I have used SiRFDemo to switch the EM408 to NMEA protocol at 4800 baud and configured just the GGA output message every 5 seconds. It works fine but I have two questions.
The SiRFDemo User Guide indicates that these settings are saved in battery-backed SRAM. How long will these settings survive after the EM408 is powered off? I can’t find any documentation and USGlobalSat tech support said the settings are “terminated once power is removed from the device.” The settings aren’t terminated but I’d like to know how long they survive.
With the EM408 sitting indoors on my desk I have watched the GGA sentence output and calculated position variations over 120 feet without moving the EM408. Is this normal when the number of satellites acquired varies between 3 and 7? Would an active GPS antenna reduce the position variations? Thanks.
Its been a while since i messed with it, but there are 2 modes you can put that in to take care of that wandering affect. i dont remember the modes off the top of my head, but check out their forum, that topic is in there and talks about it.
There are two choices : Static mode enable/disable.
EM-406 is satic mode disable by default.
In the static mode enable, SiRF chipset is designed to freeze the data when speed is under a threshold speed 4.3 Km/hr.
If your speed is under 4.3Km/hr, date updates can only occur after you moved every 50 feet.
If we disable static mode , GPS data “jumps” around when GPS unit is stationary or under 4.3 Km/hr
It is the all about SiRF chipset design. Firmware upgrade can not do anything about this issue. All GPS product or module with SiRF chipset
have the same situation.
Taiwan GlobalSat
I assume the EM408 has similar modes. The static mode won’t really help me. My application just needs to know when the GPS has traveled a fixed distance like 200 feet, which may take hours. I can live with the 10 meter accuracy but the jumping around when the unit is not moving is a problem. Should I be looking at another GPS chipset?