Using a phone as variometer
Now and then you can hear pilots talking about whether it is possible to use their phone as a standalone variometer for paragliding?
It depends! But lets break it down a little:
What is it that you expect from a flight instrument while paragliding.?
What are the key features of a vario for paragliding?
And how does a smartphone used as vario relates to these criteria?
Obviously a smartphone display is not engineered for paragliding needs! But, you won't stare at the display all the time. You will check your altitude now and then, direction of flight, direction of wind and distances. Of course, the climb rate, you need to know that but 90 percent of the time you will listen to the vario tone in order to center the thermal.
If you are flying in areas with airspace restrictions, you should have prepared your flight before. So it is not like you are opening up your screen and look up all the information while flying. It is more about an indication whether you are close by or entering an (expected) airspace. For this, we think the visual indication is sufficient not to forget that this indication is accompanied with a warning sound or some kind of alert tone.
So, not having an optimal display, the upside is: Using your smartphone as a vario, it already comes with a screen!
90 percent of your time flying you will be focusing on the audio feedback indicating an increase or decrease of the climb rate. So, whatever vario you are using it would be nice if the tone can adequately represent the (instant) changes in climb rate.
This means the frequency ot the tone, beep cycles, durations and duty times need to be able to change instantly and simultaneously to provide continuous information on your lift within a thermal. It is also worth mentioning that different flight conditions might require different sound profiles. Think of juicy spring thermals as opposed to weak flatland thermals.
Easier said: An Instant Vario should go along with an Instant Tone. For a smartphone it is easy to provide a low latency instant vario tone. The xc vario itself can be configured with different sound profiles using xctracer or leGPSBip config files and features a low latency due to optimized audio processing and synthesizing.
The Vario - Calculating Climb Rates
Ok, this one is often discussed first and with varying depths and arguments ranging from own experience up to pointing out by example why the smartphone sensors cannot be as good in providing a climb rate as some flight instrument sensor. These discussions are actually good as they show that it is a good idea to again break it down a little.
Of course a smartphone is not built with the requirements of a vario in mind. Yet, some of them come equipped with the type of sensors also found in varios and in the end it is up to the pilot to decide.
So what are the key sensors in a vario and what does it take to calculate climb rates?
Barometer: the barometer measures the pressure at certain times. Modern mid range smartphones can do this at a rate of 25ms or 40 times per seconds. Using this barometer, the vertical movement based on pressure differences at different altitudes can be calculated (how a vario works).
As for any sensor the question is how do you know how good it is. And this leads to accuracy and precision of a sensor. Let's have a look at these terms using the barometer.
Accuracy would mean that if the true pressure is 1013.25hpa, the barometer would report exactly these 1013.25hpa on average. Why on average, you might ask yourself. This is simply due to natural random errors happening when measuring continuous quantities. Unless you cannot count it exactly (like an amount of apples) you will always be a little off the accurate or true value. So you might get a
1013.2475 hpa as a first,
1013.2525 hpa as a second,
1013.2485 hpa as a third and
1013.2515 hpa as a fourth measurement.
So, is this accurate now? Generally speaking, yes! Taking the average of these values, you land at 1013.25hpa. Considering you can take 40 readings each second, you can be quite sure to have enough readings to average out at the true value.
So what are these little deviations then? These little deviations can be seen as a standard error imposed from any measurement device (in this case this is the sensor reporting back). This standard error is also called precision. The more precise your barometer, the more close to the average each single measurement will be. Precision gives a clue as to what deviations can be expected and how likely a deviation would be.
Is accuracy important then? No, accuracy is not that important. At least not for calculating the climb rate (the vertical speed in the air) for a smartphone vario. If all pressure readings from the barometer inside your smartphone are off by a constant offset the differences between the readings will be the same still. In other words 100m - 90m = (100m + 10m) - (90m + 10m) This offset is also called bias and won't affect the calculation of the climb rate. For instance, a natural offset being introduced each day is the pressure of the airmass itself (low/high pressure).
Is precision important then? Yes, precision is important. Together with the rate at what the pressure measurements from the barometer are reported it will define the possible resolution of your smartphone vario.
Wait, wait resolution? Is that the magic 0.1 meter per second climb rate some pilots argue can only be reported with professional vario. Yes. And No, it can be done with smartphone barometer too. It is just that depending on the rate of readings it will take longer or shorter to detect such a pressure difference correctly.
Now, if accuracy is not that important. How do you know at which altitude you are flying? Sad but true, a vario will never know this accurate by barometric pressure readings alone unless it is calibrated on that very date and time on a location with know altitude. The altitude with respect to mean sea level is pulled from GPS - NMEA sentence information such as the $GPGGA sentence.
Finally: Professional devices do have professional barometer inside. These are very precise and can provide an extremely high rate of barometric measurements which in turn can be used to boost precision even further. Will these ever happen to be within a smartphone? Probably not
So. how much precision is needed? We think, this question is most interesting.
Inertial Measurement Unit: The IMU contains further sensors such as accelerometer and gyroscope in one chip and enables a smartphone to perform real time motion detection. These can be used to enhance the classical vario and calculation of climb rates based on barometric readings alone to provide an instant vario. Check theses details on how an instant vario works.
Current Location, Bearing And Speed Over Ground
Current Location, bearing, speed over ground and mean sea level altitude. All of this information relevant for paragliding is stripped from Global Positioning System (GPS) or nowadays more correctly speaking of Global Navigation Satellite Systems (GNSS) information. Even the wind direction will be calculated with the help of GPS.
Fears are that GPS/GNSS in a smartphone is not precise or accurate enough. Good thing, a smartphone can tell you how big the expected variation is at your current position. Another good thing, GPS itself is not alone nowadays. There is more than NAVSTAR GPS (USA). We also have GLONASS (Russia) , Galileo (European Union) and BeiDou/COMPASS (China) satellites to get the required information from.
How many different satellites your smartphone is using to determine the relevant information, simply depends on the GNSS chip itself. You can check this with GPSTest app if you are interested. For instance on a Pixel 4a we have around 30 satellites being reported from the 4 different GNSS.
Check this nice article on Mean Sea Level, GPS and the Geoid if you want to dig deeper.
Your heading will be different to your bearing. While the bearing is the direction your are flying to calculated from the GPS locations, your heading is the direction your are looking to. The heading can be calculated from the magnetometer. The geomagnetic field declination has to be taken into account for this.
Standalone variometers are designed specifically for paragliding. So the battery should last more than a full paragliding day. This is seldom the case with smartphones. Being realistic you can expect up to 5 hours depending on how intensively the display is being used. Battery Packs do exist :-).
Charging and charging time is also a criteria. Using flight instruments you might need to carry extra cables and plugs to keep it operational. Although this is less relevant with new standalone variometers it might be worth mentioning when using legacy variometers.
So what about live tracking when paragliding? Here we have to differentiate between two purposes.
One purpose is to stay safe while flying and exploring especially new routes and terrain that is beyond the boundaries of mobile service or cell phone coverage. This is the domain of GPS satellite messengers or tracking devices.
Another purpose is to update your position to websites where other pilots can explore your history fo flights and current flights. For this to happen, you must have a mobile connection.
