Receiving the time



"GPS-Time" basics

This program will receive time information from orbiting GPS
satellites via GPS navigation equipment, and set the Psion clock and
or record the error.

The GPS set has to output data which includes "$GPGSV" sentences. 
These are likely to be included by any NMEA-capable GPS.

Time Zone

The time zone set in the Psion (not in the GPS) is not displayed
directly but will effect the calculation of local time.  The time
given by GPS is in GMT or UTC.  Both the UTC and the
local
ised versions of the time are displayed.

The time zone is not merely the geographic one but includes Daylight
Saving if this is set by the "Clock" built-in Psion application.

Error handling

The program endeavours to cope with problems with data from the GPS
set.  If the 3-Link is absent this is discovered immediately.  If the
GPS is not sending data then the program will time it out.

The data from GPS must contain "$GPRMC" for time signals to be
decoded.  This sentence must appear within the first 20 lines
or the program will give up.

The date and time are given in a simple fixed format and are parsed
by counting commas.  If digits of the date or time are not in place
then the erroneous input is displayed with the faulty characters
indicated.  If the date or time are faulty then just the input is
shown.

If problems are found, try the Psion Comms application using: Port
"TTY:A"; Setup (Baud=4800; Data=8; Stop=1; Parity=None;
Handshake=XonXoff; Newline="<$0D>")

Adjustment

The clock adjustment is made by the
num
ber of seconds shown in the dialogue.  The time is NOT set to the
localised GPS time, so any delay in answering the dialogue does not
matter.  There is no way of altering the adjustment that will be
made.

The log file

The name of the log file is chosen using a special dialogue.  If the
file already exists then its contents may be replaced by the new
single entry or the new entry added to the end.

The file selection dialogue is repeated until the answers are
consistent with the state of the file discovered.

If the write fails, perhaps because the file is currently open, the
dialogue is repeated ready for another attempt.

If the dialogue is abandoned then clock adjustment, if it was
requested, will already have taken place.

Logging

The logging of the current (UTC) time, the error in seconds and
whether the correction was applied does not currently have a defined
purpose.  It is intended in the future to support periodic adjustment
to correct the average daily error.

The current time is recorded in days and
deci
mal days since 1/1/1970.  Any calculation should be independent of
changes in time zone.

The indicator of whether a change was made is 1 for change and 0 for
none, so it can be used as a multiplier of the error to get the
alteration..

Further reading

A number of places around the world offer a dial-up time service to
anyone with a computer, modem and corresponding software.  The
service from NIST is really intended for use via PCs so PC software
is available from them.  Mike Coon has written a freeware
equi
valent for the Psion 3A.

The PC software can be obtained for free ftp download from:
time_a.timefreq.bldrdoc.gov or for $42 on diskette by post from:
Office of Standard Reference Materials, NIST, Gaithersburg, MD,
20899, (301) 975-6776

Other software for connecting GPS sets to Psion is available.  Look
for PsiGar (only for Garmin GPS) from Jonathan Duff or for TDCGPS
from Simon Taylor.

Additional technical information

More information about GPS data and NMEA in general 

Software and additional detail can
be
 obtained from WWW.

About program and author

"GPS-time" Version 1.0, written by Mike Coon of UK.

Contact 106710.2362@compuserve.com

Usual restrictions placed on freeware apply.