Satellite positions in the sky

The GPS altimeter needs to know where in the sky it’s satellites are in order to get the most accurate data for your plane ride and skydive. To get this information it will search for signals as the satellites orbit the Earth and use the position information to get a better “lock” on each satellites signal. A better signal means better reception in the airplane and the highest levels of accuracy on your skydive.

Each satellite’s position initially takes 30 sec – 2 minutes to determine. It is common for more than 15 satellites to be in view at any one time.

Start of the day

Power on your GPS altimeter (Insight / Stratos) when you arrive at the dropzone. When the altimeter first turns on it will attempt to locate satellites and download their positions for the first 20 minutes. After 20 minutes even if the altimeter didn’t find the satellite signals it will turn off the GPS to save power. The GPS will automatically turn on again at the start of each airplane ride, around 500′ above the ground, and continue to search for satellite signals for your entire plane ride and skydive until you are back on the ground.

Best practice for pre-loading information before your first jump is to turn on your altimeter outdoors and at least 50 feet away from the hanger, 1 story buildings, and trees. If you are near multi-story buildings or other very tall objects, moving farther away is better.

However our GPS altimeters use well tuned antennas that can pick up signals in “less than perfect” conditions, but it will take longer to download the satellite positions. If you can’t get away from buildings or trees it is still better to be outside while the altimeter is searching for satellites.

Once your GPS shows a signal strength of 3+ bars, allow at least 5 minutes for the bare minimum satellite download. Around 15 minutes would be even better, as this allows the altimeter to load extra data that will help find satellites more quickly throughout the day and even into the next day.

Before each skydive

Satellite information gets more and more “out of date” over the course of a couple of hours. Because of this it will take longer for the GPS to locate your position from one skydive to the next. To ensure you have the most up to date satellite information for each skydive, about 5 minutes before you load the plane (while you are waiting in the loading area ideally), switch your altimeter to the GPS screen. This screen forces the GPS to turn on even if it was in low power mode between your skydives.

The GPS screen will show bars to indicate signal strength, much like a smartphone.

In the aircraft

Metal (such as an airplane) and human bodies are bad for GPS signals. They absorb and block the satellite signal and cause problems for your altimeter. We use the best GPS components on the market to maximize our altimeter’s ability to function in an airplane, surrounded by metal and engines and other people.

If you are having trouble with the altimeter’s GPS navigation info on the way to altitude (ground track, ground speed, distance to LZ, and vertical speed), try moving your altimeter closer to a window and avoid leaving it against your body. Imagine the least obstructed path out of the airplane and up to the sky above.

Back on the ground

Once you land from your skydive, the altimeter will shutdown the GPS capabilities automatically in order to save power. While on your skydive it will have been saving satellite positions to give you the best “hot start” of the GPS capability on your next skydive.

The saved positions of those satellites is good for up to 6 hours, but in our experience it is best to turn on the GPS by switching to the GPS screen before your next skydive. Do this in the loading area for about 5 minutes and you will be doing your altimeter, and yourself, a big favor.

The post GPS Signal Best Practices first appeared on Deep and Steep - Skydiving Altimeters.

Accuracy and Differing Altitudes Between Brands

At times we get asked the question: is my altimeter accurate? The answer: It is as accurate as a pressure sensor altimeter can be — the sensors we use have a precision of 1 meter or better. But the true accuracy of an altimeter depends on much more than how precise its sensor is, which is why your Deep and Steep altimeter will not always agree with another altimeter on the airplane. Let’s cover some of the reasons for altitude display differences.

Software Differences

Every digital altimeter is a small computer taking continuous pressure readings to calculate an altitude above ground level (AGL). The most direct way to do that is to save the air pressure value at takeoff and compare it against air pressure readings during the skydive.

Manufacturers also apply their own corrections on top of that — adjusting the display altitude based on your vertivcal speed, canopy vs. freefall, burbles, and so on. Altimeters from different brands can show different altitudes on the same skydive purely because of the software they run.

Saved Pressure on Takeoff

Air pressure changes throughout the day. While on the ground, every altimeter is updating it’s ground-reference pressure every few minutes. At takeoff that value is locked in as the ground reference for the skydive.

Different altimeters may lock-in the ground pressure at slightly different moments — just before takeoff, or partway down the runway — so each one can end up with a ground reference that differs from the next by a few tens of feet.

Comparing Accuracy

So which altimeter is correct? To be honest – None of them.

We know because we build altimeters that also carry GPS, and GPS data is highly accurate and unaffected by burbles and the other problems that disturb a pressure sensor. Testing our own GPS data against third-party data from the high-end GPS units found in aircraft makes it easy to see that pressure sensor based altitude is a rough indication of your height above the ground. We tested other major brand of skydiving altimeter to confirm this is true of anything on the market today.

None of the altimeters on the plane are telling you a true distance from the ground. They are all approximations that can change with altitude, speed, burbles, air temperature and the weather. A pressure altimeter can be within 20 or 30 feet of the truth, and it can also be incorrect by several hundred feet. At higher altitudes (for instance above 6,000 feet) they will tend to be farther away from the true altitude.

Safely Transitioning to a New Altimeter

When changing altimeter models or manufacturers it is important to fly conservatively. Raise your setup altitudes slightly and use the same process of evaluation and sight picture you used to learn your previous canopy altitudes.

After a few skydives you will know what altitude is correct for your canopy on the new equipment.

The post Altimeter Accuracy, Different Brands, and Why Readings May Differ first appeared on Deep and Steep - Skydiving Altimeters.

Altitude Difference Between Pressure and GPS

The pressure altitude displayed on your altimeter will difference from the GPS track for your skydive. This is mostly due to how pressure changes with the temperature and weather systems in your area. The difference between pressure and GPS altitude will generally increase as your altitude above the ground increases. The table below gives a very rough idea of how the altitudes can differ.

Why the Two Never Match – Technical Details

If you want to get into the technical details of how these altitudes work, read on…

Pressure Altitude – Standard Air and Real Air

Pressure altitude is based on the air pressure, and the air pressure changes as you ascend in the airplane or descend on your skdyve. The altimeter converts that air pressure into a height using a fixed, standard atmosphere — the aviation model that assumes 15°C at sea level and a set rate of pressure drop as you climb. However in the real world air is almost never standard and therefor doesn’t fit the model perfectly.

Warm air is less dense, so on a hot day you are physically higher than the pressure suggests. Cold air is more dense, so on a cold day you are lower. The aviation rule of thumb is roughly 4% of your height for every 10°C (18°F) the air is away from standard — about 500 feet at a 13,000 foot exit. Pilots measuring both on warm days have logged GPS altitudes 5 to 8 percent above the pressure altitude.

This difference grows with height. Near the ground it is usually very small, which is why the two agree well under canopy and disagree most at altitude.

GPS Altitude

GPS Altitude is calculated as a height above a fixed reference called Mean Sea Level (MSL). The altitude accuracy of the GPS receiver is based mainly on how many satellites it can see. With a clear view of the sky and many satellites in view, the GPS can determine altitude within a few meters. GPS altitude is not affected by changes in temperature, weather, burbles, etc… So it is a more consistent altitude throughout the entire skydive.

You do not have to guess how good a given sample was. Every row of the GPS track carries its own vAcc figure — the receiver’s own estimate, in meters, of how good that sample’s altitude is — along with the number of satellites it used. See Understanding GPS Track Data for the full field list, and GPS Signal Best Practices for how to give the GPS the best chance of a clean fix.

Track View in the App

Knowing those two causes, here is what to expect when you review the GPS tracks:

• The app calculates and displays altitude AGL for the GPS track. This means your landing should end at roughly 0 feet AGL.
• Expect a difference between the GPS track view altitude and a video of your altimeter — up to a few hundred feet at exit. It closes as you come down, and it is smallest under canopy.

Which Altitude to Fly

The post Pressure Altitude and GPS Altitude – How Do They Differ? first appeared on Deep and Steep - Skydiving Altimeters.

Track File Fields

Each row in the file is one sample, and every sample carries the same fields:

Sending a Track to the App

The Deep and Steep app reads GPS tracks straight off the altimeter, so most jumps never need the CSV opened by hand.

For the full connection procedure, and what the altimeter does while it is connected, read App Connect.

Copying a Track File to a Computer

To work with the raw numbers, pull the CSV off the altimeter with “USB Connect” mode, which makes the altimeter appear as a USB drive. For the procedure, read the USB Connect section of Logbook.

Sample Track File

Download a sample GPS track file to see the format before you open one of your own.

Your own tracks are stored in the altimeter’s onboard memory. To learn more about you altimeter’s GPS track recording settings, see GPS Tracks and Recording Options.

The post Understanding GPS Track Data first appeared on Deep and Steep - Skydiving Altimeters.