FC/Chief, Flight Control DivisionJUL 14, 196969-FM21-191FM/Chief, Mission Planning and Analysis DivisionDescent monitoring after landing radar velocity updating
There has been considerable discussion concerning the capability to monitor powered descent with velocity residuals after landing radar (LR) velocity updating begins. This memorandum presents the recom- mendations of the Mission Planning and Analysis Division (MPAD) for powered descent monitoring during this period of powered descent.
After LR velocity updating begins, the AGS/PGNCS velocity comparisons are no longer valid. However, the powered flight processor (PFP)/PGNCS velocity residuals and the MSFN/PGNCS range rate residuals (Δρ) can be used for powered descent monitoring provided this data is valid. There- fore, if the PFP is operating satisfactorily until the time of LR velocity updating and no anomalies in the PFP are detected after velocity updating, the PFP/PGNCS and MSFN/PGNCS Δρ should be used to monitor the descent trajectory.
The objective of this monitoring is to prevent erroneous LR velocity data from destroying the PGNCS state vector to the extent that a PGNCS abort cannot be achieved. The monitoring should basically ensure that the PFP/PGNCS velocity residuals converge to a near zero value after LR velocity updating. The limits for PFP/PGNCS velocity residual monitoring after velocity updating should be the same as the values used prior to velocity updating, unless the FFP/PGNCS residuals are near the limits when velocity updating begins. In this case the PFP/PGNCS residuals limits should be increased by 10 ft/sec to ensure that the LR has sufficient opportunity to cause the PGNCS velocity to converge to the correct value.