Now, let the glue cool.
The GPS simulator code runs a
series of five nested loops to create
the GPS time. The nested loops
update the hour, minute, second,
position, altitude, and wind speed.
The code updates and sends
position reports once per second.
The parameters used to define the
flight profile are located at the
beginning of the code, so you won’t
have to go digging around for them.
The parameters are as follows.
StartHour: The starting hour of the
simulation in UTC (any value from 00 to 23).
Day: The day of the month in the simulation (any value
from 1 to 31).
Month: The month in the simulation, in two digits (any
value from 01 to 12).
Year: The last two digits of the year in the simulation (any
value from 00 to 99).
LaunchAltitude: The elevation of the launch site in meters
(any realistic value).
AscentRate: The ascent rate of the balloon in feet per
minute (any value from 300 to 1,200).
FloatAltitude: The altitude float is to occur (any value
lower than burst altitude).
BurstAltitude: The altitude balloon burst is to occur (any
value higher than LaunchAltitude).
DescentRate: The descent rate of the parachute in feet
per minute at sea level (any realistic value around 1,000).
RecoveryAltitude: The elevation of the recovery site in
meters (any value less than BurstAltitude).
reaches 40,000 feet. At this point,
the ascent rate drops to 80% like a
real balloon (the actual altitude this
occurs at is variable, however,
40,000 feet is a good value). The
balloon continues to rise until it
reaches float or burst altitude. At float,
the Status LED flashes alternately
red and green. At burst, the Status
LED turns red indicating descent.
The descent continues until
the simulator altitude descends
below the recovery elevation. The
A PC plugged into the simulator’s programming and
running the PICAXE terminal program will display the
current altitude of the simulation. There’s still room in
memory for upgrades to the simulator. However, if you make
significant changes, you may need to change the speed of
the PICAXE or shorten the code to keep position reports
appearing once per second. I plan to experiment with
software upgrades to permit the testing of cutdown devices
and of GPS-based amateur rocket altimeters. I’ll soon
make a kit available on my website (
for anyone who doesn’t want to go through the trouble of
making the PCB or locating parts.
Onwards and Upwards,
Your near space guide NV
■ FIGURE 5. Before there is a GPS
position lock, the GPS sentences
look like this.
The important thing to notice is that if the burst
altitude is lower than the float altitude, the balloon will
never become neutrally buoyant in the simulation.
USING THE GPS SIMULATOR
Disconnect the battery and plug the GPS cable into
the GPS port of the tracker under test. Then, power-up
both the GPS tracker and the GPS simulator. The GPS
simulator immediately begins producing output like a GPS
without a position lock.
When you push the Lock button, the GPS begins
reporting a stationary position and the Lock LED turns
green. There is some built-in jitter into the reports, so the
reported position jumps around slightly like a real GPS.
Once you push the Launch button, the balloon begins
rising at the ascent rate set in the software. The Status LED
is green indicating the balloon is ascending. Again, there’s
a small jitter in the position reports just like a real GPS.
The balloon continues to rise at a constant rate until it
The neutrally buoyant condition occurs when the lift
of the balloon is too close to the weight of its payload.
Initially, the balloon slowly rises, perhaps on the order of
300 feet per minute. However, at some point during the
ascent, the balloon’s lift is too weak compared to the
balloon skin’s tension against expansion. At that point, the
balloon stops rising and it just drifts downwind.
Eventually, the batteries in the GPS tracker discharge and
die. In enough time, solar ultraviolet will degrade the
latex in the balloon and pop the balloon. However, by
then, the balloon could be hundreds of miles away. The
chances are now poor that you will ever recover the
payload. To guard against this, always fill the balloon with
enough gas to lift two or three pounds more than the
payload weight. Otherwise, adding a cutdown device to
the balloon load line is in order. After activation of the
cutdown, the payload separates from the balloon and
begins its descent. Without the weight of the payload, the
balloon shoots rapidly upward in altitude and bursts. The
GPS simulator version 2.0 described here permits you to
test the operation of a cutdown device while it remains on
January 2012 17