® Take off and follow a series of retroreflectors to the area to be 
surveyed, obtaining tracker locks on each, proceed on a flight 
pattern such as shown in figure 4, and return to Wendover. 
® Refuel, realign, and fly another mission if the APTS instruments 
were performing satisfactorily and further Range clearance had 
been obtained. 
As the aircraft leaves the alignment point at Wendover, the navigation 
computations begin. This is an unsophisticated process, and position 
errors grow rather rapidly. As the aircraft approaches the first retro- 
reflector, guidance is provided by the computer to the cockpit via a 
display of heading, deviation angle, and distance. The pilot merely turns 
the autopilot control knob to fly over the retroreflector. The computer 
also directs the tracker to begin searching when the retroreflector is 
within view. A spiral search is begun about the computed location and if 
a return is obtained, the angular control of the tracker is turned over to 
an automatic lockon and tracking system. If the spiral search does not 
produce a lock, a more rapid and broader search is begun, and if a blink 
is detected, then the spiral search is repeated, centered about the loca- 
tion of the blink. Once lockon occurs, the measurements of range and 
directions are made and continue until the retroreflector passes out of 
view about 60° from the vertical. Positions are produced from the tracker 
measurements and are used to reset the navigated position. If one retro- 
reflector is missed, the chance of locking onto the next one decreases due 
to the growth of navigation errors. If two are missed, the tracker begins 
with the broader search and a lock is more difficult to obtain. The 
mission is terminated if no locks are obtained for a 20-minute period. 
Few flights were attempted during the first several weeks because the 
operations were plagued with one problem after another. Computer and 
primary power failures were the most common, 17 times altogether, but also 
an inverter and a tape drive failed. Each failure caused a 1- or 2-day 
delay while the temperature of the stable platform was restabilized. The 
few flights made during this period were not successful. The laser pro- 
filer produced inconsistent data over the salt flats, probably due to the 
low power pulse being smothered by the reflected sunlight. Data from the 
tracker were also inconsistent, with retroreflector lockons occurring 
infrequently. The cause of this last problem was never isolated, but warm 
cabin temperatures and a dirty viewing port were probably major factors. 
In late July, several actions were taken to correct these problems. In 
addition to cleaning the window (not a minor task), a retroreflector was 
placed between the Wendover Airport and retro B1, which improved the 
chances for the initial lockon; position resets were done at the Wendover 
Airport, which lowered the ground time to about 1/2 hour, including re- 
fueling; and all flights were scheduled for dawn takeoffs at Wendover in 
order to collect profile data under low sunlight conditions and cool 
temperatures. From this point on, successful profile missions were 
routinely obtained. The last two successful data collection flights 
occurred on August 18 when both missions of Section A were completed. 
To support the APTS data collection flights, a separate helicopter party 
moved the retroreflectors from one completed area to the next one sched- 
uled. The cloth panels set out at each site to be used to assist in re- 
covery were insufficient; some became encrusted with salt, and finding 
some sites became time consuming. However, this location problem almost 
284 
  
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