Sunday, March 14, 2010

Part 6 WGS-84 Versus PE-90.02 update


As a result of questions I e-mailed to Eric Gakstatter the editor of the magazine GPS World I can update the previous posts. Here is the e-mail I sent him this afternoon.

I am a pilot instructor.  Could I trouble you for guidance on how to determine the differences in location that use of the WGS-84 ellipsoid versus PE-90 ellipsoid would result if using the the US SatNav satellites location information?
Many of the pilots who attend my training presentations are flying to Russia and the Commonwealth of Independent States.  As PE-90 is the ellipsoid used by the GLONASS satellites, what is the maximum difference in locations it the SatNav system is used while flying in Russia and the CIS?
Most of the pilots have been instructed to disable their GPS sensors while flying in countries not using the WGS-84 datum.  I think it may be time to reconsider this restriction due to the nearly identical PE-90 and WGS-84 datums as described in the paper by Misra, Abbot and Gaposchkin.
It states "The difference between the coordinates of points on earth in WGS 84 and PZ-90 has been found to be less than 15 m (average: 5 m). The two coordinate frames are brought substantially into coincidence by a small rotation (0.4") of the Z-axis of either. An additional improvement, though only slight, is obtained by displacing the origin by 2.5 m along the Y-axis of WGS 84."
This paper was written 10 years ago.  Are you aware of any updated papers on the subject of PE-90 versus WGS-84?
I was surprised by the promptness of his answer about 45 minutes later.  It said:
The last I checked, the GLONASS folks made a major adjustment to PZ-90 (named PZ-90.02) in 2007 (Sept. 20) to improve its alignment with ITRF00/WGS-84 to under a 1/2 meter (Delta X = -36cm, Delta Y = +8cm, Delta Z = +18cm) and then one or more minor adjustments since then. Offhand, I don't know the precise current relationship between the two. However, I just emailed a colleague in Russia who would know the current status and hopefully he will email me a technical paper you can reference.

From GLONASS scientists I've listened to and conversed with over the past few years, it is my understanding that Russia's intent is to bring GLONASS into precise alignment with ITRF/WGS-84, if they haven't done it already.

To sum up his answer, there is just one half meter difference between WGS-84 and PE-90.02 and in some parts of Russia it is less than 6 inches difference.

4 comments:

  1. I have a query regarding Glonass + GPS combined navigation. With the differences in PZ-90 and WGS84 how will the receiver achieve a combined signal solution for navigation/receiver position estimation?
    -Nerella

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