Data Screening and Normal Point Formation

Amendment to the Herstmonceux Normal Point Procedure
for High Repetition Rate SLR Systems
May 2012

Normal Point Task Force: George Kirchner, Mike Pearlman, Peter Dunn,
Graham Appleby, and Jan McGarry

The original Herstmonceux normal point definition specifies a standard normal point interval (SNPI) for each satellite based on altitude. Normal points start and stop at prescribed times and the normal point epoch is taken as the epoch of the central point in the normal point full rate population. This definition was adopted based on our then current firing rates of 5 – 10 Hz. This amendment does not change the original SNPI and starting times (i.e. LAGEOS original normal points still have the SNPI of 2 minutes, starting at 00:00 UTC.).

Newer systems are now operating from 100 Hz to kHz rates and achieve sufficient full-rate data to accurately define normal points in time intervals far less than the SNPI.

This amendment is intended to relax some of the normal point restrictions to allow the high repetition rate SLR systems to complete normal points in less than the SNPI and then move on to another satellite, thereby increasing data pass yield through pass interleaving. It does not permit stations to start a consecutive normal point (for the same satellite) in a time less than the SNPI. Adoption of the procedure should allow some stations to double and in some cases triple pass data yield.

The procedure is based on the long-term requirement of 1 mm normal point precision from the Global Geodetic Observing System (GGOS 2020).

Procedure:

  1. A Normal Point is completed on Satellite 1 when either:
    • About 1000 valid FR points have been taken, or
    • NP precision (if computed and available in real-time) reaches 1 mm, or
    • The SNPI has elapsed, whichever comes first;
  2. If the Normal Point has been populated in less that the SNPI, move on to another satellite (e.g. Satellite 2);
  3. If the full SNPI has been required to populate the normal point, ranging can continue on the current satellite or another satellite;
  4. Do not return to Satellite 1 until the next SNPI has started.

Other related links: