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The digital differential analyzer (DDA) section handles the high 
speed computational requirements of navigational problems. The basic 
elements of the DDA are the digital integrator and servo follower. The 
digital integrator performs mathematical integration by computing a 
numerical approximation of functions. The servo follower converts 
whole numbers into an incremental form. An element is defined as 
either an integrator or servo follower by the programming contained 
on the five control tracts. The low speed DDA is controlled by the GP 
and communicates directly with the V-scan inputs, incremental buffer, 
analog buffer, and the telemetry buffer. The high speed DDA is con 
trolled by the GP and communicates directly with the incremental 
buffer and the telemetry buffer. The low speed DDA contains 192 
integrators with a rate of 100 iterations per second. The high speed 
DDA contains 8 integrators with a rate of 2400 iterations per second. 
The Input-Output Buffer Section consists of six separate and indepen 
dent buffers; telmetry, V-scan, transfer, incremental, analog and 
discrete. 
The electronic distance measuring sensor of the USQ-28 
(SHIRAN) is an intermittent C. W. system. It basically measures the 
"free space" distances between the aircraft interrogator and any four 
transponders on the ground. "Free Space" distance, or "free space" 
accuracy is used to indicate the distance or accuracy if measured in a 
vacuum. 
The interrogator set Figure 8 transmitter in the aircraft is 
modulated by four signals of coherently related frequencies. The 
transmitted signal (3. 312 me) is received by the transponder on the 
ground which retransmits the modulated signal at an offset-frequency 
carrier (3, 087 me). The four modulation frequencies are received 
and demodulated by the interrogator set in the aircraft. The phase of 
each modulation signal is compared with its phase before transmission, 
and the phase differences are directly interpreted by an electronic 
phasemeter as a free space distance.