Navigation Camera (Navcam)
The Mars Science Laboratory (MSL) rover carries 4 Navigation Cameras (Navcams). Two Navcam stereo pairs (one as a redundant backup) are mounted on the Remote Sensing Mast just below ChemCam. The base of the vertical Remote Sensing Mast is located near the front starboard corner of the rover. The upper Navcam pair has a boresight 1.99 m above the base of the rover wheels and the lower pair's corresponding height is 1.94 m, which are just above and below the Mastcam boresight of 1.97 m height. The redundant cameras are connected to the backup rover computer and are not expected to be used unless there is a problem with the primary rover computer and/or primary cameras. Each Navcam pair is separated by a 42.4 cm stereo baseline. The primary use of the Navcams is to provide terrain context for mobility planning, and they also support science operations for selecting near field targets and robotic arm operations.
Each Navcam consists of a detector head and an electronics box. The detector head houses an optical lens assembly and a Charge Coupled Device detector (CCD). The electronics box contains the CCD driver electronics, the 12-bit Analog to Digital Converter (ADC), camera/rover interface electronics, and a heater resistor that will keep the box above the minimum operating temperature of 218 K. The Navcams use a broadband visible filter and produce 1024 x 1024 pixel images.
Each camera weighs 220 grams and uses approximately 2.2 Watts of power when the heater is not on. The MSL Navcam design is identical in design to the Navcams on the Mars Exploration Rovers, which are described in [MAKIETAL2003], with the exception of a more powerful 3.5 W heater on each MSL Navcam.
The chief objectives of the Navcams are:
- to support the operation of the rover on the surface of Mars by acquiring images of the terrain and landscape,
- to acquire images for support of auto-navigation, robotic arm operations, mobility planning (wheel sinkage and obstruction), and pointing of the ChemCam and Mastcam,
- to investigate the landing sites at cm/pixel resolution,
- to acquire images of the morphology of rocks and soil in order to provide clues about past geologic processes,
- to measure the distribution of rocks and soil around the rover as it moves from site to site,
- to calibrate and validate orbital remote sensing data, and
- to place rock and soil types encountered by the rover in a geological context.
The Navcams have been calibrated over the flight range of temperatures. They were all calibrated with respect to geometric flat field response, detector dark current, camera absolute responsivity, detector noise performance, and detector gain.
Each Navcam uses a 1024 x 2048 pixel CCD with 12 micron-square pixels and a 100% optical fill factor. The CCDs operate in frame- transfer mode, dividing the detector into two regions. One of the regions is a 1024 x 1024 pixel photosensitive imaging region where the image is recorded. The other region is a 1024 x 1024 shielded storage region where the recorded image is shifted and stored during detector readout. It takes 5.1 msec to transfer data from the imaging region to the storage region, and 5.4 seconds for readout of data from the storage region. Each CCD includes 32 non-imaging pixels in the serial readout registers, which allow the monitoring of the CCD electronics offset and detector noise performance. The Navcam CCD pixels have full well capacities of approximately 170,000 electrons and are digitized at 12 bits/ pixel. The RMS read noise at cold temperatures (-55 degrees C) is approximately 20 electrons, and the detector system has gain values of approximately 50 e-/DN, which results in a system with approximately 0.5 DN of RMS read noise.
The absolute quantum efficiency (QE) of the CCDs has been measured between 400 and 1000 nm at four operating temperatures ranging from 218 K and 278 K. The QE of the MSL CCDs is typical of a silicon CCD detector, with sensitivity peaking at 700 nm with a QE value of approximately 43%. The SNR of the detector system is essentially Poisson-limited because of its low readout noise and small dark current rates in the Martian operating environments.
The Navcam CCDs use a 'clocked antiblooming' readout technique, instead of having anti-blooming circuitry. The proper choice of autoexposure parameters prevent the blooming effect.
The Navcams also have the capability for onboard image processing. The Navcams can do pixel summation in which they sum the rows and columns of an image, returning the results in a one-dimensional array of 32-bit integers whose length is the image height. These are very useful when the spatial content of an image has relatively low scene entropy and the emphasis is on the radiometry. The flight software (FSW) can also calculate and return the histogram of an image, which can be useful for atmospheric observations. Navcam images can be spatially downsampled to a user-specified image size using one of a few techniques; nearest neighbor computation of the mean, computation of the mean with outlier rejection, and median averaging. The FSW also allows the option to downlink a subframed region of an image so full resolution can be downlinked at a lower data volume. The FSW is also capable of lossy and lossless compression of MSL images. The rovers use a ICER wavelet image compressor for the lossy compression, and low complexity (LOCO) for lossless compression.
The Navcams use a combination of three filters (Schott OG590, KG5, and ND1.3) to create a red bandpass filter centered at 650 nm and a FWHM of approximately 140 nm.
The Navcams have f/12 cameras with a 14.67 mm focal length. The Navcam optics are f-theta fisheye lenses with 45 degree x 45 degree horizontal/vertical field of view and a 67 degree diagonal field of view. They have an angular resolution at the center of the picture of 0.82 milliradians/ pixel. The field depth of the Navcams ranges from 0.5 m to infinity. The nominal exposure time for a noontime image on Mars is approximately 0.25 seconds. This time is 50 times the frame transfer time of 5.1 ms. This ensures that the image signal is significantly than the image smear acquired during the frame transfer. The spectral range of the Navcams is 600-800 nm.
The Navcams are attached to metal brackets mounted on Remote Sensing Mast, just above and below the Mastcam cameras, at a wider stereo- pair separation than the Mastcam cameras.