Understanding GNSS Post-Processing Reports: A Comprehensive Guide

Introduction

GNSS (Global Navigation Satellite System) post-processing is a critical step in achieving high-accuracy geo-referencing for your Looq captures. Raw GNSS observations collected by the qCam must be processed using reference station corrections to achieve centimeter-level accuracy. The post-processing report provides a comprehensive quality assessment of this processed solution, documenting everything from coordinate systems to accuracy metrics.

This guide walks through each section of a GNSS post-processing report, explaining what each metric means and how to interpret the results for quality control purposes.

How to Download the PPK Report

Before you can review your GNSS post-processing report, you need to download it. Each capture will have an associated processing report. Here's how to download the report:

1. Navigate to the Files tab: At the top of your site view, click on the Files button in the navigation bar (located between "Site" and "Share").
2. Expand the capture details: This will display a list of all available output files for your processed capture, including Point Cloud, Ortho, Panoramic Imagery, Track, Thinned Point Cloud, and PPK Report.
3. Download the PPK Report: Locate the PPK Report row in the files list and click the download button (↓) next to it. The report will download as a PDF file to your device.

Once downloaded, you can open the PDF to review all the sections detailed in this guide. The report provides a comprehensive quality assessment that can be reviewed before using your data in downstream applications.

Section 1: General Information

The General Information section provides essential metadata about the capture and processing configuration.

Key Fields

Capture Name: The name of the capture provided by the user before starting the mission.

Site Name: The project or site where data was collected. Sites are pre-configured with coordinate system information that will be applied during processing.

Organization: The organization or entity conducting the survey.

Location: Geographic location where the survey took place (city, state/province, country).

Mission Start (UTC) and Mission End (UTC): The start and end timestamps of the data collection period in Coordinated Universal Time. These times represent when GNSS data logging began and ended.

Mission Duration: Total elapsed time of the data capture session, shown in HH:MM:SS format.

PPK Method

The PPK Method field indicates the type of differential correction strategy used during post-processing. This is one of the most important settings as it directly impacts accuracy and data requirements:

Single: Uses a single reference station for differential corrections. This is the most common method when you have access to a nearby continuously operating reference station (CORS) or have deployed your own base station. The baseline distance between your qCam and the base station should typically be under 30 km for optimal results.

Multiple (VBS): Virtual Base Station processing uses observations from multiple base stations in a network to generate a virtual reference station at or near your survey location. This technique is applied when there is no single reference station within 30 km.

PPP: Precise Point Positioning uses satellite-based corrections instead of ground-based reference stations. PPP is valuable when no suitable base stations are available in your survey area, though it typically requires longer observation times and will yield lower accuracy than the above methods.

Section 2: Coordinate System

The Coordinate System section defines the spatial reference framework used for the final deliverables. This information is either specified by the user during site creation, or derived automatically if the settings are left on “auto”.

Coordinate System Components

Horizontal Datum: The reference datum and ellipsoid used for horizontal positioning.

Projection: The mathematical transformation used to convert the curved Earth surface to a flat map. Examples include UTM zones (default when left on “auto”), or state plane.

Geoid: The geoid model used to convert between ellipsoid heights (geometric height above the reference ellipsoid) and orthometric heights (elevation above mean sea level). Common models include GEOID18, EGM2008, or regional geoid models. This is crucial for generating accurate elevation data.

Units: The measurement units for coordinates. Currently only meters and us survey feet are supported.

GNSS Ephemeris Datum: The datum of the precise satellite orbit files (ephemeris) used in processing.

GNSS Base Station Datum: The datum in which the base station coordinates are provided. This may differ from your output datum.

Datum Transformations

An important note about coordinate systems: when pulling in base station data automatically from a CORS network or other provider, our system may need to perform a datum transformation to convert from the datum of the reference station to your user-specified output datum.

For example, if your base station provides coordinates in NAD83(2011) and your site is also configured to output in a state plane system based on NAD83(2011), the transformation is straightforward. However, if there's a difference between fundamentally different datums (e.g., ITRF to NAD83), a more complex transformation with appropriate parameters must be applied. Looq's software handles these transformations automatically, but it's important to specify the correct output datum, especially when working with high-accuracy applications.

Section 3: Processed Solution

The Processed Solution section presents the core results of your GNSS processing, including timing information and a critical quality breakdown.

Processing Parameters

Processing Mode: Indicates the type of solution computed. See section 1 for more details on the different solution types.

First Epoch (UTC): The timestamp of the first successfully processed position in your dataset.

Processing Started (Local): When post-processing began (in local time), if tracked by the software.

Last Epoch (UTC): The timestamp of the final processed position.

Ephemeris Used: The quality/type of satellite orbit files used in processing:

• Broadcast: Real-time orbits transmitted by satellites
• Precise: Post-processed precise orbits from IGS or other agencies

Using precise ephemeris can improve positioning accuracy but is only released 24-48 hours after a capture takes place.

Solution Quality Breakdown

The pie chart visualization provides an at-a-glance view of your solution quality distribution. Depending on your solution type, you will see either “PPK” or “PPP” in this section. Each position epoch in your dataset is classified into one of these categories:

PPK Fixed (Certified) - Highest Quality (Dark Green)

• Integer ambiguities are resolved with high confidence
• Formal accuracy statistics meet stringent thresholds
• Target: Maximize this percentage

PPK Fixed - High Quality (Light Green)

• Integer ambiguities are successfully resolved
• Good geometry and sufficient observations

PPK Fixed (Inconsistent) - Lower Quality (Red)

• Ambiguity resolution succeeded but forward/reverse solutions show inconsistency
• May indicate multipath, cycle slips, or processing issues

PPK Float - Low Quality (Orange)

• Unable to resolve integer ambiguities
• Position computed using float ambiguity values
• Accuracy degrades to decimeter-level (10-50 cm typical)
• Common causes: poor satellite geometry, insufficient observation time, long baselines, signal obstructions (high multipath)

Autonomous - Lowest Quality (Not shown in example)

• No differential corrections applied
• Single-point positioning only
• Meter-level accuracy (1-10 meters typical)
• Indicates periods without base station data (No PPK) or precise ephemeris (No PPP).

Interpreting the Quality Distribution

A high-quality GNSS processing session should show:

• Combined PPK Fixed (Certified + Fixed): >90-95%
• PPK Fixed (Inconsistent): <5%
• PPK Float: <5%
• Autonomous: 0% or near-zero

If you see high percentages of Float or Autonomous solutions, investigate:

• Baseline distance to reference station (should be <30 km for single base).
• Satellite visibility and multipath conditions
• Base station data quality and availability

Recommendations

• Looq recommends using a local base station for applications requiring survey-grade accuracy.
• Looq's proprietary sensor fusion algorithms utilize the camera calibration to remove outliers and retain high-accuracy throughout the trajectory. Getting loop closure during capture helps improve the accuracy of the final results, especially for captures without ideal GNSS conditions.

Section 4: Accuracies

The Accuracies section quantifies the precision of your processed positions through statistical measures in both horizontal and vertical components.

Accuracy Metrics Explained

For both Horizontal and Vertical components, you'll see five key statistics:

Average: The mean accuracy value across all processed epochs. This gives you a general sense of typical accuracy. Lower values indicate better precision.

Std (Standard Deviation): Measures the variability of accuracy estimates. A low standard deviation means consistent accuracy throughout the mission. High standard deviation suggests fluctuating quality, possibly due to varying satellite geometry, multipath, or environmental conditions.

RMS (Root Mean Square): The square root of the mean of squared accuracy values. RMS provides a single comprehensive measure that accounts for both systematic bias and random error. This is often the most important single accuracy metric.

Min: The best (lowest) accuracy value achieved during the mission. This represents optimal conditions.

Max: The worst (highest) accuracy value in the dataset. Large maximum values may indicate outliers or problematic epochs. Looq's proprietary sensor fusion algorithms utilize the camera calibration to remove these outliers and retain high-accuracy throughout the trajectory.

Understanding Your Accuracy Results

Horizontal Accuracy represents the 2D positional error in the mapping plane.

Vertical Accuracy represents the error in the height/elevation component. Vertical positioning is typically less accurate than horizontal due to satellite geometry.

What Accuracy Values Tell You

These accuracy estimates come from:

1. Formal error propagation from the GNSS processing algorithm (covariance of the solution)
2. Consistency checks between multiple processing passes
3. Residual analysis from the least-squares adjustment

Important considerations:

• These are estimated accuracies based on mathematical models, not validation against ground truth
• Real-world accuracy may differ due to factors not fully captured in models (multipath for example).
• For critical applications, validate against known control points
• Accuracy typically degrades the further you are from a base station or in areas with obstructions

Section 5: Separations

The Separations section analyzes the consistency between forward and reverse processing passes.

What are Forward/Reverse Separations?

Looq processes GNSS  data in two passes:

• Forward pass: Processing from the beginning to the end of the dataset
• Reverse pass: Processing from the end back to the beginning

Ideally, both passes should produce identical or nearly identical positions. The separation is the difference between these two independent solutions at each epoch. Small separations indicate high internal consistency and reliable results.

Separation Metrics

Like the accuracy section, separations are reported with the same five statistics (Average, Std, RMS, Min, Max) for both horizontal and vertical components.

Section 6: Base Station(s)

The Base Station section documents the reference stations used to compute differential corrections. This information is critical for understanding your processing configuration and troubleshooting issues.

Base Station Information (INFO)

Receiver: The type of GNSS receiver operating at the base station.

Providers: The network or organization providing the base station data (NGS CORS for example). Looq will automatically pull base stations from publicly available networks when left on “auto” mode.

Constellations: The GNSS constellations tracked by this base station (GPS, GLONASS, Galileo, BeiDou). More constellations generally mean better satellite availability and stronger geometry.

Data Rate (s): The logging interval of the base station in seconds. Common rates are 1s, 5s, 15s, or 30s. Higher rates (1s) provide more observations and higher accuracy in your final results. If the base used to geo-reference you capture is logging at a lower rate, such as 30s intervals, you may see degraded performance.

Observation Length (h): The total duration of available base station data in hours. This should span or exceed your mission duration. Gaps in base station data will result in gaps in your processed solution.

Mean Baseline (km): The average distance between your rover trajectory and the base station. This is one of the most critical parameters affecting accuracy.

Single Base:

• <10 km: Excellent (atmospheric errors are minimal)
• 10-30 km: Good (standard single-base performance)
• 30-50 km: Exceeds recommended distance, accuracy will be degraded.

Multiple Bases (VRS):

• 0-70 km: Good (standard multi-base performance)
• 70 km+: results may vary

Antenna Information (ANTENNA)

Model: The specific antenna model at the base station, following IGS naming conventions (e.g., TRM115000.00 NONE indicates a Trimble antenna with no radome). Looq's processing software uses antenna calibration files to correct for these variations.

Reference: The antenna reference point, typically ARP (Antenna Reference Point) or sometimes APC (Antenna Phase Center). This indicates which part of the antenna the coordinates refer to.

Height (m): The measured height from the monument or mounting surface to the antenna reference point. This vertical offset must be accurately measured and entered, as errors here directly translate to vertical positioning errors. When uploading a custom local base station, this vertical offset must be found in the RINEX file.

Used Position

PositionType: Indicates how the base station coordinates were determined:

• Published: Use the base station position that is published by CORS providers/read from the RINEX file.
• Manual: Use the position that has been manually entered by the user.

Providers: The source of position information, typically matching the data provider.

Latitude (dms) and Longitude (dms): The base station coordinates in degrees, minutes, seconds. These are the reference coordinates used for differential corrections. Errors in base station coordinates directly translate to errors in the final position of your datasets.

Ellipsoid Height (m): The geometric height of the antenna reference point above the reference ellipsoid. Note this is different from orthometric height (elevation above sea level). The geoid model is used to convert between the two.

Datum: The reference datum for the base station coordinates (e.g., NAD83(2011), ITRF2014). As mentioned in the Coordinate System section, if this differs from your output datum, a transformation will be applied.

Using the Report for Quality Control

A GNSS post-processing report is your primary QC tool. Here's a systematic approach to reviewing reports:

Step 1: Check General Information

• Verify mission times and duration match expectations
• Confirm correct PPK method for your survey conditions
• Ensure proper site and location information

Step 2: Verify Coordinate System

• Confirm output datum, projection and geoid match project requirements and ground control (if applicable).
• Verify units are correct

Step 3: Evaluate Accuracy Solution Quality

• Target >90% combined PPK Fixed solutions
• Check that RMS accuracies meet project specifications

Step 6: Validate Base Station Configuration

• Verify appropriate base station(s) selected
• Check baseline distances are reasonable (<30km for single base)
• Confirm data rate and observation length are adequate
• Ensure base station coordinates are reliable

Red Flags to Investigate

• Float or Autonomous solutions >50%
• Baseline distances >40km for single base
• Gaps in base station data coverage

Conclusion

Understanding your GNSS post-processing report is essential for delivering high-quality positioning data. Each section provides valuable insights into different aspects of solution quality, from coordinate system configuration to accuracy metrics and base station geometry.

By systematically reviewing each section and applying the interpretation guidelines provided in this article, you can:

• Validate that processing completed successfully
• Identify potential quality issues and modify future capture workflows to improve in the future.
• Document quality for clients and stakeholders
• Maintain consistent standards across projects

Remember that while the metrics in this report provide objective quality measures, they are estimates and may vary from real-world conditions. When in doubt about specific results, consult with Looq support and always validate against ground control points when absolute accuracy is critical for your application.