GRAV-D is NOAA’s National Geodetic Survey (NGS ambitious program to re-define the vertical datum of the United States by the year 2021. GRAV-D is composed of two major campaigns, which includes; a high-resolution snapshot of gravity in the United States and a low-resolution movie of gravity changes (“GRAV-D: Gravity for the Redefinition of the American Vertical Datum”). The gravity-based vertical datum will be accurate at the 2 cm level where possible. The proposal is included in the NGS 10 year plan. GRAV_D is an ongoing project and is collecting gravity data across the United States and its holdings (Smith, Dru. Roman, Dan).
The need for vertical datum for the US
The NOAA’s National Geodetic Survey (NGS) started working on the GRAV-D Project in 2007. This project comprises of a complete survey of the gravity field over the USA and its territories, and is committed to track decadal changes to the gravity field. GRAV-D project’s goal is to model and monitor Earth’s geoid (“GRAV-D Field Operations Handbook”).
In order to understand the need for this vertical datum for the US or the GRAV-D project, one needs to consider the 2010 Annual Guidance Memorandum (AGM) and how this project contributes to NOAA. This briefly discussed below (Smith, Dru. Roman, Dan).
Enhancing NOAA's climate services and helping to develop a National Climate Service
Thought the GRAV-D project it is possible to determine the accurate topographic heights at tight gages around North America. It is also possible to get an understanding of local sea level changes. Furthermore, the data obtained from GRAV-D project can be used to analyze the climate driven impacts of the change and its long-term implications.
Helping to support Coastal and Marine Spatial Planning
Coastal and Marine Spatial Planning is dependent on the geodetic and tidal datum reference system and these are main goals of GRAV-D project. As a result, it will be possible to define the spatial environment in a consistent way for both maritime and terrestrial regions.
Contributing to the Sustainability of Marine Fisheries
GRAD-D project can provide important information that is essential to understand the flow of fertilizers and nutrients since they have impact on fisheries. The accurate and consistent heights will make it possible for more accurate agricultural run-off modeling. The gravimetric geoid over the oceanic regions of the United States and its territories can be used to determine the determination of dynamic ocean topography, DOT. An accurate knowledge of DOT will provide us the knowledge of geostrophic ocean circulation patterns, which will definitely have an impact on modeling ocean nutrient transport.
Strengthen Arctic Science and Service
When the GRAV-D in Alaska is completed, it will strengthen Arctic Science and Service in many different ways. It will provide a vertical reference framework that supports Alaska’s State-wide Digital Mapping.
Sustain Satellite-based Earth Observations
GRAV-D is able to provide localized information on Earth’s gravity field. This can also provide with very detailed gravimetric information over the ocean regions. The GRAV-D project has the capacity to serve as a calibration for satellite based gravity missions such as GRACE and GOCE.
Field procedures:
GRAV-D project was launched to improve how elevations or heights are determined. Airborne gravity data is required to be collected over the US and it’s holding for this purpose. The airborne survey then provides gravity field measurement from medium to long-range resolution. A blended gravity field is then obtained from combining this data with that from the measurements in the NGS database with GRACE (Gravity Recovery And Climate Experiment) and GOCE (Gravity Field and steady-State Ocean Circulation Explorer) satellite gravity. Form this blended gravity. A gravimetric geoid will be created that will form the new vertical datum, which will be released after 2022 (“GRAV-D Field Operations Handbook”).
Reconnaissance Operations
The four primary responsibilities of the reconnaissance surveyor for each location surveyed is as follows:
Assess and propose appropriate locations for base stations
This includes selection of a location, GPS Base Station Positioning, and Data Submission for three base station locations for every airport.
Perform the terrestrial gravity survey
This includes selection of location, tie for relative gravity survey and data submission. The gravity observations will be performed in accordance with the "NGS Operations Handbook".
Coordinate with airport and/or FBO management
The reconnaissance surveyor needs to ensure a smooth airborne survey at each airport location. For this purpose, a number of logistical items have to be coordinated with the airport and/or FBO and shipping address to the GRAV-D project manager should be provided. The reconnaissance surveyor will determine whether he needs to involve the airport manager and/or the FBO manager for Aircraft parking, office space, high-speed Internet access, and/or airport and FBP security.
Deliver data GRAV-D project manager
This will include field report, both mailed copy and email; which has information on the summary of survey, contact information, diagrams, maps and conclusions. Gravity and GPS data needs to be mailed as a CD or DVD.
Airborne Operations
Using the information provided from the previous reconnaissance survey, airborne gravity and GPS data will be collected. The three primary responsibility of the airborne operator is given below:
Instrument Management
This includes base station set-up and aircraft installation. The survey team will be responsible for the operation and maintenance of all instruments. The reconnaissance survey will determine the sites for GPS base station positions and will be used unless the established site have been disturbed or destroyed. In case if this happens, new base locations needs to be identified and positioned.
Airborne instruments will be installed aboard the aircraft in accordance with the pre-engineered specifications, which will be provided by the
GRAV-D project manager will provide the airborne instruments (that will be installed in accordance with the pre-engineered specifications in the aircraft) before the installation date. The reconnaissance report that will include an absolute gravity value at the location of the aircraft and at the height of the gravimeter will be provided to the survey team.
Data Collection
The field team must maintain daily logs that provide a status update to the GRAV-D project manager during the survey., Events or facts worthy of note will be kept on Google Docs. These notes in the register cab be used during data processing. The Survey and Hours Log has to be updated on Google Docs after each flight, which should include, the date, start and stop times, lines collected, latitude and longitude, operators, flight hours, and any notable flight events etc.
GPS data must be collected at 1-second intervals during airborne gravity observations. Events need to be recorded on flight logs during gravity data collection.
In the final data submission, all metadata, including the logbooks, worksheets, and final report has to be included.
Data Quality Control
The GPS and gravity data needs to be monitored to ensure a minimum quality level during the survey. At the end of the survey, all data will be downloaded and copied to the field laptop for processing of Quality Control (QC). The latest version of Waypoint Inertial Explorer (IE) is used for the evaluation of GPS data and gravity data is processed using the best software available to produce a free-air gravity QC product. However, in any case, if there is a chronic problem like an instrument or aircraft malfunction, the field team is instructed to contact the GRAV-D project manager immediately.
Airborne Operations Deliverables
This includes, final report in email that includes summary of survey, photographs, diagrams, maps, and worksheets; logbooks that include master register. Survey and hours log, QC processing log, download and backup log; Gravity and GPS data that is mailed on a hard disk; intermediate product during survey that includes flight plans and preliminary gravity plans (“GRAV-D Field Operations Handbook”).
Instruments and Software used
For the IMU and GPS airborne navigation system SPAN (made by NovAtel) and POSAV-510 is used (made by Applanix). Z-Extreme is used as the base station GPS and DL-4 Plus/ DLV-3 is the GPS inside the TAGS timing unit, which are made by Ashtech and NovAtel respectively. The airborne gravimeter is called TAGS made by Micro-g LaCoste (“GRAV-D Field Operations Handbook”).
Among the software, Inertial Explorer is used for processing of SPAN data and other kinematics GPS only data. POSPac is used for processing Applanix POSAV data and other kinematic GPS-only data. For any GPS- only data processing, GrafNav is used. Three software; e.g. AeroGrav, Isaac, and Newton, are used for gravity data processing. Aerograv is used for processing Field QC of TAGS data, pre-2012; Issac for processing Field QC of TAGS data 2012 & onward, and Newton for Silver Spring final gravity data processing. “ The Mapping Program” is used for mapping of gravity data, which uses Field QC of .csv files output from Aerograv. “ The line Comparison Program” is used for comparing gravity data lines to each other or global model and also used Field QC of .csv files output from Aerograv.
Data Products
Data products include airborne and terrestrial survey information, gravity data, and gravity-based geoids or datum.
In conclusion, the GRAV-D project will hopefully deliver significant socio-economic benefit, and strengthen and improve many diverse aspects of NOAA Science. It is a mission to understand and predict Earth’s environment.
Works Cited
“GRAV-D Field Operations Handbook.” National Geodetic Survey. 16 Oct. 2012.
Web. 22 Feb. 2016. <http://www.ngs.noaa.gov/GRAV-D/pubs/GRAVD_Field_Operations_Handbook.pdf>.
“GRAV-D: Gravity for the Redefinition of the American Vertical Datum.” National Geodetic
Survey. Web. 22 Feb. 2016. <http://www.ngs.noaa.gov/INFO/OnePagers/OnePagerGRAVDFinal.pdf>
Smith, Dru. Roman, Dan. “How NOAA’s GRAV-D Project Impacts and Contributes to NOAA
Science.” National Geodetic Survey. 29 Apr. 2010. Web. 22 Feb. 2016. <http://www.ngs.noaa.gov/GRAV-D/pubs/GRAVD_Contribution_to_NOAA_Science.pdf>
