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| gpspassion |
Posted - 08 déc. 2007 : 16:00:15
Updated 20080208 : I'm happy to report that SiRFInstantFixII is really helping my 760 a lot once it's built-up a good set of synthetic ephemeris data. As expected it did take a couple of days of regular use for that to happen, but on two short outings on two consecutive days since, the 760 locked in as quickly as the SiRFinstantFixI enabled TomTom GO920T, even a bit quicker on the second day, which makes sense since the SIFv1 data was 3 days old and is not updated onboard.
Here are some numbers : (same "protocol" as in December, see below)
- TTGO920T with SIFI : 12/12 seconds
- TTGO720T no AGPS : -/55 seconds
- nüvi 760 with SIFII : 12/8 seconds
What a change compared to the non SIFv2 2/3 minute TTFF! While Garmin clearly did not come up with a very "GPS efficient" design for the 7xx series, SiRFInstantFixII largely makes up for it in terms of TTFF, a good thing Garmin chose SiRF and not MTK or their own "bravo" chipset as they are not (yet?) "Auto-AGPS" capable...
I looked at the logs I recorded for my TTFF testing and unfortunately the 920T logs are corrupted so i'll have to them again, but I'm seeing some accuracy issues on the 760, which likely indicates that the quality of the synthetic ephemeris data computed on board is not as good as the "server data" used for SIFv1 and naturally not as good as the live ephemeris data broadcast by the satellites. Now to see if this has an impact on the user experience or not :
Updated 20080131 : While not in the notes for the v2.6 update on the Garmin site, this update adds SiRFInstantFixII to speed up the fix, provided some conditions are met, read about the pros and cons of SiRFInstantFixII in this article in the "Technical forums".
In theory this update should solve the sensivity problems on the 7x0 series that many of us found and that are discussed in the following pages.
How long for SIFII to work ? : in theory 24 hours are needed for the data to be collected and for a full set (30 satellites) of ephemeris to be calculated.
SIFII can populate the ephemerirs gradually as you use it, 2 hours here, 3 hours there, or maybe there is a "silent" mode that populates while the unit is off, I think Centrality had that option for their own "Autonomy" that likely led the way for SiRFinstantFixII. Or maybe it needs to be powered andd be in "sleep mode" like what Anders described in this topic, I think he might have been running an early version of SIFII already as my previous testing with a "standard" firmware did not show his results ;-)
I have turned off my unplugged 760 and placed it next to my reradiating antenna and will wait 24 hours to see what happens.
Posted 12/2007 before SiRFinstantFix v2 became available with FW v2.6
Analyzing TTFF on the map
This has been discussed in the main 7xx user reviews topic but I'm starting a new topic as the TTFF (time to fix) is a real concern to me on the nüvi 760 series.
Granted the 7xx uses the high sensitiviy SiRFstarIII chipset, but you still need to feed gps signals to that chipset, something the large patch antenna of the 3xx and 6xx series that faces up and is at a good distance from the RF interference of the screen and other electonics did well. On the 2xx and 7xx it's a different story...
The "ephemeris" 28db-Hz constraint
Just a quick reminder that the "bottleneck" is not at the tracking level as the SiRFstarIII chipset can track down to 14dB-Hz (the lower the signal level the lower the accuracy though), but to download the live ephemeris data from the satellites, something that requires a strong signal level of 28dB-hz, regardless of how powerful the GPS hardware is. You can overcome this limitation with what I call Internet A-GPS that downloads a 50Kb file containing predictive ephemeris data valid for 7 days, but Garmin have chosen not to implement it so far.
Other "poor man's" tips would be to hold the 7xx outside your window with the back facing up before getting started in the morning or using an external antenna. In both cases it's a constraint that we shouldn't have to put with when the previous generation didn't require it and when the competition does so much better, especially TomTom with free QuickGPS (internet A-GPS) on all it's units (Globallocate Hammerhead on the ONE and SiRFstarIII on the GO).
GO920T, 760, C620T, 660, 250
Side by Side Testing
As usual you can't beat side by side testing to get a good picture of the "GPS issues" at hand, so I did several test runs with this process:
1. place various receivers side by side in my car
3. let them boot up in my underground garage where the signal level is too low (~16dB-Hz) to download ephemeris data
4. start the data logging
5. drive out.
6. time the TTFF
7. analyse logged data
20071208 - FIELD TEST #1
Just did a side by side warm test (4+ hours without using them) by driving out in the suburbs with the 920T, nuvi 760 (with SW 2.20 and GPS FW 3.00s) and Mio C620T ->
- 920T : < 10s (thanks to QuickGPS A-GPS)
- C620T : ~ 45 seconds (standard due to ephemeris download)
- nuvi 760 : 3 minutes and it might have lasted longer if I hadn't stopped at a light.
I would hate to start out in Manhattan with the 760 in the morning, I doubt the 760 would ever get a fix...
Since the hardware design won't change, maybe Garmin will bite the bullet and license SiRFinstantFix from SiRF as TomTom have done. I'm sure they could easily make the download available via WebUpdater.
20071209 - FIELD TEST #2
Back from my test drive, exact same route as yesterday and I'm happy to report that the TTFF on the 760 is somewhat improved, whether that is due to the v2.30 software update or a better satellite "environment" will have to be verified with more testing, will be doing two more test runs, one tomorrow morning and one tomorrow night.
Here are the results :
1. TomTom GO920T < 10 seconds (thanks to A-GPS)
2. Mio C620T = 50"
3. Nuvi 660 = 1'28"
4. Nuvi 200 = 2'00"
5. Nuvi 760 = 2'05" (with SW 2.30 and GPS FW 3.00s)
Similar results to jotne's (see bottom of page 2) and there is still a notable difference with the competition (excluding TomTom that has AGPS) and again this is a favorable environment in the suburbs with little blocage, but better than the 3+ minutes.
20071210 - FIELD TEST #3
Exact same route as previously. The 660 had a bit of a "hiccup today", the 920T (with a 3 day old ephemeris file now) still steals the show and the Mio C620T shows regular performance at <1+ minute.
Here are the results :
1. TomTom GO920T < 10 seconds (thanks to A-GPS)
2. Mio C620T = 55"
3. Nuvi 760 = 2'05"
4. Nuvi 660 = 2'15"
20071213 - FIELD TEST #4
Another test run, with screenshots of the logged tracks.
Here are the results :
1. TomTom GO920T < 10 seconds (thanks to A-GPS)
2. Route 66 Maxi (uNav chipset) = 1'04"
3. Mio C620T = 1'30"
4. Nuvi 760 = 2'40"
This is what the acquisiont process looks like on the map, with a bit of "wobbling" just after getting the fix, note that the uNav based Route 66 has some tracking problems :
Another consequence is the lack of accuracy resulting from the issues downloading enough ephemeris data. Only when I had to stop at the light 2 minutes later did the 760 have enough time to download more ephemeris data and plot a more accurate track :
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| 15 L A T E S T R E P L I E S (Newest First) |
| gpspassion |
Posted - 25 oct. 2009 : 14:35:38
Definitely doesn't hurt, read all about it in the article in the first message.
As for HotFix being disabled/enabled, not sure, but the fact that no one has complained recently about sensitiviy issues on the 760 tells me it's still active. |
| pratzert |
Posted - 25 oct. 2009 : 00:41:50
How are they disabling it? Are trhey going to make some change in the next firmware version or by some other method ?
What do YOU think? Is the "Hot-Fix" good or bad or worthwhile to have ? |
| gpspassion |
Posted - 25 juin 2009 : 12:05:47
Bumping this up as Garmin indicated at one point yesterday that they were disabling HotFix, aka as "auto-AGPS"as that was causing problems maintaining the fix. |
| NanaimoRick |
Posted - 19 juin 2008 : 01:06:42
Pressing the satellite bars will bring up the satellite screen on the 760. You have to press the bars for 4 or 5 seconds. Other models the hold time has been less. |
| Jay_S |
Posted - 18 juin 2008 : 22:41:43
@IsLNdbOi - New to my 760... Just curious how you access some of the additional data you're showing. Also how you show the satellites on the 760. Garmin indicated that the touch sensitive bars are not operational on the 760. Thanks for any pointers...
Jay S. |
| gatorguy |
Posted - 09 juin 2008 : 03:57:00
As far as I know, every 700 series nuvi shipped so far includes a sirfstarIII chip. |
| synomenon |
Posted - 09 juin 2008 : 03:46:32
Do 760s still come with the SiRF Star III chips or has Garmin switched the 760s to a proprietary Garmin chip? I think the 760 I got last week has a SiRF Star III.
This is all the info. I've gotten on mine from its various menus:
nuvi 760
Software Version 2.30
Sirf: Copyright 2007 SiRF Technology, Inc. All rights reserved
SiRF B0478-0 3.00 SiRF GSC3f GPS Module 02A5C0F4 |
| gatorguy |
Posted - 08 juin 2008 : 23:02:35
What you're experiencing is not unique to Garmin. Take a look at the thread that Danham referenced above. When you turn your PND on miles away from where you turned it off, it's essentially "blind". Doesn't matter who manufactured it. As Danham suggested, if you want to check that it's operating properly, turn it on for 30 minutes or so, then turn it off for a bit, maybe 15-30 minutes, and stay at the same location. Turn it on and sat loc should be near immediate. If so, it is operating properly. |
| mangmang |
Posted - 08 juin 2008 : 21:26:55
well i live in ny, but i was away to boston yesterday and i went to a park by the harbor and i turned on the unit while i was sitting and it took 15 minutes, about the hampton situation i was stock in traffic moving 3-5 miles an hour and it took good 10 minutes to lock on signall.
thank you |
| gatorguy |
Posted - 08 juin 2008 : 20:31:19
It looks like you are moving from city to city with the nuvi turned off. Is that correct? |
| mangmang |
Posted - 08 juin 2008 : 16:17:09
hi same problem happened again, i updated to V3.0 and in Flushing NY it took unit close to 13 minutes to get a lock. |
| danham |
Posted - 08 juin 2008 : 16:16:09
@ mangmang:
gatorguy is correct. For lots more detail on this, see our FAQ and use forum search.
When you say "on the way to the Hamptons," that implies moving at highway or train speeds, so lock will almost definitely be delayed, especially after 6-7 days. In Boston, if you were on the Common, there are lots of tall trees and Beacon Hill itself can block sats (and don't get me started on what can be blocked by the Statehouse and its occupants [g]). Again, if you were moving then all bets are off.
To really test if your unit is operating properly, stand still with an unobstructed view of the sky for however long it takes to get lock, then turn off the unit for a few minutes (not hours) and try again. It should lock almost instantly.
-dan |
| gatorguy |
Posted - 08 juin 2008 : 15:19:30
It matters some if you are turning the nuvi on at a dead stop or traveling down the highway. More of a factor would be in the 750 is last turned off at one location, then turned on at some other distant place. Even as small as a 20 mile change can impact it a bit. A 300 mile change will certainly slow down sat lock, as the software is now looking for satellites at a spot they don't exist relative to last fix. |
| mangmang |
Posted - 08 juin 2008 : 14:49:00
Hello All, i have the Nuvi 750 V2.7 for 6 months now and i have been following the forum for one year. to be honest i dont get the results of getting signal as fast as other users, for example last week i was on my way to hamptons NY, the device wasnt on for 6-7 days, but it took 20 minutes to lock signals i even tried turing on and off the device which didnt help. also i was in Boston MA yesterday and the device was off for only on day and i was in the middle of a park it took the device 15 minutes to grab signal and thats a longtime. i have couple of questions: 1. DOes it matter if im in a moving car vs stopped in regards to amount of time it takes to lock signal.2. what is the problem with aquiring signal in my device?
i will really appreciate any help |
| gpspassion |
Posted - 24 mai 2008 : 21:39:40
Good question sounds like a good idea for a poll. Here goes...check the left pannel ! |
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