USB-GPS Revision X1

Connectors

J1 - USB, Mini-B

J2 - JST connector for Li-Poly battery

J3 -  Power Switch

SJ1 - Input switch for control

Revision X1 - Issues

• Vbackup on GPS should connect directly to VBATT so that when the power switch is off, the GPS still retains the settings
  

is an Arduino GPS project which fits into a "stealth" enclosure such as a Altoids tin or Tic Tac container . It is intended to be used as a platform for a GPS datalogger.

Features

• Minimal design = No battery draining display
• KML output for use with Google Earth 
• Arduino development environment compatible
• GPS
• GPS data logger application
• LiPoly battery
• Mini-USB connector for host connector and charging
• Programmable LED (can be programmed to not go on for "stealth" applications
  
• Charge complete LED
• Notes from Hardware Bringup
  

Part Selection

GPS selection - MTK3339

• Low Cost ($30 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 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
• DGPS (SBAS(WAAS,EGNOS,MSAS,GAGAN)):0.05m/s
• Without aid:0.1 m/s2
• Acceleration Accuracy  Without aid:0.1 m/
• DGPS (SBAS(WAAS,EGNOS,MSAS)):0.05m/s2
• 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)
• DGPS  SBAS (defult) [QZSS,WAAS, EGNOS, MSAS,GAGAN]  
• AGPS  Support
• Working Temperature  ‐40 °C to +85 °C
• Dimension  16 x 16 x 6.2mm, SMD  
• Weight  6g
• Arduino driver (Adafruit driver)
• Experienced with this part (GPS datalogger)

Battery Selection - 3.7V Li-Poly

• Rechargeable - Many choices
  
• 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
• AdaFruit part - ID: 258 has features:
• Cost $12
  
• 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

Candy packaging makes great stealth packaging for small electronics projects. These packages are inexpensive, easy to find and often watertight.

Small Altoids tins

• Interior dimensions 2.1"W x 1,4"D x 0.59"H
• Radius of curvature = 0.25"
• PCB size =  1.8" x 1.1" 

Large Altoids' tin

• Interior dimensions 3.5"W x 2.2"D x 0.8"H
• Radius of curvature = 0.375"
• PCB size =  1.8" x 1.1"
• Nominal placement

Height Stackup (Large Altoids' tin)

Tic Tac package

The main advantage of the Tic Tac container is that it has a watertight door which could easily allow access to the USB port. The door could be used with a USB connector on the opposite side of the PCB from the GPS. The Mini-B head of the USB cable is 0.36" x 0.39" which fits into the opening 0.39" x 0.65". alignment would need to be closely monitored.

The package is a bit small, but should fit since it is tapered from the top to the bottom. Interior minimum dimensions at the taper bottom - 1.5"W x 2.4"H x 0.48"D. The inside of the white top is 0.5" x 1.55".