CGSIC Meeting – March 11, 2004 EUROPEAN COMMISSION GPS / Galileo Time Offset ICD Development presented by Jörg Hahn (ESA), E. Powers (USNO)

CGSIC Meeting – March 11, 2004 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.

CGSIC Meeting – March 11, 2004 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

CGSIC Meeting – March 11, 2004 GPS/Galileo Interface to UTC Time Services UTC(USNO) UTC(k) USNO-AMC UTC GST-TAI, UTC-TAI CV, TWSTFT Time Service Provider

CGSIC Meeting – March 11, 2004 GPS/Galileo Timekeeping Function 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… Navigation Service Timing Service

CGSIC Meeting – March 11, 2004 Benefits of Navigation Time Scale Interoperability  In the future navigation users could benefit from a combined GPS/Galileo navigation solution (~ 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  In the future navigation users could benefit from a combined GPS/Galileo navigation solution (~ 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

CGSIC Meeting – March 11, 2004 How to account for the GPS / GALILEO system time difference Options to produce Galileo to GPS time offset: 1.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. 2.Offset estimated in GPS-Galileo user equipment at the cost of one SV tracked

CGSIC Meeting – March 11, 2004 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 system’s navigation message Scope

CGSIC Meeting – March 11, 2004  #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  #3: Incorporate GGTO into future GPS and Galileo navigation message structure (Draft GPS ICD-200D: February 2004, Galileo SIS ICD: to come). Activity Objectives

CGSIC Meeting – March 11, 2004 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

CGSIC Meeting – March 11, 2004 Draft GPS ICD-200D Message Type 33 (Galileo to GPS time offset) Table 30-IX.GPS/Galileo Time Offset Parameters ParameterNo. of Bits** Scale Factor (LSB) Effective Range*** Units A 0GGTO A 1GGTO A 2GGTO t otGGTO WN otGGTO Bias coefficient of GPS time scale relative to Galileo time scale Drift coefficient of GPS time scale relative to Galileo time scale Drift rate correction coefficient of GPS time scale relative of Galileo time scale Time data reference Time of Week Time data reference Week Number 16* 12* 6* TBD ,112Seconds sec/sec sec/sec 2 Seconds Weeks *Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; **See Figure for complete bit allocation; ***Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor.