EUROPEAN COMMISSION

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EUROPEANCOMMISSION
GPS / Galileo Time Offset ICD Development
presented by Jörg Hahn
(ESA), E. Powers (USNO)
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Galileo and GPS Time Products

• GPS and Galileo System Time
• An internal navigation time scale.
• For GPS it is specified to be kept to within 1
  microsecond modulo 1 second of UTC (USNO) and in
  practice has not exceeded 50 ns for the past
  eight years.
• For Galileo it is specified to be kept to within
  50 ns of TAI.
• typically the internal navigation system time is
  only used as part of the navigation solution and
  is not considered a standard time product.
• UTC Time is obtained from GPS or Galileo by
  adding a integral number of leap seconds and the
  fine UTC correction information contained in the
  navigation data.
• TAI Time can be obtained from Galileo by adding
  the fine Galileo TAI corrections obtained from
  the Galileo data message. TAI time may be
  obtained from GPS by adding 19 seconds to GPS
  time and applying the fine UTC correction
  information contained in the GPS data message.

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Interoperability GPS-Galileo

• Galileo Time (GST), modulo one second, is
  steered to a prediction taken from a number of
  UTC(k) via an external timing service.
• GPS Time is steered to be within one microsecond
  of UTC(USNO), modulo whole seconds. GPS time,
  like TAI is not adjusted for leap seconds.
• Both GST and GPS-Time are real-time realizations
  of UTC(k) laboratories they reflect, modulo
  whole seconds
• If the offset between GST and GPS Time is made
  available to user, interoperability is enhanced

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GPS/Galileo Interface to UTC Time Services
UTC(k)
Time Service Provider
GST-TAI, UTC-TAI CV, TWSTFT
UTC(k)
UTC
UTC(k)
UTC(USNO)
USNO-AMC
CV, TWSTFT
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GPS/Galileo Timekeeping Function
Navigation Service

• Twofold
• Navigation Timekeeping critical for
  navigation mission, needed for orbit
  determination/ prediction and internal
  satellite clock synchronization,
• not intended for timing applications.
• Metrological Timekeeping
• not critical for navigation, but needed to
  provide TAI and UTC
• timing services (time dissemination) to
  support communication
• systems, banking, power grid management, etc

Timing Service
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Benefits of Navigation Time Scale Interoperability

• In the future navigation users could benefit from
  a combined GPS/Galileo navigation solution ( 12
  - 20 SV in view)
• This requires knowledge of GPS/Galileo system
  time difference or
• Solving for GPS/Galileo time difference in
  receiver
• requires a fifth satellite (loss of one SV in
  solution)
• Important consideration for
• Urban Canyons
• E-911
• Anti-jam and interference

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How to account for the GPS / GALILEO system time
difference

• Options to produce Galileo to GPS time offset
• Determination of Time Difference by
• The underlying GPS and Galileo system time could
  be physically measured by traditional time
  transfer techniques (Two-way, common view, etc)
  and included in the systems navigation data.
• The difference could be precisely estimated in
  near real time using combined GPS/Galileo monitor
  station receivers and included in the systems
  navigation data.
• Offset estimated in GPS-Galileo user equipment at
  the cost of one SV tracked

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Scope
The purpose of the Galileo to GPS Time Offset
(GGTO) ICD is to provide a starting point for
developing a detailed ICD that will allow precise
estimation of the GGTO and inclusion of the
offset between GPS and Galileo system time in
each systems navigation message
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Activity Objectives

• 1 Produce a Draft GPS/Galileo Time Offset ICD
  (GGTO-ICD) including options for Time Offset
  computation, coordination and broadcast through
  Galileo and GPS.
• 2 This draft document was coordinated between
  GPS and Galileo representative, and consolidated
  into a first version March 2003.
• 3 Incorporate GGTO into future GPS and Galileo
  navigation message structure (Draft GPS ICD-200D
  February 2004, Galileo SIS ICD to come).

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GGTO Document History

• Drafting started beginning of December 2002
• Draft document circulated to the European experts
• Comments received in early January 2003, and
  considered in the document
• Draft document circulated to US TWG
• The TWG GGTO preparatory meeting was held at ESA
  and the Draft 1.5 produced which included US
  comments
• Draft GGTO was accepted as an official working
  paper at January 2003 EWG meeting in Paris
• ICD concept briefed as part of GPS ICE Industry
  day (Feb 2003).
• GGTO version 1.0 signed (March 2003) at TWG
  hosted by USNO in Washington DC
• GGTO incorporated into Draft ICD-200D (February
  2004)
• GGTO will be included into future Galileo SIS ICD

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Draft GPS ICD-200DMessage Type 33 (Galileo to
GPS time offset)