On January 24, 2023, NOAA released Service Change Notice 22-77, describing the change of relay satellite for all SBN services (NOAAPort and NWWS) from Intelsat's Galaxy 28 to the newly launched Galaxy 31. For ground station operators, this move is non-trivial in that the location of Galaxy 28 is geostationary over 89°W, and Galaxy 31 is geostationary over 121°W.

The following guide is intended to help operators who are not familiar with repointing a satellite dish to provide the information to a professional satellite technician.

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Unidata hosts a variety of Open Source software projects on GitHub. We use the Open Source model because we believe strongly that broad participation in all aspects of Unidata's work is essential to achieving the Unidata community's goals. Developing software that focuses on community needs is one of our main objectives, and participation by community members in all aspects of the development process — from coding to testing, documenting, and commenting — is incredibly valuable.

As community participation in Unidata's Open Source efforts grows, we are facing increasingly complex situations surrounding contributions made to Unidata-hosted projects. As a result, we have decided to begin requiring that community members who wish to contribute code to Unidata projects on GitHub agree to the Unidata Contributor License Agreement (CLA).

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This document describes the lexical elements that occur in the DAP4 grammar.

Within the Relax-NG (rng) DAP4 grammar, there are markers for occurrences of primitive type such as integers, floats, or strings. The markers typically look like this when defining an attribute that can occur in the DAP4 DDX.

<attribute name="namespace"><data type="string"/></attribute>

The "<data type="string"/>" specifies the lexical class for the values that this attribute can have. In this case, the namespace attribute is defined to have a String value. Similar notation is used for values occurring as text within an xml element. The lexical specification later in this document defines the legal lexical structure for such lexical items. Specifically, it defines the format of the following lexical items.

1. Constants, namely: string, float, integer, and character.
2. Identifiers

The specification is written using the ISO/IEC 9945-2:2003 Information technology -- Portable Operating System Interface (POSIX) -- Part 2: System Interfaces. This is the extended Posix regular expression specification.

I have augmented it in the following ways.

1. Names are assigned to regular expressions using the notation

   name = regular-expression

2. Named expressions can be used in subsequent regular expressions by using the notation {name}. Such occurrences are equivalent to textually substituting the expression associated with name for the {name} occurrence: More or less like a macro.

DAP4 Lexical elements

Notes:

1. The definition of {UTF8} is deferred to the next section.

2. Comments are indicated using the "//" notation.

3. Standard xml escape formats (&xDD) are assumed to be allowed anywhere.

Basic character set definitions

CONTROLS   = [x00-x1F] // ASCII control characters
WHITESPACE = [  	f]+
HEXCHAR    = [0-9a-zA-Z]
// ASCII printable characters
ASCII      = [0-9a-zA-Z !"#$%&'()*+,-./:;<=>?@[]^_`|{}~]

Ascii characters that may appear unescaped in Identifiers
This is assumed to be basically all ASCII printable characters except the characters ' ', '.', '/', '"', ''', and '&'. Occurrences of these characters are assumed to be representable using the standard xml '&xx;' notation.

IDASCII    = [0-9a-zA-Z!#$%'()*+,-:;<=>?@[]^_`|{}~]

The numeric classes: integer and float

INTEGER    = {INT}|{UINT}|{HEXINT}
INT        = [+-][0-9]+{INTTYPE}?
UINT       = [0-9]+{INTTYPE}?
HEXINT     = {HEXSTRING}{INTTYPE}?
INTTYPE    = ([BbSsLl]|"ll"|"LL")
HEXSTRING  = (0[xX]{HEXCHAR}+)

FLOAT      = ({MANTISSA}{EXPONENT}?)|{NANINF}
EXPONENT   = ([eE][+-]?[0-9]+)
MANTISSA   = [+-]?[0-9]*.[0-9]*
NANINF     = (-?inf|nan|NaN)

The Character classes

STRING     = ([^"&]|{XMLESCAPE})*
CHARACTER  = ([^'&]|{XMLESCAPE})

Note that the character type only supports ASCII characters because it can only hold a single 8-bit byte.

The Identifier class

ID         = {IDCHAR}+
IDCHAR     = ({IDASCII}|{XMLESCAPE}|{UTF8})
XMLESCAPE  = &x{HEXCHAR}{HEXCHAR};

Note that the above lexical element classes are not disjoint. For example, the sequence of characters 1234 can be either an identifer,a float, or an integer. So the order of testing is assumed to be this.

1. INTEGER
2. FLOAT
3. ID
4. STRING

UTF-8 Character Encodings

We discuss UTF-8 character encoding in the context of this document. https://www.w3.org/2005/03/23-lex-U.

The most correct (validating) version of UTF8 character set is as follows.

UTF8 =   ([xC2-xDF][x80-xBF])     
       | (xE0[xA0-xBF][x80-xBF])     
       | ([xE1-xEC][x80-xBF][x80-xBF])     
       | (xED[x80-x9F][x80-xBF])     
       | ([xEE-xEF][x80-xBF][x80-xBF])     
       | (xF0[x90-xBF][x80-xBF][x80-xBF])     
       | ([xF1-xF3][x80-xBF][x80-xBF][x80-xBF])     
       | (xF4[x80-x8F][x80-xBF][x80-xBF])

The lines of the expression cover the UTF8 characters as follows:

1. non-overlong 2-byte
2. excluding overlongs
3. straight 3-byte
4. excluding surrogates
5. straight 3-byte
6. planes 1-3
7. planes 4-15
8. plane 16

Note that ASCII and control characters are not included.

The above reference also defines some alternative regular expressions.

The most relaxed version of UTF8 is this.

UTF8 = ([xC0-xD6].)
      |([xE0-xEF]..)
      |([xF0-xF7]...)

The partially relaxed version of UTF8 is this.

UTF8    = ([xC0-xD6][x80-xBF])        
        | ([xE0-xEF][x80-xBF][x80-xBF])        
        | ([xF0-xF7][x80-xBF][x80-xBF][x80-xBF])

We deem it acceptable to use this last relaxed expression for validating UTF-8 character strings.

[Version: 1.0]

At the end of this document are instructions for accessing and testing a formal grammar for the DAP4 DDX using the Relax-NG schema language. I constructed it initially without any reference to any other explicit or implicit grammars so I could record my ideas. I have since modified it based on examining James' implied grammar and from comments from others and from a comparison with the xsd grammar.

Differences with DAP4 xsd Grammar

I converted the xsd grammar to an equivalent relax-ng (rng) grammar.

One major difference I see is in dimension handling.

• I just used the name "dimension" rather than "shareddimension". For me, all dimensions (except anonymous ones) are shared.

• The xsd separates out scalars from arrays. I always allowed the dimensions for a variable to be optional to handle the scalar case.

• I attempted to be as consistent as possible, so I allowed any type including sequences and structures to be dimensioned. (but see previous commentary).

• The dimensions of a variable are currently specified in the rng grammar as a sequence of elements named "Dimension" contained in the "variables" element type.

Other differences:

• The Dataset element in the xsd has a couple of extra attributes. I added these.

• The xsd appears to allow attributes to themselves have attributes. This needs discussion.

• The URL basetype is in the xsd. But I do not see the justification for keeping it.

• It appears that the Dataset contains a top level declaration. I chose to treat the Dataset itself as the top-level group.

• Attribute declarations appear to have their own "namespace" attribute. Not sure why this is needed.

• I do not understand the purpose of the "NewAttribute" attribute.

• There may still be some minor differences in representing coordinate variables.

• The xsd represents attribute values thus:

  <attribute name="a"><value>...</value><value>...</value></attribute>

  I chose to use attributes in the multi-valued case because I prefer not to use elements with content unless really necessary. So I represented the above as this.

  <attribute name="a"><value value="..."/><value value="..."/></attribute>

• There is an issue of interleaving of definitions, or equivalently, what elements must occur in a fixed order.

• Where should attributes be legal? I think the rng grammar and the xsd grammar agree on this: putting them almost everywhere, but it needs discussion.
• I dropped Blobtype. I fail to see the need for this.

 

Testing the Relax-NG Grammar

You will need to copy three files:

1. dap4.rng - this is the grammar file. it uses the Relax-NG schema language This grammar file can be obtained from https://dl.dropbox.com/u/53929684/dap4.rng.

2. test.xml - this is a test file that I am growing to cover the whole grammar. This can be obtained from https://dl.dropbox.com/u/53929684/test.xml.

3. jing.jar - Jing is a validator that takes the grammar and a test file and checks that the test file conforms to the grammar. This can be obtained from https://dl.dropbox.com/u/53929684/jing.jar.

To use this jar file, do the command:

 

java -jar jing.jar dap4.rng test.xml

No output is produced if the validation succeeds, otherwise, error messages are produced.

Looking to the future, it is clear that eventually our query language, or more generically our previous discussion of URL Annotations must encompass three classes of computations.

1. Queries in the DAP2 sense,
2. Commands to control the client-side processing of requests on the server (i.e. thing like caching),
3. Server-side processing.

I want to propose a notation for everything in the URL after the "?". I think this notation has ability to represent a wide variety of features without, I hope, being too generic.

The notation is basically nested functions combined with single assignment variables. A semantically nonsensical, but grammatical example would look something like this: "?_x=f(17,g(h(12))),f2(_x,[0:3:10])".

Everything past the "?" is in the form of a comma separated list of nested function invocations. Anything that begins with an underscore is considered a local, temporary, variable, anything that does not look like a function call (i.e. that is not a name followed immediately by a left parenthesis) is assumed to be a string constant. Each function has an arbitrary number of argument expressions separated by commas.

There would be several semantic rules.

1. A variable may only be assigned to once (single assignment), but may be referenced as many times as desired after that.

2. All functions have a defined "return type", which looks like a legal DDX minus certain things like groups, enumeration declarations, and dimension declarations. In addition, a function may be defined to have a "void" return type, which means it is executed for its side-effects on the server.

3. Any expression that is not assigned to a variable and does not have a void return type will have its return value returned to the caller as part of a DATADDX.

Notes

My hypothesis is that this notation should also be able to handle most kinds of server side processing by defining and composing functions.

The standard projection+selection constraints of DAP2 can be represented using a special query() function whose argument is the standard DAP2 constraint, or alternatively, one could define a collection of nested functions to do the same thing, or alternatively, we could split the query part into two pieces separated by a semicolon. The first piece would be a constraint expression and the second piece (after the semicolon) would be in the nest function call form defined above.

An important aspect has to do with the construction of what may be referred to as a DATADDX. It defines the structure of a DDX that is the composition of the return types of the invoked functions that will return a (possibly structured) value. I need to work this out. BUT, in any case, the resulting DATADDX may have only have a loose relation to any DDX representing the raw dataset. This is because server-side computations will not have been represented in the original DDX, but only in the DATADDX.

I also hypothesize that Ferret notations

 https://.../thredds/dodsC/hfrnet/agg/6km_expr_{}{let deq1ubar=u[d=1,l=1:24@ave]}

could be represented in my proposed function notation without having to clutter up the URL format.