GPS based instrumentation

[Don_Kirby]

I've been browsing the wonderful wide web recently looking for a GPS based speedometer to be installed in a boat. What I've found are units intended for racing craft that display speed on a very typical analog style gauge. I've also found that they cost several hundred dollars.

Before I roll my own, I'd be interested in hearing from anyone who has seen or used COTS GPS speedometer with a digital display costing less than say $200.

I have already succeeded in slapping together one myself with one of these, one of these, and a ZX24n. It would need proper watertight packaging and more robust code to be usable in the field however.

It seems that this should be a common item, along with a host of other variations on the same GPS theme. Bearing information is easy to display using the same hardware, as is the time and altitude. I did find one unit that would serve my purposes, but I have yet to be able to actually get one in my hands.

[stevech]

sorry, I don't know. But you do know that GPS at slow speeds, or when turning, is quite inaccurate.

[FFMan]

i'm not sure what COTS is but as part of my bike project it can display speed got from GPS. I used an i2c 4 digit 7 segment led display (from byvac) to make it readable in outdoor lighting, and to keep the wiring simple.

if you've got a gps input then an option to display heading might be useful although i suspect displaying elevation might not be so handy in a boat ! time is also handy though you need to allow for localalised adjustment.

be aware that if you want to use a h/w serial port (which i think you will need to keep up with a fast gps), and h/w i2c you'll need to use a zx40 as there is a pin clash on the zx24 range. however s/w i2c will work and assuming your app isn't doing too much else that will work fine on a zx24

[Don_Kirby]

[stevech]

GPS receivers default to 4800 baud. The NMEA standard lacks a means to alter the baud rate. Few people need to do so. I've written code for a SiRF GPS and use their proprietary commands to alter the baud rate and choose which sentences I want, and so on.

My brother has a large twin-engine boat (had a gas one, now a diesel one), with a large array of electronics. Radar, GPS/NAV, Loran, etc. His boat displays boat speed based on, I think, stored tables of the tides and seasonal currents for the area you are in, and what those average out to be, to correct the boat speed. It seems pretty accurate - probably better than GPS. He has a PC aboard with NAV displays on a LCD near the helm. And the radar has a moving map display too.

[Don_Kirby]

Some units also offer the ability to turn sentences on or off as needed. Most of the Sirf modules I've seen allow this, as do the more advanced NMEA devices. If I understand the specification correctly, there are standards covering the sentence structure and contents, but not necessarily which sentences a unit has to output.

I hadn't thought about compensating with tide data. It's an interesting approach. My particular application would benefit more from actual speed (and hence distance traveled) rather than water speed. Along the same lines, several of the GPS units I was looking at also incorporate a magnetic compass to provide heading information when the GPS couldn't (i.e. standing still). It turns out that this also is not ideal for my particular application due to large amounts of lateral 'slip' inherent to the craft.

-Don

[stevech]

I think that boat speed is like true air speed in an airplane which is winds aloft and the heading of the wind, versus the heading of the craft and the indicated air speed.

But alas, I'm naive about the affects of the hull/fuselage on transforming indicated speed to true speed. There's a correction factor for that. One web site says this "slip" correction is up to 16% for boats' different sizes and hull types.

EDIT:
NAV systems today often include a magnetic compass and get speed from GPS or more accuratly (OEM NAV systems) get speed from the vehicle systems. Speed and heading go into a Kalman filter with GPS fix age to use dead reckoning to extrapolate location from the last known. Car NAV systems add the twist that they do map-matching, where they assume you are on a road if you are in some proximity to a known road. This can override the compass estimate of heading in good systems.