MOMS (Modular Optoelectronic Multispectral Scanner)

MOMS was a major German imaging instrument program focused on the demonstration of new high-resolution and multispectral imaging technologies as well as the photogrammetric analysis and interpretation of the data (this included considerable efforts in algorithm development and modeling techniques). Various MOMS instrument configurations were flown on four separate spaceborne missions.

1) MOMS-01 is regarded a very early pioneering imager, flown on two Shuttle flights: STS-7 in June 1983 and STS-41B in Feb. 1984, introducing the CCD pushbroom line-scanning detector technology on a spaceborne platform.

2) The MOMS-02 instrument was flown on STS-55 (April 26 to May 6, 1993). It introduced stereoscopic along-track pushbroom imaging with a three-line imaging system (the first spaceborne multi-line stereo imager anywhere).

3) A MOMS-02 reflight took place on the MIR/Priroda module of the Russian Space Station. This mission was referred to as MOMS-2P (launch of MOMS-2P instrument on May 5, 1996, it was subsequently installed on the Priroda module of MIR).

The MOMS optical camera technology development program started in 1979, was funded by the German Ministry for Research and Technology, and designed and developed jointly by DLR, LMU (Ludwig Maximilian University of Munich, PI: J. Bodechtel), and EADS Astrium GmbH, formerly DASA (DaimlerChrysler Aerospace AG).

Some historical background on organizational affiliations: In the 1979 time frame, there was DASA's predecessor organization, namely MBB (Messerschmitt-Bölkow-Blohm) of Ottobrunn, Germany, which built MOMS-01 and became in 1989 part of the newly founded DASA conglomerate. - Similarly, the name of the predecessor organization of DLR (German Aerospace Center) was DFVLR, effective from 1969 to 1989. From 1989 to Sept. 1997, there was also DARA (Deutsche Agentur für Raumfahrtangelegenheiten), a management organization in parallel to DLR. Effective Oct. 1, 1997, DARA was re-integrated into DLR.

The development of MOMS was supervised by the so called "MOMS Science Team" coordinated by DLR. The team members for data processing and analysis came from three major institutions:

• DLR was represented with three institutes: the Institute of Optoelectronics, GSOC (German Space Operations Center), and DFD (German Remote Sensing Data Center)

• University of Stuttgart functioned as the center for coordination of topographical data evaluation

• GFZ (GeoForschungsZentrum) Potsdam functioned as the center for coordination of thematic data evaluation (note: GFZ was founded in 1992 and joined the other groups thereafter).

MOMS-01 Missions on Shuttle

MOMS-01 was a Shuttle payload, mounted on SPAS-01 (Shuttle Pallet Satellite), and flown as free-flyer configuration on two missions: STS 7 (Challenger, June 18-24, 1983), a 6-day mission; and STS-41B (Challenger, Feb. 3-11, 1984), an 8-day mission.

Orbit: Shuttle orbit, inclination = 28.5º (both flights); STS 7 = 296 km altitude; STS-41B = 289 - 300 km altitude. The low inclination orbit of 28.5º implied that a maximum latitudinal coverage of ±28.5º could only be obtained (i.e. observations in the tropical regions of the world).

The SPAS platform was initially loosely connected to the Shuttle by a specific bus system for system control. Besides MOMS-01, SPAS-01 was also a system designed and developed by MBB as a reusable free-flyer platform; it carried also other payloads besides MOMS-01 (total of 10, among them a TV camera and a 70 mm camera - taking the first pictures of Challenger in orbit from a distance of about 300 m). SPAS was deployed and retrieved using the RMS (Remote Manipulator System) of the Shuttle, referred to as Canadarm. SPAS-01 included a small cold gas propulsion system (12 x 20 mM thruster). The SPAS-01 platform had dimensions of 4.8 m x 1.5 m with a total mass of 1450 kg including the payload. SPAS was also able to supply limited power, cooling, and utilities to its payload. SPAS-01, designated officially as 1983-059F, was in fact the first spaceborne free-flyer platform. ¹⁾

Figure 1: Photo of the SPAS-01 platform and RMS arm during proximity operations of STS-7 (image credit: NASA)

During observations, the MOMS data were recorded onto a HDDT (High Density Digital Tape) recorder (part of the SPAS payload). Both Shuttle flights of MOMS-01 yielded high-resolution imagery of various regions with a pixel size of 20 m x 20 m. ^{2) 3) 4) 5)}

A most important characteristic of the MOMS instrument was the modular design of the CCD detector, electronics, optical lens system and filters - that permitted the instrument to be configured for completely different observation missions, as well as the refurbishment of the system between missions as demonstrated in practise.

MOMS-01 objectives/applications: Imaging of different ground targets with low-to-high contrast and albedo (arid regions, regions with dense and sparse vegetation coverage, coastal zones, mountainous terrain, open ocean islands) to demonstrate the instrument capabilities for thematic mapping. The multispectral feature of MOMS permitted in particular studies in such fields as: general geologic mapping, mineral resources exploration, hydrology; mapping and monitoring of renewable resources (agriculture, forestry, urban and regional planning).

MOMS-01 instrument:

The MOMS-01 instrument consisted of five major elements, mounted onto the carbon-fiber structure of the SPAS-01 free-flyer. ⁶⁾

1) The optical module (scanner head) with four objectives, eight arrays, and associated shutters. Each module, representing one spectral band, consisted of filters, dual-lens optics, four CCD detector line-scanning arrays, and preamplifier electronics.

2) A power box for overall power conditioning and thermal and shutter circuitry

3) A logic box for all sensor function control (including real-time correction, and formatting of the source data stream from the optical module)

4) A HDDT (High Density Digital Tape) recorder (model: Bell&Howell MARS-1428 LT-3B) for digital data storage (72 Gbit capacity/tape)

5) A pressurized container for the recording system.

Figure 2: Illustration of the MOMS-01 imaging system (image credit: MBB)

MOMS-01 was a two-channel optical Cassegrain system observing in two spectral bands of 575 - 625 nm for general surface imagery, and in the 825 - 975 nm band for vegetation detection. A double optical system (focal length = 237.2 mm, relative lens aperture = 1:3.5) was used per band for illuminating the CCD array consisting of four groups (arrays) with a total length of 6912 pixels (16 µm pixel size). The swath width was 138 km, corresponding to a FOV of 26.2º. The spatial resolution (pixel size on ground) was 20 x 20 m, corresponding to an IFOV of 67.2 µrad.

MOMS-01 was a pioneering CCD pushbroom line-scanning type imager. Detector: Reticon CCPD 1728 (EG&G Reticon, Sunnyvale CA). Each CCD line array had 1728 detectors. Scan line extension beyond the one CCD array was achieved with a dual-lens configuration, linking all four CCDs to a total scan line of 6912 pixels (see Figure 3). This linking technique was referred to as "butting." MOMS-01 employed the optical butting scheme of the chips using a different lens for each chip. Data quantization = 7 bits, data rate = 40 Mbit/s (onboard recording). ⁷⁾

Figure 3: MOMS-01 schematic imaging configuration of CCD arrays

Note: Both optical systems observed in reality the entire swath width in parallel (not the split arrangement as shown in Figure 3). This double exposure concept was simply needed to provide a seamless linkage of all four CCD detector arrays for each spectral band across the swath.

Table 1: Overview of MOMS-01 instrument parameters and data recording system

The two MOMS-01 missions yielded 450 individual scenes for thematic evaluation. The total experiment observation time was 26.5 minutes on STS-7, and 30 minutes on STS-41-B.

Figure 4: Example of MOMS-01 imagery of Riyadh (Saudi Arabia) in Feb. 1984 (image credit: LMU)

MOMS-02 Mission on Spacelab D-2 (Shuttle flight STS-55)

MOMS-02 was an advanced version of MOMS-01, actually a completely new instrument (Co-PIs: F. Lanzl, F. Ackermann, J. Bodechtel), flown on the Spacelab D-2 Mission (STS-55, 10 day flight with a launch: April 26, 1993). The MOMS-02 instrument package was positioned inside of Spacelab, and not SPAS-mounted (Shuttle Pallet Satellite), as conducted in the MOMS-01 missions. ^{8) 9) 10) 11) 12) 13) 14)}

Orbit: The D-2 orbit had an inclination of 28.5º and an average altitude of 296 km (this implied: observation of equatorial regions only).

The MOMS-02 objectives were:

• Stereoscopic visual observation (with a high degree of interpretability)

• Provision of high-quality topographic regional maps (scale 1:50000) and digital terrain models (< 5 m of ground pixel size)

• Test of a digital photogrammetric observation technique and processing system (prototype)

• Correlation of high-resolution panchromatic data with multispectral data.

Experiment: a) triple stereoscopy, b) along-track-stereoscopy (i.e., forward and backward tilt), c) high-resolution imagery, d) multispectral observation (refined modeling of MS classification), e) combination of stereo and multispectral imagery.

DLR (Institut für Optoelektronik) provided extensive investigation/characterization of the CCD detectors for the flight modules, the experiment design as well as data processing, while DASA built and integrated the MOMS instrument. The nadir looking CCD line array was comprised of 2 arrays with 6000 sensor elements each, which were optically combined to one array with 9000 sensor elements. The other CCD arrays of the stereo module consisted of 6000 sensor elements. There were also two detector arrays in the focal plane of each multispectral camera, together with their corresponding filters.

The optical module of the MOMS-02 instrument consisted of five lenses (see Figures 5, 8, and 9). Three were designed for stereoscopic applications, whereas the other two enabled the acquisition of multispectral data sets. The central lens (HR5 or Stereo 5), with a focal length of 660 mm, formed the core of the camera system. In combination with the central high resolution lens, there were two other stereo lenses (Stereo 6 and 7), each with a focal length of 237.2 mm. Along-track stereo imagery was obtained with the center lens (ch. 5) and two tilted (forward and backward) lenses (ch. 6 and ch.7). In addition, two other lenses (MS1/2, MS3/4), each with a focal length of 220 mm, enabled the multispectral imaging of a total of four channels. The focal length of these multispectral cameras was given by the requirement of identical pixel sizes compared to the tilted stereo channels.

Table 2: Performance parameters of MOMS-02 (300 km orbit)

Data quantization: 8 bit with seven gain steps (uncompressed for MS and stereo, 6 bit compressed for stereo). The frequency range covered by the various channels is from 450 to 815 nm. Seven variable observation modes of different band combinations are defined.

Table 3: Operational modes of MOMS-02

Figure 5: Objective configuration of the MOMS-02 instrument (image credit: DLR)

During the D-2 Shuttle mission, the MOMS-02 instrument operation was monitored and controlled by DLR/GSOC. ¹⁵⁾

Figure 6: MOMS-02 imaging geometries ¹⁶⁾

Data: Onboard recording onto HDDT (uncompressed for stereo), maximum recording time is 5.5 hours; this amounts to about 2.5 x 10¹² bit of data. The maximum recording data rate was 100 Mbit/s. The Spacelab D-2 mission produced over 1300 scenes of imagery.

The observation data of the mission were used in the following applications: Land cover (vegetated areas: land use, biomass estimation; unvegetated areas: lithology, mineral prospecting, tectonic investigations); geomorphology, ecology, basic research in the spectral signatures of rocks, soil, vegetation, etc.

Generally, the MOMS data processing provided an important impetus for photogrammetry and remote sensing in such fields as: 1) the development of methods and algorithms for image analysis, 2) the development of DEM (Digital Elevation Models), and 3) the first generation of spaceborne high-resolution topographic imagery.

Figure 7: Observation geometry of stereo triplets of the MOMS-02 instrument (image credit: DLR)

Figure 8: Line drawing of the MOMS-02 instrument (image credit: DASA)

Figure 9: Photo of MOMS-02 optical module (image credit: DASA)

Figure 10: Comparison of ground pixel sizes of major imagers in the time period 1980s and 1990s (image credit: DLR)

Figure 11: General MOMS-02 coverage regions of the Spacelab D-2 Shuttle mission (image credit: DLR)

Figure 12: MOMS-02 image of Buraida, Saudi Arabia (image credit: LMU)

Figure 13: Artist's rendition of the MOMS-02 instrument (image credit: Technical University of Munich)

MOMS-02 on MIR/Priroda Module

The MOMS-02 instrument of DLR was also flown on the Russian MIR/Priroda mission (referred to as MOMS-2P) for an extended observation period. MOMS-2P was launched on May 5, 1996 with the Progress-M31 service flight. This was preceded by the launch of the Priroda module (20 tons, 9.7 m in length, 4.35 m in diameter) on April 23, 1996 with a Proton launch vehicle. The Priroda module was the last major element of MIR completing the built-up phase of the MIR station complex (which had started in Jan. 1986 with the launch of the core module). The Priroda (nature) module represented actually the Russian equivalent of NASA's "Mission to planet Earth." The module was dedicated to Earth observation and contained a total payload mass of about 7,000 kg (i.e., Earth observation instruments).

After the D-2 mission, the MOMS-02 instrument had been refurbished and adapted to the Priroda environment (the camera system was modified to permit the cosmonauts to mount portions of MOMS-2P onto the outside wall of Priroda during EVA). Additionally, a navigation package had been included, referred to as MOMSNAV (MOMS Navigation), consisting of a high precision gyro system and a GPS system, to provide the necessary position and attitude data for supporting a high-precision data evaluation mainly for extracting digital terrain models. ^{17) 18) 19)}

Orbit of MIR Space Station with Priroda module: Average altitude of 400 km with an inclination of 51.6º. The orbit parameters resulted in a mean precession increment of -23.2º per orbit and a repetition rate between 3 and 18 days, depending on the distance to the equator.

Some background on the MIR station complex stabilization and observation capabilities: The MIR station attitude was inertially stabilized by default, i.e., during nominal operations, referred to as "duty mode." A specific inertial orientation was essential for MIR to keep its solar arrays properly illuminated by the sun. The duty mode implied the following advantages: minimum propellant consumption, maximum power generation, and normal thermal control conditions. The MIR orbital complex (about 200 tons of total mass) employed a CMG (Control Moment Gyroscope) concept for prime attitude stabilization, referred to as ROS (Gyrodyne Flywheel Orientation System). The Kvant-1, and -2 modules each carried six gyrodyne CMGs.

An important aspect of the MIR station complex operations was the provision of service to its multipurpose science program, which included the fields of astrophysical and geophysical observations as well as the support of technological programs. The typical procedure of the MIR station complex orientation to support for instance Earth observation required pointing of the line-of-sight of the station complex into the desired direction. Naturally, such service mode changes in station complex orientation required scheduling, involving a considerable amount of energy to keep the station pointed during a particular operational support period. The consequence of so many constraints at hand resulted in the allocation of fairly limited observation periods for the multitude of observation requirements onboard. The Earth-pointing mode of the station meant also less power generation capability since the solar arrays couldn't be pointed optimally into the sun direction. ²⁰⁾

During its lifetime, the MIR station experienced a number of mishaps/accidents regarding either attitude control problems or power system failures/non-availabilities. Some of them are:

- On June 25, 1997 the Progress service vehicle had struck the station (a cosmonaut lost control of Progress while practising manual docking procedures), hitting the solar array and a nearby radiator on the Spektr module (fortunately, a leak on Spektr could be closed provisionally by the cosmonaut crew). As a consequence, MIR lost use of its best batteries and the solar arrays of Spektr, generating about 50% of the electrical needs of the station. The Spektr module was damaged and disconnected from the power system. The ensuing scarcity of power lead in turn to stabilization problems of the MIR station (not enough power for nominal ROS operations) which lead to other problems in the station environment. New batteries and other spare parts were delivered on Shuttle flight STS-86 to MIR (Sept. 25 - Oct. 6, 1997).

- On May 30 1998, MIR's main onboard computer failed, which controlled the station's orbital alignment, leaving MIR adrift but in no immediate danger. The computer was later restarted.

- The last docking of a Progress service vehicle with MIR took place on Jan. 27, 2001. Progress had 2.7 tons of fuel onboard needed for the MIR station deorbit/reentry maneuvers.

MOMS-2P mission:

The major advantages of the MOMS-2P mission, compared with MOMS-02 on the Spacelab D-2 mission, were expected by the following mission parameters: ²¹⁾

• Provision of a long-term observation capability

• A much wider latitudinal Earth coverage (than with the Shuttle flights) due to the orbital inclination of 51.6º for MIR/Priroda. This permitted also observations to be taken in Germany and other parts of Europe.

• The installation of the MOMS-2P instrument on Priroda provided a multisensor observation approach (combination with the complex set of Earth observation experiments onboard the Priroda module).

Portions of the MOMS-2P (camera system) and the MOMSNAV package (two GPS antennas, two redundant gyro sensor blocks, one gyro electronics unit) were mounted onto the outside wall of the Priroda module.

Figure 14: Illustration of the MOMS-2P system on MIR/Priroda (image credit: DLR)

The spectral characteristics of MOMS-2P optical module were identical to those of MOMS-02. However, the observation geometries (swath, pixel size) changed due to the higher orbital altitude of MIR. The swath width for the high-resolution channel (HR5) could be either 36 km or 50 km, depending on the recording mode, and 50 km or 105 km for the other channels.

Table 4: Performance parameters of MOMS-2P (400 km orbit)

Figure 15: Illustration of MOMS-2P stereoscopic imaging geometries ²²⁾

MOMSNAV (MOMS Navigation). ²³⁾ An integrated navigation package built by Kayser-Threde (Munich) with the objective to provide accurate location knowledge to MOMS-2P imagery. MOMSNAV was a DGPS-based (Differential GPS) navigation package consisting of the following elements: two GPS antennas, two redundant gyro sensor blocks, one gyro electronic unit, and an electronic box. MOMSNAV used the L1 C/A signal code GPS receiver "Viceroy", manufactured by Motorola Inc. of Phoenix, AZ, and two redundant inertial systems, LWR-85, produced by LITEF GmbH of Freiburg. The instrument provided navigation data which was used in post-processing in combination with the imaging data. MOMSNAV location knowledge accuracy of the imagery was =< 5 m horizontal (1 sigma), the relative attitude accuracy was =< 10 arcseconds (1 sigma). Instrument mass = 41 kg, power = 70 W (average).

Figure 16: Illustration of the PRIRODA module on the MIR Space Station

MOMS-2P operations were limited by the data rate of the onboard tape recorder of 100 Mbit/s (source data rate). This implied that all channels couldn't be operated simultaneously. Hence, a set of four operational modes were defined combining different channels for various applications. Table 5 summarizes the four modes with the corresponding numbers of pixels per imaging line. The tape recorder allowed a maximum tape capacity of 48 GByte corresponding to a recording time of 80 minutes for an average data rate of 10 MByte/s.

Table 5: Operational modes of MOMS-2P

Table 6: Description of MOMS-2P operational modes

MOMS-2P operations

All MIR station operations (TT&C function), including all onboard instrumentation, could only be performed by ZUP, the Russian Space Control Center in Kaliningrad near Moscow. The MOMS-2P mission planning function (generation of timelines) for the various observation periods was done at DLR/GSOC in Oberpfaffenhofen. The MOMS-2P instrument data were received, and archived at the DLR/DFD station in Neustrelitz, Germany.

In Oct. 1996, the first observations of MOMS-2P were made over Australia and South Africa. This was followed by the commissioning phase during which the sensor was tested and validated.. The operational phase was due to start in spring 1997, when several problems of the MIR station hindered new data takes. Starting in May 1997, the power box of MOMS-2P produced malfunctions so that no more data could be acquired. As a consequence, only about 30 high quality scenes could be obtained in the first year of operations.

Within the MIR observation scheme, the MOMS-2P instrument was able to manage about 3 minutes/day of observation on average, equivalent to about 2000 km of ground track (4 Gbyte). The MOMS-2P instrument provided data until August 1999.

Table 7: Overview of MOMS-2P observations ²⁴⁾

The last crew said good-bye to the MIR station on Aug. 29, 1999 (a total of 27 crews had occupied the MIR station over a period of 13 years. The final MIR reentry into Earth's atmosphere took place on March 23, 2001 with a splashdown of the disintegrated station into the Pacific Ocean.

Figure 17: Overview of MOMS-2P coverage regions (image credit: DLR)

Figure 18: MOMS-2P image of the Puna de Atacama in Chile (in A Mode), image credit: DLR

2) J. Bodechtel, D. Meißner, P. Seige, H. Winkenbach, J. Zilger, "The MOMS Experiment on STS-7 and STS-11 - First Results and Further Development of the Modular Optoelectronic Multispectral Scanner," Proceedings of the Eighteenth International Symposium on Remote Sensing of the Environment, Volume I, 1984, pp. 77-85

3) "MOMS-01: First Results of STS-7 Mission," Proceedings of IGARSS'83, San Francisco, CA, Aug. 31 to Sept. 2, 1983

4) J. Bodechtel, R. Haydn, J. Zilger, "MOMS-01: Missions and Results," Monitoring Earth's Ocean, Land, and Atmosphere form Space - Sensors, Systems, and Applications, edited by A. Schnapf, Progress in Astronautics and Aeronautics, AIAA, Vol. 97 1985, pp. 524-535

5) M. Schroeder, "25 Years Space Photogrammetry in Germany - A Research Field Initiated by Gottfried Konecny," Proceedings of ISPRS Hannover Workshop 2005: Hannover, Germany, May 17-20, 2005, Commission I, WG I/5

6) http://www.iaag.geo.uni-muenchen.de/forschung/agf/moms/m_01.html

7) D. Meissner, P. Seige, "The modulator optoelectronic multispectral scanner (MOMS-01) test flight on STS-7 and STS-11 (41B)," Proceedings of SPIE, 1985

8) F. Ackermann, J. Bodechtel, F. Lanzl, D. Meissner, P. Seige, H. Winkenbach;"MOMS-02 - Ein multispektrales Stereo-Bildaufnahmesystem für die zweite deutsche Spacelab-Mission D2," Geo-Informations-Systeme, Zeitschrift für interdisziplinären Austausch innerhalb der Geowissenschaften, Wichmann Verlag, Jahrgang 2, Heft 3/1989, S. 5 - 11

9) P. Seige, "MOMS-02 - Eine hochauflösende stereoskopische und multispektrale Kamera auf der zweiten deutschen Spacelab Mission D-2," DLR-Nachrichten, Heft 77, Februar 1995

10) J. Bodechtel, S. Lutz, "Neue Wege der Erderkundung," aus Einsichten, Forschung an der LMU, pp. 38-43, 1992

11) MBB Endbericht, "MOMS-02 auf D-2," die Entwicklung von EOS über MOMS-EM, MOMS-01 bis MOMS-02, Doc. No. MOMS-02.RP.0100.0, Dec. 20, 1993

12) M. Berger, H. Kaufmann, 1995, MOMS-02 - D2/STS-55 Mission - Validation of Spectral and Panchromatic Modules," Geo-Informationssysteme, 1995, Vol 8, No 2, pp. 21-31.

13) F. Ackermann, J. Bodechtel, F. Lanzl, D. Meissner, P. Seige, H. Winkenbach;"MOMS-02/Spacelab D-2:: a high-resolution multispectral stereo scanner for the second German Spacelab mission," Proceedings of SPIE, Orlando, FLA, USA, April 1, 1991, Vol. 1490 pp.94-101 (ed. P. N. Slater)

14) M. Schroeder, W. Kornus, M. Lehner, R. Müller, P. Reinartz, P. Seige, "MOMS - the first along track stereo camera in space - a mission review," Proceedings of ASPRS Annual Conference, Washington, D.C., May 22-26, 2000

15) http://www.op.dlr.de/ne-oe/fo/moms-02t.html

16) Courtesy of Peter Seige of DLR, Oberpfaffenhofen

17) http://www.nz.dlr.de/moms2p/

18) P. Seige, P. Reinartz, M. Schroeder , "The MOMS-2P Mission on the MIR Station," http://www.nz.dlr.de/moms2p/best_of97/html/sensor/indien1.html

19) M. Berger, H. Kaufmann, P. Seige, "MOMS-02 on PRIRODA/MIR, Proceedings of EUROPTO Symposium on Satellite Remote Sensing, Toarmina, Sicily, Sept. 23-27, 1996, pp. 224-229

20) I. N. Tereshina, V. P. Teslenko, A. I. Manzheley, "Attitude Simulation During MIR Orbital Complex Flight," http://www.op.dlr.de/SpaceOps/spops96/simmod/sm-5-20/5_20.pdf

21) Data User Handbook of DARA/DLR, 1997

22) http://www.scanex.ru/data/moms/moms2p.htm

23) S. Föckersperger, et al., "MOMSNAV: Location of the Russian Space Station MIR with Differential GPS," Proceedings of the 2nd ESA International Conference on GNC, ESTEC, 12-15 April 1994, pp. 159-165

24) Information provided by Peter Reinartz of DLR, Oberpfaffenhofen, Germany

This description was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments and corrections to this article are welcomed by the author.