May 16, 2018
Time-nuts Digest, Vol 166, Issue 31
Today's Topics:
1. Re: China, GCJ-02 & cartography (Bob kb8tq)
2. Re: TruePosition GPSDO Holdover Issues (gandalfg8@aol.com)
3. Re: TruePosition GPSDO Holdover Issues (Bob kb8tq)
4. Re: China, GCJ-02 & cartography (Eric Scace)
5. Re: TruePosition GPSDO Holdover Issues (gandalfg8@aol.com)
6. Re: TruePosition GPSDO Holdover Issues (Bob kb8tq)
7. Re: TruePosition GPSDO Holdover Issues (Hal Murray)
8. Re: TruePosition GPSDO Holdover Issues (Hal Murray)
9. Re: TruePosition GPSDO Holdover Issues (Bob kb8tq)
10. TruePosition GPSDO Holdover Issues (gandalfg8@aol.com)
11. Re: TruePosition GPSDO Holdover Issues (Björn)
12. TruePosition GPSDO Holdover Issues (Mark Sims)
13. Re: TruePosition GPSDO Holdover Issues (Gary E. Miller)
14. Re: TruePosition GPSDO Holdover Issues (Chase Turner)
15. Thunderbolt code phase measurement (Mark Sims)
16. Re: Thunderbolt code phase measurement (Peter Monta)
----------------------------------------------------------------------
Message: 1
Date: Tue, 15 May 2018 12:13:19 -0400
From: Bob kb8tq
To: Discussion of precise time and
frequency measurement
Subject: Re: [time-nuts] China, GCJ-02 and cartography
Message-ID: <4186549C-D142-410F-8485-B84A6101ED4F@n1k.org>
Content-Type:
text/plain; charset=utf-8
Hi
Running with a very normal WGS-84 GPS
“by the sea shore” can easily show you underwater. That
is very much a normal result of the
model. It does not tell you what high (or low) tide level is
going to
be at your location. That stuff is
simply to complex.
The rest of it …… a lot of
countries ( and states in the US ) run their own coordinate
systems. It is
not very unusual. Was *is* unusual is
that the data providers for this or that have not
re-normalized
the data. They have had to do that
pretty much everywhere else ….
Bob
On May 15, 2018, at 9:38 AM, Eric
Scace
wrote:
The following was published on an
email list to which I subscribe. Can others on this list can
shed more light on CGJ-02 vs WGS-84, and some of the
representations made in this article?
— Eric
The Problem with Chinese GPS
If you’re in a foreign country
and try to read a map, you may find it difficult -- unless
your host nation’s language is the same as your home
nation’s, the words are going to be different and,
assuming you’re not bilingual, will require some
translation. But the locations of the roads, rivers,
buildings, and the like should be the same, regardless of
whether the map is in English, Spanish, or Chinese, right?
Language aside, Google Maps should work the same everywhere,
right?
Well… no.
Pictured above is a map of the
China/Hong Kong border via Google Maps; you can see it
yourself by clicking here .
The map is your standard road map overlaid upon a satellite
image. As you can see, the roads -- the light grey lines --
don’t match up with reality. There are roundabouts which
purport to be in public parks, bridges which don’t exist,
and multi-lane highways which seem to be underwater. The
whole thing is a big navigational mess. Go far enough into
Hong Kong, though, and the problem abates.
What’s going on? The map data,
basically, is being lost in translation.
The world -- China aside -- uses
something called the World Geodetic System 1984
(“WGS-84”) as the basis for the digital maps. Virtually
all the navigation tools we use online today -- the maps
apps on our phones, the GPS systems in our cars, the missile
guidance systems in use by the military, and yes, Google
Maps -- all use WGS-84. China, though, goes its own way.
The Chinese use something called
GCJ-02, an alternative system which the cartography world
colloquially refers to it as the “Mars Coordinates” as
it may as well be made for another planet. The Google Maps
screenshot and link, above, shows the problem: the road map
data comes from the Chinese government, which uses GCJ-02,
but the satellite data is from a non-Chinese source and uses
WGS-84. (As China exerts control over, and takes
responsibility for mapping out the border between itself and
Hong Kong, the problem bleeds into the neighboring
pseudo-sovereign state.) The two data sets, effectively, are
speaking different languages.
China isn’t just trying to be
different, though; they’re trying to be difficult. The
government has long seen map data as a matter of national
security. There’s a “Surveying and Mapping Law of the
People's Republic of China ”
which greatly restricts who can make maps. One needs a
cartography license, one which comes with many strings; if
you’re creating digital map data, for example, it needs to
use GCJ-02 and has to be hosted on servers within China. And
this isn’t one of those anachronistic laws which go
ignored and unenforced. In 2015, for example, the country
announced that those who violate the law could face fines of
200,000 yuan (about $30,000 at the time) and, according to
CityLab ,
“if the violation is deemed serious enough, [those who run
afoul of th
e law] can even find themselves booked
on criminal charges.”
So why not just make a tool which
translates GCJ-02 to WGS-84? Well, there are a few, but
they’re typically hard to come by and not all that
reliable. Multinational corporations like Google don’t
want to deploy them as it could hurt their standing with the
Chinese government. And even if they did, the results
wouldn’t be great. GCJ-02 isn’t just an alternative
coordinate system; it’s an often unpredictable one. As
Wikipedia explains ,
“it uses an obfuscation algorithm which adds apparently
random offsets to both the latitude and longitude.” And
even if you can get around those issues, it won’t matter
much if you’re in China itself. If you use a mobile device
there, per Travel and Leisure ,
“Chinese geographic regulations demand that GPS functions
must eith
er be disabled on handheld devices or
they must be made to display a similar offset.”
So if you're traveling to China,
knowing Chinese may be a lot more helpful than you'd think.
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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------------------------------
Message: 2
Date: Tue, 15 May 2018 12:38:26 -0400
From: gandalfg8@aol.com
To: time-nuts@febo.com
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID: <16364aad1eb-179f-16cf@webjasstg-vaa14.srv.aolmail.net>
Content-Type: text/plain;
charset=utf-8
Thanks Bob, and others, for comments
on this.
From my observations with this
running in position hold mode following a self survey, and
based on previous experience in this location, my general
impression is still that the TruePosition GPSDO does seem
more prone to dropping into holdover than others I've used
here under similar circumstances.
Having said that though, now running on
a better sited antenna giving consistently higher signal
strengths and with always at least 5 sats indicated, not
counting PRN120:-), it hasn't dropped into holdover in the
past 40 hours or so, so it is only under more marginal
conditions that it would be evident.
As a bonus, the slightly tilted antenna
is so far looking to be a reasonable success, with the hole
to the north noticeably reduced and tending more to a closed
circle and signal levels generally higher all round too,
partially at least perhaps due to a change in antenna gain
but either way another can of worms opened and begging
further investigation:-)
Nigel, GM8PZR
From: Bob kb8tq
Discussion of precise time and
frequency measurement
Sent: Sun, 13 May 2018 19:05
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Hi
When you are looking at timing, the
SBAS / WAAS sat’s really don’t count as part of the
total
of 4 that you need for a basic fix.
There also iis the subtle distinction of “tracking” vs
“locked to”
on some devices. Tracking means we
might get adequate data soon and locked means it is
good enough to use on those devices.In
that case, only the “locked” sats count towards the
minimum of 4 that you must have.
Past the minimum of 4 rule, most
GPSDO’s also want to see that set of devices for some
period
of time before they come out of
holdover. You will drop in very quickly ( a second or two),
but come
out slowly ( many minutes). Local noise
can in some cases be enough to put you in holdover.
Bob
On May 13, 2018, at 1:13 PM,
gandalfg8--- via time-nuts
wrote:
The location at 55N, 5W, isn't
ideal, there's quite a large hole to the north but this
isn't something I've seen here before with any other GPS
module or GPSDO.
When first noticed I'm sure it was
whilst tracking six or seven sats, it was certainly five or
more, which is why I commented in the first place, it was
only later I thought there might be some correlation with it
tracking low numbers.
There doesn't seem to be any
adjustment for elevation mask on these, at least not via
Lady H, but I've switched now from a flat survey antenna to
a Symmetricom pod on a stub mast, so I can cheat a bit and
angle it south slightly:-)
It'll take some time to build up a
picture of the effect of that but it's tracking 8 sats at
the moment.
Inmarsat-3, PRN120, seems to have
joined in the mix now and keeps popping on and off the
bottom of the list but I'm not sure whether or not that
could contribute anything useful anyway.
Nigel, GM8PZR
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions
there.
------------------------------
Message: 3
Date: Tue, 15 May 2018 12:44:01 -0400
From: Bob kb8tq
To: gandalfg8@aol.com,
Discussion of precise time and frequency
measurement
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID:
Content-Type:
text/plain; charset=utf-8
Hi
If you are in the northern hemisphere
and looking at GPS satellite tracks, there will always be a
“hole”
in the track plots to the north. The
orbits do not cross either of the poles. More or less they
make it about
to Hadrian’s Wall and that’s it.
Bob
On May 15, 2018, at 12:38 PM,
gandalfg8--- via time-nuts
wrote:
Thanks Bob, and others, for
comments on this.
From my observations with this
running in position hold mode following a self survey, and
based on previous experience in this location, my general
impression is still that the TruePosition GPSDO does seem
more prone to dropping into holdover than others I've used
here under similar circumstances.
Having said that though, now
running on a better sited antenna giving consistently higher
signal strengths and with always at least 5 sats indicated,
not counting PRN120:-), it hasn't dropped into holdover in
the past 40 hours or so, so it is only under more marginal
conditions that it would be evident.
As a bonus, the slightly tilted
antenna is so far looking to be a reasonable success, with
the hole to the north noticeably reduced and tending more to
a closed circle and signal levels generally higher all round
too, partially at least perhaps due to a change in antenna
gain but either way another can of worms opened and begging
further investigation:-)
Nigel, GM8PZR
From: Bob kb8tq
Discussion of precise time and
frequency measurement
Sent: Sun, 13 May 2018 19:05
Subject: Re: [time-nuts]
TruePosition GPSDO Holdover Issues
Hi
When you are looking at timing,
the SBAS / WAAS sat’s really don’t count as part of the
total
of 4 that you need for a basic
fix. There also iis the subtle distinction of “tracking”
vs “locked to”
on some devices. Tracking means we
might get adequate data soon and locked means it is
good enough to use on those
devices.In that case, only the “locked” sats count
towards the
minimum of 4 that you must have.
Past the minimum of 4 rule, most
GPSDO’s also want to see that set of devices for some
period
of time before they come out of
holdover. You will drop in very quickly ( a second or two),
but come
out slowly ( many minutes). Local
noise can in some cases be enough to put you in holdover.
Bob
On May 13, 2018, at 1:13 PM,
gandalfg8--- via time-nuts
wrote:
The location at 55N, 5W, isn't
ideal, there's quite a large hole to the north but this
isn't something I've seen here before with any other GPS
module or GPSDO.
When first noticed I'm sure it
was whilst tracking six or seven sats, it was certainly five
or more, which is why I commented in the first place, it was
only later I thought there might be some correlation with it
tracking low numbers.
There doesn't seem to be any
adjustment for elevation mask on these, at least not via
Lady H, but I've switched now from a flat survey antenna to
a Symmetricom pod on a stub mast, so I can cheat a bit and
angle it south slightly:-)
It'll take some time to build
up a picture of the effect of that but it's tracking 8 sats
at the moment.
Inmarsat-3, PRN120, seems to
have joined in the mix now and keeps popping on and off the
bottom of the list but I'm not sure whether or not that
could contribute anything useful anyway.
Nigel, GM8PZR
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions
there.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions
there.
------------------------------
Message: 4
Date: Tue, 15 May 2018 14:11:37 -0400
From: Eric Scace
To: Discussion of precise time and
frequency measurement
Subject: Re: [time-nuts] China, GCJ-02
& cartography
Message-ID: <9CBE96EA-F423-432B-92E8-F79B978CAED8@scace.org>
Content-Type: text/plain;
charset="utf-8"
I live at the edge of Boston Harbor, in
the Charlestown Navy Yard.
Tide stations, such as this one at
Boston Harbor’s Fort Point Channel ,
report their tides against a locally-defined datum
of mean lower low water (MLLW, the average of the lower of
the two low tides reported each day, for all days observed
in the current national tidal datum epoch of 1983-2001).
For this station, MLLW = +5.51 ft
elevation, referenced to NAVD88.
There is a National Geodetic Survey
benchmark
close to the tide gage.
The benchmark record
states:
MY0555
*CURRENT SURVEY CONTROL
MY0555
______________________________________________________________________
MY0555* NAD 83(1986) POSITION- 42 21
18.1 (N) 071 03 03.4 (W)
HD_HELD2
MY0555* NAVD 88
ORTHO HEIGHT - 4.105 (meters)
13.47 (feet) ADJUSTED
MY0555
______________________________________________________________________
MY0555 GEOID HEIGHT
- -27.738 (meters)
GEOID12B
MY0555 DYNAMIC HEIGHT
- 4.104 (meters)
13.46 (feet) COMP
MY0555 MODELED GRAVITY -
980,381.5 (mgal)
NAVD 88
MY0555
MY0555 VERT ORDER
- FIRST CLASS II
As I understand it, here comes the
messy part of getting from NAVD88 geopotential geoid to
WGS84 ellipsoid. I used NOAA's on-line vertical data
transformation tool ,
using the 2009 May 16 benchmark recovery date specified in
its record.
long = -71.0509444
lat = +42.355028
NAVD88 height = +4.105 m
The WGS84 result was:
long = -71.0509458
lat = 42.3550371
height = -24.850 m ±0.076m
If I did this correctly, to
convert from NAVD88 height to WGS84 elevation at this
location requires subtracting 28.954m. The MLLW datum of
+5.51 ft (+1.679m) becomes -27.275m in WGS84.
The benchmark record above
states that the 4.105m NAVD88 height is -27.738m height in
GEOID12B at this location. One could double check the NAVD88
→ WGS84 conversion by starting with the GEOID12B height.
Sadly, GEOID12B does not appear to be an option for the
vertical data transformation tool that I used.
But I’m a complete novice at
these transformations.
— Eric
p.s.: To confuse matter further, at the
moment my iPhone’s Compass tool reports my current height
as "50 feet”. I don’t know what is the basis for that
determination, but it doesn’t seem correct for a NAVD88 or
WGS84 height.
On 2018 May 15, at 12:13 , Bob
kb8tq
wrote:
Running with a very normal WGS-84
GPS “by the sea shore” can easily show you underwater.
That
is very much a normal result of
the model. It does not tell you what high (or low) tide
level is going to
be at your location. That stuff is
simply to complex.
-------------- next part
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------------------------------
Message: 5
Date: Tue, 15 May 2018 14:24:47 -0400
From: gandalfg8@aol.com
To: time-nuts@febo.com
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID: <163650c30b7-19ac-1e79@webjasstg-vaa54.srv.aolmail.net>
Content-Type: text/plain;
charset=utf-8
Yes, and we're quite a bit north of
Hadrian's wall:-)
I've always been aware of the hole,
it's just that I've always pointed GPS antennas straight up
without stopping before to consider whether or not that's
necessarilly the best option, but now it occurs to me that
it might not be for this location.
Whatever the optimum might be there's
obviously going to be a limit but I do think it might worth
pursuing.
Web sites such as "In The Sky.org" can
generate useful plots for any specified location and time
but I don't know if there's anything that will allow
building a projected map over a period, and I don't suppose
there's going to be anything anyway that allows
experimatation with antenna angle etc, so that really leaves
using lady H to generate the plots in real time, which she
does do really well but for something like this might be
akin to watching grass grow:-)
Nigel, GM8PZR
-----Original Message-----
From: Bob kb8tq
To: gandalfg8 ;
Discussion of precise time and frequency measurement
Sent: Tue, 15 May 2018 17:44
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Hi
If you are in the northern hemisphere
and looking at GPS satellite tracks, there will always be a
“hole”
in the track plots to the north. The
orbits do not cross either of the poles. More or less they
make it about
to Hadrian’s Wall and that’s it.
Bob
On May 15, 2018, at 12:38 PM,
gandalfg8--- via time-nuts
wrote:
Thanks Bob, and others, for
comments on this.
From my observations with this
running in position hold mode following a self survey, and
based on previous experience in this location, my general
impression is still that the TruePosition GPSDO does seem
more prone to dropping into holdover than others I've used
here under similar circumstances.
Having said that though, now
running on a better sited antenna giving consistently higher
signal strengths and with always at least 5 sats indicated,
not counting PRN120:-), it hasn't dropped into holdover in
the past 40 hours or so, so it is only under more marginal
conditions that it would be evident.
As a bonus, the slightly tilted
antenna is so far looking to be a reasonable success, with
the hole to the north noticeably reduced and tending more to
a closed circle and signal levels generally higher all round
too, partially at least perhaps due to a change in antenna
gain but either way another can of worms opened and begging
further investigation:-)
Nigel, GM8PZR
From: Bob kb8tq
Discussion of precise time and
frequency measurement
Sent: Sun, 13 May 2018 19:05
Subject: Re: [time-nuts]
TruePosition GPSDO Holdover Issues
Hi
When you are looking at timing,
the SBAS / WAAS sat’s really don’t count as part of the
total
of 4 that you need for a basic
fix. There also iis the subtle distinction of “tracking”
vs “locked to”
on some devices. Tracking means we
might get adequate data soon and locked means it is
good enough to use on those
devices.In that case, only the “locked” sats count
towards the
minimum of 4 that you must have.
Past the minimum of 4 rule, most
GPSDO’s also want to see that set of devices for some
period
of time before they come out of
holdover. You will drop in very quickly ( a second or two),
but come
out slowly ( many minutes). Local
noise can in some cases be enough to put you in holdover.
Bob
On May 13, 2018, at 1:13 PM,
gandalfg8--- via time-nuts
wrote:
The location at 55N, 5W, isn't
ideal, there's quite a large hole to the north but this
isn't something I've seen here before with any other GPS
module or GPSDO.
When first noticed I'm sure it
was whilst tracking six or seven sats, it was certainly five
or more, which is why I commented in the first place, it was
only later I thought there might be some correlation with it
tracking low numbers.
There doesn't seem to be any
adjustment for elevation mask on these, at least not via
Lady H, but I've switched now from a flat survey antenna to
a Symmetricom pod on a stub mast, so I can cheat a bit and
angle it south slightly:-)
It'll take some time to build
up a picture of the effect of that but it's tracking 8 sats
at the moment.
Inmarsat-3, PRN120, seems to
have joined in the mix now and keeps popping on and off the
bottom of the list but I'm not sure whether or not that
could contribute anything useful anyway.
Nigel, GM8PZR
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions
there.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions
there.
------------------------------
Message: 6
Date: Tue, 15 May 2018 14:32:31 -0400
From: Bob kb8tq
To: gandalfg8@aol.com,
Discussion of precise time and frequency
measurement
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID: <14414F15-42E2-4E4C-A13F-BA48337E55E1@n1k.org>
Content-Type:
text/plain; charset=utf-8
Hi
Most GPS antennas will do fine as close
to the horizon as you would ever want to get. Tilting will
simply make multi path worse.
One thing to consider - if you get far
enough north, the “hole” closes and you can get sat
tracks from the
other side of the north pole. Yes they
are a *long* ways away. It’s no worse than a lot of tracks
that GPS
thinks are perfectly fine to use. For
timing they aren’t going to do much good. They will
improve a navigation
fix.
Bob
On May 15, 2018, at 2:24 PM,
gandalfg8--- via time-nuts
wrote:
Yes, and we're quite a bit north
of Hadrian's wall:-)
I've always been aware of the
hole, it's just that I've always pointed GPS antennas
straight up without stopping before to consider whether or
not that's necessarilly the best option, but now it occurs
to me that it might not be for this location.
Whatever the optimum might be
there's obviously going to be a limit but I do think it
might worth pursuing.
Web sites such as "In The Sky.org"
can generate useful plots for any specified location and
time but I don't know if there's anything that will allow
building a projected map over a period, and I don't suppose
there's going to be anything anyway that allows
experimatation with antenna angle etc, so that really leaves
using lady H to generate the plots in real time, which she
does do really well but for something like this might be
akin to watching grass grow:-)
Nigel, GM8PZR
-----Original Message-----
From: Bob kb8tq
To: gandalfg8 ;
Discussion of precise time and frequency measurement
Sent: Tue, 15 May 2018 17:44
Subject: Re: [time-nuts]
TruePosition GPSDO Holdover Issues
Hi
If you are in the northern
hemisphere and looking at GPS satellite tracks, there will
always be a “hole”
in the track plots to the north.
The orbits do not cross either of the poles. More or less
they make it about
to Hadrian’s Wall and that’s
it.
Bob
On May 15, 2018, at 12:38 PM,
gandalfg8--- via time-nuts
wrote:
Thanks Bob, and others, for
comments on this.
From my observations with this
running in position hold mode following a self survey, and
based on previous experience in this location, my general
impression is still that the TruePosition GPSDO does seem
more prone to dropping into holdover than others I've used
here under similar circumstances.
Having said that though, now
running on a better sited antenna giving consistently higher
signal strengths and with always at least 5 sats indicated,
not counting PRN120:-), it hasn't dropped into holdover in
the past 40 hours or so, so it is only under more marginal
conditions that it would be evident.
As a bonus, the slightly
tilted antenna is so far looking to be a reasonable success,
with the hole to the north noticeably reduced and tending
more to a closed circle and signal levels generally higher
all round too, partially at least perhaps due to a change in
antenna gain but either way another can of worms opened and
begging further investigation:-)
Nigel, GM8PZR
From: Bob kb8tq
Discussion of precise time and
frequency measurement
Sent: Sun, 13 May 2018 19:05
Subject: Re: [time-nuts]
TruePosition GPSDO Holdover Issues
Hi
When you are looking at
timing, the SBAS / WAAS sat’s really don’t count as part
of the total
of 4 that you need for a basic
fix. There also iis the subtle distinction of “tracking”
vs “locked to”
on some devices. Tracking
means we might get adequate data soon and locked means it is
good enough to use on those
devices.In that case, only the “locked” sats count
towards the
minimum of 4 that you must
have.
Past the minimum of 4 rule,
most GPSDO’s also want to see that set of devices for some
period
of time before they come out
of holdover. You will drop in very quickly ( a second or
two), but come
out slowly ( many minutes).
Local noise can in some cases be enough to put you in
holdover.
Bob
> On May 13, 2018, at 1:13
PM, gandalfg8--- via time-nuts
wrote:
>
> The location at 55N, 5W,
isn't ideal, there's quite a large hole to the north but
this isn't something I've seen here before with any other
GPS module or GPSDO.
>
> When first noticed I'm
sure it was whilst tracking six or seven sats, it was
certainly five or more, which is why I commented in the
first place, it was only later I thought there might be some
correlation with it tracking low numbers.
>
> There doesn't seem to be
any adjustment for elevation mask on these, at least not via
Lady H, but I've switched now from a flat survey antenna to
a Symmetricom pod on a stub mast, so I can cheat a bit and
angle it south slightly:-)
>
> It'll take some time to
build up a picture of the effect of that but it's tracking 8
sats at the moment.
> Inmarsat-3, PRN120, seems
to have joined in the mix now and keeps popping on and off
the bottom of the list but I'm not sure whether or not that
could contribute anything useful anyway.
>
> Nigel, GM8PZR
>
>
>
_______________________________________________
> time-nuts mailing list --
time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the
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Message: 7
Date: Tue, 15 May 2018 12:27:56 -0700
From: Hal Murray
To: Discussion of precise time and
frequency measurement
Cc: hmurray@megapathdsl.net
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID:
<20180515192757.0612E406061@ip-64-139-1-69.sjc.megapath.net>
Content-Type: text/plain;
charset=us-ascii
Web sites such as "In The Sky.org"
can generate useful plots for any
specified location and time but I
don't know if there's anything that will
allow building a projected map
over a period, and I don't suppose there's
going to be anything anyway that
allows experimatation with antenna angle
etc, so that really leaves using
lady H to generate the plots in real time,
which she does do really well but
for something like this might be akin to
watching grass grow:-)
If you are far enough north, you are in
the hole, but you can see the other
side of the hole over the pole.
It's not hard to make your own
plots. This is from 66 38' north.
http://users.megapathdsl.net/~hmurray/time-nuts/Arctic/Polar-A.png
If anybody wants to play, I'll put the
code on the web.
There are several steps. First is
to collect the data. I have a python
script that grabs everything from a
NMEA device and logs each line with a
time stamp. The GPS orbit period
is 12 hours so you need 12 hours of data to
see everything. But there are 30
satellites, so a few hours will show the
hole.
The next step is to extract the data
into a useful format. That's another
python script. Then, just feed
that to gunplot.
--
These are my opinions. I hate
spam.
------------------------------
Message: 8
Date: Tue, 15 May 2018 12:52:58 -0700
From: Hal Murray
To: Discussion of precise time and
frequency measurement
Cc: hmurray@megapathdsl.net
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID:
<20180515195258.98694406061@ip-64-139-1-69.sjc.megapath.net>
Content-Type: text/plain;
charset=us-ascii
kb8tq@n1k.org
said:
One thing to consider - if you get
far enough north, the ���hole��� closes and
you can get sat tracks from the
other side of the north pole. Yes they are a
*long* ways away. It���s no
worse than a lot of tracks that GPS thinks are
perfectly fine to use. For timing
they aren���t going to do much good. They
will improve a navigation
Neat. Thanks. That raises
several questions.
How high do satellites get if
you are at a pole?
What is the best or worst
latitude for timing?
What is the best/worst latitude
for doing a survey?
--
These are my opinions. I hate
spam.
------------------------------
Message: 9
Date: Tue, 15 May 2018 15:56:17 -0400
From: Bob kb8tq
To: Discussion of precise time and
frequency measurement
Cc: hmurray@megapathdsl.net
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID: <23A5B582-BAD5-42D0-BEF2-39174297A937@n1k.org>
Content-Type:
text/plain; charset=utf-8
Hi
On May 15, 2018, at 3:52 PM, Hal
Murray
wrote:
kb8tq@n1k.org
said:
One thing to consider - if you
get far enough north, the “hole” closes and
you can get sat tracks from
the other side of the north pole. Yes they are a
*long* ways away. It’s no
worse than a lot of tracks that GPS thinks are
perfectly fine to use. For
timing they aren’t going to do much good. They
will improve a navigation
Neat. Thanks. That
raises several questions.
How high do satellites get
if you are at a pole?
What is the best or worst
latitude for timing?
What is the best/worst
latitude for doing a survey?
The answer for both timing and survey
is that you want to be on the equator as far
as sat elevation / tracks go. As long
as you are “in the tropics” it is just about as good.
Bob
--
These are my opinions. I
hate spam.
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------------------------------
Message: 10
Date: Tue, 15 May 2018 16:19:03 -0400
From: gandalfg8@aol.com
To: time-nuts@febo.com
Subject: [time-nuts] TruePosition GPSDO
Holdover Issues
Message-ID: <1636574ce88-179b-28bc@webjasstg-vab14.srv.aolmail.net>
Content-Type: text/plain;
charset=utf-8
Hi Hal, and thanks for that.
In my case at least there's no problem
with making plots from live data as Lady Heather will do
that for me,
it's just the time taken to collect the
data for each plot that could be an issue.
I'd just been wondering aloud whether
any of the online simulations for instantaneous plots
against location
could be persuaded to quickly build a
longer term prediction, but then it occured to me that as I
was looking to break
the rules anyway the chances of that
might be quite small:-)
Nigel, GM8PZR
It's not hard to make your own
plots. This is from 66 38' north.
http://users.megapathdsl.net/~hmurray/time-nuts/Arctic/Polar-A.png
If anybody wants to play, I'll put the
code on the web.
There are several steps. First is
to collect the data. I have a python
script that grabs everything from a
NMEA device and logs each line with a
time stamp. The GPS orbit period
is 12 hours so you need 12 hours of data to
see everything. But there are 30
satellites, so a few hours will show the
hole.
The next step is to extract the data
into a useful format. That's another
python script. Then, just feed
that to gunplot.
------------------------------
Message: 11
Date: Tue, 15 May 2018 22:46:16 +0200
From: Björn
To: Discussion of precise time and
frequency measurement
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID:
Content-Type:
text/plain; charset=utf-8
There are sometimes rapidly varying
ionospheric conditions in some tropical/equator areas -
older generation surveying receivers could loose phase look
(cycle slips) due to temporal variation. Spatial variation
can break the assumption that two receivers doing “RTK”
will not have equal iono delay for each satellite - this
would reduce base line lengths that could be achieved.
Don’t have a reference at hand, but
there are publications about CORS station networks in Brazil
discussing these phenomena behind the ION paywall.
/Björn
Neat. Thanks. That
raises several questions.
How high do satellites get if
you are at a pole?
What is the best or worst
latitude for timing?
What is the best/worst
latitude for doing a survey?
The answer for both timing and
survey is that you want to be on the equator as far
as sat elevation / tracks go. As
long as you are “in the tropics” it is just about as
good.
Bob
--
These are my opinions. I
hate spam.
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------------------------------
Message: 12
Date: Tue, 15 May 2018 21:20:49 +0000
From: Mark Sims
To: "time-nuts@febo.com"
Subject: [time-nuts] TruePosition GPSDO
Holdover Issues
Message-ID:
Content-Type: text/plain;
charset="iso-8859-1"
Lasy Heather can read a receiver data
capture file as a "simulation" file. Use the
/rs=filename command line option. You can also
specify the /0 command line option (don't use serial port)
and /rx= command line option to specify the receiver
type. The simulation file reads in at around 20-30
times faster than real time (goes faster when the mouse
cursor is kept out of the plot area),
------------------
First is to collect the data.
I have a python
script that grabs everything from a
NMEA device and logs each line with a
time stamp. The GPS orbit period
is 12 hours so you need 12 hours of data to
see everything. But there are 30
satellites, so a few hours will show the
hole.
The next step is to extract the data
into a useful format. That's another
python script. Then, just feed
that to gunplot.
------------------------------
Message: 13
Date: Tue, 15 May 2018 14:22:03 -0700
From: "Gary E. Miller"
To: Discussion of precise time and
frequency measurement
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID: <20180515142203.2b7ec893@spidey.rellim.com>
Content-Type: text/plain;
charset=UTF-8
Hal!
On Tue, 15 May 2018 12:52:58 -0700
Hal Murray
wrote:
Neat. Thanks. That
raises several questions.
How high do satellites get
if you are at a pole?
44.6 degrees.
RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington
Ave., Suite E, Bend, OR 97703
gem@rellim.com
Tel:+1 541 382 8588
Veritas liberabit vos. -- Quid est veritas?
"If you can’t measure
it, you can’t improve it." - Lord Kelvin
------------------------------
Message: 14
Date: Tue, 15 May 2018 17:32:08 -0400
From: Chase Turner
To: Discussion of precise time and
frequency measurement
Subject: Re: [time-nuts] TruePosition
GPSDO Holdover Issues
Message-ID:
Content-Type: text/plain;
charset="UTF-8"
On Tue, May 15, 2018 at 3:27 PM, Hal
Murray
wrote:
If you are far enough north, you
are in the hole, but you can see the
other
side of the hole over the pole.
It's not hard to make your own
plots. This is from 66 38' north.
http://users.megapathdsl.net/~hmurray/time-nuts/Arctic/Polar-A.png
If anybody wants to play, I'll put
the code on the web.
There are several steps.
First is to collect the data. I have a python
script that grabs everything from
a NMEA device and logs each line with a
time stamp. The GPS orbit
period is 12 hours so you need 12 hours of data
to
see everything. But there
are 30 satellites, so a few hours will show the
hole.
The next step is to extract the
data into a useful format. That's another
python script. Then, just
feed that to gunplot.
Hi Hal,
I'd like to see a copy of the code,
please.
Thanks!
--
Chase Turner
W4TI
http://w4ti.com
------------------------------
Message: 15
Date: Wed, 16 May 2018 00:27:38 +0000
From: Mark Sims
To: "time-nuts@febo.com"
Subject: [time-nuts] Thunderbolt code
phase measurement
Message-ID:
Content-Type: text/plain;
charset="iso-8859-1"
The Trimble Thunderbolt has a message
(0x5A) that outputs raw receiver measurement data. One
value is "code phase" (along with PRN, sample length, sig
level, dopple, and time-of-measurement). This is a
single precision floating point number in units of 1/16 of a
chip. Does anybody know how to massage this value
into either a carrier phase (in cycles) or a pseudorange (in
meters). The reported code phase values tend to
be in the 1000 ... 10000 range.
------------------------------
Message: 16
Date: Tue, 15 May 2018 21:58:50 -0700
From: Peter Monta
To: Discussion of precise time and
frequency measurement
Subject: Re: [time-nuts] Thunderbolt
code phase measurement
Message-ID:
Content-Type: text/plain;
charset="UTF-8"
... One value is "code
phase" (along with PRN, sample length, sig level,
dopple, and
time-of-measurement). This is a single precision
floating
point number in units of 1/16 of a
chip. Does anybody know how to massage
this value into either a carrier
phase (in cycles) or a pseudorange (in
meters). The reported
code phase values tend to be in the 1000 ... 10000
range.
"Code phase" represents where you are
along the 1023-bit C/A code (each bit
or "chip" of this code lasts ~1
microsecond or ~300 meters). The
scaled-by-16 code phase will thus range
from 0 to 16*1023. To get the full
pseudorange, though, suitable for
placing into a RINEX file for example,
you need to also add in the integer
number of code periods from the
satellite to you. (Another way to
put it is that the code phase is the
pseudorange modulo 1 millisecond, the
duration of 1023 chips.) Each
pseudorange also contains the
as-yet-unknown receiver clock bias---that is
the "pseudo" in "pseudorange".
The additional information comes from
the data content of the C/A message,
which has a 20 ms period, a frame
structure, and timestamps for the
frames. Hopefully all that is
included somewhere in the receiver raw data.
Carrier phase is a different beast
altogether and comes from the carrier
tracking loop, not the code tracking
loop that gives the code phase. It's
simpler in some sense, because there is
none of this burdensome,
complicated pseudorange-assembly
process from disparate data sources like
code phase, bit sync, and timestamps;
instead it's just a simple phase.
The ambiguity interval is very short,
though, which complicates its use in
a navigation solution.
If you know your position to within 150
kilometers (0.5 ms), you can
dispense with the pseudorange-assembly
arithmetic and just use the code
phase directly, after adding in the
appropriate integer number of
milliseconds, only one of which will
put you within your known
300-kilometer-diameter (1 ms) sphere.
Cheers,
Peter
------------------------------
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