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 low-latency 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 Low-Latency 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/an-introduction-to-gnss. The amount of time that the base station observations are extrapolated is in the "differential age" field of the position log. The Low-Latency RTK system extrapolates for 60 seconds. The RTKPOS log contains the Low-Latency RTK position when valid, and an "invalid" status when a Low-Latency RTK solution could not be computed. The BESTPOS log contains either the low-latency RTK, PPP or pseudorange-based 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 highest-accuracy GNSS positions. This determination is known as ambiguity resolution. Before the integer ambiguities can be resolved they are first estimated as real-numbered 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 sub-centimetre 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 multi-frequency 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 Signal-Used Mask)	Hex	1	H+70
22	GPS and GLONASS sig mask	GPS and GLONASS signals used mask (see Table: GPS and GLONASS Signal-Used Mask)	Hex	1	H+71
23	xxxx	32-bit CRC (ASCII and Binary only)	Hex	4	H+72
24	[CR][LF]	Sentence terminator (ASCII only)	-	-	-

Solution Status
Binary	ASCII	Description
0	SOL_COMPUTED	Solution computed
1	INSUFFICIENT_OBS	Insufficient observations
2	NO_CONVERGENCE	No convergence
3	SINGULARITY	Singularity at parameters matrix
4	COV_TRACE	Covariance trace exceeds maximum (trace > 1000 m)
5	TEST_DIST	Test distance exceeded (maximum of 3 rejections if distance >10 km)
6	COLD_START	Not yet converged from cold start
7	V_H_LIMIT	Height or velocity limits exceeded (in accordance with export licensing restrictions)
8	VARIANCE	Variance exceeds limits
9	RESIDUALS	Residuals are too large
10-12	Reserved
13	INTEGRITY_WARNING	Large residuals make position unreliable
14-17	Reserved
18	PENDING	When a FIX position command is entered, the receiver computes its own position and determines if the fixed position is valid PENDING implies there are not enough satellites currently tracked to verify if the FIX POSITION entered into the receiver is valid. Under normal conditions, you should only see PENDING for a few seconds on power up before the GNSS receiver has locked onto its first few satellites. If your antenna is obstructed (or not plugged in) and you have entered a FIX POSITION command, then you may see PENDING indefinitely.
19	INVALID_FIX	The fixed position, entered using the FIX position command, is not valid
20	UNAUTHORIZED	Position type is unauthorized
21	Reserved
22	INVALID_RATE	The selected logging rate is not supported for this solution type.

Position or Velocity Type
Binary	ASCII	Description
0	NONE	No solution
1	FIXEDPOS	Position has been fixed by the FIX position command or by position averaging.
2	FIXEDHEIGHT	Position has been fixed by the FIX height or FIX auto command or by position averaging
3-7	Reserved
8	DOPPLER_VELOCITY	Velocity computed using instantaneous Doppler
9-15	Reserved
16	SINGLE	Solution calculated using only data supplied by the GNSS satellites
17	PSRDIFF	Solution calculated using pseudorange differential (DGPS, DGNSS) corrections
18	WAAS	Solution calculated using corrections from an SBAS satellite
19	PROPAGATED	Propagated by a Kalman filter without new observations
20-31	Reserved
32	L1_FLOAT	Single-frequency RTK solution with unresolved, float carrier phase ambiguities
33	Reserved
34	NARROW_FLOAT	Multi-frequency RTK solution with unresolved, float carrier phase ambiguities
35-47	Reserved
48	L1_INT	Single-frequency RTK solution with carrier phase ambiguities resolved to integers
49	WIDE_INT	Multi-frequency RTK solution with carrier phase ambiguities resolved to wide-lane integers
50	NARROW_INT	Multi-frequency RTK solution with carrier phase ambiguities resolved to narrow-lane integers
51	RTK_DIRECT_INS	RTK status where the RTK filter is directly initialized from the INS filter
52	INS_SBAS	INS position, where the last applied position update used a GNSS solution computed using corrections from an SBAS (WAAS) solution
53	INS_PSRSP	INS position, where the last applied position update used a single point GNSS (SINGLE) solution
54	INS_PSRDIFF	INS position, where the last applied position update used a pseudorange differential GNSS (PSRDIFF) solution
55	INS_RTKFLOAT	INS position, where the last applied position update used a floating ambiguity RTK (L1_FLOAT or NARROW_FLOAT) solution
56	INS_RTKFIXED	INS position, where the last applied position update used a fixed integer ambiguity RTK (L1_INT, WIDE_INT or NARROW_INT) solution
57-66	Reserved
67	EXT_CONSTRAINED	INS position, where the last applied position update used an external source (entered using the EXTERNALPVAS command)
68	PPP_CONVERGING	Converging TerraStar-C, TerraStar-C PRO or TerraStar-X solution
69	PPP	Converged TerraStar-C, TerraStar-C PRO or TerraStar-X solution
70	OPERATIONAL	Solution accuracy is within UAL operational limit
71	WARNING	Solution accuracy is outside UAL operational limit but within warning limit
72	OUT_OF_BOUNDS	Solution accuracy is outside UAL limits
73	INS_PPP_CONVERGING	INS position, where the last applied position update used a converging TerraStar-C, TerraStar-C PRO or TerraStar-X PPP (PPP_CONVERGING) solution
74	INS_PPP	INS position, where the last applied position update used a converged TerraStar-C, TerraStar-C PRO or TerraStar-X PPP (PPP) solution
77	PPP_BASIC_CONVERGING	Converging TerraStar-L solution
78	PPP_BASIC	Converged TerraStar-L solution
79	INS_PPP_BASIC _CONVERGING	INS position, where the last applied position update used a converging TerraStar-L PPP (PPP_BASIC) solution
80	INS_PPP_BASIC	INS position, where the last applied position update used a converged TerraStar-L PPP (PPP_BASIC) solution

PPP requires access to a suitable correction stream, delivered either through L-Band or the Internet. For L-Band delivered TerraStar or Veripos service, an L-Band capable receiver and software model is required, along with a subscription to the desired service. Contact NovAtel for TerraStar and Veripos subscription details.

GPS and GLONASS Signal-Used Mask
Bit	Mask	Description
0	0x01	GPS L1 used in Solution
1	0x02	GPS L2 used in Solution
2	0x04	GPS L5 used in Solution
3	0x08	Reserved
4	0x10	GLONASS L1 used in Solution
5	0x20	GLONASS L2 used in Solution
6	0x40	GLONASS L3 used in Solution
7	0x80	Reserved

Galileo and BeiDou Signal-Used Mask
Bit	Mask	Description
0	0x01	Galileo E1 used in Solution
1	0x02	Galileo E5a used in Solution
2	0x04	Galileo E5b used in Solution
3	0x08	Galileo ALTBOC used in Solution
4	0x10	BeiDou B1 used in Solution (B1I and B1C)
5	0x20	BeiDou B2 used in Solution (B2I, B2a and B2b)
6	0x40	BeiDou B3 used in Solution (B3I)
7	0x80	Galileo E6 used in Solution (E6B and E6C)

Extended Solution Status
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 = Multi-frequency 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

Supplemental Position Types and NMEA Equivalents
Value	Documented Enum Name	NMEA Equivalent
68	PPP_CONVERGING	2
69	PPP	5
70	OPERATIONAL	4
71	WARNING	5
72	OUT_OF_BOUNDS	1
77	PPP_BASIC_CONVERGING	1
78	PPP_BASIC	2