Lucky Number Five (L5)

Lucky Number Five
http://sidt.gpsworld.com/gpssidt/content/printContentPopup.jsp?id=3D58=
3167

System Design & Test Newsletter, February 2009

Feb 24, 2009
By: Alan Cameron
GPS World

By this time next month, we should have witnessed GPS Modernization Truly=
Rising. In the=20
form of IIR-20(M), the GPS satellite carrying the first payload to transm=
it the L5 signal.=20
20(M), originally slated to launch in June 2008 but hampered by launch ve=
hicle problems,=20
is now purported to rise March 24 from Florida=E2=80=99s Cape Canaveral A=
ir Force Station.
Also by this time next month, I hope to have just sent off to press the A=
pril issue of GPS=20
World, containing an in-depth technical article on aspects of the L5 sign=
al, by authors at=20
a company intimately connected to the space vehicle.

Not to hold our collective breath too long, let=E2=80=99s recap what we k=
now so far about L5.

In the beginning, GPS modernization called for:

implementing military (M) code on the L1 and L2 frequencies for the Depar=
tment of Defense=20
(DoD)
providing a new L5 frequency in an aeronautical radio navigation service =
(ARNS) band with=20
a signal structure designed to enhance aviation applications
adding the C/A code to L2.
Before we get too excited, let=E2=80=99s remember that a useful L5 signal=
is still years away =E2=80=94=20
several years. The L5 signal going into orbit next month (we hope) is pur=
ely a=20
placeholder, to secure the frequency with the International Telecommunica=
tions Union.=20
Lockheed has modified one of the last Block IIR-M satellites to broadcast=
some test data=20
on the L5 frequency (1176MHz).

However, the L5 test data being broadcast won't be usable by your GNSS re=
ceiver even it it=20
is capable of receiving GPS L5. It=E2=80=99s next to certain that an L5-c=
apable GNSS receiver=20
purchased today would require a serious software update (if not hardware)=
by the time=20
usable data eventually starts broadcasting on L5.

Further life for L5, after March launch and subsequent commissioning of I=
IR(M)-20, won=E2=80=99t=20
resume until the troubled Block IIF satellites begin lofting. 12 are plan=
ned, with the=20
first optimistically slotted for sometime in 2009. The IIFs will broadcas=
t L1 C/A, L2 P/Y,=20
L2C =E2=80=94 and L5. Even at an aggressive launch rate of three per year=
, all twelve wouldn't be=20
in orbit until the end of 2012. 12 satellites broadcasting on L5 would br=
ing users some=20
benefit, but nothing like that envisioned from a full constellation.

Hurry up and wait for the next installment, Block IIIA, maybe in 2014, wh=
en the first of=20
eight or possibly 10 may start broadcasting L1 C/A, L2 P/Y, L2C, L5, and =
L1C. Full=20
operational capability may not arrive until the next decade.

Besides launching satellites, L5 faces a bevy of technical issues still t=
o be fully=20
addressed, such as the control segment infrastructure (software and hardw=
are). Not=20
forgetting non-technical issues such as money. Launching satellites is an=
expensive hobby.

Why L5 Will Rock

The oncoming new signal will bring significant benefits to the high-preci=
sion user. Its=20
most important uses will come in aviation, however.

L5 will bring to an end the era of worrying about ionospheric activity. A=
t 1176 MHz, the=20
L5 frequency separation from L1 (1575 MHz) is significant enough that hig=
h-grade, triple-=20
or even dual-frequency L5-capable receivers will mitigate the ionospheric=
refraction down=20
to a nearly negligible factor. Because ionospheric refraction error is in=
versely=20
proportional to frequency squared, ionospheric error at L2 is 65 percent =
larger than at=20
L1, and at L5 it's 79 percent larger.

L5=E2=80=99s broadcast strength will be roughly four times that of L2C. A=
stronger signal combined=20
with a superior code structure means that you'll get more robust performa=
nce in tough GPS=20
conditions. That's great news for high-precision users working in margina=
l GPS conditions.

Through the Past Darkly

Way back in September, 2001, Tom Stansell, an advisor to the magazine and=
various=20
government and private entities, envisaged this future for, well, turns o=
ut it was last=20
year. We now take you back in time, to look forward in time =E2=80=94 wi=
stfully. A prize to the=20
reader who first e-mails me the accurate count of prophecies that did not=
come true in the=20
following account:

The Scene: 2008 The meeting started at 9:00 AM in a small conference room=
at Acme=20
Industries. Fred, Acme's product development manager, had attended ION GP=
S-2008 the=20
previous week, and he wanted an update on the GPS chipset alternatives fo=
r the 2009=20
product introductions. . . . At the ION conference, GPS chipset vendors =
had impressed=20
Fred with the wide variety of options available, including single-frequen=
cy and=20
multi-frequency chipsets for all three civil GPS signals at L1, L2, and L=
5. He wanted a=20
better understanding of the new options and what they might mean for Acme=
's markets.

"Thirty satellites now transmit L1 C/A," Valerie began, "but as this slid=
e shows, only 20=20
have the L2 civil signal, and only nine have the L5 signal. I think Al ag=
rees we can't=20
sell single-frequency L2 products until there are at least 24 satellites =
in good orbit=20
positions. Until then, even with a better signal, we can't overcome the g=
eometry advantage=20
a 30-satellite constellation gives L1-only products."

"A year from now, in late 2009," she continued, "we expect to see a good =
24-satellite L2=20
constellation, so I'm starting to design for L2. But there's no guarantee=
=2E I don't know=20
whether we should put all our chips on L2 - no pun intended - or delay an=
other year until=20
we're sure of the constellation, or offer two flavors of equipment and le=
t our customers=20
decide. We also don't know what our competitors will do, so our options s=
eem to be either=20
picking one signal and taking the market risk or spending the extra money=
to cover both=20
options."

"Twelve IIR satellites were modernized into IIR-Ms to speed-up the availa=
bility of the=20
military M code on L1 and L2 and the civil code on L2. However, it wasn't=
feasible to put=20
L5 on the modified satellites. That had to wait for the IIF series now be=
ing launched. So,=20
until the twelve IIR-M satellites reach end of life and are replaced, L5 =
won't be on every=20
satellite. Any delay is a shame, however, because L5 is a great signal.

"The avionics manufacturers had a similar but even more difficult problem=
=2E Common practice=20
was for avionics to be supported for 20 years after installation on a com=
mercial airplane.=20
Can you imagine the dilemma of knowing that signals which had never been =
launched would=20
fill the sky years before the avionics was replaced? There was disagreeme=
nt in the FAA=20
about whether to use the L2 civil signal or not, because it isn't in an A=
RNS (Aeronautical=20
Radio Navigation Service) band, even though it would be available years e=
arlier than L5=20
and, by increasing signal redundancy, would give substantial protection a=
gainst GPS=20
interference. Also, it wasn't clear whether or when WAAS or LAAS would su=
pport either or=20
both of the new signals. One solution was a modular design, supporting fu=
ture upgrades,=20
including software upgrades, by adding or exchanging plug-in components."=


"What about L5, then?" Fred asked.

Charley answered, "We're excited about L5 and, if possible, we want to in=
clude it as part=20
of the overall transition to consumer chipsets. Unfortunately, we may hav=
e to do something=20
sooner than consumer chips are available, because we wouldn't want our co=
mpetitors to have=20
it first. Like Val said, so far there aren't very many signals, but in th=
e future our=20
high-end products will use all three frequencies. This will speed up ambi=
guity resolution=20
and extend baselines by permitting better ionospheric corrections over lo=
ng distances.=20
This is an important improvement, but it certainly has challenged the ant=
enna designers to=20
maintain low-multipath patterns with good sensitivity and tightly control=
led phase center=20
characteristics for all three frequencies!"

Valerie resumed, =E2=80=9CWe prefer to use the L2 civil signal now for co=
nsumer products rather=20
than stick with the tried-and-true L1. It's a better signal.

"That doesn't mean it's better for everything, so we won't abandon L1, an=
d for many future=20
applications we'll use L5. That's the beauty of having three rather diffe=
rent civil=20
signals to choose from, because we can choose the best signal for each ap=
plication.

"Relative to L1, the raw signal power on L2 is 2.3 dB weaker, but on L5 i=
t's 3.7 dB=20
stronger, for a 6 dB advantage of L5 over L2. We hope these differences w=
ill slowly=20
disappear as more GPS III satellites with increased L1 and L2 power are l=
aunched. Both L2=20
and L5 use FEC, and the data rate on L2 is 25 bits per second versus 50 o=
n L1 and L5.=20
Signal tracking threshold on both L2 and L5 is improved because one of th=
e codes has no=20
data. L2 is better than L1 C/A but not as good as L5, simply because L5 =
has four times=20
more power than L2.

"One obvious conclusion is that L5 will be a very attractive signal in a =
few years when=20
the number of signals in space catches up. However, right now it appears =
that L2 is=20
superior to L1 for many applications, it's available years earlier than L=
5, and it may be=20
better than L5 for a lot of future applications, even after there are eno=
ugh L5 signals."

Back in 2004, in an article titled =E2=80=9CAcquiring Sensitivity,=E2=80=9D=
Philip Mattos wrote:

Like the Galileo signal, new GPS signals at L2C and L5 have faster code-r=
ates, wider=20
bandwidths, longer codes, so for the same sensitivity, the receiver must =
do five to 10=20
times as much work on the more complex signals during acquisition.

GPS L5 and Galileo E5A share a common frequency, common basic code length=
of 10,230,=20
common chipping rate of 10.23 MHz. and the use of tiered codes. Tiered co=
des generate a=20
very long code by multiplying a medium length fast code by a short slow N=
eumann-Hoffman=20
(NH) code. Thus, a pre-acquisition can be done within one chip of the slo=
w code, the=20
results stored, and the results post-processed at all possible synchroniz=
ations of the=20
known, short slow code.

GPS L5 and Galileo let us in by creating the very long code from two code=
s, the second of=20
which is very slow, running one chip for each epoch of the primary code. =
This allows the=20
acquisition to be done in two stages, integrating first for only the peri=
od of the primary=20
code, and extending this to the full composite code only when high sensit=
ivity is=20
required. Of course, for tracking purposes, after acquisition, the long c=
ode is used all=20
the time.

The secondary codes are very short, 10 and 20 chips for the L5 signals, 2=
0 and 100 for the=20
E5A signals.

However the chipping rate of the L5/E5A signals means that, in the short =
term, they will=20
not be targeted by low-cost receivers. In ten years time, when full const=
ellations of both=20
systems are available, the higher speed and power and lower cost of silic=
on will have=20
overcome this, and this group of signals may well become the most used of=
all the GNSS=20
signals.

Finally, in December 2005, in a Directions essay that he titled =E2=80=9C=
The End of the=20
Beginning,=E2=80=9D Per Enge wrote:

The forthcoming diversity of signals (L1/L2/L5) will obviate the danger d=
ue to accidental=20
radio frequency interference (RFI), do much to tame the ionosphere, and m=
itigate=20
multipath. This year, GPS has launched the first of the Block IIRM satell=
ites that=20
includes the civil signal and code on the L2 frequency. Within a few year=
s, GPS satellites=20
will have three civil frequencies. The third signal, L5, will be the most=
effective of=20
all, 5 or so dB more powerful than L1, with a chipping rate of 107 chips =
per second (or 10=20
Mcps) compared to the 1-Mcps codes used by L1 and L2.

The single greatest challenge for GPS this upcoming year is to accelerate=
the delivery of=20
L5. Like the current L1 and unlike L2, L5 will reside in an Aeronautical =
Radionavigation=20
System (ARNS) band of the radio spectrum. Thus, it is much more useful to=
the=20
international civil aviation community. With its high chipping rate, L5 w=
ill also provide=20
more protection against RFI and multipath. All told, it has very high val=
ue to=20
safety-of-life applications like IFR flight. The civil aviation community=
and the FAA are=20
working hard to ensure that suitable avionics and integrity monitors are =
available, and we=20
must support their efforts.

Welcome to the future, ladies and gentlemen. It is now. Or soon will be.=
I hope I am not=20
too soon in raising a glass to L5.