# Presentation: 190568

## Comparison of ILRS Station Positions (“A” Series only)

- Van Husson

## SINEX Issues

** ****Apriori positions**
**AUSLIG – old positions for:**
**    ****Zimmerwald **(x 0.176,  y 0.024, z 0.184)
**    ****San Fernando **(x 0.244, y -0.023, z 0.172)
**    ****Greenbelt ****(**x -0.004 y 0.001, z 0.042**) from 991205 **
**Computer round-off/truncation**
**	****JCET (occasionally >0.5 mm)**
**	****CSR (occasionally >0.5 mm)**
**	****AUSLIG (occasionally >0.5 mm)**
**	****NERC (occasionally > 0.5mm)**
** ****Bias Parameters**

## Any modeling errors (e.g. GM, satellite CoM, troposphere, biases, data weighting, data treatment) will be absorbed into the station coordinates, mostly height.

What can be done to mitigate these errors?

- What can be done to mitigate these errors?

**Effect of Modeling Errors**

**Modeling Error**

**Range Bias**

**Radial**
**Height**

**Baselines**
**Scale**

## Site Position Estimation

**Case I (perfect conditions)**
- No biases (range, time, scale) in any sites
- Modeling is perfect (i.e. perfect orbit, bias estimates = 0)
- Site positions perfect

**Case I conditions do not exist.**

## Site Position Estimation (con’t)

**Case II (near perfect conditions)**
- No biases, except a fixed +RB in one site **XYZ****0**
**   ****A)** Modeling is perfect, can accurately detect the RB in site **XYZ****0**
-                    Site positions are perfect, even for site **XYZ****0**
**   ****B)** Modeling is almost perfect, but slight error in RB estimate for site **XYZ****0**
-                    Site **XYZ****0 ** will have a height error (same direction of RB estimate error),
-                    other site heights will have a slight height error in the opposite direction
**   ****C)** Modeling is almost perfect, assumed ‘0’ bias for all sites including **XYZ****0**
-                    Site **XYZ****0 ** height is lower than truth by >RB (opposite
-                    direction of  ‘true’ RB), other site heights slightly higher

**Case II conditions do not exist.**

## Site Position Estimation (con’t)

**Case III (*****TRUTH*****)**
- Few mm RBs exist in best sites, TBs ***should not*** exist in any site.
- A few sites occasionally have significant TBs.
- A few sites occasionally have scale biases.
- Modeling is not perfect. Errors in:
**Bias Estimation**
**LAGEOS/Etalon Center of Mass**
**Troposphere**
**Earth Center of Mass**
**Earth Rotation**
**GM**
**Etc.**

## Issues

- Can range bias be separated from height and to what level?
- What are the limiting factors?
- Some solutions violate ‘REALITY’. Is this a problem?
- What causing the variations in these solutions?

## 1999 NP Data Volume

## 1999 Bias Problems

## ILRS Network

**ILRS Network**

## Bias Corrections

**Bias Corrections**

## Sites in Solution

## Sites missing in 032899 Solution

## Data Treatmentby site/28 days

*There are significant differences in data treatment.*

- 1. DGFI estimated pass-by-pass biases, when the *suspected *range or time bias
-        exceeded 30mm or 30 microseconds, respectively.
- 2. IAAK assumed no biases, except a constant RB for Maidanak and Komsomolsk
- 3. For the core sites, assumed no biases, for other sites RB estimated every 7 days
-     per satellite

## Mean Range Bias of CORE Sites

## Mean Heights of CORE Sites

## ASI LAGEOS RB Analysisof the CORE Sites

## 1999 Range Bias Comparison

**1999 Range Bias Comparison**

- Estimates are in excellent agreement. Are these biases ‘real’?

- Longitude

## Greenbelt (7105) Time Bias Analysis

**Greenbelt (7105) Time Bias Analysis**

- DGFI estimates diverge
- on day 122 from ‘reality’. Day
- 122 marks the beginning of
- their next 7-day Arc

## DGFI TB Estimation

**DGFI TB Estimation**

- DGFI estimated time biases for perfectly
- good data from 6 other sites, near the
- beginning of its new LAGEOS-2 7-day Arc

**LAGEOS-1**

**LAGEOS-2**

## Influence of Modeling Error

**Influence of Modeling Error**

- DGFI’s Greenbelt Time Series

**jump in height &**
**baseline lengths**

## Dangers of Bias Estimation

- The DGFI LAGEOS-2 orbit was skewed (i.e. had an along-track error) due to the timing problems in Greenbelt.
- This along-track error was absorbed in the latitude and longitude (not height) of any sites tracking LAGEOS-2 on May 2 or May 3, 1999.
- The Greenbelt frequency bias went into Greenbelts’s height.
- Coordinate transformations solutions may **MASK** this error.

## Conservative ‘Reality’ Checks(for the ‘BEST’ Performers)

- Range Bias Estimation
- variations between LAGEOS-1 & –2  > 5mm
- Baseline Length
- variations > 20mm
- Height
- variations > 20mm

## ASI Analysis

**ASI Analysis**

**OMC(mm)**

- Monument Peak

- McDonald

- Greenbelt

- Herstmonceux

- Grasse

- Graz

- Yarragadee

- Mt. Stromlo

**OMC(mm)**

**OMC(mm)**

## CRL Analysis

**CRL Analysis**

**OMC(mm)**

- Monument Peak

- McDonald

- Greenbelt

- Herstmonceux

- Grasse

- Graz

- Yarragadee

- Mt. Stromlo

**OMC(mm)**

**OMC(mm)**

## CSR Analysis

**OMC(mm)**

- Monument Peak

- McDonald

- Greenbelt

- Herstmonceux

- Grasse

- Graz

- Yarragadee

- Mt. Stromlo

**OMC(mm)**

**OMC(mm)**

## CSR European Heights (2.5 year time series)

## DGFI Analysis

**OMC(mm)**

- Monument Peak

- McDonald

- Greenbelt

- Herstmonceux

- Grasse

- Graz

- Yarragadee

- Mt. Stromlo

**OMC(mm)**

**OMC(mm)**

## IAAK Analysis

**OMC(mm)**

- Monument Peak

- McDonald

- Greenbelt

- Herstmonceux

- Grasse

- Graz

- Yarragadee

- Mt. Stromlo

**OMC(mm)**

**OMC(mm)**

## NERC Analysis

**OMC(mm)**

- Monument Peak

- McDonald

- Greenbelt

- Herstmonceux

- Grasse

- Graz

- Yarragadee

- Mt. Stromlo

**OMC(mm)**

**OMC(mm)**

## Slide 31

## Slide 32

## Slide 33

## Slide 34

## Slide 35

## Slide 36

## Slide 37

## Slide 38

## Slide 39

## JCET vs CSR EUROLAS Heights

## Slide 41

## Slide 42

## Slide 43

## JCET vs DGFI EUROLAS Heights

## Estimated Parameters

## Conclusions/Questions

- Range bias analysis, by itself, tells us a lot about the quality of the solutions.
- Data treament is a major source of variation between analysis center solutions.
- To what level can range bias be separated from height variations?
- Should solutions pass ‘REALITY’ checks prior to be combined?

## Recommendations

- Standardize bias treatment
- For sites with stable bias either estimate a common LAGEOS-1, -2 range bias, per site, per 28-day or estimate NO BIAS
- For sites with instability and/or frequency errors and/or low data quantity, edit the data
- Develop and implement the BEST models
- LAGEOS CoM
- Tropospheric Models
- GM
- Implement data integrity checks prior to combinations
- Eccentricities
- Range bias
- Baseline lengths
- Site heights
- Implement bias corrections
- Each center should QC their own solution

##  More Accurate Site Positions

_**Improved Modeling**_
-  Satellite CoM
-  Troposphere
-  GM
-  Pressure Loading
-  Earth CoG

_**Improved Data Treatment**_
-  One RB/site/28-days
-  Edit Problem Data
-  Fix Known Bias Problems

_**Data Integrity Checks**_
-  Eccentricities
-  Range Biases
-  Baselines
-  Heights

** ****Less Variation between Solutions and**
** ****More Accurate Site Positions**