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Cellular forums Home > Archive > GPS > February 2008 > GEO SV's ephemeris
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GEO SV's ephemeris
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| Hi,
I was just wondering whether SVs with IDs like 126 or 137 are actually
used for navigation solutions or they are just WAAS/EGNOS relays?
They do transmit the C/A code at L1, and pseudoranges are observable
(for raw data capable receivers), but there is no ephemeris for them.
As these sats are geostationary their position does not change much
with respect to ECEF, but anyway we need to know an SV postion with a
sub-meter accuracy.
So, are there ephemeris for GEO GPS satellites?
Thanks,
VVX
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| claudegps 2008-02-11, 10:33 am |
| On 11 Feb, 15:07, VVX <vvx...@gmail.com> wrote:
> Hi,
>
> I was just wondering whether SVs with IDs like 126 or 137 are actually
> used for navigation solutions or they are just WAAS/EGNOS relays?
WAAS satellites can be used also for ranging, if they are enabled for
it (and if the receiver is capable).
> They do transmit the C/A code at L1, and pseudoranges are observable
> (for raw data capable receivers), but there is no ephemeris for them.
>
> As these sats are geostationary their position does not change much
> with respect to ECEF, but anyway we need to know an SV postion with a
> sub-meter accuracy.
>
> So, are there ephemeris for GEO GPS satellites?
I think they are broadcasted by the satellite itself... aren't they?
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| On Feb 11, 11:32 am, claudegps <claude...@gmail.com> wrote:
> On 11 Feb, 15:07, VVX <vvx...@gmail.com> wrote:
>
>
>
> WAAS satellites can be used also for ranging, if they are enabled for
> it (and if the receiver is capable).
>
>
>
>
> I think they are broadcasted by the satellite itself... aren't they?
Commonly refered to as 'bent pipe', the GEO signals are generated on
the ground by WAAS and uplinked to the GEO for rebroadcast.
The WAAS message (don't have the MOPS at home and I forget which
Message Type) contains the orbital parameters for the GEOs.
Regards,
Jon
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| utilsea@gmail.com 2008-02-12, 12:33 pm |
| Hi,
SBAS satellites are brodcasting exactly the same information as a GPS
one.
In fact ephemeris are broadcasted by SBAS satellites.
Some receiver are using those informations (ranging, phase and
ephemeris) to add 1 or 2 prn in the PVT.
Cheers
utilsea
http://utilsea.free.fr
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| Tim Springer 2008-02-13, 3:33 pm |
| Even if the GEOs do broadcast ephemeris this information is typically quite inacurate. The orbits of the GEOs are hard to
determine since, observed from Earth, they "do not move". Thus the "PDOP" of the GEO observation scenario is very poor. It is very
hard to get ~10meter type of orbits for GEOs.
So it is basically "fun" to use but it is not really improving you solution....
Cheers,
Tim
http://gnss.servolux.nl
<utilsea@gmail.com> schrieb im Newsbeitrag news:b072748f-e341-470c-a1a5- 735599100fe7@s8g2000
prg.googlegroups.com...
> Hi,
> SBAS satellites are brodcasting exactly the same information as a GPS
> one.
> In fact ephemeris are broadcasted by SBAS satellites.
> Some receiver are using those informations (ranging, phase and
> ephemeris) to add 1 or 2 prn in the PVT.
>
> Cheers
> utilsea
> http://utilsea.free.fr
>
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| claudegps 2008-02-13, 10:33 pm |
| On 13 Feb, 21:46, "Tim Springer" <Tim-Springer-nos...@t-online.de>
wrote:
> Even if the GEOs do broadcast ephemeris this information is typically quite inacurate. The orbits of the GEOs are hard to
> determine since, observed from Earth, they "do not move". Thus the "PDOP" of the GEO observation scenario is very poor. It is very
> hard to get ~10meter type of orbits for GEOs.
Intresting point.
> So it is basically "fun" to use but it is not really improving you solution....
It can improve your solution if it's (for example) your fifth
satellite :)
BTW it's usually not the case because they are above the equator, were
usually there already a lot of satellites.
Claudio
www.claudegps.altervista.org
[cut]
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| On Feb 13, 3:46 pm, "Tim Springer" <Tim-Springer-nos...@t-online.de>
wrote:
> Even if the GEOs do broadcast ephemeris this information is typically quite inacurate. The orbits of the GEOs are hard to
> determine since, observed from Earth, they "do not move". Thus the "PDOP" of the GEO observation scenario is very poor. It is very
> hard to get ~10meter type of orbits for GEOs.
Agreed on the performance of GEOs. The WAAS MOPS defines UDREI (UDRE
Index) and a good receiver will de-weight (the contribution of the
GEO's ranging signal) based on the index being broadcast in the WAAS
Message.
> So it is basically "fun" to use but it is not really improving you solution....
Perhaps, but it should never be worse... again for a good receiver,
read: one which properly implements the algorithms.
> Cheers,
> Timhttp://gnss.servolux.nl
Regards,
Jon
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| Thanks for the replies, everyone.
Another thing I do not understand is the GEO SVs' Dopplers.
The doppler measurements for those SVs are always around 700 Hz
meaning the satellites are actually moving relative to the receiver at
some 100 m/s. But they should not! (by the definition of GEO orbit)
Am I missing something (important) here?
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| Dale DePriest 2008-02-16, 3:33 pm |
|
Tim Springer wrote:
> Even if the GEOs do broadcast ephemeris this information is typically
> quite inacurate. The orbits of the GEOs are hard to determine since,
> observed from Earth, they "do not move". Thus the "PDOP" of the GEO
> observation scenario is very poor. It is very hard to get ~10meter type
> of orbits for GEOs.
>
> So it is basically "fun" to use but it is not really improving you
> solution....
>
There is no reason to believe the orbit ephemeris data is any different
from that of the other GPS satellites except that for short periods it
can be assumed not to move. The movement is detected by earth based SV
monitoring stations which are responsible for updating the data anyway.
In the case of the GEO SV's a master ground station uploads all of the
data. None is calculation on board. There is even differential
corrections available for those satellites as well as the standard GPS
satellites. Using the corrections they should be absolutely as accurate
as standard GPS satellites so they are quite suitable for ranging data.
Dale
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs
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| Tim Springer 2008-02-16, 10:33 pm |
|
"Dale DePriest" <Dale@gpsinformation.het> schrieb im Newsbeitrag news:13red5h5665ut73
@corp.supernews.com...
>
>
> Tim Springer wrote:
>
> There is no reason to believe the orbit ephemeris data is any different from that of the other GPS satellites except that for
> short periods it can be assumed not to move. The movement is detected by earth based SV monitoring stations which are
> responsible for updating the data anyway. In the case of the GEO SV's a master ground station uploads all of the data. None is
> calculation on board. There is even differential corrections available for those satellites as well as the standard GPS
> satellites. Using the corrections they should be absolutely as accurate as standard GPS satellites so they are quite suitable
> for ranging data.
>
> Dale
>
> --
> _ _ Dale DePriest
> /`) _ // http://users.cwnet.com/dalede
> o/_/ (_(_X_(` For GPS and GPS/PDAs
Dale,
I have to disagree with you on this. The ephemeris accuracy of GEO satellites is significantly less accurate than that of the GPS
satellites. The difference is being caused by the observation geometry! GPS (or even more general GNSS) satellites "move" when
observed from Earth. Thus they are visible from all over the world. So with permanent 24 hour tracking (as the GPS controllers do)
one gets a very good obeservability of the orbit. GEO satellites do not move, or at least not much. So even if you observed them
24 hours a day you really do not get a good orbit, or at least not easily. So where the GPS ephemeris these days is at the 1 meter
(!!!) level GEO ephemeris are at the 10 meter range (if this is not overly optimistic).
Besides the ephemeris error there are other problems (or at least "issues") with the GEO signals. I am not sure how WAAS works
exactly but at least with EGNOS the GEO functions as a transponder. So contrary to GPS the GEO does not generate the signal
itself. It just retransmits what it gets from the ground. Thus the signal "suffers" from transponder delays and double travel
times. So these transponded (sort of reflected) GEO signals are (much?) more complicated to model than the "direct" GPS signals.
Of course, you are completely right that with differential GPS (and/or GEO) the ephemeris error (and other GEO related errors)
will cancel out! So in a differential mode the GEO certainly does provide additional information!
Cheers,
Tim
http://gnss.servolux.nl
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| Dale DePriest 2008-02-16, 10:33 pm |
|
Tim Springer wrote:
>
> "Dale DePriest" <Dale@gpsinformation.het> schrieb im Newsbeitrag
> news:13red5h5665ut73
@corp.supernews.com...
>
> Dale,
> I have to disagree with you on this. The ephemeris accuracy of GEO
> satellites is significantly less accurate than that of the GPS
> satellites. The difference is being caused by the observation geometry!
> GPS (or even more general GNSS) satellites "move" when observed from
> Earth. Thus they are visible from all over the world. So with permanent
> 24 hour tracking (as the GPS controllers do) one gets a very good
> obeservability of the orbit. GEO satellites do not move, or at least not
> much. So even if you observed them 24 hours a day you really do not get
> a good orbit, or at least not easily. So where the GPS ephemeris these
> days is at the 1 meter (!!!) level GEO ephemeris are at the 10 meter
> range (if this is not overly optimistic).
>
> Besides the ephemeris error there are other problems (or at least
> "issues") with the GEO signals. I am not sure how WAAS works exactly but
> at least with EGNOS the GEO functions as a transponder. So contrary to
> GPS the GEO does not generate the signal itself. It just retransmits
> what it gets from the ground. Thus the signal "suffers" from transponder
> delays and double travel times. So these transponded (sort of reflected)
> GEO signals are (much?) more complicated to model than the "direct" GPS
> signals.
>
> Of course, you are completely right that with differential GPS (and/or
> GEO) the ephemeris error (and other GEO related errors) will cancel out!
> So in a differential mode the GEO certainly does provide additional
> information!
>
> Cheers,
> Tim
> http://gnss.servolux.nl
>
>
Well there a several ground stations that develop the differential and
in the case of GEO satellites the ephemeris data. Actually even with the
regular satellites they can correct the ephemeris data if needed. This
is done by using several satellites to compute their known location and
after comparison they can separate out the error component from the
satellites. They can use the GEO satellite to compute the known position
and adjust the data to ensure that the data at that location is correct.
Since there are several locations the actual position can be computed.
All of the GEO satellites actually traverse a known figure 8 pattern
that can be calculated mathematically since they are not perfectly above
the equator. They do not use, nor need to use observation geometry.
Dale
--
_ _ Dale DePriest
/`) _ // http://users.cwnet.com/dalede
o/_/ (_(_X_(` For GPS and GPS/PDAs
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| John R. Copeland 2008-02-16, 10:33 pm |
| "Tim Springer" <Tim-Springer-nospam@t-online.de> wrote in message =
news:fp7pe5$h2t$00$1
@news.t-online.com...
>=20
>=20
> I have to disagree with you on this. The ephemeris accuracy of GEO =
satellites is significantly less accurate than that of the GPS=20
> satellites. The difference is being caused by the observation =
geometry! GPS (or even more general GNSS) satellites "move" when=20
> observed from Earth. Thus they are visible from all over the world. So =
with permanent 24 hour tracking (as the GPS controllers do)=20
> one gets a very good obeservability of the orbit. GEO satellites do =
not move, or at least not much. So even if you observed them=20
> 24 hours a day you really do not get a good orbit, or at least not =
easily. So where the GPS ephemeris these days is at the 1 meter=20
> (!!!) level GEO ephemeris are at the 10 meter range (if this is not =
overly optimistic).
>=20
> Cheers,
> Tim
>=20
It escapes me as to why you'd say the orbital parameters of a satellite
in a 24-hour orbit would necessarily be less-accurately determined
than a satellite in a 12-hour orbit.
Earth's rotational speed is a non-issue in that measurement.
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| On Feb 16, 9:20 pm, VVX <vvx...@gmail.com> wrote:
> Thanks for the replies, everyone.
>
> Another thing I do not understand is the GEO SVs' Dopplers.
> The doppler measurements for those SVs are always around 700 Hz
> meaning the satellites are actually moving relative to the receiver at
> some 100 m/s. But they should not! (by the definition of GEO orbit)
> Am I missing something (important) here?
Ok, I got it. It was too much coffee :)
In fact raw doppler measurements for *all* SV's were biased by the
same amount - approx 700 Hz due to clock drift I guess.
Now, what would be a typical GEO SV velocity relative to the rotating
Earth surface? Is something below 0.5 m/s plausible?
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