Subject: [ids.analysis.forum]
From: "Cerri Luca" <[email-masked]>
To: <[email-masked]>
Cc: "Mercier Flavien (contact)" <[email-masked]>, "Ferrage Pascale" <[email-masked]>
Date: Mon, 15 Feb 2010 18:30:31 +0100

Dear DORIS analysts,

I'm sorry to inform you that JASON2 DORIS files in the 2.2 format
contain 
an error in the ionospheric correction field. 
This error impacts users of the 2.2 format, and not users of RINEX 
data. 

Althought the impact of this error is negligible for POD,  
it should map into station height when solving for station coordinates. 
Laurent has kindly tested the corrected file and verified that it
actually 
reduces a significant scale offset of Jason-2 solution with respect
ITRF2005. 
This offset was highlighted in early analysis of Jason-2 Doris Data
(http://ids-doris.org/documents/report/ids_workshop_2008/IDS08_s7_Soudar
in_RINEXJason2.pdf)

We propose to start the delivery of the corrected file from Jason-2
cycle 058 
(starts on Jan 28, 2010). This would allow to begin Cryosat2 processing
using
the corrected version of the software, thus avoiding any subsequent
redelivery. 

All previous JASON2 DORIS2.2 files will be corrected and delivered to
CDDIS at the 
same time.  

We apologize for any inconvenience that this error might have caused in
your 
processing. 

A more detailed explanation of the error and its impact follows. 

Best regards

Luca

__________________________________
Luca CERRI
Centre National d'Etudes Spatiales
DCT\SB\OR
18, av Edouard Belin
31401  Toulouse Cedex 09 - FRANCE
tel.   +33 (0)5 61283549
fax    +33 (0)5 61281855
bpi    1323
email  [email-masked]

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************

Currently, the ionospheric correction field is computed from the simple 
combination of the phase measurements from the RINEX file. 

e = (Lambda1*L1 - Lambda2*L2) / (gamma - 1) 

Where Lambda1 ~ c/2GHz , Lambda2 ~c/400MHz , and gamma is the square of
the 
frequency ratio (~25). 

Actually, we compute the difference between two successive epochs and
divide 
by the count interval to obtain the contribution of the ionosphere to
the range 
rate measurement, as required in the 2.2 format. 
The quantity ( e(i+1) - e(i) ) / deltaT
is the one stored in the DORIS files, where deltaT is the count
interval.

This supposes that propagation occurs between the emitter and receiver
ionofree 
phase centers. 
This is not consistent with the center-of-mass correction field, that
refers to 
the 2 GHz phase center. 
This is also not consitent with DORIS files from all other satellites, 
whose corrections always refer to the 2 GHz phase center. 

The ionospheric correction should rather be computed from 

e = (Lambda1*L1 - Lambda2*L2) / (gamma - 1) + (d2 - d1)/(gamma - 1)

where d1 is the contribution to the propagation distance of the 2GHz
offsets 
with respect to the satellite center of mass and the ground beacon
reference point, 
and d2 is the same for the 400 MHz frequency. 
The quantity (d2 - d1)/(gamma - 1) reaches 2.7 cm at zenith as shown in
the 
attached plot. 

The current ionospheric correction being too short, those ACs that
include 
JASON2 doris2.2 files should see negative scale factor of their solution
with 
respect to ITRF2005. 
How much depends on the weight that Jason2 DORIS measurements have with 
respect to the others.