# Presentation: 371860

## GPS / Galileo Time Offset 

- EUROPEANCOMMISSION

**GPS / Galileo Time Offset **
**ICD Development**

-                       **presented by **
**Jörg Hahn (ESA), *****E. Powers***** (USNO)**

## 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.

## Interoperability GPS-Galileo

**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

## UTC

**GPS/Galileo Interface to UTC Time Services**

- UTC(USNO)

- UTC(k)

- UTC(k)

- UTC(k)

- USNO-AMC

**UTC**

- GST-TAI, UTC-TAI
- CV, TWSTFT

- CV, TWSTFT

- Time Service
-  Provider

## GPS/Galileo Timekeeping Function

**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

## 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

## 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**

## 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**

## #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).

**Activity Objectives**

## 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

## Draft GPS ICD-200DMessage Type 33 (Galileo to GPS time offset)

| Table 30-IX.	GPS/Galileo Time Offset Parameters |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
| Parameter  |  | No. of
Bits** | Scale
Factor
(LSB) | Effective
Range*** | Units |
| A0GGTO

A1GGTO

A2GGTO

totGGTO

WNotGGTO

 | 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*

14

13 | 2-35

2-51

TBD

TBD

20 | 602,112 | Seconds

sec/sec

sec/sec2

Seconds

Weeks |
|       *	Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB;
    **	See Figure 30-10 for complete bit allocation;
  ***	Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. |  |  |  |  |  |