The main goal of the TerraSAR-X Add-On for Digital Elevation Measurements (TanDEM-X) mission is the generation of a global digital elevation model (DEM) of unprecedented accuracy and coverage. The global TanDEM-X DEM product became available in 2016, surpassed all expectations, and became a reference for a wide range of Earth science, commercial, and geospatial applications. In addition, new information products, such as DEM change maps (DCMs), have been developed and are available to the geoscience and remote sensing community. Beyond the operational products, new science applications have been demonstrated and are summarized in this article, along with experimental data acquisitions. This article also aims to provide an overview of science activities with TanDEM-X data and science data acquisitions planned for the coming years.

The operational calibration of TanDEM-X Digital Elevation Model (DEM) raw scenes generated from 2017 onwards, using an interferometric delta-phase approach, achieves high vertical absolute accuracy for 90% of the acquired data. The remaining DEM scenes with calibration offsets affect the accuracy of subsequent products such as the TanDEM-X DEM Change Maps (DCM). This study aims to improve the calibration of DEM scenes by using Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) altimetry data as absolute height reference and the method is demonstrated with DEM scenes at two sites: Tangkeng, China with known calibration issues, and the city of Mthatha, South Africa, with good quality data covering. We filter ICESat-2 data to get reliable reference elevation points and perform plane fitting to estimate calibration offsets. An RMSE of 0.59 m and mean of 0.41 m are found on DEM scenes taken around Mthatha city. For the data taken at Tangkeng city, the RMSE is 0.4 m with mean offset of -3.26 m. These calibration results support the goal of the TanDEM-X 4D mission to monitor changes in surface elevation using time-series products.

In 2022, the TanDEM-X Mission entered in the so-called TanDEM-X 4D phase, which will last until the end of the mission. During this phase, the highly dynamic regions of the Earth’s landmass will be regularly acquired to continuously record topographic changes. The new TanDEM-X DEM Change Maps Times Series (or stacks), which are an extension of the TanDEM-X 30m DEM Change Maps released by the end of 2023, will allow global monitoring of the changes that have occurred throughout the mission. This is a major step forward for e.g. glaciology, open pit mining and deforestation monitoring.

The TanDEM-X mission has been acquiring continuously bistatic data from which DEM are derived since 2010. The outstanding quality of these DEMs enables the observation of changes that occur on the whole Earth’s landmass. This paper presents the new product of the TanDEM-X mission, the TanDEM-X 30m DEM Change Maps, which is available for free for scientific and non-commercial use since November 2023. Various applications which are primordial for climate change monitoring are introduced and show how the DEM Change Maps can be a very useful input to derive information about e.g. glacier melting, mining monitoring or deforestation globally. Finally, the next developments towards building DEM Change Maps time series are explained.

Single-pass Interferometric SAR satellite configurations offer unique opportunities for observing dynamic features of ice masses. Future cross-track interferometry constellations of satellite missions will produce interferometric DEMs over virtually all glaciers globally and will offer significant advancements for regular monitoring their surface elevation. In the present contribution we want to address some of the main issues encountered when mapping the topographic changes with multitemporal TanDEM-X InSAR DEMs over glaciers. We illustrate open questions for InSAR DEM data acquisition when targeting height change rate calculation for mountain and outlet glaciers at large scale.

Space-borne digital elevation models (DEM) are considered as important proxy for canopy surface height and its changes in forests. Interferometric TanDEM-X DEMs were assessed regarding their accuracy in forests of Germany and Estonia. The interferometric synthetic aperture radar (InSAR) data for the new global TanDEM-X DEM 2020 coverage were acquired between 2017 and 2020. Each data acquisition was processed using the delta-phase approach for phase unwrapping and comprise an absolute height calibration. The results of the individual InSAR heights confirmed a substantial bias in forests. This was indicated by a mean error (ME) between – 5.74 and – 6.14 m associated with a root-mean-squared-error (RMSE) between 6.99 m and 7.40 m using airborne light detection and ranging (LiDAR) data as a reference. The bias was attributed to signal penetration, which was attempted to be compensated. The ME and RMSE improved substantially after the compensation to the range of – 0.54 to 0.84 m and 3.55 m to 4.52 m. Higher errors of the penetration depth compensated DEMs compared to the original DEMs were found in non-forested areas. This suggests to use the penetration compensation only in forests. The potential of the DEMs for estimating height changes was further assessed in a case study in Estonia. The canopy height change analysis in Estonia indicated an overall accuracy in terms of RMSE of 4.17 m and ME of – 0.93 m on pixel level comparing TanDEM-X and LiDAR height changes. The accuracy improved substantially at forest stand level to an RMSE of 2.84 m and an ME of – 1.48 m. Selective penetration compensation further improved the height change estimates to an RMSE of 2.14 m and an ME of – 0.83 m. Height loss induced by clearcutting was estimated with an ME of – 0.85 m and an RMSE of 3.3 m. Substantial regrowth resulted in an ME of – 0.46 m and an RMSE of 1.9 m. These results are relevant for exploiting multiple global acquisitions of TanDEM-X, in particular for estimating canopy height and its changes in European forests.

The TanDEM-X mission acquires data used for the generation of Digital Elevation Models (DEMs) since 2010. From this data two global DEMs are already generated or in generation. The DEM acquisitions used for new TanDEM-X DEM 2020 are also used to generate TanDEM-X DEM Change Maps. Furthermore, the DEM Change Maps can be combined with the additional TanDEM-X DEM datasets and used for the generation of TanDEM-X DEM Change Map Stacks. This paper presents these new products on the basis of an example of an open-pit mining area in Australia. Additionally, the potentials and challenges of the DEM Change Map stacks are presented.

The Earth is a very dynamic system and the topographic height of its landmass changes over time, especially in forested areas, glaciers, permafrost regions or where human activities take place. After the TanDEM-X mission provided a first global DEM of unprecedented quality in 2016, a new complete coverage of the Earth's landmass was acquired mainly between 2017 and 2020. This data is used to create another global DEM. In addition to providing more up-to-date elevation information, these new acquisitions also provide a great dataset to show the changes that have occurred in the few years between the two global datasets. The new product - the TanDEM-X DEM Change Maps - will be produced in 30m and 90m postings and will focus on showing these changes between the first global TanDEM-X DEM and the newly acquired time-tagged DEM scenes. It will also include the in-house automatically edited TanDEM-X DEM.

The TanDEM-X mission has acquired multiple global coverages of data over the last years in order to create digital elevation models (DEMs). The data between 2017 and 2020 is processed to Change RawDEMs (CRaw DEMs). These scenes are successfully pre-calibrated individually during the processing. However, in order to determine and quantify terrain changes between the new data and the former global TanDEM-X DEM, the calibration can be improved even further. In the case of large-scale terrain changes like glaciers or forestation areas CRaw DEMs might be calibrated on the change instead of the smaller stable regions. A comparison of to the calibration of neighboring scenes gives information on which scene has to be corrected. This paper summarizes the pre-calibration during the processing of the CRaw DEMs and analyzes the results of the calibration and corresponding change detection. Furthermore, a method for a post-calibration of the CRaw DEMs is presented. This method will be used to create TanDEM-X DEM Change Maps in the future.

This paper introduces the new TanDEM-X DEM 2020. The TanDEM-X mission systematically acquired data mainly between September 2017 and mid-2020 to create another global DEM, the so-called “TanDEM-X DEM 2020”, formerly also called TanDEM-X Change DEM. The present paper describes the generation of the TanDEM-X DEM 2020 in terms of the adopted acquisition planning strategy, the new interferometric processing and DEM generation as well as the final DEM product specifications. The main differences from the existing global TanDEM-X DEM (2010-2014) are the new and independent time frame and a new interferometric processing technique with lower phase unwrapping errors, allowing for a mainly single-coverage acquisition strategy except for more difficult terrain. The fewer acquisitions slightly increase the random height errors but still very high accuracy. Examples for height changes on forests and glaciers between TanDEM-X DEM and TanDEM X DEM 2020 round off the paper with large-scale mosaics of Iceland and New Zealand.

Over the last years the TanDEM-X mission acquired data for a second global digital elevation model (DEM) the TanDEM-X Change DEM. This new DEM is temporally independent of the former global TanDEM-X DEM and therefore yields the possibility of change detection. In order to decrease the phase noise level the interferometric processing for the Change DEM has been upgraded. This also allows a more accurate change detection. Currently, the processing of the global data is performed operationally. It includes the detection of terrain changes and first examples of detected terrain changes can be presented.

The TanDEM-X mission is currently acquiring a new dataset to provide a temporally independent DEM, the so-called “TanDEM-X Change DEM”. This set of acquisitions taken between 2017 and 2020 has a clear temporal separation with respect to the data used for the generation of the TanDEM-X global DEM which were acquired between 2010 and 2015. Therefore, this new DEM aims to enable the characterization of terrain changes. Improvements in the acquisition planning process and in the data processing have been necessary to allow the generation of this Change DEM with fewer acquisitions but still very high accuracy. For this, the use of an edited TanDEM-X DEM as a "starting point" for the interferometric processing is mandatory.

In 2017, the TanDEM-X Mission decided to generate a second - more recent - global DEM. The acquisitions took place from 2017 till mid 2020 and represent a new global coverage of the whole Earth's landmass. This global dataset is well separated in time from the data used for the first global TanDEM-X DEM. Recent terrain height information can be delivered globally again with similar accuracy and consequently, terrain changes can be monitored. Currently, this data is being processed by the Integrated TanDEM-X Processor (ITP) into pre-calibrated single scenes. A reference DEM is a pre-requisite to enable a correct interferometric processing.

The “TanDEM-X Change DEM” will be a new DEM consisting of the data globally acquired by the TanDEM-X mission from 2017 until 2020. This new DEM aims to characterize terrain changes which occurred between the acquisition of the TanDEM-X global DEM, acquired between 2010 and 2015, and the new temporally independent and up-to-date data set. The new data will mostly contain only one global coverage. Therefore, necessary improvements in the acquisition planning process as well as in the interferometric processing were made. Specifically, a new adaptive filtering approach is presented in this paper as well as its influence on the interferometric phase and the DEM for test sites over Germany and Chile.

The production of the global Digital Elevation Model (DEM) clearly showed that, at this level of accuracy, height differences are visible between interferometric SAR data acquired at different periods. As a matter of fact, height changes in glaciers, forests but also those caused by agricultural activities or infrastructure changes are well defined in the X-band Therefore, in 2017 the mission decided to acquire an additional complete coverage of the Earth's landmass. The aim is to provide an independent DEM dataset within a well-defined time span from September 2017 until the end of 2019. This additional dataset will allow monitoring topographic changes with respect to the DEM on a global scale. Hence the name of the resulting product is TanDEM-X Change DEM. It benefits from improvements in the acquisition planning process and in the data processing which enable to achieve reliable DEM data of high accuracy with fewer acquisitions. For this goal, the use of an edited DEM as starting point for the processing is mandatory allowing us to detect some 3D elevation changes.

With the global TanDEM-X DEM generation being finished in 2016, the mission is now acquiring a new dataset to provide an independent DEM, the so-called "TanDEM-X Change DEM". It is based on a completely new dataset acquired from 2017 - 2019 in contrast to the acquisitions for the global DEM between 2010 and 2015 in the aim to characterise terrain changes. It benefits from improvements in the acquisition planning process and in the data processing which enable to achieve reliable DEM data of high accuracy with fewer acquisitions. For this goal, the use of an edited TanDEM-X DEM as "starting point" for the processing is mandatory. Detectable 3d elevation changes are presented exemplarily in Indonesian forest and on an outlet glacier in Antarctica.