ARTICLE COMMENTS: TTFF Comparisons

[Dave]

Please post any comments for discussion regarding the TTFF Comparisons article in this message

[TheBoyGroucho]

As a point of interest for anyone considering the Holux GM-210...its TTFF is broadly similar to the Tomtom navigator being generally well under 30 secs for similar powered off periods.

It also seems to perform well if left for longer periods, recently producing a TTFF of 25secs after being left powered off for 8 days.

[George Pickles]

I know nothing about electronics, so forgive me if the following question seems a little stupid.



Navman say the extended TTFF on their product is due to electromagnetic interference from the PDA

s processor. Is there any way that this interefernce could be minimised physically? Could the rear of the PDA be shielded in some fashion or would it be too thick to enable the NavMan Sleeve to fit?



Just a thought!

[MikeB]

"The largest cause of poor reception / time to fix with the receiver, is electromagnetic interference from the processor on the PDA. We discovered that by increasing the baud rate at which the GPS receiver communicates with the PDA, we could transfer the NMEA data, process it on the PDA and then put the PDA into idle (and electromagnetically quiet state) allowing the GPS receiver to increase it's performance.".



This sounds pretty much like NavMan admitting that they have a serious physical design problem, they know about it, but are not prepared to do anything to fix it (as evidenced by the 3400 sleeve being the same as the 3000).



Surely if a manufacturer isolates a design fault they make attempts to rectify it, if not in existing products in product enhancements.



Most peoples experiences of the NavMan that I hear about (admittedly they may only be the bad experiences) seem to bear out the fact that the TTFF is not what NavMan quote on thier site and in litrature. How do they get away with it?



It is unacceptable to expect to fiddle around removing jackets and rebooting over a period of 20 or so minutes before getting an often unreliable fix. This unit is a tool not a toy. With a price tag of £250 it is comparable to the high range Garmin hand held products I would expect it to perform in a similar manner.
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Mike Barrett

[Dave]

I think shielding the back of the sleeve is a problem, you would need to line it with lead, or some similar metal, and thickness would be a problem, but also touching the components would also be another. I know Mike's been having major hassles today with his Navman sleeve, similar to myself testing the new SmartST software over the past few days. I tend to find it swings in roundabouts, but I haven'thad the TomTom Navigator mouse fail me yet (Dave touches the first piece of wood he can find).

[yellow1]

I've said it before and I'lls say it again...;-)

You should really be doing the testing with CE Monitor that can simulate Cold/Warm/Hot TTFF's and offers much better control than TomTom's driver.



A Cold TTFF of 5 seconds with a SiRF chipset is absolutely impossible (manufacturer specs is 45s and we know what manufacturer specs mean ;-))



If I may be so bold, take a look at my own TTFF tests made with CE Monitor



http://www.gpspassion.com/Hardware/TheFix.htm



cheers
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<a target="_blank" href= "http://www.gpspassion.com/">GpsPasSion.com</a>

It's all about sharing the info

[TheBoyGroucho]

Quote: Originally posted by yellow1 on 06 October 2002

---

A Cold TTFF of 5 seconds with a SiRF chipset is absolutely impossible (manufacturer specs is 45s and we know what manufacturer specs mean ;-))

---

I think perhaps the problem lies in the difference between how we use the phrase "cold start" in real life and how it is defined by utilities such as Sirfdemo, whichs gives its antenna reset options as:

• Hot Start


The Receiver restarts by using the values

stored in the internal memory of the Receiver.

The stored ephemeris and almanac are both valid.


• Warm Start (No Init)


This option has the same functionality as Hot Start except

that it clears the ephemeris data and retains all other data.


• Warm Start (Init)


This option clears all initialization data in the

Receiver and subsequently reloads the data that is

currently displayed in the Receiver Initialization Setup

screen. The almanac is retained but the ephemeris is

cleared.


• Cold Start


This option clears all data that is currently stored in the

internal memory of the Receiver including

position, almanac, ephemeris, and time. The stored clock

drift however, is retained.


• Factory Start


This option clears all data including position, almanac,

ephemeris, time, as well as the stored clock drift. All

Receiver parameters are also set back to the

factory defaults.

Given that after 8 days powered off I got a fix with a Holux mouse antenna in <25 secs it seems reasonably to assume that it can take on occasion a significant length of time for the data held in the antennas memory to become sufficiently unusable as to force the antenna to make a "cold start".

It might well be better to pick an arbitrary set of times at which to note TTFF times (say 0.5,8,24,48,168 hours) rather than use terms such as hot, warm and cold start.



I note on your "Thefix" webpage that you put the "factory" and "cold" times in the same box which is surely erroneous?

regards

peter

[rwbthatisme]

I've been looking at ways to reduce the EMI/RFI cross talk between the ipac & the navman sleeve.



Theoretically the best way to do this is to increase the sheilding between the back of the ipaq and the inside of the navman sleeve.



As you all probably know the silver paint on the ipaq is conductive and therfore should reduce the EMI but obviously not enough!



my quick homebuild way that I'm going to try is:



<! Disclaimer - I havn't done this yet, so its not a recomended patch! >



get some thin aluminium foil, cut out a single rectangle 90mm x 50mm, cairfully laminate the foil to the inside of the navman sleeve it should be centred lengthways using some 3M™ Stencil Mount Adhesive (so that you can remove the foil if necessary!).



The Foil will be grounded by being in direct contact with the silverpaint of the ipaq so no need to void your waranty by soldering anything!



I'm going to give this idea a bit of a test over the weekend so will post up if its any good.



Cheerio



Richard




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wibble

[Dave]

Richard, would be interested in your findings. In theory it sounds a good idea, but I don't think it will make much difference, but hey I've been proved wrong before!

[rwbthatisme]

Further to my note.



There's very little space inside the ipaq chasis (actually none!) to add more sheilding. Some of the ic components are canned but this is on the innerside of the pcb. So I guess most rf leakage is coming from the unshielded parts.



As a longer project I thought it might be possible to re-paint the navman sleeve with silver conductive paint excluding the aerial.



As this is a bit of a perminant modification I'm not going to do that. (the cost of a little pot of silver paint is about £40 and a re-radiating antena is about £50)


_________________
wibble

[Maxime]

I possess a CompactGPS of Pretec (not LP) and I contest your tests has cold for this model, this morning, after several days of no working, my CompactGPS only took 45 seconds to display 5 satellites on 8 present in the almanac of today.
_________________
Maxime

[Dave]

Are you running an external antenna with the LP ? The reason why I ask is many other readers in the forums here received similar stats from their receivers before sending them back to purchase the non LP version. However many have found using the Pretec antenna fix times are considerably better.

[DataMan]

any comparison on fortuna will be added?

[Dave]

We have the comparison databases here that compare the specs between multiple Bluetooth GPS Receivers.

[cflint]

As this was from 2003 are you going to run an update to include the latest models?

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