This section deals with the pointing performance of the Herschel spacecraft. The spacecraft Attitude Control and Measurement System (ACMS) consists of several components, as depicted in Figure 2.9. The main constituents of the ACMS are the attitude control computer (ACC), gyroscopes (GYR), star trackers (STR), reaction control system (RCS), reaction wheel assembly (RWA), Sun acquisition sensors (SAS), coarse rate sensors (CRS) and attitude anomaly detectors (AAS).
In normal operation, the spacecraft attitude is commanded by means of the reaction wheel system. It comprises four 8.6 kg wheels in a skewed configuration, each with a momentum storage capacity of 30 Nms and a maximum delivered reaction torque of 0.215 Nm in either positive or negative direction.In the baseline configuration, all four whels are powered and used for actuation, providing optimum slew performance and momentum storage. Nevertheless, the ACMS is also capable of operating with only three reaction wheels powered. In the nominal configuration, the maximum slew speed is 0.00204 rad/sec, i.e. ∼ 7 arcmin/sec.
In normal science operation, the spacecraft attitude is controlled by means of two components: the star trackers (STR) and gyroscopes (GYR). The STR comprises two cold-redundant units, nominally aligned with the -X axis. The STR hardware include:
- An objective lens.
- A baffle to protect from undesired straylight from the Sun and other bright sources.
- The focal plane assembly, containing a CCD detector and a thermo-electric cooler for CCD cooling.
- The sensor electronics.
From a functional point of view, the STR can be seen as a video camera plus an image processing unit that, starting from an image of the sky, extracts the attitude information measured with respect to the J2000 inertial reference system and delivers it to the ACC. A CPU (ERC32 microprocessor) controls the CCD sensor and also carries the image processing task.
Key characteristics of the Herschel's STR are:
The ability to determine the inertial position from "lost in space".
FoV: 16.4 × 16.4 deg².
An onboard catalogue, based on Hipparcos, of some 3000-3500 bright stars.
A minimum of 3 stars, 9 is the maximum due to HW limitations.
The STR bias is the largest contributor to absolute pointing error and is pixel-dependent (some 0.8" × √2)
The STR is provided with an enhanced performance mode the so-called "interlaced mode", only applicable if there are ≥ 15 stars in FoV. The STR samples at twice the nominal frequency (4 Hz), 9 stars at a time. A low scan rate (0.2 arcsec/sec) is required.
Gyroscopes (GYR) are devices that use a rapidly spinning mass to sense and respond to changes in the inertial orientation of it spin axis. Rate/rate-integrating gyros provide high-precision measures of the the spacecraft angular rate. The Herschel's ACMS is provided with four gyroscopes mounted in a tetrahedral configuration. The four gyroscopes are hot-redundant, and each of the four can replace any of the others. The fourth gyroscope is not used for control, but serves to detect an inconsistency in the output of the other three.
The STRs provide an absolute reference, but with limited accuracy. On the other hand, GYRs are very accurate, but only on short temporal (bias drift, 0.0016 deg/hour) and spatial (variation in the scale factor should be taken into account for distances larger than 4 deg) scales. Therefore, the GYR attitude must be recalibrated using the STR information. Therefore, in normal operation the spacecraft attitude is computed by combining the STR and GYR measurements in the ACC using a linear Kalman filter. The so-called "filtered attitude" is sampled and downloaded with a frequency of 4Hz.
Herschel pointing modes are based either on stare pointings (fine pointing mode) or moving pointings at constant rate (line scan mode). Raster maps are 'grids' of stare pointings at regular spacings; in the position switching and nodding modes, the boresight switches repeatedly between two positions in the sky. Scan maps are sequences of line scans at regular spacing. Allowed angular speed ranges from 0.1 arcsec/sec to 1 arcmin/sec. In addition, the Herschel spacecraft can track moving Solar System targets at rates up to 10 arcsec/min.
In this section, formal definitions of the spacecraft pointing accuracy parameters are provided. The term 'pointing', when applied to a single axis (e.g. the telescope boresight), refers to the unambiguous definition of the orientation of this axis in a given reference frame. When characterising the pointing performance of the telescope, it is possible to provide a figure of the absolute attitude accuracy provided by the ACMS (absolute pointing error), or how accurate the 'a posteriori' knowledge of the absolute attitude (the absolute measurement error) can be, or how stable the pointing is (the relative pointing error). Furthermore, the pointing performance can be also characterised in terms of the relative accuracy of a set of attitude measurements (the spatial relative pointing error). The latter measurement is important to characterise the accuracy of the relative astrometry in a map comprising several pointings (e.g. from a raster pointing).
Herschel pointing accuracy definitions, presented below, are based on the prescriptions given in the ESA Pointing Error Handbook (ESA-NCR-502):
Absolute Pointing Error (APE): the angular separation between the desired direction and the actual instantaneous direction.
Absolute Measurement Error (AME): the angular separation between the actual and the estimated pointing direction (a posteriori knowledge).
Pointing Drift Error (PDE): the angular separation between the average pointing direction over some interval and a similar average at a later time.
Relative Pointing error (RPE) or pointing stability: the angular separation between the instantaneous pointing direction and the short-time average pointing direction at a given time period (in this case 60 sec).
Spatial Relative Pointing Error (SRPE): angular separation between the average orientation of the satellite fixed axis and a pointing reference axis, which is defined to an initial reference direction.
The main pointing error contributors within the Herschel spacecraft are:
To AME and APE:
Position-dependent bias within STR. It is also the main contributor to SRPE.
Residuals from calibration
Thermo-elastic stability of the structural path between STR and FPU
Instrument LoS calibration accuracy w.r.t. ACA frame (best for PACS)
To PDE: Thermo-elastic stability
To RPE: The main contributor is the noise in the control loop comprising STR+Gyro noise attenuated by a linear Kalman filtering.
Table 2.4 summarises the pointing performance of the Herschel spacecraft. The most outstanding non-compliance is related to the SRPE (required 1 arscec vs. predicted/measured performance 2.44/1.45 arcsec).
Table 2.4. Herschel pointing requirements (from SRS v3.2) compared with predictions and measured performance
| Baseline (arcsec) | Goals (arcsec) | |||
|---|---|---|---|---|
| Name | Requirement | Performance | Requirement | Performance |
| Predic./Measur. | Predic./Measur. | |||
| APE point | 3.7 | 2.45/1.90 | 1.5 | 1.45/1.35 |
| APE scan | 3.7 | 2.54/n.a. | 1.5 | 1.63/n.a. |
| PDE (24 hours) | 1.2 | 0.71/n.a. | n.a. | n.a. |
| RPE point (60 sec) | 0.3 | 0.24/0.19 | n.a | n.a |
| RPE Scan (60 sec) | 1.2 | 0.88/n.a. | 0.8 | 0.81/n.a. |
| AME Point | 3.10 | 2.40/n.a. | 1.20 | 1.42/n.a. |
| AME Scan | 3.10 | 2.52/n.a. | 1.20 | 1.62/n.a. |
| AME Slew | 10.00 | 2.59/n.a. | 5.00 | 1.80/n.a. |
| SRPE | 1.00 | 2.44/1.45* | 1.00 | 1.52/n.a. |
*The SRPE has been only measured for small (1 arcmin) two-point rasters.
As commented above, the STRs provide an absolute reference, but are not accurate enough on their own to satisfy the performance requirements. In particular, they are responsible for the SRPE non-compliance. GYRs only produce accurate attitude measurements in short temporal and spatial scales and their measurements should be recalibrated using the STR information. A mechanism has been devised to perform SRPE-compliant raster pointings by using exclusively the accurate gyro information. Two variants of this mechanism can be considered:
On-board gyro-propagation mode or Calibration Pointing (CP). This procedure is implemented within the ACMS software only for the basic raster mode; gyro-propagation is performed on-board. The gyro-propagated attitude estimates are provided in S/C housekeeping telemetry.
![[Warning]](../../admonitions/warning.gif)
Warning On-board gyro-propagation is disabled until further notice. On-ground attitude reconstruction by gyro-propagation. This is a ground procedure implemented within the FDS software that reconstructs the attitude estimates based on rate information provided by the gyroscopes. It is intented to improve our a posteriori knowledge of the S/C attitude. It is applicable to any mode with OFF positions (i.e. nodding, raster with off position, line scan with off position).
Warning At the time of writting this lines (April 2010), the performance of the on-ground attitude reconstruction by gryo-propagation is below the expectations (no noticeable improvements with respect to the 'standard' filtered attitude etimates) and therefore is not offered as a common-user functionality. For specific enquiries about this topic, please contact Helpdesk http://herschel.esac.esa.int/esupport/.
If gyro-propagation is to be used within an operational day (OD), the following steps must be considered:
Once per OD, an initial fixed pointing of about 60 min is made to calibrate the GYR bias. Whenever gyro-propagation is requested, this is taken into account and a slot is included within the DTCP.
Within the next science window period (i.e. the rest of the OD), gyro-propagation observations can be scheduled, provided that they respect the following conditions:
An initial 20 sec calibration in the observation gyro calibration position (GCP, a.k.a. OFF position)
600 sec between the recalibrations of the GYR
20 sec periodic recalibration at the GCP.