OT - GPS and North
Does a stationary (not in motion) GPS receiver know where the North is?
As far
as I can understand, it doesn't, isn't it?
Antonio I8IOV
That is an interesting question.
It could be possible if it had two antennas or its antenna was
not omni-directional.
Mike - AA8K
iovane@inwind.it wrote:
Does a stationary (not in motion) GPS receiver know where the North is?
As far
as I can understand, it doesn't, isn't it?
Antonio I8IOV
iovane@inwind.it wrote:
Does a stationary (not in motion) GPS receiver know where the North is?
As far
as I can understand, it doesn't, isn't it?
Not one with a unidirectional antenna, which is the usual stuff.
If you have multiple antennas you can sort out the directions. But it is
an expensive way of achieving it.
If your antenna moves you can detect the heading relative north.
A sensor to sense the magnetic field is fairly inexpensive so that is
being used.
Cheers,
Magnus
It depends on the GPS receiver. The GPS chipset won't know (it knows where
it is and can remember where it was to know what direction it's moving), but
some consumer GPS receivers (Garmin, Magellan) have electronic compasses
built in. My Garmin eTrex Legend does NOT have a compass built in, but
taking about 2 steps in any direction will tell me which way I'm moving. It
can't do this when stationary. My buddy's eTrex can point north without
moving, but that feature drains the battery faster.
-Bob
On Sat, Nov 21, 2009 at 7:05 AM, iovane@inwind.it iovane@inwind.it wrote:
Does a stationary (not in motion) GPS receiver know where the North is?
As far
as I can understand, it doesn't, isn't it?
Antonio I8IOV
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Robert Darlington wrote:
It depends on the GPS receiver. The GPS chipset won't know (it knows where
it is and can remember where it was to know what direction it's moving), but
some consumer GPS receivers (Garmin, Magellan) have electronic compasses
built in. My Garmin eTrex Legend does NOT have a compass built in, but
taking about 2 steps in any direction will tell me which way I'm moving. It
can't do this when stationary. My buddy's eTrex can point north without
moving, but that feature drains the battery faster.
The actual GPS receiver and the navigator is two different things. A GPS
navigator may use additional sensors such as accelerometers and magnetic
sensor to aid in addition to the GPS receiver built into them.
So a navigator may know, even if on a fixed location, but the GPS
receiver does not have that information.
It is important to keep these separated so that they can be discussed
properly. I don't want to say stuff like "The actual GPS receiver in the
GPS receiver...".
Cheers,
Magnus
Am Saturday 21 November 2009 15:05:12 schrieb iovane@inwind.it:
Does a stationary (not in motion) GPS receiver know where the North is?
As far
as I can understand, it doesn't, isn't it?
Umm, as far as i understand it, a single receiver with a single
omnidirectional antenna (at least with rotational symmetry around the
vertical axis) can't know where north is.
IF you just so happen to have a more clever setup, this indeed becomes
possible. Basically, a GPS receiver happens to not compute his own location
but the one of his antenna. So if you happen to have a receiver with enough
correlators and stuff to properly connect two (or more) antennas and
calculate positions for both, it could actually deduce true north. Bearings
get better the more space you have between the antennas and the more
averaging you can apply. Time for averaging doesn't seem much of an issue in
a truly stationary setup, so just try to maximize separation of your
antennas.
HTH,
Florian
Does a stationary (not in motion) GPS receiver know where the North is?
No, a stationary object is a point, not a line or a vector.
The notion of North (or any direction) has no meaning
to a point, by definition.
There are two exceptions. If during your long polar trek
the latitude is ±90º 00' 00.0" and the longitude seems to
wander all over the map then, yes, your stationary GPS
receiver finally knows where North is. Examples:
http://www.klipsi.ch/at_Northpole/at_Northpole.htm
http://www.icetent.com/south_pole.htm
http://www.4x4offroads.com/south-pole-expedition-world-record.html
/tvb
----- Original Message -----
From: "Magnus Danielson" magnus@rubidium.dyndns.org
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Saturday, November 21, 2009 4:05 PM
Subject: Re: [time-nuts] OT - GPS and North
Robert Darlington wrote:
It depends on the GPS receiver. The GPS chipset won't know (it knows
where
it is and can remember where it was to know what direction it's moving),
but
some consumer GPS receivers (Garmin, Magellan) have electronic compasses
built in. My Garmin eTrex Legend does NOT have a compass built in, but
taking about 2 steps in any direction will tell me which way I'm moving.
It
can't do this when stationary. My buddy's eTrex can point north without
moving, but that feature drains the battery faster.
The actual GPS receiver and the navigator is two different things. A GPS
navigator may use additional sensors such as accelerometers and magnetic
sensor to aid in addition to the GPS receiver built into them.
Agreed Magnus, but I dont think any gizmos are required. If the the
positions of the satellites are known, as they must be to enable the
antennas position to be calculated, I think just an extra set of
calculations is necessary to indicate the direction to anywhere else on the
planet (or elsewhere) including the geographic poles. Getting the magnetic
pole directions would need something else I suppose.
So a navigator may know, even if on a fixed location, but the GPS receiver
does not have that information.
It is important to keep these separated so that they can be discussed
properly. I don't want to say stuff like "The actual GPS receiver in the
GPS receiver...".
Cheers,
Magnus
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Tom Van Baak wrote:
Does a stationary (not in motion) GPS receiver know where the North is?
No, a stationary object is a point, not a line or a vector.
The notion of North (or any direction) has no meaning
to a point, by definition.
You are mixing things a litte too much here. There is no direction
within a 0-dimensional space, but a point as it is positioned in a
3-dimensional space has no problem to have an associated vector pointing
either to a location or along some field such as the magnetic field.
An electron is a point-charge, and reacts to a magnetic field or the
electrostatic field.
The antenna is certainly not a point, it is a sizeable object and it's
phase centrum isn't a point either, it's just a handy approximation.
It's just that normal GPS antennas and receivers isn't built for this
purpose. A much smaller object than either the antenna or the receiver
is the SO-8 packaged magnetic sensor you can buy cheaply and sense the
magnetic field. A GPS receiver can be used to compensate for magnetic
deviation if needed.
Cheers,
Magnus
mike cook wrote:
----- Original Message ----- From: "Magnus Danielson"
magnus@rubidium.dyndns.org
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Saturday, November 21, 2009 4:05 PM
Subject: Re: [time-nuts] OT - GPS and North
Robert Darlington wrote:
It depends on the GPS receiver. The GPS chipset won't know (it knows
where
it is and can remember where it was to know what direction it's
moving), but
some consumer GPS receivers (Garmin, Magellan) have electronic compasses
built in. My Garmin eTrex Legend does NOT have a compass built in, but
taking about 2 steps in any direction will tell me which way I'm
moving. It
can't do this when stationary. My buddy's eTrex can point north without
moving, but that feature drains the battery faster.
The actual GPS receiver and the navigator is two different things. A
GPS navigator may use additional sensors such as accelerometers and
magnetic sensor to aid in addition to the GPS receiver built into them.
Agreed Magnus, but I dont think any gizmos are required. If the the
positions of the satellites are known, as they must be to enable the
antennas position to be calculated, I think just an extra set of
calculations is necessary to indicate the direction to anywhere else on
the planet (or elsewhere) including the geographic poles. Getting the
magnetic pole directions would need something else I suppose.
Yes, but you don't know in what direction from the unidirectional
antenna. The antenna has no sense of direction, but the receiver knows
very well in what direction relative north each sat is. The NMEA stream
even include the coarse directions.
If you has a steerable directional antenna, several antennas or moveable
antenna, then you can convert the internal directions of the antenna
into a north heading or heading towards anything else.
But a normal unidirectional GPS antenna and associated receiver will be
able to know where it is and from it provide very high precision
direction relative north, but unless you move it or make any other form
of aid, it will not be able to show the heading... but when you move
around in with the car, then it can show it relative to the current
heading of the car... as that forms a vector for which you can relate an
angle... to north or to your intended destination.
Cheers,
Magnus
Yes, but it is rather difficult to manufacture an
isotropic antenna. Unless the antenna is oriented
so as to present a normal hemisphere of same gain,
it might be theoretically possible to model the gain
versus direction, coupled with the knowledge of
location of satellites and received strength, to
determine the antenna's orientation with respect
to the world.
Now, there are lots of sources of error and
potential ambiguities for this process. As
you say, it is more straightforward to use a
magnetic field sensor. With position information
it should be easy to correct for true North.
Mike - AA8K
Magnus Danielson wrote:
Yes, but you don't know in what direction from the unidirectional
antenna. The antenna has no sense of direction, but the receiver knows
very well in what direction relative north each sat is. The NMEA stream
even include the coarse directions.
If you has a steerable directional antenna, several antennas or moveable
antenna, then you can convert the internal directions of the antenna
into a north heading or heading towards anything else.
But a normal unidirectional GPS antenna and associated receiver will be
able to know where it is and from it provide very high precision
direction relative north, but unless you move it or make any other form
of aid, it will not be able to show the heading... but when you move
around in with the car, then it can show it relative to the current
heading of the car... as that forms a vector for which you can relate an
angle... to north or to your intended destination.
Cheers,
Magnus
If the
positions of the satellites are known, as they must be to enable the
antennas position to be calculated, I think just an extra set of
calculations is necessary to indicate the direction to anywhere else on the
planet (or elsewhere) including the geographic poles.
Mike,
The calculations tell you where on the globe you are. Correct,
from this you can easily calculate angle and distance to either
pole.
But the receiver cannot "indicate" this angle. Meaning if you
hold a GPS antenna in your hand you may know you have
to aim 75 degrees and walk 5000 km to the North pole but you
still have no idea how to turn around to take that first step.
A static GPS receiver is a point device, not a pointing device.
On the other hand a magnetic compass is a pointing device,
not a point device. That's why some navigation units combine
both.
If you just want to travel then as Confucius say: journey of a
thousand miles begins with single step. But if you want to
reach a particular destination then journey of a thousand
miles must begin with single step in correct direction. ;-)
/tvb
OK. Sme GPS receivers have magnetic sensors. What do they do with/about
magnetic deviation.
-John
==============
Tom Van Baak wrote:
Does a stationary (not in motion) GPS receiver know where the North is?
No, a stationary object is a point, not a line or a vector.
The notion of North (or any direction) has no meaning
to a point, by definition.
You are mixing things a litte too much here. There is no direction
within a 0-dimensional space, but a point as it is positioned in a
3-dimensional space has no problem to have an associated vector pointing
either to a location or along some field such as the magnetic field.
An electron is a point-charge, and reacts to a magnetic field or the
electrostatic field.
The antenna is certainly not a point, it is a sizeable object and it's
phase centrum isn't a point either, it's just a handy approximation.
It's just that normal GPS antennas and receivers isn't built for this
purpose. A much smaller object than either the antenna or the receiver
is the SO-8 packaged magnetic sensor you can buy cheaply and sense the
magnetic field. A GPS receiver can be used to compensate for magnetic
deviation if needed.
Cheers,
Magnus
Would Time Difference of Arrival techniques combined with an array of four closely spaced antennas work with Gps signals as a means of determing the orentation of the antenna array vs the gps satellites ? (I'm thinking traditional TDOA techniques may not work with gps signals.)
Mike Naruta AA8K wrote:
Yes, but it is rather difficult to manufacture an
isotropic antenna. Unless the antenna is oriented
so as to present a normal hemisphere of same gain,
it might be theoretically possible to model the gain
versus direction, coupled with the knowledge of
location of satellites and received strength, to
determine the antenna's orientation with respect
to the world.
Now, there are lots of sources of error and
potential ambiguities for this process. As
you say, it is more straightforward to use a
magnetic field sensor. With position information
it should be easy to correct for true North.
Mike - AA8K
Magnus Danielson wrote:
Yes, but you don't know in what direction from the unidirectional antenna. The antenna has no sense of direction, but the receiver knows very well in what direction relative north each sat is. The NMEA stream even include the coarse directions.
If you has a steerable directional antenna, several antennas or moveable antenna, then you can convert the internal directions of the antenna into a north heading or heading towards anything else.
But a normal unidirectional GPS antenna and associated receiver will be able to know where it is and from it provide very high precision direction relative north, but unless you move it or make any other form of aid, it will not be able to show the heading... but when you move around in with the car, then it can show it relative to the current heading of the car... as that forms a vector for which you can relate an angle... to north or to your intended destination.
Cheers,
Magnus
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Clearly, two three antennas can tell you everything you need to know. I've
seen video of a model helicopter flying itself with three GPS antennas.
NASA, I think.
-John
===========
Would Time Difference of Arrival techniques combined with an array of four
closely spaced antennas work with Gps signals as a means of determing the
orentation of the antenna array vs the gps satellites ? (I'm thinking
traditional TDOA techniques may not work with gps signals.)
Tom, this would be taking advantage of the
irregularities of the GPS receive antenna
to determine the orientation of the antenna.
For example, if the GPS antenna were a Yagi,
and it was pointed with the major lobe in
an Easterly direction, when you listen to
a satellite in the East, you know it's location
and you can observe a high signal strength.
A satellite in the West might have a low
signal strength, or be missing.
Now this thought experiment is loaded with
issues; for example, external attenuation,
multi-path, multiple lobes, small gain
differences, etc., but we're just having
fun here.
Mike - AA8K
Tom Van Baak wrote:
Mike,
The calculations tell you where on the globe you are. Correct,
from this you can easily calculate angle and distance to either
pole.
But the receiver cannot "indicate" this angle. Meaning if you
hold a GPS antenna in your hand you may know you have
to aim 75 degrees and walk 5000 km to the North pole but you
still have no idea how to turn around to take that first step.
A static GPS receiver is a point device, not a pointing device.
On the other hand a magnetic compass is a pointing device,
not a point device. That's why some navigation units combine
both.
If you just want to travel then as Confucius say: journey of a
thousand miles begins with single step. But if you want to
reach a particular destination then journey of a thousand
miles must begin with single step in correct direction. ;-)
/tvb
Mark Spencer wrote:
Would Time Difference of Arrival techniques combined with an array of four closely spaced antennas work with Gps signals as a means of determing the orentation of the antenna array vs the gps satellites ? (I'm thinking traditional TDOA techniques may not work with gps signals.)
There are commercial products available:
http://www.hemispheregps.com/Default.aspx?tabid=379
http://www.hemispheregps.com/Default.aspx?tabid=412
These devices claim less than 1 degree RMS heading accuracy.
Here the antennas are integrated and a fixed 0.5 meter apart. They do
(or did) make a board that could be used with separate antennas, but I
can not find it right now.
I have no idea of how the math works for computing heading.
Gary
Gary Chatters wrote:
Mark Spencer wrote:
Would Time Difference of Arrival techniques combined with an array of
four closely spaced antennas work with Gps signals as a means of
determing the orentation of the antenna array vs the gps satellites ?
(I'm thinking traditional TDOA techniques may not work with gps
signals.)
There are commercial products available:
http://www.hemispheregps.com/Default.aspx?tabid=379
http://www.hemispheregps.com/Default.aspx?tabid=412
These devices claim less than 1 degree RMS heading accuracy.
Here the antennas are integrated and a fixed 0.5 meter apart. They do
(or did) make a board that could be used with separate antennas, but I
can not find it right now.
I think this is the one:
http://www.hemispheregps.com/Products/PrecisionProductsGroup/PrecisionProducts/CrescentOEMModules/tabid/136/Default.aspx
I have no idea of how the math works for computing heading.
J. Forster wrote:
OK. Sme GPS receivers have magnetic sensors. What do they do with/about
magnetic deviation.
There is a field in one of the NMEA messages for magnetic deviation
which some GPS receivers fill in.
I do not know where they get the values. There is a mathematical model
available from an appropriate agency (I forget which one right now) that
can be used to compute magnetic deviation anywhere on earth. The model
is supposed to be good for about five years before the coefficients need
to be updated.
Gary
Gary Chatters wrote:
I have no idea of how the math works for computing heading.
Checkout FORWARD and INVERSE (and their 3d equivalents) at
http://www.ngs.noaa.gov/TOOLS/Inv_Fwd/Inv_Fwd.html
While these are hosted on a NOAA site, I'm sure I've seen
it on a NASA site as well. IIRC, there is also a C port
somewhere.
Iain
Mark Spencer wrote:
Would Time Difference of Arrival techniques combined with an array of
four closely spaced antennas work with Gps signals as a means of
determing the orentation of the antenna array vs the gps satellites ?
(I'm thinking traditional TDOA techniques may not work with gps
signals.)
There are commercial products available:
http://www.hemispheregps.com/Default.aspx?tabid=379
http://www.hemispheregps.com/Default.aspx?tabid=412
These devices claim less than 1 degree RMS heading accuracy.
Here the antennas are integrated and a fixed 0.5 meter apart. They do
(or did) make a board that could be used with separate antennas, but I
can not find it right now.
I have no idea of how the math works for computing heading.
Gary
Any (almost) pair of GPS-receivers with phase measurement outputs can be
used to make an attitude GPS receiver - sometimes called a GPS compass.
You need decent antennas for good results.
This is a special case of a phase ambiguity problem with moving base
receiver and extremely short baseline. If the antennas are mounted at a
fixed relative position, knowing the baseline gives a simpler problem.
I have read papers about using only one receiver and one antenna. The
trick is then to use the antenna diagram and SNR from the currently
tracked satellites to estimate an orientation. I have seen no commercial
product trying this. Accuracy was not spectacular - a few degrees - if I
remember correctly. Would need a very stable environment to work. A
groundapplication (say car moving in urban environment) will influence the
received SNRs to randomly to make a one antenna approach possible.
Most GPS manufacturers have attitude GPS receivers. If not dedicated its a
special case for their RTK capable versions.
--
Björn
J. Forster wrote:
OK. Sme GPS receivers have magnetic sensors. What do they do with/about
magnetic deviation.
There is a field in one of the NMEA messages for magnetic deviation
which some GPS receivers fill in.
I do not know where they get the values. There is a mathematical model
available from an appropriate agency (I forget which one right now) that
can be used to compute magnetic deviation anywhere on earth. The model
is supposed to be good for about five years before the coefficients need
to be updated.
Gary
One model is called the World Magnetic Model
http://en.wikipedia.org/wiki/World_Magnetic_Model
These models are the magnetic equivialent of the gravity models used to
determine the geoid.
--
Björn
Iain Young wrote:
Gary Chatters wrote:
I have no idea of how the math works for computing heading.
Checkout FORWARD and INVERSE (and their 3d equivalents) at
http://www.ngs.noaa.gov/TOOLS/Inv_Fwd/Inv_Fwd.html
While these are hosted on a NOAA site, I'm sure I've seen
it on a NASA site as well. IIRC, there is also a C port
somewhere.
I am not sure that this would be useful in this application. Getting
< 1 degree heading error from two positions would require about 8mm
absolute accuracy. I suspect that such accuracy requires something like
the phase measurements as mentioned by Björn.
Gary
bg@lysator.liu.se wrote:
J. Forster wrote:
OK. Sme GPS receivers have magnetic sensors. What do they do with/about
magnetic deviation.
There is a field in one of the NMEA messages for magnetic deviation
which some GPS receivers fill in.
I do not know where they get the values. There is a mathematical model
available from an appropriate agency (I forget which one right now) that
can be used to compute magnetic deviation anywhere on earth. The model
is supposed to be good for about five years before the coefficients need
to be updated.
Gary
One model is called the World Magnetic Model
http://en.wikipedia.org/wiki/World_Magnetic_Model
That is the one I was thinking of. Follow the Wikipedia link to the
NGDC webpage: http://www.ngdc.noaa.gov/geomag/WMM/
New model due 15 December.
Gary
When you think of time specifications from GPS, the GPS system is a
poor way to find north.
Even with a base line of 1000 metres you only have a fraction of a
degree.
The GPS system may be useful to get accurate time to simplify a star
observation, from a known (GPS)
position on this planet, but finding north is still a problem because
of the accuracy of a small
number of observations from a star fix.
Gyrocompasses take some time to get a measurement
( one hour) but even their estimate of North cannot match the
precision that the GPS system can get us with time.
cheers, Neville Michie
Interesting thanks !
----- Original Message ----
From: Gary Chatters gcarlistaa@garychatters.com
To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Sat, November 21, 2009 2:57:15 PM
Subject: Re: [time-nuts] OT - GPS and North
Mark Spencer wrote:
Would Time Difference of Arrival techniques combined with an array of four closely spaced antennas work with Gps signals as a means of determing the orentation of the antenna array vs the gps satellites ? (I'm thinking traditional TDOA techniques may not work with gps signals.)
There are commercial products available: http://www.hemispheregps.com/Default.aspx?tabid=379
http://www.hemispheregps.com/Default.aspx?tabid=412
These devices claim less than 1 degree RMS heading accuracy.
Here the antennas are integrated and a fixed 0.5 meter apart. They do (or did) make a board that could be used with separate antennas, but I can not find it right now.
I have no idea of how the math works for computing heading.
Gary
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bg@lysator.liu.se wrote:
Mark Spencer wrote:
Would Time Difference of Arrival techniques combined with an array of
four closely spaced antennas work with Gps signals as a means of
determing the orentation of the antenna array vs the gps satellites ?
(I'm thinking traditional TDOA techniques may not work with gps
signals.)
There are commercial products available:
http://www.hemispheregps.com/Default.aspx?tabid=379
http://www.hemispheregps.com/Default.aspx?tabid=412
These devices claim less than 1 degree RMS heading accuracy.
Here the antennas are integrated and a fixed 0.5 meter apart. They do
(or did) make a board that could be used with separate antennas, but I
can not find it right now.
I have no idea of how the math works for computing heading.
Gary
Any (almost) pair of GPS-receivers with phase measurement outputs can be
used to make an attitude GPS receiver - sometimes called a GPS compass.
You need decent antennas for good results.
This is a special case of a phase ambiguity problem with moving base
receiver and extremely short baseline. If the antennas are mounted at a
fixed relative position, knowing the baseline gives a simpler problem.
By having the additional antennas hooked to the same GPS core with
multiple frontends, the central antenna can act like a traditional
receiver. The carrier and code tracking-loops for additional antennas
can then be aided by the central antenna, which will help to reduce
ambiguities. After inital ambiguities have been resolved, maintaining
tracking should not be too hard.
Using multiple receivers does not give the same effect as tight integration.
I have read papers about using only one receiver and one antenna. The
trick is then to use the antenna diagram and SNR from the currently
tracked satellites to estimate an orientation. I have seen no commercial
product trying this. Accuracy was not spectacular - a few degrees - if I
remember correctly. Would need a very stable environment to work. A
groundapplication (say car moving in urban environment) will influence the
received SNRs to randomly to make a one antenna approach possible.
Such an approach is indeed possible but fragile. You could also include
shift in phase center, in which case a "bad" antenna would be good.
Knowing the orientation of the antenna could allow for post-processing
to compensate for phase-shift. You would loose precission from position
tracking but gain an estimate in heading.
Cheers,
Magnus
Hi Neville
When you think of time specifications from GPS, the GPS system is a
poor way to find north.
I do not understand. Please elaborate. A baseline of 10 meter will often
give better than 1mrad accuracy.
Even with a base line of 1000 metres you only have a fraction of a
degree.
This is just not true.
The GPS system may be useful to get accurate time to simplify a star
observation, from a known (GPS)
position on this planet, but finding north is still a problem because
of the accuracy of a small
number of observations from a star fix.
Gyrocompasses take some time to get a measurement
( one hour) but even their estimate of North cannot match the
precision that the GPS system can get us with time.
While legacy mechanical gyrocompasses on a ship, might take time to
converge - it does not take a modern strapdown INS more than about 4
minutes to find north while stationary. But having 4 minutes at hand with
some decent GPS-receivers its easy to get a north-measurement that will be
much better than the best available intertial systems.
cheers, Neville Michie
--
Björn
Neville Michie wrote:
When you think of time specifications from GPS, the GPS system is a
poor way to find north.
Even with a base line of 1000 metres you only have a fraction of a
degree.
The GPS system may be useful to get accurate time to simplify a star
observation, from a known (GPS)
position on this planet, but finding north is still a problem because
of the accuracy of a small
number of observations from a star fix.
Gyrocompasses take some time to get a measurement
( one hour) but even their estimate of North cannot match the
precision that the GPS system can get us with time.
cheers, Neville Michie
If you are taking star shots a stellar compass can easily provide a
boresight pointing accuracy of a few arcsec.
Bruce
Hi
If you only have one antenna and one receiver, the answer is fairly simple. Swing it around you head on the end of a long string. Plot the position reading vs time. Correlate the readings to the phase of the rotation.
It does indeed work (it's a doppler scanner ...). You could easily argue that it's not exactly a stationary situation any more.
Making it work correctly would involve a lot of work figuring out just how much lag the receiver has. You might have to swing it at a 10 rpm rate ...
Bob
On Nov 21, 2009, at 6:59 PM, Magnus Danielson wrote:
bg@lysator.liu.se wrote:
Mark Spencer wrote:
Would Time Difference of Arrival techniques combined with an array of
four closely spaced antennas work with Gps signals as a means of
determing the orentation of the antenna array vs the gps satellites ?
(I'm thinking traditional TDOA techniques may not work with gps
signals.)
There are commercial products available:
http://www.hemispheregps.com/Default.aspx?tabid=379
http://www.hemispheregps.com/Default.aspx?tabid=412
These devices claim less than 1 degree RMS heading accuracy.
Here the antennas are integrated and a fixed 0.5 meter apart. They do
(or did) make a board that could be used with separate antennas, but I
can not find it right now.
I have no idea of how the math works for computing heading.
Gary
Any (almost) pair of GPS-receivers with phase measurement outputs can be
used to make an attitude GPS receiver - sometimes called a GPS compass.
You need decent antennas for good results.
This is a special case of a phase ambiguity problem with moving base
receiver and extremely short baseline. If the antennas are mounted at a
fixed relative position, knowing the baseline gives a simpler problem.
By having the additional antennas hooked to the same GPS core with multiple frontends, the central antenna can act like a traditional receiver. The carrier and code tracking-loops for additional antennas can then be aided by the central antenna, which will help to reduce ambiguities. After inital ambiguities have been resolved, maintaining tracking should not be too hard.
Using multiple receivers does not give the same effect as tight integration.
I have read papers about using only one receiver and one antenna. The
trick is then to use the antenna diagram and SNR from the currently
tracked satellites to estimate an orientation. I have seen no commercial
product trying this. Accuracy was not spectacular - a few degrees - if I
remember correctly. Would need a very stable environment to work. A
groundapplication (say car moving in urban environment) will influence the
received SNRs to randomly to make a one antenna approach possible.
Such an approach is indeed possible but fragile. You could also include shift in phase center, in which case a "bad" antenna would be good. Knowing the orientation of the antenna could allow for post-processing to compensate for phase-shift. You would loose precission from position tracking but gain an estimate in heading.
Cheers,
Magnus
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