The Astrophysical Journal Supplement Series, 152:137–162, 2004 May # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.
SUBMILLIMETER WAVE ASTRONOMY SATELLITE PERFORMANCE ON THE GROUND AND IN ORBIT V. Tolls, G. J. Melnick, M. L. N. Ashby, E. A. Bergin,1 M. A. Gurwell, S. C. Kleiner, B. M. Patten, R. Plume,2 J. R. Stauffer,3 Z. Wang, and Y. F. Zhang Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138;
[email protected]
G. Chin NASA Goddard Space Flight Center, Greenbelt, MD 20771
N. R. Erickson and R. L. Snell Department of Astronomy, University of Massachusetts, Amherst, MA 01003
P. F. Goldsmith Department of Astronomy and National Astronomy and Ionosphere Center, Cornell University, Ithaca, NY 14853
D. A. Neufeld Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218
and R. Schieder and G. Winnewisser I. Physikalisches Institut, Universita¨t zu Ko¨ln, Zu¨lpicher Strrasse 77, D-50937 Ko¨ln, Germany Received 2003 October 16; accepted 2004 January 5
ABSTRACT The Submillimeter Wave Astronomy Satellite (SWAS ), which was launched in 1998 December, is a NASA mission dedicated to the study of interstellar chemistry and star formation. SWAS is conducting pointed observations of molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: O2, C i, H218O, 13CO, and H216O at approximately 487, 492, 548, 551, and 557 GHz, respectively. The SWAS instrument is comprised of a 54 cm ; 68 cm off-axis Cassegrain telescope feeding two independent heterodyne receivers with Schottky barrier diode mixers, passively cooled to about 175 K. An Acousto-Optical Spectrometer (AOS) provides 1 MHz (0.6 km s1) frequency resolution and 1400 MHz (840 km s1) total bandwidth with 350 MHz (210 km s1) per line for spectral analysis. SWAS was fully characterized during ground-based testing, and all performance parameters were verified on-orbit. During its onorbit operation, SWAS observed more than 200 astronomical objects with more than 5000 lines of sight. This paper describes the tests conducted and compares the ground-based test results with the on-orbit test results. Subject headings: instrumentation: detectors — instrumentation: spectrographs — radio lines: general — submillimeter — techniques: spectroscopic — telescopes 1. INTRODUCTION
particular, the SWAS instrument. Section 3 contains a description of the ground-based and on-orbit tests performed and compares their results as applicable.
The Submillimeter Wave Astronomy Satellite (SWAS ) mission is dedicated to observing Galactic sources in the ground state or low-lying transitions of five astrophysically important species: O2, C i, H218O, 13CO, and H216O. The key parameters of these spectral lines are listed in Table 1. By conducting these observations, SWAS has provided data that (1) tested long-standing theories that predicted which species were the dominant coolants of molecular clouds during the early stages of their collapse to form new stars and planets and (2) improved existing chemical models of the dense interstellar gas by suppling previously missing information about the abundances of these species. A review of the SWAS science objectives and early results is given in Melnick et al. (2000a). Examples of SWAS observations for each of the five species are shown in Figure 1. This paper focuses on the technical performance of the SWAS hardware. Section 2 describes the SWAS spacecraft and, in
2. SUBMILLIMETER WAVE ASTRONOMY SATELLITE 2.1. Overview SWAS (Fig. 2) consists of the scientific instrument and the spacecraft system. The scientific instrument (Fig. 3) is composed of (1) the telescope, (2) two heterodyne receivers, (3) the Acousto-Optical spectrometer, (4) the instrument control electronics, (5) the star tracker, (6) the bright object sensor, (7) the magnetometer, (8) the thermal control system, and (9) the instrument structure. The spacecraft system is composed of (1) the attitude control system (ACS), (2) the solar arrays and power regulation circuitry, (3) the on-board command, data handling, and ACS computer system, (4) the solid-state memory for data storage, (5) the telemetry system, and (6) the spacecraft structure. SWAS does not require any expendables. All systems including the science instrument are passively cooled and the spacecraft is maneuvered by reaction wheels and torque rods. A worst-case scenario of SWAS’s orbital lifetime predicts reentry in the year 2013. Thus, SWAS’s operational lifetime of 7–8 yr is determined by the failure of a critical instrument component (for details see x3.6). The
1 Now at Department of Astronomy, University of Michigan, Ann Arbor, MI 48109. 2 Now at Department of Physics and Astronomy, University of Calgary, Calgary, AB T2N IN4, Canada. 3 Now at IPAC, California Institute of Technology, Pasadena, CA 91125.
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TABLE 1 Spectral Lines Observed by SWAS
Species O2 ................................ C i ............................... H2 18O ......................... 13CO ............................ H2 16O .........................
Transition Upper State–Lower State
Energy above Ground State (E=k) (K)
Frequency (GHz)
Wavelength (m)
3,3–1,2 3 1 – P0 110 –101 5–4 110 –101
26 24 26 79 27
487.249 492.161 547.676 550.926 556.936
615.276 609.134 547.390 544.161 538.289
3P
Fig. 1.—Examples of SWAS observations: (top left) W49: ¼ 19h10m13:s5, ¼ 9 60 2900 (J2000), integration time tint ¼ 371 hr; (top right) Orion: ¼ 5h35m14:s5, ¼ 5 220 3700 (J2000), tint ¼ 122 hr, Melnick et al. (2000b); (middle left) M17: ¼ 18h20m22:s1, ¼ 16 120 3700 (J2000), tint ¼ 57:5 hr, Howe et al. (2000); (middle right) DR21: ¼ 20h39m0:s9, ¼ 42 190 3800 (J2000), tint ¼ 18:8 hr, Ashby et al. (2000); (bottom left) Earth atmosphere: tint ¼ 2 s, Goldsmith et al. (2002); (bottom right) W3: ¼ 2h25m29s:9, ¼ 62 50 5400 (J2000), tint ¼ 17:4 hr.
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Fig. 2.—SWAS spacecraft during ground testing with unfolded solar arrays. (Courtesy NASA/GSFC)
Fig. 3.—Cutaway view of the SWAS instrument (after Melnick et al. 2000a)
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following subsections provide a detailed description of the SWAS instrument subsystems, a description of the spacecraft, and a overview of the essential observing modes. 2.2. SWAS Instrument A cutaway view of the SWAS instrument is shown in Figure 3, a block diagram of the heterodyne receiver system and the back end is shown in Figure 4, and a summary of the key instrument parameters is given in Table 2. The SWAS optical subsystem consists of a 54 cm ; 68 cm diameter f =0:76 ; f =0:59 offaxis Cassegrain elliptical primary mirror, a secondary mirror with chopping mechanism, a warm calibration load, and a flip mirror to redirect the SWAS beam onto the calibration load. The primary and the secondary mirrors are made from aluminum. The combined rms surface accuracy is 11 m within the inner 80% (by area) of the primary mirror and increases to about 15 3 m at the outer edge. A more detailed discussion of the surface accuracy is given in x 3.5.2. The SWAS secondary mirror can be chopped for observations of pointlike sources resulting in an 8A5 movement on the sky along a fixed axis coinciding with the smaller dimension of the primary mirror. The chopping rate is selectable among three options: 2 Hz, 14 Hz, and off. The 2 Hz rate can only be used with the total power continuum detector in each receiver channel and is synchronized with the read-out rate (the time per position is 0.25 s and the effective integration time per position is 0.236 s). The 14 Hz rate can be used for spectroscopic observations employing the AOS and for total power continuum measurements. The continuum detector is synchronized to the 2 s read-out rate of the AOS and has 1.887 s of integration time with the remaining time used for read-out and reset. In the nodding observation mode, the chopping mechanism is inactive, the secondary mirror is on-axis, and the spacecraft is used to move the telescope over spatial angles greater than 8A5 as is required for extended sources. The SWAS spacecraft has been designed to provide spatial nodding over angles up to several degrees on short timescales. As a trade-off between achieving short nodding times and using only minimal power, the nodding speed has been set to a slew-and-settle time of 15 s for a 3 nod. Smaller and larger angle nods require less and more time, respectively. The SWAS instrument block diagram is shown in Figure 4 and consists of the telescope, the cold front end (CoFE), the mm-wave local oscillator/synthesizer, the dual IF system, and the acousto-optical spectrometer. The CoFE end consists of the input optics, two mixers, two triplers, and an intermediate frequency (IF) high-electron–mobility transistor (HEMT) amplifier for each of the two receiver channels. The beam from the SWAS telescope is split by a polarizing wire grid beam splitter and redirected into the feed horns of the two SWAS mixers. The polarization in the high-frequency channel is rotated 90 by a reflective half-wave plate between the beam splitter and the feed horn. Thus, both mixers have the same physical linear polarization orientation while being sensitive to orthogonal linear polarizations on the sky. The reference signals are provided by the mm-wave LO/synthesizer utilizing two redundant InP Gunn oscillators for each receiver channel. Each Gunn oscillator is controlled by a frequency synthesizer and derives its output frequencies from a single reference frequency of 5.11 GHz, which in turn is derived from a high-precision 10 MHz reference oscillator. The output signals of 81.5 GHz for receiver channel 1 and 92.3 GHz for receiver channel 2 are frequency tripled in the
CoFE and the second harmonic of the resulting signals, at 489.5 and 553.8 GHz in receiver channel 1 and receiver channel 2, respectively, are mixed with the astronomical signals from the telescope. Through the careful choice of IF frequencies, SWAS is able to observe simultaneously O2 at 487.2 GHz and C i at 492.2 GHz in the lower and upper sidebands of receiver channel 1, respectively, and 13CO at 550.9 GHz and H216O at 556.9 GHz simultaneously in the lower and upper sidebands of receiver channel 2, respectively. For observations of H218O, receiver channel 2 is tuned to an LO frequency of 550.7 GHz. For the purpose of placing the spectral lines in the center of the band for sources with varying LSR velocities, both receivers can be tuned by 300 MHz about their nominal frequency in steps of 12 MHz. This corresponds to a tuning range of 182 km s1 in steps of 7.3 km s1 in receiver channel 1 and of 164 km s1 in steps of 6.6 km s1 in receiver channel 2. The Gunn oscillators are connected via WR-10 and WR-12 waveguides to the triplers. The tripler and mixer blocks are physically attached to each other resulting in very short waveguides for the high-frequency signals. The output power of the Gunn oscillators is about 30 mW and the LO power in the mixers is approximately 0.5–1 mW. The second harmonic mixers utilize T13 Schottky barrier diodes and the triplers 5P8 diodes, both made by the University of Virginia. During the development phase, the SWAS team decided to select Schottky mixers over superconductor-insulator-superconductor (SIS) mixers for the following reasons: (1) Schottky diode mixers can be operated cooled or uncooled at ambient temperature enabling ground-based performance testing; (2) cooling the mixers required only a simple passively cooled thermal design discussed below; (3) although SIS mixers would have had a higher sensitivity, they would have required an operating temperature of a few Kelvin. This temperature could only have been achieved through cryogenic cooling either with a closedcycle cooler or by using expendable cryogens. At the time SWAS was developed, there were no space-qualified closedcycled coolers available and the existing, non–space-qualified technology would have consumed too much power not to mention the additional development costs. Utilizing expendable cryogens would have severely limited the SWAS operational lifetime to only a few months because of mass constraints, whereas the lifetime of passively cooled Schottky mixers is many years. Also, many important astronomical targets would not have been available for observation with the short operational lifetime. The CoFE is passively cooled on-orbit to about 175 K. The CoFE is connected via flexible multilayer S-link cold straps to three coldplate radiators that are shaded by compound parabolic Winston cones. These cones, which point to cold space, have a 40 half-angle field of view, which is tipped 5 away from the spacecraft z-axis in the direction of the y-axis (see Fig. 6 below). The inside of the cones have a 10 m thick layer of gold plating to retro-reflect scattered radiation. The mixer and tripler blocks, the CoFE, the mm-wave LO/synthesizer, and the dual IF system were built and assembled by Millitech Corporation. The HEMT amplifiers were built by Berkshire Technologies, Inc. The telescope and instrument was designed and assembled by Ball Aerospace, and the primary reflector was built by SSG, Inc. The receiver system is calibrated using the common y-factor method. The warm calibration load measurement is performed utilizing an oversized blackbody radiator at
Fig. 4.—Block diagram of the SWAS instrument
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TOLLS ET AL. TABLE 2 SWAS Spacecraft and Instrument Summary Parameter
Telescope................................... Aperture efficiency.................... Main-beam efficiency................ Beam size: 490 GHz................................ 550 GHz................................ Mirror surface accuracy............ Absolute pointing accuracy ...... Pointing jitter ............................ Receiver type ............................ Receiver temperature ................ Operating frequencies: Receiver 1: O2 ...................... Receiver 1: C i...................... Receiver 2: H216O................. Receiver 2: 13CO .................. Receiver 2: H218O................. Receiver noise temperature: Receiver 1 ............................. Receiver 2 ............................. Back end ................................... Bandwidth ............................. Resolution ............................. Weight: Instrument ............................. Spacecraft.............................. Power: Instrument ............................. Spacecraft.............................. Launch vehicle.......................... Launch site................................ Launch date............................... Orbit .......................................... Design mission lifetime ............
Value 54 cm ; 68 cm oA-axis Cassegrain 66% 90% 3A7 ; 5A2 3A3 ; 4A5 10 dB m1 at 557 GHz, which made the far-field measurements for the high-frequency receiver impossible. The advantage of the near-field measurement is that the phase information in a scan contains nearly all information concerning the alignment. With appropriate software tools, any possible alignment problem could be identified and corrected. The near-field measurements are described in more detail in the following section. After measuring the SWAS beams and their co-alignment on the ground, the results had to be verified on-orbit. For this purpose, Jupiter and Saturn were used as pointlike calibration sources. The details of these measurements are described in x 3.5.3. 3.5.2. SWAS Antenna Near-Field Measurements
Near-field range (NFR) measurements to characterize the SWAS beam (Erickson & Tolls 1997) were performed as part of the SWAS ground testing program. To perform these measurements, the SWAS program used a compact near-field range built by Nearfield Systems, Inc. (Slater 1994), and modified by Millitech Corp. to meet the SWAS test requirements. The compact range used a granite reference slab with granite rails and carriages with air bearings. This configuration achieved a flatness of its travel of about 2 m rms. The carriage could be moved by horizontal and vertical stepper motors over 86 cm in either direction with a maximum speed of 10 cm s1. A
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transmitter was mounted on the carriage and connected via flexible coax cable and a coax rotatory joint to the 5.11 GHz reference source in the SWAS receiver system. The combined use of the coax rotatory joint and flexible coax cable added the least phase variation to the signal while the carriage moved over the scanning area. The signal strength of the radiation source was about 0.5 W at 490.6 GHz and about 10 times less at 551.9 GHz. The radiation source and the compact range were protected by a 0.8 m by 0.8 m shield covered with FIRAM submillimeter–wave absorption material (Giles et al. 1993) to suppress possible reflections of radiation that could have compromised the measurements. Only the emitting horn of the transmitter was visible through the absorber material. The SWAS antenna was positioned approximately 5 cm in front of the NFR. The signals measured by the two SWAS receivers were downconverted and amplified. Power couplers in the first IF stages (signal frequencies were 2.1 and 2.496 GHz in receiver channels 1 and 2, respectively) provided access ports to the signal for the compact test range (see Fig. 4). The reference signal was derived from the phase lock loop IFs (350 MHz and 416 MHz) used to lock the instruments Gunn oscillators and multiplied by 6 to match the signal frequencies. These two signals were initially measured with a HP 8753 network analyzer used as a vector voltmeter. During the first test it was found that there was a line width mismatch between the reference and the source signals. This mismatch was compensated for by an additional mixing stage with the 1 GHz reference signal provided by the network analyzer. The number of points in the measurement grid of the nearfield measurements could be varied in steps of a factor of 2. However, because of the amount of memory in the analysis computer, the grid was limited to a maximum of 256 points by 256 points. The total scan range was 0.86 m by 0.86 m compared with the SWAS primary mirror size of 0.54 m by 0.68 m and, hence, provided ample coverage. A typical measurement took approximately 50 minutes for a 256 point by 256 point, 0.8 m by 0.8 m scan including repeated measurements (every 10 minutes) of four reference points near the center of the mirror to provide corrections for phase drifts. Data were alternately taken for both frequencies, 490.6 and 551.9 GHz. The 490.6 GHz data set was considered more reliable because of the stronger radiation source signal and the lower atmospheric attenuation. Maps at 551.9 GHz were used to verify the focus of the high-frequency receiver and to verify the co-alignment of the two SWAS receivers. The results of the SWAS NFR measurements are summarized in Figure 18. The compact range required a tilt relative to the SWAS beam axis to remove the effects of a small leakage signal from the last RF mixing stage of the NFR test setup. Thus, all measurements showed a periodic phase variation across the measurement plane. This phase variation was removed by transforming the near-field data into the far-field, compensating for the tilt by recentering the data and transforming the far-field back into the near-field. The resulting SWAS amplitude pattern is shown in Figure 18 (top left). Unfortunately, residual effects of the removed phase pattern can still be seen. Another weakness of the NFR measurement becomes clear in this plot. As a result of the finite width of the NFR transmitter and the imperfection of the SWAS main mirror due to hand polishing, the edge taper of about 15 dB as shown in the plot can be seen as a lower limit (SWAS was designed with an edge taper of 11 dB). The plot shown is for 491 GHz. An overlay of the amplitude contour lines for 491 and 552 GHz (not shown) indicates that the co-alignment of
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both beams was within a few arcseconds. The same result was achieved by comparing the corresponding far-field pattern (again, only the 491 GHz far-field pattern is shown in Fig. 18, top right). The 491 GHz far-field pattern shows that there are no sidelobes with an amplitude larger than 15 dB relative to the main beam peak. Another important result of the NFR measurements was the verification of the surface accuracy. The surface accuracy was measured after fabrication of primary and secondary mirror using the Hartman test. The averaged measured wavefront error (seven positions on the primary mirror were measured) was 14 2 m rms resulting in a surface error of 7 1 m rms. By combining this result with the contribution of all other optical elements in the receiver system (e.g., wire grids), the total wavefront error yielded 16 3 m corresponding to 8 2 m surface error. The NFR measurement results are shown in Figure 18. The bottom left panel of Figure 18 shows the rms surface accuracy of the main mirror (it is assumed that the primary mirror is the main source of the rms wavefront error measured by the NFR yielding the plotted rms surface accuracy), and the bottom right panel shows the cumulative rms surface error within an increasing elliptical area around the beam center (0% is the beam center and 100% is the mirror rim). The solid line shows the unweighed rms surface error, and the dashed line shows the amplitude weighed rms surface error. Assuming a measurement error of 1 m, the results for 492 and 552 GHz track very well. Minor deviation can be caused by the different sensitivity to environmental changes and by half-pixel offsets between the measurements. For the central 60% of the primary mirror, the rms surface error is about 6 m rms, which increases to 12 m rms (amplitude weighed result) or to 15 m rms (unweighed result) for the full primary mirror size. These values are about twice as large as the wavefront errors derived from the Hartman test. It was concluded that the optical method did not see all error sources that contributed to the submillimeter performance. Besides calculating the surface accuracy as described above, the same data set was used to obtain an estimate of the aperture efficiency A of the SWAS telescope. In its simplest form, the aperture efficiency is the product of the taper efficiency T and the spillover efficiency S : A ¼ T ; S ;
ð6Þ
with R R 2 Ap EA (x; y)dxdy T ¼ R R ; RR 2 Ap j EA (x; y)j dxdy Ap dxdy
ð7Þ
and RR S ¼ R R
2 Ap j EA (x; y)j dxdy 2 Ap Plane j EA (x; y)j dxdy
:
ð8Þ
The integrals above are calculated over the telescope aperture, ‘‘Ap,’’ and over the whole aperture plane, ‘‘Ap Plane.’’ For the application of the SWAS near-field data, all integrals were replaced by sums, the electrical field EA (x; y) ¼ EA0 ( cos þ i sin ) with the near-field amplitude EA0 and the near-field phase , and the step widths dx ¼ x and dy ¼ y. The integration range caused a problem. The integration over the physical area of the telescope did not lead to conclusive results. The reason is that the near-field measurements suffer
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Fig. 18.—Results of the SWAS NFR measurements. Top left: Relative near field power pattern derived from measurement of the SWAS near-field beam at 492 GHz. Top right: SWAS relative far-field power pattern derived from the near-field data to the right via FFT. Bottom left: Surface deviation of the SWAS telescope at 492 GHz. Bottom right: Cumulative rms surface accuracy as a function of increasing elliptically shaped area around the mirror center. The dashed lines show the amplitude weighed rms surface accuracy, and the solid line shows the unweighed rms surface accuracy for receiver channels 1 and 2, respectively.
from decreased sensitivity measuring the amplitude and phase at the edge of the telescope. The measured amplitude is too small because only part of the beam from the test probe hits the telescope and the phase measurement shows increased random values due to small signal strength. Therefore, the integration was performed using the edge taper as a cutoff. The SWAS telescope is designed for a 11 dB edge taper. Assuming that only half of the probe beam was on the telescope at the edge of the telescope, the cutoff amplitude was set to 14 dB (however, the resulting used physical diameter of the telescope was still about 2.5 cm too small). Applying this to the data, the taper efficiency is 0:81 0:01 and 0:76 0:06 at 490 and 550 GHz, respectively. The uncertainty in the 550 GHz measurement is significantly higher since it was affected by atmospheric water vapor. The near-field data could not be used to calculate the spillover efficiency because there were not sufficient data
available from outside the telescope aperture. However, the SWAS feed horns were measured at Millitech Corporation before integration into the SWAS receivers. These data were fitted to a theoretical feed horn pattern of a TE11 mode conical horn. This theoretical model was then used to calculate the spillover efficiency. The resulting values are 0:92 0:03 and 0:88 0:03 for 490 and 550 GHz, respectively. Combining these results, the SWAS aperture efficiencies are 0:75 0:03 and 0:66 0:03 for 490 and 550 GHz, respectively. These value were regarded as upper limits for the aperture efficiency because they were derived from a smaller telescope aperture than the physical aperture. 3.5.3. Beam Measurements on Jupiter
The SWAS beamwidth for each receiver channel could not be well determined using the NFR test setup because of a lack of resolution in the far field data. Thus, the beam size had to
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Fig. 19.—SWAS line scan beam measurements on Jupiter. The azimuth scan was performed along the spacecraft x-axis, and the elevation scan was performed along the spacecraft y-axis. During the elevation scan, Jupiter appeared in the on-beam and in the off (chopped)-beam. The resulting FWHM are listed in Table 3.
be measured on-orbit. The beam measurements required a strong submillimeter point source to achieve a sufficient signal-to-noise ratio for a single map observation. This limited the list of possible sources to only the nearby planets. As a result of Sun avoidance limitations, which largely excluded Mars and Venus, only Jupiter and Saturn could be used, of which Jupiter was the stronger and thus preferred source. Figure 19 shows the results of chopped linear scans across Jupiter. The line scans were performed with respect to the spacecraft x-axis and y-axis. The y-axis scan shows Jupiter in the on-beam and in the off-beam since the scan was performed along the primary mirror-chopping secondary axis. Fits to the data showed that both beams were Gaussian in the center and that the full width at half-maximum (FWHM) beamwidths in either plane were within 10% of the design values (the resulting beamwidths are summarized in Table 3). The FWHM results from the NFR testing were comparable even though the elevation and azimuth cross sections of the beams did not look Gaussian. The center positions of the 492 and 557 GHz beams are coincident to within 0A10 0A02, or within 1/30 beamwidth. In addition to the beam shape, the elevation scans on Jupiter were used to determine the angular distance between the unchopped and chopped SWAS beams. The resulting chopping angles were 8A4 0A2 for 492 GHz and 8A5 0A2 for 557 GHz.
3.5.5. Beam Efficiency
The aperture efficiency of the SWAS observatory was measured using Mars as the primary calibrator. The aperture efficiency A can be calculated from (see e.g., Rohlfs & Wilson 2000)
A ¼
k2 TA ; Ap TRJ Mars
ð9Þ
where the observation wavelength k ¼ 612 m, the physical area of the SWAS primary mirror Ap ¼ 0:295 m2, the Rayleigh-Jeans corrected brightness temperature of Mars is TRJ , the antenna temperature is TA , and the solid angle of Mars is Mars. SWAS observed Mars during two extended observing periods: from 1999 April 17 to May 3 and from 2001 May 23 to September 13. During both observing periods, Mars was near opposition and at its largest angular extent. The angular diameter varied from 15B6 to 16B2 and from 11B9 to 20B7, respectively, during these epochs. The data presented here were taken during the second observing period in balanced beam continuum 2 Hz chopping mode (Mars was alternately in the unchopped beam with the chopped beam as reference and in the chopped beam with the unchopped beam as reference). The brightness temperature TRJ of Mars was derived from a model of the thermal emission characteristics of the Martian atmosphere at centimeter wavelengths (Rudy et al. 1987), which was extended into the submillimeter (Gurwell et al. 2000). The model calculated a disk-averaged Planck brightness temperature of 206 10 K in the 490–560 GHz range (with an average physical surface temperature and emissivity of 225 K and 0.915, respectively).
3.5.4. Pointing Accuracy
The final pointing accuracy of the SWAS telescope depended on how well the attitude control system could control the spacecraft orientation. The specification was 3800 pointing accuracy (one-fifth of the SWAS beam, 2 ) and 1900 jitter (onetenth of the SWAS beam, 2 ). On-orbit tests showed that the final pointing accuracy was approximately 500 (1 ) and that the jitter was of comparable magnitude.
TABLE 3 SWAS Beam Full Width at Half-maximum (FWHM) in Arcminutes 490 GHz
553 GHz
Test
Azimuth
Elevation
Azimuth
Elevation
Error
Design ........................................ NFR............................................ Jupiter scans...............................
3.8 3.8 3.7
4.8 5.0 5.2
3.3 3.4 3.3
4.2 4.2 4.5
0.2 0.1
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Fig. 20.—Measured antenna temperature TA of Mars vs. SWAS mission day (mission day 1000 is 2001 July 28). The measurements were conducted in balanced beam switch mode using the continuum detector with a 0.25 s integration time. The measurement error is smaller than the plot symbols. The solid lines shows the expected TA for an aperture efficiency of 0:68 0:04 at 490 GHz. The measurements for MD 970 to MD 1020 are lower than expected because a global dust storm on Mars lowered the surface temperature, which was not predicted by the thermal emission model used.
Figure 20 shows the measured antenna temperatures TA . for the lower frequency receiver as a function of observation time (SWAS mission day 1 was 1998 November 1). The measurement uncertainty per data point is smaller than the diamond shaped symbol. The solid lines represent theoretically expected antenna temperatures using equation (9) and the brightness temperature from our Mars model. For the final calculation of the aperture efficiency at 490 GHz, only the first six data points were used since a global dust storm started on Mars on 2001 June 26, mission day 967, affecting all later measurements. The data showed that the dust storm did cool the surface. Thus, the aperture efficiency derived from the measurements between mission days 936 and 964 is 0:68 0:04 at 490 GHz. An aperture efficiency for receiver channel 2 could not be calculated since a very broad water absorption feature with a half-width wider than the receiver bandwidth made the calibration impossible. However, cross-calibration measurements on the Moon showed that the efficiencies in both receiver channels were equal to within 10% as predicted by the Ruze equation (e.g., Rohlfs & Wilson 2000), which yields aperture efficiencies of 0.74 and 0.72 for 490 and 553 GHz, respectively, for a surface accuracy of 15 m. Compared with the aperture efficiencies derived from the NFR data, the aperture efficiencies derived from the Mars observations are, as expected, smaller but still in very good agreement considering the assumptions that have been made. The main beam efficiency was derived from the aperture efficiency: M ¼
M A Ap M ¼ ; A k2
ð10Þ
where the main beam solid angle is M and the aperture solid angle is A . The resulting main beam efficiency for the
492 GHz receiver is M ¼ 0:96 0:07. However, the theoretical main beam efficiency for an unblocked beam with perfect Gaussian shape and 11 dB edge taper is 0.90, which is within the error of our measurement. Thus, for SWAS we assume a main beam efficiency of M ¼ 0:90 0:07. 3.6. SWAS Lifetime The SWAS instrument was designed for a lifetime of 3 yr. However, since SWAS does not carry any expendables, its lifetime is limited only by the lifetime of its components. During the design phase the possible lifetime limiting components were identified to be the following: the mixers and triplers in the receivers, the local oscillator, and the laser diode in the AOS. Figure 21 shows the mixer bias voltages (the bias current being held constant) and the tripler bias currents (the voltage being held constant) for the first 1400 mission days. The variations in the bias data were caused either by temperature variations or frequency tuning, which changes the local oscillator power and consequently the tripler bias current and the mixer bias voltage. The receiver channel 1 tripler shows a small increase since mission start. This tripler showed a much higher temperature sensitivity than the receiver channel 2 tripler causing an increase in the bias current with increasing temperatures. Since the average temperature of the SWAS receiver has increased by about 2–3 K since beginning of the mission, the receiver channel 1 tripler bias current has increased too. Therefore, there can only be a minor degradation. The system performance is not affected because the receiver channel 1 mixer bias voltage has not changed and the receiver channel 1 system noise temperature is still in the range where it was at the beginning on the mission. Hence, it is concluded that there has been no degradation of the local oscillator as well.
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Fig. 21.—SWAS mixer bias current and tripler bias voltage as a function of time. The data show only variations due to temperature changes and local oscillator power changes due to frequency tuning, but no significant long-term degradation can be seen.
After 3 yr of on-orbit operation, the CCD sensor in the AOS was checked for radiation damage. AOS zero scans (the receiver signal is blanked) from ground testing were compared with zero scans taken after 3 yr on-orbit. The rms noise of these spectra increased from 0.81 counts to 1.39 counts. However, this increase is insignificant compared with the average count number of 20,000–30,000 during regular operation and the quality of the science data is not affected. The only degradation observed has been the slow decrease in laser diode power. Figure 22 shows the output signal of a monitor diode that measures the intensity of the laser light emitted through the rear facet of the laser diode. A major degradation of the monitor diode can be ruled out because the mean of the SWAS receiver signal power on the CCD decreased by the same factor as the monitor signal (the signal from the SWAS receivers stayed constant as verified by the SWAS continuum detector, which operates completely
independently from the AOS). The Hitachi HL 7851G laser diodes in the AOS were selected from a batch of 20 laser diodes according to their performance after a 100 hr burn-in. An additional 30 laser diodes were used for 3500 hr acceleratedlifetime tests at 50 C and 70 C. The measured expected mean time to failure (MTTF) was calculated to be 4780 and 1950 hr at 50 C and 70 C, respectively. In order to increase the lifetime, the laser diodes were operated at 40 mW compared with 50 mW typical optical output power, and at an operating temperature of 20 C. The MTTF for this part derating is about 20,000 hr or 2.3 yr. The current accumulated operating time of laser diode 1 is approximately 1 yr of ground testing and 3 yr of on-orbit operation, exceeding the estimated lifetime by 70%. In addition to the decreased optical output power, laser diode 1 showed small power fluctuations during the last 6 months of operation. These power fluctuations were not caused by mode jumps since the frequency calibration of the
Fig. 22.—Degradation of the optical output power of the SWAS laser diodes. Plotted is the monitor diode current (measuring the emission through the rear facet of the laser diode) as a function of time. The solid line shows a fit to the laser diode 1 response. The fitting model is a superposition of a linear (mostly during mid-life) and an exponential degradation (during end-of-life).
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SWAS PERFORMANCE
AOS did not change. As a result, operation was switched to laser diode 2 while saving laser diode 1 for test purposes only. It is expected that laser diode 2 will degrade slightly faster since its optical output power is slightly higher resulting in a slightly higher operating temperature. The details of the degradation mechanisms are not well understood. It seems that the additional on-orbit radiation pollution of about 0.34 krad (Si) did not accelerate the MTTF of the laser diode. Figure 22 shows that the degradation is initially almost linear with time, increasingly exponentially during the end-of-life with a sharp drop-off at the end. Laser diode 2 seemed to have gone through a burn-in phase during the first 100 days and is currently operating in the linear regime. The burn-in phase is missing in laser diode 1 since it had
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substantially more operating time on the ground (about 1 yr vs. a few month for laser diode 2). The linear portion of the curve for laser diode 2 shows a slightly steeper slope, which also supports the assumption of a shorter expected lifetime.
This work was supported by NASA contract NAS5-30702. R. Schieder and G. Winnewisser would like to acknowledge generous support provided by the DLR through grants 50 0090 090 and 50 0099 011. The authors thank all members of the SWAS teams at Millitech, Ball Aerospace, at GSFC, and at SAO for their tremendous efforts, which has made SWAS such a successful mission.
APPENDIX A FREQUENCY UNCERTAINTY IN SWAS SPECTRA All SWAS spectra are frequency calibrated. Since the pixel spacing of the AOS spectra is about 1 MHz, one would assume that the frequency calibration results in a much smaller uncertainty. However, there are multiple sources that contribute to the frequency uncertainty, which will be discussed in this section. The uncertainty sources are (1) precision of the frequency reference oscillator, (2) frequency calibration of the AOS scans, (3) frequency drift of the AOS, (4) velocity determination of the SWAS spacecraft, and (5) data processing. 1. SWAS uses an ovenized 10 MHz crystal oscillator as frequency reference. The prelaunch analysis of the error budget of this oscillator was 1.8 Hz (or rrel 1:8 ; 107 relative error). It was comprised of temperature- and pressure-induced drifts, load and supplied power-induced drifts, aging effects, and radiation-induced drifts. This reference frequency is used to generate the local oscillator frequencies of the first LO and second LO stages and the comb lines used for frequency calibration of the AOS spectra. The resulting error is for the worst case: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi f ¼ ( fmaxlo1 rrel )2 þ ( fmaxlo2 rrel )2 þ ( fmaxcomb rrel )2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ (554 GHz 1:8 ; 107 )2 þ (5:5 GHz 1:8 ; 107 )2 þ (3:3 GHz 1:8 ; 107 )2 ¼ 0:1 MHz:
ðA1Þ
2. The frequency calibration of the AOS scans is obtained utilizing the results from comb scans. A comb scan consists of inserting a frequency comb with comb lines 100 MHz apart into the regular receiver bandpass. For the analysis of the comb scans, a regular scan is subtracted from the comb scan leaving a pure comb line spectrum. A Gaussian line fit is applied to each of the comb lines to retrieve the line centers in pixel space. Since the frequencies of the comb lines are known (the uncertainty is given in [1]), a frequency can be calculated for each pixel. Because the frequency axis of the AOS is inherently nonlinear (a second-order polynomial would describe the frequency axis with negligibly small error), and because the astronomical analysis software CLASS can only handle linear frequency axes, a linear fit is applied to the frequency/pixel data pairs. The fit is applied such that the least error occurs in the middle of the band, where the astronomical lines are expected. If the spectral line is not near the center, either a frequency correction has to be applied or the frequency calibration in the SWAS pipeline software has to be centered around the line’s frequency. The frequency uncertainty for lines centered in the band can be assumed to be fcal ¼ 0:1 MHz. This uncertainty increases to fcalunmax ¼ 2 MHz at the band edges if uncorrected, or to fcalcorrmax ¼ 0:2 MHz if corrected. 3. The frequency scale of the AOS spectra may drift between comb calibration measurements. Comb measurements are typically performed on timescales of 5 minutes. Since the main reason of this short-term change is the temperature drift of the AOS, we assume as the worst case a 2 value of the temperature change during a 5 minute interval. Derived from a similar analysis as performed for the CoFE temperature in Figure 11 we get a change rate of the AOS temperature (T˙ ) ¼ 0:2 hr1 . Since a temperature change of 1 C causes a shift of the comb lines by 1 pixel equivalent to 1 MHz, the frequency uncertainty introduced in the AOS through drifts during a 6 minute sampling interval as used in the SWAS pipeline (see x 2.5) is fAOSmax ¼ 0:01 MHz. 4. The position and the velocity of the SWAS spacecraft can be determined with high precision. The uncertainty of the spacecraft velocity is usually better than 3 ms1. Therefore, the uncertainty of the frequency calibration due to the spacecraft velocity uncertainty is fs=c 5:5 ; 103 MHz. 5. As described in x 2.5, the measured 2 s AOS scans are calibrated in the SWAS pipeline and are reduced to 6 minute co-adds. This latter reduction introduces a frequency uncertainty in the final spectra. Each 2 s scan of the 6 minute co-add has a slightly different frequency calibration since the spacecraft velocity in direction of the observed target changes for every scan. This Doppler shift is compensated for in the data reduction by frequency shifting spectra in increments of pixels to align their frequency axis. A shift by increments of pixels results in a smaller loss in spectral resolution than a shift by fractions of pixels, which increases the correlation between adjacent pixels. The spectral shifting is performed in the software such that almost equal numbers of spectra are shifted to higher and to lower frequencies. Therefore, the effective shift in the final spectrum is very small. We estimate that this shifting introduces an additional uncertainty of fdopp 0:2 MHz.
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The final uncertainty including all four contributions is F ¼ 0:22 MHz. There are a few more frequency uncertainties that have to be considered for special observing targets, e.g., comets, and uncertainty introduced by the data analysis, e.g., quality of spectral line fits. These additional sources have to be looked at separately from the baseline uncertainties described above (which affect all spectra). APPENDIX B SPECTRAL RESOLUTION AND FLUCTUATION BANDWIDTH The spectral resolution of the SWAS receiver system is primarily determined by the resolution of the AOS. The spectral resolution of the AOS was measured during subsystem tests at the University of Cologne. In order to calculate the spectral resolution, the filter function of a single AOS pixel was measured applying a very narrow (
