The obvious approach is to scale the battery voltage down with a voltage divider, but that uses current and so has to be switched. This "simple" approach requires three resistors and another transistor. Any voltage drop across the transistor degrades the measurement.
Another approach uses a zener diode or other calibrated voltage drop, a resistor, an output pin, and an analog input pin. Connect the zener from the battery to the input pin, and the resistor from the input pin to the output pin. Normally the output pin is high, and with a 2.1V zener, no current flows. Pull the output pin low, and the input pin will be 2.1V less than battery voltage, a maximum of 2.4V.
In a parking garage, out of coverage, tracking mode becomes expensive. The GPS has to stay on for about two minutes to be sure there is no coverage, burning over 1 milliamp-hour per check. The user may want to know immediately when the tracked vehicle leaves the garage, so he faces an unpleasant trade-off between quick response and battery life.
The ideal solution is a mercury switch or similar kinetic motion detector. Once the GPS reports blocked, the Tracker could go to sleep until it detects motion, using no power. Mouser does not sell mercury switches; they must be considered an environmental hazard. Some green alternative should be used instead.
The current power supply is a wasteful linear regulator. The production design should use a micropower boost switching regulator. This consists of a chip, an inductor, and some capacitors. In sleep mode, the regulator remains off and the MCU runs on battery power. When the MCU wakes up, it turns on the regulator, bringing the voltage up to 3.3 volts, and then enables the GPS. This design could run on two alkaline cells instead of three, or on rechargeable batteries. The design would require the ATTINY84-10 variant of the MCU, which works down to 1.8 volts. Mouser does not sell it, but DigiKey does.
For a very-long-life application, it could use D-cells or an expensive Tadiran 3.6V lithium battery, and recharge the mobile phone as needed. A GSM module could be used instead of a mobile phone.
The lithium battery and module have been used successfully in the car security prototype.
The Aarlogic GPS 3A works fine. Set the tracker baud rate to 9600. The Aarlogic stores its settings in battery-backed RAM. Since the tracker only runs it for a few seconds at a time, the battery does not stay charged and the Aarlogic forgets its settings and defaults to 9600 baud.
The Antenova M10214-A is a possible alternative GPS module. It runs on 3.6V up to 5 volts and can switch itself on and off. It should last quite a while on a 3.6V Tadiran battery without a regulator. The connector looks tricky to solder to, and the device may come up in SiRF Binary mode, requiring a 57,600 bps transmission to switch it into NMEA mode. The interrupt-driven serial transmitter cannot run that fast, so a cycle-counted loop would have to be written.