Data Correction in the Database

[1] SPZ Correction

The SPZ (Short-period vertical component) data were recorded at a sampling rate of 0.0188678 sec (53 samples/sec) at the ALSEP Central Stations. On the PSE Tape files, one data frame consits of 64 ALSEP data words, and they were acquired at a rate of 1.65625 frames/sec. (These are the nominal values, and the actual rates varied a little from time to time, due mainly to temperature varies.) This means that the 32 SPZ data should be stored in one data frame and all the even ALSEP words should be allocated to the SPZ data (See the "Exabyte Tape Data Format"). However, some even ALSEP words were occupied by other data. They are ALSEP words 2, 46 and 56 in Apollo 12 and 16 data, words 2, 24, 46 and 56 for the Apollo 15 data and words 2 and 56 for the Apollo 14 data. Thus we need to provide the SPZ data to recover the full data. For this correction, we used the 4th degree Lagrange interpolation suggested by Dr. Yosio Nakamura.

Lagrange interpolation

We can represent the curve line which passes four points (p1,q1), (p2,q2),(p3,q3),(p4,q4) using following equation.
      (x-p2)(x-p3)(x-p4)            (x-p1)(x-p3)(x-p4)          (x-p1)(x-p2)(x-p4)           (x-p1)(x-p2)(x-p3)
y = ----------------------- q1 + ----------------------- q2 + ----------------------- q3 + ----------------------- q4
(p1-p2)(p1-p3)(p1-p4) (p2-p1)(p2-p3)(p2-p4) (p3-p1)(p3-p2)(p3-p4) (p4-p1)(p4-p2)(p4-p3)

The point (x,y) to be interpolated is represented by the four points (two points are immediately prior to and the other two points are immediately following the point to be the interpolated).
where dt is the sample interval. Then, we can derive
     4(q2 + q3) - (q1+q4)
y = ----------------------
6 .

The value in digital unit of SPZ must be an integer. For this reason, we added 0.5 to the interpolated value of [y], and truncated the value below the decimal points. That is
     4(q2 + q3) - (q1+q4) +3
y = --------------------------
6 .

We interpolated all the SPZ data in the database using the above equation.

[2] P&E Supplement Data

P&E supplement tapes were created to compensate for the lost data that were not recorded in the PSE and Event tape files of the Exabyte tapes because of the initial difficulies in reading some old 7-track tapes. These lost data were recovered after the tapes were cleaned and were recorded in the P&E supplement tapes. The data files recorded on the P&E supplement tape are listed in "Catalog of Lunar Seismic Data from Apollo Passive Seismic Experiment on 8-mm Video Cassette (Exabyte) Tapes" (Nakamura, 1992). All the PSE data recorded in the P&E supplement tapes are included in our database. (The data on the Event tape files are not specifically included in our database, but the same data are archived as PSE data in this database.)

[3]Year Conversion

Some Exabyte format data files include an invalid value for the [year]. On these files, the value of the year is incorrectely recorded as 1975 when it should be 1976. These values are apparently different from those listed in UTIG Technical Report No.118. We corrected the value of the year from 1975 to 1976 for these data in our database. The names of the Exabayte format data files which indicate invalid year are listed below.


[4] Time Error Correction on Ground Stations

During the Apollo missions, 14 ground stations were used in rotation to receive the ALSEP data from the Moon.The time on each frame was stamped at JSC based on the standard time signals recorded on the range tapes at each ground station where the ALSEP data were received. However, when the ground stations switched from one to the next, the ALSEP data were received on both ground stations. Then, the same data that have nearly the same time stamps may be recorded.

Range Station Binary Identifier (In Octal)
Octal MNENStation Name
refered from 'The ALSEP Data Analog to Digital Transcription System (1976)'

Currently, we don't correct for this problem, and you may have to take care of this problem. Of course, in original Exabyte files, the same problem happens. You may refer to the ALSEP tracking station ID in the frame header of the downloaded data.

[5] Software Time Stamp Error

A potentially more seriouse problem is the occasional use of the 'software clock' for the time stamping. When the operator had a problem reading the standard time signal recorded on a range tape, a time stamp generated by a computer software was substituted. When this happened, the recorded time stamp may be off by a fraction of a second to as much as several tens of seconds. This can easily be detected by the recorded sample rate that is outside the expected range. If necessary, this can be corrected to a certain degree. This data system, however, did not attemt to do this correction.

[6] Bit Error Correction

Last Modified: 21 December 2020