Finished Building my GPS, now some ?s for the GPS gurus

Dominik · 10-22-2004, 04:30 AM

Well, after weeks of procrastination i finally decided to build the GPS that i was planning. Someone had given me a Trimble Lassen LP GPS Module, and i have wired it to a MAX232 chip to make a Serial GPS (3.3v & 5v power taken from my PSU, saves having 2 voltage regs).

So anyway, i finally finished it last night. Here are the pics before i jammed everything into a jiffy box which fits inside my PC case:

Powering it up, i was extatic to find it worked first time. Data streaming from the GPS visible in Hyperterm. Destinator 3 also found the GPS as NMEA 4800 baud... But here is the problem, it wouldnt lock any satellites. Last night it would give me my location, and even traced my driving (holding the antenna out the sunroof), but it didnt show any satellites locked.

Today i cant get any location info. Is the antenna too weak? Is it possible to get location info without 'locking' onto a satellite? Anyone know?

wi77iam · 10-22-2004, 05:57 AM

What are you using for antenna?

Dominik · 10-22-2004, 08:01 AM

The antenna isnt in the picture (D'oh! i forgot to get a photo). I dismantled a large GSM/GPS Antenna and i am using the GPS Patch Antenna from that (about 1" x 1"). It is in a small jiffy box at the end of about 5m of antenna cable

I suspect this is part of my problem. The antenna is a regular 5V active antenna (also given to me), however the GPS is a low power (3.3V) unit. This means the active antenna only gets 3.3V; although i was told that it would operate down to 2.9v or something like that... Although i am beginning to doubt this; i havent picked up a signal since last night.

jcdillin · 10-22-2004, 08:10 AM

is it actually enclosed inside the box? maybe try taking it out and see what happens, it might not like the stuff the box is made out of.

Zip-Lock · 10-23-2004, 09:28 AM

If it's tracking your position then it must have locked onto at least 3 satellites. How do you know it’s reporting that it hasn’t locked onto any satellites? Are you looking at the raw NMEA stream?

You said you where using Destinator. I don’t know which NMEA sentence Destinator uses to get its satellite locking information, but perhaps your GPS module doesn’t enable that NMEA sentence by default and you have to enable it.

Dominik · 10-24-2004, 12:18 AM

Zip-Lock, thats exactly right! My mate also came up with the same suggestion last night, and i checked it out. The GPS module only had location and time output strings, so i had to enable the Signal strength string using the configuration software.

That was a headache in itself - i thought i had fried my board a couple of times. But it seems to be working now after a reboot.

Its just a pity this one doesnt have the SiRF mode - i heard that has faster updates (more than 1hz), and more accurate.. Though i shouldnt complain - this hasnt cost me anything yet!

it seems to get lock fairly quickly (<2 mins cold start) with the antenna on the boot, so i will probably mount it there permanently

Thanks for your help guys!

pate60 · 10-24-2004, 07:05 PM

i've got an OEM G8 GPS Ashtech board with an active antenna. The GPS runs off 5volts. In the tech specs for the board it clearly states that the active antenna should have a different power supply input from the board. The input voltage for the antenna ranges from 3.3v to 26v DC.

I would google and see if you can find the data sheet for the board, i tried using the same input but found that it took forever to lock, and dropped the signal many a time, im at a loss as to why this is, can anyone explain?

Dominik · 10-24-2004, 07:28 PM

I have the datasheet for mine (Trimble has comprehensive documentation!), and unfortunately it does not give you the option for external power for the antenna (would be nice!). I have a GSM Modem which has a separate input for Antenna power...

I have no idea why you cant use the same power source for both the Board Power and Antenna Power though.

Dont forget that Cold start takes up to 5 minutes to get a lock, so you need to put a battery backup in the circuit so it can get quicker locks on subsequent power-ups

Also, where was the patch antenna when you were testing it? Mine refuses to see any satellites from inside my car... Even through the sunroof. On the boot seems to work for now...

pate60 · 10-24-2004, 11:48 PM

well mines got a magnetic base so its on my boot, and gets perfect lock, i've had it in the back window which was ok, but took ages to start up, probably because of the metal film in the windows. and yes it was battery backed up.

Has ur board got an onboard amp for the antenna?

Dominik · 10-25-2004, 01:34 AM

No, the amplifier is on the actual patch antenna. My board just supplies 3.3v down the centre core of the antenna lead.

I think i've ruined any chances of getting lock inside my car on the weekend - i put a heavy full metallic tint on my car (blue mirror tint); i can hardly see out, so how can the GPS??

How much did you get your board/antenna for?