[IGSMAIL-180] The GPS site at Kokee

[Ulf]

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IGS Electronic Mail       20-JAN-1993 09:56:15       Message Number 180
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FROM: Ulf J. Lindqwister
SUBJECT: The GPS site at Kokee
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In the last 2-3 months the GPS data from Kokee have been of sub-standard
quality. Only about 50% of the expected data volume have been obtainable. the 
pre-fit P-code range multipath have been at the several meter level, up
from ~20 cm for each band. The post-fit residuals for pseudorange have
hoovered around 200 cm, which is an order of magnitude worse than the
norm. The lack of data volume are probably mostly due to power outtages
(after-effects of the severe hurricane damage to the Island),
but possibly also from frequent blockage of GPS signals. The latter
measures are closely related to multipath problems.

These observations are of serious concern for us, since Kokee is such
a critical site in the GPS Global Tracking Network. This site ties in
other sites around the Pacific rim, i.e., it provides crucial mutual
visibility for sites in Japan, Australia, Alaska, and California - 
such mutual visibility greatly improves our global GPS solutions.
Based on these concerns we are considering moving the GPS receiver
to a more benign environment. The plan is to first move the receiver
to a temporary site (further away from the new VLBI antenna) and evaluate
the multipath there - to see if it changes significantly. Then in a second
stage move the antenna to a new, low-multipath & low-blockage, permanent site.

I have received a couple of comments on the planned moves so far.
One investigator suggested (off-line) that even the temporary site should
be tied to the old site. We are looking in to this possibility right now.
A note from Tom Clark sent over IGS mail suggested that most of the blockage
and multipath was due to a crane, which is building the new 20 m VLBI antenna
and hence is only temporary (1 month ?) - so the GPS antenna may not have to
move. Unfortunately, his report contains several factual errors, some of which
could prove to be significant:

1) According to the station manager, Clyde Cox, the GPS antenna is located
on a 19 feet (5.7 m) tall concrete tower, whose center is 45 m from the
center of the VLBI antenna. GPS signals from the North-West direction
are blocked below 23 degrees, when the VLBI antenna is facing zenith  
[ (20-5.7)/(45-10) => 23 degrees ]. The VLBI receiver feed box is
situated higher and causing blockage below 30 degrees [ (20+9.5-5.7)/45 =>
30 degrees ]. When the new VLBI antenna is pointing towards the horizon,
it causes masking for the GPS antenna below 25 degrees 
[ (25-5.7)/45 => 25 degrees ]. Hence, obstruction in the GPS signals
below 23-30 degrees North-West direction can be expected, which is
substantially larger that the desired 10 degree horizon cut-off.
Clark assumed that the tower was 9 m tall, which significantly under-
estimates the blockage due to the new VLBI antenna.

2) The azimuth range of the blockage is ~26 degrees, between 312 - 338
degrees in the North-West direction. A sky-track of GPS satellites
over Kokee shows that quite a few GPS satellites are present in this
quadrant (312-338 degrees). A significant number of these satellites
will experience various degrees of blockage from the new VLBI antenna.
The sky-track and the masking profile of the GPS antenna are available
by FAX if anyone is interested.

3) We are in frequent touch with the station manager and we certainly 
would not move the antenna without his concurrence.

It has been suggested that the multi-path problems are only
temporary and will vanish as soon as the crane comes down.
The crane in fact went down for a day last week and we are in the process 
of analyzing the data and comparing with the conditions from
before the new VLBI antenna went up.

We certainly would prefer to not move the antenna, since it
involves a significant effort and cost on our part - but, we are
also committed to providing the GPS community (and ourselves)
with the best possible GPS data obtainable from Kokee - so we are
examining all the evidence and carfully weighting all options.

I would propose that we continue to carry out this discussion 
regarding potential antenna moves at Kokee off-line (there are already
a lot of IGS email as it is) and reserve IGS messages for direct actions
which will affect the IGS community at large.
So, please direct your comments directly to my email address
below. We will consider all suggestions carefully.

Sincerely,

- Ulf 



Ulf J. Lindqwister
GPS Networks & Operations Group
Jet Propulsion Laboratory
4800 Oak Grove Dr.
Pasadena, CA 91109

(818) 354 1734
(818) 393 4965, FAX
ujl at logos.jpl.nasa.gov