Surveyor Flight Path Analysis and Command (FPAC)–John Gans

Surveyor mission operations were conducted in JPL’s Space Flight Operations Facility (SFOF) in Pasadena.   Technical support groups in the SFOF included the Flight Path Analysis and Command (FPAC) Group, the Space Performance and Command (SPAC) Group, and the Space Science Analysis and Command (SSAC) Group. FPAC and SPAC were the responsibility of Hughes.

The FPAC organization chart for Surveyor 6 is shown below. For earlier missions Mal Meredith was the FPAC director. FPAC was organized into five groups:  Computer Support, Tracking Data Analysis, Orbit Determination, Trajectory and Maneuver Analysis. Computer Support, Tracking Data Analysis and Orbit Determination were manned by JPL personnel. For earlier missions John Ribarich was the head of the Maneuver Analysis group.


The Atlas-Centaur launch trajectories were designed to provide a lunar transit trajectory that will impact at the desired lunar landing site –as selected by NASA in the desired Apollo landing zone. Errors in the Atlas-Centaur boost resulted in missing the desired landing site.

The primary responsibility of FPAC was to determine these errors and correct them. Spacecraft range rate (doppler) and angle data were gathered by the three stations of the Deep Space Net located at Goldstone, California, Canberra, Australia and Johannesburg, South Africa.  JPL’s Orbit Determination Program (ODP) was used to process the tracking data using a weighted least-squares technique to generate an estimate of the spacecraft trajectory and produce a state vector.  The state vector consisted of position and velocity of the spacecraft defined in an Earth-centered Cartesian coordinate system at a defined epoch. The state vector provided the Trajectory Group the initial conditions for the calculation of a precision trajectory using the program JPL Trajectory software (JPTRAJ). to determine the resulting lunar landing location.

Using this trajectory the Maneuver Analysis Group determined a midcourse maneuver that will correct the landing location error and investigated parameters that might affect the probability of a successful terminal descent and landing. A computer program, Midcourse and Terminal Guidance Operations (MTGS), designed and built by Hughes was used for these analyses.

The earth-centered lunar transit trajectory upon approach to the moon resulted in a hyperbolic trajectory that can be described in a selenographic B, T, and R coordinate system. For a given trajectory the lunar impact location can be defined in terms of the two components B.T and B.R (vector dot products). This is a very useful concept as the miss vector is very nearly a linear function of changes in the initial conditions at the time of the midcourse correction.

A critical plane was determined so that ∆Vs in that plane only affect the landing location while ∆Vs normal to the plane affect only the velocity at lunar impact and not the landing location. The determination of the midcourse maneuver will then proceed in two stages: first the ∆V in the critical plane required to correct landing location errors will be determined and second using the ∆V normal to the critical plane as a parameter to improve the probability of a successful terminal descent.

A time of the midcourse correction is selected and a critical plane established with TRS coordinates:  the B vector was aligned along the spacecraft’s velocity vector and the T and R vectors comprised the critical plane which was perpendicular to the flight path.  Any thrust by the spacecraft in the critical plane would result in a change in the landing point on the moon. A  K-matrix was formed of partial derivatives in the critical plane which would then be used to find a suitable thrust vector (described by B.T and B.R). The use of a K matrix allowed MTGS to find the optimum thrust vector in a relatively quick fashion to change the trajectory to the desired lunar site in just a few seconds of MTGS IBM 7090 execution time.  Then the equations of motion were integrated to the moon (many minutes of MTGS execution time using the IBM 7090) to insure that the vehicle would land at the required site.

After the critical plane maneuver to correct the landing location was determined possible maneuvers normal to the critical plane are investigated as shown in the attached figure for Surveyor 6. This velocity increment, designated U3, was varied parametrically to determine the resulting flight time (compared to the Goldstone DSN rise and set times), the main retro solid rocket burnout velocity, vernier propellant reserve, and landing site dispersions as shown in the figure. For Surveyor 6 the critical plane maneuver was 1.18 m/sec and a maneuver of 10 m/sec normal to the critical plane was selected to reduce the main retro burnout velocity to 480 ft/sec. The total ∆V was 10.06 m/sec.U3ScanThis thrust vector was then used to determine the roll, pitch and yaw maneuvers to reorient the S/C for the course correction maneuver.  This information with the required vernier engine thrust duration was given to SPAC to generate the commands for transmission to the S/C.  Following the midcourse maneuver the S/C was again reoriented to its translunar attitude, tracked, and the trajectory determined and the landing site verified.  Then as the S/C approached the moon MTGS was again employed to calculate the thrust vector to slow the S/C for the lunar landing.  Roll, pitch and yaw maneuvers were determined, and a delay time in seconds calculated to fire the solid rocket motor and initiate terminal descent.  This delay time was computed based upon the 60 mile mark obtained by the S/C Altitude Marking Radar (AMR) sensor.  The maneuvers and the delay time, in seconds were transferred to SPAC for transmission to the S/C via the DSN.

This completed the actions by FPAC.  The S/C completed all maneuvers, fired the retro motor and the vernier engines to begin the gravity turn until the S/C was 12 feet above the moon’s surface. Following vernier engine shut down the S/C then free fell to the surface where it remains to this day.

Among other things we wrote and sang a ditty which went, ” B.T and B.R, How we wonder what they are, Way up in the sky so blue…”  Just a little bit of levity to relieve the tension in FPAC.


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