What Were They Thinking?

[Regarding the Navigate! e-newsletter lead story of February 20, PRN-32 — Not Quite Yet,” recapped in “The System] As the possessor of several older GPS receivers (used for teaching students), which probably won’t pick up PRN-32 (but might pick up PRN-0), I have to wonder why the new satellite wasn’t designated “zero.”

And while I’m at it, here’s a gripe from years past: How did a bunch of otherwise bright people settle on a 10-bit week code, creating a rollover problem in two decades that could have been pushed to 40 or 80 years with one or two more bits?

— Michael Kennedy Dept. of Geography, Univ. of Kentucky author of The Global Positioning System and GISContributing Editor Richard Langley Replies: The GPS satellite PRN numbers, since the inception of the system going back to the 1970s, are 1 through 32. This has been published in all official documentation including the ICD-GPS-200 user interfaces document (now called IS-GPS-200) and available on the USCG Navcen website.

If you look at http://gge.unb.ca/Resources/GPSConstellationStatus.txt and note #8, you will see that SVN-32 did operate as PRN-32 for a short while after launch before being changed to PRN-01. All GPS receiver manufacturers should have allowed for the fact that there might have been a new PRN-32 at any time. And note that PRN-00 is an invalid PRN number.

Concerning the GPS Week rollover, I did a GPS World Innovation column on this topic in November 1998. Once again, the rollover was explained in the ICD and should have been noted by all GPS receiver manufacturers.

More on 32

In your [Navigate! e-newsletter] article you talk about a healthy 32-satellite constellation. But setting PRN-32 healthy would only result in 31 healthy satellites. There is currently no satellite assigned PRN-7, although PRNs 1-6 and 8-32 are currently assigned.

— T.S. Kelso, Senior Research AstrodynamicistCenter for Space Standards & Innovation, Analytical Graphics, Inc.Editor: Several astute readers noticed our error. The online news story has been corrected, and the news article in this issue’s The System section computes (we hope) correctly.

Anomaly Straight Talk

Nice interview with Col. Crews! Very clear explanations of the anomaly that should quickly dissipate the “inevitable paranoid rumors” of such an anomaly.

— Phil Ward, Navward ConsultingEditor: The interview by Don Jewell ran in the February 27 System Design & Test e-newsletter.

Don: Great job on this [Military & Government e-newsletter] column!  This is what the community had been missing. Keep up the good work.

— Elliott Kaplan, Co-editor/co-author of Understanding GPS: Principles and Applications

WAAS and ADS-B

In the February Business section there is an article on ADS-B and how UPS will be the first to use it. I work as the Operations Lead for the WAAS and was wondering why there is no mention of WAAS in this article.

Seems like WAAS would need to be an integral part of Next Gen. WAAS is not an integral part of ADS-B (ITT contract).

— Jorge J. Boubion, WAAS Operations Lead (POC)Federal Aviation Administration

Avionics Contributing Editor Bill Thompson Replies: GPS avionics do not require the use of augmentation such as the Wide Area Augmentation System (WAAS) or Local Area Augmentation System (LAAS) to provide a position report. The use of augmentation improves their Required Navigation Performance (RNP) value. For the UPS first use, WAAS probably wasn’t used as it wasn’t required for what they were doing.

I did a survey of the news of the first use and nowhere was WAAS mentioned since I don’t think they use it at this time.

Further, I have heard that not many airlines are equipping with WAAS receivers since they are not mandated by the FAA.