From Ulf Tue Feb 8 16:59:42 1994 From: Ulf (Ulf) Date: Tue, 8 Feb 1994 16:59:42 PST Subject: [IGSMAIL-0484] Rogue SNR-8, 800, 8C tracking problems during AS Message-ID: ****************************************************************************** IGS Electronic Mail Tue Feb 8 16:59:42 PST 1994 Message Number 0484 ****************************************************************************** Author: Ulf J. Lindqwister/JPL Subject: Rogue SNR-8, 800, 8C tracking problems during AS The Rogues in the Network seem to have settled down from last week. At least we are seeing fewer tracking problems while the GPS signal is encrypted (AS on). It is not clear if the events of last week were transient (part of some special adjustments done by the Air Force) or mainly part of some Rogue firmware bug with a random behavior. We think we may have, however, found one cause of the tracking problems for Rogues when AS is on. One of the features (in addition to several bug fixes) of the last firmware release (V7.5) was to make the Rogue switch from P-codeless (x-correlation) to P-code tracking as soon as the satellites stopped encrypting the signal. The idea was to maximize the amount of P-code data collected, since in earlier versions the Rogue would contionue to track in P-codeless mode until the satellite set on the horizon (independent of when the satellite quit encrypting the signal). This feature already exists on the TurboRogues (and are believed to be working fine there). We suspect that the feature was incorrectly implemented, possibly by reading the wrong data bit in the navigation message. This could cause the Rogue to try to switch into P-code mode at random times (while the satellite is still transmitting encrypted data). At this point we are still evaluating the extent of the problem and we are also trying to figure out a couple of workarounds, as described below. For now I would suggest that we simply wait and observe for another week before taking any action. A new software release would take at minimum a month to accomplish, since this includes diagnostics & testing, PROM re-programming for 30 sites, firmware shipping, and installation at the sites. Hence, before embarking on a new release we want to be reasonably sure that we have found the problem and that the error is serious enough. There are a few possible alternatives to simply observing for now: 1. P-codeless tracking At the 'FIELD>' or 'ROGUE>' prompt on the Rogue receiver type: terminate configure -x (where is a carriage return) The above commands will do the following: a) terminate the existing tracks, and b) start up tracking on all satellites in P-codeless mode (thereby bypassing the switch bug - thanks to Danny Van Loon/KOSG, for noticing this - it was very helpful for our debugging efforts) Note that this command will have to be re-issued every time power is lost or the receiver reset. This also has the additional disadvantage of tracking the Block I satellites (PRN 3, 12, 13) in P-codeless mode (although they transmit P-code signals). To enable tracking in P-code mode for the Block I satellites only (while the Block II track in P-codeless mode), please contact me for further details (it is a bit complicated). Also, remember that the Air Force may at any time remove encryption on a Block II satellite. An advantage of tracking in P-codeless mode for all satellites is that the existing biases between the P-code and P-codeless pseudoranges will not be a concern (could be troublesome when you are filtering mixed code data and only solving for 1 clock). For carrier phase only solutions, however, the biases are not an issue. 2. Raising the horizon-mask to 15 degrees At the Rogue prompt issue the commands: pps e 15 This will set the horizon-mask to 15 degrees. Some of you have noticed a reduction in loss-of-lock events after doing this - this could be related to the fact that the tracking switch to P-code mode often seems to happen at the beginning of arcs (although, this is still not well confirmed). Again, to reiterate, I think it is best at this point to wait and observe, however, the suggestions above are not likely to cause much harm as long as the constellation remains in AS mode. I'll be in touch with more information in the near future - feel free to contact me for questions/suggestions. Thanks for your patience. - Ulf ======================================================================= Ulf J. Lindqwister Telephone: (818) 354 1734 GPS Networks & Operations Group FAX: (818) 393 4965 Jet Propulsion Laboratory Email: ujl at logos.jpl.nasa.gov 4800 Oak Grove Dr. Pasadena, CA 91109 =======================================================================