BESTPOS
Best position
Platform: 
OEM719, OEM729, OEM7500, OEM7600, OEM7700, OEM7720, PwrPak7, CPT7, CPT7700, SMART7, SMART2 
When positioning with GNSS, there are four parameters being solved for: latitude, longitude, height and receiver clock offset from GPS time. The quality of the solution for all four parameters depends on the geometry of where the satellites are with respect to the antenna (and receiver). The strength of the positioning geometry is indicated by Dilution of Precision (DOP) values, with lower DOP numbers indicating better geometry. Because all the GNSS satellites are above terrestrial receivers, the VDOP (vertical DOP) is the largest DOP value. This is why the reported standard deviation for height is usually larger than for latitude or longitude.
Accuracy is based on statistics and reliability is measured in percentages. When a receiver states it can measure height to one metre, this is an accuracy measure. Usually this is a one sigma value (one SD). A one sigma value for height has a reliability of 68%. In other words, the error is less than one metre 68% of the time. For a more realistic accuracy, double the one sigma value (one metre) and the result is 95% reliability (error is less than two metres 95% of the time). Generally, GNSS heights are 1.5 times poorer than horizontal positions. See also the note in the GPGST log for CEP and RMS definitions.
This log contains the best position computed by the receiver. In addition, it reports several status indicators, including differential age, which is useful in predicting anomalous behavior brought about by outages in differential corrections. A differential age of 0 indicates that no differential correction was used.
SPAN Systems
On systems with SPAN enabled, this log contains the best available combined GNSS and Inertial Navigation System (INS  if available) position computed by the receiver.
With the system operating in an RTK mode, BESTPOS reflects the latest lowlatency solution for up to 60 seconds after reception of the last base station observation. After this 60 second period, the position reverts to the best solution available and the degradation in accuracy is reflected in the standard deviation fields. If the system is not operating in RTK mode, pseudorange differential solutions continue for the time specified in the PSRDIFFTIMEOUT command. If the receiver is SPAN enabled, the GNSS+INS combined solution is also a candidate for BESTPOS output.

The RTK system in the receiver provides two kinds of position solutions. The Matched RTK position is computed with buffered observations, so there is no error due to the extrapolation of base station measurements. This provides the highest accuracy solution possible at the expense of some latency which is affected primarily by the speed of the differential data link. The MATCHEDPOS log contains the matched RTK solution and can be generated for each processed set of base station observations. The LowLatency RTK position is computed from the latest local observations and extrapolated base station observations. This supplies a valid RTK position with the lowest latency possible at the expense of some accuracy. The degradation in accuracy is reflected in the standard deviation and is summarized in An Introduction to GNSS available on our website at novatel.com/anintroductiontognss. The amount of time that the base station observations are extrapolated is in the "differential age" field of the position log. The LowLatency RTK system extrapolates for 60 seconds. The RTKPOS log contains the LowLatency RTK position when valid, and an "invalid" status when a LowLatency RTK solution could not be computed. The BESTPOS log contains either the lowlatency RTK, PPP or pseudorangebased position, whichever has the smallest standard deviation. RTK positioning uses the carrier phase observations from the receiver. The carrier phases are precise but ambiguous: the measurement includes an unknown integer number of cycles known as the “ambiguities”. Determining these ambiguities is the key to unlocking the highestaccuracy GNSS positions. This determination is known as ambiguity resolution. Before the integer ambiguities can be resolved they are first estimated as realnumbered values, “floats” in computing parlance. After some period that depends on RTK baseline length, ionosphere activity, and other observing conditions, the ambiguities can be resolved into integers, making centimetre or even subcentimetre positioning possible. 
Different positioning modes have different maximum logging rates, which are also controlled by model option. The maximum rates are: 100 Hz for RTK, 100 Hz for pseudorange based positioning, 20 Hz for GLIDE (PDP) and 20 Hz for PPP.
On SMART antennas, the position in the BESTPOS log may be corrected for antenna height. If so this will be indicated in the Extended Solution Status. See Terrain Compensation for details.
Message ID: 42
Log Type: Synch
Recommended Input:
log bestposa ontime 1
ASCII Example:
#BESTPOSA,USB1,0,58.5,FINESTEERING,2209,502061.000,02000020,cdba,16809;SOL_COMPUTED,PPP,51.15043706870,114.03067882331,1097.3462,17.0001,WGS84,0.0154,0.0139,0.0288,"TSTR",11.000,0.000,43,39,39,38,00,00,7f,37*52483ac5
Field 
Field type 
Description 
Format 
Binary Bytes 
Binary Offset 
1 
Log header 
BESTPOS header For information about log headers, see ASCII, Abbreviated ASCII or Binary. 

H 
0 
2 
sol stat 
Solution status, see Table: Solution Status 
Enum 
4 
H 
3 
pos type 
Position type, see Table: Position or Velocity Type 
Enum 
4 
H+4 
4 
lat 
Latitude (degrees) 
Double 
8 
H+8 
5 
lon 
Longitude (degrees) 
Double 
8 
H+16 
6 
hgt 
Height above mean sea level (metres) 
Double 
8 
H+24 
7 
undulation 
Undulation  the relationship between the geoid and the ellipsoid (m) of the chosen datum When using a datum other than WGS84, the undulation value also includes the vertical shift due to differences between the datum in use and WGS84. 
Float 
4 
H+32 
8 
datum id# 
Datum ID number 61 = WGS84 63 = USER 
Enum 
4 
H+36 
9 
lat σ 
Latitude standard deviation (m) 
Float 
4 
H+40 
10 
lon σ 
Longitude standard deviation (m) 
Float 
4 
H+44 
11 
hgt σ 
Height standard deviation (m) 
Float 
4 
H+48 
12 
stn id 
Base station ID 
Char[4] 
4 
H+52 
13 
diff_age 
Differential age in seconds 
Float 
4 
H+56 
14 
sol_age 
Solution age in seconds 
Float 
4 
H+60 
15 
#SVs 
Number of satellites tracked 
Uchar 
1 
H+64 
16 
#solnSVs 
Number of satellites used in solution 
Uchar 
1 
H+65 
17 
#solnL1SVs 
Number of satellites with L1/E1/B1 signals used in solution 
Uchar 
1 
H+66 
18 
#solnMultiSVs 
Number of satellites with multifrequency signals used in solution 
Uchar 
1 
H+67 
19 
Reserved 
Hex 
1 
H+68 

20 
ext sol stat 
Extended solution status (see Table: Extended Solution Status) 
Hex 
1 
H+69 
21 
Galileo and BeiDou sig mask 
Galileo and BeiDou signals used mask (see Table: Galileo and BeiDou SignalUsed Mask) 
Hex 
1 
H+70 
22 
GPS and GLONASS sig mask 
GPS and GLONASS signals used mask (see Table: GPS and GLONASS SignalUsed Mask) 
Hex 
1 
H+71 
23 
xxxx 
32bit CRC (ASCII and Binary only) 
Hex 
4 
H+72 
24 
[CR][LF] 
Sentence terminator (ASCII only) 
 
 
 
PPP requires access to a suitable correction stream, delivered either through LBand or the Internet. For LBand delivered TerraStar or Veripos service, an LBand capable receiver and software model is required, along with a subscription to the desired service. Contact NovAtel for TerraStar and Veripos subscription details.
Bit 
Mask 
Description 
0 
0x01 
If an RTK solution: an RTK solution has been verified If a PDP solution: solution is GLIDE Otherwise: Reserved 
1‑3 
0x0E 
Pseudorange Iono Correction 0 = Unknown or default Klobuchar model 1 = Klobuchar Broadcast 2 = SBAS Broadcast 3 = Multifrequency Computed 4 = PSRDiff Correction 5 = NovAtel Blended Iono Value 
4 
0x10 
RTK ASSIST active 
5 
0x20 
0 = No antenna warning 1 = Antenna information is missing See the RTKANTENNA command 
6 
0x40 
Reserved 
7 
0x80 
0 = Terrain Compensation corrections are not used 1 = Position includes Terrain Compensation corrections 