The accuracy of solar radiation measurements, for direct (DIR)
and diffuse (DIF) radiation, depends significantly on the precision of the
operational Sun-tracking device. Thus, rigid targets for instrument
performance and operation have been specified for international monitoring
networks, e.g., the Baseline Surface Radiation Network (BSRN)
operating under the auspices of the World Climate Research Program (WCRP).
Sun-tracking devices that fulfill these accuracy requirements are available
from various instrument manufacturers; however, none of the commercially
available systems comprise an automatic accuracy control system allowing
platform operators to independently validate the pointing accuracy of
Sun-tracking sensors during operation. Here we present KSO-STREAMS
(KSO-SunTRackEr Accuracy Monitoring System),
a fully automated, system-independent,
and cost-effective system for evaluating the pointing accuracy
of Sun-tracking devices. We detail the monitoring system setup, its design
and specifications, and the results from its application to the Sun-tracking
system operated at the Kanzelhöhe
Observatory (KSO) Austrian radiation monitoring network (ARAD) site.
The results from an evaluation campaign from March to June
2015 show that the tracking accuracy of the device operated at KSO lies within BSRN specifications
(i.e., 0.1
A precise knowledge of the surface energy budget, which comprises the solar and terrestrial radiation fluxes, is essential for understanding Earth's climate system (e.g., Wild et al., 2015). The surface radiation budget itself is defined by the difference of the downward and upward components of short- and long wave irradiance (e.g., Augustine and Dutton, 2013). To date, ground-based measurements provide the most reliable information on short- and long wave irradiance. They are routinely utilized for retrieval optimization, the evaluation of satellite radiation products (Pinker et al., 2005; Gupta et al., 2004; Wang et al., 2014; Yan et al., 2011), and the evaluation and parameterization of radiative fluxes in global and regional climate models (e.g., Wild et al., 1998; Marty et al., 2003; Donner et al., 2011; Freidenreich and Ramaswamy, 2011) and reanalysis products (e.g., Allan, 2000).
Driven by the increasing need for high-accuracy surface radiation data for scientific and technical applications, e.g., to enhance the performance of solar photovoltaic plants (e.g., Fontani et al., 2011), national and international radiation monitoring networks have been established over recent decades. The most prominent international radiation monitoring network is the so-called Baseline Surface Radiation Network (BSRN) operating under the auspices of the World Climate Research Program (WCRP), e.g., Ohmura et al., (1998). BSRN sites are equipped with instruments of the highest accuracy. Targets for instrument performance and operation are specified in the BSRN guidelines (McArthur, 2005). Furthermore, BSRN guidelines are (closely) adopted by national radiation monitoring networks, e.g., ARAD (Austrian radiation monitoring network) in Austria (Olefs et al., 2016), SACRaM in Switzerland (Wacker et al., 2011), or SURFRAD in the US (Augustine et al., 2005).
BSRN guidelines require the operation of radiation sensors on Sun-tracking devices with specified accuracy, available, in various designs, from different instrument manufacturers. BSRN guidelines recommend the use of (i) single-axis synchronous-motor tracking devices, (ii) dual-axis passive-tracking (algorithm-controlled) devices, or (iii) dual-axis active-tracking (quadrant-sensor-controlled) devices. For a detailed overview about advantages and disadvantages of these tracking devices, we refer the interested reader to Sect. 4 in McArthur (2005).
Among the suite of solar radiation measurements, the accuracy of
pyrheliometer (direct solar radiation, DIR) and pyranometer measurements
(diffuse solar radiation, DIF) depends significantly on the accuracy of the
operational Sun-tracking device. Precise alignment is of the highest priority
for the monitoring of DIF, as small misalignments can significantly
affect the measurement accuracy. For measurements of DIF, one strives to
solely shade the pyranometer's glass dome to mask as little of the diffuse
component as possible while simultaneously shielding direct solar
irradiance. As the BSRN network strives to achieve measurements at the highest
possible accuracy, its guidelines recommend using a Sun-tracking device
with an accuracy of
Sun-tracking devices fulfilling these BSRN recommendations are available from various instrument manufacturers. Nevertheless, none of the commercially available platforms comprise an automatic accuracy control system allowing platform operators to check whether the operational pointing accuracy of the Sun-tracking device indeed fulfills BSRN targets. The lack of a pointing accuracy control system is not unique to Sun-trackers used to accommodate solar radiation sensors. In fact, the determination of pointing accuracy is a common challenge for most types of Sun-pointing instruments, and various approaches have been presented to address this issue. For example, innovative camera-based (e.g., Gisi et al., 2011) and camera-free (e.g., Reichert et al., 2015) approaches to monitor tracking accuracy have been presented in the field of solar Fourier transform infrared (FTIR) spectrometry. While Gisi et al. (2011) documents a camera setup with real-time image-evaluation and tracking software (CamTrack), Reichert et al. (2015) presents an approach based on subsequent FTIR measurements with a different orientation of the solar rotation axis relative to the zenith direction, which allows us to obtain both mispointing components, the component in the zenith direction, and the component perpendicular to the solar rotation axis.
Here we present a related, camera-based, fully automated, system-independent, and cost-effective observing system to determine the pointing accuracy of Sun-tracking devices used in solar radiation monitoring networks which can be easily added to existing monitoring platforms, and its application to the Sun-tracking device operated at the Kanzelhöhe Observatory (KSO), Austria. We note that KSO-STREAMS (KSO-SunTRackEr Accuracy Monitoring System) is solely intended to monitor tracking accuracy and not to adjust the alignment of an operational Sun-tracking device.
Components of the KSO-STREAMS device and their characteristics.
The proposed observing system for continuous monitoring of the alignment
(i.e., pointing accuracy) of the Sun-tracking device, hereinafter referred
to as KSO-STREAMS, consists
of five key components: (i) a circular aperture, (ii) an optical filter
block, (iii) an achromatic lens (fixed focal length of 60 mm), (iv) an
adapted compact network camera with corresponding web connectivity, and (v) a fitted housing and mounting system. The observing system and a schematic
illustration of the system components are shown in Fig. 1a and b. Details
on system components are provided in Table 1. During operation
KSO-STREAMS needs to be mounted like a pyrheliometer on the
Sun-tracking device (Fig. 1c) to ensure correct imaging of the Sun's
position as identified by the tracker (computed and adjusted in the case of
a four-quadrant sensor correction). The focal length of KSO-STREAMS is
chosen to allow the registration of a misalignment of the imaged solar disk
of up to 0.5
The instrumental setup is described as follows:
Solar disk images on the sensor array (VGA resolution) of the
compact network camera of KSO-STREAMS.
Illustration of individual steps in KSO-STREAMS image processing to
derive a circular fit to solar-limb pixels:
KSO-STREAMS is operated by an automated script and takes, between sunrise and sunset, a snapshot every 15 s of the solar disk. The images taken are immediately processed as detailed below. A typical image taken by KSO-STREAMS is given in Fig. 2b. We note that high image contrast and minimal stray light, not image type (i.e., color), are important in further steps for solar-limb detection. Thus all KSO-STREAMS pictures are first converted to grayscale (see Fig. 3a) and derotated during post-processing. Derotation is necessary to convert image pixel coordinates into azimuth and zenith coordinates, as it is not possible to mount KSO-STREAMS in perfect horizontal alignment on the Sun-tracking system. To determine the amount of image rotation necessary to achieve the horizontal alignment, we follow a four-step procedure: (i) the Sun-tracking device is positioned and fixed to its local noon position 3 min before actual local noon, (ii) images are recorded in 5 s intervals while the Sun is moving across the whole image plane, (iii) the center of the solar disk is determined (see method described below), and (iv) a line is fitted through the sequence of recorded solar disk centers, which represents the true solar path. Each picture has to be derotated for the angle between this fitted line and the image border to achieve horizontal image alignment.
For each image, the solar disk center (
Next, we apply a classical least-squares circle fit (Ludwig, 1969) to the
solar-limb pixels identified to calculate the radius and the center of the
solar disk. Each of the first order solar-limb pixels identified is
characterized through coordinates in the
The best fit through the set of
The error on the circle fit has to be less than 1 pixel,
considering the observing conditions (astronomical seeing). As the solar radius
varies throughout the year uncertainty limits for the detection of the solar
limb have been defined as
Solar disk center positions (black dots) determined by KSO-STREAMS, used for the determination of the mean daily zero-point centers on 12 clear-sky days in 2015. The zero-point center of KSO-STREAMS is defined as the mean value of these 12 zero-point centers.
If both accuracy conditions are fulfilled, an image is considered valid and used for further analysis. It is obvious that turbidity and cloudiness (and here, especially broken cloud coverage in front of the Sun) complicate and compromise solar-limb detection. This is further investigated in Sect. 3.2, where we analyze Sun-tracker pointing accuracies over a wide range of cloud-cover conditions ranging from clear-sky to perpetual overcast.
To determine the average solar disk center, we utilize data from 12 days from
mid-March to mid-June 2015 (3 days each in March, April, May, and June; see
Fig. 4) with perpetual clear-sky conditions and a high (and continuous)
availability of observational data. Furthermore, we restrict the accuracy of
limb detection to less than half a pixel. We then determine the average of
the 12 individual daily-mean solar disk center positions and use this
average as the initial zero point. We note that the difference among
individual daily-mean zero positions (in both azimuthal and zenithal
directions) is small, i.e., less than 8 pixels (which corresponds to
approximately 0.03
KSO-STREAMS was installed on 12 March 2015 on the Sun-tracking device
(type SOLYS2, Kipp & Zonen) of the Kanzelhöhe Observatory ARAD
station (1540 m a.s.l.), see Fig. 1d.
The Sun-tracking device is equipped with a Sun sensor which allows the fine-tuning
of the alignment to the Sun if DIR is at least
300 W m
Relative daily sunshine duration (yellow) and valid KSO-STREAMS results (black) during the evaluation period from 14 March to 27 June 2015.
Over the 15-week evaluation period, a total of 100 939 valid observations by KSO-STREAMS were available. This corresponds to 28 % of the astronomically possible observations (360 228). The remaining observations (72 %) have been discarded due to exceedance of the accuracy requirements, detailed in Sect. 2.2. An overview of data availability (and relative sunshine duration) per day during the evaluation period is given in Fig. 5. We note that only 43.7 % of the theoretically possible sunshine duration was observed during the evaluation period because of ambient weather conditions.
KSO-STREAMS allows identifying the fraction of observations within
manufacturer-specified (i) active-tracking accuracy during periods with
direct solar radiation exceeding 300 W m
Performance of the Sun-tracking device at the Kanzelhöhe Observatory ARAD site
for days with nearly continuous clear sky (7 May 2015, left
column) and clear sky interrupted by frontal movement (22 April 2015, right
column):
In the following, we illustrate the performance of the Sun-tracking device at the Kanzelhöhe Observatory in four selected situations, illustrating (i) nearly continuous clear sky, (ii) clear sky interrupted by frontal movement, (iii) variable cloud cover, and (iv) nearly perpetual overcast conditions.
Nearly continuous monitoring of the Sun-tracking device pointing accuracy
was possible on 7 May 2015, with prevailing clear-sky conditions. Figure 6
shows the result of the zero-point distance determined by KSO-STREAMS (in
15 s intervals; panel a), and the result of direct solar radiation (panel b),
derived from ARAD (1 min averages) and the actual total output of
the Sun sensor of the Sun-tracking device (in 10 min increments) for
this day. All available Sun disk centers, according to the selected
restrictions (see Sect. 2.2), monitored on 7 May 2015 have been within the
0.1
On 22 April 2015, zero-point center distances are comparable to 7 May 2015, although clear-sky conditions were interrupted through frontal movement from around 07:15 to 08:15 UT, indicated by the abrupt decline in DIR (Fig. 6d). No evaluation of the Sun-tracking device pointing accuracy was possible during the frontal passage as thick cloud coverage affected solar-limb detection. Before and after the frontal passage, clear skies prevailed and KSO-STREAMS-monitored pointing accuracies were within BSRN targets and largely (89.2 % of the observations) within manufacturer specifications for active tracking (see Fig. 6c).
As Fig. 6, but for days with variable cloud cover (left, 12 April 2015; right, 10 May 2015).
Next, we focus on the evaluation of Sun-tracking accuracy during variable
cloud cover as well as on days with nearly continuous cloud coverage, where
active-tracking mode (manufacturer requirement is denoted by a total output
of the Sun sensor of at least 300 W m
As Fig. 6, but for days with nearly perpetual overcast conditions (left, 20 May 2015; right, 3 June 2015).
12 April and 10 May 2015 are representative for days with variable
meteorological conditions, and, therefore, large variations in cloud cover.
Periods with high (thick) cloud coverage, and limited direct radiation (Fig. 7b and d),
affect KSO-STREAMS' ability of solar-limb detection.
During times with thinner clouds, limb detection is possible, just
as under the clear-sky conditions discussed above. Pointing accuracies are
within manufacturer specifications for passive-tracking
(0.1
Similar results are found on days with prevailing overcast conditions, where
only small gaps in cloud cover occur. Figure 8 shows data on pointing
accuracy and direct radiation on 20 May and 3 June 2015, which are
representative of overcast days during the evaluation period. On both days,
the evaluation of the pointing accuracy of the Sun-tracking device was only
possible during small gaps in cloud cover. We evaluate tracking accuracy
within manufacturer targets for active tracking on these days, utilizing all
observational data where the minimum of the ARAD direct radiation
measurements (performed at a sample rate of 10 Hz) within a minute exceeds
300 W m
If the analysis is extended to the entire 3-month period, we find that
96 % of the observations (during periods where the minimum of the ARAD
direct radiation measurements within a minute exceeds 300 W m
Summary of the achieved tracking accuracy for the determined sky-cover categories.
Finally, we characterize the overall attainment of tracking accuracy within
active-tracking targets on days comprising the sets of days with (i) nearly
continuous clear sky, (ii) clear sky interrupted by frontal movement,
(iii) variable cloud cover, and (iv) nearly perpetual overcast conditions. On
days with nearly continuous clear-sky conditions and with dominating
clear-sky conditions interrupted by frontal movement, the BSRN targets are fully
met, and the active-tracking requirements are met for 76.4 % of valid
observations. Active-tracking accuracy requirements are less frequently met
during variable cloud-cover conditions (defined as days with
Precise Sun-tracking is necessary for high-accuracy measurements of direct
and diffuse solar radiation. Therefore, rigid targets for Sun-tracking
pointing accuracies are specified in national and international radiation
monitoring networks, e.g., the Baseline Surface Radiation Network,
which specifies pointing accuracy requirements within 0.1
To determine the pointing accuracy of the Sun-tracking device operated at the
Kanzelhöhe Observatory Austrian radiation monitoring network (Olefs et al., 2016)
site, observations by KSO-STREAMS, taken over a 15-week period
from March to June 2015, were analyzed. Instrument performance was
evaluated for valid KSO-STREAMS observations during four sets of ambient
meteorological conditions: (i) nearly continuous clear sky, (ii) clear sky
interrupted by frontal movement, (iii) variable cloud cover, and (iv) nearly
perpetual overcast conditions. The results show that 72.9 % of all
observations made during periods with DIR more than 300 W m
The data presented in this article are available at
The authors declare that they have no conflict of interest.
The authors thank three anonymous referees for their useful suggestions and their help in improving this article during the discussion phase. Edited by: M. Van Roozendael Reviewed by: three anonymous referees