BT GPS74R (Holux BT321) Reception Question(s) + Altitude

[WigWag]

Folks,

BACKGROUND
I have one of the above and it works okay with TomTom Nav 6. Device sits on passenger seat and comms via bluetooth to Nokia N80.

I believe its a 32ch device, I live in NW England and only ever get 7 or 8 channels if I'm lucky (inside the car and with the device sat in the garden). Is this "usual" for such a unit - I've no way of knowing.

However, I've also bought AFtrack91 which effectively turns my N80 into a GPS device - great for walking and cycling as I can store waypoints, and track my route. Its quite useful to download my track into the PC when I find my way back home to see distance travelled, where I've been on Goog Earth and the altitude profile. However, the altitude accuracy sucks. Its relatively flat round here, but a 6mile cycle can produce a 50m delta, min to max, .e.g. -20m to +30m. Is this usual?

QUESTION + DESIRE
Q1 - If my experience perceived by yourselves as as good as it gets then fine, I'll grumble less and stick with what I've got. BUT IF NOT.......

I'd desire a bluetooth device with:

- much better altitude accuracy
- compatible with tomtom6
- pref. a manual on/off with an antenna socket to enable me to leave / forget the device in the boot, permanently wired to a switched 12V supply and a rear screen mounted antenna?

Q2 - Any Recommendations would be gratefully received.....

Thanks in Advance

[robertn]

These observations are typical and 7 or 8 sats is more than enough to get an HDOP of 5 meters or less. Have a look at your HDOP as this is a better indication as to the quality of fix than number of Satalites, and regardless, your maps wont be that accurate anyway.

GPS altitude precision is not as accurate as lat/long precision, so what you are seeing is common, depending again on the satalite constalation configuration at the time. I doubt you would be much happier with another device. Also the GPS altutude is based on the GPS elipsiod, not the Mean Sea Level, so the absolute value will differ depending where on Earth you are

If you really want much better altitude accuracy, some of the Garmin Range of hand helds have a Barometric sensor.

As for your 'desired" device - Thats what I have wanted for ages, but I have not found one. I am sure that someone now does it, but I gave up looking a few years back.

[WigWag]

Many Thanks

You've saved me a bit of money for now - I'll have to go and find something else to spend it on.

I'll probably have a casual look re: the desired GPS unit but will worry less about the antenna.

I've an enquiring mind and am very interested in finding out more about GPS such as mean sea level vs GPS elipisoid, PDOP, HDOP, VDOP, HACC and VACC. I've not explored the forum here yet but if there are any pointers for good info resources I'd be grateful.

[philpugh]

There are quite a few resources on the net. Google will find most of them. Try "GPS HDOP" and get a flask of coffee ready Some of the manufacturers web sites have good info on them. To be honest most readily available GPS receivers with modern technology (eg SiRF III chipset) should give consistent results irrespective of manufacturer. What you need to take into account are the additional features you desire. (I would like to see a datalogger GPS unit, using SD card with OSGB grid ref display - but there isn't one about yet - but they are getting close).

At any one time there are 24 active satellites in orbit (plus a few spares ready to take over). At any time there will be (on average) 12 "visible" from a point on earth. Due to topology some of these will be below the local horizon so 7 or 8 is quite typical and will give a reasonable horizontal fix ( within a circle of about 5-10 metres). Altitude is a problem because to get a similar positional fix would need you to be able to receive satellites from below you as well as above - and good old Mother Earth gets in the way.
_________________
Phil

[WigWag]

I've started to have a look - there's a lot of "geeky" websites out there, not always the easiest to read and I have a technical background.

Re: the number of satellites "received", the spec for my unit may explain why I only get 8 and no more - it uses the best 8 signals for the solution?

Off to read a bit on the chipset - its apparently RFMD.

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