SparkFun Forums

[hkwmon]

Hi all GPS users,

Which GPS Eval Board I ought to buy due to your experiences.
I will use the GPS with the ADuC832 Eval Board.
This board have a Industry Standard 8052 Microcontroller and a Serial Interface Port (RS232) with 5V.
I have enough place and power and I want to use external magnetic antenna.
Thank you in advance for your helps.
Best regards from Switzerland
Willy

[jaime]

well... i'm not a expert, but i bought a lassen iq rs232 ev board and works good. I connect to my pic with rs232 and if i want i can connect to the pc.

~The only problen is because i cant compare lassen iq with other moduls like sirf3. I'm doing a simple log file. I log nmea data from the gps. Sometimes the track in the map is a litle wrong. Dont know if the fault is in the gps or in the map.

Hope it helps, and hope to have answers about experiences with this and other gps moduls...


from Portugal

Jaime

[hkwmon]

Dear Jaime
Thanks for your answer.
Many questions in the GPS column refer to the EM-406 .
Is this module with 20 channel more exact or is lassen iq more easier to use and therefore gives it no questions?
Best regards from Switzerland.
Willy

[jaime]

I have the same question... i dont know wich is the best...

I hope someone can help you (and me) and tell more about is own experience...

But i'm happy with Lassen


Jaime

[ckuethe]

First off, it would be helpful to know what you're doing. I doubt you'd see any difference in the performance of these modules if you were sitting on the top of the tallest building in town, or out in a wide open field. Down in an "urban canyon" with cars and tall buildings all around, and you very likely would see some differences.

SiRFstar3 is called "20 channel" because it has enough CPU power to track 20 PRNs in realtime. In a way, SS3 is a fast embedded DSP running a software-gps to track the satellite, with the on-board ARM running the navigation solution and handling the I/O. I think you'd be hard-pressed to find places where you're going to get more than 11 usable satellites. There are times during the day when I can track 11 + WAAS, but there is always one down on the horizon that's pretty much useless.

When logging GPS data, make sure you are mindful your fix quality. GPGSA gives you number of satellites used to calculate your postion and your Dilution Of Precision. If your DOP numbers are high , you need to be aware that there could be some significant discrepancies between your reported position and your position on a map. For my purposes, HDOP >= 2.0 is high. That's still perfectly acceptable for driving around town, though.

Things that can affect your fix, even with the best receivers include a poor antenna (or the antenna housing), multipath error from signals reflecting off water/buildings/metal objects, atmospheric interference from moisture or charged particles, poor visible constellation geometry, and differences in the implementation of the satellite tracking and navigation solution algorithms. Higher sensitivity receivers sacrifice some accuracy and speed for the ability to lock on to a weaker (less reliable) signal.

My SS3 receivers have been quite reliable, and I should have a Lassen iQ shortly. I suspect that if you're looking for a quick fix (pardon the pun) make sure that your antenna cable is not bent or kinked, there is no dirt or oil on your antenna, and you have an unobstructed view of as much sky as possible. Even a little bit of adhesive tape on your antenna can adversely affect signal reception.

[hkwmon]

Hi ckuethe,
Thank you for the very interesting explanations.
I will use the GPS-module on my sailing boat on the lake of Constance.
I want to measure the speed and the direction.
The antenna I will fix aft at the ship at the railing and not at the top of mast.
But I'm not a professional computer freak.
Best regards from Switzerland
Willy

[jaime]

Hello ckuethe

Thanks for your help. It was a nice post.

I will take a look at lassen iq manual about HDOP and if i have any question i will try to ask you.

When you get lassen iq you could tell us aboit your experience...

My aplication is in a car. I'm doing a log with gpgga and gpvtg sentence, and then convert to kml or kmz file and open with google earth.

[ckuethe]

hkwmon:
Not putting the antenna on top of the mast is a good choice, you won't get additional positional error due to the motion of the mast. My biggest worries in your application would be multipath and water exposure. The radio waves can reflect off water and may interfere slightly with signal reception. You can mitigate this with an expensive choke ring antenna, or just put down a 19cm (1 wavelength) metal plate underneath the antenna as a ground plane.

As for the issue of water; be sure you keep water away from the circuitry. I suppose if you put the your receiver itself and the display engine in a waterproof box (like a clear pelican case) and only had the antenna exposed, you'd be OK. If you plan on using an auxiliary radome (a fancy way of saying "putting the antenna inside a plastic container so it doesn't get wet") be sure you test the effect your housing has. You'd think it wouldn't matter, but even something as simple as a bit of packing tape can mess up your reception. Various kinds of glass or plastic have different effects...

Jaime:
DOP figures are unitless - they don't measure so much as give you some indication of how badly your receiver is lying. I don't remember where I read this, but I once saw a recommendation to the effect of "if your HDOP <= 8.0 you're fine for street navigation".

I assume you'll be using something like gpsbabel to convert the NMEA from your receiver to google earth format? What you may notice is that when reception is poor your plotted track will jump all over the place. If you are using gpsbabel, it has filters to remove points with high DOP and to interpolate between points... you may find these useful.

Gut feeling: don't worry too much about it.

[ckuethe]

I'm doing a simple log file. I log nmea data from the gps. Sometimes the track in the map is a litle wrong. Dont know if the fault is in the gps or in the map.

This has been dealt with earlier on but it bears repeating. NMEA positions are
DDDMM.mmmm, not DDDMM.ssss.

1234.5678 means
12 degrees, 34.5678 minutes, not
12 degrees, 34 minutes, 56.78 seconds.

Interpreting it the second (wrong) way could put you nearly half a minute out... I'm too tired to do the math right now (it's 0140h) but that can easily be a hundred meters of error.

[jaime]

i dont do math... i record every 5 seconds the nmea sentence gpgga and dpvtg. I dont do calculations. Then i use gpsvisualizer to convert to kmz.

U dont have gpgsa. Do u recommend to use gpgsa? will gpsvisualizer exclide the sentences with with HDOP??

thanks

[ckuethe]

You might not be doing any calculations yourself, but your processing tools might be.

GPRMC gives you pretty much everything you'd need: time, position, speed, course. GPGSA gives you information about the quality of your solution. I would recommend using this message. These are the only two you should need, though some logging applications might insist on GPGGA and GPGLL.

I see no evidence that gpsvisualizer will exclude positions with poor fix quality.

[AliasMrJones]

I will use the GPS-module on my sailing boat on the lake of Constance.
I want to measure the speed and the direction.

My suggestion would be to buy a standard off-the-shelf handheld GPS. I use a Garmin Legend for sailing on a lake here in the US. It is waterproof (no problems getting splashed on) it will measure heading and direction, you can drop waypoints and it will navigate you to the waypoints, it has mapping capability and you can have it track your position so when you get home you can download the tracks to your computer. And, it also has a serial port output of NMEA data so if you want to interface to a microprocessor/datalogger/etc. you can.