Subject: [IGSMAIL-2150] work near USNO antenna
From: Jim Ray
Date: Tue, 9 Feb 1999 7:19:29 PST

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IGS Electronic Mail      Tue Feb  9  7:19:29 PST 1999      Message Number 2150
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Author: Jim Ray
Subject: work near USNO antenna

A large piece of air-handling mechanical equipment was lifted onto the
rooftop of Bldg 52 near the USNO GPS antenna this morning, 09 February
1999.  The air-handler passed above the horizon of the antenna although
not directly over it.  It is very possible that observations were
affected by this work.

Even worse, though, the air-handler was placed only a short distance
from the antenna on a raised platform area.  The separation between 
the antenna and air-handler is about 12.8 meters.  Below is a schematic
of the new geometry:

                              <- approx. North
            ____________
            |          |
            |   Air-   |                                  
 1.6 m      | handler  |                              Ant. 
            |          |                               ==     ^
        ---------------------                          ||    1 m
 0.7 m  |                   |                          ||      
        |                   |__________________________||     v
                                                        |

                       <------------- 12.8 m ---------->

     [Southeast corner area of USNO Bldg 52 rooftop]

As of today, the top of the metal air-handler rises about 1.3 meters
above the choke-ring antenna.  At a distance of 12.8 meters, the
structure is only about 6 degrees above the antenna horizon.  However,
the potential for multipath problems is presumably great.

I have only recently learned that during the course of related work
on the rooftop over the last couple of months (since December 1998 at
least) significant amounts of sheet metal and other metal pieces have
been moved around the rooftop near the GPS antenna.

We will try to determine the possible impact of these activities on
the quality of the USNO observations.  If significant degradation is
detected, the antenna may need to be relocated.

We apologize for the short notice and inconvenience to users.

=========================================================================
Jim Ray                                   e-mail: [email-masked]
U.S. Naval Observatory, EO Dept.          phone : 202-762-1444
3450 Massachusetts Avenue, NW             fax   : 202-762-1563
Washington, DC  20392-5420
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[Mailed From: Jim Ray (USNO 202-762-1444) <[email-masked]>]