What is the difference between Apex and Ultra corrections?

Apex and Ultra have slightly different update rates due to the way in which the corrections are transmitted.  The Apex corrections use separate RTCM type messages for the orbit and for the clock, whereas the Ultra corrections transmit both orbit and clock in the same message.

The corrections for both Apex and Ultra are transmit in blocks of 8 satellite corrections at a time. For Ultra in 1 block it will include the orbit and clock corrections for 8 satellites.  With Apex the block will include the orbit correction for 8 satellites and every 30 seconds the clock corrections for every satellite will be broadcast.

When the corrections get received there is already latency on the correction due to the processing time of the correction prior to being sent out over the air. This latency is usually seen to be between 10-15 seconds.

When viewing the latency of Apex and Ultra corrections in Verify QC the latency for Ultra refreshes any time the Ultra corrections are received, where with the Apex latency will only update when the corrections for the clock are received.   To look at the latency output with the GGA message the Ultra and Apex are similar update timings.

It’s worth noting that the slight increase in latency has no impact on the position performance.

The following graph depicts the difference in update rates between Apex, and Ultra corrections:

The difference in update rates between Apex, and Ultra corrections

The blue line represents the Apex corrections and the green line represents the Ultra corrections.