is little sense of relative
motion over the ground and a pilot might otherwise be tempted to simply point
the aircraft’s nose directly at the target waypoint. This technique is also
especially useful during flight over a cloud deck, or at night, in a powered
airplane.
3) Notes on
“heading”, “track”, “bearing”, and “course”
The terms
“heading”, “track”, “bearing”, and “course” are used in ways that vary from one
GPS manufacturer to the next, and in some cases, from one model of GPS to the
next. Be sure to understand exactly
what these displays show on your own GPS unit.
It is important
that GPS users understand that in the absence of a magnetic compass sensor, a
GPS unit has no way of knowing the exact direction that the nose of the
aircraft, or the nose of the GPS unit, is pointing. The satellite-derived GPS data only reveals the direction of
travel over the ground. This can be
quite different from the actual heading of the aircraft, especially
in ridge-soaring situations.
If a particular
GPS unit does include a magnetic compass sensor, it should be switched off
before flight, because it is subject to the same banking-related errors that a
conventional wet compass is subject to.
Leaving the magnetic compass sensor switched on during flight will cause
the GPS’s “heading” displays to behave very erratically during turns. I’ve confirmed this to be true first-hand
with in-flight experiments involving the Garmin Etrex Vista, Garmin GPSmap 76S,
and Magellan Meridian Platinum GPS units.
See the related article on this website entitled "Compass errors in flight" for more
on this. Sometimes the magnetic compass
sensor’s on/off status is controlled by a set of interlocking speed and
distance parameters. Do whatever is
necessary to ensure that your GPS’s magnetic compass sensor stays off in
flight, even if your groundspeed happens to drop near zero momentarily. A GPS-driven heading display will naturally
behave erratically whenever the groundspeed approaches zero, but in the context
of a vehicle that turns by banking, the situation becomes even worse if the
heading display is switching back forth between being driven from the GPS
satellites and being driven from the magnetic compass sensor, or if the heading
display is being driven entirely from the magnetic compass sensor. For notes on how to ensure that the magnetic
compass display of the Garmin GPSmap 76S/CS/CSx or the Etrex Vista/Vista C/Cx
remains switched off in flight, see the related articles on this website
entitled “More on the Garmin GPSmap 76S” and “More on the Garmin Etrex
Vista”.
Nearly all GPS's
do have a compass-like heading display screen, and the value displayed on the
screen is usually called the “heading”, but when the magnetic compass sensor is
absent or inactive, this “heading” value actually reflects the direction of
travel over the ground. Properly speaking
this really ought not be called the “heading”, but in this article we’ll follow
the convention of many GPS manufacturers and use the word “heading” to describe
the direction of travel over the ground, at least in all cases where the
magnetic compass sensor is inactive or absent.
Since we’ll also assume that the magnetic compass sensor is absent or
inactive, for our purposes the term “heading” will mean the direction of travel
over the ground.
Occasionally a
GPS manufacturer will use the word “track” is used instead of the word
“heading” to describe the direction of travel over the ground. Another suitable term for the direction of
travel over the ground might perhaps be “course”, but many GPS manufacturers
use the word “course” to mean something entirely different, which we’ll explore
in more detail below.
Most GPS
manufacturers use the word “bearing” to mean the direction to the target
waypoint at any given moment, and we’re following that convention in this
article.
In virtually all
soaring applications, the concept of a defined “course” line is
meaningless. In my own flying, even in
powered airplanes, when I want to fly toward a target waypoint, I generally
want to take the most direct path from my present location, rather than
returning to some pre-defined “course” line.
Therefore I almost never allow my GPS to display any information
relating to the “course”, either as numerical data field or as a graphic
display on the map screen or on the compass-like screen. But for clarity, we’ll take a moment to
define the concept of a “course” line, as used by the manufacturer in the
context of the GPS units that I’m most familiar with (Garmin GPSmap 76 series
and Etrex series.) At the moment that
a “goto” function is activated on one of these GPS units, a “course” line is created,
which is a fixed line in space, extending from the aircraft’s location at that
moment, to the target waypoint. As the
aircraft continues to fly, the “bearing” to the target waypoint may change, but
the “course” line remains fixed in place over the ground. If one of the numerical data fields has been
configured to display the azimuth of the “course” line, this number will remain
constant, until the navigation to the waypoint is discontinued or re-started
with the same waypoint or with a different waypoint, at which point a new,
fixed, “course” line will be created.
On most GPS units (including the GPSmap 76C/Cx/CS/CSx) , the user must
choose to display either a “bearing” line or a “course” line on the map screen;
the GPSmap 76S is somewhat unusual in that can be configured to display both a
“bearing” line and a “course” line on the map screen if the user wishes. On many GPS units the “distance off course”
is one of the values that can be displayed in a numerical field or as part of a
HSI-style display. Here are some
examples of situations where a pilot of a powered aircraft may be interested in
returning to a defined “course” line rather than simply aligning the current
direction of travel over the ground with the bearing to the target waypoint: when
flying at low altitude through mountainous areas, or when trying to stay in
visual contact with a particular set of pre-selected checkpoints, or when
flying along defined airways, or when flying instrument approaches.
4) “Current glide
ratio” and “glide ratio to destination”
Some GPS's with
barometric altimeters (pressure sensors) include "current glide
ratio" and "glide ratio to destination" displays. These functions are quite useful. By comparing these two numbers, one can get
a sense of whether the glider will reach the target with altitude to spare, or
will not have enough altitude to reach the target.
The “current
glide ratio” display on the pressure-sensor-equipped GPS’s that I’m familiar
with—namely the Garmin GPSmap 76S/CS/CSx, GPSmap 60CS/CSx, and Etrex
Vista/Vista C/CX --is extremely responsive.
This is a mixed blessing. In
very smooth air, the effects of a change in airspeed can be seen almost
instantly, after waiting just a few seconds for the aircraft's sink rate to
stabilize. In turbulent air the display
is so “twitchy” that it is not really very useful for fine-tuning the pilot’s
choice of speed-to-fly. It would be
nice if the user could select for the “current glide ratio display” to be
averaged over a slightly longer time interval on these GPS’s, so that it would
function more like the digital “current glide ratio” display on some
GPS-compatible variometers like the Brauniger IQ Comp GPS. This would make the “current glide ratio”
display slightly more useable for fine-tuning the pilot’s choice of speed-to-fly. However, even in turbulent air, and even
given the “twitchiness” in the “current glide ratio” displays of the GPS units
mentioned above, a rough comparison of the “current glide ratio” with the
“glide ratio to destination” will give a good idea of whether or not the glider
is currently on a glide path that will reach the target destination with
altitude to spare or fall short of the target destination, assuming that the
current atmospheric conditions continue all the way to the target.
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.”
A long-term trend in the “glide ratio to destination” function
gives a pilot some useful information.
For example, in a sailplane, if a pilot sees the “glide ratio to
destination” slowly scroll from “30” down to “25” over the course of several
minutes, this indicates that the glider will overfly the target with altitude
to spare, assuming that the current atmospheric conditions continue all the way
to the target. Conversely, if the pilot
sees the “glide ratio to destination” slowly scroll from “30” up to “35” over
the course of several minutes, this indicates that the glider will run out of
altitude before reaching the target, assuming that the current atmospheric
conditions continue all the way to the target.
For hang gliding and paragliding applications where glide ratios are
often below 10:1 and can even drop to 5:1 or less when a strong headwind is
present, a “tenths” digit in the “glide ratio to destination” display is very
useful for helping a pilot to detect slow trends in the “glide ratio to
destination” display. For example, if
over a period of several minutes, the “glide ratio to destination” figure
slowly scrolls from "4.8" to "4.7" to "4.6", this
lets the pilot know that he will overfly the target with altitude to spare,
assuming that the current atmospheric conditions continue all the way to the
target. On the other hand, if over a
period of several minutes, the “glide ratio to destination” figure slowly scrolls
from "4.7" to "4.8" to "4.9", this lets the pilot
know that he will not be able to reach the target, if the current atmospheric
conditions continue all the way to the target.
For hang gliding and paragliding applications, a “glide ratio to
destination” display is significantly more useful if it has a “tenths” digit,
than if it does not.
The “glide ratio
to destination” display on the pressure-sensor-equipped GPS’s that I’m familiar
with—namely the Garmin GPS units listed above--does not include a “tenths”
digit. Perhaps in a future software
update for these GPS units, Garmin will create a “tenths” digit for the “glide
ratio to destination” function, at least in cases where the glide ratio to
target has dropped below 10:1. In fact
this would be my number one suggestion to Garmin for improving the
functionality of their GPS’s for hang gliding and paragliding
applications. One GPS unit whose “glide
ratio to destination” display does include a “tenths” digit is the MLR SP24 XC
VL.
Pilots may
occasionally encounter a rather peculiar problem with the “current glide ratio”
function on handheld Garmin GPS’s with barometric pressure sensors-- if the
satellite reception is poor, the “vertical speed” display will scroll to zero
and the “current glide ratio” display will scroll to infinity. For more, see the related article on this
website entitled “Notes on the glide ratio functions of some Garmin GPS receivers
with pressure sensors.” For most soaring applications, the satellite
reception is good enough that this problem is rarely encountered.
At first glance
it may seem redundant for a pilot to fly with both a GPS-interfaced variometer
with glide ratio displays, and a pressure-sensor-equipped GPS unit that has its
own independent glide ratio displays. But
in actual practice it often happens that there is no way to make the vario
alone, or the GPS alone, simultaneously display a useable, well-damped “current
glide ratio” display that is somewhat useable even in moderately turbulent air,
as well as a “glide ratio to destination” display that includes a “tenths”
digit. For example the Brauniger IQ
Comp GPS vario that I use for hang gliding has a digital "current glide
ratio" display that is averaged over a long enough time period that it is
somewhat useable even in rough air, and has a digital "glide ratio to
destination" display that includes a "tenths" digit. It would be great to be able to see both of
these displays at the same time. However,
with this vario, these two displays are never both visible at the same time—if
the attached GPS is in “goto” mode toward a waypoint that is coded in a manner
that the vario can recognize, then the digital “glide ratio to target” display
is visible, and in all other situations, the digital “current glide ratio”
display is visible. (See the related article
on this website entitled "An expanded manual for the Brauniger IQ Comp GPS variometer"
for much more.) Therefore I find it
useful to also display one or both of “current glide ratio” and “glide ratio to
destination” values on my GPS, even though neither of these displays function
quite as well as the equivalent displays on my variometer. If my vario could simultaneously display
both of these digital glide ratio values, I would most likely be flying with a
simpler GPS with no pressure sensor, especially since I never use the
accompanying magnetic compass sensor.
Some handheld
Garmin GPS’s without barometric pressure sensors—including the
96/96C/196/296/396 aviation series, as well as some general-purpose units like
the Etrex Legend C--now also feature “current glide ratio” and “glide ratio to
destination” displays. I’m not familiar
with how well these features work for use in paragliders, hang gliders,
sailplanes, and light airplanes. These
features would be ideally suited for use in a high-speed aircraft with a
pressurized cockpit. In a low-speed
aircraft, at any reasonable glide angle the descent rate will be quite low and
will need to be measured quite accurately to produce a useable “current glide
ratio” display, so I’m a bit skeptical that GPS-derived altitude data would be
suitable for this application. On the
other hand, the “glide ratio to destination” display will not be particularly
sensitive to small altitude errors, at least so long as the destination is
still some distance away.
5) Mounting a GPS
for hang gliding, polarization issues, protecting the display face
I’ve made a mount
that attaches to the right down tube of my hang glider and holds both my vario
and GPS. I prefer to mount my
instruments on the right down tube of my hang glider rather than on the base
bar, and I prefer to place the long axis of the instruments parallel to the
horizon rather than perpendicular to the horizon. I find the map display and compass-like heading display on the
GPS to be more intuitive to use in this orientation than if the instruments
were mounted on the down tube in a more upright position.
I always wear
polarized sunglasses in flight, because they darken the sky, so that distant
clouds in the haze near the horizon and nearby patches of mist that mark the
beginnings of new cumulus clouds all become much more visible. However, LCD screens are polarized, and
there are some orientations where they are not visible when viewed through
polarized sunglasses. The LCD screen on
my vario (a Brauniger IQ Comp GPS) happens to be polarized in an orientation
that is about 45 degrees off from the polarization of the screen of my GPS
(either a Garmin Etrex Vista or a Garmin GPSmap 76S). However, the vario is tolerant of quite a wide range of viewing
angles, so it turns out to be practical to mount the vario and the GPS parallel
to each other. When the instruments are
mounted on the right down tube of my hang glider, with their long axes horizontal
as described above, I find that I don’t have any problem viewing either screen
through polarized sunglasses. The exact
orientation of the polarization of LCD screens in GPS and variometers varies
from one manufacturer to the next, and also sometimes varies among the various
product lines offered by a given manufacturer.
Also, some LCD displays are only visible through polarized sunglasses
when they are viewed at the optimal orientation that yields the minimum
interference between the glasses and the display, and other LCD displays remain
visible through a much wider range of viewing angles. Before purchasing an expensive instrument mount, or before
finalizing the design of a home-made mount, pilots who fly with polarized
sunglasses should definitely check that all the instruments are visible when
the pilot and instruments are all positioned in realistic in-flight
attitudes.
For protection
against scratches, I always cover the LCD screen of my GPS with a piece of
wide, transparent tape, which I replace every few months. This keeps the screen in like-new condition,
though the tape does slightly detract from the visibility of the screen.
To mount my
GPS’s, I simply drill a hole through the battery cover and pass a 6-32 bolt
through the hole, with the head of the bolt trapped inside the battery
cover. (I use as single bolt with my
Etrex Vista and 2 bolts with my GPSmap 76S.)
I then pass the bolt through a hole in a piece of sheet metal on my
home-made vario/GPS mount, and fasten the GPS in place with a wing nut on the
other side of the piece of sheet metal.
This arrangement relies heavily on the fastener that keeps the battery
cover closed—if this fastener were to fail, the battery cover would stay attached
to the mount and the rest of the GPS unit would fall from the aircraft. However, I’ve never had any trouble of this
nature.
6) A simple leg-strap mount for use in sailplanes
To mount my
Garmin GPSmap76S in a rented or club-owned sailplane, I simply pass two 6-32 bolts through the battery cover from the inside, as described above, and then pass these bolts through a wide velcro strap which I wrap around my left thigh and velcro down tight. Again, I use wingnuts to secure the bolts to the straps, so that it is easy to disassemble the set-up to change batteries etc. Mounted in this way, the GPS receiver isn't as easily viewed as if it were mounted somewhere on the instrument panel, but it still provides extremely valuable assistance for thermal flying or wave flying.
7) Naming
waypoints
I generally prefer not to use the coding system
that is recognizable by my Brauniger IQ-Comp GPS variometer (6 digits in total,
consisting of 3 letters followed by three numbers, coding for the altitude of
the waypoint to the nearest 10 feet, e.g. “DOG180”.) By not allowing my variometer to recognize that my GPS is locked
onto a target waypoint, I ensure that the vario will continue to show me my time-averaged
climb rate and my current glide ratio, instead of my glide ratio to
destination. The pros and cons of this
setup are explored in more detail in a related article on this website entitled "An expanded manual for the Brauniger IQ Comp variometer.”
Many pilots will undoubtedly want to use waypoints
that are configured in a format that their GPS-compatible variometer can
recognize. At first glance this can
appear to present a problem in a when a pilot downloads a set of pre-selected
“official” waypoints at a contest. If a
given day’s task only involves a few waypoints, it is no problem for the pilot
to use his GPS to quickly create a set up “dummy” waypoints, coded in whatever
format he prefers, that lie only a feet away from their “official”
counterparts. He can then build a route
from the “dummy” waypoints and take advantage of all the
glide-to-destination-related features of his variometer. If the contest will feature the style of
task that allows pilots to choose their own goals in flight, then a bit more
advance preparation will be needed to convert all the possible “official”
waypoints into appropriately coded “dummy” waypoints.
For my own
recreational flying, to facilitate quickly finding waypoints corresponding to
hang glider launch sites, hang glider landing sites, and airports, I always use
the prefixes “Y-“, “Z-“, and “AP-“ in the names of waypoints representing these
features, respectively. This ensures
that all the hang glider launch and landing sites will appear in one single
consecutive block when I’m scrolling through an alphabetical list of
waypoints. This greatly speeds the
process of finding a waypoint in flight.
When I’m coding
waypoints in this manner, I also use the last 3 digits of the waypoint name as
a numerical code for elevation, to the nearest 10 feet (e.g. “Z-ANDS001” for a
hang gliding landing site named “Andersons” located at 10’ above sea
level). Neither my GPS nor my
variometer pay any attention to this coding system, but it’s nice to be able to
quickly see the elevation of a given waypoint at a glance, without needing to
call up the detailed waypoint data page.
Naturally, when I'm actually creating a waypoint in flight-- e.g. to mark an area of good lift while wave-soaring or ridge-soaring-- I normally don't attempt to modify the simple 3-digit numerical name that the GPS unit automatically assigns to the waypoint. However, in the case of a waypoint denoting sink rather than lift, I typically change the first character of the waypoint name from a numeral to a letter. On my GPSmap76S, 4 keystrokes are required to change the first character of a waypoint name from a numeral to the letter "Z". As noted above, for soaring purposes, it would be convenient if only a single keystroke were needed to create an easily-distinguishable "lift" waypoint or "sink" waypoint.
8) Giving a location in an emergency
In a well-reasoned article entitled “Using the GPS as a Safety
Tool", Peter Gray has discussed the best way to transmit location
information by radio in an emergency, such as when a paraglider or hang glider
pilot is descending by parachute over rugged terrain. Poor radio reception, and in an international competition or
flying safari situation, even language barriers may come into play in such a
situation. The author points out that
there are significant advantages to transmitting one’s distance and bearing to
a waypoint that is known to all the pilots in the area (e.g. “Pine Mountain
Launch is 272 degrees true from me, and 6.7 miles”) rather than transmitting
one’s latitude and longitude. These
advantages include: a bearing-and-range location is easier to understand,
remember, and conceptualize, because it contains many fewer consecutive numbers. There is no ambiguity about the map datum
that is in use (this is a non-trivial factor in some parts of the world) or the
latitude/longitude format (degrees and minutes and decimal minutes, or
something else?) that is in use.
However, the waypoint should be relatively nearby (within 10 miles or so
if the pilot will descend into thick forest) or there will be significant
ambiguity due to the fact that most GPS receivers only display bearings to the
nearest degree.
Therefore, in emergency, using the “find nearest waypoint” to
select a waypoint that is known to other pilots in the area, and transmitting a
bearing and range to that waypoint, might be more appropriate than scrolling to
a screen that gives lat/lon information.
In a non-contest situation where pilots are not flying with a shared
list of waypoints, a nearby hang glider/paraglider launch area or landing zone
might be a good choice for a reference feature, as it would likely be entered
on the GPS’s of most pilots in the area.
A nearby small town might be an even better choice. Once a pilot has transmitted bearing and
range information to a known feature, it is easy for anyone else with a GPS
that contains the known feature to use the “measure distance” function to
create a new waypoint, representing the pilot’s location, at the appropriate
bearing and distance from the known waypoint.
Keep in mind that an addition or subtraction of 180 degrees will be
required to find the reciprocal of the bearing originally transmitted by the
pilot in distress.
It would be a good idea for a pilot to include the word “true”
when transmitting bearing information in this manner, to emphasize that he has
not set his GPS to read in terms of magnetic degrees rather than true degrees.
Pilots who envision describing their position in an emergency by
giving the bearing and distance to a waypoint should practice this method in
advance. With some GPS’s (e.g. the
Garmin GPSmap 76CS or Etrex Vista), simply calling up a list of nearest
waypoints and moving the cursor onto the desired waypoint is all that is needed
to see the continually-updated bearing-and-range to that waypoint. This can be done very quickly. With other GPS’s (e.g. the Garmin GPSmap
76S), to see the bearing and distance to a waypoint, the user will actually
need to initiate a “goto” function toward that waypoint, and will need to have
a numerical data field on the map screen (or elsewhere) configured to show
“bearing”, and will need to have a second numerical data field on the map
screen (or elsewhere) configured to show “distance.”
For non-emergency situations where good communications are
available and a pencil and paper are handy and everyone is using the same
datum, it may be simplest to communicate positions in terms of a latitude and
longitude. All GPS users should
familiarize themselves with how to create a waypoint to match a given latitude
and longitude. One way to do this is to
simply create a waypoint at the present location, and then change the latitude
and longitude to match the desired values.
9) The GPS as an
emergency cloud-flying aid
It is possible to
use a GPS unit as an emergency cloud-flying aid in certain conditions with
certain types of aircraft, though this practice is quite dangerous and success
is not by any means assured. (In other
words, you might not survive; this technique is for emergency use only.) The heading display screen (with the
magnetic compass sensor turned off) often gives a pilot the best chance at
quickly detecting a small change in heading, which indicates that the wings are
no longer level. However, on many
handheld non-aviation GPS units, the heading display screen is only updated about once per
second, which will make the heading display jump around very erratically once a
slow-flying aircraft like a hang glider or paraglider has developed an
appreciable bank angle, along with the associated high rate of change in
heading. In this situation, the pilot
has a better chance of determining the direction of turn by looking at the
curving track on the map screen, zoomed in to a fine scale. While hang gliding, I sometimes carry 2
GPS’s so that I can have both the map screen and the heading display screen
available in case of an inadvertent entry into clouds. (The Garmin Etrex Venture is a compact,
inexpensive choice for a second GPS to show the heading display.)
Note that in
high-wind ridge-soaring situations where an aircraft’s groundspeed may approach
zero, or where the aircraft may drift backwards, a GPS’s heading indicator will
behave extremely erratically. (Recall
again that if the magnetic compass sensor is turned off, as it should be for
all flight applications, or if there is no magnetic compass sensor, all the
“heading”-related displays will actually show the aircraft’s current path of
travel over the ground.) If a pilot
wants to get useable heading guidance from his GPS in this situation, he must
increase his airspeed enough to maintain a good positive groundspeed.
My experience is
that regardless of whether the heading display screen or the map screen is
used, it is very difficult to use a GPS to keep the wings of a hang glider
level in cloud. Eventually the glider
is likely to end up in a steep spiral or some other uncontrolled maneuver. Obviously this is very hazardous: even if
the glider does not collide with terrain, the glider may tumble or may
overstress and fail due to excessive airspeed.
(Again, in other words, you might not survive.) Paragliders have a much better chance of staying out of a spiral
dive, since they are one of the few types of aircraft that are actually
spirally stable rather than spirally unstable.
However, since a paraglider experiences a low groundspeed when flying
into even a moderate wind, a GPS will tend to be hypersensitive to small changes
in the aircraft’s actual heading when flying upwind. By the same token, the GPS will tend to be quite insensitive to
small changes in heading when a paraglider is flying downwind. All this will make it very challenging to
use the GPS to keep the glider pointed in a constant direction. The risks created by a pilot’s failure to
control his aircraft’s heading are obvious, especially if terrain is near, or
if there are large areas in the downwind direction where no safe landing would
be possible.
Obviously, hang
glider pilots who contemplate using their GPS’s for guidance while “stuffing”
the control bar to increase the sink rate to escape being “whited out”
completely, or to regain visibility after being briefly “whited out”, or to
maintain a good positive groundspeed in a high-wind situation ridge-soaring
situation, will need to mount the GPS in a position where it can be seen even
when the control bar is “stuffed”. Here is an extreme example of a mount that is optimized for easy viewing
with the control bar “stuffed” (photo #1, #2, #3, pilot's-eye view). This GPS is mounted on a long carbon-fiber rod that fits into a socket on the base bar. The whole apparatus rests in a tube that is taped to one of the lower front wires for launch and landing and any other time that it is not needed. Here (photo #1, #2) is a less extreme example of a mount allows the
GPS to be visible even when the control bar is well pulled-in.
One problem in
the use of a GPS as an emergency blind-flying aid in a hang glider or
paraglider is that it is almost impossible to find opportunities to safely, let
alone legally, practice these skills in a realistic manner. In the world of “conventional” aviation no
pilot would expect to successfully use the gyro instruments in cloud without
any prior practice, and hang glider and paraglider pilots should employ an
equally cautious attitude toward the prospects of successfully controlling a
glider with only a GPS for guidance.
Even taking into account the inherent spiral stability of paragliders
and the inherent load-shedding characteristics of flex-wing hang gliders, the
dangers inherent in loss of visual contact with the ground in either type of
aircraft are very severe.
In a light
airplane (Cessna 152), flying with the rudder pedals only and keeping my hands
off the yoke, in smooth air, with the gyro instruments and compass hidden from
my view, in cloud (with an IFR-rated check pilot in the other seat), I’ve found
that it is possible to use the heading display screen on a handheld GPS unit
like the Garmin GPSmap 76S or Etrex Vista to keep the aircraft out of a
spiral dive. The resulting flight path
didn’t resemble anything like a straight line, but rather constituted a series
of reversing gentle S-turns that slowly meandered in the intended direction of
travel. I have some doubts that this
would have worked at all in rough air, or with a more spirally unstable
aircraft. Again, it’s clear that this
sort of thing is extremely hazardous and is only appropriate as a strategy of
last resort: due to the large risk of a spiral dive followed by structural
failure, I would never dream of intentionally carrying out this type of
experiment in a “conventional” airplane or sailplane unless working gyro
instruments were instantly available as a back-up.
For more
strategies and more warnings pertaining to this topic, see the related article
on this website entitled "Using a GPS as an emergency cloud flying aid in hang
gliders, paragliders, and ‘conventional’ aircraft.”
10) Additional links
For more GPS-related information, see the following articles on
the Aeroexperiments website:
“More on the Garmin GPSmap 76S”
"More
on the Garmin Etrex Vista"
“Map screen size
comparison of some handheld Garmin GPS units with numerical data fields
enabled.”
"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"
"Using a GPS as an emergency cloud flying aid in
hang gliders, paragliders, and 'conventional' aircraft"
"Compass errors in flight"
"An expanded
manual for the Brauniger IQ Comp GPS variometer"
And for still more GPS-related information, see the following
links:
"Using the GPS as a Safety
Tool" by Peter Gray
