Specifications

Differential and RTK Operations 35
Differential and RTK
For Ashtech DBN messages, the required minimum baud rate is the DBN rate
multiplied by 10/8. Table 3.9 lists the required baud rates.
Table 3.8 and Table 3.9 list the minimum baud rates, assuming no other data is sent on the data link.
If other messages are transmitted, then the minimum standard baud rate may increase.
The recommended optimal setting is to transmit type 18 and 19 messages once every
second on a high-speed link.
If a high speed data link is not available, you have indirect control over the number of
satellites used, by setting elevation mask angles. The elevation angle for any
particular satellite changes by 1° for every 100 km of baseline length. For baselines of
less than 100 km, you should set the base station elevation mask at 1° less than the
remote receiver elevations masks to guarantee that the base station sends data for all
satellites the remote might use, while not sending data for low elevation satellites that
the remote does not use.
Recommended mask angle settings for RTK:
Remote: 10° (Default)
Base: 9°
Use Mission Planner to determine the maximum number of satellites visible above a
given mask angle. Table 3.10 shows the maximum number of satellites above a 4°
mask angle, with the constellations available August 11, 1997, (25 GPS satellites)
12 30*30*2*8/6*10/8*1/T 600 bps 4800 bps
Table 3.9: Minimum Baud Rates for Ashtech DBN Messages
Number of
Satellites
Minimum baud rate
(message period = T)
Minimum standard
baud rate (T = 5 sec)
Minimum standard
baud rate (T = 1 sec)
7 1240*10/8*1/T 600 baud 2400 baud
9 1528*10/8*1/T 600 baud 2400 baud
12 1960*10/8*1/T 600 baud 4800 baud
Table 3.8: Minimum Baud Rates for RTCM Messages 18 & 19 or 20 & 21
Number of
Satellites
Minimum baud rate
(message period = T)
Minimum standard
baud rate (T = 5 sec)
Minimum standard
baud rate (T = 1 sec)