GPS Modules, which to choose?

[Joe2003]

Hi

Just browsing for a GPS module, I have found 3, there is a big price difference and im not sure why.

http://www.totalrobots.com/access_files/gps.htm

http://www.coolcomponents.co.uk/index.php?page=1&category=G

AND

http://www.crownhill.co.uk/product.php?prod=1474

The last one has an external antenna, which I think is a big +

Apart from that I dunno. Just want it to do waypoint navigation, lat long, height and speed.

I guess, (that is a big guess) that you can do way point navigation with any gps, its just abit of maths manipulating the lat long?

anyway, any pointers would be great, thanks.

[Tuner]

Everyone I know uses UBlox GPS units?

[mat01]

Yes, the price difference is a little confusing.

In terms of performance the EM406 tracks 20 satellites at -159dB sensitivity. This means it's much more likely to get a fix in bad conditions.

It has a built in battery so is much faster to get a fix on power up. For lots of people the built in antenna is a big plus - means no extra cost or weight.

I work for www.coolcomponents.co.uk

[Unterhausen]

the first link goes to a gps that has a lot of extra circuitry. That being said, I don't think it's worth it, it's much larger and heavier than you want. We have some of those, and they take just about forever to start tracking. We have some of the units at the second link. Sparkfun sells that one. It seems to be very sensitive. don't know about the third unit.

[bbstriker]

Hi

I'm building a predator scale model UAV.

I've chosen the tiny (and very cheap at £28+VAT) Trimble LAssen IQ GPS module and even smaller high gain extran antenna (costing £11 plus VAT).

I did not go for the dev kit as it is priced at a ridiculous £399. Anyway I got it workig after alot of trial and error. I thought I'd share a schematic for any would be GPS project.

The aim of the GPS unit is to interface (via RS232) with a video overlay card (so I can see position, velocity and altitude on video signal) and a PC using a NULL modem cable. When the UAV takes to the air, the physical serial link will be replaced by a wireless serial link using Aerocomm licence free transceivers. The link to the ground will provide a moving map display using Fugawi maps program or any other freeware GPS map program.

The schematic shows the connection schema to send NMEA signals from GPS Module to PC. I hope this helps. It's taken three whole days to get to the diagram below! :rolleyes:

[Tom Harper]

bbstriker,

Thanks - seeing what you are doing definitely helps!

I have essentially the same set up but with a Global Sat SiRF lll. I get a +- 6V signal at the output of the RS232 driver. There are transmission bursts on the line. But the buffer in the PC is empty. My program does read the output of my Magellan 315.

Did you have to invert the data?

Tom

[Tom Harper]

bbstriker,

New deal! I found a problem - now it fills the buffer but not with anything that looks like a sentence.

The LED is blinking which indicates it has a fix. The data bursts look reasonable.

Checked the output of the Magellan. It is a 0V to +6V swing. Hooked it up to the port and it works fine.

Tried inverting the data - no help.

Any thoughts?


Tom

[bbstriker]

bbstriker,

New deal! I found a problem - now it fills the buffer but not with anything that looks like a sentence.

The LED is blinking which indicates it has a fix. The data bursts look reasonable.

Checked the output of the Magellan. It is a 0V to +6V swing. Hooked it up to the port and it works fine.

Tried inverting the data - no help.
Any thoughts?
Tom
Hi Tom
The converter I'm using (www.robomicro.co.uk) converts TTL voltage levels to RS232 and back to TTL. It also carries out a logical inversion of 1 to 0 and 0 to 1.

I'm not 100% sure if this is the root of your problem.
BR
Tony

[Joe2003]

They all seem alot cheaper than I would have expected, but im not complaining.

[Tom Harper]

bbstriker,

I found it! After a frustrating evening of trial and error I did some Googling and found that the RCV input has to be tied to + through a resistor. I had grounded it. Picky, picky, picky.

Now the thing is happy and works fine.

Thanks for the response.

Tom