# Presentation: 946359

## Geosat Follow-On Waveforms: 
Retracking for Hydrology Applications

John Lillibridge
NOAA Lab. for Satellite Altimetry, Silver Spring, MD, USA

Stephane Calmant & Gabriel Lellouch
IRD/LEGOS/OMP, Toulouse, France

**Geosat Follow-On Waveforms: **
**Retracking for Hydrology Applications**
- John Lillibridge
*NOAA Lab. for Satellite Altimetry, Silver Spring, MD, USA*
- Stephane Calmant & Gabriel Lellouch
*IRD/LEGOS/OMP, Toulouse, France*

## Freshwater - a Precious Resource

## GFO Spacecraft & Sensors

- Ku-band Altimeter
- Dual-ƒ Radiometer
- 4 GPS Receivers
- Doppler Beacon
- Laser Retro-reflector
- Fixed Solar Array
- Solid State Memory

## Navy GFO Ground Segment

## A Brief History of GFO

- Launch: 10-Feb-1998 from Vandenberg AFB
- 8-year nominal lifetime, just celebrated its 8th anniversary
- Initial hardware difficulties
- Onboard CPU resets during first two years
- GPS receivers failed in spite of 4-unit redundancy
- Loss of primary orbit determination system
- Loss of precision time-tagging
- Ground-based time-tag unit at NAVSOC: June, 1999
- Precision orbit determination based on SLR tracking
- CPU reset problems solved: November, 1999
- Navy acceptance: November 29, 2000
- Momentum wheel thermal sensitivity & Battery aging

## Altimetry Data Return

## GFO Waveform Data Collection

## GFO Waveforms - Lake Tanganyika

## Typical Ocean Waveform

- Range estimates in GDRs « Ocean » tracking

**Notes:**
Explain the tracker pb: original tracker

## Lagrange Interpolation of waveform bins

## Ice-1 “OCOG” Retracker

**Notes:**
The alternative trackers: 1/ ICE-1

## Ice-2 Retracker

**Notes:**
The alternative trackers: 2/ ICE-2

## GFO Mississippi River Study

## Specular Waveform Sequence over Mississippi

**Notes:**
Wf cross strike the Mississipi: they are very different from assumed by the « ocean tracker » leading to wrong restimates of the range

## Ice-1 vs. Ice-2 Retracking

**Notes:**
Results of along-pass ranges for both algo.

## GFO ranges from ICE-1 & ICE-2

*GFO ranges from **ICE-1** & **ICE-2*
*     **Along the Tanganika Lake*

## “Hooking” on a Lake Surface

## Application to monitoring river stages

- Mississipi River

## GFO Passes overflying Lake Nasser (Egypt)

**Notes:**
Zoom on the « virtual station »: all passes crossing the river

## Application to monitoring lake levels

**Notes:**
Times series (dots stands for median values) : the large # of outliers wrt to dataset requires that median is used instead of mean
Note also the relative bias: ICE-2 ranges > ICE-1 ranges

## Conclusions

- GFO ranges retracked with ICE1 & 2 algorithms are clearly valuable for the monitoring of continental water stages.
- 	- They usefully complement existing datasets between 2000 and 2005
- Availability of these waveforms provides another independent dataset for better understanding the algorithm-related biases
- With the end of the T/P mission and disappointing results of Jason-1 over continental waters, retracked GFO data are essential to complement ENVISAT (35 day only)
- To be done :
- 	- include and regularly update these data in web-accessible databases
- 	- unbias the ranges (using levelled gauges or GPS surveys)
- 	- Retracking the Geosat data would extend the series back to the late 1980’s