There will be no April Fools materials this year. The author of the GPS Overview has been called away to help solve a GPS problem that may have global positioning significance. Here is his preliminary report from the field.
 

Bombay India (18:58:35.345 N 72:50:31.339 E) April 1, 1999

GPS Failures
    Global Positioning System (GPS) receivers have been known to take many minutes to acquire signals from time to time. Other reports indicate that in many regions of the world, occasionally GPS receivers will lose lock for no apparent reason.

Walter Depressions
    Gravity field depressions, or geoid parabolic reflectors may be part of the reason why.
Off the southern tip of the Indian subcontinent is a large parabolic reflector formed by the WGS-84 geoid shape. The WGS-84 gravity field of the earth is illustrated here:

    Note the smooth parabolic dip in the geoid off the southern tip of India. Geoid dips that occur where the topographic surface of the earth is above mean-sea-level do not form any physical parabolic shape. The earth’s topographic surface does not follow the low frequency terms of the geoid model. At sea, however, gravity depressions are coincident with the mean sea surface in the region. These geoid dips, or parabolic reflecting surfaces in the sea surface are known as Walter Depressions, after their discoverer, Clarence Walter.
    Recent measurements have indicated that Walter Depressions in the geoid may be partly responsible for temporal failures in GPS receivers.

The Indian Dip
    The Indian Dip is just one of the large Walter Depressions that occur around the globe. Others have been measured off the south-western coast of the United States, near Bermuda, just off the northern coast of Brazil, and west of New Plymouth, New Zealand (The Taranaki Depression).

    A South to North Cross Section of the India Dip shows the parabolic nature of the Walter Depression

    The West to East profile shows the parabolic shape in the other axis:

Uridiumb Solar Flashes
    The current theory suggests that the Walter Depressions cause a reflection of GPS satellite signals to focal points above the depressions. These focal points are coincident with the orbital altitudes of the Low Earth Orbiting (LEO) satellite system known as Uridiumb. These communications satellites have been observed to reflect sunlight from their solar panels that can be predicted and observed on earth. These solar Uridiumb Flash events are reported worldwide:

http://www.assa.org.au/iridium.html

http://www-star.st-and.ac.uk/~fv/sag/iridium.htm

http://www.gsoc.dlr.de/satvis/
 

Uridiumb C/A Flashes
    Predictions of reflections from Uridiumb satellites passing through the focal points of Walter Depressions have coincided with moments when GPS receivers have failed in the field.
    A mechanism for these failures suggests that the large size of the parabolic sea surface reflectors causes variable path lengths for the satellite reflections as the satellites pass over the regions. At specific moments in time the path length from the near side of the Walter depression can differ from the path length from the far side by 299.79 kilometers.
    These path length differences can cause GPS spread-spectrum signals that would ordinarily be received as low level noise, to be exactly one millisecond apart in propagation path time. This causes perfect auto-correlation between a GPS satellite C/A code from the near side reflection and the far side reflection. When coincident with Uridiumb passovers, correlated de-spread GPS L1 carrier signals are beamed to points on earth. depending on the incidence angle of the signals at the focal point and the attitude of the Uridiumb solar panels, the beam can even be measured over land surfaces near the Walter Depressions. It is surmised that these unexpectedly large 1.57542 GHz signals at the center of the 1MHz C/A code spread spectrum signal normally seen by GPS receiver, simply swamps, or overloads the front end (RF amplifiers) of the GPS receiver.

Plasma Bubbles or Walter Depressions/Uridiumb C/A Flashes?
    Dr Jose Humberto Sobral of the Instituto Nacional de Pesquisos Espacios (the Brazil
Space Agency) reported on Ionosphere Plasma Bubbles in the South American Sector.
These can occur from October to March and effectively block radio signals from satellites.

http://www.imca-int.com/seminar.htm

    It may well be that "Plasma Bubbles" and other proposed causes of short-duration GPS receiver failures are nothing more than the occasional  Walter Depression/Uridiumb C/A code Flash Events.

Initial Recommendations
    Continuing research is necessary to assess the full extent of the effects of Walter Depression/Uridiumb C/A Flash Events.