APRS Pocket Tracker

           April - May  2006

This is the first tracker built for the SABLE balloon flights and was used for SABLE-1 and SABLE-2.

A different tracker was built and used for SABLE-3.

The tracker is a Byonics Pocket Tracker kit from James (VE6SRV) and
the BEAR Lassen LP GPS Receiver in a .010" copper sheet enclosure.

Outside & inside views of the GPS antenna jack & rubber grommet that was placed over it to make the opening somewhat water resistant.
 
The pocket tracker kit's BNC antenna connector with plastic insulation was replaced with a Teflon one that could be soldered to the case to eliminate any mechanical/electrical connection problems plus make the case water resistant as the GPS antenna jack & battery conductor grommets do. These moisture prevention measures and sealing the enclosure lid with tape will hopefully prevent problems like BEAR 2 experienced if a wet landing is ever experienced again. Arrow 1 identifies inductors L1 & L2, and arrow 2 identifies the VCO oscillator coil which are all likely fine being simply soldered in place, but the slightest vibration of these components causes the VCO frequency to shift due to changing capacitance with nearby items and the resulting modulation could possibly cause data errors so these 3 components were mechanically stabilized with hot glue & wax. I used hot glue to stabilize the coil first, but should have used wax to stabilize all three components as it is much easier to work with for this application.
 
Pwr -  J9 or Power ON/OFF Jumper

CD -

 Carrier Detect Input Pin. Other pin is +5V.
 Jumper these 2 pins to suppress automatic transmissions while testing, if required.

4Pin -

 4 pin connector strip to power & connect the GPS or connect to a PC serial port.
 Brown is Ground, Red is +5V, Orange is Tracker Data Out, Yellow is Tracker Data In.

J6 -

 Ground & Power Switch Output Pins.
 DO NOT place a shorting jumper on these 2 pins.

Note:
Serial Power Enable Jumper J7 on the bottom of the PCB must be shorted to provide +5V on Pin 3 of the 4 pin GPS connector.

P/S -

 J5 or Primary/Secondary Configuration Jumper.
 Jumper these 2 pins to select Secondary Configuration.
LED -  J4 or LED Indicator Enable Jumper.
Gnd -  Two ground pins that can be used to store spare jumpers.
+5 -  Two +5V pins that can be used to store spare jumpers.

FS -

 J11 or Frequency Select Jumper.
 Jumper center pin and pin next to "9" for 144.390 MHz
 Jumper center pin and pin next to "4" for 144.340 MHz.
J13 -

 Jumper these 2 pins to keep transmitter on during alignment.

 

 

 

 

 

 

 

 

 

 

 

 

 

Power Requirement & Battery Tests

Weight
Pocket Tracker -
Lassen LP GPS -
Copper Enclosure -
BNC Connector -		 30g
22g
60g
6g
Tracker Assembly   Total Weight -
GPS Antenna -
VHF Antenna -
9V Battery -		 118g
45g
48g
48g
Total System Weight - 259g
Unit Notes:
Pocket Tracker

Reverse Polarity Protection Diode Voltage Drop = 0.8 volts.
Voltage Regulator Input-Output Differential = 0.3 volts.
RF Output Power -
Supply Voltage -
Minimum Voltage -
Idle Current (No LEDs) -
Transmit Current -		 250 mW
9 VDC -
6.1 VDC
4 to 5 ma
138 ma
Lassen LP GPS Receiver

GPS is powered from a 3.3V linear regulator powered
from the Pocket Tracker 5V linear regulator output.
Supply Voltage -
Receiver Current -
Active Antenna Current -		 3.3 VDC
54 ma
10 ma
6.1 to 9 VDC Total
 Continuous Current  =   69 ma
              Peak Current  = 202 ma		 A 9V Duracell Alkaline Battery provided 5.5 hours of operation during a battery test
with the tracker beaconing once / minute.

 

Using 6 AA cells to increase the operating time adds significant weight so some experimenting was done with switching regulators, converters and different battery sources to try to increase efficiency, operating time and reduce weight.

 

The 3.3V GPS receiver requires 64ma which is over 91% of the total tracker power requirement with the Pocket Tracker requiring a continuous average current of only **6.1 ma. so the largest improvement would be to replace the GPS 3.3V linear voltage regulator with a switching regulator.  (**5ma continuous + 1.1ma average current resulting from 138ma for ≈1/2 sec every 60 sec's while transmitting APRS data).


The is a (National Semiconductor) LM2954 5V simple switcher evaluation board after its regulator chip was replaced with a 3.3V one. The original capacitors had to also be replaced due to excessive ESR likely from old age even though this board had never been used. A post ripple filter (the inductor & capacitor in upper left corner) was also added and reduced the output ripple to below a measurable level.

Table 2 lists the required current vs. supply voltage using the switching regulator to power the GPS Receiver and percent difference from using a linear regulator.
Table 2 - 3.3V Switching Vs Linear Regulator Continuous Current Requirement
  * 4.1 volts is minimum supply voltage.
** add 133 ma for Peak Current
(while transmitting data)		 Supply
Voltage		 GPS Receiver ma. Tracker
ma.		 **Total % change
in ma with a
Switching
Regulator
With
Linear
Regulator		 With
Switching
Regulator
*4.1V 64 ma 87.0 ma 5 ma 92.0 ma +36%
6.3V 56.8 ma 61.8 ma −11%
7.0V 51.5 ma 56.5 ma −20%
8.0V 45.5 ma 50.5 ma −29%
9.0V 40.8 ma 45.8 ma −36%
12.0V 31.3 ma 36.3 ma −51%
13.8V 28.0 ma 33.0 ma −56%

This is how the tracker was powered for SABLE-1, with a switching regulator for the GPS and from a 9V alkaline battery. There was no time to perform a proper battery life test before the flight, but the tracker was used while travelling to Hanna to launch SABLE-1 and it is unclear as to when the tracker actually quit operating, but it appears the 9V alkaline battery lasted about 6 hours. With less current required at voltages greater then 6.1 volts (minimum pocket tracker operating voltage) it was hoped a 9 volt battery would last a little longer then it appears to have, but having the battery on the very hot dash board of my car that day likely didn't help.

 

Some work was also done with this converter which converts 2 to 4.5 VDC from 3 AA cells, which weigh the same as a 9V alkaline battery, to 6.3 VDC. Inexpensive no-name alkaline AA cells were used for a test and provided 7.8 hours of operation which is a 48% increase in operating time over using a 9V battery.

No further work was done with this regulator, but the next step would have been to change it to provide 5V and simply remove the Pocket Trackers linear regulator for a further improvement.

 

To BEAR Home Page