GPS first LO need to be locked?
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
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 paulswedb@gmail.com 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.
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
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 paulswedb@gmail.com 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
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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)
Hi
On Mar 30, 2017, at 7:05 PM, jimlux jimlux@earthlink.net 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 paulswedb@gmail.com 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
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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 kb8tq@n1k.org wrote:
Hi
On Mar 30, 2017, at 7:05 PM, jimlux jimlux@earthlink.net 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 paulswedb@gmail.com 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.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/time-nuts
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To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/time-nuts
and follow the instructions there.
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
On Thu, 30 Mar 2017 19:56:31 -0400
Bob kb8tq kb8tq@n1k.org 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
On Thu, 30 Mar 2017 20:00:31 -0700
Peter Monta pmonta@gmail.com 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