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..... - HAN OLE VIA 

TOP SECRET BYEMAN 

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..fB1 NATIONAL RECONNAiSSANCE OFFICE 

WASHINGTON, D.C. 

THE NRO STAFF March 23, 1970 

NOTE TO DR. McLUCAS 

SUBJECT: TAG BOARD Navigation 

You asked for a layman's description of why the fancy 

computation to satisfy DP's. 

The drone senses Mach through pitot sensors and adjusts 

altitude and attitude to hold constant Mach. This mechanization is necessary due to the sensitivity of the engine performance to Mach but results in a somewhat unpredictable altitude 

and wind history. This consideration plus the 3000 n mi range 

cause any preprogrammed auto pilot only scheme to have errors 

considerably in excess of the 28 n mi (+ 14) camera swath. 

Therefore some system of on-board navigation is required. 

The inertial navigator selected has been performing very 

well, it has an allowable error of 1.5 n mi/hr but has been 

yielding only 2250 ft cumulative max error at end of mission. 

The navigator operates by constantly computing present 

position and steers to maintain the great circle cours'e between 

selected destination points (DPs). These destination points 

(up to 32) are preprogrammed and cannot be changed in flight. 

The INS computer is mechanized to compute and cause execution 

of steering correction to maintain course between DPs. A 

separate computer routine compares present position with next 

DP to determine "satisfy" conditions which cause the following 

DP to be entered into computer. It was this separate routine 

which used latitude and longitude angle functions in a manner 

such that truncation errors occurred and, since the DP never 

was adequately satisfied no new DPs were ever entered. The 

changes were simply to open up the "satisfy" conditions and 

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include a "fail-safe" prov1.s1.on so that if the drone goes 

too far past a DP fuel is shut off. 

BYEMAN 

CONTROL S VSTEM 

You asked whether one could simply use predicted time 

between DPs and a clock to determine DP satisfaction. Actually, 

such a scheme could work pretty well. The drone would steer 

the course between DPs as now, it would satisfy the DP with an 

error proportional to velocity error but after "satisfy" it 

would steer back to the desired course to next DP at an intercept angle of up to 45°. Thus the erroneous portion of the 

track would be in a circle around the DP of radius equal to 

the in-track difference between actual pOSition at the predicted 

time and the DP. Although this would work, I am not sure of 

implementation difficulty and I think they have the problem 

in the "satisfy" computation worked out satisfactori~ , now. 

Lew Allen, Jr" 

Brigadier General, USAF 

Director 

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