Re: [time-nuts] GPS first LO need to be locked?

[paul swed]

Perhaps someone could start a new thread with a different title since this
has evolved to something else then my request.
By the way the response to my question was answered by the group and its
true the first LO does not need to be locked for at least code tracking. I
really didn't prove its needed for carrier tracking.
Thanks everyone.
Paul
WB8TSL

On Tue, Apr 4, 2017 at 9:55 AM, jimlux <jim...@earthlink.net> wrote:

> On 4/3/17 9:32 PM, Logan Cummings wrote:
>
>> Dave,
>>
>> I was able to find [2] here:
>> http://web.stanford.edu/group/scpnt/gpslab/pubs/papers/Akos_
>> IONGPS_2003_3FreqRX.pdf
>>
>>
>
> So those folks were trying to use 1 ADC for all three bands, so they had
> to choose a sampling rate that lets them separate the signals later in
> software.
>
> But that ADC is a MAX104 - a 1GSPS, 8 bit converter - that draws 5 Watts!!!
>
> I'm not sure that's a good trade against a 1 or 2 bit converter for each
> band, in terms of the downstream data rate and processing.
>
>
>
>
>
> [3] was harder, and I don't have a link but a google search for the title
>> in quotes got me a link on semanticscholar that let me download the PDF.
>>
>> Interesting stuff!
>>
>> Hope that helps,
>> -Logan
>>
>> On Fri, Mar 31, 2017 at 9:17 AM, David C. Partridge <
>> david.partri...@perdrix.co.uk> wrote:
>>
>> links [2] and [3] give 404 errors
>>>
>>> Dave
>>>
>>> -Original Message-
>>> From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Attila
>>> Kinali
>>> Sent: 31 March 2017 12:35
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] GPS first LO need to be locked?
>>>
>>>  [2] "A Prototyping Platform for Multi-Frequency GNSS Receivers", by
>>> Akos,
>>> Ene and Thor, 2003 http://waas.stanford.edu/~wwu/papers/gps/PDF/
>>> AkosIONGPS033FreqRX.pdf
>>>
>>> [3] "Design of a GPS and Galileo Multi-Frequency Front-End", by Parada,
>>> Chastellain, Botteron, Tawk, Farine, 2009 http://202.194.20.8/proc/
>>> VTC09Spring/DATA/04-04-01.PDF
>>>
>>>
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Re: [time-nuts] GPS first LO need to be locked?

[jimlux]

On 4/3/17 9:32 PM, Logan Cummings wrote:

Dave,

I was able to find [2] here:
http://web.stanford.edu/group/scpnt/gpslab/pubs/papers/Akos_IONGPS_2003_3FreqRX.pdf




So those folks were trying to use 1 ADC for all three bands, so they had 
to choose a sampling rate that lets them separate the signals later in 
software.


But that ADC is a MAX104 - a 1GSPS, 8 bit converter - that draws 5 Watts!!!

I'm not sure that's a good trade against a 1 or 2 bit converter for each 
band, in terms of the downstream data rate and processing.







[3] was harder, and I don't have a link but a google search for the title
in quotes got me a link on semanticscholar that let me download the PDF.

Interesting stuff!

Hope that helps,
-Logan

On Fri, Mar 31, 2017 at 9:17 AM, David C. Partridge <
david.partri...@perdrix.co.uk> wrote:


links [2] and [3] give 404 errors

Dave

-Original Message-
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Attila
Kinali
Sent: 31 March 2017 12:35
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS first LO need to be locked?

 [2] "A Prototyping Platform for Multi-Frequency GNSS Receivers", by Akos,
Ene and Thor, 2003 http://waas.stanford.edu/~wwu/papers/gps/PDF/
AkosIONGPS033FreqRX.pdf

[3] "Design of a GPS and Galileo Multi-Frequency Front-End", by Parada,
Chastellain, Botteron, Tawk, Farine, 2009 http://202.194.20.8/proc/
VTC09Spring/DATA/04-04-01.PDF


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Re: [time-nuts] GPS first LO need to be locked?

[Logan Cummings]

Dave,

I was able to find [2] here:
http://web.stanford.edu/group/scpnt/gpslab/pubs/papers/Akos_IONGPS_2003_3FreqRX.pdf

[3] was harder, and I don't have a link but a google search for the title
in quotes got me a link on semanticscholar that let me download the PDF.

Interesting stuff!

Hope that helps,
-Logan

On Fri, Mar 31, 2017 at 9:17 AM, David C. Partridge <
david.partri...@perdrix.co.uk> wrote:

> links [2] and [3] give 404 errors
>
> Dave
>
> -Original Message-
> From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Attila
> Kinali
> Sent: 31 March 2017 12:35
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPS first LO need to be locked?
>
>  [2] "A Prototyping Platform for Multi-Frequency GNSS Receivers", by Akos,
> Ene and Thor, 2003 http://waas.stanford.edu/~wwu/papers/gps/PDF/
> AkosIONGPS033FreqRX.pdf
>
> [3] "Design of a GPS and Galileo Multi-Frequency Front-End", by Parada,
> Chastellain, Botteron, Tawk, Farine, 2009 http://202.194.20.8/proc/
> VTC09Spring/DATA/04-04-01.PDF
>
>
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Re: [time-nuts] GPS first LO need to be locked?

[Bob Camp]

Hi


> On Apr 1, 2017, at 11:18 AM, paul swed  wrote:
> 
> Thanks everyone but I am working on an austron 2201a so all the discussions
> on modern methods won't help. Whats is interesting is indeed the 2201 down
> converts to 80KHz and the does sample in an IQ fashion. Its all discreet
> chips and such.

If you have a free running VCO in the head end, then indeed you need to lock it 
to 
something. An unlocked VCO will not be close enough to frequency to do you any 
good.
The 8660 may or may not be close enough in free run mode. It depends a lot on 
what standard you have in yours. 


> Easily traceable and logical.
> I think I have what I asked for and am experimenting with active mixers and
> IFs made of minicircuit gain stages.
> I am using a commercial antenna with 34 db of gain. It says 50db I question
> that.

That is a very normal antenna gain spec and it was quite common in the era the
device you have was designed. I run a number of GPS gizmos that need a 50
db antenna on them. They *might* work with a 40 db setup. They do not work 
with something in the 20 to 30 db range. If you watch for a while (as in 6 to 
12 
months) you can indeed get good old 50 db gain antennas on eBay pretty cheap.

Bob

> But lots of gain to a HP IAM 81008 active mixer low drive LO. Then a 40db
> at least 75 MHz IF. (Pretty sure this is overkill.)
> The LO is a HP 8660c for now. Locked to a TBolt.
> Thats the reason for the question. I can shift the 8660 to the ausytron 10
> MHz.
> Regards
> Paul.
> WB8TSL
> 
> On Fri, Mar 31, 2017 at 7:47 PM, Bob kb8tq  wrote:
> 
>> Hi
>> 
>> 
>> There are a lot of GPS chips that do an I/Q mix down to a low IF. It’s
>> then (re) sampled from there. The “LO” in this case would down convert to
>> the low IF ….
>> 
>> Bob
>> 
>>> On Mar 31, 2017, at 6:08 PM, Magnus Danielson <
>> mag...@rubidium.dyndns.org> wrote:
>>> 
>>> God natt Attila,
>>> 
>>> On 03/31/2017 11:29 PM, Attila Kinali wrote:
 God kväll Magnus,
 
 On Fri, 31 Mar 2017 21:19:00 +0200
 Magnus Danielson  wrote:
 
> Still fills the function of LO, as the sample and hold operates as a
> mixer and the fold-down can be seen as an overtone mix followed by a
> sampling of the mix product, so well, it's about the same thing.
 
 "Harmonic mixer" is the word you are looking for :-)
>>> 
>>> Not necessarily. It could be a locked oscillator too.
>>> Harmonic mixer is another way to go.
>>> 
>>> Cheers,
>>> Magnus
>>> ___
>>> time-nuts mailing list -- time-nuts@febo.com
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Re: [time-nuts] GPS first LO need to be locked?

[paul swed]

Thanks everyone but I am working on an austron 2201a so all the discussions
on modern methods won't help. Whats is interesting is indeed the 2201 down
converts to 80KHz and the does sample in an IQ fashion. Its all discreet
chips and such.
Easily traceable and logical.
I think I have what I asked for and am experimenting with active mixers and
IFs made of minicircuit gain stages.
I am using a commercial antenna with 34 db of gain. It says 50db I question
that.
But lots of gain to a HP IAM 81008 active mixer low drive LO. Then a 40db
at least 75 MHz IF. (Pretty sure this is overkill.)
The LO is a HP 8660c for now. Locked to a TBolt.
Thats the reason for the question. I can shift the 8660 to the ausytron 10
MHz.
Regards
Paul.
WB8TSL

On Fri, Mar 31, 2017 at 7:47 PM, Bob kb8tq  wrote:

> Hi
>
>
> There are a lot of GPS chips that do an I/Q mix down to a low IF. It’s
> then (re) sampled from there. The “LO” in this case would down convert to
> the low IF ….
>
> Bob
>
> > On Mar 31, 2017, at 6:08 PM, Magnus Danielson <
> mag...@rubidium.dyndns.org> wrote:
> >
> > God natt Attila,
> >
> > On 03/31/2017 11:29 PM, Attila Kinali wrote:
> >> God kväll Magnus,
> >>
> >> On Fri, 31 Mar 2017 21:19:00 +0200
> >> Magnus Danielson  wrote:
> >>
> >>> Still fills the function of LO, as the sample and hold operates as a
> >>> mixer and the fold-down can be seen as an overtone mix followed by a
> >>> sampling of the mix product, so well, it's about the same thing.
> >>
> >> "Harmonic mixer" is the word you are looking for :-)
> >
> > Not necessarily. It could be a locked oscillator too.
> > Harmonic mixer is another way to go.
> >
> > Cheers,
> > Magnus
> > ___
> > time-nuts mailing list -- time-nuts@febo.com
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> mailman/listinfo/time-nuts
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Re: [time-nuts] GPS first LO need to be locked?

[Bob kb8tq]

Hi


There are a lot of GPS chips that do an I/Q mix down to a low IF. It’s then 
(re) sampled from there. The “LO” in this case would down convert to the low IF 
….

Bob

> On Mar 31, 2017, at 6:08 PM, Magnus Danielson  
> wrote:
> 
> God natt Attila,
> 
> On 03/31/2017 11:29 PM, Attila Kinali wrote:
>> God kväll Magnus,
>> 
>> On Fri, 31 Mar 2017 21:19:00 +0200
>> Magnus Danielson  wrote:
>> 
>>> Still fills the function of LO, as the sample and hold operates as a
>>> mixer and the fold-down can be seen as an overtone mix followed by a
>>> sampling of the mix product, so well, it's about the same thing.
>> 
>> "Harmonic mixer" is the word you are looking for :-)
> 
> Not necessarily. It could be a locked oscillator too.
> Harmonic mixer is another way to go.
> 
> Cheers,
> Magnus
> ___
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Re: [time-nuts] GPS first LO need to be locked?

[Magnus Danielson]

God natt Attila,

On 03/31/2017 11:29 PM, Attila Kinali wrote:

God kväll Magnus,

On Fri, 31 Mar 2017 21:19:00 +0200
Magnus Danielson  wrote:


Still fills the function of LO, as the sample and hold operates as a
mixer and the fold-down can be seen as an overtone mix followed by a
sampling of the mix product, so well, it's about the same thing.


"Harmonic mixer" is the word you are looking for :-)


Not necessarily. It could be a locked oscillator too.
Harmonic mixer is another way to go.

Cheers,
Magnus
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Re: [time-nuts] GPS first LO need to be locked?

[Attila Kinali]

God kväll Magnus,

On Fri, 31 Mar 2017 21:19:00 +0200
Magnus Danielson  wrote:

> Still fills the function of LO, as the sample and hold operates as a 
> mixer and the fold-down can be seen as an overtone mix followed by a 
> sampling of the mix product, so well, it's about the same thing.

"Harmonic mixer" is the word you are looking for :-)

Attila Kinali

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor
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Re: [time-nuts] GPS first LO need to be locked?

[Magnus Danielson]

Hi Jim,

On 03/31/2017 01:07 AM, jimlux wrote:

BTW a lot of GPS receivers don't have a "first LO".. they are more like
a Tuned RF receiver - an input BPF for L1, L2, or L5, then direct
sampling at around 30-40 MHz -  something that makes the GPS signals
alias down somewhere convenient (and always have positive frequency
offset from zero, even at max negative Doppler)


Still fills the function of LO, as the sample and hold operates as a 
mixer and the fold-down can be seen as an overtone mix followed by a 
sampling of the mix product, so well, it's about the same thing.


Cheers,
Magnus
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Re: [time-nuts] GPS first LO need to be locked?

[Magnus Danielson]

Hi,

On 03/30/2017 03:46 PM, paul swed wrote:

I am curious if the first local oscillator on a GPS receiver must actually
be locked or coherent to the reference oscillator in the GPS receiver
typically running at some 10 MHz approximately. Or as long as the first LO
is quite stable it doesn't matter because the receiver can track the code.
This is a question for very classic receivers like Austrons, Odetics etc.
Discreet. Modern fully integrated receivers are not in question.
Thank you for your insights.


If you only do a code receiver, you can do this, but the carrier 
tracking will need to compensate the LO. This is naturally possible, but 
some of the precision will be lost. You can play some tricks to average 
this from the channels and sort things out.


If you want to do a carrier phase receiver, you would suffer too much, 
then you would need to lock the LO to get the gain.


For old receivers, I'd just assume the LO is locked, as it saves 
compensation tricks later, which would save computation cycles.

However, there where some crude receivers back in the days.

Cheers,
Magnus
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Re: [time-nuts] GPS first LO need to be locked?

[David C. Partridge]

links [2] and [3] give 404 errors

Dave

-Original Message-
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Attila Kinali
Sent: 31 March 2017 12:35
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS first LO need to be locked?

 [2] "A Prototyping Platform for Multi-Frequency GNSS Receivers", by Akos, Ene 
and Thor, 2003 
http://waas.stanford.edu/~wwu/papers/gps/PDF/AkosIONGPS033FreqRX.pdf

[3] "Design of a GPS and Galileo Multi-Frequency Front-End", by Parada, 
Chastellain, Botteron, Tawk, Farine, 2009 
http://202.194.20.8/proc/VTC09Spring/DATA/04-04-01.PDF


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Re: [time-nuts] GPS first LO need to be locked?

[jimlux]

On 3/31/17 4:35 AM, Attila Kinali wrote:

On Thu, 30 Mar 2017 20:00:31 -0700
Peter Monta  wrote:


BTW a lot of GPS receivers don't have a "first LO".. they are more like a

Tuned RF receiver - an input BPF for L1, L2, or L5, then direct sampling at
around 30-40 MHz -  something that makes the GPS signals alias down
somewhere convenient (and always have positive frequency offset from zero,
even at max negative Doppler)




True.  I've been wanting to try this with an FPGA transceiver; even the
cheap ones go to 6 Gb/s now, but binary only.  The newest transceivers
support PAM-4, which would be great, but they're not affordable yet.  Also
that's a lot of gain at one frequency.


I guess you know of [1] already?

Alternatively, instead of using a MAX2021, you can use a discrete
mixer and use the high analog bandwidth of todays ADCs to use them
as downmixers. [2] and [3] describe how to do this in detail.

The epitome of this is using direct sampling of the signals without
previous downmixing (e.g. [4]). Though I have no idea how easy or hard that
is with todays electronics. It will definitely need good preselection
filters to keep SNR high.



Our space GPS receiver flying on SCaN Testbed/ISS is a direct sampling 
at 38 MHz. Filters are nothing special as I recall.  A chain of 
amplifiers with filters - Lots of gain into the thresholders, so good 
layout is important.


https://trs.jpl.nasa.gov/handle/2014/41781

https://trs.jpl.nasa.gov/bitstream/handle/2014/41781/11-0046.pdf?sequence=1=y




Attila Kinali

[1] http://www.aholme.co.uk/GPS/Main.htm

[2] "A Prototyping Platform for Multi-Frequency GNSS Receivers",
by Akos, Ene and Thor, 2003
http://waas.stanford.edu/~wwu/papers/gps/PDF/AkosIONGPS033FreqRX.pdf

[3] "Design of a GPS and Galileo Multi-Frequency Front-End",
by Parada, Chastellain, Botteron, Tawk, Farine, 2009
http://202.194.20.8/proc/VTC09Spring/DATA/04-04-01.PDF

[4] "Design and Implementation of a Direct Digitization GPS Receiver Front End",
by Akos, and Tsui, 1996



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Re: [time-nuts] GPS first LO need to be locked?

[Attila Kinali]

On Thu, 30 Mar 2017 19:56:31 -0400
Bob kb8tq  wrote:

> > Doppler is pretty big when the spacecraft is coming or going at the 
> > horizon, about 5 kHz (out of 1.5 GHz, so 4-5 ppm).
> > Relatively speaking, GPS satellites are moving slowly (a few km/s)
> > 
> 
> So somewhere in the baseband processor code somebody said “we’ll handle +/- 5 
> KHz”. If your LO is < (say) 500 Hz it’s still inside the likely doppler 
> handling range. 
> 
> If you want to do carrier phase then maybe you want to get a bit fancier ….

You have a carrier PLL anyways. What we call "carrier phase" is just
using the information from that subsystem to get a better estimate
for the time differences.

Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [time-nuts] GPS first LO need to be locked?

[Attila Kinali]

On Thu, 30 Mar 2017 20:00:31 -0700
Peter Monta  wrote:

> > BTW a lot of GPS receivers don't have a "first LO".. they are more like a
> >> Tuned RF receiver - an input BPF for L1, L2, or L5, then direct sampling at
> >> around 30-40 MHz -  something that makes the GPS signals alias down
> >> somewhere convenient (and always have positive frequency offset from zero,
> >> even at max negative Doppler)
> >
> >
> True.  I've been wanting to try this with an FPGA transceiver; even the
> cheap ones go to 6 Gb/s now, but binary only.  The newest transceivers
> support PAM-4, which would be great, but they're not affordable yet.  Also
> that's a lot of gain at one frequency.

I guess you know of [1] already?

Alternatively, instead of using a MAX2021, you can use a discrete
mixer and use the high analog bandwidth of todays ADCs to use them
as downmixers. [2] and [3] describe how to do this in detail.

The epitome of this is using direct sampling of the signals without
previous downmixing (e.g. [4]). Though I have no idea how easy or hard that
is with todays electronics. It will definitely need good preselection
filters to keep SNR high.

Attila Kinali

[1] http://www.aholme.co.uk/GPS/Main.htm

[2] "A Prototyping Platform for Multi-Frequency GNSS Receivers",
by Akos, Ene and Thor, 2003
http://waas.stanford.edu/~wwu/papers/gps/PDF/AkosIONGPS033FreqRX.pdf

[3] "Design of a GPS and Galileo Multi-Frequency Front-End",
by Parada, Chastellain, Botteron, Tawk, Farine, 2009
http://202.194.20.8/proc/VTC09Spring/DATA/04-04-01.PDF

[4] "Design and Implementation of a Direct Digitization GPS Receiver Front End",
by Akos, and Tsui, 1996

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [time-nuts] GPS first LO need to be locked?

[Peter Monta]

>
> BTW a lot of GPS receivers don't have a "first LO".. they are more like a
>> Tuned RF receiver - an input BPF for L1, L2, or L5, then direct sampling at
>> around 30-40 MHz -  something that makes the GPS signals alias down
>> somewhere convenient (and always have positive frequency offset from zero,
>> even at max negative Doppler)
>
>
True.  I've been wanting to try this with an FPGA transceiver; even the
cheap ones go to 6 Gb/s now, but binary only.  The newest transceivers
support PAM-4, which would be great, but they're not affordable yet.  Also
that's a lot of gain at one frequency.

Cheers,
Peter
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Re: [time-nuts] GPS first LO need to be locked?

[paul swed]

Thanks everyone for your comments. It will be a GPSDP TBolt or Z3801
reference.
I just wanted to eliminate some variables at this stage.
Regards
Paul
WB8TSL

On Thu, Mar 30, 2017 at 7:56 PM, Bob kb8tq  wrote:

> Hi
>
>
> > On Mar 30, 2017, at 7:05 PM, jimlux  wrote:
> >
> > On 3/30/17 10:32 AM, Bob Camp wrote:
> >> Hi
> >>
> >> There is a limited tracking range for Doppler. You would need to stay
> inside that.
> >>
> >
> > Doppler is pretty big when the spacecraft is coming or going at the
> horizon, about 5 kHz (out of 1.5 GHz, so 4-5 ppm).
> > Relatively speaking, GPS satellites are moving slowly (a few km/s)
> >
>
> So somewhere in the baseband processor code somebody said “we’ll handle
> +/- 5 KHz”. If your LO is < (say) 500 Hz it’s still inside the likely
> doppler handling range.
>
> If you want to do carrier phase then maybe you want to get a bit fancier ….
>
> Bob
>
> >
> > in LEO you're buzzing along at 7km/s, which is about 20-25 ppm.  That is
> the usual limiting case for bandwidth/tracking loops; you might want to go
> up to 11-12 km/s so you can get things moving at escape velocity.
> > (there just aren't many people putting GPS on hypersonic projectiles -
> if you've got the bucks to shoot something at Mach 45, you can probably
> afford a custom GPS receiver)
> >
> > This is a bit tricky for older receivers because their tracking loop has
> to acquire in the face of the Doppler uncertainty and the range (code
> phase) uncertainty - there's a whole lore of optimum search strategies and
> how to get the fastest time-to-first-fix.
> >
> > Does the first LO have to be locked to something?  the signal you're
> acquiring is MHz wide, so a 10ppm error in the LO frequency isn't a big
> deal. Short term stability does help, while you're acquiring.
> >
> > But one of the things about GPS that made it attractive is that the
> local clock can be pretty crummy.
> >
> >> Bob
> >>
> >> Sent from my iPhone
> >>
> >>> On Mar 30, 2017, at 9:46 AM, paul swed  wrote:
> >>>
> >>> I am curious if the first local oscillator on a GPS receiver must
> actually
> >>> be locked or coherent to the reference oscillator in the GPS receiver
> >>> typically running at some 10 MHz approximately. Or as long as the
> first LO
> >>> is quite stable it doesn't matter because the receiver can track the
> code.
> >>> This is a question for very classic receivers like Austrons, Odetics
> etc.
> >>> Discreet. Modern fully integrated receivers are not in question.
> >>> Thank you for your insights.
> >>> Regards
> >>> Paul
> >>> WB8TSL
> >>> ___
> >>> 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.
> >> ___
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> >> and follow the instructions there.
> >>
> >
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Re: [time-nuts] GPS first LO need to be locked?

[Bob kb8tq]

Hi


> On Mar 30, 2017, at 7:05 PM, jimlux  wrote:
> 
> On 3/30/17 10:32 AM, Bob Camp wrote:
>> Hi
>> 
>> There is a limited tracking range for Doppler. You would need to stay inside 
>> that.
>> 
> 
> Doppler is pretty big when the spacecraft is coming or going at the horizon, 
> about 5 kHz (out of 1.5 GHz, so 4-5 ppm).
> Relatively speaking, GPS satellites are moving slowly (a few km/s)
> 

So somewhere in the baseband processor code somebody said “we’ll handle +/- 5 
KHz”. If your LO is < (say) 500 Hz it’s still inside the likely doppler 
handling range. 

If you want to do carrier phase then maybe you want to get a bit fancier ….

Bob

> 
> in LEO you're buzzing along at 7km/s, which is about 20-25 ppm.  That is the 
> usual limiting case for bandwidth/tracking loops; you might want to go up to 
> 11-12 km/s so you can get things moving at escape velocity.
> (there just aren't many people putting GPS on hypersonic projectiles - if 
> you've got the bucks to shoot something at Mach 45, you can probably afford a 
> custom GPS receiver)
> 
> This is a bit tricky for older receivers because their tracking loop has to 
> acquire in the face of the Doppler uncertainty and the range (code phase) 
> uncertainty - there's a whole lore of optimum search strategies and how to 
> get the fastest time-to-first-fix.
> 
> Does the first LO have to be locked to something?  the signal you're 
> acquiring is MHz wide, so a 10ppm error in the LO frequency isn't a big deal. 
> Short term stability does help, while you're acquiring.
> 
> But one of the things about GPS that made it attractive is that the local 
> clock can be pretty crummy.
> 
>> Bob
>> 
>> Sent from my iPhone
>> 
>>> On Mar 30, 2017, at 9:46 AM, paul swed  wrote:
>>> 
>>> I am curious if the first local oscillator on a GPS receiver must actually
>>> be locked or coherent to the reference oscillator in the GPS receiver
>>> typically running at some 10 MHz approximately. Or as long as the first LO
>>> is quite stable it doesn't matter because the receiver can track the code.
>>> This is a question for very classic receivers like Austrons, Odetics etc.
>>> Discreet. Modern fully integrated receivers are not in question.
>>> Thank you for your insights.
>>> Regards
>>> Paul
>>> WB8TSL
>>> ___
>>> 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.
>> 
> 
> ___
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Re: [time-nuts] GPS first LO need to be locked?

[jimlux]

On 3/30/17 11:06 AM, Peter Monta wrote:

I am curious if the first local oscillator on a GPS receiver must actually
be locked or coherent to the reference oscillator in the GPS receiver
typically running at some 10 MHz approximately. Or as long as the first LO
is quite stable it doesn't matter because the receiver can track the code.



It doesn't matter, so long as the first LO is in the ballpark so that the
Doppler search is not needlessly large.  I'm not so familiar with the early
receivers, but I imagine a single reference oscillator serves for
everything---there would seem to be no reason to have more than one unless
the antenna/downconverter were physically separate from the rest of the
receiver.  If an older receiver used a physical source at 10.23 MHz, it
would still need to be offset slightly for each satellite because of "code
doppler", but this choice of frequency might slightly simplify the
circuitry.  Current receivers would use any convenient physical rate, then
synthesize the code rates.



BTW a lot of GPS receivers don't have a "first LO".. they are more like 
a Tuned RF receiver - an input BPF for L1, L2, or L5, then direct 
sampling at around 30-40 MHz -  something that makes the GPS signals 
alias down somewhere convenient (and always have positive frequency 
offset from zero, even at max negative Doppler)


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Re: [time-nuts] GPS first LO need to be locked?

[jimlux]

On 3/30/17 10:32 AM, Bob Camp wrote:

Hi

There is a limited tracking range for Doppler. You would need to stay inside 
that.



Doppler is pretty big when the spacecraft is coming or going at the 
horizon, about 5 kHz (out of 1.5 GHz, so 4-5 ppm).

Relatively speaking, GPS satellites are moving slowly (a few km/s)


in LEO you're buzzing along at 7km/s, which is about 20-25 ppm.  That is 
the usual limiting case for bandwidth/tracking loops; you might want to 
go up to 11-12 km/s so you can get things moving at escape velocity.
(there just aren't many people putting GPS on hypersonic projectiles - 
if you've got the bucks to shoot something at Mach 45, you can probably 
afford a custom GPS receiver)


This is a bit tricky for older receivers because their tracking loop has 
to acquire in the face of the Doppler uncertainty and the range (code 
phase) uncertainty - there's a whole lore of optimum search strategies 
and how to get the fastest time-to-first-fix.


Does the first LO have to be locked to something?  the signal you're 
acquiring is MHz wide, so a 10ppm error in the LO frequency isn't a big 
deal. Short term stability does help, while you're acquiring.


But one of the things about GPS that made it attractive is that the 
local clock can be pretty crummy.



Bob

Sent from my iPhone


On Mar 30, 2017, at 9:46 AM, paul swed  wrote:

I am curious if the first local oscillator on a GPS receiver must actually
be locked or coherent to the reference oscillator in the GPS receiver
typically running at some 10 MHz approximately. Or as long as the first LO
is quite stable it doesn't matter because the receiver can track the code.
This is a question for very classic receivers like Austrons, Odetics etc.
Discreet. Modern fully integrated receivers are not in question.
Thank you for your insights.
Regards
Paul
WB8TSL
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Re: [time-nuts] GPS first LO need to be locked?

[Bob Camp]

Hi

There is a limited tracking range for Doppler. You would need to stay inside 
that. 

Bob

Sent from my iPhone

> On Mar 30, 2017, at 9:46 AM, paul swed  wrote:
> 
> I am curious if the first local oscillator on a GPS receiver must actually
> be locked or coherent to the reference oscillator in the GPS receiver
> typically running at some 10 MHz approximately. Or as long as the first LO
> is quite stable it doesn't matter because the receiver can track the code.
> This is a question for very classic receivers like Austrons, Odetics etc.
> Discreet. Modern fully integrated receivers are not in question.
> Thank you for your insights.
> Regards
> Paul
> WB8TSL
> ___
> 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.
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Re: [time-nuts] GPS first LO need to be locked?

[Peter Monta]

> I am curious if the first local oscillator on a GPS receiver must actually
> be locked or coherent to the reference oscillator in the GPS receiver
> typically running at some 10 MHz approximately. Or as long as the first LO
> is quite stable it doesn't matter because the receiver can track the code.
>

It doesn't matter, so long as the first LO is in the ballpark so that the
Doppler search is not needlessly large.  I'm not so familiar with the early
receivers, but I imagine a single reference oscillator serves for
everything---there would seem to be no reason to have more than one unless
the antenna/downconverter were physically separate from the rest of the
receiver.  If an older receiver used a physical source at 10.23 MHz, it
would still need to be offset slightly for each satellite because of "code
doppler", but this choice of frequency might slightly simplify the
circuitry.  Current receivers would use any convenient physical rate, then
synthesize the code rates.

Cheers,
Peter
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