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 PRO or TerraStar-X solution

69

PPP

Converged 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 PRO or TerraStar-X PPP (PPP_CONVERGING) solution

74

INS_PPP

INS position, where the last applied position update used a converged 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