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I have a dream. A GPS/SBAS/QZSS dream ! In honour of MLK day.



 
 
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  #1  
Old January 17th 11, 06:54 PM posted to sci.geo.satellite-nav,rec.aviation.ifr
macpacheco
external usenet poster
 
Posts: 29
Default I have a dream. A GPS/SBAS/QZSS dream ! In honour of MLK day.

I have a dream. All IIA satellites retired. All IIF satellites in
orbit.
L2C signal set fully healthy with 19 operational satellites (12 IIF +
7 IIRM).
L5 signal set healthy even though only 12 IIF satellites are
broadcasting it. But their ranging signals are as perfect as
predicted, not enough for stand alone L5 ranging, but who will use L5
without L1 C/A ??? Nobody in their right mind.

That would be a dream for Jan 2016 actually. Two IIF launches per year
starting this year.

Back to reality, with the current status quo, the last IIF launch
won't happen before 2020. I'm willing to bet on this. L5 FOC, not
before 2030. Even if the doomsday mass IIA failure hypothesis from the
GAO happens, that would only mean accelerated IIF launches, but FOC L5
requires another 12 GPS III launches, that will only happen as IIR
satellites fail on mass, not likely at all.

Back to the dream, a steady IIF launch schedule would first retire all
undesirable IIA satellites without a working Rb clock. That would
improve average GPS clock performance substantially. Then replace the
rest of the IIA, even those with usable Rb clocks and enough buses and
reaction wheels working, those would become residuals.

A new informal 36 slot orbital scheme would be made. 30-32 active
satellites with 4+ residuals. No more pairs due to fear of failing
satellites, 6x6 orbital arrangement (today there's 3 primary slots for
each orbit, an additional 3 secondary slots would be created half way
between each consecutive pair of slots there is today). With 32
healthy satellites optimally spaced out and 4 residuals in the
remaining 4 slots, any satellite failure/maintenance is a non-event.
95% of the possible dual satellite failures would also be a non event,
with residuals brought online within 24 hrs of the failure event. No
need to delta V satellites around, when a satellite fails, just
schedule to replace that satellite and leave that slot open until
launch.

Remember there are today 3 WAAS, 4 EGNOS and 2 MSAS L1/L5 ranging
sources, with 3 QZSS on the way. If those sources are considered part
of the civil GPS constellation, even with 12 regular GPS L5 signals,
an IOC can be declared.

Since I'm pretty sure no one will listen... Why not float another
crazy idea, err, DREAM. The american aviation authorities poll
resouces and licence QZSS and launch a 4 (or perhaps 5) satellite
american QZSS, using a flat wide eight figure orbit, and use those
satellites to broadcast a replacement WAAS signal, replacing the
current GEOs, with an additional 20 reference stations in south
america, Caribean and a few more in central america (since Mexico is
already covered right now). My understanding is that QZSS is
broadcasting an L1 SBAS signal and since each satellite will have a Rb
clock, their ranging should be on par with a IIR ranging, except that
they broadcast L2C, L5 and L1C from the get go. With their wide orbit,
Alaska and northern Canada integrity signals would be improved to
perfect, as well as coverage all the way down to Antartica (wide
figure 8 orbits are much higher than GPS orbits = wider signal
coverage). Users outside the current WAAS coverage are will be
required to have L1/L5 GBAS receivers, so they can generate their own
IONO corrections = no longer need a station within 250nm. With
stations spread all over south america and data exchange with EGNOS
and MSAS all healthy GPS satellites should stay at a 3 meter UDRE all
the time, with the extra QZSS ranging also at 3 meter UDRE, that would
allow for LPV 200 coverage from -90 to +90 longitude in the American
side of the globe.

Remember that the current 3 GEO WAAS layout (assuming CRW back online
on it's original location) uses satellites so close to each other that
even with the 3 GEOs at their best ranging accuracy, they're just
about as useful as a single GPS satellite for ranging purposes. And
since they don't have an internal atomic clock, their ranging is
limited to 7.5 meter UDRE instead of the usual 3 meter UDRE of regular
GPS satellites. With an onboard Rb clock, hourly clock updates, QZSS
ranging could reach better quality than regular GPS satellites due to
triple frequency and on board laser reflector, sub inch ephemeris
accuracy possible all the time.

There, I said it, I have a dream. Suitable for MLK day.

Cheers,

Marcelo Pacheco
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  #2  
Old January 18th 11, 08:02 PM posted to sci.geo.satellite-nav,rec.aviation.ifr
HIPAR
external usenet poster
 
Posts: 6
Default I have a dream. A GPS/SBAS/QZSS dream ! In honour of MLK day.

I haven't seen proposals for a QZS type operation over the western
hemisphere, Many years of preparation will be required to convince
the technical community and political decision makers of a requirement
that leads to appropriations for that project. Currently funded GPS
projects operating along with forthcoming GNSS constellations remains
the best option for civilian PNT enhancements and US government policy
allows that.

I'm still skeptical about the technical quality of the IIFsatellites.
GPS IIF-1 might be experiencing thermal problems with respect to long
term clock stability. There are measurable distortions in the L5
signal. A recent report suggests IIF-1 didn't gracefully enter its
eclipse season. Now I'm suspicious if some last minute rework is in
progress prior to the next launch this summer.

The Air Force contends technical problems aren't an issue anymore ..
maybe one launch per year if the satellites are delivered and funding
is available to launch them.

Consequently, those IIA satellites will be with us for several years.
If the recent trend continues, one per year will be retired and
replaced. That really doesn't matter because modernized satellites
cannot be fully operated owing to limitations of the master control
station. So, I'm thinking L5 and L2C will not be certified until
about 2015 when OCX master control begins to come on-line. Meanwhile,
the legacy portion of the GPS constellation performs as advertised.

GPS III is pragmatically on schedule having achieved important
milestones. Now comes the hard part .. making the actual hardware
perform in accordance with modernized standards. I'm agreeing with
the GAO expecting several years slippage to 2016-2017 before the first
GPS IIIA is launched. Hopefully, with OCX, the 32 satellite limit
will be lifted and the modernized signals can finally be set usable.

That's my analysis.

--- CHAS





  #3  
Old January 19th 11, 01:39 AM posted to sci.geo.satellite-nav,rec.aviation.ifr
macpacheco
external usenet poster
 
Posts: 29
Default I have a dream. A GPS/SBAS/QZSS dream ! In honour of MLK day.

On Jan 18, 5:02 pm, HIPAR wrote:
I haven't seen proposals for a QZS type operation over the western
hemisphere, Many years of preparation will be required to convince
the technical community and political decision makers of a requirement
that leads to appropriations for that project. Currently funded GPS
projects operating along with forthcoming GNSS constellations remains
the best option for civilian PNT enhancements and US government policy
allows that.

I'm still skeptical about the technical quality of the IIFsatellites.
GPS IIF-1 might be experiencing thermal problems with respect to long
term clock stability. There are measurable distortions in the L5
signal. A recent report suggests IIF-1 didn't gracefully enter its
eclipse season. Now I'm suspicious if some last minute rework is in
progress prior to the next launch this summer.

The Air Force contends technical problems aren't an issue anymore ..
maybe one launch per year if the satellites are delivered and funding
is available to launch them.

Consequently, those IIA satellites will be with us for several years.
If the recent trend continues, one per year will be retired and
replaced. That really doesn't matter because modernized satellites
cannot be fully operated owing to limitations of the master control
station. So, I'm thinking L5 and L2C will not be certified until
about 2015 when OCX master control begins to come on-line. Meanwhile,
the legacy portion of the GPS constellation performs as advertised.

GPS III is pragmatically on schedule having achieved important
milestones. Now comes the hard part .. making the actual hardware
perform in accordance with modernized standards. I'm agreeing with
the GAO expecting several years slippage to 2016-2017 before the first
GPS IIIA is launched. Hopefully, with OCX, the 32 satellite limit
will be lifted and the modernized signals can finally be set usable.

That's my analysis.

--- CHAS


Hi Chas,

Did you look at L1 CA/L2C ICD ? PRN33 through 37 is reserved for
pseudolites and other ground applications. Even if OCX can control 36
(or more) satellites, in order to have 33 or more active PRNs, the ICD
will have to be changed in an incompatible way.

My bet is that even if OCX allows for control over 36 (or more) GPS
satellites, then 32 PRN limit will still be with us 10-20 yrs from
now. Perhaps it's valuable to keep online track of all residual
satellites, allowing for faster reactivation if needed (ephemeris
calculated at all times using only downlink control returns and cross
link ranging). Wait for them to fail and replace them. That's the USAF
current modus operandi. 24 active satellites is the baseline, so the
extra 8 satellites are used to allow offset the risk of using aging
IIA satellites.

My hope is that you're correct and I'm wrong, and the slow pace of IIF
launches is due to some IIF glitch that USAF isn't acknowledging, not
money (and launch manifesto) saving measures. Perhaps both money
saving and IIF concerns. Perhaps in 2/3 years USAF will declare the
built but unlaunched IIF satellites an strategic reserve and move on
to IIIA satellites.

There goes my bi monthly ranting, you've been warned... :-)

When IIF-2 is launched, if USAF is more interested in improving the
GPS constellation than trying to salvage SVN49, then SVN49 will be put
into residual status and PRN1 will be assigned to IIF-2. In this
scenario we'll finally have 32 healthy PRNs. But that's not an awesome
improvement. If you look carefully at the current GPS constellation,
the basic layout is still 18 primary slots + backups. Not even 24
primary slots. Right now there are 18 primary satellites + 14 backup
satellites (including SVN49). Those 14 backup satellites are not
contributing 20% of what they could contribute to PDOP if they were
positioned primarily to relief moments of PDOP weaknesses. The timing
of the PRN30 delta V shows me they don't care. They just allowed PRN30
to go from a great spot midway between PRN 16/PRN 25 to becoming a
useless (for PDOP) close pair to PRN 16. Even if you forget about a
6x6 constellation arrangement, just allowing each pair of satellites
to be positioned around the primary slot (with 10 degrees orbital
phase plus or minus **** for both members of the pair) in a wide pair
arrangement would give about 30-40% of the improvement between the
current constellation layout and an optimal 6x6 layout, maintaining
90% of the backup effectiveness they have today, since all pairs would
be kept fairly close to their primary slots. And no more triplets,
*please*, the PRN19/3/6 is a huge waste of satellites. Plus the
relative positioning of PRN29/11, when they fly close to PRN20/32
daily causing spikes in PDOP 4x daily (that's four SVs flying so close
that they're just about as useful as a single one at that 30 minute
interval, 4x daily = 2hrs daily impact). I hope PRN24 and PRN3 are at
the top of the GPS retirement list, since they're both IIA with Cs
only freq.standards and are positioned in hugely wasteful positions.
Just launching 3 IIF birds reusing PRN1/3/24 taking optimized mid way
positions would be a huge GPS constellation improvement. That doesn't
require OCX, that doesn't require 33+ active PRNs. That just requires
prioritizing constellation enhancements versus the PR nonsense of
"look, the GPS constellation is so great right now", so lets delay
launches that will have to happen anyways, and leave L2C and L5 as an
after thought.

Since there's still 11 IIA active SVs, 1 launch per year takes us to
2022 with no GPS IIIA SVs launched, perhaps just the first one for
testing purposes. Even considering SVN49 healthy, L2C IOC will only
happen when IIF SV-10 is operational (SV-11 if SVN49 isn't healthy by
then), circa 2020. To reach this conclusion all you need to do is to
read USAFs defense against the GAO report. It's pretty clear that
SVN23 isn't the only IIA SV that will live beyond 20 yrs old, and
given the reliability improvements made from IIA to IIR components,
IIRs average service life should be close (or better than) 25 yrs.
That delays launches of GPS III SVs for at least 10 yrs from the
current schedule. USAF has no plans to replace GPS SVs that are
compliant. As long as their signal is just good enough that their
signal is correctable with USAFs own augmentation system, then why the
hell do we need to waste US$ 100+ million on an early launch ???

From: http://www.insidegnss.com/node/2418
“From a constellation perspective, there is no compelling need for us
to launch the next IIF satellite prior to fiscal year 2012. However,
GPSW continues processing IIF SV-2 and we are following a plan to
launch by summer 2011.”

They modus operandi is: MONEY is SHORT ! Let's postpone a hundred
million expenditure (not a real savings, just a postponement) here and
there, while delivering a billion less in GPS signal value.
As long as there's 31 or 32 healthy compliant GPS satellites, USAF
will launch one satellite per year. GPS modernization = after thought.
If L5 takes another 20 yrs to reach FOC, than tough luck.
This maintains a heavy dependence on old IIA satellites in pairs to
survive satellites failures, which prevents any substantial
improvements to the GPS constellation.

A reorganization in a 6x6 layout with 32 active + 4 residual SV would
achieve:

1 - Worst case worldwide PDOP around 2 (last 24 hrs = 5)
2 - 99.99% median PDOP is 1.5 (last 24 hrs = 2.6)
3 - Worst case WAAS/EGNOS VPL = 15 meters, with 10 meters 99% median
(of course that's on the current reference station uses semi-codeless
and users use L1 C/A only). That's good enough for auto-land
operations that are forbidden today without GBAS (LAAS).
And that is true even on days with SV delta Vs or clock pumping
maintenance activity.

I said, American QZSS = crazy idea... The subject was "I have a
dream". The current WAAS GEO layout is very poor for ranging
performance (high UDRE and very limited geometry). Using QZSS like
satellites with a single on-board Rb clock would deliver strong full
Americas (and Hawaii) coverage and improve full americas PDOP and WAAS
VPL/HPL hugely. With a 5 satellite system, additional L5 and L1
ranging sources, dual frequency SBAS would be possible with just the
12 IIF launches (with a 5 degree mask, 3 satellites would be visible
from anywhere in the Americas at all times, and all 5 of them at
Equatorial and nearby latitudes). And I was not advocating that the US
pay for it, actually, I think it would be only fair that if the US is
paying for GPS and most (if not all) WAAS costs, than such an
improvement would be paid by the other countries. My Brazil has 200+ US
$ billion in currency reserves. Chile isn't bad either. Canada is
doing pretty well. Mexico can afford it. Colombia too. Countries would
chip in based on their relative volume of air traffic, so it would be
fair for everybody else, considering the savings in retiring ILS
systems all over the Americas. Of course, then all countries would
want full knowledge transfer and it would become a non-starter (see
the Brazilian air force fighter competition 10 year soap opera).
Again, crazy idea. 6x6 GPS constellation much more likely. Much
cheaper to just add GLONASS corrections to WAAS, and require dual
frequency WAAS receivers to have dual frequency GLONASS and Galileo
capability, much like EGNOS is doing (today it broadcasts GPS +
GLONASS corrections, and it will include EGNOS when the constellation
reaches required maturity). But then, FAA using a Russian system...
Not invented here syndrome... Also crazy idea. Perhaps more feasible
once Galileo comes only and GLONASS has dual frequency CDMA signals
with at least a dozen operational satellites. Augmentations don't
require full constellations from each original system. IOC GPS L5 +
IOC Galileo + IOC Glonass CDMA = MUCH BETTER than 30 operational L5
GPS satellites

Regards and thanks for following,

Marcelo
 




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