The GPS Simulator enables the testing of flight computers on the ground. The behavior
of the flight computer and its program can be tested before a near space launch. High
fidelity testing reduces the chances of failures during a near space mission. A GPS
Simulator combined with thermal and vacuum testing is ideal for increasing the success
of near space missions and saving money.
Onwards and Upwards,
Your near space guide
Overview of the GPS Simulator
The GPS Simulator is a PICAXE-18M2 programmed to create the GPS sentences of a
GPS receiver traveling into near space. GPS sentences are produced once per second. By
connecting the GPS Simulator to a flight computer of APRS tracker, the device can be
tested on the ground. Failures identified on the ground are far less expensive than failures
occurring in near space. The GPS Simulator terminates in a DB-9 connector, like a real
GPS. Therefore, it can be plugged into any tracking system or flight computer without
modifying the device. It also includes LED indicators and push buttons to create events
like loss of GPS lock, wait to launch, balloon ascent, balloon float, balloon burst, and
landing. The ascent rate and maximum altitude of the balloon is programmed into the
GPS Simulator prior to connecting it to a flight computer or APRS tracker.
Figure 1. The NearSys GPS Simulator for near space testing.
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Kit Components
The following parts are included with the GPS Simulator kit.
R1 10k resistor
R2 22 k resistor
R3 (no longer used)
R4 680 ohm resistor
R5 4.7 k resistor
R6 4.7 k resistor
R7 680 ohm resistor
R8 680 ohm resistor
C1 22 uF tantalum capacitor
U1 18 pin IC socket
U2 LP 2950 voltage regulator
D1 LED
D2 bicolor LED
D3 bicolor LED
SW1 push button tactile switch
SW2 push button tactile switch
Female DB-9 PC-mount connector
The following components are not marked on the PCB
Male DB-9 solder cup connector
DB-9 plastic jacket
Four “AAA” cell holder
Theory of Operation
Figure 2. Schematic for the NearSys GPS Simulator.
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The PICAXE-18 is programmed to calculate positions in near space following the
characteristics of a high altitude balloon flight. The exact ascent rate, float altitude, and
burst altitude are set in variables at the beginning of the program and are therefore easy to
change. The descent speed and altitude is modeled after the descent characteristics of a
parachute and how it is affected by air density. In addition, the fluctuations in GPS are
also modeled in the program’s output. The GPS sentences generated by the PICAXE are
transmitted over a serial link at a baud rate of 4800 and includes the following GPS
sentences to simulate a UniTraQ GT-320,
GPGGA
GPGLL
GPGSA
GPGSV
GPRMC
GPVTG
GPZDA
.
The PICAXE needs to know when GPS lock is achieved and when launch occurs.
Therefore, therefore the GPS Simulator incorporates two push buttons and LED status
indicators. The first button is used to signal when the GPS is to have a lock. If a
simulated flight is in progress, the lose of GPS does not stop the balloon from rising. The
GPS Simulator just produces the sentences of a GPS without a lock. Lock and be lost and
regained at any time and also at multiple times during a flight. A positive GPS lock is
indicated with a green LED and lose of lock is indicated with a red LED.
The second push button indicates when a launch occurs. Prior to launch, but after a GPS
position lock, the altitude of the GPS fluctuates slightly to more closely simulate the
behavior of a GPS receiver.
The remaining components on the GPS Simulator provide power regulation and I/O for
the PICAXE-18.
Assembly Directions
1. Resistors
Bend the resistor leads before inserting them into the PCB. Then install and solder the
following resistors
□ R1 10k resistor (brown, black, orange, gold)
□ R2 22 k resistor (red, red, orange, gold)
□ R4 680 ohm resistor (blue, gray, brown, gold)
□ R5 4.7 k resistor (yellow, violet, red, gold)
□ R6 4.7 k resistor (yellow, violet, red, gold)
□ R7 680 ohm resistor (blue, gray, brown, gold)
□ R8 680 ohm resistor (blue, gray, brown, gold)
Note: R3 is no longer used. It was required for the PICAXE-18X as a pull up resistor.
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2. IC Socket
Do not insert the PICAXE-18M2 into the IC socket prior to soldering it to the PCB. In
fact, resist installing the PICAXE until after checking your soldered connections.
□ U1 IC Socket
3. Voltage Regulator
The proper orientation of the voltage regulator is indicated on the PCB top silk
□ U2 LP2950
4. Capacitor
The capacitor is an electrolytic capacitor, and therefore polarized. Look for the + marking
for the capacitor’s positive lead if your kit includes a yellowish tantalum capacitor. If the
capacitor is an aluminum electrolytic capacitor, then look for the band that marks the
negative lead.
□ C1 22 uF electrolytic capacitor
5. LEDs
LEDs are polarized, although this is less important for the kit’s two bicolor LEDs. Look
for the flat spot along one side of the LED’s base which indicates the LED’s cathode
lead. The other lead is the anode and its proper position is indicated by the solder pad
next to the top silk A. Bicolor LEDs are a milky white in color.
□ D1 Green LED
□ D2 Bicolor LED
□ D3 Bicolor LED
6. Tactile Push Button Switches
The tactile switches only fit the PCB one of two ways. Either way works in this PCB.
□ SW1 push button tactile switch
□ SW2 push button tactile switch
7. Female DB-9
The female DB-9 soldered to the PCB is the PICAXE programming port.
□ Insert a #2-56 bolt into one of the DB-9 mounting holes
Note: The head of the bolt is on the bottom side of the PCB.
□ Slide a nylon spacer over the bolt
□ Repeat for the other DB-9 mounting holes
□ Slide the DB-9 over the bolts and its pins into the PCB
□ Firmly attach the DB-9 using two nylocks
□ Solder the leads on the DB-9
8. Battery Holder
To ensure sufficient capacity, the GPS Simulator uses a 4 “AAA” cell pack rather than a
9V transistor battery. The leads of the cell holder are red and black. Red is the positive
terminal and black is the negative terminal. To prevent the leads form breaking off from
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normal use, the leads are strained relieved in the PCB. Each lead enter the strain relief
hole form the bottom of the PCB and then bends over to proper solder pad.
Figure 3. A strain relieved wire ready for soldering
□ Insert the bare end of the red wire through the large pads (strain relief pads) marked +V
□ Bend the lead and insert them into the neighboring smaller pads
□ Solder the lead
□ Repeat for the black wire
9. GPS Port
□ Cut two wires to equal lengths
□ Bare ¼ inch of insulation from both ends of each wire
□ Insert each wire into a pad of the GPS port using the strain relief holes
Note: One pad of the GPS Port is marked 2 and the other is marked G
Locate the female DB-9 connector and look on the back and you’ll see fine numbering
next to each solder cup. The numbers label the pins of the DB-9 from 1 to 9.
Figure 4. The back of a DB-9 connector and its solder cups.
□ Tin the following two solder cups of the DB-9 (2 and 5)
□ Tin the other ends of the two wires
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□ Place tinned #2 wire against the DB-9 #2 solder cup□
□ Heat the solder cup while pressing the wires into the molten solder
□ Remove the soldering iron and hold the wire in place until the solder cools
□ Repeat for the G wire and DB-9 solder cup #5
Figure 5. The back of a DB-9 with a wire soldered to cup #3.
□ Open the DB-9 shell bag and set the shell, shell bolts, and shell nuts aside
□ Fill the bottom half of one shell with hot glue and press the DB-9 inside
□ Fill around the solder cups with hot glue
□ Fill the other shell with hot glue and close the shells around the DB-9 and cable
□ Bolt the shells together
□ Back fill the end of the shell where the cable exits
Checking Your Work
After soldering the GPS Simulator, flip the PCB over and look for poor solder joints or
shorts. The last check is to verify the voltage on the GPS Simulator. A DDM will
measure between 4.75 and 5.25 volts between pins #5 and #14. After this test, you can
insert the PICAXE-18M2 into itsscoket.
Programming the GPS Simulator
The program for the GPS Simulator is available on the NearSys.com website, under the
Catalog link. Click the GPS Simulator link for the latest assembly directions and
programs. At some point, there should be programs to simulate other models of GPS
receivers.
You will need the PICAXE programming editor to program the PICAXE-18M2. The
software is available for free at www.picaxe.com.
Using the GPS Simulator
Plug the GPS port connector into the flight computer’s GPS port (or the APRS tracker’s
GPS Port). As soon as four “AAA” cells are inserted into battery holder, the GPS
Simulator will start up. The Lock LED indicator will be red indicating the GPS does not
have a lock. The Launch LED indicator will be off, indicating the balloon has not lifted
off yet. At this time, the GPS Simulator produces the same GPS sentences as a GPS
recently turned on and before it has achieved a GPS lock.
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After pressing the Lock button, the GPS Simulator begins producing GPS sentences
similar to a GPS stationary on the ground. The starting altitude is set in the
LaunchAltitude variable. Once the Launch button is pressed, the GPS Simulator
produces GPS at increasing altitudes and the Launch LED turns green to indicate balloon
ascent. The latitude and longitude of the GPS changes as if the balloon was drifting in a
straight line. The ascent rate for the balloon flight is set in the AscentRate variable. If the
FloatAltitude variable is set to a lower value than the BurstAltitude variable, the GPS
Simulator will stop producing GPS Sentences at increasing altitudes. Instead, the altitude
will fluctuate slightly. If the BurstAltitude variable is set lower than the FloatAltitude
variable, the GPS Simulator will begin producing GPS sentences at lower altitudes. At
this time, the Launch LED will be red to indicate descent. The altitude will drop rapidly
at first and gradually slow down as the parachute approaches the ground. The landing
speed according to the GPS Simulator is set in the DescentRate variable. The descent
rate at high altitude is increased in response to the lower air density at altitude. The GPS
Simulator stops producing lower altitudes once the altitude drops below the setting of the
RecoveryAltitude variable.
Note that at any time, you can press the GPS Lock button to force the GPS Simulator to
lose a GPS lock. The calculated is still set in the PICAXE, but not displayed in the GPS
sentences. When the GPS Lock button is pressed again, the GPS Simulator begins
producing proper GPS sentences with the up to date altitude.
The program written for the GPS Simulator is well documented. Look it over and set the
variables at the beginning of the program to create the high altitude flight profile you
desire.
Updating the GPS Simulator for other GPS Receivers
The code written of the GPS Simulator replicates the output of the UniTraQ GT-320
(high altitude model). The program can be modified to replicate other GPS Receivers, if
you are familiar with their output.
10 December 2012
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