Cheap and used Sirf II e LP GPS Module from Ebay

I have idea to build precise clock, and GPS module is here because GPS satellites has atomic clock on board. I have chosen particularly this module, because it was the cheapest, that I could find on internet, only 15 USD with free shipping. While new GPS module costs 60 USD, thought You could find completed GPS  navigator for 70 USD.

Module has simple 5 wires connection. It’s powered from 5V, but can also work form 3.3V power supply. Data from/to device flows via well known RS-232 interface’s RX and TX data lines. Module can be directly connected to PC using RS-232 (com port), but is also needs 5V power source, USB works well. RS232 have the same data structure as UART, but voltage levels are different, so it  can’t be connected directly to microcontroller, logic level converter MAX232 must be used.

Module use standard NMEA-0183 standard interface for data transmission. One reason it’s good, because module outputs data continuously  without any request, so really it can be used only one RS232 data line.

Cold start (18 minutes until GPS fix)

To test bought GPS module I had connected it to my desktop PC RS232 port. It’s very easy to test. because seller send my with documentation and test software – GPS Diagnostics V1.05. It decrypts data in NMEA format and shows it in user friendly way.

18 minutes elapsed until I got location from module, at so called Cold start. Cold start means, that GPS module doesn’t have primary  data about time, and location, so it is impossible to calculate which of 24 satellites are in module “view”.  In this scenario module searches for signal of all 24 satellites, consequently it takes some time. Seller recommended to test device with clear view of sky, but I placed module near open window, so it is why 18 minutes 🙂

After 7 minutes I got data from 2 satellites, no location info still, but UTC date and time from satellite.

After 18 minutes location is found.

Here is module’s best result – 6 satellites, but still near open window.

Warm start (5-6 seconds)

Module saves date, time and location information in memory. Next time it is turned on it uses that information for faster GPS fix. With built in RTC (Real time clock) used time is current, not the same as was when module last time worked.

2010.10.14 update.

There is  GPS module pinout instructions from seller:

  1. Black (GND)
  2. Red (+5V)
  3. White (GPS RX Data input)
  4. Green (GPS TX Data output)
  5. Black (Shield, GND)
  6. No connect.