Terra MODIS Frequently Asked Questions

General Questions

Is MODIS better than AVHRR?

MODIS has a number of advantages over AVHRR and SeaWiFS in terms of spectral resolution, number of channels and spatial resolution. Also, there are a number of ready developed products being produced from MODIS data by NASA. The difficulty with MODIS is that it is new and unlike SeaWiFS the file format is not similiar to AVHRR. This means that the data requires a bit more careful handling.

What File Formats are available?

As standard the MODIS data for L0, L1A and L1B comes in a Hierachical Data Format (HDF) file. HDF is a file format developed by NCSA for storing multi type datasets. Unfortunately, it requires a lot more than a simple import filter to read. At present, most image processing packages struggle to read MODIS data well. Also, the data still has the bowtie problem and most likely the calibration coeffecients will not be applied properly when the data is read. This is a problem with the import method and the chosen HDF implementation not the actual data. The different resolution datasets are stored within different files although subsampled versions of the higher resolution files are available in the low resolution files. Routines do exist for reading MODIS L1B data in IDL here.

We can extract the MODIS data, apply calibration coeffecients, and reproject to a choice of reprojections for you. In this case, the data is supplied in a flat binary, band sequential, floating point format. Other data formats may be produced if requested. We can ensure that if you want the highest resolution possible that the reprojection is done from the highest available resolution for that particular channel.

Is MODIS restricted? Do I need to apply to NASA to use it?

No, unlike SeaWiFS, MODIS data is available without need for approval.

Where can I find out about the status of MODIS?

There are weekly reports published at http://www.mcst.ssai.biz/IOT/weekly/index.shtml

How much data does MODIS produce?

MODIS produces data at two rates depending on  the spacecraft orbital position. When the spacecraft is over the sunny side of the Earth, MODIS produces 10.6 million bits of data per second (Mbps).This rate drops to 3.2 Mbps over the night side of the earth. At  present, we are only taking Day time data but we have sucessfully received night time data. If you have use for night MODIS data please let us know and we will receive it.

Whats the difference between Raw Data, Level 0, Level 1A and Level 1B?

We archive the Level 0 data so that any advances in the calibration and geolocation code are available to those requesting historical data.

Do you receive Night Time data?

We are regularly receiving Night Time data, although we have not done so for the whole of our MODIS archive.
Note that during night time NASA's policy is for all of the visible detectors to be switched off and only thermal emmisive channels are collected.

Geolocation

What is the Panoramic Bowtie Effect?

This is the effect whereby the image seems distorted near the edges. One of our clients refers to it as the earthquake effect and this is what it looks most like at the edges. This results from the fact that MODIS scans 10 lines at a time, unlike AVHRR and SeaWiFS which only scan 1 line at a time. What the satellite sees as a pixel, the footprint, increases with distance. Unfortunately the distance to a pixel increases with scan angle mainly due to earth curvature. This means that the pixels near the edge of an image are bigger than the ones in the middle. The satellite is configured to move forward 10km in the time that it takes to scan once. This is so that pixels in the middle of the scan line match up next to each other. At the edges though,the pixels are bigger, up to 6 times wider and 4 times longer. This results in oversampling, i.e. the same bit is imaged twice. The reason that such effects are not obviously seen with LANDSAT MSS data is that the earth curvature effect is smaller as the swath width of the instrument is smaller.

How do you remove the Panoramic Bowtie Effect?

The bowtie should really be removed after all scientific processing of the data has been completed, i.e. after atmospheric correction and product generation. This is so that the you know which look and solar angle is used for each pixel. An acceptable solution is to use a reprojection. The reprojection takes the geolocation product and uses it to produce a image with a standard projection. The reprojected image is free from the bow tie artifacts, the MODIS quicklooks are an example of this. The problem with this is that you aren't able to calculate solar and zenith angles easily from this and for scientific work it is suggested that these angles be requested at the same time and thus for each pixel as well as having the data value the user will also have all the angles.

What is Definitive Ephemeris?

The level1 processing software makes use of ephemeris from the data stream. It has been found that this can sometime result in poor geolocation. Version 1.2 of the IMAPP software is much better at dealing with poor geolocation than previous version however that can still be problems at the beginning and the end of the pass. The definitive ephemeris is produced by Goddard and corrects the majority of the issues that are found with the geolocation.

We have recently received a patch for the IMAPP 1.2 software that corrects the innaccuracies at the beginning and the end of the pass.

How accurate is the geolocation?

The geolocation accuracy available will depend on a number of factors such as scan angle. NASA use a high resolution Digital Elevation Model in the geolocation which IMAPP supports. Currently Dundee does not implement this as standard because it requires a longer processing time that is contrary to the aims of near realtime data dissemination. It is unlikely therefore that the geolocated data will be as accurate as is claimed by NASA, 150m within regions of variable terrain. Discussions with the IMAPP team suggest that an accuracy of around 250m should be expected. Ordered data is reprocessed to ensure that the software and calibration applied is most up to date and if real time delivery is not required the DEM is used in geolocation.

Why don't you like Spain, it's always cut out?

This is the result on a rather unfortunate conflict between the MODIS direct broadcast (DB) system on Terra and the Deep Space Network (DSN) receiver in Madrid. The DB system, which is separate from NASA's MODIS reception using relay satellites, is used to transmit data to gound stations such as ourselves. NASA's scientific investigation of the Solar System is being accomplished mainly through the use of unmanned automated spacecraft. The DSN provides the vital two-way communications link that guides and controls these planetary explorers, and brings back the images and new scientific information they collect. All DSN antennas are steerable, high-gain, parabolic reflector antennas. The problem is that MODIS DB operates on a similiar frequency to certain aspects of the DSN and obviously the signal from MODIS is much larger than the deep space probes. As a compromise NASA determines when the DSN and MODIS will conflict and switches off the DB during these periods. Aqua has a filter which prevents such interference so Aqua DB is operational over the DSN antennae. However Aqua DB is not operational during data dumps to the NASA antenna at Svalbard.

Why aren't some Northern regions visible in the reprojected quicklooks when you can see the area in the non-reprojected quicklook?

This only happens with the visible bands and is because it is dark during acquisition! Each scan from MODIS is categorised as either a night time scan or a day time scan. During the winter certain parts in the far north fall into the night time scan category so the data acquired might not be reliable. The data is present in the data stream but removed by processing to Level1. When reprojected the data is not available so its filled even though the scan is present within the file. If you look you will see that area is covered by the thermal channels.

Resolution and channel selection

What channels are available?

See the table showing the available MODIS channels.

Whats this about 250m, 500m and 1000m channels?

Certain channels are available at different resolutions (1 & 2 at 250m, 3 - 7 at 500m). Also the HDF files for the lower resolution data contains the higher resolution data down-sampled to the lower resolution. It should be remembered that 250m, 500m and 1000m is a nominal resolution and unless the image is reprojected the actual size of the pixel will vary with scan angle.

Why are there two channel 2's in the MODIS quicklook directory?

The reason for this is that they are generated at different times. Our system puts the unprojected quicklook into the directory as soon as the pass has been received. The projected quicklooks are generated from the Level1B data which takes up to an hour to run. We try to give users an image as soon as possible so they can determine whether it it worth checking back later.

Why are Bands 13 and 14 listed as high and low gain?

Bands 13 and 14 are each set up to provide two different sensitivities with the same bandpass, to satisfy different scientific needs. In order to maintain acceptable signal-to-noise ratio, each band consists of two parallel arrays of 10 identical detectors. The Time Delay Integration (TDI) circuit provides a precise delay between the signal for two adjacent detectors so that the signal from each detector samples the same geography. The two signals are added, then split and put through two different amplifications.

The signal sequence can be thought of as follows: The first detector receives light from a 1-km square area on the earths surface. One frame (333µs) later, the scan mirror has rotated to illuminate the adjacent detector with light from the same area The TDI circuit delays the first signal by exactly 333µs, and adds the two signals. The combined analog signal is split into two paths. A different gain is applied to each signal voltage path. Each analog signal is digitized. Each digital signal is read as a separate band.

Note that what we sometimes call a "detector" in Bands 13 and 14, high and low, would be more properly called a "channel" to distinguish signalpath from physical chunks of silicon.

Products

There are 44 distinct MODIS products which can be created from level-0 data.

Currently we are in discussions with NASA as to how to best bring the Product Generation software to Dundee. Once all the issues have been decided we will let you know.

In the mean time, we aim to provide the most useful channels as quicklooks and to create other simple product quicklooks. The products that we initially will provide are a Colour Brightness Temperature, a simple weekly NDVI composite and possibly a simple snow mapping composite. If you have any explicit requirements for quicklook channels or product quicklooks please contact support@sat.dundee.ac.uk.

The IMAPP team have released software for generating Level-2 Cloud products (cloud mask and cloud top properties) which we are currently investigating.


Home Page Dundee Satellite Receiving Station Home Page      Send Us A Comment Send Us A Comment