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FAQs on GPS Systems and Flight Recorders

1.   I have heard that the IGC Approval of some IGC approved flight recorders will be downgraded from 1 October 2012. What impact is this likely to have on using it to validate Badge flights?

The first flight recorders were designed during the mid 1990's and conformed to the security standards then prevailing.   These recorders could be used to validate all IGC flights, including World records.   During the last five years, the security standards have been increased, predominantly by requiring a security system based on a RSA or asymmetrical algorithm.   While all the recorders designed and marketed during the last five years comply, certain early models have a symmetrical security algorithm and/or problems with the mechanical security micro switch.

From 1 October 2012, the IGC has decreed that some early will have their IGC approval revoked.  These include the Print Technik GR1000, EW Models A, B and D (but not the EW Micro) and early models of the Filser LX20,   Other recorders such as the the Cambridge GPS10, GPS20 and GPS25, Colibri v1 through v4, and Zander GPS940 are downgraded to FAI Badges and Distance Diplomas only. 


2.   Can you explain how UK airspace is entered and updated in Glide Navigator II, See You and See You Mobile?

Glide Navigator imports airspace from the Tim Newport-Peace .air files.   These are published each year by Tim and published on the BGA website.   They can also be found in John Leibacher's World Soaring Turn Point Exchange.  

See you and See You Mobile use airspace files which are compiled specifically for the See You and LX Navigation products.   These files have a .cub extension (See You binary) and can be found on both the See You and LX Navigation web sites.   Alternatively, they can be compiled using a compiler, LX Airspace Browser.   This free program compiles airspace from a variety of formats including the Tim Newport Peace version and the  DAFIF format.   Both these formats are ASCI text formats and can, if required, be easily modified with a simple word processor before compilation.


3.   Why does my IGC approved flight recorder have two serial numbers?

When the International Gliding Commission (IGC) created the standards for approved flight recorders, they decreed that the serial numbers would be a trigraph consisting of three alpha numeric characters using the numbers 0 to 9 and the letters A to Z. These three character alpha numeric serial numbers are mathematically described as 'base 36' numbers, and use the digits 0 to 9, with 10 represented as 'A', 11 as 'B' etc to 35 represented as 'Z'. They can be converted to an equivalent base 10 number which only uses the characters 0 to 9, and which we all recognize. This means that every IGC approved flight recorder will have both an alpha numeric base 36 three character serial number, as well as an equivalent base 10 numeric serial number. Flight recorders manufactured by Cambridge and Volkslogger were all assigned a base 36 three character serial number when built. However, the early recorders made by Filser, EW and LX Navigation carried the base 10 serial number. This is converted to a base 36 number when the IGC file is downloaded. For example, a LX Colibri may be designated with a base 10 serial number of 15657. This is equivalent to a base 36 serial number C2X.  In the fullness of time, all base 10 serial numbers will be displayed as base 36 three character serial numbers.  In passing, it should be noted that a flight recorder can have a base 36 number comprised of all numeric characters.  For example a Cambridge recorder might carry the S/No 535.  This is a base 36 number and equates to a base 10 S/No of 6593.  Still confused?  

The .IGC files that are generated by an IGC approved flight recorder will always have a file name of xxxxxxxx.IGC where the 'x' are decoded as follows:

The first character represents the last digit of the year the flight was made.  For a flight made in 2005 (or 1995) this digit will be 5.

The second character represents the month of the flight.  1 to 9 denote Jan to Sep respectively, while Oct is denoted by 'A', Nov by 'B' and Dec by 'C'.

The third character represents the day of the month.  1 to 9 denote the first 9 days of the month, while the 10th is denoted by 'A', the 11th by 'B' and so on, until the 31st which is denoted by 'V'.

The fourth character is a single letter denoting the manufacturer of the flight recorder.  'A' denotes Garrecht (Volkslogger), 'C' Cambridge, 'E' EW Avionics, 'F' Filser, 'L' LX Navigation, 'P' Peschges and so on.

The fifth, sixth and seventh characters is the base 36 alpha numeric serial number of the flight recorder.

The eighth digit represents the number of the flight on any day

For instance, an IGC flight log file might have the file name of '5AGLC2X1.IGC'. Decoded this means that the flight was made in 2005 (or 1995), in the month of October on the 16th of the month.  The recorder was manufactured by LX Navigation and carried the serial number 'C2X'.  It was the first flight recorded on that day.

Arn't you glad you asked!


4.  What is the accuracy of GPS altitude?

GPS altitude has about three times the error of horizontal position.   This
is due to the satellites being organized to prioritize a horizontal
solution rather than a vertical one.   Average horizontal error is better
than 15m for 95% of the time,  therefore GPS height error is 3 x 15 = 45m
(150ft).   This means that for 95% of the time, GPS altitude error will be
less than 150ft. 

However, you must remember that Altimeters and GPS are not measuring the same thing.  An altimeter measures pressure altitude, which is the height of an atmospheric pressure level according to an ISA atmosphere (International Standard Atmosphere).   As the actual atmosphere rarely conforms to ISA, the actual height above MSL shown on a pressure altimeter can vary some hundreds of feet from the true height above MSL.   GPS altitude is measured as the height above the surface of the geoid (MSL for all practical purposes).   The result is that GPS altitude can vary many hundreds of feet from the pressure altitude as shown by the altimeter.   This is not an error; just the difference between two totally different measuring systems.

 If the GPS has access to a WAAS signal (Wide Area Augmentation), then both the horizontal and vertical errors are decreased by a factor of 10


5.  Why do some Naviter/LX Airspace Files "Crash".

In the beginning, Naviter created a binary airspace file format.  The format was good and they gave it a .cub file extension.   This proved popular and LX Navigation then adopted the same .cub file to update the airspace in their instruments.   Unfortunately, their firmware architecture could not handle the original .cub format, so they made one or two minor changes to the format but retained the .cub file extension.  And it came to pass one day, an unsuspecting glider pilot upgraded his See You Mobile out of date airspace with a current .cub file compiled for LX Navigation instruments.  When he next used his See You Mobile, the PDA crashed with many random lines on the screen.

It transpired that the two versions of the airspace files, both using the same file extension of .cub, were not compatible.   Unfortunately, by this time venerated airspace file sources such as John Leibacher's World Wide Soaring Exchange had only one version of the .cub files without any explanation of the problems.   Fortunately, there are ways to re-compile the airspace files if you think you have the wrong version.

See You for the PC will accept both versions of the .cub airspace file.  See You Mobile, Oudie, LXNAV 8000, 8080 and 9000 all use the original Naviter .cub file.  These files can be downloaded from the Naviter website, or compiled by loading an unknown airspace format into See You for the PC and then exporting them as .cub files.   Files for the LX7007 and earlier instruments can be downloaded from the LX Navigation website or compiled using the LX Airspace Browser program. 

Understanding brought sweetness and light back to users of .cub airspace files