Tips & Tricks on Using GPS
This page is intended to help you to choose and set up a GPS receiver to record skydives for later evaluation with Paralog based on what works best for me. Most of this information is specific to the unit I am using, but can be easily transferred to all the other units on the market. There is more in-depth information available out there, especially in Scott Campos' great book on wingsuit flying 'Skyflying - Wingsuits in Motion' or in several threads on DZ.com.
This page will not tell you how to use your unit as most devices come with a detailed manual which describes all the necessary steps. So does Paralog. Use it!
The Receiver
The
Garmin ForeTrex 101 is small,
can be wrist mounted and runs on AAA batteries available everywhere. It
supports WAAS/EGNOS for improved accuracy and records up to 10.000 data
points in 1 sec intervals. Data can be downloaded from the unit through
a serial cable. The ForeTrex 201 is more or less identical, except that
it has a built-in rechargeable battery.
The Garmin ForeRunner or Edge series can not be recommended as they are tailored to be used on the ground for running and bicycling which renders the data they record useless for our purpose.
In the meantime, the ForeTrex and similar units are outdated. A new generation of GPS receivers with a much higher sensitivity has hit the market. While the old receivers only held their satellite fix with a clear view of the sky (e.g. by sitting next to the pilot), modern receivers hold their fix no matter where you sit in a Caravan, Twin Otter or even a Skyvan!
My current favourite is the Wintec WBT-201, a matchbox
sized, logging GPS receiver. It has a built-in battery which
is rechargeable throught the USB port. Data can be downloaded
through USB or BlueTooth. Example tracks recorded with this unit can be
seen in my logbook.
The unit is the same size as an audible, so you can simply put the unit
into the secondary(!) audible pocket of your helmet or you can
mount
it on the back of your helmet using a Neptune quick release bracket.
Set Up
Enable WAAS/EGNOS, enable recording of tracks and set the recording interval to 1 sec. If your unit has a barometric altimeter, disable it, as they do not cope well with our descent speeds.
For the Wintec WBT-201, use the follwoing settings (using the latest version of TimeMachineX):
- Under GPS Setting/SBAS, check "Enable", "Apply Integrity Information", "Apply SBAS...", and "Ranging...". Also, "1 Channel", and "WAAS" for the US, "EGNOS" for Europe (Click "Set" when done).
- Under GPS Setting/Navigation Setting, select "5 - Airborne under 1g", and "3 - 3D only". Do not check "Allow Almanac Navigation", but selct "Enable RAIM" (Click "Set" when done).
- Under Log Settings, select "Time" button and set Cond 1 for 0 to 2000KMH. and Cond 2 to 1 sec (Click "Set" when done).
See this thread on how to set up your unit to log more than one point per second.
Preparing for the Jump
Well before boarding the plane turn on the unit and have it acquire a good fix. Clear the track memory and turn the unit off again. For the next 30 mins. the unit will do a hot fix, which will usually last only 15 secs. Update: With the modern sensitive units with their large track buffer, simply leave the unit turned on during the whole flight.
On Jump Run
Turn the unit back on on the 2 min call. Place the unit close to a window with a clear view of the sky (i.e. not a window under the wing). The unit will usually acquire a solid fix within 30 secs and keep it while you walk to the door. Update: With the modern sensitive units with their large track buffer, simply leave the unit turned on during the whole flight.
During Freefall/Canopy Flight
Keep the receiver oriented towards the sky. Mounting the ForeTrex on the mud flap or the main lift web will result in poor data.
After Landing
Turn the unit off before you walk back to the hangar. Do not save the track to memory! On some units, this will reduce the resolution and strip the elevation data. Some units even strip the timestamps!
Downloading
Connect the unit to your computer: The ForeTrex 101 requires a special cable which needs to be purchased seperately. If your computer does not have a serial port, you need an USB-Serial adapter as well, for example this one). The WBT-201 can be connected with the supplied USB cable or Bluetooth. See this blog on how to use a WBT-201 with OSX/Linux.
Turn the unit on and download the tracks. Paralog will split the saved tracks, crop superfluous data before and after the track, extract the usual jump details like exit and open altitude, delay, etc. as well as horizontal and vertical distances and speeds, glide ratio, course, etc. and save the converted tracks into its logbook.
Analyzing the Jump
Every downloaded track can be analyzed individually using Paralog's jump profile chart and jump profile details view.