Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Chuck said
I got the notion that it was turned off during Desert Storm,
by virtue of being involved in the e-warfare effort that lead
up to, and followed the event.
I haven't been paying much attention since. I knew that they
had intended to turn SA back on after production of the p-code
units was up to speed, but I hadn't heard whether or not they
did.
Yes, it was turned off for a brief period during DS, largely because the
DoD had to scurry around to buy mortal commercial units to fill the
need. Also during DS (and the present excursion) lots of parents sent
COTS GPS widgets to their kids.
It turned out that one of the most important uses of cheap GPS receiver
in DS was by the food trucks. Troops were deployed in the desert all
along the Iraq & Kuwait border. The mess tents were behind the lines,
and hot meals needed to be delivered to the remote outposts. The
delivery trucks found they could navigate across the roadless desert
very well by using GPS receiver intended for navigating civilian boats.
S/A is a dithering of the clock with a pseudorandom phase jitter. The
key to disentangling it was to have the same code generator available on
the ground. I use the analogy that DoD had a smart mouse in each
satellite running around on a phase resolver. To de-jitter it, you need
the mouse's clone inside the receiver.
The dithering of S/A had nothing to do with the encryption of the P code
to make the Y code. The P-code is a LONNNNG code (37 weeks until a
repeat) at 10.23 Mbits/sec. Each of the satellites uses the same code
stream, offset by some integer number of weeks. The Y-code is an
additional secret code that uses a shorter code to (pseudo)randomly flip
the phase of the P-code. On the ground, the civilian "code crackers"
have found out that the convolution code is running at a rate ~500
kbits/sec. This means that the Y-code may be the correct P-code for ~20
bits, and then it (may|may not) flip phase to become "anti-P" code.
AFAIK, Ashtec's patented "Z-code" receivers generate a hardware estimate
of this code and (nearly) coherently demodulate the signal. Other brands
have similar tricks up their sleeve.
Tom
From: "Tom Clark, K3IO (ex W3IWI)" K3IO@verizon.net
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Date: Mon, 13 Mar 2006 16:44:51 -0500
Message-ID: 4415E7D3.8000106@verizon.net
Chuck said
I got the notion that it was turned off during Desert Storm,
by virtue of being involved in the e-warfare effort that lead
up to, and followed the event.
I haven't been paying much attention since. I knew that they
had intended to turn SA back on after production of the p-code
units was up to speed, but I hadn't heard whether or not they
did.
Yes, it was turned off for a brief period during DS, largely because the
DoD had to scurry around to buy mortal commercial units to fill the
need. Also during DS (and the present excursion) lots of parents sent
COTS GPS widgets to their kids.
It turned out that one of the most important uses of cheap GPS receiver
in DS was by the food trucks. Troops were deployed in the desert all
along the Iraq & Kuwait border. The mess tents were behind the lines,
and hot meals needed to be delivered to the remote outposts. The
delivery trucks found they could navigate across the roadless desert
very well by using GPS receiver intended for navigating civilian boats.
S/A is a dithering of the clock with a pseudorandom phase jitter. The
key to disentangling it was to have the same code generator available on
the ground. I use the analogy that DoD had a smart mouse in each
satellite running around on a phase resolver. To de-jitter it, you need
the mouse's clone inside the receiver.
The dithering of S/A had nothing to do with the encryption of the P code
to make the Y code. The P-code is a LONNNNG code (37 weeks until a
repeat) at 10.23 Mbits/sec. Each of the satellites uses the same code
stream, offset by some integer number of weeks. The Y-code is an
additional secret code that uses a shorter code to (pseudo)randomly flip
the phase of the P-code. On the ground, the civilian "code crackers"
have found out that the convolution code is running at a rate ~500
kbits/sec. This means that the Y-code may be the correct P-code for ~20
bits, and then it (may|may not) flip phase to become "anti-P" code.
AFAIK, Ashtec's patented "Z-code" receivers generate a hardware estimate
of this code and (nearly) coherently demodulate the signal. Other brands
have similar tricks up their sleeve.
The Y-code is the P-code xored with the A-code (sometimes also referred to as
the W-code). The A-code is indeed ~500 kbis/sec. The first "codeless" receivers
just squared out the A-code from the equation, but then they had a worse
problem to fight regarding ambiguity. Also, it does not form a very good
receiver. The Ashtec solution is to make the L1 handover from C/A-code to
P-code and predict the A-code, delay that a suitable amount to the L2 Y-code
and attempt to lock up to that. The delay is trimmed to match up with the
L1-L2 delay in P(Y)-code. You could say that the Ashtec receivers cracks the
code, but they really don't since they do not disclose the state of the A-code
generator or its architecture. Infact, they don't even get it rigth all the
time, but sufficiently often for a good lock since each success has a good
quality.
It is interesting that what they did to figure things out was hunting GPS
satellites with a big parabol antenna tracking the satellite and getting a
much better S/N than normal semi-omnidirectional antennas. With that they
could make advanced guesses.
Cheers,
Magnus
I have a 75 page PDF briefing from Zyfer on SAASM P/Y which has loads of
useful information on GPS signal structure, acquisition, jamming, spoofing
etc.
Can either post it to the group (approx 3MB) or send it on request.
Rob Kimberley
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: 13 March 2006 22:32
To: K3IO@verizon.net; time-nuts@febo.com
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
From: "Tom Clark, K3IO (ex W3IWI)" K3IO@verizon.net
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Date: Mon, 13 Mar 2006 16:44:51 -0500
Message-ID: 4415E7D3.8000106@verizon.net
Chuck said
I got the notion that it was turned off during Desert Storm, by
virtue of being involved in the e-warfare effort that lead up to,
and followed the event.
I haven't been paying much attention since. I knew that they had
intended to turn SA back on after production of the p-code units was
up to speed, but I hadn't heard whether or not they did.
Yes, it was turned off for a brief period during DS, largely because
the DoD had to scurry around to buy mortal commercial units to fill
the need. Also during DS (and the present excursion) lots of parents
sent COTS GPS widgets to their kids.
It turned out that one of the most important uses of cheap GPS
receiver in DS was by the food trucks. Troops were deployed in the
desert all along the Iraq & Kuwait border. The mess tents were behind
the lines, and hot meals needed to be delivered to the remote
outposts. The delivery trucks found they could navigate across the
roadless desert very well by using GPS receiver intended for navigating
civilian boats.
S/A is a dithering of the clock with a pseudorandom phase jitter. The
key to disentangling it was to have the same code generator available
on the ground. I use the analogy that DoD had a smart mouse in each
satellite running around on a phase resolver. To de-jitter it, you
need the mouse's clone inside the receiver.
The dithering of S/A had nothing to do with the encryption of the P
code to make the Y code. The P-code is a LONNNNG code (37 weeks until
a
repeat) at 10.23 Mbits/sec. Each of the satellites uses the same code
stream, offset by some integer number of weeks. The Y-code is an
additional secret code that uses a shorter code to (pseudo)randomly
flip the phase of the P-code. On the ground, the civilian "code crackers"
have found out that the convolution code is running at a rate ~500
kbits/sec. This means that the Y-code may be the correct P-code for
~20 bits, and then it (may|may not) flip phase to become "anti-P" code.
AFAIK, Ashtec's patented "Z-code" receivers generate a hardware
estimate of this code and (nearly) coherently demodulate the signal.
Other brands have similar tricks up their sleeve.
The Y-code is the P-code xored with the A-code (sometimes also referred to
as the W-code). The A-code is indeed ~500 kbis/sec. The first "codeless"
receivers just squared out the A-code from the equation, but then they had a
worse problem to fight regarding ambiguity. Also, it does not form a very
good receiver. The Ashtec solution is to make the L1 handover from C/A-code
to P-code and predict the A-code, delay that a suitable amount to the L2
Y-code and attempt to lock up to that. The delay is trimmed to match up with
the
L1-L2 delay in P(Y)-code. You could say that the Ashtec receivers cracks the
code, but they really don't since they do not disclose the state of the
A-code generator or its architecture. Infact, they don't even get it rigth
all the time, but sufficiently often for a good lock since each success has
a good quality.
It is interesting that what they did to figure things out was hunting GPS
satellites with a big parabol antenna tracking the satellite and getting a
much better S/N than normal semi-omnidirectional antennas. With that they
could make advanced guesses.
Cheers,
Magnus
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Rob, I'd like a copy of that. 3 MB is no problem.
Please send to bill@iaxs.net
Thanks,
Bill Hawkins
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]On
Behalf Of Rob Kimberley
Sent: Tuesday, March 14, 2006 3:36 AM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
I have a 75 page PDF briefing from Zyfer on SAASM P/Y which has loads of
useful information on GPS signal structure, acquisition, jamming, spoofing
etc.
Can either post it to the group (approx 3MB) or send it on request.
Rob Kimberley
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: 13 March 2006 22:32
To: K3IO@verizon.net; time-nuts@febo.com
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
From: "Tom Clark, K3IO (ex W3IWI)" K3IO@verizon.net
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Date: Mon, 13 Mar 2006 16:44:51 -0500
Message-ID: 4415E7D3.8000106@verizon.net
Chuck said
I got the notion that it was turned off during Desert Storm, by
virtue of being involved in the e-warfare effort that lead up to,
and followed the event.
I haven't been paying much attention since. I knew that they had
intended to turn SA back on after production of the p-code units was
up to speed, but I hadn't heard whether or not they did.
Yes, it was turned off for a brief period during DS, largely because
the DoD had to scurry around to buy mortal commercial units to fill
the need. Also during DS (and the present excursion) lots of parents
sent COTS GPS widgets to their kids.
It turned out that one of the most important uses of cheap GPS
receiver in DS was by the food trucks. Troops were deployed in the
desert all along the Iraq & Kuwait border. The mess tents were behind
the lines, and hot meals needed to be delivered to the remote
outposts. The delivery trucks found they could navigate across the
roadless desert very well by using GPS receiver intended for navigating
civilian boats.
S/A is a dithering of the clock with a pseudorandom phase jitter. The
key to disentangling it was to have the same code generator available
on the ground. I use the analogy that DoD had a smart mouse in each
satellite running around on a phase resolver. To de-jitter it, you
need the mouse's clone inside the receiver.
The dithering of S/A had nothing to do with the encryption of the P
code to make the Y code. The P-code is a LONNNNG code (37 weeks until
a
repeat) at 10.23 Mbits/sec. Each of the satellites uses the same code
stream, offset by some integer number of weeks. The Y-code is an
additional secret code that uses a shorter code to (pseudo)randomly
flip the phase of the P-code. On the ground, the civilian "code crackers"
have found out that the convolution code is running at a rate ~500
kbits/sec. This means that the Y-code may be the correct P-code for
~20 bits, and then it (may|may not) flip phase to become "anti-P" code.
AFAIK, Ashtec's patented "Z-code" receivers generate a hardware
estimate of this code and (nearly) coherently demodulate the signal.
Other brands have similar tricks up their sleeve.
The Y-code is the P-code xored with the A-code (sometimes also referred to
as the W-code). The A-code is indeed ~500 kbis/sec. The first "codeless"
receivers just squared out the A-code from the equation, but then they had a
worse problem to fight regarding ambiguity. Also, it does not form a very
good receiver. The Ashtec solution is to make the L1 handover from C/A-code
to P-code and predict the A-code, delay that a suitable amount to the L2
Y-code and attempt to lock up to that. The delay is trimmed to match up with
the
L1-L2 delay in P(Y)-code. You could say that the Ashtec receivers cracks the
code, but they really don't since they do not disclose the state of the
A-code generator or its architecture. Infact, they don't even get it rigth
all the time, but sufficiently often for a good lock since each success has
a good quality.
It is interesting that what they did to figure things out was hunting GPS
satellites with a big parabol antenna tracking the satellite and getting a
much better S/N than normal semi-omnidirectional antennas. With that they
could make advanced guesses.
Cheers,
Magnus
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Rob,
I for one would like a soft copy of this document as well. Three (3) MB is
okay by me at:
csutc.observatory@earthlink.net
Many thanks,
Gar NE8S
ne8s@earthlink.net
----- Original Message -----
From: "Rob Kimberley" time.bandit@btinternet.com
To: "'Discussion of precise time and frequency measurement'"
time-nuts@febo.com
Sent: Tuesday, March 14, 2006 3:36 AM
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
I have a 75 page PDF briefing from Zyfer on SAASM P/Y which has loads of
useful information on GPS signal structure, acquisition, jamming, spoofing
etc.
Can either post it to the group (approx 3MB) or send it on request.
Rob Kimberley
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: 13 March 2006 22:32
To: K3IO@verizon.net; time-nuts@febo.com
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
From: "Tom Clark, K3IO (ex W3IWI)" K3IO@verizon.net
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Date: Mon, 13 Mar 2006 16:44:51 -0500
Message-ID: 4415E7D3.8000106@verizon.net
Chuck said
I got the notion that it was turned off during Desert Storm, by
virtue of being involved in the e-warfare effort that lead up to,
and followed the event.
I haven't been paying much attention since. I knew that they had
intended to turn SA back on after production of the p-code units was
up to speed, but I hadn't heard whether or not they did.
Yes, it was turned off for a brief period during DS, largely because
the DoD had to scurry around to buy mortal commercial units to fill
the need. Also during DS (and the present excursion) lots of parents
sent COTS GPS widgets to their kids.
It turned out that one of the most important uses of cheap GPS
receiver in DS was by the food trucks. Troops were deployed in the
desert all along the Iraq & Kuwait border. The mess tents were behind
the lines, and hot meals needed to be delivered to the remote
outposts. The delivery trucks found they could navigate across the
roadless desert very well by using GPS receiver intended for navigating
civilian boats.
S/A is a dithering of the clock with a pseudorandom phase jitter. The
key to disentangling it was to have the same code generator available
on the ground. I use the analogy that DoD had a smart mouse in each
satellite running around on a phase resolver. To de-jitter it, you
need the mouse's clone inside the receiver.
The dithering of S/A had nothing to do with the encryption of the P
code to make the Y code. The P-code is a LONNNNG code (37 weeks until
a
repeat) at 10.23 Mbits/sec. Each of the satellites uses the same code
stream, offset by some integer number of weeks. The Y-code is an
additional secret code that uses a shorter code to (pseudo)randomly
flip the phase of the P-code. On the ground, the civilian "code crackers"
have found out that the convolution code is running at a rate ~500
kbits/sec. This means that the Y-code may be the correct P-code for
~20 bits, and then it (may|may not) flip phase to become "anti-P" code.
AFAIK, Ashtec's patented "Z-code" receivers generate a hardware
estimate of this code and (nearly) coherently demodulate the signal.
Other brands have similar tricks up their sleeve.
The Y-code is the P-code xored with the A-code (sometimes also referred to
as the W-code). The A-code is indeed ~500 kbis/sec. The first "codeless"
receivers just squared out the A-code from the equation, but then they had
a
worse problem to fight regarding ambiguity. Also, it does not form a very
good receiver. The Ashtec solution is to make the L1 handover from
C/A-code
to P-code and predict the A-code, delay that a suitable amount to the L2
Y-code and attempt to lock up to that. The delay is trimmed to match up
with
the
L1-L2 delay in P(Y)-code. You could say that the Ashtec receivers cracks
the
code, but they really don't since they do not disclose the state of the
A-code generator or its architecture. Infact, they don't even get it rigth
all the time, but sufficiently often for a good lock since each success
has
a good quality.
It is interesting that what they did to figure things out was hunting GPS
satellites with a big parabol antenna tracking the satellite and getting a
much better S/N than normal semi-omnidirectional antennas. With that they
could make advanced guesses.
Cheers,
Magnus
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Rob,
Could you upload it to ftp.ko4bb.com, login: manuals, password: manuals
so I could put it with the other manuals at www.ko4bb.com/ham_radio/Manuals?
Thanks in advance
Didier KO4BB
Rob Kimberley wrote:
I have a 75 page PDF briefing from Zyfer on SAASM P/Y which has loads of
useful information on GPS signal structure, acquisition, jamming, spoofing
etc.
Can either post it to the group (approx 3MB) or send it on request.
Rob Kimberley
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: 13 March 2006 22:32
To: K3IO@verizon.net; time-nuts@febo.com
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
From: "Tom Clark, K3IO (ex W3IWI)" K3IO@verizon.net
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Date: Mon, 13 Mar 2006 16:44:51 -0500
Message-ID: 4415E7D3.8000106@verizon.net
Chuck said
I got the notion that it was turned off during Desert Storm, by
virtue of being involved in the e-warfare effort that lead up to,
and followed the event.
I haven't been paying much attention since. I knew that they had
intended to turn SA back on after production of the p-code units was
up to speed, but I hadn't heard whether or not they did.
Yes, it was turned off for a brief period during DS, largely because
the DoD had to scurry around to buy mortal commercial units to fill
the need. Also during DS (and the present excursion) lots of parents
sent COTS GPS widgets to their kids.
It turned out that one of the most important uses of cheap GPS
receiver in DS was by the food trucks. Troops were deployed in the
desert all along the Iraq & Kuwait border. The mess tents were behind
the lines, and hot meals needed to be delivered to the remote
outposts. The delivery trucks found they could navigate across the
roadless desert very well by using GPS receiver intended for navigating
civilian boats.
S/A is a dithering of the clock with a pseudorandom phase jitter. The
key to disentangling it was to have the same code generator available
on the ground. I use the analogy that DoD had a smart mouse in each
satellite running around on a phase resolver. To de-jitter it, you
need the mouse's clone inside the receiver.
The dithering of S/A had nothing to do with the encryption of the P
code to make the Y code. The P-code is a LONNNNG code (37 weeks until
a
repeat) at 10.23 Mbits/sec. Each of the satellites uses the same code
stream, offset by some integer number of weeks. The Y-code is an
additional secret code that uses a shorter code to (pseudo)randomly
flip the phase of the P-code. On the ground, the civilian "code crackers"
have found out that the convolution code is running at a rate ~500
kbits/sec. This means that the Y-code may be the correct P-code for
~20 bits, and then it (may|may not) flip phase to become "anti-P" code.
AFAIK, Ashtec's patented "Z-code" receivers generate a hardware
estimate of this code and (nearly) coherently demodulate the signal.
Other brands have similar tricks up their sleeve.
The Y-code is the P-code xored with the A-code (sometimes also referred to
as the W-code). The A-code is indeed ~500 kbis/sec. The first "codeless"
receivers just squared out the A-code from the equation, but then they had a
worse problem to fight regarding ambiguity. Also, it does not form a very
good receiver. The Ashtec solution is to make the L1 handover from C/A-code
to P-code and predict the A-code, delay that a suitable amount to the L2
Y-code and attempt to lock up to that. The delay is trimmed to match up with
the
L1-L2 delay in P(Y)-code. You could say that the Ashtec receivers cracks the
code, but they really don't since they do not disclose the state of the
A-code generator or its architecture. Infact, they don't even get it rigth
all the time, but sufficiently often for a good lock since each success has
a good quality.
It is interesting that what they did to figure things out was hunting GPS
satellites with a big parabol antenna tracking the satellite and getting a
much better S/N than normal semi-omnidirectional antennas. With that they
could make advanced guesses.
Cheers,
Magnus
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Didier,
Uploaded this morning.
Rob K
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Didier Juges
Sent: 15 March 2006 03:57
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Rob,
Could you upload it to ftp.ko4bb.com, login: manuals, password: manuals so I
could put it with the other manuals at www.ko4bb.com/ham_radio/Manuals?
Thanks in advance
Didier KO4BB
Rob Kimberley wrote:
I have a 75 page PDF briefing from Zyfer on SAASM P/Y which has loads
of useful information on GPS signal structure, acquisition, jamming,
spoofing etc.
Can either post it to the group (approx 3MB) or send it on request.
Rob Kimberley
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: 13 March 2006 22:32
To: K3IO@verizon.net; time-nuts@febo.com
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
From: "Tom Clark, K3IO (ex W3IWI)" K3IO@verizon.net
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Date: Mon, 13 Mar 2006 16:44:51 -0500
Message-ID: 4415E7D3.8000106@verizon.net
Chuck said
I got the notion that it was turned off during Desert Storm, by
virtue of being involved in the e-warfare effort that lead up to, and
followed the event.
I haven't been paying much attention since. I knew that they had
intended to turn SA back on after production of the p-code units was
up to speed, but I hadn't heard whether or not they did.
Yes, it was turned off for a brief period during DS, largely because
the DoD had to scurry around to buy mortal commercial units to fill
the need. Also during DS (and the present excursion) lots of parents
sent COTS GPS widgets to their kids.
It turned out that one of the most important uses of cheap GPS
receiver in DS was by the food trucks. Troops were deployed in the
desert all along the Iraq & Kuwait border. The mess tents were behind
the lines, and hot meals needed to be delivered to the remote
outposts. The delivery trucks found they could navigate across the
roadless desert very well by using GPS receiver intended for
navigating
civilian boats.
S/A is a dithering of the clock with a pseudorandom phase jitter. The
key to disentangling it was to have the same code generator available
on the ground. I use the analogy that DoD had a smart mouse in each
satellite running around on a phase resolver. To de-jitter it, you
need the mouse's clone inside the receiver.
The dithering of S/A had nothing to do with the encryption of the P
code to make the Y code. The P-code is a LONNNNG code (37 weeks until
a
repeat) at 10.23 Mbits/sec. Each of the satellites uses the same code
stream, offset by some integer number of weeks. The Y-code is an
additional secret code that uses a shorter code to (pseudo)randomly
flip the phase of the P-code. On the ground, the civilian "code crackers"
have found out that the convolution code is running at a rate ~500
kbits/sec. This means that the Y-code may be the correct P-code for
~20 bits, and then it (may|may not) flip phase to become "anti-P" code.
AFAIK, Ashtec's patented "Z-code" receivers generate a hardware
estimate of this code and (nearly) coherently demodulate the signal.
Other brands have similar tricks up their sleeve.
The Y-code is the P-code xored with the A-code (sometimes also referred
to as the W-code). The A-code is indeed ~500 kbis/sec. The first "codeless"
receivers just squared out the A-code from the equation, but then they
had a worse problem to fight regarding ambiguity. Also, it does not
form a very good receiver. The Ashtec solution is to make the L1
handover from C/A-code to P-code and predict the A-code, delay that a
suitable amount to the L2 Y-code and attempt to lock up to that. The
delay is trimmed to match up with the
L1-L2 delay in P(Y)-code. You could say that the Ashtec receivers
cracks the code, but they really don't since they do not disclose the
state of the A-code generator or its architecture. Infact, they don't
even get it rigth all the time, but sufficiently often for a good lock
since each success has a good quality.
It is interesting that what they did to figure things out was hunting
GPS satellites with a big parabol antenna tracking the satellite and
getting a much better S/N than normal semi-omnidirectional antennas.
With that they could make advanced guesses.
Cheers,
Magnus
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I got it and moved it to "http://www.ko4bb.com/ham_radio/Manuals/GPS Stuff"
Thanks a lot
Everyone is welcome to get a copy. Check my manuals while you are
there... Feel free to upload stuff you want to share.
Didier
Rob Kimberley wrote:
Didier,
Uploaded this morning.
Rob K
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Didier Juges
Sent: 15 March 2006 03:57
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS Selective Availability. Is it On or Off?
Rob,
Could you upload it to ftp.ko4bb.com, login: manuals, password: manuals so I
could put it with the other manuals at www.ko4bb.com/ham_radio/Manuals?
Thanks in advance
Didier KO4BB
Rob Kimberley wrote:
I have a 75 page PDF briefing from Zyfer on SAASM P/Y which has loads
of useful information on GPS signal structure, acquisition, jamming,
spoofing etc.
Can either post it to the group (approx 3MB) or send it on request.
Rob Kimberley