SXBlue II GPS Series Technical Reference Manual
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coverage and ionospheric map coverage. In fact, it’s desirable to have a few degrees of
latitude and longitude of ionospheric map coverage around your location to ensure that
the satellites available have these correctors.
Q - How do I know if the SXBlue II GPS module has acquired a SBAS signal?
A - The “DIFF” LED is the primary indicator that SBAS signal has been acquired and
locked. The SXBlue II GPS allows you to request the output of the $RD1 message that
contains the SBAS bit error rate (BER) for both receiver channels. The BER value
describes the rate of errors received from SBAS. Ideally, this should be zero; however,
the SXBlue II GPS
should provide good performance up to a 150 BER. The PocketMAX
utility discussed in the PocketMAX Manual is a useful tool that provides this information
without needing to use NMEA commands.
Q - How do I know if the SXBlue II GPS is offering a differentially corrected position?
A – The “DGPS” LED is the primary indicator that the receiver is outputting differentially
corrected positions. The SXBlue II GPS 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 SXBlue II GPS 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 SXBlue II GPS 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 SXBlue II
GPS
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.
6.7 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
