Nine GPS Satellites May Fail in the Next 20 Months!

GPS Insights -- January 2008
http://mg.gpsworld.com/gpsmg/content/printContentPopup.jsp?id=483020

One of the most common questions I get from readers concerns the
disparity over the "official" number of GPS satellites. This seems like
a very simplistic question on the surface, but it has a
less-than-simple explanation.

Jan 10, 2008
By:Don Jewell
GPS World

GPS Tracking the New Year

As you read this, we are about two weeks into 2008, and I hope you
have a great year. Certainly for GPS World this is going to be a
phenomenal year, if for no other reason than the myriad new
developments surrounding GPS. And that's part of what we will explore
in this, my first column of the New Year.

For instance, did you know that between now and October 2009, the GPS
Wing is predicting that as many as nine GPS series IIA satellites may
fail. It sounds worse than it is, and I will explain that later in
this newsletter.

But first I must start off the New Year thanking all you who read GPS
World, both the print and electronic versions. I have discovered many
of you subscribe to and read both formats, especially the Military
and Government section, where we endeavor to keep you up to speed on
the latest developments and happenings in the world of GPS.

Thanks also for all the letters and e-mails. I started to say the
hundreds of e-mails, but to be more accurate, thanks for the
thousands of e-mails during the last year. They usually fall in the
category of thanks for a particular column or wanting more
information, but of course a few have been critical -- thankfully,
that is an extremely small number. It's usually just a
misunderstanding, but even so, we just can't make all the people
happy all the time, but we will continue to do our best. The bottom
line is, keep those cards and letters coming! I enjoy hearing from
you, especially those who suggest new ideas for columns.

For example, one of the most common questions I get from all levels
of readers, inside and outside of the government, concerns the
disparity over the "official" number of GPS satellites. This seems
like a very simplistic question on the surface, but in actuality has
a less-than-simple explanation.

The actual number of on-orbit and active satellites should never be
in question, because it is always available via NANUs or GPS Notice
Advisory to Navstar users and on numerous websites, of which the
"most official" is the United States Coast Guard (USCG) official
NAVCEN website: http://www.navcen.uscg.gov/gps/nanu.htm -
www.navcen.uscg.gov

If you look there today, you will (hopefully) see that there are 32
active GPS satellites on orbit, five of which are the latest Lockheed
Martin-built GPS IIRM satellite vehicles that have several advantages
over their predecessors. This is one of those great new developments
in the GPS world I was speaking of earlier. Indeed, the GPS Wing says
these five IIRM satellites provide global coverage and increased
performance of the GPS services to users worldwide. The modernized
series delivers increased signal power to receivers on the ground,
two new military signals for improved accuracy, enhanced encryption
and anti-jamming capabilities for the military, and a second civil
signal to provide users with an open access signal on a different
frequency.

While I applaud Lockheed Martin for these great new satellites, our
current question really concerns the number 24. In almost all the
literature you read about the GPS NAVSTAR constellation, it says
something like this: "The GPS constellation consists of 24 satellites
orbiting the Earth to provide worldwide coverage, 24 hours a day,
free of charge. GPS receivers are passive, so the system can serve an
unlimited number of users."

I have a lot of problems with this statement, because several facts
are technically misstated, but for now let's deal with the number 24.
Why does the literature say the GPS constellation consists of 24
operational satellites when we currently have 32 on orbit, and have
had more than 24 for several years?

The first part of the answer is very simple: there are 24 primary GPS
orbital slots, and the best satellites on orbit are typically
assigned to these slots if possible. For the rest of the answer, we
need to go back to the beginning of the NAVSTAR GPS program in the
late 1970s. Fortunately, a friend and colleague of mine was the first
GPS SPO commander in those days, and actually negotiated the size of
the constellation as well as most aspects of the system we have on
orbit today. So, according to Professor Bradford W. Parkinson,
currently at Stanford, who was then Colonel Parkinson USAF, avid
aviator and navigation guru, the original constellation size was set
at 21 satellites, with three active spares on orbit for a total of
24.

You need to know that in those days, GPS was seen as a military
system only, and there were no thoughts of granting public, much less
worldwide, free access to the GPS signals. It was basically a
military en route navigation system, and not much was thought about
it beyond that. Certainly there was no serious consideration given to
GPS becoming the de facto world standard for time.

The 21 GPS satellites with three on-orbit spares were deemed a
workable number for military purposes, and that was that. I won't go
into the long history of the NAVSTAR GPS, other than to say that a
Korean airliner shot down by the Soviets -- and a very savvy
President Ronald Reagan -- made public access to the GPS signals a
priority (see my May 2007 GPS Insights column in GPS World for the
complete story).

Since that time, the NAVSTAR GPS program has never been the same and
it's become a critical system in a "system of systems" approach by
the U.S. government. And I mean that in a good way. That was about
the same timeframe that GPS World magazine came about, and I was a
lucky recipient of the very first issue, which I still have
somewhere. Suffice it to say it was not very large, in the number of
pages anyway, but was huge in concept. I also remember thinking, at
the time, what are they going to find to write about next month?

Well, many years have passed (eighteen for GPS World this month), and
here we are with a magazine and a GPS constellation that have grown
by leaps and bounds, and all for the better. But I digressu2026

When you continually see the number 24 for the size of the NAVSTAR
GPS constellation, it is simply the number of satellites on orbit
that the USAF has guaranteed will always be there for constellation
sustainment. It is the minimum number of satellites with spares that
will guarantee worldwide coverage of the navigation signal. Notice I
said "navigation signal" and not global 3-D or 4-D coverage, because
even with 24 satellites on orbit there are spots on the surface of
the Earth that for brief periods of time would not have the necessary
number of satellites in view (three) for a satisfactory GPS position
solution. I think I am safe in saying that the coverage for basic
navigation is in the 99th percentile with 24 satellites, but that
does not provide 4-D coverage or accurate heading, altitude, and
velocity vectors.

The increase to 32 active satellites on orbit supplies almost global
4-D coverage and allows more PDOPs (Position Dilution Of Precision)
closer to 90 degrees, and that generates much greater position
accuracy. Indeed, to answer yet another common question, PDOP is
actually a function expressing the mathematical quality of solutions
based on the geometry of the satellites. PDOP has a best-case value
of 1, with higher numbers being worse. The best PDOP occurs when one
satellite is directly overhead and three others are evenly spaced
about the horizon. The higher the PDOP number, the less accurate the
position. Theoretically PDOP could be infinite, if all the satellites
were in the same plane. But in the NAVSTAR GPS there are six evenly
spaced planes, so this never occurs. Actually the current GPS
constellation was designed and operated as a 24-satellite system,
consisting of six planes, with a minimum of four satellites per plane
for many years. Twenty-four was never the absolute optimum number; it
was a number that worked and fulfilled most of the requirements of
the day.

When all is said and done, 21 satellites with three on orbit
operating spares is all the USAF is officially committed to, but
unofficially they hope never to get down to that number again. In the
current orbital configuration scheme, 32-33 satellites actually work
out to be the optimum number. As the number of planes changes, so
does the number of satellites for optimum coverage.

I know I went on a bit here, but this has given me the opportunity to
answer several hundred e-mails, and it segues nicely into another
common question.

Current Constellation

The second most common question I get asked is, what is the optimum
constellation size for the current configuration, and why doesn't the
constellation size change with each additional GPS launch?

In the current constellation there are indeed 32 satellites, and
normally that would be nearly the perfect constellation
configuration, but a few of the older satellites and payloads are
"single string" in space parlance (or on their last legs) and require
substantial care and feeding, including power management, by the very
talented personnel/crews at the 50SW (Space Wing), 2SOPS (Space
Operations Squadron), at Schriever AFB in Colorado, and the intrepid
engineers at the GPS Wing at Los Angeles Air Force Base in
California.

Each GPS satellite is designed with an "A" and "B" side that
approaches 100% redundancy for critical systems. Several of the
satellites were switched to the "B" side years ago and have
significantly outlived their design life, which differs with each
series of satellites launched.

Of course, the various contractors are ecstatic when a GPS satellite
remains operational beyond its design life because it gives them
bragging rights, but more importantly they get paid more money for
every extra year of longevity. The government usually considers it to
be a good thing because it helps us maintain that optimum
constellation number, but the down side is that most older satellites
are not as programmable or as capable as the newer versions, so you
generally wind up with a constellation that requires much more work
to maintain and actually provides less capability on orbit. So, as
you've heard me say before, "The good news is the GPS satellite
lasted 14 years and the bad news is the GPS satellite lasted 14
years." Kind of depends on which side your bread is buttered.

Therefore, don't be surprised that as we launch more and more GPS
satellites (IIRM+s), the number of active satellites in the
constellation stays the same. Since we have 32 satellites on orbit
(remember, that is almost the optimum number), we are in a
replenishment mode, and attempting to maintain the constellation at
the optimum number while still adding new capabilities, or
modernization -- a good thing for war fighters when we are involved
in several hot conflicts/wars around the globe.

Nine GPS Satellites May Fail in the Next 20 Months!

Now, what about the nine possible failures of the IIA series GPS
satellites? The satellites in question are all at or beyond their
design life, and have critical failures. They are being kept alive by
heroic means that require exceptional amounts of time and money. If
the worst should happen and all nine IIA birds fail, then we would be
down to 23 satellites, which is far from the optimum number -- but
remember, we will be launching the rest of the IIRM satellites at the
same time, and that should put the number of on-orbit GPS satellites
at about 29. Colonel Dave Madden says the goal is to stay as near the
optimum number as possible, but to certainly never go below 27
satellites if possible.

OK, I think that takes care of the numbers thing. Now on to the
Perfect Handheld GPS Transceiver (PHGPST), and more cutting-edge
technology.

Several of our war fighters have written and complained bitterly
about battery life on the PLGRs (Precision Lightweight GPS Receivers)
and DAGRs (Defense Advanced GPS Receivers). Batteries really don't
like heat and let's face it, most of the year this is a serious
problem in Iraq and in parts of Afghanistan. Plus, according to the
war fighters in the field, running the SAASM or Selective
Availability Anti-Spoofing Module on the current receivers depletes
the battery so quickly that users rarely see half the claimed battery
life on their military GPS receivers.

Several calls to major manufacturers helped me determine that
everyone is working on new battery technologies, but at the same time
we are adding more and more requirements and capabilities to our
receivers which equals even less or at a minimum, no increase in
battery life.

One possible solution is solar power. Simply add a solar charging
system to the GPS unit, and the batteries recharge automatically.
Face it -- the current military GPS units only work when they can
"see" the open sky, so voila, during the day solar charging extends
the battery life on the unit you are using and charges the batteries
on spare units.

Sounds like a simple solution to me, and evidently commercial GPS
providers agree because there are numerous solar solutions for
extending battery life that claim a good solar cell design can extend
the GPS device usage time up to 30 hours. Now that is an incredible
increase of 100% over what our war fighters are currently
experiencing with their PLGRs and DAGRs today with the SAASM turned
off. It could even mean that a DAGR with SAASM activated might turn
out to be a much more useful device.

Next time we will talk more about the PHGPST and what it can do for
our war fighters. See you right here next month.