Dear All,

1. Moblas 4 has shown an extra ordinarily large Range Bias (RB) since Feb 23, 2012, due to a hardware change occurred that day. The problem is potentially due to a time interval counter issue or the frequency feed going to the counter.

2. We consider it prudent to quarantine the “problematic” data since Feb 23, due to the magnitude of the observed error (>7cm for Lageos). Those who want to still access the data can have it from CDDIS or EDC,

3. The above RB problem is in addition to the multi-year monotonically increasing RB (~2cm in 2011) observed at the station

4. As some of you may know, M4 has been under increased engineering scrutiny for a while. As a consequence of the larger RB problem, Engineering needs to conduct systematic study and make necessary hardware changes at the station before station can resume its routine SLR operations. During this time, we will collect data in the old and new configurations. This is estimated to take approx. 10 days (weather co-operating). Any changes to configuration will have associated flags.

5. The quarantined data flow to CDDIS will be as per the standard protocol. There are no changes to LRO operations or its data flow.

6. During the quarantine period, the operations of the station will not follow the ILRS priority of tracking. Rather, it will cover: (1) 20 degrees to 20 degrees Lageos 1&2 passes, (2) longer duration (30 minutes) Etalon passes, and (3) Ajisai passes at the highest priority and order. During the remaining time, other available satellites will be tracked at ILRS priority.

7. We are exploring ways to correct the data since Feb 23 at the 1 cm level; there is NO guarantee at this time whether this is possible or not.

8. We will give you an operational status update upon completion of the due process.

If you have any questions, please feel free to contact me.

Thanks

Tom Varghese

NASA SCNS SLR