Galileo, GPS and satellite count

Hello.
I'm trying to figure out why, when comparing Galileo with GPS, there
is often a reference to GPS indicating that it's constellation is only
composed by 24 satellites...
Example:
http://www.spacedaily.com/reports/Factfile_on_Galileo_Europes_rival_to_GPS_999.html

Galileo will consist of 30 satellites, six more than the US Global
Positioning System (GPS). The system will offer several services from
2014, becoming fully operational in 2020 when a constellation of 27
satellites, supported by three spares, is deployed.

It seems only Galileo marketing to me...
The GPS 24 satellites constellation was the original design, but I
don't even remember when there was only 24 sats!
Moreover they count the 3 spares on 30 Galileo satellites!

On 10/19/11 11:00 AM, claudegps wrote:
http://www.spacedaily.com/reports/Factfile_on_Galileo_Europes_rival_to_GPS_999.html

Here's the current GPS Count: 31


GPS OPERATIONAL ADVISORY 292.OA1
SUBJ: GPS STATUS 19 OCT 2011
1. SATELLITES, PLANES, AND CLOCKS (CS=CESIUM RB=RUBIDIUM):
A. BLOCK I : NONE
B. BLOCK II: PRNS 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14
PLANE : SLOT D6, D1, C2, D4, E3, C6, A4, A3, A1, E6, D2, B4, F3, F1
CLOCK : RB, RB, CS, RB, RB, RB, RB, CS, CS, CS, RB, RB, RB, RB
BLOCK II: PRNS 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 28, 29
PLANE : SLOT F2, B1, C4, E4, C3, E1, D3, E2, F4, D5, B2, F5, B3, C1
CLOCK : RB, RB, RB, RB, RB, RB, RB, RB, RB, CS, RB, RB, RB, RB
BLOCK II: PRNS 30, 31, 32
PLANE : SLOT B5, A2, E5
CLOCK : RB, RB, RB
2. CURRENT ADVISORIES AND FORECASTS :
A. FORECASTS: FOR SEVEN DAYS AFTER EVENT CONCLUDES.
NANU MSG DATE/TIME PRN TYPE SUMMARY (JDAY/ZULU
TIME START - STOP)
B. ADVISORIES:
NANU MSG DATE/TIME PRN TYPE SUMMARY (JDAY/ZULU
TIME START - STOP)
2011083 301528Z Sep 2011 24 UNUSUFN 273/1437-/
2011085 142008Z Oct 2011 01 USABINIT 287/1953-/
C. GENERAL:
NANU MSG DATE/TIME PRN TYPE SUMMARY (JDAY/ZULU
TIME START - STOP)
2011074 042312Z Sep 2011 GENERAL /-/
3. REMARKS:
A. THE POINT OF CONTACT FOR GPS MILITARY OPERATIONAL SUPPORT IS THE GPS
OPERATIONS CENTER AT 719-567-2541 OR DSN 560-2541.
B. CIVILIAN: FOR INFORMATION, CONTACT US COAST GUARD NAVCEN AT
COMMERCIAL 719-567-2541 24 HOURS DAILY AND INTERNET
HTTPS://WWW.NAVCEN.USCG.GOV
C. MILITARY SUPPORT WEBPAGES CAN BE FOUND AT THE FOLLOWING
HTTPS://gps.afspc.af.mil/GPS OR HTTPS://gps.afspc.af.mil/GPSOC

al_t...

GPS is currently limited to 31 satellites on the almanac. But that
limitation will be lifted with the OCX control segment becomes
operational latter this decade.
Plus the GPS III birds will be able to operate PRN codes higher than
the current PRN32 limit.
So its entirely conceivable that by 2020 we could have around 36
operational satellites. Right now there are 30 healthy satellites,
with PRN24 on the almanac unhealthy and PRN27 offline.
At the same time, once Galileo has 30 operational satellites, it could
be more reliable due to the lower average satellite life. But 30
operational for Galileo is far from guaranteed.
If you disregards all paired satellites, then GPS would have less than
24 satellites.
Right now only 18 Galileo satellites are funded.
Its important to realize that even if we have 36 operational GPS
satellites, at any given time we should have around dozen satellites
with single string status (some critical system have no redundancies
left, if that last critical system module fail, then the satellite is
dead).

Marcelo Pacheco

al_t...


I quite often see that annoying reference to 24 GPS satellites.
That's the baseline constellation as defined in the performance
specification .. 4 satellites at the prime slots in 6 planes. But,
also defined is the expanded constellation where B1, D2 and F2
positions are expanded into satellite pairs in fore and aft
locations. That's the 24+3 configuration that's flying now. So if
you want to quantify the NAVSTAR constellation in terms of 'effective'
satellites, more accurately it consists of 27 satellites. Remaining
satellites are placed to backup weak ones in prime slots.

--- CHAS

ival_t...

If you look at the positioning of those the constellation today, there
are 18 prime slots plus pairs. Some pairs are spread a little more
than others, but beyond those 18 truly prime (3 per orbital plane) all
others are pairs (or in some cases triplets).
A constellation composed of 36 uniformly spread slots (6x6) with 9
slots empty would perform hugely better than the current 30
operational satellites, even if three birds were to fail all at once
(but not neighboring satellites). But there are many issues preventing
from moving forward with such an improvement, the primary one is the
funding uncertainty of GPS launches beyond the absolute minimum
requirement to maintain current constellation performance, second
there are always much more important issues facing the brass of
SpaceCom that would propose such an improvement to the civilian
bosses, and third, the military leadership is conservative by nature,
they always want a crazy level of fault tolerance in the system, and
having no pairs doesn't look to the military like a safe system (they
like safety in numbers). But the fact is that uniformly spreading the
available satellites would result in multiple mutual backups between
satellites, reducing the impact of all failures, except for twin
neighbor failures (in which case it would still be less of an impact
than a pair of satellites were to fail).
If you look at the contribution of PRN30 as its closer to midway
between the prime slots to the east and west and look at PDOP maps
during its westerly move, you'll see that just PRN30 moving west has
resulting in sensible improvements to PDOP where PRN30 is flying over.
During the next two months as PRN30 drifts slowly through the midway
point, PDOP improvements will be maintained, but as PRN30 gets closer
to its intended destination this PDOP improvement will be reduced.

The substantial advantage that a full Galileo constellation would have
over GPS is it will have 3 orbital slots with a total of 27 prime
slots plus 3 backups. But that's not going to happen in less than 5
years, considering that only 18 birds are funded right now.

Marcelo Pacheco (always trying to stir some healthy controversy to
make people see through the typical PR of all large organizations of
the world - always an equal opportunity offender).