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HEO and Daylight Ranging “Reality and Wishes”

Ramesh Govind

ILRS Fall Workshop, 4th October 2005

HEO and Daylight Ranging “Reality and Wishes”

• Reality – SLR is an expensive game – have to get the best out

of it – little room to compromise– Must be seen to provide value to the community --

with what we have:• Network of 40 stations• Geodetic Targets (including GPS, GLONASS, Etalon)• Analysis capability• Must be able to defend and improve (not deteriorate)

the existing infrastructure with its shortcomings (eg poor geometry)

HEOs and Daylight Ranging “Reality and Wishes”

• Some Fundamental Questions:– Why observe HEOs – Etalon, GPS, GLONASS and in

daylight?– What is the contribution of HEOs to the fundamental

ILRS analysis and science products?– Is this requirement fulfilled i.t.o. fundamental

geodetic and unique ILRS products?– Is the HEO data requirements being met to produce a

state-of-the-art analysis product?

HEOs and Daylight Ranging “Reality and Wishes”

–What are the gaps in terms of measurements (Network Geometry, station performance) and analysis techniques that limit HEO contribution?

–What needs to be done to fulfil these requirements for ILRS products?

– Are there competing priorities that need to be addressed – particularly tracking commitments?

HEOs and Daylight Ranging“Reality and Wishes”

• The Context (Cue):– Towards 1mm Accuracy– Absence of Science Strategy– GGOS Initiative– External Challenges– Opportunities -- Strategic Perspective

Reality and Wishes“Towards 1mm Accuracy”

• 1mm accuracy of the final ILRS product– geocentre, scale, EOP, station coordinates at the

1mm level –

• Measurement error (range, time, refraction, bias, com) + Orbit error (static and dynamic geopotential, SRP, etc)

+ Reference frame error (coordinates, loading, EOP,

Precession, Nutation, etc)

= 1 mm

Reality and WishesILRS Science Strategy & GGOS

• ILRS Science Strategy to fit the GGOS vision:– “A system which integrates different geodetic

techniques, different models, and different approaches … in order to achieve a better understanding of geodynamic and global change processes … as a basis for Earth science research” – Herman Drewes (2005)

• The need for SLR in this vision could not be over-emphasised – dependency on SLR

Reality and WishesGGOS Initiative

• GGOS – Modern Approach to Space Geodesy – • Scientific Requirements to be met for efficient integration

(GGOS):• The need for Consistent (including SLR)

– Observations– Reference Frames– Constants, conventions, models– Processing Methodologies and Algorithms– Parameter sets– Products– Applications

• Sets the framework for the operational standards for the ILRS components (stations, analysis)

HEOs and Daylight Ranging The External Challenge &

Opportunities• Improve the definition of the TRF by a

magnitude

• Provides a relevance for the SLR technique – dependency for TRF definition

Reality and WishesILRS Opportunities

• Strategic Perspective to exploit all available geodetic targets

• State of the art science products from consistent data and analysis

Comparision of Analysis Techniques

• GPS– 150 stations– 26 satellites– 1000’s observations

• Estimate (24-hr arc)– Stations (450)– EOP (6 semi-diurnal)– Receiver & satellite clocks– State vector (6 per sat)– SRP (1 per day)– Y-Bias (1 per day)– Bias/Ambiguity/Cycle slips (2000)– Troposphere (24 per site)

• TOTAL 7000 parameters

• SLR– 40/<20 0bserving stations– 2 GPS + 3 Glonass satellites

tracked (+ 2 Etalon)– few NP

• Estimate (7-day arc)– Stations– EOP – State Vector– SRP– Y Bias– Measurement Biases ??– Troposphere ??

• TOTAL 125 parameters/sat/arc

Data Analysis Procedure

• GPS Measurement Error Sources– Station and satellite antenna phase centre

variations– Residual Ionosphere –Multipath

Data Analysis Procedure

• SLR Issues (observations)– Not enough observations – Network Geometry -- Not enough stations

observations– Almost all daylight passes are to LEOs – not

contributing to the geodetic products–Minimum impact on the current quality of the

solutions – not improving the Lageos only solutions

Data Analysis Procedure SLR Issues (observations)

• HEO Tracking Requirements– Require daylight tracking for:• Continuous tracking (GPS strength)–Consecutive passes–Consecutive days–Substantial increase in #NP data for mm level

solutions–CONSISTENT data volume and quality from the

network

Data Analysis Procedure SLR Issues (analysis)

• Measurement biases– Not enough data to estimate any measurement biases– Biases – station dependent– If biases applied – which one (different per satellite)– Downweight station related measurements by a

consistent/agreed set of weights – applied to all satellites

– Tuesday problem – measure of the effectiveness of the QC reports

Challenges

• Significantly improve the tracking output of the HEOs – consecutive passes/days – Consistent data volume and quality

• Rationalise Tracking priorities on basis of fundamental SLR outputs – address competing tracking priorities if they exist

• (Initially) Produce GPS (Glonass + Etalon + Lageos) ILRS products at the highest quality – at least matching the IGS for the common products

Challenges

• Exploiting current GPS, GLONASS and future Galileo satellites (analogous to microwave observations – for best definition of the TRF

• Can we do the job and fulfil the GGOS requirements – mm accuracy products from HEO observations

HEO and Daylight Ranging “Reality and Wishes”

• Wishes–Meet the 1mm Accuracy for ILRS products–Meet the science challenges presented by the

GGOS Initiative– External Challenges – Improvement of the

TRF definition by a magnitude– See these challenges met in the short term

HEO and Daylight Ranging “Reality and Wishes”