State vector update situation for AS-501 and AS-502

FM/Chief, Mission Planning and Analysis DivisionJanuary 20, 196767-FM7-6FM7/Acting Chief, Guidance and Performance BranchState vector update situation for AS-501 and AS-502

1. The AS-501 and 502 missions have been designed to update the state vector (position and velocity) in the Apollo Guidance Computer (AGC) with a state vector from the Manned Spaceflight Tracking Network (MSFN) prior to the Saturn (S-IVB) out-of-orbit burn and again prior to the second Spacecraft Propulsion System (SPS) burn. If these updates are not transmitted from the ground the AGC will use position and velocity estimated from onboard navi- gation equations. Because of the inaccuracies of the navigation equations, even with perfect Inertial Measurement Unit (IMU) performance, a perfect vehicle will fly different trajectories depending on which combinations of updates are transmitted.

2. The attached table illustrates the deviations of four trajectory cases with various combinations of updates. Case #2 (no updates) has actual reentry conditions close to Case #1 (both updates) but the AGC navigated state vector is in error. Case #2 was propagated through the reentry phase and the Command Module could not achieve the desired landing point and the actual impact point (IP) was off from nominal by 225 nautical miles. At splash the AGC's estimate of the IP was in error by about 120 nautical miles (a similar run from MIT resulted in 90 nautical miles for this error).

3. Case #3 (first update only) indicated a large improvement over the no update case and Case #4 (second update only) brings condition back to the nominal (both updates) performance; however, in order to draw proper conclusions from these four cases, the performance of these cases with IMU errors must be examined.

4. The two update mission (Case #1) with the IMU performing at specification values has an entry flight-path angle error of ± .53° (3σ), which produces a marginal first heat rate test. Case #2 with dispersions has a good probability that reentry will not be achieved. (That is, the minimum altitude is greater than 400,000 feet.) Case #3 will have about a thousand mile IP dispersion. Case #4 will have entry errors slightly larger than Case #1.

5. The second update is mandatory to insure a high probability of achieving the required 501 test objectives. The first update assures that the vehicle can be recovered in case the second update is not received as planned. In other words, you must send both updates.