New GPS Configuration: 24+3=Win/Win
January 12, 2010
By: Don Jewell
Defense PNT Newsletter, January 2010
http://www.gpsworld.com/gnss-system/...243winwin-9369

Moving three existing GPS satellites to new orbit locations will have a
profound effect on GPS capabilities for all civil, commercial, and
military users worldwide.

Numbers and geometry matter where GPS is concerned, and many of you have
read these words in many of my columns or heard them in my
presentations. Fortunately the U.S. military must have been listening as
well, because just last week the CDRUSSTRATCOM, or the Commander of U.S.
Strategic Command, formerly the Commander of Air Force Space Command,
General Kevin P. ‘Chili’ Chilton approved “…a reconfiguration of the
GPS constellation to improve global coverage, as well as regional
coverage in areas where coverage can be degraded. [Think mountainous
regions of Afghanistan for example]. [Since] “ the current GPS
constellation has six fully operational satellites above the primary 24
satellites needed for a full, globally useful constellation …accepting a
plan to put three additional SV's into the operational constellation,
CDRUSSTRATCOM is improving overall global coverage.”

So much for the military jargon. What does this announcement really
mean? I can just see many of you scratching your head and mumbling
something about, so what, we already have 30 GPS satellites on orbit.
And you’re correct, that is the numbers side of the equation, but just
as important is the geometry portion, and that’s why this announcement
is so important. There are indeed 30 GPS satellites in MEO or medium
earth orbit that are used globally. However, many of the additional
satellites are currently flown in tandem, side by side, with
considerably older satellites, effectively limiting the constellation
geometry to that of 24 satellites.

Having augmentation GPS satellites on orbit is not a new state of
affairs for the GPS. While GLONASS, which was once fully operational, in
1996 for about six months, struggles to return to FOC or Full
Operational Capability sometime this year with 24 satellites (21+3), and
Galileo struggles over numerous financial and international hurdles with
plans to launch its first operational satellite this year (don’t hold
your breath) and start operations in 2014, the GPS constellation size
has been at 24 or more satellites for the last 14 years. These auxiliary
satellites on orbit, and our historical replenishment policy, now have
the added advantage of giving the U.S. military the capability to
significantly improve the geometry and consequently the accuracy of GPS
for U.S. and allied warfighters, especially in mountainous AORs (area of
responsibility) and urban canyon situations. Simply moving three
existing GPS satellites to new orbit locations will have a profound
effect on GPS capabilities for all civil, commercial, and military users
worldwide.

Timeline

The official military announcement laid out the 24+3 or “Expandable 24”
as the military has branded it, timeline as follows:

“The [24+3] initiative will take up to 24 months to fully implement as
satellites are repositioned within the constellation based on
constellation health. The beneficial impact to all GPS users, including
civilian users, will be slowly realized during that time period. Over
the next two years, the number of GPS satellites in view from any point
on earth will increase, potentially increasing accuracy of GPS receivers.”

This all came about when an independent GPS panel, comprised of GPS
experts, conducted feasibility studies and along with the experts at the
GPS Wing and the 50th Space Wing made the 24+3 recommendation to General
Robert Kehler and his staff at HQ Air Force Space Command (AFSPC) and to
Lt General Larry James the Commander of 14th Air Force and the Joint
Functional Commander for Space or JFCC Space. General’s Kehler and James
approved the plan and presented it to General Chilton at USSTRATCOM.
Things happened quickly after that and, according to Colonel David
Buckman, the AFSPC command lead for GPS, the first GPS SV to be moved
(SVN 24) actually begins its long journey on January 13. This is the
satellite that has the farthest to travel, and we will not see SVN 24 in
its new slot for approximately 12 months (January 2011). The two
additional satellites being moved, SVN 49 and SVN 26, will affect the
geometry much sooner. SVN 49 will start its journey on January 21 and be
in place in as little as four months (May 2010), and SVN 26 will begin
to move on February 8 and should be in place in approximately three
months (May 2010), if all goes as planned. The bottom line is the
decision has been made and it is a good one that will positively affect
all GPS users on a global basis.

I won’t go into all the technical details in this column, because I can
guarantee you this will be studied and written about in great detail for
the next 12 months. And I should point out that several augmentation
options have been discussed for several years, but it was just recently
that all the trade and technical studies were completed to make 24+3 a
viable option. Again, the important thing is it is happening as you read
this column, and it will benefit you as a GPS user.

New Control Software

About the same time the pivotal 24+3 decision was being announced, the
2SOPS (2nd Space Operations Squadron) at Schriever AFB made its own
announcement. The professionals who operate and maintain the GPS
constellation on a daily basis announced they are upgrading the ground
control segment software provided to them by Colonel David Madden and
the folks at the GPS Wing in Los Angeles. The current GPS ground control
system known as AEP, or Architecture Evolution Plan, has been
controlling the constellation since September 2007. AEP was upgraded on
January 11 to enable telemetry, tracking, and commanding for the new GPS
IIF satellites (yet to be launched) and provide over-the-air (OTAR)
distribution of encryption keys for military user P(Y)-code equipment,
better known as SAASM-based UE or selective availability anti-spoofing
module user equipment. OTAR has been a long time in development and will
be heartily welcomed by our warfighters.

Next-Generation Boeing IIF GPS Satellites

While the control segment upgrade announcement may sound innocuous, it
is actually of critical importance operationally, and is also indicative
of several important schedule slips. Originally the first Boeing IIF
satellite (IIF-1), which has been plagued with numerous problems, was to
be transported to Cape Canaveral in December 2009 for launch in the
first quarter of 2010. Now it appears that IIF-1 won’t make the
cross-country trip to the Cape until February. This totally expected,
indeed predicted, schedule slip moves the earliest possible inaugural
launch date, in an already crowded launch schedule, to the middle of
2010 and a probable operational date for IIF-1 around the August 2010
timeframe.

The new capabilities of the Boeing built IIF satellites include the new
L5 navigation signal for civil users, continued deployment of an
encrypted military-only code known as M-Code, on-orbit crosslink
(between GPS satellites) improvements, and overall signal power
increases. Hopefully these enhancements along with the 24+3 geometry
will lead to a more robust constellation and greater user accuracy. Now
we just need to get them launched and on-orbit.

OCX Competition: Contract Award

For those of you who are keeping track, there was a contract award
scheduled for December 2009 of the next=generation or OCX control
segment software. The long-overdue control segment system software will
thankfully finally replace the already outdated AEP system. Best
estimates are the contract award will now take place in February 2010;
and it can’t be too soon, especially for the two prime competitors,
Northrop Grumman and Raytheon.

Civil GPS Improvements

While I have concentrated on support to the warfighters, because that’s
what I do, it should be obvious that 24+3 and the new control segment
software will benefit all GPS users globally. Indeed when all the trade
and technical studies were made it was with the civil user in mind as
well as the warfighters. What makes this a WIN/WIN for everyone is that
we all benefit. For example 24+3 will be especially beneficial to the
RTK (real time kinematic) users who currently require six satellites in
view for a very precise (centimeter accuracy) position. In the last
couple of years some users that require long dwell times and experience
high mask angles have been forced to use GLONASS satellites as an
augmentation, which works, of course, but GLONASS satellites have
historically been less accurate than the GPS. When 24+3 is fully
implemented, hopefully GLONASS augmentation will no longer be necessary.

Policy

My hat is off to all those who helped make the 24+3 decision possible.
This is a timely strategic decision made to support warfighters while
our nation is fighting a war on two fronts in Southwest Asia. It is a
bold and timely move; the decision makers stood up and made the right
choice. I want to emphasize that this is not a major policy shift or
change, it is Air Force Space Command and USSTRATCOM supporting the
warfighters, and the rest of us are beneficiaries of a smart and timely
decision. Stay tuned for more. Any takers for 27+3?

Until next time, happy navigating.