ZBasic Forum

Does anyone have a preference for GPS engines that can be used with the ZX24? Prices and capabilities are all over the lot so any guidance/experience would be greatly appreciated.

TIA,
Paul

Hello Paul,
A rather open ended question...

A lot depends on your project requirements.
Do you need indoor GPS capability?
Do you want an integrated antenna, or will you be using an external antenna?
Will you be parsing the standard NMEA strings, or do you require access to several unit's specific, additional capabilities?
Will you be using standard, civilian GPS modules, or the higher precision, or military units?
Is general GPS adequate, or do you need a module which incorporates WAAS or other similar improvements?
Do you need an accurate 1 pulse per second time base?
Do you mind level shifting 3 v GPS signals interfaced to a 5 v CPU?
Are you building 1 or a million ?

I have several "Sandpiper" eBay surplus GPS units. They require an active, external antenna. They are clearly old generation, and do not have the sensitivity required for indoor operation. They allowed me to inexpensively learn about the NMEA messages, and how to parse and process them.

I have several EM-406 modules, obtained through SparkFun. (www.sparkfun.com) They are 3 V units, and include an active patch antenna. They work great, and I routinely have a signal indoors. I have not personally used the advanced capabilities of this unit.

I have a bluetooth unit, and a Garmin aviation unit. The EM-406 is based on the SiRF III core, and outperforms both of these units by far.

There is another GPS thread on this site, and Sparkfun has a gps forum which may also be of interest to you.

There are also GPS chip sets available, instead of pre-packaged modules. Laying out a board for the low noise amp and antenna interface exceeded (by far) my capabilities, but would be of interest for mass production.

The attached photo shows an EM-406 with its patch antenna at the 12 O"Clock position. A ZX-24 sits to its right. 3 & 5 volt regulators are in the middle of the board.

There are several members on this forum with GPS experience, hopefully they will chime in with their thoughts and experience.

JC

DocJC wrote:Hello Paul,
A rather open ended question...

A lot depends on your project requirements.
Do you need indoor GPS capability?
<...>

There are several members on this forum with GPS experience, hopefully they will chime in with their thoughts and experience.

JC

Thanks for the reply, JC. I did a GPS project last year and learned quite a bit about using the technology. The project used both NMEA and SiRF protocols and worked out quite well. I used a Laipac unit for that project but a new project won't bear that price level. The Sparkfun units look like they might work out quite well.

One question: is there any advantage to placing a module in it's own housing as opposed to placing it inside the case as shown in you pic? I presume that the case must be plastic in order for the unit to work if placed within the case.

Again, thanks for the info,
Paul

My project uses a plastic case, and for the small quantity I built the board area the GPS module took up wasn't an issue. This also made data transmission to the CPU trivial, and I did not have to deal with a second case, or cabling and data transmission back to the CPU.

SparkFun sells this GPS module with a several inch data connector cable. I purchased several surface mount connectors to put on the boards, to plug the cables into. In the end, however, I just cut the connector off one end of the cable, and soldered the wires into holes on the board. This was much easier that mounting one surface mounted connector on the board, which is otherwise a through hole project. I can unplug the end of the cable from the GPS module end if I need to swap out / test / etc the GPS module.

If you use a GPS unit with an external antenna then you can use a metallic case, but you have to mount the antenna, and route the cable. Fortunately most of the small, active antennas can get their power through their RF lead.
For this project it was easier to stuff it all together.

JC

I also have used the Sparkfun EM-406, which is SiRFStar-III-based. SiRFStar-III is the currently hot GPS chipset, easily outperforming other affordable devices. It's sensitivity is incredible, even tracking indoors where other units can't see a single bird. I recommend it.

However, SiRFStar-III is said to be SBAS (WAAS/EGNOS/MSAS) compatible, but only the current firmware version (v3.2.2) is, in fact, SBAS-functional. Earlier versions are not, despite the claim. Go to http://www.gpspassion.com for thorough information, in the forum technical areas.

If you need WAAS, don't get the EM-406 unless Sparkfun can assure you that you'll get v3.2.2 or later firmware. Users cannot flash new firmware themselves and I don't think Sparkfun can, either.

If you can use a Bluetooth GPS receiver, I have much experience with USGlobalSat's BT-359, SiRFStar-III-based, and can also recommend it - with the same firmware concern. I returned four BT-359Ws to USGlobalSat (no WAAS despite advertising) before I made enough noise to be the first to receive the replacement BT-359Cs, which work.

Do these receivers decode WAAS (from INMARSAT) indoors? (You'd see the corrected/DGPS tag in the NMEA message if so).

Lots of GPS multipath indoors to introduce errors - maybe it's moot.

stevech wrote:Do these receivers decode WAAS (from INMARSAT) indoors? (You'd see the corrected/DGPS tag in the NMEA message if so).

Lots of GPS multipath indoors to introduce errors - maybe it's moot.

Yes, I was surprised to find that the WAAS signals from the Inmarsats are actually better than those from the GPS satellites themselves, which are at half the altitude. Seeing PRN122, 135 and 138 in the US is easy. Still, as you say, position error is higher indoors; WAAS can't do anything to correct that.

Inmarsats are fairly high power - but 22,000 miles out there.
GPS are at what, 800 mi.

We love the inverse square law for radio!

stevech wrote:GPS are at what, 800 mi.

GPS vehicles are in half-sidereal day orbits.
"The Block II GPS satellites have been deployed:
* In 6 nearly circular orbital planes.
* With 4 satellites equally spaced within the plane (the 3 in-orbit spares are spread across different orbital planes).
* In orbital planes at an inclination of 55 degrees.
* At altitudes of approximately 20200km above the earth."
http://www.gmat.unsw.edu.au/snap/gps/gp ... 22sats.htm

Both BlockII GPSvs and the Inmarsat SBAS package radiate about 50Watts.