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USB-GPS Features

  • Minimal design
    • No battery draining display
  • Indicators
    • 2 Programmable LEDs
      • For instance, LED can be programmed to reflect the GPS lock signal
      • Can be programmed to not go on for "stealth" applications
    • Charge in progress (red)/charge complete (green) LED
  • MT3339 GPS - Capable of Data logging up to 16 hours at 32 seconds per log entry
  • ATMega32U4, 8-bit Microprocessor
    • Built-in USB interface
  • Li-Polymer battery charger
    • Charges from USB
    • JST connector for direct battery connection
  • Compatible with Li-Poly Batteries
  • Measured 35 mA current draw
    • Allows for 36+ hr battery life with 1300 ma-hr battery or 24+ hours with the 850 ma-hr battery
      • Well beyond the 32 hour log time of the GPS (in 15 second mode)
    • Battery life could be extended while logging by putting the AVR into sleep mode (20-25 mA for GPS only)
  • Very Small form factor
    • 1.79x1.27 inch
    • Design goal was to make the device and battery small enough to fit into an Tic Tac candy dispenser


J1 - USB, Mini-B

Standard USB Mini-B pinout.

J2 - JST connector for Li-Poly battery

J3 - Power Switch

The power switch has a two pin header. When the pins are connected the power is applied from the battery to the rest of the card.

J4 - Reset

Reset has a two pin header. To reset the Microprocessor temporarily short the two pins. To enter programming mode, the HWB jumper should be installed prior to activating the reset.

J5 - HWB

HWB is used for programming the Microprocessor.

Pin Description
1 HWB (pulled up tp +5V)
2 Ground

Install shunt from 1-2 to allow board to go into programming mode (also requires cycling the reset line to the Microprocessor.

SW1 - Input switch for control


Indicator Function
DS2 On = USB power is good
DS1 Microprocessor PB2 output pin
DS3 Microprocessor PB1 output pin

Part Selection

GPS selection - MTK3339

File:PA6C 009-1.jpg

  • Low Cost ($30) MTK3339 GPS part from Adafruit
  • -165 dBm sensitivity, 10 Hz updates, 66 channels
  • Ultra low power usage: 20mA current draw while tracking, 25mA acquisition
  • Power Supply, VCC: 3.0V to 4.3V, (VBACKUP: 2.0V to 4.3V not used)
  • RTC battery-compatible (LiPoly battery connects to this)
  • Built-in "LOCUS" datalogging (up to 16 hours)
  • PPS output on fix
  • Tested at 27 Km altitude
  • Internal patch antenna + connection for optional external active antenna
  • Fix status output
  • Ultra small size: only 16mm x 16mm x 5mm and 4 grams
  • Frequency L1, 1575.42MHz
  • Sensitivity1 Acquisition: -148dBm, cold start, Reacquisition: -163dBm, Hot start
  • Tracking: -165dBm
  • Channel 66 channels
  • TTFF Hot start: 1 second typical, Warm start: 33 seconds typical, Cold start: 35 seconds typical
    • (No. of SVs>4, C/N>40dB, PDop<1.5)
    • Without aid:3.0m (50% CEP)
    • DGPS (SBAS(WAAS,EGNOS,MSAS)):2.5m (50% CEP)
  • Position Accuracy
  • Velocity Accuracy Without aid : 0.1m/s
    • Without aid:0.1 m/s2
    • Acceleration Accuracy Without aid:0.1 m/
  • Timing Accuracy 10 ns RMS
  • (1PPS Output)
  • Altitude Maximum 18,000m (60,000 feet)
  • Velocity Maximum 515m/s (1000 knots)
  • Acceleration Maximum 4G
  • Update Rate 1Hz (default), maximum 10Hz
  • Baud Rate 9600 bps (default)
  • AGPS Support
  • Working Temperature -40 °C to +85 °C
  • Dimension 16 x 16 x 6.2mm, SMD
  • Weight 6g

Battery Selection - 3.7V Li-Poly


  • Rechargeable - Many choices of mA-hr capacities
  • Slow self discharge rate - LiPo best fit
  • GPS stores up to 16 hours of breadcrumbs.
  • GPS uses 20 mA when in sync
  • Arduino can go into sleep mode using 0 mA
  • 16 hrs x 22 mA / hr = 32;
  • 0 mAh
  • ID AdaFruit part - : 258 has features:
    • Cost $10
    • Weight: 25g
    • Size: 1.35" x 2.55" x 0.23" (35mm x 65mm x 5.5mm)
      • Size means part won't fit into a small Altoids' tin.
    • Output: 1300mAh at 3.7V nominal
      • 1300 mAhr / 20 mA = 65 hrs (2+ days)
  • Sparkfun part has features:
    • Cost $9
    • 860 mAhr
    • 1.16" x 1.9" x 0.23"

Charger Selection - MCP73833

  • Charges Lithium Polymer batteries from USB power (+5V at 500 mA)
  • Charges in < 3 hrs

Enclosures - Candy


Revision X2 Improvements over the X1 board

  • NTC resistor pads 0603 to match available parts (X1 had 0805 pads)
  • Added reset header holes to make firmware development easier
  • Moved resistor near JST power connector to make it easier to move JST in
  • Move around parts near LiPoly charger to make more room
  • Eliminated the connections from the LiPoly charger to the Microprocessor so to eliminate backpowering the charger from the Microprocessor
  • Changed the LED associated with the charger from indicating that there is +5V to the PG (power good) output of the charger
  • Battery connected directly to the GPS VBATT power connection to keep the GPS in low-power mode when shut off

Revision X1 Board

File:Usb-gps-x1 scan scaled.png


Connectors (Rev X1)

J1 - USB, Mini-B

Standard USB Mini-B pinout.

J2 - JST connector for Li-Poly battery

Standard JST connector mates to the JST connector at the end of the battery wires.

J3 - Power Switch

The power switch has a two pin header. When the pins are connected the power is applied from the battery to the rest of the card.


Revision X1 - Issues