[IGSMAIL-166] KPGO situation

Tom Tom
Mon Jan 11 10:37:43 PST 1993

IGS Electronic Mail       11-JAN-1993 10:37:43       Message Number 166

From:     Tom Clark
Subject:  KPGO situation: Reply to Ulf's comments

In IGS Mail #163, Ulf Lindqwister from JPL noted problems at our Kokee
Park Geophysical Obsv. (KPGO) site and stated that:

> The GPS receiver (Rogue SNR-8) at Kokee, hawaii, has had numerous
> problems with power interruptions, severe multipath and physical
> obstructions (nearby cranes) causing losses of lock. The power
> interruptions are due to the after-effects of the hurricane which
> hit Kauai a few months ago. Moreover, a 34 m VLBI antenna is being
> built ~30 m from the GPS antenna causing severe blockage and multipath
> problems. For the near-term the antenna will be moved to a more benign
> location to maintain the site as a tracking site and for the long-term
> the antenna will be placed on a geodetic monument again. The process of
> moving the antenna to a better place will occur in two stages as follows:
> 1. On January 28, 1993, we are planning to move the antenna away from the
> 34 m VLBI antenna to a temporary site. This should significantly
> reduce signal blockage and multipath and improve the receivers
> tracking performance. After this date, data from
> Kokee should NOT be held fixed at the old locations coordinates
> until further notice. Hence during this period the Kokee site
> should only be used for tracking support - not as a fiducial.
> 2. We are currently exploring two possible permanent locations for
> the Kokee site and once the new site have been selected and checked
> out (monumentation, infrastructure support, etc.) we will move the
> antenna to this new, permanent location. A site tie will be conducted
> to the VLBI monument so that the site can become a fiducial GPS site
> once again.

This report has several factual errors. The new VLBI antenna is a 20M dish.
The GPS antenna is located on top of a ~9M tall concrete tower which is 
~45M from the center of the new antenna. When the new antenna is in the 
stow position (pointed at the zenith) the rim of the dish is ~20M above 
ground. This makes nearest point on the new antenna be ~11M higher than the 
GPS antenna and ~35M away, causing blockage in the north-west at elevation
angles below ~(20-9)/(45-10) radians (18 degrees). The dish's feed support
legs (dish f/D = 0.43) and receiver feed box extend another ~9.5M higher
(45M from the GPS antenna) blocking to (20+9.5-9)/45 radians (26 degrees).

When the new antenna is pointed at the horizon, the top-most point is 
~25M high and ~45M from the GPS antenna causing masking below (25-9)/45
radians (20 degrees) elevation. 

In both cases the blockage (corresponding to the 10M dish radius) occurs over 
a total azimuth range of about +/- 10/45 radians (+/- 13 degrees). Part of
this blockage includes the +/- 30 degree azimuth range centered on north
in which no satellite passes occur due to the inclination of the GPS orbits.

The needs of GPS were carefully considered (by Marshall Eubanks and me) 
when we picked the location for the new KPGO VLBI antenna. Given the physical 
real estate we control at KPGO, any other site we might have picked for the 
new antenna would have been worse!

The new antenna certainly causes some blockage to the present GPS antenna
in the northwest. This is unavoidable. However all the metal pieces in
the VLBI antenna which can reflect signals and cause multipath are curved 
and have a small fraction of their surface area normal to the raypaths from 
GPS which can cause specular reflections ("glints").

My guess is that any multipath problems being experienced are mainly
due to the tall (~35-40M) crane that is temporarily at the site for the
construction of the new antenna. If/when the rain at the site will stop 
so that the construction of the new antenna can be completed, the crane 
will be removed. This will be sometime in the next month since only 3-4 
clear days are needed to mount the rest of the antenna panels.

Ulf stated that the GPS antenna would be moved on Jan.28. I have checked with
Clyde Cox, who is my station manager at KPGO, and he knows nothing of
this plan. Finding a better site than we currently use for the GPS within
the real estate we control (or on neighboring "turf") will be difficult and 
will require the concurrence of our "landlords", the U.S.Navy and the State 
of Hawaii.

Given the difficulty of finding a better site and that the crane should be
moved soon, I'd recommend against moving the GPS antenna.

Regards, Tom Clark
clark at tomcat.gsfc.nasa.gov

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