New Results from Strapdown Airborne Gravimetry Using Temperature Stabilisation

In recent years, the use of a strapdown Inertial Measurement Unit (IMU) for airborne gravimetry has proven itself to be an accurate and resilient measurement system, improving the operational flexibility. The main concern is erroneous long-wavelength information in the resulting estimates, which is suspected to originate from uncompensated long-term drift of the accelerometers, probably originating from temperature variation. For this reason, iMAR navigation has designed a temperature stabilisation box, which allows for temperature stabilisation of their IMU systems. On a regional airborne gravity survey over the Kattegat Sea between Denmark and Sweden, such a temperature stabilised strapdown IMU was operated alongside a traditional spring-type platform-stabilised gravity system from ZLS. An analysis of the difference in gravity estimates at cross-over locations yielded a mean value of −0.3 mGal for the iMAR system with an indicated accuracy of 1.0 mGal. The temperature stabilisation unit therefore effectively limits the accelerometer drift and improves the long-wavelength information. However, a straightforward merging approach, adjusting the line-based mean values of the iMAR estimates to match that of the ZLS estimates, improved the accuracy to 0.8 mGal. This indicates that the long-wavelength information of the stabilised-platform system is still superior to that of the strapdown system.

Improvement of Downward Continuation Values of Airborne Gravity Data in Taiwan

An airborne gravity survey was carried out to fill gaps in the gravity data for the mountainous areas of Taiwan. However, the downward continuation error of airborne gravity data is a major issue, especially in regions with complex terrain, such as Taiwan. The root mean square (RMS) of the difference between the downward continuation values and land gravity was approximately 20 mGal. To improve the results of downward continuation we investigated the inverse Poisson’s integral, the semi-parametric method combined with regularization (SPR) and the least-squares collocation (LSC) in this paper. The numerically simulated experiments are conducted in the Tibetan Plateau, which is also a mountainous area. The results show that as a valuable supplement to the inverse Poisson’s integral, the SPR is a useful approach to estimate systematic errors and to suppress random errors. While the LSC approach generates the best results in the Tibetan Plateau in terms of the RMS of the downward continuation errors. Thus, the LSC approach with a terrain correction (TC) is applied to the downward continuation of real airborne gravity data in Taiwan. The statistical results show that the RMS of the differences between the downward continuation values and land gravity data reduced to 11.7 mGal, which shows that an improvement of 40% is obtained.

PENGGUNAAN KINEMATIK GNSS PRECISE POINT POSITIONING (PPP) PADA SURVEI GAYABERAT AIRBORNE SULAWESI

<p class="judulabstrakindo"> ABSTRAK</p><p class="abstrakindo">Metode <em>Precise Point Positionin</em>g (PPP) adalah metode penentuan posisi teliti yang hanya menggunakan sebuah receiver GNSS dual frekuensi. Metode ini dapat digunakan untuk menentukan posisi teliti objek-objek yang diam (<em>static</em>) maupun bergerak (<em>kinematic</em>). Pada penelitian ini, akan dipaparkan mengenai penggunaan kinematik PPP dalam penentuan posisi pesawat terbang pada survei gayaberat <em>airborne</em> di Sulawesi tahun 2008. Data yang digunakan adalah jalur terbang pesawat pada <em>day of year</em> (DOY) 291 dan 274. Perangkat lunak yang digunakan adalah Waypoint^® Grafnav. Hasil pengolahan menggunakan metode PPP tersebut kemudian dibandingkan dengan hasil pengolahan data Diferensial GPS (DGPS) dengan 1 titik ikat untuk DOY 291 dan 2 titik ikat untuk DOY 274. Hasil perbandingan pada DOY 291 menunjukkan nilai RMS untuk arah timur, utara dan tinggi masing-masing sebesar 0,024 m; 0,020 m dan 0,039 m. Pada DOY 274, RMS yang diperoleh adalah 0,032 m; 0,011 m dan 0,058 m masing-masing untuk arah timur, utara dan tinggi. Hasil-hasil tersebut mengindikasikan bahwa metode PPP dapat digunakan untuk menentukan posisi pesawat terbang dengan fraksi ketelitian sentimeter. Tingkat ketelitian posisi ini sudah memenuhi syarat untuk digunakan pada survei gayaberat <em>airborne</em>.</p><p class="katakunci"><strong>Kata kunci</strong>: GNSS, kinematik PPP, gayaberat airborne, DGPS</p><p class="katakunci"> </p><p class="abstrak"> ABSTRACT</p><p class="abstraking">The Precise Point Positioning (PPP) is a positioning method which only use a dual frequency GNSS receiver. This method can be used to determine the precise position of either static (static) or moving objects (kinematic). In this paper, we will discuss the application of kinematic PPP for the 2008 Sulawesi airborne gravity survey. By using a commercial GNSS processing software called Waypoint® Grafnav, we determine the PPP solutions for the aircraft trajectory of the day of year (DOY) 291 and 274. Each solution then be compared to the Differential GPS (DGPS) results, which use one base station for DOY 291 and two reference stations for DOY 274. The PPP solution of DOY 291 gives RMS error of 0.024 m eastward, 0.020 m northward, and 0.039 m upward. Moreover, the comparison of DOY 274 gives RMS error of 0.032 m eastward, 0.011 m northward, and 0.058 m upward. These centimeter level RMS errors show that PPP is a compatible positioning method for airborne gravity survey.</p><p class="katakunci"><strong><em>Keywords</em></strong><em>: GNSS, </em><em>k</em><em>inematic PPP, airborne gravity, DGPS</em><em></em></p>