Surveyors will be hit hardest by the SVN 49 L1 anomaly

Take a look at
http://www.insidegnss.com/auto/mayjune09-gao.pdf

On page 35, "Figure 14 indicates that the SVN 49 anomaly is primarily
in the L1 band. Compared to Figure 15, the L1 code-minus-carrier
(CMC) with ionosphere correction for PRN 1 has a bias highly
correlated with the satellite elevation. Consistent with Figure 13,
the bias has a relative shift of 0.7 meters from a low elevation of 20
degrees to a high elevation of 60 degrees."

At higher elevations (the article mentions Australia at 90 degrees)
the bias *is* larger; no "can be" about it.

The key is that the anomaly affects carrier phase users much more than
code phase users. (See http://www.novastars.com/gps/codevscarrier.htm
). Old guys in their Cadillac sedans will be less affected than
surveyors. Surveyors rely heavily on carrier phase and so will be
much harder hit by this anomaly. If Surveyors don't manually set
their receivers to treat SVN 49 as having bad health than their
databases could become corrupt over time if and when the AF sets the
health of SVN 49 to "good for all users".

--Mike Jr

Mike Jr wrote:


GPS Modernization: L5 Signal First Light (PDF)

A Preliminary Analysis of SVN49=92s Demonstration SignalGreat excitement =
surrounds the=20
activation of a new transmitter from a satellite, an occasion dubbed firs=
t light. Research=20
groups around the globe joined the government=92s GPS Wing in monitoring =
and analyzing the=20
first L5 signals from space. In our June Innovation column, one group des=
cribes the=20
equipment and procedures used to capture and analyze SVN49=92s signals an=
d gives an=20
assessment of their characteristics. Click to download PDF
http://www.questexenews.com/t.do?id=3D2697481:16071314

surrounds the
t light. Research
and analyzing the
cribes the
d gives an

The section titled "Legacy Signal Anomaly" on page 58 states,
"The cause of the bias is unclear but resides apparently in the
design of the transmitter antenna or signal generation chain. It is
exclusively seen on SVN49 and not on other GPS (or GIOVE)
satellites, which excludes a possible problem of the receiver antenna
or environment."

Looks more and more like the problem was introduced when marrying the
new L5 transmitter to the existing phased array antenna.

--Mike Jr

,
0
n
t surrounds the
rst light. Research
g and analyzing the
escribes the
and gives an

One would assume parameters associated with the antenna farm pattern
such as gain/pattern, phase response and group delay would have been
thoroughly tested on the antenna range prior to releasing M7 for
launch. The developers contend the L5 modification was indeed tested
to assure it would have no impact upon the satellite primary
navigation mission. What did they miss? Perhaps the satellite was
damaged during launch.

Recently, PRN01 satellite error has been consistently less than a
meter so there must be some capability to remotely calibrate it. It
now consistently outperforms PRN30 which is in a nearly identical
orbit. However, if this is being done by application of some sort of
bias to 'split the difference' between the low and high angle range
vectors that solution can be, at best, 'fudging' the solution so the
Air Force will be reluctant to set PRN01 usable until the consequences
of doing so are fully understood.

Meanwhile, M7 has the more important mission; holding down the
international frequency allocation for L5.

If there is a design flaw associated with L5, let's hope it hasn't
been passed on to the GPS-IIF satellite series.

--- CHAS

ly
13,
20
han
m
e
e
ent surrounds the
first light. Research
ing and analyzing the
describes the
s and gives an

Or they were in a hurry, did an analysis showing that using a
component from another bird should work and winged it.


Unlikely. The L5 bolt on was to a(n) IIRM bird and was not integrated
to the SCP/NDU. IIRM-20's L5 is designed to operate more like
Sputnik. IIF should be engineered completely differently.

[]

--Mike Jr