GPS Sensor Specifications: Sensitivity, TTFF, and More

This document outlines key specifications for GPS sensors, including sensitivity, Time to First Fix (TTFF), form factor, and the number of channels. We’ll also cover power consumption, update rate, output type, supply voltage, and current consumption.

Sensitivity

• Definition: Sensitivity refers to the minimum signal power level at which a GPS sensor can operate effectively and provide accurate data.
• Units: Measured in dBm (decibels relative to one milliwatt).
• Explanation: A lower (more negative) dBm value indicates a more sensitive receiver, capable of acquiring and tracking signals even when they are weak.

TTFF (Time to First Fix)

• Definition: TTFF is the time it takes for a GPS receiver to acquire satellite signals, process navigation data, and calculate its initial position (“fix”).

• Process: GPS satellites continuously transmit messages (every 30 seconds) containing two essential data components:

  • Ephemeris: Precise orbital data for each satellite, used to calculate its position.
  • Almanac: General information about the status and approximate location of all satellites in the constellation.

• TTFF Scenarios: TTFF varies based on the receiver’s starting state:

  • Cold Start:
    • Time: Approximately 12.5 minutes.
    • Explanation: This occurs when the GPS receiver has been off for a long time, or its stored data is invalid. The receiver must download the complete almanac, which requires receiving about 25 messages (taking around 12.5 minutes).
  • Warm Start:
    • Time: Ranges from 30 seconds to a couple of minutes.
    • Explanation: The receiver has a valid almanac but may need to update the ephemeris data.
  • Hot Start:
    • Time: Between 0.5 and 20 seconds.
    • Explanation: The receiver has both valid almanac and ephemeris data, allowing for a very quick fix.

Number of Channels

• Definition: The number of channels represents the maximum number of satellites that a GPS receiver module (housing the sensor) can track simultaneously.
• Evolution: Early GPS receivers typically had only 4 channels.
• Modern Sensors: Modern GPS sensors can often handle 22 or more channels. More channels allow the receiver to maintain a fix even with obstructions or weak signals from some satellites.

Form Factor

• Definition: The physical size and shape of the GPS sensor chip.
• Influence: Often determined by the dimensions of the ceramic antenna integrated into the module.
• Typical Size: Dimensions can be around 15 mm x 15 mm or larger.

Power Consumption

• Units: Expressed in milliwatts (mW).
• Example: A G.top GPS module consumes approximately 82 mW during satellite acquisition and 66 mW during tracking.
• Typical Consumption: A 4V DC chipset might consume about 20mA.

Update Rate

• Definition: The frequency at which the GPS sensor provides position measurements per second.
• Typical Rate: 1 update per second is common. Some specialized applications require faster update rates.
• Units: Measured in Hz (Hertz).

Output Type

• Format: Typically a TTL-level serial output, providing NMEA (National Marine Electronics Association) data.
• Baud Rate: Common baud rates are 4800 or 9600 bps (bits per second).
• Data Encoding: Usually employs {8 data bits, no parity, 1 stop bit} for decoding the received satellite data.

Supply Voltage

• Typical Range: Usually below 5V DC.

Current Consumption

• Variations: Higher during the satellite acquisition phase compared to the tracking phase.