Djibouti

PW DJIA data should be deleted from 24-NOV-1998 to (end). Following comparison of DORIS residuals, using GPS velocities from CV

DM#454 DJIB corrosion problem detected. Tilted antenna. Problem occurred between 2000 and JUN-2006. Following previous detection from PW

DM#456 DJIB station reinstalled after corrosion problem on 28-JUN-2006

Geodetic displacement : N = +105 mm, E = +21 mm, V = 0 mm

JMN: problem from corrosion was ignored

DJIB is not in ITRF2005

Preliminary computations:

DJIA in ITRF2005 is

DJIA 2000.0 4583119.333 4250952.265 1266247.788 -19.80 24.83 17.65

In comparison, DJIA from DORIS solution X/VX (tf_070515a) is

DJIA 2000.0 4583119.345 4250952.250 1266247.790 -19.33 21.43 17.06

So the position and velocities are quite coherent

Using recent DORIS weekly solutions (ign07d02) expressed in ITRF2005 and fixing the velocity to the above ITRF2005 values, we obtain :

2000.0 DJIA 4583119.3457 4250952.2690 1266247.7758 -19.80 24.83 17.65

statistics DJIA XYZ 0.0016 0.0016 0.0012 297

so this is in good agreement with the original ITRF2005 coordinates that can be adopted as such (even if the formal error on the velocity is rather large)

Even reprocessing very old data (1990-1992), the agreement is still good

2000.0 DJIA 4583119.3615 4250952.2660 1266247.7659 -19.80 24.83 17.65

statistics DJIA XYZ 0.0099 0.0114 0.0078 33

Geodetic local tie from SIMB is

DJIB 39901S003 DJIA 39901S002 -13.705 8.282 18.767 0.001

So, using original ITRF2005 + local tie, we get:

DJIB 2000.0 4583133.050 4250943.968 1266229.023 -19.80 24.83 17.65

Fixing ITRF2005 and estimating DORIS position we get (tf_070515a)

2000.0 DJIB 4583133.0357 4250943.9172 1266229.0251 -19.80 24.83 17.65

statistics DJIB XYZ 0.0027 0.0028 0.0018 41

the difference is XYZ/mm -14.5 -50.8 2.1

The difference in Y probably come from the difference in the velocity between ITRF2005 and tf_070515a (based on a longer data span).

Using more recent DORIS solution, we obtain :

2000.0 DJIB 4583133.0137 4250943.9197 1266229.0331 -19.80 24.83 17.65

statistics DJIB XYZ 0.0015 0.0014 0.0008 80

So the difference in Y is still significant. As there was some question concerning the centering of the old beacon (DJIA), I suggest to use for DJIB the DORIS-derived value that seem to be more coherent from week to week.

Recommendation:

DJIA delete data from 24-NOV-1998 to (end)

DJIB delete data from (start) to 28-JUN-2006

Outside these period, use original ITRF2005 for DJIA and use DORIS-derived solution for DJIB

2000.0 DJIB 4583133.0137 4250943.9197 1266229.0331 -19.80 24.83 17.65

If the deleted period seems too long for POD (24-NOV-1998 - 28-JUN-2006), current positions and velocities for DJIA and DJIB may be used with caution.