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APEX - Airborne Prism EXperiment

APEX is a hyperspectral imaging spectrometer for use in an airplane at an altitude of 7km.

APEX spectrometer is intended as a simulator and a calibration and validation device for future spaceborne hyperspectral imagers and will be used to determine the quality of vegetation.

The APEX spectrometer consists of two spectral channels: one in the visible and one in the infrared range. It has a field of view of 28° imaged on a detector of 1000 by 400 pixels. About 300 spectral bands are recorded in the wavelength area between 380 and 2500nm, with a spectral resolution of 10nm and at a spatial ground resolution of 2m to 5m.

APEX Instrument

The APEX OSU spectrometer is a state-of-the-art hyperspectral imaging sensor for use in an airplane at an altitude of 7km and consists of two spectral channels: one in the visible wavelength range (380-970nm) and one in the short wave infrared range (940-2500nm). The spectral separation is realized by means of prisms. The total FOV is 28°, recording 1000 spatial elements across track at a maximum of 300 contiguous spectral bands simultaneously. The spectral sampling interval is designed to be better than 10 nm in the SWIR and 5 nm in the VNIR range of the spectrum. The final radiance data products will be well characterized and calibrated traceable to absolute standards. The push broom technique enables the instrument to gather spectral and spatial information at the same time.

APEX is a dual-prism dispersive pushbroom imaging spectrometer using one ground imager with a slit in its imaging plane. Light enters the spectrometer part through a curved slit and a collimator redirects the light towards a beamsplitter, which separates the VNIR wavelengths from the SWIR. The VNIR wavelengths are then dispersed from another face of the beamsplitter/prism and imaged by a CCD (Charged Coupled Device) detector after passing through a customized VNIR optics.
The SWIR wavelengths enter a further prism and are dispersed from a second surface of this prism. A focusing SWIR optic then projects the spectral components onto a CMOS (Complementary Metal Oxide Semiconductor) detector. The VNIR array detector can record up to 334 unbinned bands and SWIR 199 unbinned bands. Customized binning patterns can be applied in order to satisfy specific scientific applications.

 

Key words

Solution: Spectrometer

Type: Hyperspectral

Application field: Earth observation

Mission: Various, flight with Dornier aircraft

Life: Operational since 2010

Instrument Specifications

  • Spectral range: VNIR (380-970nm) and SWIR (940-2500nm)
  • VNIR spectral bands:  Default 114 bands. Max. unbinned bands: 334
  • SWIR spectral bands:  199 bands (unbinned)
  • VNIR Spectral Sampling Interval : 0.55 – 8 nm over spectral range (unbinned)
  • SWIR spectral Sampling Interval : 5 – 10 nm over spectral range
  • VNIR Spectral resolution (FWHM): 0.6 – 6.3 nm over spectral range (unbinned)
  • SWIR Spectral resolution (FWHM): 6.2 – 11 nm over spectral range
  • Spatial pixels: 1000
  • FOV (across track):  28°
  • IFOV: 0.48mrad
  • Spatial Sampling Interval (across track): 1.75 m @ 3500 m AGL (Above Ground Level)(2 – 5 m at flight altitudes of 4 – 10 km)
  • Sensor dynamic range: VNIR CCD, 14 bit encoding ; SWIR CMOS, 13 bit encoding
  • Pixel size:  VNIR 22.5 μm x 22.5 μm ; SWIR 30 μm x 30 μm
  • Smile: 0.35 pixels (average over FOV)
  • Frown/Keystone: 0.35 pixels (average over FOV)
  • Co-Registration: 0.6 pixels (average over FOV)
  • Data transfer Spectral frames: 30 Mbit/s via optical link
  • Housekeeping data: 20 kbit/s via SR

 

APEX system description

The APEX system consists of several sub-units. The OSU (Optical Sub-Unit) is the core element of the instrument including the sensitive optics, properly interfaced with customized FEE (Front-End Electronic) boards. The OSU is operated on a stabilized platform (STP) in order to dampen all the externally induced vibrations and ensure stable vertical measurements. The platform is controlled by the navigation system, which receives orientation information from an IMU (Inertial Measurement Unit) implemented on the OSU and position signals from a GPS receiver. The orientation and position information are then synchronized with the image data by the CSU (Control and Storage Unit). Each data frame is thus time and day tagged and stored on a hard disk array. This information is finally transferred to the PAF (Processing and Archiving Facility), either over a Gigabit Ethernet or via a storage medium.

The instrument is temperature and pressure stabilized. The OMU (Opto-Mechanical Unit, composed of OSU and FEE) is enclosed by the ETC (Environmental Thermal Control) box. The TCU (Thermal Control Unit) controls the temperature of the OMU cooling circuits and of the ETC box atmosphere. The SWIR (Short Wavelength Infrared) detector is directly linked with a dedicated cooling system that keeps its temperature at about -100°C, thus drastically reducing the thermal noise.

Mission

Imaging spectroscopy greatly extends the scope of traditional remote sensing. It is based on the detection of many narrow, contiguous spectral bands. This presents opportunities for more precise identification of surface materials than is possible with broadband multispectral sensors.

APEX has been designed to serve the needs of a broad palette of Scientific Users in the fields of simulation, calibration and validation of variables; atmospheric chemistry and physics; vegetation and ecology; geology, soils and minerals; coastal and inland waters; snow and ice; urban areas; algorithm development.

In addition, APEX was intended as a simulator and a calibration / validation device for future spaceborne hyperspectral imagers. Although the intended spaceborne sensors (PRISM, LSPIM, SPECTRA, ..) were cancelled during the APEX development, the hyperspectral imager was used to simulate and complement important Earth Observation missions such as Sentinel, PROBA-V, MEDUSA, FLEX, ..
APEX will easily team up with future spaceborne missions.

OIP’s Participation

OIP, as subcontractor to RUAG [CH], was responsible for the design, development and testing of the imaging spectrometer.

Status:

The first flight in a Dornier Do-228 Aircraft of DLR (Deutsches Luft- und Raumfahrt) took off from Oberpfaffenhofen airfield (Munich, Germany) on October 13th, 2008 and acquired 13 flight lines over Wittenverge (Germany).

APEX was intensively tested with multiple test flights over Switzerland, Germany and Belgium and with pre- and after flight CHB (Calibration Home Base) campaigns in October 2008 and in June 2009. The Swiss and Belgian target areas were selected in order to cover a variety of possible scientific applications. Flight lines have been executed over forests, crops, inland water, urban and coastal areas to derive a wide spectrum of products of interest. Vicarious calibration campaign with extensive ground truth measurements has been carried out for all flight lines.

APEX became the first airborne imaging spectrometer of ESA available to the user community from 2011 onwards. Fully operational since 2011 and still providing exquisite data.

Partners

The APEX instrument is developed by a Swiss-Belgian consortium on behalf of ESA. OIP was subcontractor to RUAG [CH].

The most important users are RSL (Remote Sensing Laboratories) [CH] and VITO (Flemish Institute for Technological Research) [B].

The project was funded by ESA/PRODEX.

 

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