■ It’s a simple schematic;
the real work is in the software.
be small and simple (yeah, right).
To be realistic, the GPS simulator
would need to start at some altitude
other than zero feet, ascend at a near
constant rate until the knee altitude is
reached, and then continue ascending at a slightly slower rate until the
peak altitude — apogee — is reached.
The initial descend would have to be
very fast, but gradually slow down as
the landing was approached. The
descent would have to conclude at
some altitude above zero feet MSL.
Throughout this simulated mission,
the latitude, longitude, and time
fields would have to change in a
reasonable way. Because of memory
limitations, the GPS Simulator doesn’t
reproduce the small, fast fluctuations
in ascent speed. Nor does it produce
all the sentences of a GPS receiver —
it’s limited to just the GGA sentence.
The components for the GPS are few
in number and all are common items:
•Nine volt battery snap
•Eight pin socket
•10K resistor (1/4W)
•22K resistor (1/4W)
•Female DB- 9 connector
(use one with solder cups)
•DB- 9 housing
•1K resistor (two of these)
• 22 µF capacitor
•LP2950 (TO- 92 low dropout
•Wire cut into three equal
lengths (#24 AWG stranded
•GPS Simulator printed circuit
Insert components with the lowest
lying one first before moving on to
the taller ones. I recommend adding
components in the following order:
1. Resistors (10K, 22K, 1K,
2. Nine volt battery snap
3. Wires (three of equal length)
4. IC socket (watch orientation)
5. Voltage regulator (polarized,
6. 22 µF capacitor (polarized,
7. Voltage regulator (polarized,
8. LED (polarized, watch
9. Two-pin header
The PCB design includes strain
relief for the DB- 9 connector and the
nine-volt battery snap. So, pass their
wires through the strain relief holes
(from the underside of the PCB)
before soldering them to their pads.
The DB- 9 is the programming
port and GPS output port for the
simulator. After soldering the three
serial port wires to the PCB, add the
DB- 9 connector to the end of the
wires this way.
— Strip 1/4 inch of insulation
from the ends of the three wires.
— Solder the wires to the solder
cups in the back of the DB- 9
— Note that the cups are
numbered so you can match the
wires to the proper cups by noting
the markings on the top silk diagram.
— Squirt a little hot glue over the
solder cups and inside on half of a
DB- 9 housing.
— Set the DB- 9 connector into
the housing before the glue cools.
— Add additional hot glue over
— Fill most of the other half of
the DB- 9 housing with glue.
— Cover the DB- 9 connector
with the other half of the housing.
— Bolt the two halves together.
— Squirt hot glue into the
opening in the back to seal off the
DB- 9 housing.
When it cools, you’ll have a solid
DB- 9 serial connector with wires that
won’t break from normal usage.
Before snapping the PICAXE-08M
into the socket, check the circuit for
errors with this procedure:
— Check the soldering for shorts.
July 2009 83