u-blox 6

AssistNow Offline works in locations without any wireless connectivity as the correction data files reside in the
receiver or the host. This makes them immediately available upon start-up, eliminating connection set-up
delays, download waiting times and call charges.
The simplest set-up is for GPS receivers including an internal Flash Memory or an external SPI Flash Memory
where ALP data can be stored. In this case, the UBX-AID-ALP message is used.
When the GPS receiver has neither an internal Flash Memory nor an external SPI Flash Memory, the ALP file
must be stored to the host CPU. The GPS receiver can then request data from the host when needed. This
arrangement is implemented using the UBX-AID-ALPSRV message.
In both cases, status reporting on ALP data currently available to the GPS receiver can be taken from message
AID-ALP (STAT).
AssistNow Offline data are published at http://alp.u-blox.com/.
13.7.1 Flash-based AlmanacPlus Overview
Flash-based AlmanacPlus functionality means that AlmanacPlus data is stored in the program flash memory
connected to the u-blox 6 chip.
The task of a server is simply to download the data from an Internet server or other sources, and then deliver
the full file piece by piece to the GPS receiver. This is different to the method described in UBX-AID-ALPSRV
where the file would remain within the host and the GPS receiver would request chunks from that file when
needed.
The message AID-ALP exists in several variants, combining all functionality needed to download data and report
status within one Class/Message ID.
13.7.1.1 Download Procedure
The following steps are a typical sequence for downloading an ALP file to the receiver:
• The server downloads a copy of a current ALP file, and stores it locally
• It sends the first N bytes from that file, using the AID-ALP (TX) message
• The server awaits a AID-ALP (ACK) or AID-ALP (NAK) message
• If can then continue, sending the next N bytes if the message was acknowledged
• Once all data has been transferred, or a NAK has been received, the server sends an AID-ALP (STOP)
message
Note that:
• N should not be larger than ~700 bytes (due to the input buffers on the RS232/USB lines). Smaller values of
N might improve reliability
• N must be a multiple of 2
• There is no re-send mechanism; if a NAK message is received, the full downloading process must be
restarted
• There is no explicit checksum, but an implicit one, as the ALP file already includes a checksum to verify
consistency
Overview of the different versions of AID-ALP messages
Short Name Content Direction
AID-ALP (TX) ALP server sends Data to client Server -> Client
AID-ALP ALP server terminates a transfer sequence
Server -> Client
(STOP)
AID-ALP (ACK) ALP client acknowledges successful receipt of data.
Client -> Server
GPS.G6-SW-10018-A Public Release Page 36 of 201