Mars Pathfinder APXS EDR Dataset Description

Table of Contents

Data Set Overview

The APXS EDRs are a collection of 27 measurements (173 data files) obtained by the Alpha Proton X-ray Spectrometer (APXS). Of these, nine are measurements of rock, seven are of soil (if Scooby Doo is an indurated soil rather than a rock), and eleven are of atmosphere. The atmosphere was measured on Sol 1 to determine background cosmic ray levels, on a few occasions for electronics noise tests, and sometimes as a result of the APXS deployment mechanism not reaching the intended rock surface.

During the rover's 83 Sol mission on Mars, it nearly circumnavigated the lander, staying within a 12 meter radius of the lander. Early APXS measurements were taken on rocks to the north of the lander (Barnacle Bill and Yogi) and a variety of soils near the end of the ramp and to the east of the end of the ramp (cloddy deposits between the ramp and Yogi, Scooby Doo, wheel-disturbed soil next to Scooby Doo, and soil next to Lamb).

The unusually high sulfur content of the rock analyses for Barnacle Bill and Yogi indicated that the APXS may have sampled weathered rock or rock covered by sulfur-rich dust or soil, so the rover was sent towards the 'Rock Garden' for more rock APXS analyses. This is an area to the southwest of the lander containing several large rocks, some of which have low red/blue ratios (less oxidized iron) and what appeared to be near-vertical rock faces (less likely to be heavily dust covered). En route, an APXS measurement was taken on the 'dune-like' feature called Mermaid Dune. The 'Rock Garden' terrain was difficult for rover driving and the loss of the rover battery on Sol 56 slowed down operations, but during the last 46 Sols, seven additional rock APXS analyses were obtained. Two analyses were taken on one rock (Half Dome), although the rover battery was alive for the first one and dead for the second one. Preliminary X-ray composition results for eleven APXS analyses (5 rocks and 6 soils) are given in [RIEDERETAL1997B].

An inventory of the 173 APXS_EDR data files is found in a table provided in three formats: APXS_INV.HTM (HTML), APXS_INV.ASC (formatted ASCII text), and APXS_INV.TAB (comma separated value ASCII). (The former two are found in the DOCUMENT directory on the MPRV_0001 CD, and the latter is in the INDEX directory of the same CD.) The inventory lists for each data file: APXS measurement number; rover command number; rover command sequence number; VICAR data file name; whether the file contains cumulative data, is the start of a new data collection, or is a re-read of old data from the memory; measurement start and stop times (in SCLK and Local True Solar Time); target; and measurement comments (including duplicate insurance readouts of the APXS memory, sequence requeue mistakes, electronics noise tests, and power problems).


During each measurement session four spectra are accumulated, each containing up to 256 channels. Each channel consists of two bytes, organized as an event counter (each channel can contain a maximum of 65535 counts; the channel number corresponds to the amplitude of the event, i.e. the energy of the registered particle/photon. The first spectrum, called the 'alpha spectrum', contains events registered by the alpha detector only. The second spectrum, called the 'proton spectrum', contains events registered simultaneously by the alpha and the proton detector (the amplitude being the sum of the amplitude of both signals). The third spectrum ('X-ray spectrum') contains events registered by the X-ray detector and the fourth spectrum ('background spectrum') contains events registered by the proton detector only. This fourth spectrum is essentially cosmic ray background events. Further details are provided in [RIEDERETAL1997A].

Other parameters measured are the temperature of the X-ray preamplifier in the sensor head, the temperature of the alpha detector preamplifier in the rover's warm electronics box (in °C), and the sampling duration of the alpha, proton, and X-ray measurements (hh:mm:ss).


The only processing performed on the data was Real Time processing, using the VICAR (Video Image Communication and Retrieval) program MPFTELEMPROC. This program obtained the Standard Formatted Data Unit (SFDU) records from the Telemetry Delivery Subsystem (TDS), and reconstructed the data files (each of which contained 4 spectra) from the telemetry data. The data files were compared to any previously downloaded versions to determine which contained the most data. The versions with the greatest amount of data were retained. The program resulted in VICAR formatted files with a subset of descriptive label items. The label information was then supplemented with data derived from the mission catalog and SPICE kernels. Finally, each file was run through the VICAR program MPFPDSLBL to convert the labels to PDS format.

The APXS Experiment Data Records were created at the Multimission Image Processing Laboratory of the Jet Propulsion Laboratory.


The APXS EDR data are stored as image data files, each file containing four lines (four spectra) and 256 samples (256 16-bit unsigned numbers).

Some data files are completely new starts, as indicated by ACCUMULATION_COUNT=1 in the file header, and these only contain temperature measurements at the beginning and end of the APXS measurement session. Other files are 'cumulative spectra' and consist of accumulations of multiple measurement sessions added together, and the number of sessions is indicated by the accumulation count. For cumulative spectra, temperatures at the beginning and end of each measurement session are recorded and counts for each detector are summed with those for previous accumulations. Up to 10 accumulations are possible.

Ancillary Data

Clock conversion software was taken from NAIF (Navigation and Ancillary Information Facility) software tools. The most up-to-date spacecraft and instrument kernels available can be obtained from the anonymous FTP site of the Navigation and Ancillary Information Facility at JPL, For further assistance, contact Boris Semenov of the PDS NAIF node:

Address: Boris Semenov
M/S 301-100
Jet Propulsion Laboratory
Pasadena, CA 91109-8099
Phone: (818) 354-8136
FTP Site:

Coordinate Systems

Estimates of the locations of the 5-cm diameter spots measured on rocks and soils on Mars are reported here as XYZ coordinates in the Martian Local Level Coordinate Frame:

X Y Z Rock/Soil APXS Target
A-2 1.89 -1.95 0.31 soil off the end of the ramp
A-3 1.30 -2.45 0.18 Barnacle Bill rock
A-4 2.79 -2.64 0.28 soil near Yogi
A-5 3.29 -2.48 0.28 soil near Yogi
A-7 4.58 -2.91 -0.18 Yogi rock
A-8 2.85 1.13 0.32 Scooby Doo indurated soil or rock
A-9 3.08 1.32 0.28 disturbed soil next to Scooby Doo
A-10 3.74 -0.43 0.28 dark soil next to Lamb
A-15 -5.87 2.80 0.52 Mermaid dune
A-16 -3.79 -1.31 0.12 Wedge rock
A-17 -5.56 -3.25 -0.35 Shark rock
A-18 -4.81 -3.81 -0.54 Half Dome rock, first location
A-19 -4.82 -4.13 -0.59 Half Dome rock, second location
A-20 -4.20 -4.10 -0.38 Moe rock
A-23 -3.48 -3.86 -0.43 Stimpy rock
A-27 -9.03 -2.59 -0.07 Chimp rock

The Mars Pathfinder Lander (L) Coordinate Frame

The Mars Pathfinder Lander is a tetrahedral structure. One of its faces, the one upon which it sits, is called the base petal and houses most of the lander equipment. The other three faces, or petals, open after surface impact to expose these systems. The rover is mounted on one of these petals. The Mars Pathfinder Lander Coordinate Frame, or 'L' Frame, has the lander base petal as its reference plane and its center coincident with the geometric center of the base petal. The YL-axis of this coordinate system passes through the geometric center of the rover petal, and defines the reference direction. The ZL-axis is normal to the reference plane and coincident with the nominal spacecraft spin vector. When the lander is upright on the surface, the ZL-axis is directed positively downward into the ground.

The Martian Local Level (M) Coordinate Frame

The Martian Local Level Coordinate Frame is a right handed, orthogonal, frame whose origin is co-incident with the origin of the Lander Coordinate Frame. The XM axis points north, the YM axis points east, and the ZM axis points down.

For more information on Mars Pathfinder coordinate systems, see the [MELLSTROM&LAU1996], [WELLMAN1996B], and [VAUGHAN1995] references. However, please note that as of the time this APXEDRDS.CAT file was written, [WELLMAN1996B] had not yet updated his discussion of elevation measurements to match that agreed upon by the Project. Where he used elevation ranges of 0° to 180°, the MPF Project used -90° to +90°.


The APXS data can be displayed on UNIX, Macintosh, and PC platforms using the PDS developed program, NASAView. This software is freely available from the PDS Central Node and may be obtained from their web site at For more information or help in obtaining the software, contact the PDS operator at the following address:

Address: Planetary Data System, PDS Operator
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
Phone: (818) 354-4321

Media / Format

The APXS EDR data will be stored on compact disc-read only memory (CD-ROM) media. The CDs will be formatted according to ISO-9660 and PDS standards. The data files will not include extended attribute records (XARs), and will therefore not be readable on some older VMS operating systems.

Confidence Level Overview

The Alpha Proton X-ray Spectrometer (APXS) functioned well during the 83 sols of operation on the Martian surface. For nighttime X-ray measurements, a temperature variation of more than 100°C during the data accumulation produced a shift in peak position smaller than a fraction of an energy channel.


Prior to release, the data will be reviewed by the APXS instrument team and the Planetary Data System.

Data Coverage and Quality

The highest quality measurements for soils were A-4, A-5, A-8, A-10, and A-15. The highest quality measurements for rocks were A-3, A-7, A-16, A-17, and A-18. The preliminary X-ray results for all of these high quality measurements are shown in [RIEDERETAL1997B]. Results for the soils A-2 and A-9 are not as good, due to poor contact with the sample by the APXS deployment mechanism and lower counting rates for alpha particles, protons, and X-rays. The rover battery died in the early morning of Sol 56, and all further APXS measurements had to be made during the daytime, which had an adverse affect on the X-ray portion of the data for rock measurements A-19, A-20, A-23, and A-27. These four suffer from increased noise and low counting rates in the X-ray spectra (especially adverse for the low energy events and low atomic number elements such as sodium), but the alpha and proton portions of the data retain high quality. Some of the files after Sol 55 have defects, with proton, or X-ray data not always appearing in the same file, and the time placed at the start of the data stream is incorrect.

When measuring rock and soil samples, the desire was to obtain at least 10 integrated hours. Only 3 hours of nighttime measurement were needed for a good X-ray analysis. X-ray spectra obtained during the night, when ambient surface temperatures were low, were unaffected by electronics noise. Ten hours of measurement during the day or night provide good alpha and proton analyses. Shorter times still provide useful results. Actual spectral accumulation times (listed in the table below) for the rock and soil analyses on Mars range from 5 to 16 hours.

A summary of the 27 APXS measurements is shown in the table below.

Measurement initial start
time and final stop time
(Local True Solar Time)
meas. time
APXS spectra
time (hrs)
A-1 Sol 1 07:17 - Sol 2 09:58 2.8 1.9 atmos.
A-2 Sol 2 14:53 - Sol 3 10:00 19.6 15.9 soil
A-3 Sol 3 15:00 - Sol 4 07:01 16.5 13.6 rock
A-4 Sol 4 16:59 - Sol 5 01:32 8.8 8.1 soil
A-5 Sol 5 16:01 - Sol 6 06:55 15.3 9.2 soil
A-6 Sol 10 09:01 - Sol 10 09:40 0.7 0.1 atmos.
A-7 Sol 10 14:17 - Sol 11 02:37 12.7 5.7 rock
A-8 Sol 14 14:03 - Sol 15 02:55 13.2 5.7 soil
A-9 Sol 15 14:04 - Sol 16 03:15 8.6 7.7 soil
A-10 Sol 20 14:03 - Sol 21 02:59 8.3 7.0 soil
A-11 Sol 22 15:23 - Sol 23 02:40 11.6 4.8 atmos.
A-12 Sol 24 17:07 - Sol 24 17:17 0.2 0.1 atmos.
A-13 Sol 24 18:01 - Sol 24 18:12 0.2 0.1 atmos.
A-14 Sol 25 00:06 - Sol 25 00:16 0.2 0.1 atmos.
A-15 Sol 28 14:05 - Sol 29 02:44 8.0 5.3 soil
A-16 Sol 37 14:07 - Sol 38 03:05 8.2 6.5 rock
A-17 Sol 52 14:18 - Sol 53 03:05 8.0 7.0 rock
A-18 Sol 55 14:06 - Sol 56 00:05 7.2 5.9 rock
A-19 Sol 58 12:08 - Sol 64 10:45 12.6 8.9 rock
A-20 Sol 64 12:04 - Sol 66 10:40 10.3 6.5 rock
A-21 Sol 66 12:04 - Sol 67 10:32 5.2 3.0 atmos.
A-22 Sol 67 12:04 - Sol 68 09:32 4.0 2.0 atmos.
A-23 Sol 68 12:05 - Sol 70 10:47 9.5 4.9 rock
A-24 Sol 76 12:09 - Sol 77 09:06 2.7 0.5 atmos.
A-25 Sol 77 12:08 - Sol 78 09:06 3.0 0.8 atmos.
A-26 Sol 78 12:03 - Sol 79 10:17 4.6 2.6 atmos.
A-27 Sol 79 12:08 - Sol 82 09:47 11.1 6.3 rock

Measurement initial start and stop times were obtained from the SCLK times in the downlink telemetry for the acknowledgement of the exact commands that were issued to trigger the start and stop of each APXS measurement (usually Meas_Start, Meas_Stop, Reset, or Shutdown). SCLK times were converted to Local True Solar Time using the script sclk2ltmst (see Ancillary Data discussion). This time is only as accurate as the rover's clock, and is a close approximation to the exact initial start and stop time of each APXS measurement. In a few cases, the downlink was lost, and uplink predictions were used instead. Some of the cumulative APXS measurements were interrupted by other rover activities, in which case, the first accumulation start time and last stop time are indicated. Integrated measurement time, as indicated by ALPHA_SAMPLING_DURATION, PROTON_SAMPLING_DURATION, and XRAY_SAMPLING_DURATION in the data file headers, is always less than the accumulation final stop time minus the initial start time, because the sampling durations do not include quiet periods when the APXS was powered off and detector 'dead time.'

In 15 of the APXS_EDR files, the APXS spectra are filled with zeros. This is the case when rover command sequence 50520D was requeued by mistake on Sol 6, when there was not enough power to turn on the APXS (some of the A-19 measurements). The files filled with zeroes are indicated by N/A listings for measurement start and stop times in the inventory table of the 173 APXS_EDR data files (mentioned in the Data Set Overview).


Further calibration and special processing is needed to further reduce the APXS_EDR data and convert the alpha, proton, X-ray, and background spectra to elemental abundances.


In each APXS data file header, the PACKET_CREATION_SCLK is the time at which the rover formatted the packet for delivery to the lander. In most cases, this is just a few seconds after the APXS measurement was actually completed. Sometimes, though, it was much later, and this can be discovered by examining the inventory table of the 173 APXS_EDR data files (mentioned in the Data Set Overview). The situations that resulted in longer time discrepancies were: (1) duplicate insurance readouts of previously read out data were often commanded on the morning following a series of nighttime readouts just in case the previous one was lost in transmission (these are indicated in the comments section of the inventory table), (2) re-reads of old data to recover from lander telemetry data that was lost but still held in the APXS memory (the A-19 files indicated as 'reread of memory'), and (3) problems with rover power resulting in shutdown of the rover before APXS measurements had ceased (several A-2 files: commands 2749, 2762, 2775, and 3020 and one A-19 file: command 6138).

One known discrepancy was found in the APXS time stamps, for A-2 rover command 2723. The ALPHA_SAMPLING_DURATION (and PROTON and XRAY) indicate a minimum duration between measurement start and stop time of 2 hrs 45 minutes, but the stop minus start time duration, as indicated by rover time stamps is 2 hrs 10 minutes. The cause of this discrepancy is unknown.

In addition, the rover clock was sometimes significantly in error when it wasn't synced up with the lander clock beforehand. Syncing usually occurred early each morning. The only way to check for this is to carefully examine the rover downlink telemetry. Timestamps for A-19 are the most uncertain of the APXS files, because the rover battery stopped working early in the morning of Sol 56 and the rover had difficulty powering back up in the first few days following this event.

Related Information


Mars Pathfinder

Instrument Host

Mars Pathfinder Rover (Microrover Flight Experiment)


Alpha Proton X-ray Spectrometer
Rover Cameras


PDS Welcome to the Planets: Mars
PDS High Level Catalog: Mars