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GEOPRIV                                                       M. Thomson
Internet-Draft                                        Andrew Corporation
Intended status: Experimental                              June 29, 2011
Expires: December 31, 2011


Global Navigation Satellite System (GNSS) Reference Information Protocol
        (GRIP) - Global Positioning System (GPS) Assistance Data
                 draft-thomson-geopriv-grip-gps-02.txt

Abstract

   This document defines assistance data formats for the Global
   Positioning System (GPS).  These formats can be used with the Global
   Navigation Satellite System (GNSS) Reference Information Protocol
   (GRIP) by a GPS receiver to acquire assistance data.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 31, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions used in this document  . . . . . . . . . . . . . .  3
     2.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  3
     2.2.  Angular Measures . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Polynomial Expressions . . . . . . . . . . . . . . . . . .  4
     2.4.  Expressing Reference Time  . . . . . . . . . . . . . . . .  5
   3.  GPS Assistance Data Types  . . . . . . . . . . . . . . . . . .  6
     3.1.  UTC Model  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Navigation Model . . . . . . . . . . . . . . . . . . . . .  7
       3.2.1.  Navigation Model Clock Model . . . . . . . . . . . . . 10
       3.2.2.  Navigation Model Orbital Model . . . . . . . . . . . . 10
       3.2.3.  Example Navigation Model . . . . . . . . . . . . . . . 12
     3.3.  Ionosphere Model . . . . . . . . . . . . . . . . . . . . . 12
     3.4.  Acquisition Assistance . . . . . . . . . . . . . . . . . . 13
     3.5.  Differential GPS Corrections . . . . . . . . . . . . . . . 14
     3.6.  Almanac  . . . . . . . . . . . . . . . . . . . . . . . . . 14
     3.7.  Extended Navigation Model  . . . . . . . . . . . . . . . . 16
   4.  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
     6.1.  URN Sub-Namespace Registration for
           'urn:ietf:params:xml:ns:grip:gps'  . . . . . . . . . . . . 27
     6.2.  XML Schema Registration  . . . . . . . . . . . . . . . . . 27
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 28
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 28






















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1.  Introduction

   The Global Positioning System (GPS) is a navigation system that
   provides the means to determine the location of a receiver in space
   and time with high accuracy.  With a large number of satellites, it
   is the most widely used Global Navigation Satellite System (GNSS).

   Server-assisted GPS provides a number of advantages including
   reducing the time required to measure satellites; reducing the time
   required to obtain the information necessary to calculate a position;
   and improving the ability of a receiver to successfully measure
   satellites in low signal conditions.

   This document defines a series of XML elements that, in combination
   with the GNSS Reference Information Protocol (GRIP) protocol
   [I-D.thomson-geopriv-grip], can be used to provide a receiver with
   assistance data.

   Readers of this document are warned that detailed knowledge of GPS is
   likely necessary to understand this document.  It is intended that
   this document be read in conjunction with the GPS Interface
   Specification [GPS-IS-200], which defines all the significant
   parameters.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The GPS Interface Specification [GPS-IS-200] describes the navigation
   message and the parameters contained therein.  This document does not
   repeat definitions and it cannot be interpreted without reference to
   the GPS Interface Specification.  Only enough information is provided
   to unambiguously identify the corresponding variable in the Interface
   Specification.

2.1.  Notational Conventions

   In naming parameters, the GPS Interface Specification frequently uses
   mathematical symbols and characters that cannot be represented in
   this document format.  In particular, Greek characters are used to
   represent many parameters.  The following conventions are used in
   this document to aid in correlating the two texts:

   o  This document uses the full English name of the character (for
      instance, the argument of periapsis is represented by the Greek
      letter "omega").



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   o  The case of English names follows the case of the Greek character.
      Uppercase Greek characters are represented in full uppercase
      (e.g., the longitude of the ascending node is represented by the
      Greek letter "OMEGA").

   o  Difference variables, which are represented as a character
      preceded by the Greek letter delta, are expressed in the same
      manner, with a hypen separating the delta from the variable
      definition (e.g., the mean motion difference from computed value
      is represented as "DELTA-n").

   o  Subscripts on symbols are represented using square brackets (e.g.,
      the reference time used in satellite clock correction is
      represented by "t[ot]").

   o  Variables that represent a time rate of change, shown in the
      Interface Specification with a small dot above the character are
      succeeded by the string "dot" using consistent case (e.g., the
      rate of right ascension is represented by "OMEGADOT").

   o  The following mathematical operators are used: add "+", subtract
      "-", multiply "*", divide "/", and power "^".  Spaces are used to
      separate these from identifiers in cases where it might otherwise
      be ambiguous.

   o  The following mathematical functions are represented by common
      abbreviations: square root "sqrt", sine "sin", and cosine "cos".

2.2.  Angular Measures

   The formats described in this document are expressed in units of
   radians, not semi-circles.  When converting, it is important to use
   the same approximation for the mathematical constant pi as is used by
   all GPS systems.  Using this approximation ensures that the
   assistance data is generated and applied using the same values.

   The fixed approximation for pi used in GPS is 3.1415926535898
   [GPS-IS-200].

2.3.  Polynomial Expressions

   The GPS navigation message is updated infrequently, but it models
   values that change over time.  Thus, the message includes values that
   model how values change over time.

   Many values in GPS assistance data are expressed with a base value
   that is set at a particularly point in time, plus a value for the
   rate of change of that value in time.  Some values are further



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   expressed with coefficients for the rate of change of the rate of
   change.  In other words, values are expressed as a polynomial
   function in terms of time.

   This extends to other values, such as those in the Ionosphere Model
   that change depending on longitude.  In the GPS navigation message,
   the Ionosphere model has four polynomial terms, allowing for a more
   complex model of ionosphere effects across different longitudes.

   This document uses a generalized model for univariate polynomial
   expressions.  These expressions are used extensively.  Each element
   that includes a polynomial expression rather than a fixed value
   includes the definition of the target value (and its units) plus the
   input variable (and its units).

   Polynomial expressions are expressed as simple lists in XML schema,
   or a space-separated sequence of numbers.  The first value in the
   list is the constant expression; the second value is the first order
   term; the nth value is the coefficient of the (n-1)th power of the
   input, giving:

      value(x) = sum from i=1..n of p[i]*x^(i-1)

   Polynomials can be any length, but since every term needs to be
   considered when the input variable is anything other than zero,
   receivers MAY limit their support to as many coefficients as are
   included in the GPS navigation message.  The mandatory number of
   supported coefficients is included in the definition of polynomials.

      For instance, the polynomial expression "4 0.5 0.06" with an input
      value of 2 produces: 4 + 0.5 * 2 + 0.06 * 2 ^ 2 = 5.24.

2.4.  Expressing Reference Time

   Where the input variable to a polynomial expression is time, a
   reference value is commonly used.  The input to the polynomial
   expression is the difference between the current (or applicable) time
   and the reference time.  The value for reference time is usually
   provided as a separate element in these cases and this element is
   identified in the definition of the polynomial expression.

   The "tow" element provides a reference time for assistance data that
   requires a reference.  This element includes the GPS time of week in
   milliseconds to use as a reference.  The input variable to time-based
   polynomials is the difference between the current time of week and
   the reference time of week.

   A GPS week has 604800000 (6.048e8) milliseconds.  Unless specified



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   explicitly, reference time is specified within half a week
   (3.024e8ms) of the time that assistance data is created.  The GPS
   Interface Specification [GPS-IS-200] describes how to accomodate week
   rollovers into calculations of time differences.

   Alternatively, an explicit GPS week number can be indicated in the
   "week" attribute of the "tow" element.  Note that GPS week rollovers
   also occur ever 1024 weeks (approximately 20 years).

3.  GPS Assistance Data Types

   This section defines assistance data types for GPS.  The following
   definitions are provided:

   o  UTC Model (Section 3.1)

   o  Navigation Model (Section 3.2)

   o  Ionosphere Model (Section 3.3)

   o  Acquisition Assistance (Section 3.4)

   o  Differential GPS Corrections (Section 3.5)

   o  Almanac (Section 3.6)

   o  Extended Navigation Model (Section 3.7)

3.1.  UTC Model

   The UTC model contains information on the relationship between
   Coordinated Universal Time (UTC) time and GPS time.  The realization
   of the UTC model used in GPS is the US Naval Observatory realization:
   UTC(USNO).  The UTC model is always global.

   tow:  The tow (Section 2.4) element includes a the reference time
      value (t[ot]).

   offset:  A polynomial in time for the time offset between GPS time
      and UTC time.  This produces a value in seconds.  Two values are
      required, which correspond to A[0] and A[1].

   leapsec:  UTC occasionally introduces a leap second.  GPS time does
      not.  The "leapsec" element without attributes indicates the
      current number of leap seconds difference between the two time
      systems.

      Additional instances of the "leapsec" element may be provided to



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      indicate when new leap seconds come into effect.  The "week" and
      "day" attributes respectively indicate the week and day that the
      included value is introduced.

   The following example shows an instance of the UTC model in XML form,
   including information on the introduction of a new leap second:

     <utc xmlns="urn:ietf:params:xml:ns:grip:gps">
       <tow week="477">436559</tow>
       <offset>0.76014e-4 -0.21722e-12</offset>
       <leapsec>14</leapsec>
       <leapsec week="2" day="3">15</leapsec>
     </utc>

3.2.  Navigation Model

   The "navigation" element contains the navigation model, information
   on the clock and position of each satellite, plus satellite status
   information.  The navigation model is specific to a satellite and
   forms the bulk of the information transmitted by a satellite.

   Navigation model data can be global or local.  Global data contains
   information for all satellites; local data contains on those
   satellites that can be seen from the input location.

   Multiple "satellite" elements are included in the navigation model,
   each containing the information for a single satellite.  If the
   information is local, only relevant satellites are included.  Each
   satellite included in the set is identified by number in the "number"
   attribute.

   The "satellite" element for navigation model has the following
   elements:

   ura:  User Range Accuracy (URA) includes an estimation of the net
      error in pseudorange measurements from this satellite, in meters.
      This value MAY be set to "INF", indicating that no accuracy
      prediction is available and that the satellite is not suitable for
      use.  INF corresponds to a value of 15 in the transmitted
      navigation message.

   health:  The health of the satellite and the signals that it
      transmits.  The bit map from the Interface Specification is
      represented in the value of this element and the value of the
      "bad" and "signals" attributes.






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      The following values for the "bad" attribute relate to all of the
      navigation data:

      none:  No data is bad (default value)

      parity:  Some or all of the parity is bad

      tlm-how:  The TLM or HOW is bad, apart from the Z-count

      z-count:  The Z-count is bad

      sf123:  Some or all data in subframes 1 through 3 is bad

      sf45:  Some or all data in subframes 4 and 5 is bad

      most:  Some or all data in any subframe is bad, excluding the TLM/
         HOW

      all:  Some or all data in any subframe is bad, including TLM/HOW

      The "signals" attribute identifies specific signals that might be
      affected:

      L1P:  The L1P signal

      L1C:  The L1C signal

      L1:  Both L1P and L1C signals

      L2P:  The L2P signal

      L2C:  The L2C signal

      L2:  Both L2P and L2C signals

      all:  All signals

      The following values for the element describe problems with the
      identified signals, or with the entire satellite:

      ok:  All signals are OK

      weak:  The identified signal is weak

      dead:  The identified signal is dead






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      nodata:  The identified signal has no data modulation

      out:  The entire satellite is temporarily out of service

      soonout:  The entire satellite will soon be out of service

      spare:  Unknown, reserved value

      combination:  Multiple errors that require a combination of
         indications

   l2codes:  A space separated list identifying which codes (C/A or P)
      are transmitted on the L2 signal.  This MAY be empty.  The "pdata"
      flag is set to "true" if the L2P signal contains data (note that
      this is indicated with a bit value of 0 in the GPS navigation
      message).

   sf1reserved:  A hexadecimal representation of the 87 reserved bits
      from subframe 1.  This value is padded with a zero bit at the
      start of the sequence.

   aodo:  The age of data offset, in seconds.  A value of "INF"
      corresponds to the maximum value from the GPS signal (27900) and
      indicates that the navigation message correction table (NMCT)
      cannot be used.

   clock:  Parameters that model the satellite clock.  These are
      described below.

   ephemeris:  Satellite orbital parameters, or ephemeris.  These are
      described below.

   The"iod" attribute contains the Issue Of Data, Clock (IODC) value
   from the navigation message.  The lower 8 bits of this 10 bit value
   indicate the Issue of Data, Ephemeris (IODE).  This is only included
   if the information is derived from the navigation message broadcast
   by the satellite.

   Servers MAY choose to omit navigation model information for
   satellites that have a URA of "INF" or bad health when providing
   local assistance data.  Satellite data SHOULD always be provided for
   global data.

   The "health", "l2codes", "sf1reserved" and "aodo" elements contain
   information that might not be relevant to some receivers.  These
   elements are optional, and MUST be omitted if the information
   provided is not directly taken from the navigation message broadcast
   by the satellite.  This ensures that when present these values can be



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   used by a receiver to compansate for the effect of navigation message
   modulation on the signal it receives.

3.2.1.  Navigation Model Clock Model

   The "clock" element of the satellite navigation model contains
   information on the clock in the satellite.

   tow:  The tow (Section 2.4) element includes a the reference time
      value (t[oc]).

   groupdelay:  The group delay differential (T[GD]), used in
      calculating time offsets in single frequency receivers.  This is a
      value in seconds.

   offset:  A polynomial expression in time, modelling the difference
      between the satellite time and GPS time.  This produces a value in
      seconds.  Three values are required, which correspond to a[f0]
      a[f1] and a[f2].

3.2.2.  Navigation Model Orbital Model

   The "ephemeris" element of the satellite navigation model contains
   information on the orbit of the satellite.

   tow:  The tow (Section 2.4) element includes the epehemeris reference
      time (t[oe]).

   semiMajor:  The size of the semi-major axis of the satellite orbit,
      in meters (A).  The navigation message includes the square root of
      this value in A^(1/2).

   eccentricity:  The eccentricity of the satellite orbit (e), which is
      dimensionless.

   longitude:  A polynomial espression in time for the longitude of the
      ascending node.  This produces a value in radians.  Two values are
      required, which correspond to OMEGA[g] and OMEGADOT[g].

      Note that this deviates from the navigation message, which
      includes the longitude of the ascending node at weekly epoch
      (OMEGA[0]) and the rate of right ascension (OMEGADOT).  The values
      used are derived as follows:

       OMEGA[g] = OMEGA[0] - OMEGADOT[e] * t[oe]
       OMEGADOT[g] = OMEGADOT - OMEGADOT[e]

      Where the constant OMEGADOT[e] is 7.2921151467e-5 radians/second.



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      This polynomial expression is applied using:

       OMEGA[k] = OMEGA + OMEGADOT[g] * t[k]

   inclination:  A polynomial espression in time for the angle of
      inclination.  This produces a value in radians.  Two values are
      required, which correspond to i[0] and idot.

   periapsis:  A polynomial espression in time for the argument of
      periapsis.  This produces a value in radians.  One value is
      required, which corresponds to omega.

   anomaly:  A polynomial espression in time for the mean Keplerian
      anomaly.  This produces a value in radians.  Two values are
      required, which correspond to M[0] and n.

      The value of n is derived from DELTA-n using the following
      expression:

       n = n[0] + DELTA-n

   harmonicCorrection:  This element contains harmonic correction values
      for the argument of latitude, the orbit radius and the angle of
      inclination.  Each is expressed as a list with two terms.  The
      first term contains the amplitude of the cosine harmonic
      correction; the second term contains the amplitude of the sine
      harmonic correction.

      The following harmonic correction values are provided:

      latitude:  The amplitude of harmonic correction for the argument
         of latitude, corresponding to C[uc] and C[us].

      radius:  The amplitude of harmonic correction for the orbit
         radius, corresponding to C[rc] and C[rs].

      inclination:  The amplitude of harmonic correction for the angle
         of inclination, corresponding to C[ic] and C[is].

   The "fit4hr" indicates whether the model is fit over the standard
   period of 4 hours.  If false, the model is based on a longer time
   frame.









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3.2.3.  Example Navigation Model

   The following example shows an instance of the navigation model in
   XML form, including a single satellite only:

     <navigation xmlns="urn:ietf:params:xml:ns:grip:gps">
       <satellite number="12" iod="75">
         <ura>4.85</ura>
         <health bad="none" signals="all">ok</health>
         <l2codes pdata="true">p</l2codes>
         <sf1reserved>180f7e3aaa2a7062c8d3a2</sf1reserved>
         <aodo>7200</aodo>
         <clock>
           <tow week="477">436559</tow>
           <groupdelay>3.949e-9</groupdelay>
           <offset>8.034e-9 -2.209e-10 9.17e-14</offset>
         </clock>
         <ephemeris fit4hr="true">
           <tow week="477">436559</tow>
           <semiMajor>6.15861e5</semiMajor>
           <eccentricity>0.98519</eccentricity>
           <longitude>2.10554 -4.674e-9</longitude>
           <inclination>0.02469827347 -1.265e-9</inclination>
           <periapsis>0.978932</periapsis>
           <anomaly>0.122742 0.366697e-8</anomaly>
           <harmonicCorrection>
             <latitude>-0.6958e-4 -0.7828e-4</latitude>
             <radius>0.7604e-4 0.3125e-4</radius>
             <inclination>0.437e-4 0.6893e-4</inclination>
           </harmonicCorrection>
         </ephemeris>
       </satellite>
     </navigation>

3.3.  Ionosphere Model

   The "ionosphere" contains a model of the signal transmission delays
   introduced by the Earths ionosphere.  Alternative models are
   available, but this element uses the Klobuchar model that is
   broadcast in the satellite navigation message.  The ionosphere model
   is always global.

   vdelay:  A polynomial expression in latitude for the vertical delay
      imposed by the ionosphere.  This produces a value in seconds.
      Four values are required, which correspond to alpha[0] through
      alpha[3].





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   period:  A polynomial expression in latitude for the period of the
      ionosphere model.  This produces a value in seconds.  Four values
      are required, which correspond to beta[0] through beta[3].

   The following example shows an instance of the ionosphere model in
   XML form:

     <ionosphere xmlns="urn:ietf:params:xml:ns:grip:gps">
       <vdelay>1.23e-7 1.1819e-6 1.167e-5 1.16e-6</vdelay>
       <period>7.445e4 3.188e6 1.34e7 1.188e7</period>
     </ionosphere>

3.4.  Acquisition Assistance

   The "acqAssist" element contains an estimate of the measurement a
   receiver is expected to make at a specified location.  Acquisition
   assistance is always local.

   The "tow" element includes the reference time used in constructing
   the acquisition assistance.

   The "satellite" element for acquisition assistance has the following
   elements:

   rtow:  An estimate of satellite time as seen by the receiver at the
      reference time.  This can be used to gain an estimate of the
      range.

   codephase:  An estimate of the code phase at the receiver at the
      given time.  This value is in units of whole chips (equivalent to
      1/1023 milliseconds).  The "uncertainty" element includes an
      estimate of the error in this estimate, at 95% confidence.

   doppler:  An estimate of the frequency shift due to the Doppler
      effect at the receiver at the given time.  This value is in units
      of Hertz.  The "uncertainty" element includes an estimate of the
      error in this estimate, at 95% confidence.

   direction:  The direction of the satellite from the centroid of the
      provided location, using the directional notation from [RFC5962].
      The first value is horizontal azimuth from Northing to Easting,
      the optional second value is elevation above (or below) the plane
      of the horizontal.








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   The following example shows an instance of acquisition assistance in
   XML form, including a single satellite only:

     <acqAssist xmlns="urn:ietf:params:xml:ns:grip:gps">
       <tow week="477">436559</tow>
       <satellite number="12">
         <rtow>436480</rtow>
         <codephase uncertainty="56">147</codephase>
         <doppler uncertainty="25">1020 0.309</doppler>
         <direction>28 38.71</direction>
       </satellite>
     </acqAssist>

3.5.  Differential GPS Corrections

   The "dgps" element contains an estimate of the pseudorage measurement
   error.  Differential GPS corrections are generated by fixed
   receivers, which are able to compensate for errors that are not
   accounted for in other GPS data.  Because this information degrades
   quickly as the distance from the fixed receiver increases,
   differential GPS corrections are always local.

   The "tow" element includes the reference time used in the
   differential GPS corrections.

   The "satellite" element for differential corrections has a single
   "range" element.  This contains a polynomial expression in time for
   the range correction.  At least two coefficients are required.  The
   "uncertainty" element includes an estimate of the remaining error in
   this estimate, at 95% confidence.

   The following example shows an instance of differential GPS
   corrections in XML form, including a single satellite only:

     <dgps xmlns="urn:ietf:params:xml:ns:grip:gps">
       <tow week="477">436559</tow>
       <satellite number="12">
         <range uncertainty="0.2">0.623 0.5e-3</range>
       </satellite>
     </dgps>

3.6.  Almanac

   The "almanac" element contains long term information about satelite
   clock and orbit.  Almanac data is always global.

   In the navigation message, almanac data is a simplified, low accuracy
   version of the navigation model.  In this XML representation, the



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   data MAY include additional information in the provided fields,
   providing that the model remains usable over a period equivalent to
   that of the almanac in the navigation message (182 days).

   Almanac is of limited use when a navigation model is available.  All
   satellites transmit this information, so global information is
   available even where the region covered by the server is not global.
   Receivers use almanac to provide a rough indication of satellite
   location after a long period where the receiver does not update other
   data.

   The "satellite" element contains per-satellite almanac data.  The
   "healthy" attribute identifies whether the satellite is currently fit
   for use; a value of "false" indicates the presence of a problem.  The
   "healthy" attribute can be omitted if the satellite is healthy.

   The "satellite" element contains the following elements:

   tow:  The reference time for the almanac data.

   clockOffset:  A polynomial expression for clock offset, containing
      information identical to that in the "offset" element from
      Section 3.2.1.

   semiMajor:  The semi-major axis of the orbit.  Identical in
      definition to the same element in Section 3.2.2.

   eccentricity:  The eccentricity of the orbit.  Identical in
      definition to the same element in Section 3.2.2.

   longitude:  The longitude of the ascending node.  Similar to the same
      element in Section 3.2.2, with the exception that only one
      coefficient is required.

   inclination:  The inclination angle.  Similar to the same element in
      Section 3.2.2, with the exception that only one coefficient is
      required.

   periapsis:  The argument of periapsis.  Similar to the same element
      in Section 3.2.2, with the exception that only one coefficient is
      required.

   anomaly:  The mean Keplerian anomaly.  Similar to the same element in
      Section 3.2.2, with the exception that only one coefficient is
      required.






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   The following example shows an instance of the almanac in XML form,
   including a single satellite only:

     <almanac xmlns="urn:ietf:params:xml:ns:grip:gps">
       <satellite number="12" healthy="true">
         <tow week="477">436559</tow>
         <clockOffset>8.034e-9 -2.209e-10</clockOffset>
         <semiMajor>6.15861e5</semiMajor>
         <eccentricity>0.98519</eccentricity>
         <longitude>2.10554</longitude>
         <inclination>0.02469827347</inclination>
         <periapsis>0.978932</periapsis>
         <anomaly>0.122742</anomaly>
       </satellite>
     </almanac>

3.7.  Extended Navigation Model

   The extended navigation model allows for predictions of ephemeris
   over a longer period of time than might be allowed for with the basic
   navigation model.  For many parameters, a curve fit using a
   polynomial with a limited number of coefficients only works for a
   short time.  While data might be sufficiently accurate within the
   curve fit interval, outside this interval the data becomes
   increasingly inaccurate.

   Extended navigation model data is always global.

   The "extNavigation" element includes multiple predictions for
   satellite clock and ephemeris.  These are predictions of future
   state, based on more elaborate models that the server might maintain.
   Each prediction has a specific period of time that it is valid for.
   Thus, a receiver is able to use a new model as the previous model
   expires.

   The "extNavigation" element contains multiple "estimate" elements,
   each of which contains the a complete model, plus an indication of
   when the model is predicted to be valid.

   The "validity" element includes two GPS time of week (Section 2.4)
   elements, "start" and "end", that respectively indicate when the
   estimate becomes valid and when it becomes invalid.

   Each "satellite" element then contains a clock model (Section 3.2.1)
   and an orbit model (Section 3.2.2).

   Scheduled events might occur that cause predictions about certain
   satellites to become invalid.  The server MUST omit predictions for



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   affected satellite from the affected period onwards.

   The following example shows an instance of extended navigation model
   data in XML form, including a single estimate and a single satellite
   only:

     <extNavigation xmlns="urn:ietf:params:xml:ns:grip:gps">
       <estimate>
         <validity>
           <start week="477">872465</start>
           <end week="477">1066134</end>
         </validity>
         <satellite number="12">
           <clock>
             <tow week="477">872465</tow>
             <groupdelay>3.949e-9</groupdelay>
             <offset>8.034e-9 -2.209e-10 9.17e-14</offset>
           </clock>
           <ephemeris>
             <tow week="477">872465</tow>
             <semiMajor>6.15861e5</semiMajor>
             <eccentricity>0.98519</eccentricity>
             <longitude>2.10554 -4.674e-9</longitude>
             <inclination>0.02469827347 -1.265e-9</inclination>
             <periapsis>0.978932</periapsis>
             <anomaly>0.122742 0.366697e-8</anomaly>
             <harmonicCorrection>
               <latitude>-0.6958e-4 -0.7828e-4</latitude>
               <radius>0.7604e-4 0.3125e-4</radius>
               <inclination>0.437e-4 0.6893e-4</inclination>
             </harmonicCorrection>
           </ephemeris>
         </satellite>
       </estimate>
     </extNavigation>

4.  XML Schema

   <xs:schema
       targetNamespace="urn:ietf:params:xml:ns:grip:gps"
       xmlns:xs="http://www.w3.org/2001/XMLSchema"
       xmlns:gps="urn:ietf:params:xml:ns:grip:gps"
       elementFormDefault="qualified"
       attributeFormDefault="unqualified">

     <xs:annotation>
       <xs:appinfo
           source="urn:ietf:params:xml:schema:grip:gps">



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         Global Positioning System (GPS) Schema for GRIP
       </xs:appinfo>
       <xs:documentation source="http://www.ietf.org/rfc/rfcXXXX.txt">
   <!-- [[NOTE TO RFC-EDITOR: Please replace above URL with URL of
                              published RFC and remove this note.]] -->
         This document defines assistance data for GPS.
       </xs:documentation>
     </xs:annotation>

     <!-- Common types -->
     <xs:element name="tow" type="gps:towType"/>
     <xs:complexType name="towType">
       <xs:simpleContent>
         <xs:extension base="gps:towBaseType">
           <xs:attribute name="week" type="gps:weekType"/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
     <xs:simpleType name="towBaseType">
       <xs:restriction base="xs:nonNegativeInteger">
         <xs:maxExclusive value="604800000"/> <!-- 7*24*60*60*1000 -->
       </xs:restriction>
     </xs:simpleType>
     <xs:simpleType name="weekType">
       <xs:restriction base="xs:nonNegativeInteger">
         <xs:maxExclusive value="1024"/> <!-- 10 bits -->
       </xs:restriction>
     </xs:simpleType>
     <xs:simpleType name="dayType">
       <xs:restriction base="xs:positiveInteger">
         <xs:maxInclusive value="7"/> <!-- starts at 1 -->
       </xs:restriction>
     </xs:simpleType>

     <xs:simpleType name="nonNegativeDouble">
       <xs:restriction base="xs:double">
         <xs:minInclusive value="0.0"/>
       </xs:restriction>
     </xs:simpleType>

     <xs:simpleType name="polyType">
       <xs:restriction base="gps:polyBaseType">
         <xs:minLength value="1"/>
       </xs:restriction>
     </xs:simpleType>
     <xs:simpleType name="polyBaseType">
       <xs:list itemType="xs:double"/>
     </xs:simpleType>



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     <xs:complexType name="polyUncType">
       <xs:simpleContent>
         <xs:extension base="gps:polyType">
           <xs:attribute name="uncertainty"
                         type="gps:nonNegativeDouble"/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>

     <xs:complexType name="perSatelliteBaseType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element name="satellite" type="gps:satelliteBaseType"
                         minOccurs="0" maxOccurs="unbounded"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>
     <xs:complexType name="perSatelliteTowType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element ref="gps:tow"/>
             <xs:element name="satellite" type="gps:satelliteBaseType"
                         minOccurs="0" maxOccurs="unbounded"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>
     <xs:complexType name="satelliteBaseType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:any namespace="##any" processContents="strict"
                     minOccurs="1" maxOccurs="unbounded"/>
           </xs:sequence>
           <xs:attribute name="number" type="xs:nonNegativeInteger"
                         use="required"/>
           <xs:anyAttribute namespace="##any" processContents="strict"/>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <!-- UTC -->
     <xs:element name="utc" type="gps:utcType"/>
     <xs:complexType name="utcType">
       <xs:complexContent>



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         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element ref="gps:tow"/>
             <xs:element name="offset" type="gps:polyType"/>
             <xs:element name="leapsec" type="gps:leapsecType"
                         minOccurs="1" maxOccurs="unbounded"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>
     <xs:complexType name="leapsecType">
       <xs:simpleContent>
         <xs:extension base="xs:integer">
           <xs:attribute name="week" type="gps:weekType"/>
           <xs:attribute name="day" type="gps:weekType"/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>

     <!-- Navigation Model -->
     <xs:element name="navigation" type="gps:navigationType"/>
     <xs:complexType name="navigationType">
       <xs:complexContent>
         <xs:restriction base="gps:perSatelliteBaseType">
           <xs:sequence>
             <xs:element name="satellite"
                         type="gps:navigationSatelliteType"
                         minOccurs="0" maxOccurs="unbounded"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:complexType name="navigationSatelliteType">
       <xs:complexContent>
         <xs:restriction base="gps:satelliteBaseType">
           <xs:sequence>
             <xs:element name="ura" type="gps:nonNegativeDouble"/>
             <xs:element name="health" type="gps:healthType"
                         minOccurs="0"/>
             <xs:element name="l2codes" type="gps:l2codesType"
                         minOccurs="0"/>
             <xs:element name="sf1reserved" type="gps:sf1reservedType"
                         minOccurs="0"/>
             <xs:element name="aodo" type="gps:nonNegativeDouble"
                         minOccurs="0"/>
             <xs:element ref="gps:clock"/>
             <xs:element ref="gps:ephemeris"/>



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           </xs:sequence>
           <xs:attribute name="iod" type="gps:iodType"/>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:complexType name="healthType">
       <xs:simpleContent>
         <xs:extension base="gps:healthBaseType">
           <xs:attribute name="bad" type="gps:healthBadType"
                         default="none"/>
           <xs:attribute name="signals" type="gps:healthSignalsType"
                         default="all"/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
     <xs:simpleType name="healthBaseType">
       <xs:restriction base="xs:token">
         <xs:enumeration value="ok"/>
         <xs:enumeration value="weak"/>
         <xs:enumeration value="dead"/>
         <xs:enumeration value="nodata"/>
         <xs:enumeration value="out"/>
         <xs:enumeration value="soonout"/>
         <xs:enumeration value="spare"/>
         <xs:enumeration value="combination"/>
       </xs:restriction>
     </xs:simpleType>
     <xs:simpleType name="healthBadType">
       <xs:restriction base="xs:token">
         <xs:enumeration value="none"/>
         <xs:enumeration value="some"/>
         <xs:enumeration value="parity"/>
         <xs:enumeration value="tlm-how"/>
         <xs:enumeration value="all"/>
       </xs:restriction>
     </xs:simpleType>
     <xs:simpleType name="healthSignalsType">
       <xs:restriction base="xs:token">
         <xs:enumeration value="L1"/>
         <xs:enumeration value="L1P"/>
         <xs:enumeration value="L1C"/>
         <xs:enumeration value="L2"/>
         <xs:enumeration value="L2P"/>
         <xs:enumeration value="L2C"/>
         <xs:enumeration value="all"/>
       </xs:restriction>
     </xs:simpleType>



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     <xs:complexType name="l2codesType">
       <xs:simpleContent>
         <xs:extension base="gps:l2codesBaseType">
           <xs:attribute name="pdata" type="xs:boolean" use="required"/>
         </xs:extension>
       </xs:simpleContent>
     </xs:complexType>
     <xs:simpleType name="l2codesBaseType">
       <xs:list>
         <xs:simpleType>
           <xs:restriction base="xs:token">
             <xs:enumeration value="p"/>
             <xs:enumeration value="c/a"/>
           </xs:restriction>
         </xs:simpleType>
       </xs:list>
     </xs:simpleType>

     <xs:simpleType name="sf1reservedType">
       <xs:restriction base="xs:hexBinary">
         <xs:pattern value="[0-7][\da-fA-F]{21}"/> <!-- 87 bits -->
       </xs:restriction>
     </xs:simpleType>

     <xs:element name="clock" type="gps:clockType"/>
     <xs:complexType name="clockType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element ref="gps:tow"/>
             <xs:element name="groupdelay" type="xs:double"/>
             <xs:element name="offset" type="gps:polyType"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:element name="ephemeris" type="gps:ephemerisType"/>
     <xs:complexType name="ephemerisType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element ref="gps:tow"/>
             <xs:group ref="gps:keplerianOrbitBase"/>
             <xs:element name="harmonicCorrection"
                         type="gps:harmonicCorrectionType"/>
           </xs:sequence>
           <xs:attribute name="fit4hr" type="xs:boolean"/>



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         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:group name="keplerianOrbitBase">
       <xs:sequence>
         <xs:element name="semiMajor" type="gps:nonNegativeDouble"/>
         <xs:element name="eccentricity" type="gps:nonNegativeDouble"/>
         <xs:element name="longitude" type="gps:polyType"/>
         <xs:element name="inclination" type="gps:polyType"/>
         <xs:element name="periapsis" type="gps:polyType"/>
         <xs:element name="anomaly" type="gps:polyType"/>
       </xs:sequence>
     </xs:group>

     <xs:complexType name="harmonicCorrectionType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element name="latitude" type="gps:harmonicType"/>
             <xs:element name="radius" type="gps:harmonicType"/>
             <xs:element name="inclination" type="gps:harmonicType"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>
     <xs:simpleType name="harmonicType">
       <xs:restriction base="gps:polyType">
         <xs:length value="2"/> <!-- cosine sine -->
       </xs:restriction>
     </xs:simpleType>

     <xs:simpleType name="iodType">
       <xs:restriction base="xs:nonNegativeInteger">
         <xs:maxExclusive value="1024"/>
       </xs:restriction>
     </xs:simpleType>

     <!-- Almanac -->
     <xs:element name="almanac" type="gps:almanacType"/>
     <xs:complexType name="almanacType">
       <xs:complexContent>
         <xs:restriction base="gps:perSatelliteBaseType">
           <xs:sequence>
             <xs:element name="satellite"
                         type="gps:almanacSatelliteType"
                         minOccurs="0" maxOccurs="unbounded"/>
           </xs:sequence>



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         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>
     <xs:complexType name="almanacSatelliteType">
       <xs:complexContent>
         <xs:restriction base="gps:satelliteBaseType">
           <xs:sequence>
             <xs:element ref="gps:tow"/>
             <xs:element name="clockOffset" type="gps:polyType"/>
             <xs:group ref="gps:keplerianOrbitBase"/>
           </xs:sequence>
           <xs:attribute name="healthy" type="xs:boolean"
                         default="true"/>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <!-- Extended Navigation -->
     <xs:element name="extNavigation" type="gps:extNavigationType"/>
     <xs:complexType name="extNavigationType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element name="estimate" type="gps:estimateType"
                         maxOccurs="unbounded"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:complexType name="estimateType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element name="validity" type="gps:validityType"/>
             <xs:element name="satellite"
                         type="gps:extNavigationSatelliteType"
                         minOccurs="0" maxOccurs="unbounded"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:complexType name="validityType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element name="start" type="gps:towType"/>



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             <xs:element name="end" type="gps:towType"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:complexType name="extNavigationSatelliteType">
       <xs:complexContent>
         <xs:restriction base="gps:satelliteBaseType">
           <xs:sequence>
             <xs:element ref="gps:clock"/>
             <xs:element ref="gps:ephemeris"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <!-- Acquisition Assistance -->
     <xs:element name="acqAssist" type="gps:acqAssistType"/>
     <xs:complexType name="acqAssistType">
       <xs:complexContent>
         <xs:restriction base="gps:perSatelliteTowType">
           <xs:sequence>
             <xs:element ref="gps:tow"/>
             <xs:element name="satellite"
                         type="gps:acqAssistSatelliteType"
                         minOccurs="0" maxOccurs="unbounded"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:complexType name="acqAssistSatelliteType">
       <xs:complexContent>
         <xs:restriction base="gps:satelliteBaseType">
           <xs:sequence>
             <xs:element name="rtow" type="gps:towType"/>
             <xs:element name="codephase" type="gps:polyUncType"/>
             <xs:element name="doppler" type="gps:polyUncType"/>
             <xs:element name="direction" type="gps:directionType"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:simpleType name="directionType">
       <xs:restriction base="gps:polyType">
         <xs:maxLength value="2"/>



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       </xs:restriction>
     </xs:simpleType>

     <!-- DGPS Corrections -->
     <xs:element name="dgps" type="gps:dgpsType"/>
     <xs:complexType name="dgpsType">
       <xs:complexContent>
         <xs:restriction base="gps:perSatelliteTowType">
           <xs:sequence>
             <xs:element ref="gps:tow"/>
             <xs:element name="satellite" type="gps:dgpsSatelliteType"
                         minOccurs="0" maxOccurs="unbounded"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <xs:complexType name="dgpsSatelliteType">
       <xs:complexContent>
         <xs:restriction base="gps:satelliteBaseType">
           <xs:sequence>
             <xs:element name="range" type="gps:polyUncType"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

     <!-- Ionosphere Model -->
     <xs:element name="ionosphere" type="gps:ionosphereType"/>
     <xs:complexType name="ionosphereType">
       <xs:complexContent>
         <xs:restriction base="xs:anyType">
           <xs:sequence>
             <xs:element name="vdelay" type="gps:polyType"/>
             <xs:element name="period" type="gps:polyType"/>
           </xs:sequence>
         </xs:restriction>
       </xs:complexContent>
     </xs:complexType>

   </xs:schema>

5.  Security Considerations

   This document is subject to the security considerations described in
   [I-D.thomson-geopriv-grip].





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6.  IANA Considerations

   The XML namespace used for GPS assistance data is registered in
   Section 6.1, the corresponding schema definition is registered in
   Section 6.2.

6.1.  URN Sub-Namespace Registration for
      'urn:ietf:params:xml:ns:grip:gps'

   This section registers a new XML namespace,
   "urn:ietf:params:xml:ns:grip:gps", per the guidelines in [RFC3688].

      URI: urn:ietf:params:xml:ns:grip:gps

      Registrant Contact: IETF, GEOPRIV working group,
      (geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com).

      XML:

         BEGIN
           <?xml version="1.0"?>
           <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
             "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
           <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
             <head>
               <title>GPS Assistance Data</title>
             </head>
             <body>
               <h1>Namespace for GPS Assistance Data Definitions</h1>
               <h2>urn:ietf:params:xml:ns:grip:gps</h2>
       [NOTE TO IANA/RFC-EDITOR: Please replace XXXX
       with the RFC number for this specification.]
               <p>See RFCXXXX</p>
             </body>
           </html>
         END

6.2.  XML Schema Registration

   This section registers an XML schema as per the guidelines in
   [RFC3688].

   URI:  urn:ietf:params:xml:schema:grip:gps

   Registrant Contact:  IETF, GEOPRIV working group, (geopriv@ietf.org),
      Martin Thomson (martin.thomson@andrew.com).





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   Schema:  The XML for this schema can be found as the entirety of
      Section 4 of this document.

7.  Acknowledgements

   This document is part of the definition of GRIP.  The original GRIP
   protocol was developed by the University of New South Wales through
   the OSGRS project <http://osgrs.sourceforge.net/>.  The GPS expertise
   of Neil Harper was invaluable in assembling this document.

8.  References

8.1.  Normative References

   [RFC2119]                       Bradner, S., "Key words for use in
                                   RFCs to Indicate Requirement Levels",
                                   BCP 14, RFC 2119, March 1997.

   [RFC3688]                       Mealling, M., "The IETF XML
                                   Registry", BCP 81, RFC 3688,
                                   January 2004.

   [RFC5962]                       Schulzrinne, H., Singh, V.,
                                   Tschofenig, H., and M. Thomson,
                                   "Dynamic Extensions to the Presence
                                   Information Data Format Location
                                   Object (PIDF-LO)", RFC 5962,
                                   September 2010.

   [GPS-IS-200]                    GPS Navstar Joint Program Office,
                                   "Navstar GPS Space Segment /
                                   Navigation User Interfaces",
                                   Interface Specification IS-GPS-200D,
                                   December 2004.

8.2.  Informative References

   [I-D.thomson-geopriv-grip]      Thomson, M., "Global Navigation
                                   Satellite System (GNSS) Reference
                                   Information Protocol (GRIP)",
                                   draft-thomson-geopriv-grip-02 (work
                                   in progress), Jun 2011.

   [W3C.REC-xmlschema-1-20010502]  Maloney, M., Thompson, H.,
                                   Mendelsohn, N., and D. Beech, "XML
                                   Schema Part 1: Structures", World
                                   Wide Web Consortium FirstEdition REC-
                                   xmlschema-1-20010502, May 2001, <http



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Internet-Draft                  GRIP GPS                       June 2011


                                   ://www.w3.org/TR/2001/
                                   REC-xmlschema-1-20010502>.

Author's Address

   Martin Thomson
   Andrew Corporation
   Andrew Building (39)
   Wollongong University Campus
   Northfields Avenue
   Wollongong, NSW  2522
   AU

   Phone: +61 2 4221 2915
   EMail: martin.thomson@andrew.com




































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