| Author |
Message |
peter
Guest
|
 Posted:
Mon Oct 17, 2005 8:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Sam wrote:
Thanks, much appreciated. |
|
| Back to top |
|
 |
Sam Wormley
Guest
|
| Posted:
Mon Oct 17, 2005 8:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
peter wrote:
| Quote: | Sam wrote:
All correlators my not grab their respective satellites at the
same time and my have to wait for the next 30 second interval.
Why would it have to wait for the full 30 seconds? Once the correlator
has locked onto the satellite signal the unit can start receiving and
storing the bits in the data channel. Then it looks for the frame
information (synch code in the Telemetry word) to determine where it is
in the message so it can interpret the various bits. The message in
subframes 1 - 3 repeats so once it has 30 seconds worth of data it
should have all the clock and ephemeris data necessary from that
satellite. In the unlikely event that there's an update to the subframe
1 - 3 data (only happens every hour or two) right at the time the unit
starts getting that data then I could see the need to repeat a 6-second
subframe, but I still don't see why you'd ever need to wait an entire
extra 30 seconds beyond the 30 seconds required to read one entire
frame's worth of data.
|
You are certainly correct--The manufacturers algorithm could
certainly put together the data in the right order to save
time.... the questions is do they? Probably some do and some don't.
| Quote: |
Worse yet, you may drive under a tree or otherwise encounter
an obstruction when the ephemeris data was being grabbed... and
may have to try again.... and again.... and...
Sure, but the conditions that Jack and I were postulating were for
error-free reception. With enough errors the time to get a fix could
well be infinite.
See GPS User Equipment Introduction - Sept 1996 (PDF Format)
http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf
This gives only a cursory overview of the navigation message format
(page 1-7) and refers to "Technical Characteristics of the Navstar GPS"
for more details. Do you have a link to that?
|
Try: http://www.navcen.uscg.gov/pubs/gps/icd200/default.htm
http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm
-Sam Wormley
http://edu-observatory.org/gps/gps_books.html |
|
| Back to top |
|
|
peter
Guest
|
| Posted:
Mon Oct 17, 2005 8:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Sam wrote:
| Quote: | All correlators my not grab their respective satellites at the
same time and my have to wait for the next 30 second interval.
|
Why would it have to wait for the full 30 seconds? Once the correlator
has locked onto the satellite signal the unit can start receiving and
storing the bits in the data channel. Then it looks for the frame
information (synch code in the Telemetry word) to determine where it is
in the message so it can interpret the various bits. The message in
subframes 1 - 3 repeats so once it has 30 seconds worth of data it
should have all the clock and ephemeris data necessary from that
satellite. In the unlikely event that there's an update to the subframe
1 - 3 data (only happens every hour or two) right at the time the unit
starts getting that data then I could see the need to repeat a 6-second
subframe, but I still don't see why you'd ever need to wait an entire
extra 30 seconds beyond the 30 seconds required to read one entire
frame's worth of data.
| Quote: | Worse yet, you may drive under a tree or otherwise encounter
an obstruction when the ephemeris data was being grabbed... and
may have to try again.... and again.... and...
|
Sure, but the conditions that Jack and I were postulating were for
error-free reception. With enough errors the time to get a fix could
well be infinite.
This gives only a cursory overview of the navigation message format
(page 1-7) and refers to "Technical Characteristics of the Navstar GPS"
for more details. Do you have a link to that? |
|
| Back to top |
|
|
Guest
|
| Posted:
Mon Oct 17, 2005 2:24 pm Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Whining about it sux more. You're never going to persuade
anyone not to do so by whining, so live with it.
On Sun, 16 Oct 2005 22:42:20 -0400, Steve Calvin
<calvins@optonline.net> wrote:
| Quote: | TLoewenberg wrote:
Hmmm, on placement, I try to position the gps as far forward on the dash as
possible. I also have a ram mount that positions the gps further back but
with the antenna still under the windshield. How would I know (or find out)
if the pcb battery has failed? Is it user replaceable? If the battery is
okay, how long does the III+ retain the almanac data? As for the small
bluetooth gps, I place it all the way forward in the center of the
windshield. The bluetooth gps is a Dell, who has agreed to replace it under
warranty. If the new one, acts the same way, I can only assume too many
trees and/or a gps unfriendly windshield.
Thx
TL
Gawd, top postint sux |
|
|
| Back to top |
|
|
Dale DePriest
Guest
|
| Posted:
Mon Oct 17, 2005 4:08 pm Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Sam Wormley wrote:
| Quote: | peter wrote:
Sam wrote:
All correlators my not grab their respective satellites at the
same time and my have to wait for the next 30 second interval.
Why would it have to wait for the full 30 seconds? Once the correlator
has locked onto the satellite signal the unit can start receiving and
storing the bits in the data channel. Then it looks for the frame
information (synch code in the Telemetry word) to determine where it is
in the message so it can interpret the various bits. The message in
subframes 1 - 3 repeats so once it has 30 seconds worth of data it
should have all the clock and ephemeris data necessary from that
satellite. In the unlikely event that there's an update to the subframe
1 - 3 data (only happens every hour or two) right at the time the unit
starts getting that data then I could see the need to repeat a 6-second
subframe, but I still don't see why you'd ever need to wait an entire
extra 30 seconds beyond the 30 seconds required to read one entire
frame's worth of data.
You are certainly correct--The manufacturers algorithm could
certainly put together the data in the right order to save
time.... the questions is do they? Probably some do and some don't.
|
However you still have to wait 30 seconds for that particular subframe
to be repeated. So it is not a full minute but could be very close.
Dale
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs |
|
| Back to top |
|
|
Jack Yeazel
Guest
|
| Posted:
Sat Oct 22, 2005 12:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Dale DePriest wrote:
| Quote: |
However you still have to wait 30 seconds for that particular subframe
to be repeated. So it is not a full minute but could be very close.
|
-Would you buy, then, 59 seconds?? (-8
--
Jack
Get general GPS information at: http://www.gpsinformation.net/ |
|
| Back to top |
|
|
peter
Guest
|
| Posted:
Sat Oct 22, 2005 12:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Jack wrote:
| Quote: | -Would you buy, then, 59 seconds??
|
Assuming a reasonably designed receiver that doesn't throw away data
already received the most I can see buying is 36 seconds in very rare
circumstances and generally a maximum of 30 seconds plus processing
time. Also assuming error-free reception of course. |
|
| Back to top |
|
|
Sam Wormley
Guest
|
| Posted:
Sat Oct 22, 2005 12:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
peter wrote:
| Quote: | Jack wrote:
-Would you buy, then, 59 seconds??
Assuming a reasonably designed receiver that doesn't throw away data
already received the most I can see buying is 36 seconds in very rare
circumstances and generally a maximum of 30 seconds plus processing
time. Also assuming error-free reception of course.
|
Or even 31 seconds! |
|
| Back to top |
|
|
Jack Yeazel
Guest
|
| Posted:
Sun Oct 23, 2005 9:33 pm Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Sam Wormley wrote:
| Quote: | -Would you buy, then, 59 seconds??
Assuming a reasonably designed receiver that doesn't throw away data
already received the most I can see buying is 36 seconds in very rare
circumstances and generally a maximum of 30 seconds plus processing
time. Also assuming error-free reception of course.
Or even 31 seconds!
|
Yes, 31 seconds is the practical minimum time... 59 seconds is the
maximum cold-start time under ideal conditions -as I understand it...
JY
--
Jack
Get general GPS information at: http://www.gpsinformation.net/ |
|
| Back to top |
|
|
peter
Guest
|
| Posted:
Tue Oct 25, 2005 12:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Jack wrote:
| Quote: | Assuming a reasonably designed receiver that doesn't throw away data
already received the most I can see buying is 36 seconds in very rare
circumstances and generally a maximum of 30 seconds plus processing
time. Also assuming error-free reception of course.
Or even 31 seconds!
Yes, 31 seconds is the practical minimum time... 59 seconds is the
maximum cold-start time under ideal conditions -as I understand it...
|
No, as explained previously, under normal conditions with error-free
reception the *maximum* time should be 30.02 seconds (one frame plus
one bit position, rounded off to 30 seconds) plus the processing time.
This is for what I consider a 'warm start' i.e. the unit has a valid
almanac but not a sufficiently current ephemeris. |
|
| Back to top |
|
|
Jack Yeazel
Guest
|
| Posted:
Tue Oct 25, 2005 12:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
peter wrote:
| Quote: |
Yes, 31 seconds is the practical minimum time... 59 seconds is the
maximum cold-start time under ideal conditions -as I understand it...
No, as explained previously, under normal conditions with error-free
reception the *maximum* time should be 30.02 seconds (one frame plus
one bit position, rounded off to 30 seconds) plus the processing time.
This is for what I consider a 'warm start' i.e. the unit has a valid
almanac but not a sufficiently current ephemeris.
|
-I give up... We're talking about cold starts... Warm starts can start
within just a few seconds...
Incidentally, I've been e-mailing Garmin asking if they will tell me
what constitutes a cold start and a warm start, but nothing yet... I'll
let the group know when they 'divulge' it...
--
Jack
Get general GPS information at: http://www.gpsinformation.net/ |
|
| Back to top |
|
|
Bob Greschke
Guest
|
| Posted:
Tue Oct 25, 2005 12:08 am Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
"Jack Yeazel" <jack@finalapproach.net> wrote in message
news:435D6033.204DCA8E@finalapproach.net...
| Quote: |
Incidentally, I've been e-mailing Garmin asking if they will tell me
what constitutes a cold start and a warm start, but nothing yet... I'll
let the group know when they 'divulge' it...
|
A "cold start" on the internal GPS on one of our seismic recorders (don't
know who makes the GPS engine) is defined as dumping the almanac and
ephemeris data and any sense of what time the GPS thinks it is and searching
from scratch.
Bob |
|
| Back to top |
|
|
Jack Yeazel
Guest
|
| Posted:
Tue Oct 25, 2005 4:08 pm Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Bob Greschke wrote:
| Quote: |
"Jack Yeazel" <jack@finalapproach.net> wrote in message
news:435D6033.204DCA8E@finalapproach.net...
Incidentally, I've been e-mailing Garmin asking if they will tell me
what constitutes a cold start and a warm start, but nothing yet... I'll
let the group know when they 'divulge' it...
A "cold start" on the internal GPS on one of our seismic recorders (don't
know who makes the GPS engine) is defined as dumping the almanac and
ephemeris data and any sense of what time the GPS thinks it is and searching
from scratch.
|
Well, here's Garmin's reply:
"Thank You for contacting Garmin International. I will be happy to help
you.
Anywhere between 30 minutes to an hour is considered a "warm start."
The
longer the unit remains powered off the more the satellites will change
overhead. This will take the unit longer to re-acquire the signal thus
indicating a "cold start.""
-As I remember it, they used to have a hard and fast time limit for
warm starts, but maybe they now they 'examin the situation' before
doing a cold start...??? The satellites themselves will indicate when
their last valid ephemeris was obtained and if different from that
stored in the unit will cause a cold start (at least for that
satellite)...
--
Jack
Get general GPS information at: http://www.gpsinformation.net/ |
|
| Back to top |
|
|
Dale DePriest
Guest
|
| Posted:
Wed Oct 26, 2005 4:08 pm Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
peter wrote:
| Quote: | Jack wrote:
Assuming a reasonably designed receiver that doesn't throw away data
already received the most I can see buying is 36 seconds in very rare
circumstances and generally a maximum of 30 seconds plus processing
time. Also assuming error-free reception of course.
Or even 31 seconds!
Yes, 31 seconds is the practical minimum time... 59 seconds is the
maximum cold-start time under ideal conditions -as I understand it...
No, as explained previously, under normal conditions with error-free
reception the *maximum* time should be 30.02 seconds (one frame plus
one bit position, rounded off to 30 seconds) plus the processing time.
This is for what I consider a 'warm start' i.e. the unit has a valid
almanac but not a sufficiently current ephemeris.
|
The maximum time is closer to 36 seconds while the minimum time is 18
seconds to collect the data. The 36 seconds comes about if the gps
starts at exactly the top of a minute plus just enough time to miss the
hand over word that is at the start of each message. Thus it will miss
that full 6 second interval. It will pick up the next 12 seconds of data
and then clock through the 12 seconds of not ephemeris data, finally it
will read the first 6 seconds of data that it missed. Thus 6 + 12 + 12 +
6 equals 36 seconds in the maximum condition. To this should be added
the time the unit needs to compute the fix data from the information it
just received. This is why Garmin typically says 45 seconds.
Dale
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs |
|
| Back to top |
|
|
Dale DePriest
Guest
|
| Posted:
Wed Oct 26, 2005 9:11 pm Post subject:
Re: GPS Satellite Searching in Connecticut |
|
|
Jack Yeazel wrote:
| Quote: |
peter wrote:
Yes, 31 seconds is the practical minimum time... 59 seconds is the
maximum cold-start time under ideal conditions -as I understand it...
No, as explained previously, under normal conditions with error-free
reception the *maximum* time should be 30.02 seconds (one frame plus
one bit position, rounded off to 30 seconds) plus the processing time.
This is for what I consider a 'warm start' i.e. the unit has a valid
almanac but not a sufficiently current ephemeris.
-I give up... We're talking about cold starts... Warm starts can start
within just a few seconds...
Incidentally, I've been e-mailing Garmin asking if they will tell me
what constitutes a cold start and a warm start, but nothing yet... I'll
let the group know when they 'divulge' it...
|
Garmin and the rest of the industry have different definitions of warm
and cold. It is best not to use these terms. I wish Garmin hadn't
redefined what Navstar said but we are stuck with it now. The discussion
is about the time it takes to get a fix when the almanac is known, the
current possition is known approximately, the time is known
approximately, the ephemeris is not known. That time ranges from 18
seconds to 36 seconds to download the emphemeris data plus the initial
find time for the satellite (usually a second or so) plus the time to
compute the fix (on the order of 5-8 seconds or so).
Dale
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs |
|
| Back to top |
|
|
|
|
|
|
FAQ
Memberlist
Register
Profile
Log in to check your private messages
Log in