iSXblue/SXBlue II GNSS Series Technical Reference Manual
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Q - How do I know if the iSXBlue/SXBlue II GNSS is offering a differentially corrected position?
A – The “DGPS” LED is the primary indicator that the receiver is outputting differentially corrected
positions. The iSXBlue/SXBlue II GNSS outputs the GGA message as the main positioning data
message. This message contains a quality fix value that describes the GPS status. If this value is a 2,
then the position is differentially corrected. The PocketMAX utility is a useful tool that provides this
information without needing to use NMEA commands.
Q - Does it matter much if the iSXBlue/SXBlue II GNSS is frequently losing lock on SBAS due to
obstructions and the low satellite elevation angles at my geographic location?
A - No, provided that the receiver is receiving a full set of corrections relatively often. Using the COAST
technology, the iSXBlue/SXBlue II GNSS will be able to perform well for up to 40 minutes with old
correction data (similar to Auto-Dif, accuracy towards the end will not be as accurate as the beginning.)
In order to obtain a full set of corrections, the iSXBlue/SXBlue II GNSS
antenna receives the ionospheric
map over a period of a few minutes (about 5 minutes). This is the minimum amount of time required to
get a full set of corrections for SBAS operation. After this, the receiver can COAST until the next set of
corrections has been received.
7.7
Auto-Dif
Q – Why was Auto-Dif developed?
A – Once Selective Availability was removed in May of 2000, it opened the door to develop this
technology as environmental errors change much more slowly than SA did. The purpose of Auto-Dif was
to market an alternative to users in place of costly subscription-based differential services around the
world where free services such as DGPS beacon and SBAS did not exist.
Q – Will Auto-Dif perform as well as other correction services?
A – Auto-Dif will perform with a similar level of performance as other differential services at first. There
will be a slow drift to the positioning performance with increasing time. We normally allow Auto-Dif to
age for a maximum of 30 to 40 minutes; however, it’s up to you on how long you will allow it to age. A
simple update to the reference position will again provide best performance.
Q – Will Auto-Dif correct for multipath?
A – No, there’s no method of differentially correcting multipath signals. There’s only ways of reducing
their impact on the measurements gathered by the receiver.
7.8
External Corrections
Q - My iSXBlue/SXBlue II GNSS system doesn’t appear to be using corrections from an external
correction source, what could be the problem?
A - This could be due to a number of issues:
•
Make sure that the corrections are of an RTCM SC-104 protocol.
•
Make sure that the differential source is set to receive RTCM by issuing a $JDIFF,THIS<cr><lf> on the RS-
232 Serial port.