Transearth Spacecraft Navigation

See list attachedOctober 17, 196868-PA-T-220APA/Chief, Apollo Data Priority CoordinationTransearth Spacecraft Navigation

During Jim McPherson's Transearth Spacecraft Navigation Mission Techniques meeting of October 8 and 15, a potpourri of ground rules, working agreements and constraints was established. I may be dupli- cating other reports with this memo but figure better too many reports than not enough. All of the following apply specifically to the first batch of sextant sightings – star/lunar horizon – after TEI on the way back to earth. Many may also apply to later navigation observations, but I won't attempt to identify them here.

a. Prior to initiation of transearth onboard spacecraft navigation, the pre-TEI MSFN state vector navigated through TEI will be stored in the CMC LM slots and will be used to initialize the navigation. That is, no new state vector will be uplinked.

b. Navigation using star/lunar horizon observations give approxi- mately the same accuracy as star/lunar landmarks – at least as far as hitting the entry corridor is concerned. Accordingly for purposes of mission simplifications – both pre-flight preparation and real time operation – all star/lunar landmark observational exercises will be deleted from lunar missions starting with C'.

c. This exercise is to start at TEI + 1½ hours.

d. Altitude, which is not a constraint, should initially be about 6,000 nautical miles.

e. Stars of 2.3 magnitude or brighter are required for lunar observa- tions.

f. Due to the required spacecraft attitude, the hi-gain antenna will probably be out-of-lock. Therefore, low bit rate telemetry will probably be used to transmit the data in real time. If so, marks must be made no more frequently than one for each 10 seconds – procedures are required to assure proper downlink antenna is selected.

g. After completion of this exercise, the crew will obtain sextant photographs of the lunar horizon – to see what the horizon looks like at altitudes of 10,000 to 20,000 nautical miles – not to determine its location.

h. The W-matrix will be initialized to 3,300 feet and 3.3 fps. If possible, they will be initialized at TEI and propagated from there. These are the same values to be used after TLI and included in the E memory load.

i. MPAD and MIT will establish the ΔR, ΔV threshold the crew should use for data selections – hopefully, it will be simple but perhaps must be a function of geometry and time in the mission. (The data is on the downlink regardless of whether the crew accepts the update or not.) It should be noted that no good simulation facility will ever be available to provide the crew any pre-flight judgment. Although the V83 rendezvous RR display gives relation of pre-navigation versus navigated state vectors, this kind of activity shall not be a part of the decision logic. If someone comes in with a good, useful proposal, this will be reconsidered.

j. A P52 align shall be performed immediately prior to this exercise.

k. The sextant calibration shall be repeated until agreement of at least two checks (not necessarily sequential ones) are within .006° before “preceeding.”

l. Sextant calibrations will be performed every one-half hour.

m. The CMC clock shall be updated by the MCC-H whenever in “error” by more than .040 seconds.