SXBlue II GPS Series Technical Reference Manual
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via a terminal program. It appears as ‘garbage’ data on-screen since it is a binary format and
not ASCII text. The following is an example of how the RTCM data appears on-screen:
mRMP@PJfeUtNsmMFM{nVtIOTDbA^xGh~kDH`_FdW_yqLRryrDuhcB\@}N`ozbSD@O^}nrGqk
eTlpLLrYpDqAsrLRrQN{zW|uW@H`z]~aGxWYt@I`_FxW_qqLRryrDCikA\@Cj]DE]|E@w_ml
roMNjkKOsmMFM{PWDwW@HVEbA^xGhLJQH`_F`W_aNsmMFM[WVLA\@S}amz@ilIuPqx~_IZhT
CpLLrYpdP@kOsmMFM[kVDHwVGbA^P{WWuNt_SW_yMsmMnqdrhcC\@sE^ZfC@}vJmNGAHJVhT
CqLRryrdviStW@H_GbA^P{wxu[K
RTCM has various levels of detail; however the highest level is the message. RTCM defines
numerous messages that contain specific information. The SXBlue II GPS receiver processes
the C/A code and does not support more advanced methods of differential positioning, such as
real-time kinematic (RTK) that uses different standard RTCM message types. Instead, the
SXBlue II GPS uses proprietary RTCM message to achieve cm-level RTK. Considering this fact,
only certain RTCM messages are important for use with the SXBlue II GPS for sub-meter
applications:
•
Type 1 and Type 9 messages, both of which contain similar information. These two messages
contain pseudorange corrections and range rate corrections to each GPS satellite.
•
The Type 2 message contains delta differential corrections that are used when the remote receiver
is using a different satellite navigation message than used by the base station.
•
The Type 5 message contains GPS constellation health information used for improving tracking
performance of a GPS receiver
•
The Type 6 message contains null information, and is broadcast so that a beacon receiver
demodulating the data from the broadcast does not lose lock when the beacon station has no new
data to transmit.
Note - RTCM is a local area data standard. This means that when positioning with
external correction input to the SXBlue II GPS from an external source or outputting
corrections from the SXBlue II GPS to another GPS receiver, performance will degrade as
a function of distance from the base station. The additional degradation will depend on
the difference in observed orbit and ionospheric errors between the reference station
and the remote unit. A general rule of thumb would be an additional 1 m error per 100
miles. This error is often seen as a bias in positioning, resulting in a position offset. The
scatter of the receiver is likely to remain close to constant.
The RTCM SC-104 data output by the SXBlue II GPS is converted from the RTCA SC-159 data
broadcast by SBAS networks. Appendix D - Resources contains the contact information should
you wish to purchase a copy of the RTCM SC-104 specification.
3.3 Configuring the SXBlue II GPS
The SXBlue II GPS features three bi-directional ports referred to as Bluetooth (Port A), Serial
(Port B) and USB (Port C). GPS data messages for all ports are easily configured by sending
NMEA commands to the SXBlue II GPS through either of its communication ports (the output of
the Serial Port can be configured through Bluetooth, for instance and vice versa). The $JASC
NMEA message discussed in detail in Chapter 4 allows you to turn messages on and off as you
require.
Although free utilities are available to configure the SXBlue II GPS, a terminal emulation
software, such as HyperTerminal, can be used to send commands and query the receiver.
