AN OVERVIEW ON PRISMA PAYLOAD

Marrucci Paolo(1), Dami Michele(1), Giunti Lorenzo(1), Ponticelli Beatrice(1), Fossati Enrico(1)

(1) SELEX GALILEO, Space LoB – Via A. Einstein 35, I-50013 Campi Bisenzio (FI), Italy, Email: paolo.marrucci@selexgalileo.com;michele.dami@selexgalileo.com; lorenzo.giunti@selexgalileo.com;

beatrice.ponticelli@selexgalileo.com;enrico.fossati@selexgalileo.com.

ABSTRACT PRISMA (PRecursore IperSpettrale della Missione Applicativa) Payload is an Electro-Optical instrument composed of an advanced, high spectral selective imaging spectroradiometer, optically integrated with a medium resolution panchromatic camera. The instrument is the focus of the new Earth observation mission that a consortium of Italian companies, including Selex Galileo who has the responsibility of the payload and relative L0-L1 products, is developing under contract with Italian Space Agency. Key features of the instrument are the very high requirement for Signal-to-Noise Ratio and the high quality of data that have to be provided. To meet these demanding figures, the optical system has been based on a high transmittance optical system, including a three mirror anastigmatic telescope and two prism spectrometers. To provide the required data quality for the entire mission lifetime, an accurate calibration unit (radiometric and spectral) has been included in the instrument opto-mechanical assembly. Furthermore, some proper satellite manoeuvres has been foreseen to complete the calibration process, including also the geometrical aspects, utilising external natural and man-made sources. The thermo-mechanical design of the instrument is based on innovative concepts, considering that the use of prism spectrometers implies a tight control of temperature variations to guarantee the stability of all instrument features once in orbit. The presented paper describes the concepts and the design principle of the instrument, a sketch of the internal calibration unit and the L0 and L1 data processors.

1. INTRODUCTION

PRISMA (Precursore IPerspettrale della Missione Operativa) is a remote sensing space mission in development under the Italian Space Agency (ASI) authority, devoted to derive information about land cover and agriculture landscape, pollution, quality of inland waters, status of coastal zones and Mediterranean Sea, soil moisture, carbon cycle. PRISMA program general information, including industrial team organization, is widely available in open literature [1].

The PRISMA Payload is an Electro-Optical instrument composed of an advanced, high spectral selective imaging

spectroradiometer, optically integrated with a medium resolution panchromatic camera.

PRISMA is based on the Pushbroom concept (no scanning mechanism on board), and the summary of the performances of the instrument are represented in the following table:

Table 1. Performances of the instrument.

The payload will be utilised on a sun synchronous orbit

with a Local Time Descending Node (LTDN) 10:30, at altitude of about 620 km,

Hyperspectral range covered are Visible and Near Infrared (VNIR) and Short Wave Infrared (SWIR) bands, with Panchromatic (PAN) images provided at higher resolution (5m), co-registered to the hyperspectral ones, so as to allow testing of images fusion techniques.

The instrument is composed by two units properly located on the PRISMA satellite structure: - Hyperspectral/PAN Optical Head (OH - Fig.1) - Main Electronics box (ME)

The HYP/PAN Optical Head collects the radiation by a telescope common to hyperspectral and panchromatic channels, disperses the radiation by two spectrometers, converts photons to electrons by means of appropriate detectors, amplifies the electrical signal and converts it into digital data stream. The Main Electronics box controls the instrument and handles the bit stream representing the spectral images up to the interface with the S/C transmitter.

Figure 1. Hyperspectral/PAN Optical Head layout

The instrument optical system consists of a Three

Mirrors Anastigmatic (TMA) telescope with an entrance aperture of 210mm and a field of view (FOV) of 2.77° that assures excellent optical quality with a minimum number of optical elements

The use of refractive prisms optimised for each band allows a much higher efficiency and lower polarization sensitivity than those achievable with gratings. In this way it was possible to maximize the SNR with a relatively small clear entrance pupil diameter for the telescope, with evident advantage in sizing the whole instrument. The spectral dispersion is achieved by prisms placed in parallel beams designed in order to achieve the required high optical quality, in terms of Point Spread Function (PSF), Smile and Keystone effects. Indeed, for each wavelength and for each field of view (central, axial and marginal) the spot size is well contained inside the pixel dimension and the MTF at the Nyquist frequency is better than 0.8 and 0.7 for the VNIR and SWIR respectively.

The Panchromatic channel is obtained separating the main beam coming from the TMA telescope by an in-field separation technique implemented by a second slit parallel to the spectrometer. The PAN image is obtained using a three mirrors optical relay that is diffraction limited for wavelengths greater than 400 nm. To provide the required data quality for the entire mission operational lifetime, the instrument is provided with an in-flight internal calibration unit (ICU), designed so as to allow operations of absolute and relative radiometric calibration as well geometric and spectral calibrations.

2. CALIBRATION ASPECTS The calibration campaign is performed on-ground and has the purpose to characterize the instrument by means of the Key Data Parameter set (KDP): for example the Instrument Transfer Function (ITF), Edge Response Function (ERF), Line Spread Function (LSF), Modulation Transfer Function (MTF), Field of View (FOV), Instantaneous Field of View (IFOV). KDP represent the set of parameters necessary to convert the raw data into the relevant physical units (i.e. from Digital Number to calibrated Spectral Radiance). KDP are further updated by L1 processor using the in-flight calibration acquisitions.

3. IN-FLIGHT CALIBRATION

In-flight calibration is performed by means of the ICU

with a regular repetition rate, properly matched to the normal satellite manoeuvring phases. During the in-flight calibration the temperature is continuously monitored.

Additional in-flight calibration reference sources are foreseen: Moon and Flat-Field and Dedicated targets on the Earth. For these sources, specific S/C manoeuvres are needed: this fact causes a more rarely execution respect to the ICU’s one. These sources are observed through the instrument main entrance door, giving a direct measurement of the main optical path transmittance and, allowing, simultaneously and independently, a cross-calibration of the ICU optical path transmittance.

Atmospheric features also provide an external information source to extend in-flight spectral calibration task.

Figure 1. ICU opto-mechanical accommodation and optical paths

The driving design of the ICU is the illumination of the

whole instrument pupil and FOV by the reference radiation passing through the same optical path of the Earth-Observed signal.

The ICU has the purpose to stimulate the instrument both with known spectral radiance and with filters characterized by spectral features in order to represent a reference signal. The ICU input spectral radiance for calibration is generated by the Sun (ICU_S Mode) or by Internal Sources (ICU_IS Mode), through a dedicated optical path.

The following table summarizes the main characteristics and purposes of the ICU modes.

Table 2. ICU_S and ICU_IS Purposes

4. L0-L1 DATA PROCESSING

Figure 3. IDHS Processing System Overview

PRISMA Mission Image Data Handling System (IDHS) main functions are Data acquisition, Data archiving, Data processing and User Interaction management. IDHS main block organization is shown into Fig. 3. The data processing function is devoted to generate Level 0, Level 1 and Level 2 products defined as reported in [1]. In case of L0a files marked as Special Product for Calibration/Validation, the IDHS will provide activating automatically the L1 processing. In case of L0a files marked as Not Special Product, the processing L1 and L2 will occur only in case of user demand. L1 processor can produce also the KDP file. The initial KDP file is inserted into the IDHS Archive after the on-ground calibration campaign. Any time the L1 processor runs, it can produce a new updated KDP file. The new KDP file is archived and marked with a flag as “to be validated”. The file is also forwarded to the Calibration Facility, where the Calibration Working Group can validate it. Once the KDP file has been checked, the Calibration Working Group shall return it back to the Archive with the validation flag as “validated”. L0 Processor is composed of following sub-processing:

1. unpacketizer (to process the DDF file and to sub-divide it in several L0a files. It produces also the Product Report);

2. cloud coverage percentage (to insert the CC% in the Catalogue Metadata file of each L0a EO/EOS file produced by the unpacketizer).

The Input of L1 processor is made by a set of L0a and KDP files that represent the Scene of Interest (SOI). L1 Processor is composed of following sub-processing:

1. frame builder algorithm (to transform the incoming L0a files into a list of Frames, ordered according by the UTC Time);

2. dark subtraction task ( to transforms the DN into Float Dark Subtracted values);

3. KDP updating (to update the Input KDP with the in-flight calibration acquired data: any time the L1 processor is activated, a new set of KDP is automatically computed. In case of a significant

difference respect to the input KDP file, the new KDP shall be sent to IDHS Archive with the validation flag at high: the updated KDP file shall be tagged with the UTC time that corresponds to the UTC time of the first frame of the SOI);

4. radiance generation (to process the EO and EOS frames together with the Updated KDP file, in order to convert the Dark Subtracted Values into Radiance Frames);

5. mask generation (to create the Generic Cover masks for each incoming L0a EO and EOS file: it has the purpose to classify each pixel according to Cloud, Sun-Glint, Snow, Water, Vegetation and Not Vegetation Lands);

6. end user binning (to select only those bands that the IDHS user has ordered).

5. REFERENCES

[1] C. Galeazzi, C. Ananasso, R. Loizzo - Italian Space Agency ASI - The PRISMA Mission -, 6th EARSEL 16-18 March 2009 Tel Aviv -Israel [2] M. K. Griffin , “Examples of EO-1 Hyperion Data Analysis”. Lincoln laboratory journal, volume 15, number 2, 2005 [3] J. A. Goodmann, M. J. Montes and S.L. Ustin “Applying Tafkaa for atmospheric correction of AVIRIS over coral ecosystems in the Hawai’Ian Island”, 2003 [4] G.F. Epema “Determination of Planetary Reflectance for Landsat-5 Thematic-Mapper Tapes processed by Earthnet (Italy)” ESA Journal 1990, vol 14.