General Structure of Product

The structure of the products produced for delivery to the PDGS follows the requirements of [AD-12] as represented in Figure 1 below.

Figure 1: General Product Structure

Each product is composed of two physical files:

• XML Header file having extension .HDR

• Product file having extension .DBL

The XML Header file is an ASCII file containing data information that users can easily access for identifying the product without needs to look inside the product file.

It consists of:

• Fixed Header, a common header for all files in the Swarm Ground Segment

• Variable Header, including

• Main Product Header (MPH) containing general information.

• Specific Product Header (SPH) containing product specific information. The Specific Product Header will also contain references to auxiliary and/or input files relevant for the specific product.

The product file is the real product containing the processing results.

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XML Header File

The XML Header file contains information identifying the product.

The Fixed Header (hereafter called Standard Swarm Header) is the common header for all files in the Swarm Ground Segment, which means it is applied to all files flowing amongst the sub-systems composing the PDGS.

The format of the Standard Swarm Header is under ESA responsibility and is specified in [AD-12] and [AD-13].

The Variable Header (hereafter called Product Header) is the header with format and content depending on the file type and kind of product.

The next sub-paragraphs specify the content of these headers.

Level 2 Products Fixed Header (Standard Swarm Header)

The Standard Swarm Header is completely ASCII and based on XML syntax and conventions proposed in [AD-12].

Table 1: Level 2 Fixed Header

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Level 2 Products Variable Header (Product Header)

The XML Variable Header (hereafter called Product Header) for the Level 2 products is composed by:

• an XML Main Product Header (XML MPH), and
• an XML Specific Product Header (XML SPH)

Each header is completely ASCII and based on XML syntax and conventions proposed in the [AD-12].

The following sections describe the MPH and SPH for Level 2, Cat-1 Auxiliary products, and Cat-2 Auxiliary products AUX_LIT_2F, AUX_COR_2F and AUX_IGR_2F. MPH and SPH for all other CAT-2 AUX products are described in [AD-16].

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XML Main Product Header (XML MPH)

The Main Product Header (MPH) has the following format - very similar to the Level 0 and Level 1b Main Product Header ([AD-5], [AD-14]), see also [AD-13]:

Table 2: Level 2 Main Product Header (MPH)

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XML Specific Product Header (XML SPH)

The formats of the Specific Product Headers (SPHs) are described. The SPHs are separately described for Level-2 Products (Cat-1 and Cat-2) and for CAT-1 AUX products. The SPH for CAT-1 AUX products applies also to the CAT-2 AUX products AUX_IGR_2F, AUX_COR_2F and AUX_LIT_2F. The SPH's for all other CAT-2 AUX products are described in [AD-16].

Table 3: Level 2 Specific Product Header (SPH) - For L2 Cat-1 and L2 Cat-2 products

Table 4: Level 2 Specific Product Header (SPH) - For CAT-1 AUX products and for AUX_IGR_2F, AUX_COR_2F and AUX_LIT_2F

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Input Files

Input files to the Level 2 Processor (Level 1b and auxiliary products) used in the generation of the products are specified in the specific product header (XML Specific Product Header (XML SPH)).

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Product File Formats

The L2 and Auxiliary Product File consist of data sets with specific formats that have been agreed during the SCARF Technical Meeting 2 on February 22/23, 2010. Five different formats exist:

• ASCII Listing (SHC format)
• ASCII Listing (SP3 format)
• ASCII Listing (XML)
• ASCII Listing (others)
• CDF-format
• PDF-files

The following section describes the format categories in more detail. The format category for the individual products is provided in Table 5-1 – Table 5-3 in [AD-15].

ASCII Listing (SHC format)

All core and lithosphere magnetic field model products (final and intermediate) obey the SHC format. Missing data values are marked with NaN.

The Gauss coefficients g and h of spherical harmonic models are given as snapshots at different time instants. A static field (i.e. the high-degree lithospheric field) is given as one snapshot; the core field is provided as a series of snapshot models.

Each file contains one or more blocks. Each of these blocks describes a certain range of Gauss coefficients with the same description of time dependence (i.e. either static, linear time dependent, cubic splines, or order-6 splines, or others).

Each block contains a header part and a coefficient part of length K = N_max*(N_max+2) – (N_min-1)*(N_min+1) lines with the Gauss coefficients stored row-wise in natural order (g₁⁰, g₁¹, h₁¹, g₂⁰ g₂¹ h₂¹, …). The first column of this coefficient part contains spherical harmonic degree n, the second contains spherical harmonic order m. Non-negative values of the order refer to the Gauss coefficients g_n^m while negative orders indicate the Gauss coefficients h_n^m. Rows 3 to N_times+2 of this coefficient part contains the Gauss coefficients of the N_times snapshot models, provided at the time instants t (given in decimal years).

Such a series of snapshot models is sufficient to completely describe a model if its time dependence is constant, a Taylor expansion in time, or a piecewise linear varying spline (order 2 spline).

In the case of a time dependence described by splines of order larger than 2 (e.g. cubic splines, i.e. spline of order 4), additional information is needed to reconstruct the time dependence from the N_times snapshot models. Within SCARF, following format applies: providing spline_order snapshot models for each knot interval (i.e. an "oversampling" of the model), indicated by the value N_step (which is equal to spline_order-1 in this case). This is explained in Table 4-5 SHA format used in SCARF in [AD-15]. Every N_step value of time t (in decimal years) are the spline knots, and therefore N_step = spline_order - 1.. It is possible to read the whole block using space-delimited free format read instructions.

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